PropertiesandRadiationResponse ofOpticalFibers: RoleofDopants · 2013-02-18 · UNIVERSITÉ JEAN MONNET OF SAINT-ETIENNE (FRANCE) and UNIVERSITÁ DEGLI STUDI OF PALERMO (ITALY) Cotutelle

UNIVERSITÉ JEAN MONNET OF SAINT-ETIENNE (FRANCE)

and

UNIVERSITÁ DEGLI STUDI OF PALERMO (ITALY)

Cotutelle Ph.D. Thesis

Giusy Origlio

Properties and Radiation Responseof Optical Fibers:Role of Dopants

TUTORS:Prof. Youcef OuerdaneProf. Marco Cannas

Ph.D. RAPPORTEURS:Prof. Roberto BoscainoProf. Linard Skuja

To my new and to my old family

Contents

Contents 1

Introduction 1

I State of the art 3

1 The silica optical fibers 5

1.1 General structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1.1 Light propagation in step-index optical fibers . . . . . . . . . . . . . . 6

1.1.2 Single-Mode and multi-mode optical fibers . . . . . . . . . . . . . . . . 8

1.1.3 Dispersion and losses in fibers . . . . . . . . . . . . . . . . . . . . . . . 11

1.1.4 Fiber Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2 Dopants in optical fibers 17

2.1 Germanium doped optical fibers . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2 Fluorine doped optical fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3 Phosphorus doped optical fibers . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4 The optical fibers under irradiation exposure . . . . . . . . . . . . . . . . . . . 22

3 Point defects in optical fibers 25

3.1 Intrinsic point-defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1.1 Oxygen Deficient Centers . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1.2 Oxygen associated hole centers . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Extrinsic point-defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.1 Ge-related defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.2 P-related defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

II Materials and methods 37

4 The canonical samples 39

4.1 Tested optical preforms and fibers . . . . . . . . . . . . . . . . . . . . . . . . . 40

5 Experimental set-ups 47

5.1 Irradiations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.1.1 UV laser irradiations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.1.2 γ-ray and X-10 keV irradiations . . . . . . . . . . . . . . . . . . . . . . 48

5.2 Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.3 Photoluminescence and Raman spectroscopy . . . . . . . . . . . . . . . . . . . 50

5.3.1 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.3.2 Stationary and time resolved luminescence setup . . . . . . . . . . . . . 52

5.3.3 Photoluminescence under synchrotron radiation excitation . . . . . . . 54

5.3.4 Raman measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.3.5 Confocal Micro-spectroscopy setup . . . . . . . . . . . . . . . . . . . . 55

5.4 Electron Paramagnetic Resonance measurements . . . . . . . . . . . . . . . . . 57

III Ge-doped fibers and preforms 59

6 Measurements on non-irradiated samples 61

6.1 Discussion: the drawing effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

7 Effects of the UV and X-ray irradiation 69

7.1 EPR results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

7.2 Optical absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

7.3 Discussion: radiation effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

7.3.1 Localization of defect species . . . . . . . . . . . . . . . . . . . . . . . . 75

7.3.2 Generation processes of GECs defects . . . . . . . . . . . . . . . . . . . 77

7.3.3 The drawing effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

IV Influence of further dopants: fluorine and phosphorus 83

8 F-doped fibers and preforms 85

8.1 Raman results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

8.2 EPR measurements on irradiated samples . . . . . . . . . . . . . . . . . . . . 89

8.2.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

8.2.2 Discussion: generation of E′ centers . . . . . . . . . . . . . . . . . . . . 91

9 P-doped fibers and preforms 93

9.1 Optical activity of P-related point defects . . . . . . . . . . . . . . . . . . . . . 93

9.1.1 Absorption and photoluminescence analysis . . . . . . . . . . . . . . . 93

9.2 Discussion: luminescent P-defects structure . . . . . . . . . . . . . . . . . . . . 101

9.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Conclusions 107

List of related papers 109

List of communications to congresses 111

Bibliography 113

Introduction

Today the circulatory system that sustains our communication society is made up by opticalfibers. These low-loss glass fibers facilitate worldwide broadband communication such as theInternet. Light travels in thin guides of glass, and it carries almost all of the telephony anddata traffic in every direction. Text, music, images and video can be transferred around theworld in a fraction of second.

If unraveled, all of the glass fibers that wind around the globe would turn into a singlethread over one billion kilometers long-which is sufficient to encircle the globe more than 25000times-and which is still increasing by thousands of kilometers every hour. Global communi-cation, and in particular internet and long-distance telephony, is now based mainly on opticalfiber technology.

The main benefit resulting from the use of optical waves with respect to radio waves isthe high frequencies that allow high data transmission rate. Today, it is possible to transmitseveral terabits per second in a single fiber and that represents an improvement by a factorof one million to what could be obtained fifty years ago with radio signal transmission. Thenumber of optical fiber cables being installed all over the world is increasing rapidly. Fiberoptics is also important for a huge number of other applications in medicine, laser technologyand sensors.

In order to be developed and manufactured, the optical fiber needed modern glass tech-nology. Furthermore, a reliable light source was also needed and this was provided by semicon-ductor technology. Finally, a clever network needed to be assembled and extended, consistingof transistors, amplifiers, switches, transmitters and receivers, as well as other units, all work-ing together. This telecommunications revolution was made possible by the work of thousandsof scientists and inventors from all around the world. Even if the optical fibers have been sointensively investigated over the years, the interest of the scientific community is still alive:in fact the Nobel Price in Physics 2009 was awarded to C. K. Kao, whose discoveries havepaved the way for optical fiber modern technology. In 1966, Kao understood that it was notimperfections in the fiber thread that was the main responsible for losses, instead it was theglass that had to be purified, because of the presence of defects. He admitted that this wouldbe feasible but very difficult. The goal was to manufacture glass of a transparency that had

2 Introduction

never been attained before.

Even in the glass fiber of the highest purity, the signal, however slightly, is reduced alongthe way and needs reinforcement when it is transmitted over longer distances. This taskpreviously required electronics, while it is nowadays performed by optical amplifiers. This hasallowed to overcome the unnecessary losses that occur in the transformation of light to andfrom electronic signals.

Furthermore, choosing which fiber to use is subject to so many different technical consid-erations, communication needs and costs, that it is not possible to speak of only one singlekind of fiber. The fibers are based on a complex interplay between size, material properties,and wavelengths of light.

Following the interest in the field, this Thesis deals with the experimental study of thespectroscopic properties of three types of prototype preforms and associated fibers. The sam-ples have been designed and fabricated to investigate the role of germanium (Ge), fluorine(F) and phosphorus (P) doping elements on the fiber attenuation and eventually on the ra-diation sensitivity of silica-based glasses. We characterized the behaviors of these canonicalsamples before, during and after irradiation through several spectroscopic techniques, to ob-tain global information (electron paramagnetic resonance) or spatially-resolved information(confocal microscopy, absorption and luminescence on preform).

The Thesis is organized in four parts. Part I, comprising Chapters from 1 to 3, dealswith the general system of optical fiber communication providing an extensive overview ofthe history, construction, operation, and benefits of optical fiber, with particular emphasis onthe importance of the doping procedure to enhance the fiber characteristics. An overview ofthe main intrinsic and extrinsic defects in silica is also presented. Part II includes Chapters4 and 5 and it is devoted to the description of the prototype samples and of the adoptedexperimental techniques. Part III, including Chapters 6 and 7, reports on the experimentson Ge-doped samples and their main results. The results concerning F-doped and P-dopedsamples are reported and discussed in Part IV, comprising Chapters 8 and 9. Finally, themost relevant "conclusions" are summarized. A "list of the scientific papers" comprising theresults presented in this Thesis and a few others on closely related topics are reported at theand of the manuscript, together with a list of communications to congresses.

Part I

State of the art

Chapter 1

The silica optical fibers

Optical fibers lie at the very heart of modern society, providing the information superhighwaysrequired within our global communication systems. Fiber-optic communication is based on theprinciple that light in a glass medium can carry more information over longer distances thanelectrical signals can do in a copper or coaxial medium or radio frequencies through a wirelessmedium. The purity of today’s glass fiber, combined with improved system electronics, enablesfiber to transmit digitized light signals hundreds of kilometers without amplification. Withfew transmission losses, low interference, and high bandwidth potential, optical fiber is analmost ideal transmission medium. Thanks to high transmission speed, low attenuation andinterference and the large bandwidth, optical fibers represent at now the major progress indata transfer.

The purpose of this chapter is to introduce the main basic fiber features starting with adescription of some general properties of silica optical fibers and showing the reasons of themassive scientific investment in optical fiber telecommunications technology nowadays.

1.1 General structure

An optical fiber is essentially a dielectric waveguide used for information transfer. The fibercommunication is based on the principle that the light in a glassy medium can carry moreinformation and at longer wavelengths in comparison to the electrical signal transferred byclassical cables. The usual telecommunication optical fibers are made of two cylindrical parts:the interior part is called core, the exterior one is the cladding (Figure 1.1).

Core and cladding have different refractive index: the exterior part has a smaller refrac-tive index than the inner one, allowing light reflection according to the classical geometricaloptics laws. The two fiber portions are generally made of the same glassy material where the

6 1. The silica optical fibers

coren1

buffer

jacketcladdingn2

Figure 1.1: Structure of a classical optical fiber for telecommunications.

refractive indexes are varied and accurately controlled during the fiber fabrication throughdopant incorporation in the silica-based matrix. A classical fiber for telecommunication hasan exterior diameter of about 125 µm, while the core diameter varies from few µm to 60 µm,depending on the network requirements a. Such a fiber could prove to be mechanically fragile,so it is necessary to cover it with two protective coatings: a first vitreous buffer and a poly-meric exterior jacket.

1.1.1 Light propagation in step-index optical fibers

The most basic function of a fiber is to guide light, i. e., to keep light concentrated over longerpropagation distances despite the natural tendency of light beams to diverge, and possiblyeven under conditions of strong bending.

A very important concept in fiber optics is that of waveguide modes. These are fieldconfigurations which maintain their intensity profile during propagation, apart from possiblepower losses. Of highest interest are usually the guided modes, i. e. those modes which havesignificant intensity only in or near the core. Depending on the fiber design and the opticalwavelength, some number of guided modes may exist, or only a single one, or even no guidedmode at all. A fiber with only one guided mode is called a single-mode fiber, and multi-modefibers support several guided modes (section 1.1.2).

aWe can also find multimode fibers with cores of 100, 200 µm, or more. Such fibers are used for peculiarapplications (like sensors) and they are not routinely used for telecommunication networks.

1.1. General structure 7

The propagation mechanism inside an optical fiber can be approximatively described bygeometrical optics principia. The used ray picture cannot be applied to fibers with a smallcore or a small refractive index contrast between core and cladding: the approximation scaleimproves with reduction in λ/r ratio, where r is the optical fiber core and λ is the lightwavelength propagating inside. The reason is that wave effects occur: a real beam has somefinite width, and the incident and reflected wave interfere with each other. Furthermore, theoptical field somewhat extends beyond the core/cladding interface. Therefore, the ray pictureis only a rough approximation for strongly guiding large core fibers, while a wave analysisthrough Maxwell equations is required for the more general case.

The research activity described in this PhD Thesis is related to multi-mode optical fibers,for which the ray optics approximation is effective: therefore only a description of light prop-agation on fiber by means a geometrical approach is here supplied. The loss of generalitythat such choice implies is partially balanced by the immediate physical interpretation of theresults and the simple visualization of the propagation processes.

It is common to explain the guiding effect as a result of total internal reflection. A lightbeam S approaching the separation interface between two transparent and homogenous media,with refractive index n1 and n2 respectively, is partially reflected and partially refracted. Ifθ1 is the incident angle against the normal direction, the refractive beam will propagate in n2

medium in accordance with the Snell law:

n1 sin(θ1) = n2 sin(θ2) (1.1)

The same mechanism is involved in the optical fiber: the core refractive index n1 is greaterthan the cladding one n2 and the refracted angle is greater than the incident one (θ1<θ2).For incidence angle values greater than θ1=θc=arcsin(n2/n1), called critical angle, there isno refracted angle. This is the total internal reflection phenomenon which is at the basis ofthe optical fibers working. All the rays propagating inside the core with an angle θ > θc,will be totally reflected and trapped inside the fiber. Typical values for the optical fibers aren2 = 1.475, n1 = 1.5; θc = 79.5◦.

It is possible to define an acceptance cone (Figure 1.2) containing all the rays propagatinginside the core through total internal reflection. The cone vertex is the center of the entryface of the fiber and the vertex angle is called acceptance angle θA.It is also possible to get a measure of the coupling efficiency between the source and the fiberdefining the so called numerical aperture (N.A.) defined byb:

N.A. = sin(θA) =√n2

1 − n22 (1.2)

bIf the core radius a is much larger than the wavelength λ, a geometrical-optics description for the propa-gation of light is valid. However, when a is in the order of λ, a wave-propagation theory is needed.


θA

Ray lost into

the cladding

Ray outside the

acceptance cone

Acceptance cone

Figure 1.2: Acceptance cone in an classical optical fiber

1.1.2 Single-Mode and multi-mode optical fibers

As anticipated in section 1.1.1, based on the number of modes propagating through thefiber, there are multi-mode and single mode fibers [1].

Figure 1.3: Optical fiber sizes

Multi-mode fibers

Multi-mode fiber was the first type of fiber to be commercialized. It has a much largercore than single-mode fiber, allowing hundreds of modes of light to propagate through thefiber simultaneously. Multi-mode fibers routinely used for telecommunication networks havecore sizes of 50 to 62.5 µm in diameter, while the overall diameter is about 125 to 200 µm


(Figure 1.3). Based on the refractive index profile we have two types of fibers: (a) Step indexfiber (b) Graded index fiber.

(a) Step index fiber : in the step index fiber, the refractive index of the core is uniformthroughout and undergoes an abrupt or step change at the core cladding boundary.The light rays propagating through the fiber are in the form of meridional rays whichwill cross the fiber axis during every reflection at the core cladding boundary and arepropagating in a zig-zag manner as shown in Figure 1.4a. When light is launched into amulti-mode fiber, multiple guided modes can be excited, and at the fiber exit, there isan intensity profile which arises from the interference of light in all these modes. In thiskind of fiber there is a considerable modal dispersion (section 1.1.3): even rays with thesame wavelength but emitted at a different incident angles (lower than the acceptanceangle) propagate with the same speed into the fiber but across different length paths.They will arrive at the fiber end at distinct times, producing a temporal broadening ofthe transmitted pulse.

(b) Graded index fiber : in the graded index fiber, the core refractive index is made to varyin a parabolic manner so that the maximum value of refractive index is at the center ofthe core. The light rays propagating through it are in the form of skew rays or helicalrays which will not cross the fiber axis at any time and are propagating around the fiberaxis in a helical or spiral way as shown in Figure 1.4b. In the case of multi-mode gradedindex fiber, signal distortion is very low because of self-focusing effects. Here the lightrays travel at different speeds in different paths of the fiber because of the parabolicvariation of refractive index of the core. As a result, light rays near the outer edge travelfaster than the light rays near the center. In fact, the rays are continuously refocused asthey travel down the fiber and almost all of them reach the exit end of the fiber at thesame time due to the helical path of the light propagation.

Multi-mode fibers are strongly required when light from a source with poor spatial co-herence has to be transported. As an example, the output of a high power diode bar containsthousands of modes, and requires a correspondingly large number of fiber modes. Additionally,the larger core diameter facilitates the use of lower-cost optical transmitters and connectors.

In most applications, the standard multi-mode graded index optical fibers have significantperformance advantages over conventional copper-based systems: they are very useful whenmultimodal transmission is needed for relative long distances. However, performance require-ments and cost restraints may prohibit the use of these fibers in certain applications. Firstof all it is very complex and expensive to realize a graded-index fiber in which the refractiveindex varies continuously during all the fabrication process. Sometimes fiber manufacturersmodify standard material composition and structural design to meet these additional require-ments. The intent of each change is to increase performance and reduce cost. For instance it


Figure 1.4: Different propagation modes in (a): multi-mode step index, (b): multi-mode graded-indexand (c): single-mode step-index fibers.

is possible to obtain the optimal characteristics of a graded-index fiber in the so called multi-step index fiber. As its name indicates, the structure, showed in Figure 1.5, uses multiple stepindexes which approximate the parabolic curve of the refractive index profile. Although the

Figure 1.5: Refractive index profile in multi-step index fiber.

basic principle is the same as that of step index fiber because the index of refraction changesin multiple steps, the locus of the light is shifted toward the center at the same time. In anycase, with enough number of steps, differences from a graded-index become small and theycould be neglected. So it is possible to reconcile the advantage of a little modal dispersionwith a more easy fiber production at reasonable prices.

Single-mode Fibers


In a single mode fiber, only one mode can propagate through its core (Figure 1.4c). Thesingle mode fiber has a smaller core diameter (10 µm, Figure 1.3) and the difference betweenthe refractive indices of the core and the cladding is very small. Its fabrication procedurecould be very difficult and the launching of light into single mode fibers is also hard. Theadvantages of single mode optical fibers lie in the very low transmission loss and dispersion ordegradation, thus resulting very useful in long distance communication.

1.1.3 Dispersion and losses in fibers

Dispersion in the fiber means the broadening of the signal pulse width due to dependence ofthe refractive index of the material of the fiber on the wavelength of the carrier. If we senddigitized signal pulses in the form of square pulses, they are converted into broadened gaussianpulses due to dispersion. The dispersion leads to the distortion or degradation of the signalquality at the output end due to overlapping of the pulses. There are two kinds of dispersionmechanisms in the fiber: intramodal dispersion and intermodal dispersion.The first one arises due to the dispersive properties of the optical fiber material (materialdispersion) and the guidance effects of the optical fiber (waveguide dispersion). Further itincreases with the increase in spectral width of the optical source.Intermodal dispersion or multi-mode dispersion arises due to the variation of group velocity foreach mode at a single frequency. Different modes arrive at the exit end of the fiber at differenttimes. So there is multi-mode dispersion and hence there is broadening of the signal pulses.The multi-mode step index fibers exhibit a large value of dispersion due to the enormousamount of multi-mode dispersion which gives the greatest pulse broadening. At the same timethe multi-mode graded index fiber exhibits an overall dispersion which is 100 times lesser thanthe multi-mode step index fiber’s dispersion. This is due to the shaping of the refractive indexprofile in a parabolic manner. In the case of single mode step index fibers, they have onlyintramodal dispersion.

Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km) and it isa function of wavelength as shown in Figure 1.6.Attenuation is caused by several different factors, but primarily diffusion (Raileigh scattering)and absorption. It can be classified into two types: intrinsic and extrinsic losses generated byseveral mechanisms:

• Tail of infrared (IR) absorption by Si-O coupling that it is present at higher wavelengthsaround 1.4 µm to 1.6 µm.

• Tail of ultraviolet (UV) absorption due to electron transitions and present at lowerwavelengths near 0.8 µm. This produces a loss of 0.3 dB/km.


Figure 1.6: Spectral attenuation of a silica optical fiber.

• Rayleigh scattering (Figure 1.7) originates from microscopic irregularities in the glass

Figure 1.7: Illustration of Rayleigh scattering effect.

structure; it is inversely proportional to λ4 and in many cases it can be expressed asc:

αR[dB/km] = 1.7

(0.85

λ[µm]

)4

(1.3)

It produces high losses mainly in the ultraviolet region. In the wavelength region around0.8 µm to 1 µm, it gives a loss of 0.6 dB/km.

• Absorption: conversion process of electromagnetic wave energy into other forms (i. e.lattice vibration). Intrinsic silica glass absorption occurs in both ultraviolet and infrared

cEquation 1.3 is sample-dependent: actually Rayleigh losses depend on the core composition. The formulapredicts 0.15 dB/km at 1.57 µm, while lower Rayleigh losses of 0.12 dB/km have been reported by Nagayamaet al. [2].


bands, in particular infrared absorption tail causes attenuation for the wavelengths longerthan 1.6 µm. Further attenuation is caused by light absorbed by residual species, suchas metals or OH ions, within the fiber core and inner cladding. In particular OH causesthe water peak region on the attenuation curve, typically around 1.4 µm. The removal ofOH ions is of primary interest to fiber manufacturers as this water peak has a broadeningeffect and contributes to attenuation loss for nearby wavelengths. Figure 1.8 shows thespectral attenuation of different material fibers.

Figure 1.8: Spectral attenuation of different material fibers.

For silica fiber, the lowest losses of about 0.12 dB/km can be obtained in the region around1.55 µm [2]: at longer wavelengths, the attenuation increases. An optical signal transmittedthrough fiber, could travel more than 100 km without regeneration or amplification.

Other attenuation mechanisms are due to macroscopic bends, occurring when installingfibers, microscopic bends, due to local distortions of the fiber geometry, and nonlinear scatter-ing.

Optical power propagating in a fiber decreases exponentially with distance:

P (z) = P0 exp(−α′z) (1.4)

where P is the optical signal power and α′ is the attenuation coefficient [1/km].Using a logarithmic scale we obtain:

logP (z) = −αz/10dB + logP0 (1.5)

where α is the logarithmic attenuation coefficient measured in [dB/km].

Overall optical fibers offer superior performances over other transmission media becausethey combine high bandwidth with low attenuation. These properties allow the transmissionof signals over longer distances while using fewer regenerators or amplifiers, thus reducing costand improving signal reliability.


1.1.4 Fiber Fabrication

The manufacture of an optical fiber takes place into two steps: the preform fabrication andthe drawing process. Preform is a cylinder of silica composition from 10 mm to some cm indiameter and from 60 to 120 cm lengthd. It consists of a core surrounded by a cladding witha desired refractive-index profile; in other words, this is a desired optical fiber, but on a muchlarger scale. The main reason a preform is prepared is to have a drawable material that is clean,low in OH concentration, low in metallic-ion contaminants, and inexpensive. Many techniqueshave been developed to prepare these preforms. Some common commercially used methodsare Outside Vapor-Deposition (OVD), Modified Chemical Vapor Deposition (MCVD), VaporPhase Axial Deposition (AVD), and Plasma Chemical Vapor Deposition (PCVD) and PlasmaModified Chemical Vapor Deposition (PMCVD) [3]. All these methods are based on thermalchemical vapor reaction in which two gases, SiCl4 and O2, are mixed at a high temperature(>800 ◦C) to produce silicon dioxide (SiO2):

SiCl4 +O2 → SiO2 + 2Cl2 (1.6)

Silicon dioxide, or pure silica, is usually obtained in the form of small particles (about 0.1 µm)called soot. This soot is deposited on the target rod or tube layer upon layer and it forms ahomogeneous transparent cladding material. To change the value of the cladding’s refractiveindex, some dopants are used. For example, fluorine (F) is used to decrease the cladding’srefractive index in a depressed-cladding configuration. The soot for the core material is madeby mixing several gases which results in a mixture of SiO2 and of the core dopant. The degreeof doping is controlled by changing the amount of dopant gas added to the mixture. Sincedeposition is made by the application of silica layers, the manufacturer can control the exactamount of dopant added to each layer, thus controlling the refractive-index profile.

The different preparation methods differ mainly by the way the soot is deposited. Thepreforms studied in this PhD thesis were all made by MCVD process which provided a simpleand straightforward means of manufacturing high-quality optical fibers.This method was developed by Bell Laboratories [4]. The soot is deposited on internal wall ofthe tube (Figure 1.9) and then vitrified by the traversing burner to provide a thin glass layer.The procedure is repeated many times as the cladding and core layers are formed. Whenthe deposition is finished, the temperature of the burner is increased (≈1700 ◦C) to collapsethe tube into a solid preform [5]. The entire process is highly automated and all processparameters are precisely controlled.

Optical fibers are obtained by drawing from the preform at high temperature (≈2000 ◦C).The drawing process must be integrated with the coating process to avoid contamination offiber surface. These processes are shown schematically in Figure 1.10. The tip of the preform is

d It remains difficult to have an idea of the maximum preform diameter as this is confidential for the fibermanufacturers


Figure 1.9: Deposition by modified chemical vapor deposition (MCVD) process.

Figure 1.10: Optical Fiber Drawing Process

heated in a furnace to a molten state. Formed molten gob falls down under the force of gravitywhile shrinking in diameter into a proper diameter strand. It is controlled continuously duringthe drawing process. Diameter drift cannot exceed 0.1%. The strand is threaded through aseries coating applicators immediately after drawing. Liquid prepolymer coatings are curedby thermal or ultraviolet apparatus. Dual coating, soft inner and hard outer, is needed toprotect against impact and crushing forces in both manufacturing process and installation.


The fiber with coatings is pulled down and wound on a winding drum. The drawing processmust take place in controlled atmosphere, because air pollution influences fiber attenuation.Both stages of fiber manufacturing are fully automated and are performed in a clean, climate-controlled room. Obviously, the manufacturers use high-precision measuring equipment toautomatically control each step of the fabrication process. For example, preform analyzersmeasure the critical characteristics of the optical-fiber preform. Also, specific measurementsystems control fiber geometry, the refractive-index profile, and the coating geometry.

Chapter 2

Dopants in optical fibers

As reported in section 1.1, fundamental condition to having light propagation in optical fibersis the different refractive index between core and cladding. To realize an index variation ina−SiO2, dopants are usually added in the glass matrix.

Depending on the use and characteristics of the optical fibers, several elements can beadded to modify the fiber characteristics. GeO2 and P2O5 are dopants commonly used fordoping the core region, raising the refractive index. On the other hand B2O3 or F are dopantschosen for the cladding region that in turn lower the refractive index (see Figure 2.1). Several

Figure 2.1: Refractive index as a function of dopant materials and their concentration (from ref. [6]).

dopants can be added in more special fibers to functionalize the glass, as rare-earth ions(erbium, ytterbium [7]) for fiber-based amplifiers, or fluorine to improving the fiber radiationhardness (see section 2.4).

The nature of the elements (impurities or dopants) contained in fibers deeply influences

18 2. Dopants in optical fibers

the optical properties of the fibers themselves: dopants can modify the fiber hardness underradiation exposure or simply influence the drawing process.The dopants need to have the following characteristics [8]:

• It is of high purity and easily available.

• It is easy to liquify.

• It differs from the transition metal in vapor pressure.

• It is easy to vitrify with silica and gives a proper refractive index.

• After being vitrified, its coefficient of thermal expansion is nearly equal to that of SiO2.

• When vitrified, it has stable properties.

The following sections are devoted to a review of the influence of three particular dopantsoften used in optical fiber technology: germanium, fluorine and phosphorus.

2.1 Germanium doped optical fibers

The addition of germanium in the silica matrix, disguised as GeO2, allows to increasethe refractive index of the glass. This property is often used for the elaboration of the opticalfiber core and Ge has been the first traditional dopant used in fiber. Ge-presence does notaffect the fiber losses in the telecommunication windows (1300-1500 nm) (save for the increaseof Rayleigh scattering due to density fluctuations), but it can produce the apparition of newenergy levels within the silica band gap, thus leading to detrimental losses of part of thetransmitted signals into the fibers (see section 3.2.1 for details).

The scientific interest for germanosilicate glass increased even more after the experimentaldiscovery of the property of photosensitivity of this material. Photosensitivity of a medium isdefined as its capacity to have its refractive index permanently changed by a modification ofits physical or chemical properties through UV light exposure. Photosensitivity is a complexphenomenon and it is not well understood yet because of the influence of many parameters:fiber composition, fabrication process, operation wavelength and even light sources. Photo-sensitivity was first observed in 1978 by Hill et al. [9] at the communication Research Centrein Canada. The experiment consisted of injecting light from a single frequency Argon laser(514 nm) into the core of a Ge-doped silica fiber. Hill observed that a fraction of the inputpower was reflected by the fiber itself and this phenomenon was attributed to the formationof a permanent index grating. Progress in optical fiber photosensitivity research developedrapidly after the discovery of the possibility to write Fiber Bragg Gratings (FBG) into the

2.1. Germanium doped optical fibers 19

fiber illuminating the core from the fiber’s side with the interference pattern of two beams ofcoherent UV radiation [10], as shown in Figure 2.2. Spectroscopic studies of Ge-doped fibers

Figure 2.2: Inscription of a Fiber Bragg Grating on the core of an optical fiber

before and after intense UV exposure have been interpreted as pointing to a color center modelfor photosensitivity, in which a Ge-related defect optical activity (see section 3.2.1) at the ex-posure wavelength (242 nm) is bleached [10, 11]. Nevertheless many studies provided someadditional clues about the microscopic mechanisms of photosensitivity, such as laser-induceddensification [12,13,14,15].

Apart from the photoinduced change of the isotopic refractive index, it was discoveredin 1985 by Parent et al. that photoinduced birefringence could also be written into fibers bypolarized radiation [16]. Additionally, in 1986 Osterberg et al. discovered that a prolongedexposure of an optical fiber to 1064 nm light from a Nd:YAG laser results in the generation ofsecond harmonic light at 532 nm [17,18]: optical nonlinearity was discovered in germanosilicateglasses.

These features have a deep impact in several applications and therefore they stimulatethe strong interest in the study of Ge-doped amorphous silica (a−SiO2), to understand themicroscopic mechanisms at the basis of the properties of the material with the aim to controland enhance them. Usually the largest part of scientific investigation on Ge-related glassesfor optical fibers consists in the direct study of bulk samples and the subsequent transfer ofinformation to the fibers [19, 20, 21]. However, this approach cannot take into account thepeculiarities implied in the fiber preparation procedure, such as the drawing process, whichcan generate precursors and influence the defect generation [22] and the necessity of directstudies on fibers samples strongly emerges.


2.2 Fluorine doped optical fibers

Fluorine is an important dopant in optical fiber technology. In contrast to most of otherdopants, F decreases the refractive index of silica glass [23] (Figure 2.1). This property is ofgreat practical importance for designing optical fibers with an undoped high-purity silica core.Such fibers exhibit the best performance in the UV and IR spectral regions and have a betterdurability in environments with an increased level of ionizing radiation. So fluorine-doped silicais a promising key material in optical fiber technology directed to applications requiring highand stable transmission over a broad spectral range from infrared to ultraviolet. Indeed, recentstudies have shown that radiation toughness of silica samples is achieved by incorporatingSi − F groups (Figure 2.3) whose positive effect is assumed to be the reduction of defectprecursors [24,25], such as strained bonds (≡ Si−O) from which is likely generated the pairof silicon dangling (≡ Si•) and oxygen dangling (≡ Si−O•), where (≡) and (•) indicate bondswith three oxygen atoms and an unpaired electron, respectively (see section 3.1). Fluorine is a

Figure 2.3: Schematic illustration of fluorine incorporation in a−SiO2 matrix (from ref. [24]).

silica network modifier, because it considerably affects the viscosity and softening temperatureof silica glass [26,27]. Recently, thanks to high transparency in the vacuum ultraviolet (VUV)range without any increase of optical defects [28,29,30] (Figure 2.4), the F-doping has receiveda large attention through the application of silica glass as an optical material for projectionphotolithography at 157 nm of F2 excimer laser [25, 31]. It is well known that the shape ofthe fundamental absorption edge in the exponential (Urbach) region can yield informationon the disorder effects [32]. Skuja et al. [30] demonstrate that fluorine doping affects UrbachVUV absorption edge by increasing its steepness. It is evident from Figure 2.4 that when Fwas doped to 1 wt%, the transmittance at 157 nm increased to ∼80 % in comparison to nondoped glass. This gain in VUV transparency by F-doping is mainly due to a reduction ofconcentration of strained bonds in the silica network. Because the Si − F bond is strongerthan the Si − O bond, a monotonic increase in the optical bandgap of fluorine doped SiO2

would be expected with increasing the F content [25, 31].

The replacement of a single bridging oxygen atom with a terminal fluorine, results in theformation of SiO3\2F tetrahedra, that produces a depolymerization of the silicate network and

2.3. Phosphorus doped optical fibers 21

Figure 2.4: VUV absorption spectra of SiO2:F glasses as a function of F content. From ref. [25].

consequently the lowering the viscosity of silica [33].

The positive effects of F-doping seems to have at least 4 mechanisms: (a) by quenching ofdistinct color centers absorbing in the edge region, (b) by reducing of the structural disorderby breaking up the strained bonds in glass network, (c) by increasing of the band gap due tothe higher energy of Si-F bond as compared to Si-O bond, (d) by reducing the glass viscosity,thus allowing to achieve more easily a lower fictive temperature of the glass. Actually, thefeasibility to exploit these properties in the fabrication of F-doped silica fibers is conditioned byseveral queries including the dependence on F-concentration also on the basis of its influencein modulating the silica refractive index and the role of F after drawing silica preform. Despitethe great importance assumed by silica glasses doped with F, their structure is not yet wellcharacterized. It is known, as above mentioned, that fluorine is incorporated in the glass matrixessentially as Si-F bonds, taking the place of a bonding oxygen. For all glasses containing≥1 wt% fluorine, a small fraction of the fluorine is bonded to silicon atoms containing fourbridging oxygen atoms, resulting in fivefold coordinated silicon of the type SiO4\2F [27].

2.3 Phosphorus doped optical fibers

Phosphorus-doped a−SiO2 is a material of fundamental importance in fiber optics com-munications and in microelectronics. First of all, the addition of P2O5 allows to improve therefractive index of the glass (Figure 2.1), for this reason phosphorus doping is often used inoptical fibers to achieve an optimal refractive index profile [6]. Phosphorus is indeed used inoptical fibers to ameliorate the internal glass structure [34] thanks to its ability in modifyingthe viscosity of the core and cladding regions [34]. Phosphate glasses are potentially goodultraviolet (UV) transmitting materials allowing the fabrication of thin glass films for appli-


cation in microlithography and laser systems [34]. They are also good materials for highlyeffective optical amplifiers, especially via co-doping with rare-earth elements [7,35], and phos-phosilicate glasses are promising candidates as radiation sensors due to their closely linearresponse to radiation dose [36]. It has been reported that strong photosensitive propertiescan be induced in P-doped silica by hydrogen loading or high temperature treatment in ahydrogen-oxygen flame [37, 38]. A thorough understanding of the microscopic arrangementsby which P impurities are incorporated in silica, as well as of the properties of the result-ing P-related point defects, would be useful to optimize the performance of P-doped SiO2 inapplications (see section 3.2.2).

Due to the fact that phosphorus is first of all used as co-dopant in fiber technology, itis particularly difficult to separate its contribution in optical fiber attenuation in the UV-visible region [39]. Only a few papers in literature have investigated this issue, so that littleinformation is available at the moment on P-related point defects and on their generation andtransformation mechanisms [40, 34, 41]. The situation is very different from the case of otherdopants, like germanium for which, as seen in section 2.1, a vast amount of knowledge derivingfrom experimental and theoretical work has been accumulated.

2.4 The optical fibers under irradiation exposure

The appearance of new radiative environments integrating silica components, such as opticalfibers [42, 10], requires their immunization under ionizing radiation. In particular two impor-tant applications of optical fibers in the nuclear industry are related to plasma diagnosticsin fusion reactors [43] and transmission of signals from inaccessible parts of nuclear instal-lations [44]: in this field the relevant doses are above 1 MGy [45]. When optical fibers aresubjected to radiation, whether it consists of high energy light, X-rays, γ-rays, neutrons orhigh energy cosmic particles, their optical properties change due to the interaction of the ra-diation in the fiber core and in the cladding material. The main effects result from electronicprocesses: electrons are excited to leave their normal (bound) position, changing the physicaland chemical properties of the silica glass. The most obvious among the optical effects is theradiation induced optical attenuation, that depends essentially from wavelength: this is ingeneral a not desirable effect because it causes degradation of the performance of the opticalfiber systems. Moreover, it is also possible to use the fiber response under radiation exposureas a detector for radiation.

At the present time, there are several applications for the optical fibers under radiationenvironments. As an example, fiber diagnostic and imaging are new interesting fields for thedevelopment of the optical fibers technology under radiation exposure. Historically, the firstinterest in fiber response under ionizing radiation comes from the military sphere. Moreover,

2.4. The optical fibers under irradiation exposure 23

due to the confidential nature of these information, very few literature data exist on this subject[46]. In contrast the fiber applications in space environment or civil nuclear environment havebeen largely investigated.

The interaction of the radiation with the fiber material is a complex process with quitea number of dependencies on parameters related to the fibre fabrication process, operatingenvironment and radiation type. Exposure to radiation can induce stable alterations of thematerial [47, 48], often related to point defects generation and conversion processes [21] (seesection 3).


Chapter 3

Point defects in optical fibers

In 1966 Charles K. Kaoa and A. Hockham, two English researchers of the British Post Office,found and demonstrated that the high-loss, till then observed in the existing optical fibers,arose from impurities in the glass, rather than from an underlying problem with the technologyitself [49].

The presence of defects in optical fibers often causes the appearance of new energy levelslocated inside the band gap of the dielectric [50, 51]. As a consequence, the glass absorbs amore important part of the transmitted signal giving rise to an attenuation of the light guidedinside and consequently in a degradation of the fibers themselves.

The fiber radiation response depends on many intrinsic parameters: core and claddingdopants, impurity content, strain [52, 22], which are generally not accessible for researchers.Usually classical optical fibers for telecommunications are used in the IR, from 835 to 1600 nm,where the optical transmission is the largest. Moreover new technological fields, like medicalapplication and plasma diagnostic, need the use of light guides in the visible and UV regionwere optical transmission is affected by many losses [50, 53, 54, 55]. All these aspects moti-vate the necessity to investigate the exact nature of point defects, checking their origin andproperties and so reducing degradation effects also in the UV-visible domain.

Point defects and their precursors in the amorphous silica network are introduced duringthe fabrication process, through dopants and the interaction with ionizing radiation (highenergy, photons including UV laser irradiation, particles). Numerous publications are relatedwith defects in a−SiO2 [56, 57, 50, 58]. Most types of defects have optical absorption andluminescence bands and could be detected by optical absorption (OA) in the visible, UV,or infrared spectral range, Raman and photoluminescence (PL) spectroscopies. Detailed in-formation and identification on the subset of paramagnetic defects is obtained by electronparamagnetic resonance spectroscopy (EPR). Often, the combined use of different techniques

aNobel price in physics 2009

26 3. Point defects in optical fibers

allows to infer information not available by examining separately the results of single observa-tions. The formation of paramagnetic point-defects in silica glass has been studied from twopoints of view: transformation of diamagnetic precursors (also point defects) and the breakingof intrinsic Si-O bonds.

Defects can be distinguished in intrinsic, when they are due to a variation of the basicsilica elements (silicon or oxygen) and extrinsic, if they are related to presence of impuritiesin the silica matrix (H, Ge, P, etc.). Extrinsic defects due to the presence of impurities (Cl, Hand so on) are always present in variable concentrations in the material. On the other hand,how above explained (see section 2), selected impurities can be deliberately added by dopingto induce many useful properties [59,51,60].

To provide a background for the presentation of the results, the following sections of thischapter are devoted to review in more detail the current understanding of defects in a−SiO2,particularly with regard to the generation and conversion of defects related to the opticalfibers.

3.1 Intrinsic point-defects

Amorphous silica is the principal building material for glassy fiber waveguides. The structureof the glass network and point defects has been the subject of extensive studies through a largevariety of experimental techniques and theoretical modelling [61, 62, 63, 64]. An illustrativepicture of an amorphous silicon dioxide network is shown in Figure 3.1. The SiO2 network is

Si

O

Figure 3.1: structure of the amorphous silica, with Si atoms in grey and O atoms in black. The angleα define the spatial configuration of two connected tethraedra.

built with SiO4 tetrahedra joined at the corners so that each Si-atom is bound to four O-atomsand each O atom is the bridge between two Si-atoms. The angles defining the relative spatialorientation of each pair of connected tethraedra are statistically distributed between 120◦ and180◦ [65,66]. This description of the microscopic structure of amorphous silica is known as the

3.1. Intrinsic point-defects 27

Continuous Random Network (CRN) model, and is mainly based upon the evidences comingfrom X-ray and neutron diffraction [62,61,64,67]. The structural order in glass can generallybe divided into different stages or ranges [62]. The first stage is the tetrahedron structural unitSiO4 followed by the interconnection of adjacent units. A third stage is the network topologyfor describing the intermediate range order in shortest path ring structures. Finally, the longrange density fluctuations over several tens of are the fourth stage of structural order. Apoint defect in the intrinsically disordered structure of silica can be defined as any deviationfrom the perfect glass structure defined by the CRN model, provided that it is localized in aregion whose dimensions are comparable to the interatomic distance [62].

In the following the characterization of the main intrinsic defects are summarized.

3.1.1 Oxygen Deficient Centers

Oxygen deficient centers (ODC) are formed when an oxygen is missing or removed for instanceby irradiation from its Si-bonding position.

The silicon dangling bond, or E ′ center, is the most widely investigated oxygen deficientdefect in a−SiO2. It consist of a silicon atom with six electrons in three pairs and one unpairedelectron: ≡Si•, the symbol (≡) represents three bonds to oxygen atoms, (•) represents oneunpaired electron (Figure 3.2 (B,C,D)). E ′ defect was observed for the first time in 1956 by

Figure 3.2: Oxygen deficient centers in silica (from [50]). (A): Relaxed oxygen vacancy (ODC(I)center). (B,C): silicon dangling bond (E′ center) relaxed into the plane of the neighboiing oxygens(B) or relaxed towards neighboring bridging oxygen atom (C). (D): Surface-type SiE′ center. (E):Twofotdcoordinated Si atom (ODC(II) center).

R.A.Weeks in neutron-irradiated α-quartz (E ′1) and in silica [68] (Figure 3.2(B)). Thus far,at least four different types of E ′ centers have been observed in silica: E ′γ, E ′δ [69, 70, 71, 72],E ′α [73], E ′β [69,59] (Figure 3.2(B,C,D)). They differ from each other in the second coordinationenvironment around the respective silicon atom and they are distinguishable, at least in prin-ciple, either by their spectroscopic properties or on the basis of their generation mechanism.Additionally there exists a surface type E ′ center [74] due to isolated silicon dangling bonds(Figure 3.2(D)). E ′ center is found almost in every specimen exposed to radiation, but it can


be also formed by the fibre drawing process [59]. The E ′ family of centers are paramagneticand give rise to strong EPR signals. The EPR spectrum of the E ′ center consists of a singlemain resonance line (Figure 3.3) and of four hyperfine doublets with splitting of ∼40 mT(strong hyperfine), ∼0.8 mT and ∼0.9 mT (weak hyperfine) and ∼0.05 mT (very weak hy-perfine) [58, 75, 76]. An optical absorption band centered at 5.8 eV with FWHM=0.8 eV and

Figure 3.3: X-band electron paramagnetic resonance spectrum of E′ center (from [75]).

oscillator strength f=0.14, well correlates with the growth of E ′ EPR signature [77,78] and itis usually ascribed to E ′, even if the nature of the optical transition involved is still contro-versial [60, 79]. The formation efficiency of E ′ defects strongly depends both on the contentof the hydroxyl radicals (OH) in the glass and and on the irradiation energy. It was shownby Hanafusa [80] and Hibino [81] that E ′ defects also exist in non-irradiated optical fibers.The strong tensions during the preform drawing process seem to be at the origin of the defectformation. Also the drawing conditions, like temperature or drawing speed, influence the E ′

concentration in optical fibers: in particular it was shown that the E ′ concentration growthas a function of the drawing temperature, follows the Arrhenius law [80,81].

The neutral oxygen vacancy (ODC(I)) [82] consists of a bond between two Si atoms andit is indicated as ≡ Si − Si ≡ (Figure 3.2(A)). This diamagnetic ODC is electrically neutraland intrinsic to oxygen deficient silica since it disappears by oxidation. It gives rise to strongoptical absorption bands around 7.6 eV [82, 83]. By hydrogen loading of silica samples it wasshown that the oxygen vacancy converts itself to Si-H groups according to the reaction [50]:

≡ Si− Si ≡ +H2 −→≡ Si−H +H − Si ≡ (3.1)

The twofold coordinated Si (ODC(II)) [84] consists of a Si coordinated with two O atomsand denoted by = Si•• where (••) represents two paired electrons in the same orbital (Fig-ure 3.2(E)). This defect is also called silicon lone pair center or divalent Si. It shows arelatively weak absorption band, called B2 band, with peak at 5.03 eV and FWHM 0.4 eVand two photoluminescence emissions at 4.3 eV and 2.7 eV to singlet-singlet and triplet-singlettransitions occurring in the same defect [50].

3.1. Intrinsic point-defects 29

3.1.2 Oxygen associated hole centers

The oxygen dangling bond or Non Bridging Oxygen Hole Center (NBOHC) (≡Si−O•) [50,85]is shown in Figure 3.4(A). It is detectable by its characteristic EPR signal, as well as by

Figure 3.4: Oxygen excess-related color centers in a−SiO2 (from [50]). (A): Non Bridging Oxygen HoleCenter (NBOHC). (B): Peroxy radical (POR). (C): Peroxy bridge. (D): Interstitial oxygen molecule.(E):Interstitial ozone molecule.

its optical activity, consisting in three absorption bands at 2.0 eV, 4.8 eV and 6.4-6.8 eV,which excite a photoluminescence emission peaked at 1.9 eV. All these absorbtion bands, andin particular the intense bands at 4.8 eV and 6.8 eV, make NBOHC the defect that moreinfluences the transmission of silica in the UV and VUV spectral ranges. There are severalformation mechanisms for the NBOHC. In optical fibers these centers could be created duringthe drawing process [86] and their concentration grows in particular with the O2 [86] fluxand the drawing tension [87]. In bulk silica the NBOHC are usually created after energeticradiation exposure (X, γ, UV) [85,88].

The peroxy radical (POR), independently from its formation mechanism, consists in asilicon atom linked to an oxygen molecule: ≡ Si − O − O• [58] (Figure3.4(B)). It has anunpaired electron delocalized on two oxygen atoms that are not equivalent from a chemicalpoint of view: the electron spends 75% of the time on the more distant oxygen atom from thesilicon one. Several formation mechanisms were proposed in literature to explain the formationof these defects. Like NBOHC and E ′, the peroxy radical can be induced during the drawingprocess and it can be revealed by EPR measurements at low temperature [86]. The followingreaction was proposed for the POR formation:

Si−O −O − Si −→ Si−O −O• + Si+ e− (3.2)

Some authors supposed that the same mechanism could be responsible for the POR formationunder ionizing radiation [59,89].

Other oxygen excess related defects are the peroxy bridge, the interstitial oxygen moleculeand the interstitial ozone molecules (Figure3.4(C, D, E)). The presence of O2 [50] in silica hasbeen inferred from out-gassing experiments, from reaction with H2 forming Si-OH groups andfrom conversion from E ′ centers to POR centers.


Finally, the self-trapped hole (STH) may be the first defect to form under the influenceof ionizing radiations. Its principal characteristic is the capture of a hole on a 2p orbital froma doubly linked oxygen atom [90]. Two different STH species were identified: the STH1 andthe STH2 [91], consisting on a self trapped hole on one or two oxygen atoms respectively.

3.2 Extrinsic point-defects

Among the impurities present in silica fibers, hydrogen, germanium, phosphore and fluorineare the most diffuse. How above explained, Ge, P and F are very important dopants in fibertechnologies. On the other hand several other impurities are often integrated in the silicamatrix whether for the difficulty in eliminated them during the preparation procedure or forimproving the fiber properties.

3.2.1 Ge-related defects

Germanium may be arranged within a−SiO2 in many different configurations, each of whichconstitutes a specific point defect. Since Ge and Si are isoelectronic elements, it is qualitativelyexpected that many Ge-related point defects are structurally identical to Si-related centersapart from the substitution of Si with Ge [92]. Starting from the comparison between a Ge-doped silica glass and a pure silica glass, it is possible to show that defects related to germaniumare predominant on the intrinsic ones [19]. This property implies an UV absorption from twoto three order od magnitude more intense in germanosilicate glasses, even before irradiationexposure [93], as compared to pure silica.

Actually defects in germanium doped silica, and than in germanosilicate optical fibers,are the main subject of many experimental and theoretical works in order to know the originof the photosensitivity in these glasses (see section 2.1). Several researchers have shown thata contribution to the photosensitivity is due to the variation of the UV optical absorptionspectra associated with the so called Ge-lone pair center (GLPC ) [55,54,21]: a dicoordinategermanium atom with a lone pairs (= Ge••) [94, 95]. This defect is characterized by an OAband at 5.1 eV related to the optical transition S0 →S1 [54], from the ground state to the firstexcited singlet state. It has been suggested that the bleaching after radiation exposure (UVlaser, γ-rays) of this OA band, often referred as B2β, associated with the growth of several newabsorption signals (Figure 3.5), is at the basis of the permanent glass refractive index change.This observation clearly suggests that the defect responsible for the B2β band are convertedby UV radiation to other centers [55,97,96,94,95]. GLPC defect is also characterized by twophotoluminescence (PL) bands at 4.2 and 3.1 eV, related to the transitions from the excitedelectronic states of singlet (S1) and triplet (T1), respectively, to the ground state (S0) [92,98].

3.2. Extrinsic point-defects 31

Figure 3.5: Difference absorption spectrum, showing the bleaching of the 5.1 eV band and the growthof two components at 4.5 eV and 5.8 eV. From Fujimaki et al. [96].

Figure 3.6: Evolution of PL signal associated with GLPC after UV pulsed laser irradiation. Theinset shows the OA spectra acquired before (dashed line) and after (solid line) the irradiation. Figureadapted from Cannas and Origlio [21].

Then the energetic level scheme, pictured in Figure 3.7 and associated with GLPCs, consistsof a ground singlet S0 and the excited S1 and T1 states. The radiative decay channels from S1

and T1 are described by the rates KS and KT respectively, while the ISC process linking S1

and T1 is characterized by KISCb. Though the determination of the GLPC spatial distribution

in optical fibers is crucial to probe the silica refractive index variation, convenient experimentshave not been performed yet, the main obstacle being the small fiber dimensions. Experimentalliterature data on the defect spatial distribution in fiber, exist mainly for elements of intrinsicnature [99, 100].

bother non-radiative channels can be neglected [92].


O

PL 4.2 eV

S1

S 0

PL 3.1 eV

~ 10-8

s

OA 5.2 eV

T1

KISC

KS

Figure 3.7: General scheme of GLPC diamagnetic defect. Solid arrows indicate the radiative transitionin absorption and luminescence. Dashed arrows indicate the Inter System Crossing (ISC) non-radiativetransition.

The most common Ge-related paramagnetic defects that are detected by EPR in irradiatedGe-doped a−SiO2 are the GeE ′ center and the Germanium Electron Centers (GECs) Ge(1)and Ge(2) (Figure 3.8).

(b)

(a) (c)

Figure 3.8: Microscopic structures proposed by Neustrev [19] as models for (a): Ge(1), (b):Ge(2) and(c): GeE′ defects.

The microscopic structures of GeE ′ and Ge(1) have been unambiguously identified byEPR studies, further supported by theoretical calculations. The GeE ′, which is observed alsoin pure GeO2, is structurally identical to the E ′ center apart from substitution of Si with Ge(≡ Ge•) [101,102,55] (Figure 3.8 (c)). This center was put in relationship with an absorptionband at 6.2 eV-6.4 eV [19,103].The Ge(1) consists in an electron trapped at the site of a substitutional 4-fold coordinatedGe precursor (GeO•4) [19,104] (Figure 3.8 (a)). An absorption band at 4.4 eV-4.6 eV has beenattributed to this center [19,19,105,106].Finally the structure of the defect responsible of the latter EPR signal, the Ge(2) center,is still debated. Indeed, its structural model was first ascribed to a trapped electron centerat the site of a GeO4 unit, such as the Ge(1), on the basis of the similarities of their 73Gehyperfine coupling constants, differing from Ge(1) for the number of Ge nearest neighbors


ions [107]. According to this attribution, an absorption band at 5.8 eV was assigned to Ge(2)center [54, 105]. However, subsequent studies, based on the defect annihilation, suggested analternative model for Ge(2): an ionized twofold coordinated Ge (= Ge•) [19, 96]. Even thecircumstance that the g value of Ge(2) is smaller than 2.0023 does not permit its conclusiveassignment to a trapped electron center, because this line of reasoning is rigorously valid onlyfor very simple paramagnetic centers, and generally cannot be extended to point defects insilica [108]. The EPR signals related to GeE′ and GECs centers are reported in Figure 3.9.

Figure 3.9: EPR signature of the GeE′, Ge(1) and Ge(2) paramagnetic defects in germanosilicateirradiated silica. From Fujimaki et al. [106].

3.2.2 P-related defects

Several literature papers are focused on the study and characterization of paramagnetic P-related point defects generated by ionizing radiation [40,109]. In a defect-free SiO2 glass eachoxygen would bridge two SiO4 tetrahedra. On the other hand, the ideal P2O5 glass would becharacterized by one nonbridging and three bridging oxygens per phosphorus: any deviationfrom this structure can be considered as a defect [40].

Most of the current understanding of P-related defects in SiO2 derives from electronparamagnetic resonance (EPR) experiments on irradiated phosphosilicate glasses. Electronparamagnetic resonance allowed to identify 4 main P-related paramagnetic point defects, re-ferred to as P1, P2, P4 and Phosphorus Oxygen Hole Center (POHC ) centers [110, 40] thatwere not observable before irradiation exposure [40]. Figure 3.10 shows the supposed struc-tures for the above mentioned P-related defects, together with the precursors suggested byGriscom et al. in ref. [40] for all these defect structures.


Figure 3.10: Main phosphorus related paramagnetic defects induced by radiation in P-doped a−SiO2

silica. The supposed precursor structures are also showed (from ref. [40]).

In P4, P1 and P2, the unpaired electron is localized on the central P atom, bonded toa different number of oxygen atoms, 2, 3, 4 respectively [40, 111, 112, 38, 109]. Hence, theirstructure can be represented as [(O−)2P

•]0, [(O−)3P•]+, and [(O−)2P

•(−O)2]0 respectivelyc.

The paramagnetic signal of POHC is ubiquitous in P2O5-containing glasses. In the simplestmodel of this defect, the P atom is bonded to three bridging O atoms and to a fourth non-bridging O which hosts the unpaired electron: [(O−)3P − O•]+. However, this structure(here referred to as l-POHC) has been argued to be stable only at low temperature, whilethe room-temperature stable form of POHC (here referred to as r-POHC) was proposed tofeature an electron shared by two non-bridging oxygen atoms bonded to the same phosphorus[(O−)2P (−O)•2]

0 [40]. l-POHC and r-POHC supposedly feature two slightly different EPRsignals. Figure 3.11 shows the EPR signature of the POHC center: arrows indicates themetastable PHOC form. Figure 3.11 also shows in the central part of the EPR spectrum asignal attributed by Griscom et al. [40] to the so called Si(E′)(P), a species of Si(E′) centerwith phosphorus next-nearest-neighbors.

After clarifying by EPR the microscopic structure of these defects, data obtained byoptical absorption (OA) studies of irradiated P-doped silica were interpreted by proposingassociations between some of the observed OA bands (Figure 3.12) and the paramagneticcenters [40, 38]. In particular Griscom proposed the attributions presented in Figure 3.13 forthe OA bands of Figure 3.12, observed in the range 3÷6 eV. According to these assignments,r-POHC centers absorb at 2.2, 2.5 and 5.3 eV, while l-POHC have an absorption band atabout 3.1 eV.

In contrast, much less is known about diamagnetic P-related centers in SiO2. Based onthe results obtained by several independent experimental techniques, including Raman and

cIn P4, the P atom hosts an additional lone pair, not represented.


Figure 3.11: EPR signature detected at low temperature (77 K) of the stable form of POHC defect.Additional peaks indicated by the arrows are supposed to be due to the metastable POHC variant. Inthe central part of the spectrum the lineshape related to Si(E′)(P) is also visible (from ref. [40]).

Figure 3.12: Radiation induced optical absorption spectrum in a P-doped silica sample (from ref. [40]).

Infrared measurements, phosphosilicate glass is generally believed to consist of an intermixedrandom network of [(O-)2Si(-O)2]0 and [(O-)3P=O]0 tetrahedra randomly bonded by sharingO atoms, this being consistent with the fact that [(O-)3P=O]0 is the basic building block ofpure stochiometric (P2O5) phosphate glass [113, 114, 115, 116]. In this model each P atom isbonded to 3 bridging O atoms and a single doubly-bond non-bridging O, and thus each site canbe argued to be a potential precursor for l-POHC via ionization of the non-bridging oxygen. Incontrast, r-POHC should be formed by ionization of a defective site [(O-)2P(=O)2]− where theP atom bonds two bridging oxygen atoms with single bonds and two more non-bridging oxygenwith double bonds. Finally, P4, P1, P2 centers are supposedly formed by irradiation via holeor electron trapping on hypothetical diamagnetic precursor defects where P is 2-, 3-, and 4-fold


Figure 3.13: Attribution of the main P-related absorption bands showed in Figure 3.12 in irradiatedphosphosilicate glasses. Also the average peak energy (E), the full with at half maximum (W ) and theoscillator strength (f) are reported. From [40].

coordinated respectively (Figure 3.10):d [(O-)2P:]−, [(O-)3P:]0, [(O-)2P(-O)2]+ [40,111,112,38].In the intermixed random network model, also these sites should be considered as randomlyoccurring point defects.

dIn the 2-fold coordinated precursor of P4 center, the P atom hosts an additional lone pair, not represented.

Part II

Materials and methods

Chapter 4

The canonical samples

This chapter is focused on the description of the particular specimens used to perform theexperiments discussed in the rest of the work. An important part of the interest in our approachis based on the choice in the design of our samples that hereafter we will call canonical samples.These samples have to be representative of commercial fibers that have already been testedand will be used in future facilities [117]. They have also to offer an easier interpretationof their responses thanks to their custom designs. Previously, two different studies used aset of homemade samples to understand the influence of several process and compositionparameters on the radiation response of single-mode germanosilicate optical fibers at 1.3 and1.55 µm [118,22]. Due to the good knowledge of their sample characteristics, E.J. Friebele etal. [118] were able to obtain statistically significant correlations between the γ-ray steady-stateRadiation Induced Attenuation (RIA) and some of the fabrication parameters. As an example,they found that for doses of 2×103 rad at -35 ◦

C, the RIA level at the end of the irradiationis correlated with the Ge content in the fiber core (for a Ge/F doped cladding). The secondstudy was devoted to the transient X-ray radiation response of Ge-doped fibers and showedthat, on lowering the standard preform deposition temperature from 2000 to 1600 ◦

C andthe drawing tension from 140 to 20 g, the induced losses slightly decrease wavelengths [119]and the influence of various cladding co-dopants. However, these two studies were limitedby the difficulty to obtain the samples with the characteristics needed to get unambiguouscorrelations. For the present work, we design the structures to overcome these difficulties.First, a quantitative analysis of the influence of a dopant can only be achieved if all thedifferently-doped glasses have been made with strictly identical processes. From a practicalpoint of view, due to the non-negligible influence of MCVD process parameters [119], this cannot be achieved by investigating several preforms and fibers. It must be done within a singlesample. Secondly, new spectroscopic techniques are now accessible, thus allowing to spatiallyresolve the radiation-induced changes in the fiber with micrometer resolution. For example,we show the efficiency of the confocal microscopy of luminescence (CML) to characterize the

40 4. The canonical samples

radiation-induced point defects in passive [100] or rare-earth doped [120] optical fibers. Thesespatially-resolved techniques enable the characterization of new fiber designs (such as thesamples studied in this Thesis) that would have not been possible in the past.

4.1 Tested optical preforms and fibers

The purpose of this Thesis is to study the role of the dopants in the properties and in theradiation response of the multi-mode optical fibers. To this aim the experiments were carriedout on three types of silica optical fiber and preform samples, doped with germanium, fluorineand phosphorus respectively. These samples are prototype not commercialized yet: their futureuse can be planned for specific fields, like space or nuclear power plants, where expositionsto high irradiation doses are expected. All preform and associated fiber samples were madethrough the Modified Chemical Vapor Deposition process (see section 1.1.4) by iXFiber SAS[121]. About 50 mm of each prototype preform has been kept for analysis whereas the otherpart of the preform has been drawn to obtain several hundreds meters for each fiber. Therefractive index profiles for the three types of samples are presented in Figure 4.1. Standardconditions of fiber manufacturing (preform deposition and drawing process) have been usedfor these waveguides. To quantitatively investigate the influence of the dopants concentration,the structure of each preform, and then of each fiber, has been designed with several stepsof concentration of one doping element, Ge or F or P, in the core. This particular samplestructure was thought and realized ad hoc with the purpose of studying the dopant influence,thus maintaining the other fabrication parameters as fixed. The fiber and preform maincharacteristics are listed in Table 4.1.

Using spatially-resolved techniques [100,120,122], we will then be able to study the fiberor preform properties and the radiation response to the dopant concentration for samples withstrictly identical MCVD process parameters.

Dramatic importance in the sample fabrication had the choice of the dopant concentrationlevels. Part of the dopant concentration values have been chosen to reproduce the classicalrange of concentrations measured on commercial fibers (e.g.,from 2 to 12 wt.% for the Ge-doped fibers). The other ones have been defined in relation with our ab initio calculationsconducted on a 108 atoms silica-based supercell [123].

At the fabrication stage, the obtained multi-step radial distribution of the dopant alongthe fiber diameter can be roughly estimated through measurements of the fiber and preformrefractive- index profiles (Figure 4.1). A more accurate estimation of the concentration valuesof the dopants is obtained by electron microprobe analysis (EMPA) which also allows to inspectthe impurities content inevitably present in the samples. Fiber and preforms are made up offour cylindrical layers (core part, zones 1-4) of high pure synthetic silica differently doped,

4.1. Tested optical preforms and fibers 41

Tab

le4.

1:Param

etersrelatedto

thecano

nicalfib

eran

dpreform

samples.

∆nrefers

totherefractive

indexchan

geatλ=633nm

withrespectto

a−SiO

2.Dop

ants

andim

purities

averageconcentrations

wereevalua

tedby

electron

microprob

ean

alysis.

(a)

Gecanon

ical

samples

Pcanon

ical

samples

Zone

<Ge>

<Cl>

∆n

Pref.

<P>

<Cl>

∆n

Pref.

Fiber

(wt%

)(w

t%)

(×10−

3)

diam

.(w

t%)

(wt%

)(×

10−

3)

diam

.diam

.(m

m)

mm

(µm

)

Cladd

ing

00.0

0.01

0.33

10.16

0.00

0.23

0.46

12.8

125.0

12.5

0.07

2.19

5.02

1.63

0.07

1.37

5.0

62.5

24.5

0.10

4.33

4.24

3.17

0.06

2.58

4.2

52.8

Core

38.0

0.09

7.97

3.10

5.05

0.05

4.80

3.1

38.6

411

0.06

11.95

1.48

7.09

0.02

7.08

1.6

18.4

(b)

Fcanon

ical

samples

Zone

<F>

<Cl>

∆n

Pref.

Fiber

(wt%

)(w

t%)

(×10−

3)

diam

.diam

.(m

m)

(µm

)

Coa

ting

00.0

0.00

0.33

9.47

125.0

Cladd

ing

12.5

0.08

-5.9

4.44

62.5

21.3

0.10

-4.1

3.98

52.8

Core

30.7

0.08

-2.1

3.10

38.6

40.2

0.00

-0.5

1.50

18.4


- 4 - 3 - 2 - 1 0 1 2 3 40

2

4

6

8

1 0

1 2

- 3 - 2 - 1 0 1 2 3- 6

- 5

- 4

- 3

- 2

- 1

0

1

- 4 - 3 - 2 - 1 0 1 2 3 401234567

G e

∆n (x

10-3 )

( a )

( b )

F

∆n (x

10-3 )

( c )

P

∆n (x

10-3 )

R a d i a l d i s t a n c e ( m m )

Figure 4.1: refractive index change (∆n) measured at λ=633 nm with respect to a−SiO2 in: (a)germanium, (b) fluorine and (c) phosphorus canonical preform samples.

following a multiple step distribution. The layers were deposited in a tube of undoped fusedsilica which forms the cladding (zone 0). The preform samples have a diameter of about 10 mm,with a 5 mm inner doped region, and they were cut and polished into plates of approximately1.5 mm thickness. The fiber/preform length ratio is about 4×103 and the fiber core diameteris 62.5 µm.


Ge-doped canonical samples

Ge-doping increases the refractive index of a−SiO2, so germanium doping profile grows fromthe boundaries to the center. The samples have been doped with several amounts of germanium

Figure 4.2: Microscopic preform view obtained with an optical microscope. Numbers from 0 to 4 referto the zones listed in Table 4.1 with different amounts of germanium.

from ∼2 wt.% in the exterior part (zone 1) and up to ∼11 wt.% in the inner-center part (zone4), as shown by the microscopic vision in Figure 4.2.

Doped parts of this fiber contain typical levels of chlorine impurity (∼1200 part per million(ppm)) and OH-groups (∼60 part per billion (ppb)). Figure 4.3 shows the Ge and Cl trendinside the fiber and preform samples. The average Ge and Cl concentrations measured byEMPA for each zone are given in Table 4.1.

The drawing speed was ∼ 40 m/min, the drawing tension ∼ 70 g and the temperature ofthe furnace ∼ 1600◦C. These fibers exhibit pre-irradiation optical characteristics at 1.55 µmclose to that of commercial fibers, that is 0.34 dB/km.

F-doped canonical samples

In the fluorine doped samples the core consists in three zones (zones 2 to 4) with three differentF-concentrations (see Figure 4.4).

The F-incorporation inside a−SiO2 decreases its refractive index, so F-doping profile hasan opposite trend with respect to Ge as shown in Figures 4.1 and 4.5. Optical cladding (zone1) corresponds to the zone with the highest F-doping region (∼1.8 wt%). More details arepresented in Table 4.1.

The outer cladding (zone 0) is made of pure-silica. Only a little part of the signal is guidedin this part of the waveguide. As a consequence, its contribution to the global transmission canbe considered as negligible. Doped parts of this fiber contain very small amounts of chlorineimpurity (∼1000 ppm) and OH-groups (∼200 ppb) typical of MCVD glasses. In this fiber the


- 4 0 - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 4 00

2

4

6

8

1 0

1 2

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5- 4 - 2 0 2 4

0

12

3

0

4

3

2

GeO 2(%

wt)

D i s t a n c e f r o m f i b e r c e n t e r ( µm )

1

Cl (%wt)

D i s t a n c e f r o m p r e f o r m c e n t e r ( m m )

Figure 4.3: Impurities trend inside canonical Ge-doped fibers (lower x scale) and preforms (upperx scale). Empty squares represent the Ge content, full circles the Cl content. The estimation of theconcentration values of the elements is obtained by EMPA analysis. Numbers from 0 to 4 refer to thezones listed in Table 4.1.

1

2

3

4

0

1

Figure 4.4: Microscopic preform view of the F-doped canonical sample. Numbers from 0 to 4 refer tothe zones listed in Table 4.1. The different sample zones are clearly visible.

attenuation at 1.55 µm is about 1.9 dB/km.

P-doped canonical samples

The P canonical samples are made up of an outer undoped high purity silica layer, andfour internal cylindrical layers (core part, zones 1-4) of highly pure synthetic silica doped


- 4 0 - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 4 0- 2 . 0

- 1 . 5

- 1 . 0

- 0 . 5

0 . 0

- 4 - 2 0 2 4

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5 F

F-con

tent (w

t%)



C lCl-content (wt%)

2

4

3

1

03

2

1

0

Figure 4.5: Fluorine (empty squares) and chlorine content (full circles) in F-doped fibers (lower xscale) and preforms (upper x scale). Impurities concentrations were evaluated by EMPA analysis.

with different P-amounts. Phosphorus doping profile grows from the boundaries to the center

- 4 - 3 - 2 - 1 0 1 2 3 4

( a )

0

4 3 2 1

µm

Figure 4.6: Enlarged view of the P-doped fiber canonical sample. Numbers from 0 to 4 refer to thevarious sample zones listed in Table 4.1 with different P-amounts. The various zones are clearly visible.

(Figure 4.1) following the desired multiple step distribution, as shown in the microscopic imageof the fiber sample in Figure 4.7.

The core-cladding part does not contain a relevant concentration of extrinsic impurities,except for chlorine that is present with a maximum concentration of ∼0.2 wt%. The preformshad an initial diameter of 12.84 mm with a 5 mm doped region and they were subsequently


012345678 - 3 - 2 - 1 0 1 2 3

0 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

- 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0

P


P-con

tent (w

t%)

C l

Cl-content (wt%)


0

1

2

3

4

3

2

1

0

Figure 4.7: P-content (empty squares) and Cl-content (full circles) obtained by electron microprobeanalysis at various distances from the fiber (lower x scale) and preform (upper x scale) center.

cut into 6×6×1.5 mm3 samples and polished. Phosphorus and chlorine concentrations werechecked by EMPA, giving the results shown in Table 4.1 and in Figure 4.7.

The spectral attenuation at 1.5 µm, obtained with the cutback method, is about 0.5 dB/kmfor this fiber. The OH content is evaluated as sensibly inferior to 10 ppb.

Chapter 5

Experimental set-ups

This chapter is concerned with the description of the instruments and setups used to performthe experiments discussed in the rest of the work.

Several irradiation sources were used to analyze the generation processes of defects bothon fibers and on preforms. Additionally, various spectroscopic techniques have been employedto identify the precursor sites in non-irradiated samples or stable point defects in irradiatedsamples. Some of these techniques have been applied both on fibers and on preform samplesallowing the study of the influence of the drawing effects on defects generation. Other tech-niques could be applied only on preform (time resolved luminescence, absorption) or on fibersamples (radiation induced attenuation), for the particular sample structure.

5.1 Irradiations

5.1.1 UV laser irradiations

UV exposures at 5 eV (248 nm) were carried out at room temperature with two distinct set-ups:a pulsed KrF laser and a continuum (CW) Ar-laser and .UV pulsed laserIrradiation exposures with the high power KrF pulsed laser were performed at a repetitionrate of 10 Hz, a duration time of 30 ns for every pulse and a pulse energy varying from 100to 400 mJ. Irradiations on fibers were conducted by moving the samples at a constant speed,transversally to the UV laser. The energy of the laser pulses is measured with a pyroelectricdetector; the accuracy, taking into consideration the laser fluctuations, is ±10%.Continuum UV laserThe setup used for the continuum UV exposures was developed at the Hubert Curien Labo-ratory. Irradiations were performed using an UV Argon Laser, emitting at 244 nm (5.1 eV)

48 5. Experimental set-ups

and having a gaussian intensity profile. For preform samples the beam was unfocused on thecenter of the preform thanks to a spherical lens. The laser energy was measured by a powermeter with an accuracy of ∼5%. For the fiber samples the irradiation system is more compli-cated [117,124] and it is described in Figure 5.1. The fiber samples are mechanically uncoated

Figure 5.1: Schematic representation of the experimental setup used for the CW ultraviolet (244 nm,5.1 eV) exposures of optical fibers.

to prevent the UV light absorption by their acrylic coatings. A variable fiber length (from fewmillimeters to several meters) filed past the CW laser beam. To this purpose, the fiber wasmaintained in a V-groove and was interdependent with a tended thread by a counterweight.This thread was pulled by a rotating motor. The 244 nm light was focused by a spherical lens,leading to a spot size of some mm diameter on the fiber. By an appropriate choice of the fibertranslation speed (0.2−0.6 cm/s) and of the laser power (5−100 mW), it was possible to varythe fluence value. The uncertainties in the fluence evaluation were mainly attributed to themechanical part of this setup and they were estimated within ±20%.

5.1.2 γ-ray and X-10 keV irradiations

The samples were exposed to γ-rays produced by a 60Co source available in the IGS-3 irradiatorof the Department of Nuclear Engineering of University of Palermo. γ-rays have energiesbetween 1.17 MeV and 1.33 MeV. Irradiation was performed at room temperature, in ordinaryatmosphere at a dose rate of 1.39 kGy/h.

The 10-keV X-ray irradiations of fiber and preform samples were performed at roomtemperature using an ARACOR Semiconductor X-ray irradiator [125,126] at the French atomicenergy center (CEA). The irradiated zone is homogeneous over a diameter size of ∼2.5 cm; thedose was varied from 50 Gy up to 2 MGy (two different dose rates: 10 Gy/s and 0.1 kGy/s)in fibers and from 1 kGy up to 2 MGy (dose rate = 0.1 kGy/s) in preform samples.

5.2. Absorption 49

5.2 Absorption

How shown in the introduction section, point defects in a−SiO2 can introduce new electroniclevels inside the valence and the conduction band. So, using electromagnetic radiation atenergy lower than the gap, it is possible to induce transitions corresponding to the absorptionbands of these materials.

The optical absorption experiments were conducted sending electromagnetic radiation onthe sample, varying continuously the light energy during a fixed interval and than analyzingthe transmitted light. According to the Lambert-Beer law, the transmitted radiation intensityIT is linked to the incident intensity I0 by the relation 5.1 [127]:

IT (~ω) = I0(~ω) exp{−α(~ω) · d} (5.1)

where d is the sample thickness and α(~ω) is the optical absorption coefficient, measured incm−1. Knowing the absorbing species concentration, N , the absorption coefficient can beexpressed by the following equation:

α(~ω) = Nσ(~ω) (5.2)

where σ(~ω) is the absorption cross section [50].

Optical absorption spectra presented in the result section were performed with two dif-ferent spectrometers: the JASCO V-560 and the AVANTES S2000 spectrophotometers.

The JASCO spectrophotometer

The JASCO V-560 spectrometer is a double beam spectrophotometer providing the measuredabsorption values in optical density (O.D.), defined as:

O.D. = log10

(I0(λ)

I(λ)

)(5.3)

The incident beam I0 is split thanks to a beam splitter and it is alternatively sent to thesample and to the detector. In this way we can have the control, and than the correction, ofevery source fluctuation. The source is made of two lamps: a deuterium lamp, working in theultraviolet (UV) region from 340 nm to 190 nm, and a Xenon lamp, operating in the visibleand infrared range (340÷2500 nm). The detector is a photomultiplier (PMT).

The AVANTES spectrophotometer

In the optical fiber AVANTES S2000 spectrophotometer the source is a deuterium lamp (D2)that injects light into an optical fiber that splits up in two channels, referred to as Master


and Slave. The optical fibers are multimode pure silica core\F2-doped silica cladding withdiameter of 200 µm. They are loaded with H2 to better resist to the prolonged exposure toUV light without being deteriorated. The light carried by the master channel gets out of thefiber and is used as the probe beam (PB). The PB is collimated by two lens and it is coupledto another fiber that brings it to the detector. The two lenses are mounted on independentmicrometric positioning controls (xyz), which permit both to control the alignment of the PBto the sample and to optimize the collection efficiency after the sample. The slave channelpasses through a variable attenuator, after which it goes to the detector. Since the slavechannel does not traverse the sample, it could be used to correct experimental data for thetemporal drift of the lamp. The detector consists in a 1200 lines/mm grating with blaze at300 nm, dispersing on a 2048 channels Charge Coupled Device (CCD) array. The instrumentworks in the 200 nm−500 nm range with a spectral resolution of 5 nm. Before acquiringa spectrum it is necessary to obtain a dark reference signal D(λ) from the master channelwhen the D2 lamp is disconnected from the fibers. If I0(λ) and I(λ) are the signals acquiredrespectively without and with the sample, the absorption profile of the specimen is given by:

O.D. = log10

(I0(λ)−D(λ)

I(λ)−D(λ)

)(5.4)

Absorption in the VUV spectral range

VUV absorption spectra in the 6.0÷7.5 eV range were obtained using an ACTON SP-150single-beam spectrophotometer, equipped with a 30 W D2 lamp and two 1200 lines/mmmonochromators, and working in N2 flux. The acquired spectra were corrected by subtractingthe contribution due to surface reflectance.

5.3 Photoluminescence and Raman spectroscopy

5.3.1 Photoluminescence

An optical property of great importance in the study of point defects in a−SiO2 is the pho-toluminescence (PL), i.e. the process by which a system excited by light with wavelength λexemits light at λem > λex while decaying back to its ground state [50, 128, 129]. To picturethe physical processes determining the PL emission band, let us consider the two-level systemof Figure 5.2. Due to the absorption process OA, a number N(λex) of defects will be in theexcited state during exposure of the sample to the excitation light. Some of these defects candecay radiatively with a rate kr, thus originating the photon emission or luminescence. Theremaining excited defects relaxes back to the ground state by a temperature dependent non-radiative process, with a rate knr, in which the energy is dissipated by emission of phonons.

5.3. Photoluminescence and Raman spectroscopy 51

O

kR

kNR

ZPL

OA

Figure 5.2: Electronic-vibrational level scheme of a two levels point defect. The continuous arroworiented upward represents the absorption transitions OA from the ground state to the exited state.The continuous arrow oriented downward represents spontaneous emission from the excited state to theground state. The dotted arrow represents the non-radiative decay process. The grey arrow indicatesthe ZPL transition and the dashed arrow within the excited levels represents the internal relaxationprocess.

In Figure 5.2 the transition at lowest energy (that is from (0, 0) to (1, 0)) is called the zerophonon line (ZPL). The PL intensity is given by:

IPL(λex, λem) = krN(λex)LPL(λem) (5.5)

where LPL(λem) is the emission lineshape determined by homogeneous and inhomogeneouscontributions [51,50,15]. The variation rate of the excited state population, N , depends on theabsorption and decay processes, both radiative and non radiative, according to the followingequation:

dN(λex, T )

dt= I0(λex) [1− exp {−α(λex)d}]− (kr + knr(T ))N(λex, T ) (5.6)

Steady state luminescence experiments are performed when the system undergoes a continuousexcitation. In this case dN/dt = 0 and combining Equations 5.6 with 5.5 we get:

IPL(λex, λem, T ) = ηI0(λex) [1− exp {−α(λex)d}]LPL(λem) (5.7)

where η = kr

kr+knris the luminescence quantum yield, defined as the ratio between emitted

photons and absorbed photons.

Two basic types of measurements are possible:

• measuring the intensity IPL(λex, λem) as a function of λem for fixed λex we obtain theshape and intensity of the band emitted by the center, that is the emission spectrum(PL);


• acquiring IPL(λex, λem) as a function of λex for fixed λem, we have the excitation spectrum(PLE), which represents a measurement of the efficiency of the emission process independence of the excitation wavelength.

Differently from stationary PL measurements, in the time-resolved PL measurement thetime decay of the emitted light after an exciting light pulse is studied. After the excitation ofa point defect with a light pulse, that produces a population of N(0) of the excited state, thelight source is switched off (I0 = 0) and N(t) decays as:

N(t) = N(0)e−t/τ (5.8)

with τ = 1/(kr + knr). From equations 5.5 and 5.8 we obtain the luminescence timedecay:

IPL(λex, λem, T, t) = krIPL(λex, λem)N(0)e−t/τ (5.9)

At sufficiently low temperatures, the non-radiative decay channels are usually quenched,i.e. knr � kr, so that the measurement directly yields the radiative decay time τ = 1/kr. Theimportance of the knowledge of τ relies in the possibility of calculating the oscillator strengthf of the center [50]:

f =1

E2τ

m~2c3

2e2(5.10)

Not all the point defects that feature a measurable absorption band decay by emittingluminescence, but when this occurs, their study by PL spectroscopy has some importantadvantages with respect to OA. In particular, PL is more selective, as it often allows to isolatea center whose absorption band overlaps to those arising from other defects, based on thedifferent emission properties.

5.3.2 Stationary and time resolved luminescence setup

In this section we are going to describe the experimental setup used for acquiring the lumi-nescence data of preform samples. As shown in the schematic representation of Figure 5.3the equipment is mainly constituted by a laser source, a sample chamber, a dispersion systemand a detection one. The tunable laser (VIBRANT OPOTEK) [130] is an integrated systemthat emits pulses of 5 ns duration with a maximum repetition rate of 10 Hz in the range(210-2400) nm. The excitation beam with a spot size of ∼1mm2 hits the sample mountedin the so-called 45◦ back-scattering geometry. The emitted light is collected by a lens andthen arrives at the detection system. The energy of the laser pulses is measured with a py-roelectric detector capable of giving in output a short electric pulse for each laser shot (somemus), whose amplitude is read by a digital meter. It is positioned before the sample holder


Tunable Laser

OPOTEK

Xe-lamp

Sample

SpectrographCCD+ Delay generator

Figure 5.3: Schematic representation of the equipment used for the luminescence measurements.

to measure the intensity of incident laser radiation. The accuracy of pulse energy measures,taking into consideration the laser fluctuations, is ∼10%.

Time resolved luminescence spectra are performed with a detection system consisting ofa Spectrograph and an Intensified CCD Camera. This latter amplifies the input luminescencesignal: for each photon that strikes the photocathode surface many photons are produced.Moreover, the possibility of varying the photocathode voltage allows to enable or to disablethe CCD: in the GATE ON mode the photocathode voltage is -200 V and the CCD seesthe light, in the GATE OFF mode the photocathode voltage is 0 and the CCD does notsee the light. This peculiarity permits the detection of time resolved luminescence spectrasynchronized with the laser excitation pulses. In fact, the CCD is triggered by an electronicsynchronization signal produced by the laser 60 ns before the pulse. The CCD can accumulate

4 ns(0.1-1)s

I PL

λem

I PL

λem

Δtτ’

Figure 5.4: Diagram of the CCD timing.

in a time window defined by the width parameter ∆t (Gate Width) and by its delay τ (GateDelay) from the origin of the time scale. So, as shown in the diagram of Figure 5.4, the Gate


Width determines the amplitude of the time window during which the CCD is enabled toreveal the luminescence light (GATE ON mode); while the Gate Delay regulates the temporalshift of the acquisition window with respect to the trigger signal.

In the next chapters all luminescence spectra are presented as a function of the energy Einstead of the wavelength λ, so they require a specific correction procedure: the CCD countsare directly proportional to the luminescence spectral density dI/dλ that is the intensitycollected with a constant spectral bandwidth dλ. Since energy and wave number are linked bythe relationship E = hc/λ, the spectral density dI/dE with respect to E must be multipliedfor spectral dispersion λ2 because to a constant spectral bandwidth in λ corresponds a spectralbandwidth which depends from the emission energy E.

5.3.3 Photoluminescence under synchrotron radiation excitation

Excitation (PLE) end emission spectra excited in the Vacuum UV (VUV) range on preformsamples were carried out under excitation by pulsed synchrotron radiation at the SUPER-LUMI station on the I-beamline of HASYLAB, DESY (Hamburg) [131]. Measurements wereperformed in the spectral range 4.5÷9.0 eV, with a pulse width of 130 ps, an interpulse of500 ns, and a spectral width of 0.3 nm. The excitation beam is directed into the primarymonochromator with a 1200 lines/mm grating (Figure 5.5; the excitation wavelength can be

Figure 5.5: Schematic representation of the experimental station used for PL, time resolved PL andPLE under synchrotron radiation (figure adapted from [131]).

varied from 310 nm to 50 nm (4.0 to 24.8 eV). The emitted light was spectrally dispersed bya 300 grooves/mm grating blazed at 300 nm and acquired by a liquid nitrogen cooled CCDcamera (1100 Princeton Instruments) for PL spectra.

Luminescence spectra were corrected both for the spectral response and dispersion of thedetecting system, while excitation spectra were corrected for the spectral efficiency of the


exciting light, using a sodium salicylate sample as a reference. Excitation bandwidth was0.3 nm, while emission bandwidth was 20 nm.

During laser-excited luminescence and measurements, we put the sample behind a prop-erly built mask so as to allow spatial selection of the various preform zones (see Section 4.1).

In time resolved PL and in PL under synchrotron radiation, temperature dependencies(from 10 to 300 K) were investigated using continuous flow helium cryostats.

5.3.4 Raman measurements

The Raman effect is the anelastic scattering of light (by a molecule or a point defect) dueto emission or absorption of a vibrational quantum [132, 127, 133]. If Ei is the energy ofthe incident photons, scattering at Es < Ei implies the excitation of a vibrational mode ofenergy ~ω = Ei − Es. A Raman spectrum consists of a plot of the scattered intensity as afunction of ω. This spectroscopy allows to probe the vibrational modes of a molecule or apoint defect, sometimes bearing some advantages with respect to common IR spectroscopy.For example, it can happen that a vibrational mode is Raman-active but non IR-active, orvice versa [132, 127]. Raman spectra presented in this PhD Thesis were all performed by themicroRaman spectrometer described here below.

5.3.5 Confocal Micro-spectroscopy setup

Confocal microscopy luminescence (CML) measurements and microRaman measurements wereperformed with the LabRAM Aramis (Jobin-Yvon) integrated confocal microRaman systemwhose scheme is presented in Figure 5.6.

The confocal microscope is coupled to a 460 mm focal length spectrograph equipped witha four interchangeable gratings turret. The different excitation wavelengths are supplied by upto one internal laser (He-Ne, 633 nm, 2.0 eV) and by two external lasers: a He-Cd laser workingat 442 nm (2.8 eV) and 325 nm (3.8 eV) and an Argon laser working at 488 nm (2.5 eV). Brieflythe principle of confocal microscopy consists of focusing the laser source through the objectiveof the microscope and carrying out a spatial filtering of the signal coming from the illuminatedvolume, by using a diaphragm of small diameter placed in the conjugated plane where themagnified image of the sample is formed by the objective (see Figure 5.7).

On the incoming path, the laser beam is reflected towards the microscope by the means of aspecial filter (holographic notch filter or dielectric edge filter) used in injection/rejection mode.On the return path to the spectrograph, the Raman backscattered light is fully transmitted


External Lasers CCD detector Grating turret

Filter turret

Autofocus

Camera

White light source

Laser HeNe

Figure 5.6: Top view of the the LabRAM Aramis (Jobin-Yvon) integrated confocal microRamansystem with the optical path.

Figure 5.7: Scheme of micro-luminescence and micro-Raman analysis set-up.

through the filter towards the confocal slit-hole located at the entrance of the spectrograph.The spectrograph disperses the multichromatic Raman signal onto the CCD multichanneldetector.

The optical drawer constitutes the coupling platform between the laser, the samplingchamber (microscope or macro chamber) and the spectrograph. In order to reduce the laserpower at the sample, a density filters wheel driven by the software, can be used. Duringour measurements we make sure that the power level of the probe light was reduced to few

5.4. Electron Paramagnetic Resonance measurements 57

hundreds of µW to avoid photobleaching effects.

For Raman measurements, the Raman signal is then collected by the same microscopeobjective (backscattered configuration) and follows the return path to the spectrograph. Theraman filter filters out the backscattered laser light (Rayleigh scattering) whereas the Ramansignal is transmitted to the spectrograph entrance slit.

The sample can be translated, under computer control, with an accuracy of about 0.1 µm.The excitation beam penetrates of a few micrometers into the sample. The spot diameterfocalized on the samples varies as a function of the microscope objective used. Using a ×50objective, the spot size is some µm. With this spot size it is possible to inspect in detail notonly the preform samples, but also the fibers. This is a new tool of our study: previously thelargest part of scientific investigation on defects in optical fibers consisted in the direct study ofbulk samples and the subsequent transfer of information to the fibers [19,20,21]. However, thisapproach cannot take into account the peculiarities implied in the fiber preparation procedure,such as the drawing process, which can generate precursors and influence the defect generation[22, 134, 135]. Micro-Raman and micro-luminescence investigations, allowing inspection ofdefect directly inside optical fibers, permit us of overcoming this limit.

5.4 Electron Paramagnetic Resonance measurements

The Electron Paramagnetic Resonance (EPR), also referred to as Electron Spin Resonance(ESR) spectroscopy measures the absorption of microwave radiation corresponding to theenergy splitting of an unpaired electron when it is placed in a strong magnetic field. EPR isa spectroscopic technique that detects the presence of these unpaired electrons in a chemicalsystem [136, 137, 138]. This can yield meaningful structural and dynamic information, evenfrom ongoing chemical or physical processes (i.e. kinetics, etc.) without influencing the processitself. In the absence of an external magnetic field the two possible electron spin states (spinup and spin down) are degenerate. When an atom or molecule with an unpaired electron isplaced in a magnetic field, the spin of the unpaired electron can become aligned either in thesame direction (spin up) or in the opposite direction (spin down) of the applied field. Thesetwo possible electron alignments have different energies (i.e. are no longer degenerate) and aredirectly proportional to the applied magnetic field strength. This is called the Zeeman effect.

The EPR measurements reported in this Thesis were performed by a Bruker EMX spec-trometer working at ω0=9.8GHz. In Figure 5.8 it is reported a simplified scheme of theinstrument. The microwave radiation travels down a waveguide to the sample, which is heldin place in a microwave cavity held between the poles of two magnets. A variable attenuatorpermits to regulate the actual power Pi incident to the cavity from a maximum value 200mWdown to 200 nW. In this way, Pi is usually chosen so as to avoid the saturation of the observed


Resonant cavity

Magnet

Attenuator

A/D

Detector

A/D

EPR Signal

Integrator

RC Filter

Source

Amplifier

Lock-in

Modulator

Figure 5.8: Schematic representation of the Bruker EMX spectrometer. The main sections are visible:magnet, cavity, source, attenuator, modulation system and detector.

magnetic resonance transition. The microwaves arriving on the entrance of the cavity arepartially absorbed by the sample and partially reflected to join the revelation system. Thereflected power PR is measured by a detector that gives a current signal I proportional tothe square root of PR. Spectra are obtained by measuring the absorption of the microwaveradiation while scanning the magnetic-field strength. EPR spectra are usually displayed inderivative form to improve the signal-to-noise ratio.

It is important to underline that the doubly-integrated intensity of the EPR spectrum isproportional to the number N of paramagnetic centers. In this sense, if a reference sample isavailable, EPR may be used to provide a measurement of the absolute concentration ρ = N/V

of every paramagnetic defect, where V is the volume of the sample. To this purpose, in thiswork it was used a specimen where the absolute number of E ′ centers (purposely generatedby γ irradiation) was known by spin-echo measurements [139, 140]. The accuracy of theabsolute concentration measurements obtained by ESR, based on comparison with the spin-echo reference sample, is estimated as 20%a.

athis error never explicitly appears when reporting in the following the uncertainties on the concentrationmeasurements. The reason for this choice is that the uncertainty on the concentration of E′ in the spin-echo reference sample plays the role of a systematic error, affecting in the same way all the concentrationmeasurements here reported.

Part III

Ge-doped fibers and preforms

Chapter 6

Measurements on non-irradiated samples

As discussed in detail in the introductory chapters (section 2.1), identification of de-fects responsible for the permanent change of the refractive index in Ge-doped glasses underradiation exposure is a clue to make clear the microscopic origin of transparency loss and pho-tosensitivity. Consequently, the study and the characterization of point defects in Ge-dopeda−SiO2 is a very effective subject to understand the fiber properties and to improve theirperformances.

We have seen that germanium lone pair centers are the main responsible defects forphotosensitivity property. The GLPC optical activity variation under UV-radiation exposurecontributes to the permanent refractive index variation of the glass. So the exact knowledgeof the GLPCs concentration before radiation exposure and the modifications induced by thedrawing process, result to be a fundamental aspect in optical fibers technology. The study ofGLPCs variations in optical fibers can so be adopted as probe for testing the refractive indexmodulation induced by external factors.

To this aim, this section is devoted to introduce the optical properties of the as-grown ma-terials performing direct investigation of GLPCs inside our germanosilicate step-index canon-ical fibers and preforms.

First of all we performed absorption measurements on preforms samples before irradiationin order to investigate GLPC presence in the different sample zones. Figure 6.1 shows the OAspectra detected by the use of the AVANTES spectrometer in the spectral range 2.5-5.5 eV;in the figure the variation of the OA intensity as a function of the different sample zones,that is as a function of the Ge-content, is also shown. For GeO2>5% the OA signal saturates,indicating defects concentration higher than our detection system (∼70 cm−1). As expected,the OA band centered at 5.2 eV and related to GLPCs, grows with the Ge-content.

Though the determination of the GLPC spatial distribution in optical fibers is crucial to

62 6. Measurements on non-irradiated samples

2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0 5 . 5

0

3 0

6 0

9 0

[ G e O 2 ] ( w t % )1 1 8 5 3 0

Abso

rption

Coeff

icient

(cm-1 )

E n e r g y ( e V )

G L P C

Figure 6.1: OA spectra detected in pristine Ge-doped canonical sample. The arrow specifies the GeO2

content from the exterior to the inner sample part. The OA band at ∼5.2 eV is due to GLPC centerswhose microscopic structure is showed in the inset. For [GeO2]>5 wt% the OA signal saturates.

probe the silica refractive index variation, convenient experiments have not been performedyet, the main obstacle being the small fiber dimensions. So far, studies dealing with GLPCswere extracted from bulk samples and than transferred to fibers [19, 20, 21], thus remainingaffected by an intrinsic deficiency caused by the information lack related to the drawing pro-cess. Nevertheless the use of Confocal Microscopy Luminescence (see section 5.3.5) allows toexamine the variation of the PL bands linked to GLPCs even on micrometric scale.

Than we performed direct investigation of GLPCs inside germanosilicate step-index op-tical fibers by using CML technique with the purpose of examining their radial distributionalong the core. The comparison with the corresponding preforms permits to recognize theactual role of the drawing process in modifying the defect formation, and consequently tocontrol the fiber photosensitivity.The recognition of GLPC concentration in preform samples is not possible via OA B2β banddue to our measurement conditions: in the central sample region the absorption coefficient istoo high and cause the saturation of the detected signal (see Figure 6.1), thus preventing acareful evaluation of defects concentration.There exists opinion that there could be two very closely spaced optical bands at about 5.2 eVdue to Ge-related oxygen-deficient centers, and that only one of them could give rise for

63

PL [55]. Otherwise, using the linear correlation between B2β band and the GLPC PL activity,we assumed that the GLPC absorption band was unique. This assumption could systemat-ically affect the absolute evaluation of GLPC concentration, nonetheless, for our purposes,the relative concentration variation is still meaningful. In principle this problem could beovercome estimating concentration by the singlet-triplet GLPC absorption band at ∼3.8 eV.Nevertheless, this OA band is much weaker than the singlet-singlet one (about 104 times) andit is not detectable in our samples.

GLPCs in fibers and preforms were revealed by the He-Cd ion laser excitation line ofthe LabRam Aramis spectrometer (photon energy 3.8 eV, power 0.5 mW). We used a ×40objective and a diaphragm diameter of 100 µm. The excitation beam penetrates of a few µminto the sample as to be sure that the collected signal was not due to abnormal surface defectconcentration detection.GLPCs were checked monitoring the intensity variation of the 3.1 eV phosphorescence bandby direct excitation S0→T1 (Eex = 3.8 eV) (see GLPCs levels scheme in the left inset ofFigure 6.2). As an example, in Figure 6.2 OA at 5.2 eV and PL at 3.1 eV (right inset)detected in zone 2 of preform sample ([GeO2]∼5 wt%) are depicted. The use of a laser probe

4 . 6 4 . 8 5 . 0 5 . 2 5 . 4 5 . 6 5 . 8

0

1 0

2 0

3 0

4 0

5 0

2 . 7 3 . 0 3 . 3

S 1

P L 3 . 1 e V

O A 5 . 2 e V

T 1

S 0

E E X 3 . 8 e V

Abso

rption

Coeff

icient

(cm-1 )

E n e r g y ( e V )

PL (a

rb. un

its)

E n e r g y ( e V )

Figure 6.2: OA spectrum detected in zone 2 ([GeO2]∼5 wt%) of Ge-doped pristine preform. Theright inset shows the PL spectrum al 3.1 eV obtained exciting at 3.8 eV in the same preform zone. Inthe left inset transitions giving rise to the observed bands are schematically depicted.

at 3.8 eV has a double advantage:


• First of all we have seen that the germanosilicate exposition to UV laser induces thebleaching of the optical activity related to GLPCs [21] (see section 3.2.1). Consequentlyexciting the GLPC activity by the use of a laser beam at the energy of 5.2 eV, that isby the excitation S0→S1, could produce an intensity diminution of the 3.1 and 4.2 eVPL bands not related to an actual defect concentration decrease in pristine samples, butrather to an undesirable sample irradiation. The result will be an invasive measurementthat alters the evaluation of defects concentration. in contrast, as shown in Figure 6.1,our samples are almost transparent in the excitation spectral range from 2.5 to 4 eV,making sure that the use of a laser probe at 3.8 eV results in a non invasive analysis.

• In the second place, it is worth to note that in our samples the absorption at∼5 eV is veryhigh: comparing Figure 6.1 we can see that at 5.2 eV the absorption coefficient exceeds70 cm−1 in the central core region. As a consequence, the luminescence signal obtainedexciting in the peak of the OA band, will not be proportional to GLPC concentration, dueto the fact that the sample will be not uniformly illuminated during PL measurements.

GLPC-related emissions at ∼ 3.1 eV detected in the fiber layers with different Ge-contentare shown in Figure 6.3. CML spectra were recorded from 370 nm (3.4 eV) to 500 nm (2.5 eV)

Figure 6.3: 3D plot of the CML band related to the T1→S0 GLPC transition, measured at differentdistances from the fiber core center and obtained under excitation at 3.8 eV in the fiber sample. ThePL intensity decreases with the distance from the center, i.e. with the GeO2 content.

6.1. Discussion: the drawing effect 65

with a spatial resolution of about 1 µm. Qualitatively, we observe that the PL intensitydecreases with the distance from the center, i.e. with the Ge content.

6.1 Discussion: the drawing effect

To perform a quantitative calculation of defects concentration, [GLPC], we convert the PLintensity PL(GLPC) to an absolute concentration measurement thanks to the evaluation ofthe oscillator strength, f , for the GLPC centers. Starting from Smakula’s equation [50], the5.2 eV absorption intensity in the preform is determined by its linear correlation with the PLbands and its oscillator strength, estimated to be f = 0.07 [21] from the radiative decay timeof the 4.2 eV (τ ' 7.8 ns) transition, measured at T=10 K under synchrotron radiation [141]:

[GLPC] = PL(GLPC)× 3.62× 1013 [cm−3] (6.1)

The calculation giving rise to Equation 6.1 was performed for preform samples and thantransferred to fibersa.

GLPC-related emissions at ∼ 3.1 eV detected in the fiber layers with different Ge-contentare shown in Figure 6.4. To draw the GLPC radial distribution in the fiber sample, theside (a) of Figure 6.5 shows their concentration measured at various distances from the fibercenter. For sake of clarity in the lower part of Figure 6.5 (side (b)) the microscopic fiber viewis sketched, so as to localize the various zones where these defects are detected.

To point out the influence of drawing effects on the defect generation, Figure 6.5 showsthe comparison of [GLPC] as a function of [Ge], as measured in preform and in fiber. In thepreform such a dependence has an almost linear trend over the whole range with a coefficientβpref ' 2×10−3. In contrast, in the fiber is observed a discontinuity at [Ge]≈ 14×1020 cm−3

(8 wt%): over the lower range the experimental points overlap with those of the preform,whereas at larger Ge concentrations the ratio [GLPC]/[Ge]=βfiber exceeds βpref up to aboutfive times (βfiber ' 9×10−3) for [Ge] ' 2×1021 cm−3 (11 wt%).

The interpretation of this result could be related to the differences between radial stressof the fiber and of the preform. Both materials are characterized by a thermoelastic stressthat increases linearly with the Ge concentration [142, 143, 144]: as this kind of stress islinked to the temperature expansion coefficient, it results in a positive tensile stress [143].This means that highly doped core regions, as those investigated in our experiment, are morelikely to exhibit tensile core stresses. On the other side, the drawing process can introducean additional negative compressive stress in the fiber samples [145]. The interplay betweenthese two contributions influences therefore the net fiber stress, which could result to be either

aOur experimental set-up does not allow to perform spatial resolved OA measurements on fibers.


0

5

1 0

1 5

- 6 0 - 4 0 - 2 0 0 2 0 4 0 6 0��

[GLP

C] (10

18 cm

-3 )

( b )

( a )

- 4 0 - 2 0 0 2 0 4 0

Figure 6.4: (a): GLPC concentration obtained by a transversal mapping of the fiber sample at variousdistances from the core center. (b): microscopic view of the fiber cross section. The different layers aredistinguishable. The x scales in the upper and lower figures coincide.

negative or positive in core zones with a different Ge content [145, 143], differently from thepreform where the radial stress is always positive. We suggest that a sudden change of thestress in the core inner part causes the observed discontinuity of GLPC concentration, due toa new defect generation mechanism.

This result is relevant for its connection with the refractive index change in the core zones.As the drawing process dramatically modifies the distribution of UV optically defects insidethe fiber, the photosensitivity properties cannot be deduced simply by the fiber compositionor by measurements on preform samples. From a handy point of view, it seems possible to

6.1. Discussion: the drawing effect 67

2 3 4 5 6 7 8 9 1 0 1 1 1 2

0

4

8

1 2

1 6

2 0

[GLP

C] (10

18 cm

-3 )

[ G e O 2 ] ( w t % )

P r e f o r m F i b e r

Figure 6.5: GLPC concentration trend detected in the different sample layers as a function of Gecontent both in fiber (empty squares) and preform (full circles).

Figure 6.6: Left: Assial stress as a function of GeO2 content in a graded index Ge-doped fiber. Right:Stress profile of a graded index Ge-doped fiber. Figures from Lee et al. [142].

modulate the fiber photosensitivity during the fabrication process by a systematic control ofthe fiber drawing conditions.

In conclusion, our experimental results point out that the CML is a powerful and notinvasive tool to probe the defect spatial distribution directly in optical fibers. We have demon-


strated that the GLPC radial distribution is different in fibers and preforms. This differencecan change the photosensitivity of the fiber regarding to the preform sensitivity. A possibleexplanation for this effect is the changes in the mechanical stress introduced by the drawingprocess. Such a stress can lead to an enhancement of the defect generation in some specificpart of the fiber. Our study shows that the drawing process has to be considered to explain thedefect generation mechanisms at the origin of the glass photosensitivity. More detailed analy-sis with fibers designed with variable process parameters and stress measurements have to bedone to control this phenomena in order to enhance the photosensitivity of germanosilicatefibers or to reduce their degradation under radiative environments.

Chapter 7

Effects of the UV and X-ray irradiation

As pointed out in section 3.2.1 the identifications of centers causing the degradation of the fiberproperties under radiation exposure is one of the main subject in the current study of opticalfibers response. Many studies, both experimental and computational, have investigated thegeneration and conversion processes of Ge-related point defects under radiation exposure. Theresearch interest is mainly motivated by the practical importance of these materials in photonicapplications, from standard optical fibers as waveguides for telecommunications to non-linearoptical devices. This multi-use of Ge-doped silica is strongly influenced by its response toradiation; for instance, it is known that radiation exposure induces Ge-related defects thatare cause of attenuation for the waveguides, thus leading to detrimental losses of part of thetransmitted signals. The identification of the specific defects, sensitive to radiation, is thereforeof crucial importance to make clear the microscopic origin of technologically relevant processes.Nerveless the current understanding of many important aspects is not at all complete, andseveral relevant questions are still debated.

In the following we present the results of a series of experiments on the effects of theradiation exposure on canonical Ge-doped fibers and preforms [126, 123, 146, 122] to get adeeper clarification of the microscopic origin of the photoinduced structural changes observedin these materials.

7.1 EPR results

One of the main techniques employed to investigate the effects of irradiation on Ge-dopedcanonical samples is the electron paramagnetic resonance (see section 5.4).

After X-ray exposure we observe the presence of paramagnetic centers, as an exampleFigure 7.1 shows the EPR spectra detected in fiber and preform samples after 20 kGy X-ray

70 7. Effects of the UV and X-ray irradiation

irradiation.

3 4 6 3 4 7 3 4 8 3 4 9

- 8

0

8

1 6

2 4

g = 1 . 9 9 3g = 1 . 9 9 9

M a g n e t i c F i e l d ( m T )

X d o s e 2 0 k G y P r e f o r m F i b e r

g = 2 . 0 0 7

Figure 7.1: EPR spectra in fiber (open circles) and preform (full line) Ge-doped samples after a X-doseof 20 kGy.

We can note that in both samples the irradiation induces the same defect species butwith a different efficiency: defect concentration in fiber is higher than in preform. It is alsoapparent that these EPR spectra are composite signals, resulting from the overlap of differentcontributions. As shown in Figure 7.1 we can distinguish the g values of three germaniumrelated defects: 2.007, 1.999, 1.993. To obtain information on the concentration of every defectsspecie contributing to the total EPR lineshape, we performed a deconvolution of the measuredspectra. We found that the EPR spectra can be least-square-fitted, after any irradiation, bya linear combination of the lineshapes associated with the following Ge-related paramagneticpoint defects, already described in section 3.2.1: (i) Ge(1) (GeO•4), (ii) GeE′ (≡ Ge•) and(iii) Ge(2), whose structure is still questioned between models consisting either in a trappedelectron center or in a hole center (see section 3.2.1 for details). As an example, Figure 7.2shows the fitting result on fiber (panel (a)) and preform (panel(b)), after a X-ray dose of20 kGy. Red solid line plots the best fitting function. The normalized EPR lineshapes ofthese three defects, used in the best fitting procedure, are shown separately in panel (c) ofthe Figure 7.2. Ge(1) and GeE′ lineshapes are experimental curves measured in γ-irradiatedGe-doped samples [147], where it has been possible to single out the two defects. The Ge(2)

7.1. EPR results 71

3 4 6 3 4 7 3 4 8

0

1 0

2 0

0

1 0

2 0

3 0

3 4 6 3 4 7 3 4 8 3 4 9 M a g n e t i c F i e l d ( m T )

( a )

EPR I

ntens

ity (ar

b. un

its)( c )

( b )

G e ( 1 )

G e ( 2 )

G e E ’

Figure 7.2: EPR signal detected in (a): fiber and (b): preform samples after 20 kGy X-ray exposure.Solid red lines plot the best fitting functions obtained as a superposition of the three lineshapes related toGECs and GeE′ centers. Panel (c) reports the three base lineshapes used for the best fitting procedure.

lineshape is a curve taken from literature, obtained by Friebele et al. [101] by a simulationprocedure aimed to fit EPR lineshapes observed in γ-irradiated multimode Ge-doped fibers.We point out that fitting an EPR signal with a linear combination of single-defect lineshapesis founded on the assumption that the centers are sufficiently far from each other that theirlineshape is not influenced by mutual interactions. This condition is usually satisfied at defectsconcentrations up to 1018 cm−3.

Figure 7.3 shows the changes in EPR spectra after different X doses in fibers and inpreforms. Qualitatively we can deduce that the EPR signal intensity increases with increasingthe X-ray dose both in fibers and in preforms, nevertheless the lineshape does not significantlychange.

We show that after UV pulsed laser, continuum UV laser irradiations or γ exposure theEPR spectra show the appearance of the same defect species with similar trend, even if withdifferent intensity. As an example in Figure 7.4 the comparison between a spectrum obtainedin preform sample after a X-dose of 2 MGy and the signal detected in gamma irradiatedpreform sample (total dose=91 kGy) is reported. In the rest of this section, only results


3 4 6 3 4 7 3 4 8 3 4 9- 1 0

0

1 0

2 0

3 0

3 4 6 3 4 7 3 4 8 3 4 9- 1 0

0

1 0

2 0

3 0


X d o s e ( k G y )2 0 0 0 2 0 2

( a )P r e f o r m

( b )F i b e r

X d o s e ( k G y )2 0 0 0 2 05 2

Figure 7.3: EPR signal intensity as a function of the total deposited X-dose in (a): preform and (b):fiber canonical Ge-doped samples.

obtained after X-ray irradiations will be discussed.

From the best fit coefficients appearing in the linear combination of the GEC defects(Figure 7.2(c)), we are able to evaluate the contribution of the three defect species to theoverall signal measured upon exposure at each total deposited X-dose. We point out thatEPR measurements give results on the whole sample, regardless the different sample layerswith various Ge amount. The contribution of each paramagnetic specie, obtained with theabove described procedure as a function of the X-ray dose is reported in Figure 7.5. It isevident that the GECs grow with the same trend on increasing the X-dose, Ge(2) being thedefect induced in largest concentration over the considered dose range. Both in fiber and inpreform Ge(1) and Ge(2) grow with the X irradiation dose till a saturation value, in contrastGeE′ centers do not reach a saturation value, at least at the considered doses.

7.2 Optical absorption

Due to the reduced fiber dimensions, absorption measurements in the UV-visible region wereperformed only on preform samples. Results will be than extended to fibers. In our specificcase, this experimental procedure is supported by results showed in section 7.3.3, in whichthe effects of the drawing process are illustrated: the results clearly show that only at low

7.2. Optical absorption 73

3 4 6 3 4 7 3 4 8 3 4 9

X i r r a d i a t e d ( 2 M G y ) γ i r r a d i a t e d ( 9 1 k G y )

EPR I

ntens

ity (Ar

b. Un

its)

M a g n e t i c f i e l d ( m T )

Figure 7.4: EPR spectra detected in preform samples after 10 keV-X-ray exposure (total depositeddose 2 MGy, full line) and after γ exposure (total dose=91 kGy, open circles).

irradiation dose the drawing procedure can influence defect generation.

The transmission losses from 2.5 eV to 5.5 eV of the preform sample after an X dose of2 MGy, detected with the AVANTES spectrometer, are shown in the panel (a) of Figure 7.6.As the beam spot size diameter is smaller than the different zones dimensions, we are able toselectively investigate the distinct sample layers. The spatial resolution of this measurementis obviously limited by the diameter of the beam spot, that is about 0.5 mm.

Through this procedure we can easily obtain the OA spectra as a function of the averageGeO2 content, as shown in Figure 7.6(a). The OA spectra referred to the different samplezones evidence the wing of an induced band centered at energies higher than 4.5 eV, more andmore intense with increasing the Ge-content. We notice that the detection of the whole bandin the heavy doped zones is limited by the presence of the OA band centered at 5.1 eV andrelated to the GLPCs, whose amplitude is higher than our detection limit (∼50 cm−1).

To single out the different OA components, the spectra have been fitted by Gaussiancurves. Panel (b) of Figure 7.6 shows the analysis of the spectrum detected in the preformzone containing 5 wt.% of GeO2; it is accounted for by three bands: the first centered at(4.6±0.1) eV, FWHM of (1.8±0.2) eV, the second centered at (5.14±0.02) eV, FWHM of


1 0 0 1 0 1 1 0 2 1 0 3

1 0 - 1

1 0 0

1 0 1

1 0 2

( b )

EPR I

ntens

ity (ar

b. un

its)

X d o s e ( k G y )

1 0 - 1

1 0 0

1 0 1

1 0 2

( a )

G e ( 1 )G e E ’G e ( 2 )

Figure 7.5: Integrated EPR intensity related to GECs and GeE′ centers as a function of the X-raydose detected in (a): fiber and (b): preform samples.

(0.50±0.02) eV and the last centered outside the investigated range (∼6 eV) to take intoaccount higher energies contributions.

In regard to the absorption tail centered at ∼4.5 eV, Figure 7.7 shows its dependencefrom the X total deposited dose. The graph illustrated the OA increasing, from 2.7 to 4.8 eV,in the central part of the preform sample, containing the higher GeO2-content (zone 4 inFigure 4.2 and in Table 4.1, GeO2 ∼11%). Similar trends are observed in the other samplezones. From the Figure 7.7 we can qualitatively conclude that, with respect to pristine sample,the radiation exposure causes an increasing in transmission losses at about 4.5 eV.

7.3 Discussion: radiation effects

The use of several spectroscopic techniques allows from one side to identify defect speciescausing radiation losses in the considered spectral range and on the other side to localize

7.3. Discussion: radiation effects 75

3 4 50

1 0

2 0

3 0

4 0

5 0

6 0

3 4 50

1 0

2 0

3 0

4 0

5 0

Abs.

Coeff

. (cm-1 )

[ G e O 2 ] ( w t % ) 1 1 8 5 3 0

( a )[ G e O 2 ] = 5 w t %

( b )

E n e r g y ( e V )

Figure 7.6: (a): Transmission losses detected in the various preform zones after 2 MGy X-ray exposure.The arrow specifies the GeO2 content from the exterior to the inner sample part. (b): Absorptionspectrum detected in the zone 2 (5 wt.% of GeO2) superimposed to the best fit curve, dashed linesrepresent the Gaussian components.

them inside the fiber sample in the considered spectral range. This result is a very originaland interesting feature in the study of the optimization of fiber manufacture and it will bediscussed in the following two points:

• localization of defect species

• generation of GECs defects

7.3.1 Localization of defect species

In the previous section we have shown the EPR results on preform and fiber Ge-doped canon-ical samples after radiation exposure. The results allow to identify various defect speciespresent in the samples, nevertheless this measurement procedure does not permit to localizethe defects in the various sample layers. We have seen in Section 4, that our specific canonicalsamples were made with concentric layers containing different amount of germanium. Wecan reasonably expect that the defect distribution along the sample layers is not uniform:Ge-related species will be concentrated in the central sample zone with the grater Ge-content(<[GeO2]>∼11%).

In the case of absorption measurements, this assertion is immediately confirmed by the


2 . 7 3 . 0 3 . 3 3 . 6 3 . 9 4 . 2 4 . 5 4 . 80

5

1 0

1 5

2 0

2 5

3 0

Abs.

Coeff

. (cm-1 )

E n e r g y ( e V )

X - d o s e ( k G y ) 2 0 0 0 2 0 2 0

Z o n e 4[ G e O 2 ] ~ 1 1 w t %

Figure 7.7: Transmission losses from 2.7 to 4.8 eV, detected in the central preform zone (zone 4,[GeO2]∼11%) as a function of the X-ray exposure. The arrow specifies the X total deposited dose.

radiation induced losses trend illustrated in Figure 7.6. While the absorption band centeredat 5.1 eV is undoubtedly attributed to GLPC centers, defects causing losses in the region3÷4.5 eV are not unambiguously identified. Neustruev [19] and Fujimaki et al. [106] tentativelyattributed the absorption at ∼4.5 eV to Ge(1) centers. As shown in section 7.1, our samplesreveal the presence of the Ge(1) EPR signature after radiation exposure (see Figure 7.2).Plotting the absorption variation at 4 eV detected in the central sample layer with respectthe Ge(1) EPR intensity, we obtain the graph illustrates in Figure 7.8 (lower x-scale). It isapparent that Ge(1) EPR signal is proportional to OA absorption around 4 eV. The linearcorrelation is confirmed by the best fit function represented as a full line in Figure 7.8. Thisconfirms that Ge(1) is the main responsible defect for the transmission losses at 4 eV.

So, while EPR measurements do not allow to spatially resolve defect concentration, byrelating Ge(1) OA band to their EPR signal, we can locate Ge(1) presence in the central partof the sample (the more doped part). Ge(1) concentration can be calculated considering thecentral part of the sample (x-upper scale in Figure 7.8).

It is also possible to measure the Ge(1) cross section, σ, at 4 eV , defined as [50]:

σ4eV = ∆α4eV /N (7.1)

where ∆α is the variation of the absorption coefficient compared to pristine sample and N


- 5 0 5 1 0 1 5 2 0 2 523456789

1 01 11 2 - 2 0 2 4 6 8 1 0 1 2

∆α4e

V(cm-1 )

G e ( 1 ) E P R I n t e n s i t y ( A r b . U n i t s )

∆[ G e ( 1 ) ] ( 1 0 1 8 c m - 3 )

Figure 7.8: Correlation between the EPR intensity of the Ge(1) signal (lower x scale) or the Ge(1)concentration (upper x scale) and the OA absorption variation at 4 eV. Full line represents the linearbest fit curve.

represents the defect concentration.To get the σ4eV value for Ge(1) centers, we can evaluate the slope of the correlation curve ofFigure 7.8, between Ge(1) concentration and the OA absorption variation at 4 eV. By a linearfit of the type:

[Ge(1)] = A+ σ ·∆α4eV (7.2)

we find that the cross section is σ4eV ≈6×1019 cm2. Hence it so possible to demonstrate thatGe(1) centers are responsible for the observed transmission losses and that they are mainlylocalized in the central part of the samples where the larger germanium content favors theelectron trapping on GeO4 sites.

7.3.2 Generation processes of GECs defects

In section 7.3.1 we have seen that Ge(1) defects are concentrated in central sample zone, withthe higher Ge-content. Hence, starting from the global EPR measurements, we are able toevaluate their effective concentration in the different preform zones. The result is illustratedin Figure 7.9. Comparing Figures 7.5 and 7.9, we can observe that the Ge(1) concentration,


1 1 0 1 0 0 1 0 0 00 . 0 10 . 1

11 0

1 0 0 P r e f o r m

Defec

ts co

ncen

tratio

n (10

18sp

in/cm

3 )

X d o s e ( k G y )

G e ( 1 ) G e E ’ G e ( 2 )

1 1 0 1 0 0 1 0 0 00 . 0 10 . 1

11 0

1 0 01 0 0 0

F i b e r

Figure 7.9: Concentration of the GECs centers as a function of the X-ray dose detected in (a): fiberand (b): preform samples. Defect concentrations are evaluated as concentrated in the central samplezones (<[GeO2]>∼10%).

both in fibers than in preforms, is larger than the value expected considering only the EPRintensity trend. In particular, after an X-ray exposure of 2 MGy the Ge(1) concentration isabout 1019 cm−3 in the inner core fiber part, that is about two order of magnitude larger thanthe value obtained considering the defects as equally distributed on the whole sample. ForGe(2) the defect concentration in fibers at the saturation value is ∼1×1019 cm−3. For theconsidered total deposited doses, the GeE′ centers do not reach a saturation value: after atotal X deposited dose of 2 MGy, [GeE′]'4×1017 cm−3, both in fibers and in preforms.

We note that the Ge(1) localization and the calculation of their concentration can beindirectly applied to fiber samples. This outcome is crucial to find out the radiation effectin modulating the optical properties of step index fibers, both the light transmission and therefractive index change.

Regarding GECs generation mechanism, many authors have supposed the following reac-


tion as the basic creation process of Ge(1) and Ge(2) centers in Ge-doped silica after UV orgamma exposure:

= Ge•• +GeO4 + hν −→ (= Ge•)+ + (GeO4)• (7.3)

A germanium lone pair center (= Ge••) and a 4-coordinate germanium (GeO4) generate, afterradiation exposure, an ionized GLPC ((= Ge•)+), that is a Ge(1) center, and a hole center ona 4-coordinate germanium ((GeO4)

•).

Evidently Equation 7.3, implies that the structure for Ge(2) center is a hole center andnot a trapped electron center. This also means that, to confirm the generation mechanismproposed in Equation 7.3, a direct proportionality between Ge(1) and Ge(2) concentration hasto exist.

According to our measurements, the dependence of Ge(1) concentration from Ge(2), bothevaluated from EPR measurements, has the trend shown in Figure 7.10. We point out that,

0 2 4 6 8 1 0 1 2 1 4 1 60

1 02 03 04 05 06 07 08 09 0

1 0 0

[Ge(2

)](1018

spin/

cm3 )

[ G e ( 1 ) ] ( 1 0 1 8 s p i n / c m 3 )

F i b e r P r e f o r m

Figure 7.10: Correlation between Ge(1) and Ge(2) center in fiber (open circles) and preform (fullsquares) Ge-doped canonical sample after irradiation exposure.

to obtain the graph in Figure 7.10, we have considered also the Ge(2) centers as concentratedin the central sample zone, even if rigorously we have demonstrated this assertion only forGe(1) centers. We justify this hypothesis on the basis of Equation 7.3: if Ge(1) and Ge(2)generation mechanisms coincide and if we find that the Ge(1) centers are only present in thecentral part of our canonical samples, this must be true also for Ge(2).


7.3.3 The drawing effect

Up to now, only few studies investigated the influence of the drawing process on the fiberradiation response by comparing the behaviors of fiber and preform. Most of them havecharacterized the effect of varying the drawing tension, speed or temperature on the radiationresponse of optical fibers [119, 118, 148]. In our study, all our canonical samples have beenmade with standard MCVD drawing conditions. By comparing the concentrations of room-temperature stable paramagnetic defects at the same doses in a preform and its correspondingfiber, we can estimate the global influence of the drawing process on the glass sensitivity. Theobtained results are illustrated in Figure 7.11 for the Ge canonical samples.

1 1 0 1 0 0 1 0 0 0

1

1 0

p o s i t i v e i n f l u e n c e

[Fibe

r]/[Pre

form]

X - D o s e ( k G y )

G e ( 1 ) G e E ’ G e ( 2 )

n e g a t i v e i n f l u e n c e o f d r a w i n g p r o c e s s

Figure 7.11: Defect concentration ratio on fibers and preforms in Ge-doped samples at different doses.

Our results clearly show that the influence of the drawing process on the generation of Ge-related centers is dose-dependent. The dose-dependence is observed separately for the threeparamagnetic defects: Ge(1), Ge(2) and GeE′. The impact of the drawing process is stronglynegative at low doses and it becomes nearly negligible at higher doses: at 2 MGy this ratio isabout 1 and it becomes 10 for a dose of 2 kGy. These results show that at lower doses (space,military applications) the drawing process governs the generation of paramagnetic defects inGe-doped glass whereas the glass composition seems to be the most influential parameter forhigh-dose applications (nuclear power plants, high energy physics).

A possible explanation for this dose dependence is that drawing strongly increases the


number of defect precursors such as GeODC(II) and the so called neutral oxygen vacancy(NOV) (≡ Ge− T ≡; T=Si or Ge) [19, 106] which can turn into Ge(1), Ge(2) or GeE′ underirradiation following the process described by Equation 7.3 and by the following relation:

≡ Ge− T ≡ +hν −→= Ge• ++ T ≡ +−e (7.4)

where ≡ Ge−T ≡ is the NOV, T is either Ge or Si, and = Ge• is the GeE′ center. At low doses,the contribution of defects generated from these precursor sites to the total concentration ofdefects is predominant whereas it may become less important at higher doses due to defectgeneration via other mechanisms. As above shown, our CML measurements on the Ge-dopedfiber with UV excitation provided evidence for the presence of GeODC(II) in both pristine(see Section 6) and irradiated samples. By the way, Ge(1) and Ge(2) defects may be createdfrom the preexisting and X-ray radiation-induced GeODC(II). We have previously shownthat the Ge(1) and Ge(2) concentrations saturate at higher doses (>20 kGy) whereas GeE′

concentrations continuously increase up to 2 MGy (Figure 7.9) providing evidence for severalgeneration mechanisms for this defect.

Additional tests have to be done to fully understand the drawing influence on the fiberradiation response. Complementary EPR measurements have to be done at lower doses toinvestigate this effect for low-dose environments. Furthermore, different fiber samples have tobe drawn from the same Ge-doped preform to determine the most favorable drawing conditionsfor this kind of optical fiber.


Part IV

Influence of further dopants: fluorine andphosphorus

Chapter 8

F-doped fibers and preforms

In this chapter we will analyze the fluorine role in changing the structure and the radiationeffects in the optical fibers. As shown in Section 2.2, there are several plausible reasons forjustifying the addition of fluorine to the a−SiO2 matrix:

• F substitutes to OH groups or Cl, always present in optical fiber structure, withoutinducing any optical absorption in the transparent region of a silica glass.

• Si− F bond is stronger than Si−O, resulting in widening the optical band gap.

• Formation of Si− F bonds makes the glass network to be more stable, i.e., it decreasesstructural disorder, which shift the vacuum-ultraviolet silica fundamental absorptionedge (Urbach edge) to shorter wavelengths [30].

• F favours structural relaxations, and makes it easy to fabricate silica glasses with lowerfictive temperatures (Tf ), which is defined as "the temperature at which the glass wouldfind itself in equilibrium if suddenly brought to that temperature from its given state"[149].

The first part of the Chapter is devoted to the materials characterization by a microRaman analysis. Successively the fluorine ability of improving the material radiation hardnessis investigated. Specifically, after fiber and preform exposition to ionizing radiations of differentnature, the samples are spectroscopically analyzed by electron paramagnetic resonance (EPR)to reveal the possible presence of point defects thus evaluating the fiber radiation resistance.

86 8. F-doped fibers and preforms

8.1 Raman results

It is known that fluorine is incorporated into the silica glass structure as SiF with the siliconatom bonded to three oxygen atoms in the network [150]. Raman spectroscopy is an useful toolto determine the presence of SiF linkages in silica glass, since they give rise to a spectroscopicline around 945 cm−1. In the following we take advantage of the spatial resolution of theconfocal micro-spectrometer elsewhere described (Section 5.3.5) to check F-presence in oursamples.

In Figure 8.1 we show the Raman spectrum of the zone 1 (see Table 4.1(b)), obtainedin the spectral region (150-1100) cm−1. It is clearly visible the peak centered at 945 cm−1

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

Rama

n Inte

nsity

(arb.

units)

R a m a n S h i f t ( c m - 1 )

S i - Fp e a k

Figure 8.1: Preform cladding (zone 1) Raman spectrum in the spectral range (100-1100) cm−1 . Thepeak at 950 cm−1 is related to Si− F stretching mode.

corresponding to the Si − F stretching vibration mode of a SiO3F tetrahedron [151]. Thevariation of this F-related line in the other zones is plotted in Figure 8.2, all spectra beingnormalized to the intensity of the peak around 800 cm−1 which is ascribed to the stretchingmode of Si − O bond [152]. Table 8.1 gives the nominal average F-concentrations for eachzone, as evaluated by electron microprobe analysis, and the ratio of the Raman 945 cm−1 bandand the 800 cm−1 reference band, for the various layers. As known from literature [27], forlow F content (inferior to 2 wt%) only Si − F links are present; this agrees with our resultsthat point out a correlation between F content and the Si− F Raman signal.

8.1. Raman results 87

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0

( z o n e 0 )

( z o n e 1 )

( z o n e 2 )

( z o n e 3 )

Rama

n Inte

nsity

(arb.

units)

R a m a n S h i f t ( c m - 1 )

D i s t a n c e f r o m c e n t e r2 . 3 5 m m2 . 1 5 m m1 . 6 m m1 m m0 . 4 5 m m( z o n e 4 )

Figure 8.2: Raman spectra detected in the different sample zones listed in Table 4.1. The variation ofthe Si − F band (950 cm−1) at different distances from the preform center is clearly visible. For sakeof comparison, spectra are shifted and they are normalized to the 800 cm−1 band.

All the other peaks are intrinsic features associated with the glassy silica matrix [153].The peaks at 495 cm−1 and 606 cm−1 in Figures 8.1 and 8.2, superposed to the larger band at440 cm−1, are the so-called defect bands D1 and D2. The D2 and the D1 bands are attributedto the three- and the four-membered ring structures, respectively [154]. In these small ringstructures the Si − O − Si bond angle (130.5◦ for the three-membered ring and 160.5◦ forthe four-membered ring) [25] is significantly different from the stablest angle (150◦) [154,155].As a consequence, these ring structures are composed of heavily strained S − O − Si bonds.According to estimates by Galeener [154], the atomic fraction of these rings is of the order of1%.

It has been demonstrated that strained Si − O − Si bonds, which are created on den-sification of SiO2 glass by high pressure [156], cause the absorption edge shift to a longerwavelength [25]. The densified SiO2 glasses are extremely sensitive to defect formation by ir-radiation. The formation of strained three- and four-membered ring structures has been foundto be suppressed effectively by doping of a small amount of F because fluorine is incorporatedin the form of Si−F bonds that terminate in a continuous silica network structure [25]. This isevident from the Raman spectra depicted in Figure 8.3, where a comparison (without verticalshift) is reported between the F-free zone (zone 0 in Figure 4.4) and the 11 wt% F-doped zone


Table 8.1: Fluorine average concentration and ratio of the Raman bands at 945 cm−1 and at 800 cm−1

band for the different preform zones. N.D. stands for not detected.

Zone <F> I(945 cm−1

I(800 cm−1

(wt%)

Coating 0 0.0 N.D.Cladding 1 2.5 0.14±0.03

2 1.3 0.12±0.02Core 3 0.7 0.09±0.02

4 0.2 0.03±0.01

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

D 2

Rama

n Inte

nsity

(arb.

units)

R a m a n S h i f t ( c m - 1 )

D i s t a n c e f r o m c e n t e r : 2 . 1 5 m m

( z o n e 1 ; [ F ] = 1 . 8 w t % ) 2 . 3 5 m m

( z o n e 0 ; [ F ] = 0 w t % )

D 1

Figure 8.3: Raman spectra of the F-free and the F-doped (11 wt%) zones of the pristine canonicalpreform sample. D1 and D2 defect bands are highlighted.

(zone 4 in Figure 4.4). The main difference between the two spectra, apart from the presenceof the 945 cm−1 Si − F band, is the intensity reduction of the D1 and D2 bands. The roleof F is similar to that of OH groups in this respect. However, laser damage is drasticallyenhanced for OH doping because OH groups have strong optical absorption at a wavelength of157 nm. The breaking of the continuous silica network structure by F doping largely facilitatesstructural relaxation in the process of cooling from the melt: F doping may be regarded aschemical annealing [25].

8.2. EPR measurements on irradiated samples 89

8.2 EPR measurements on irradiated samples

As pointed out in Section 2.2, recent studies have shown that radiation toughness of silicasamples is achieved by incorporating Si−F groups whose positive effect is assumed to be thereduction of defect precursors [150,27,24], such as strained bonds (Si−−O−−Si) from whichis likely generated the pair of silicon dangling (≡ Si•) and oxygen dangling (≡ Si−O•), where(≡) and (•) indicate bonds with three oxygen atoms and an unpaired electron, respectively.These two paramagnetic defects, that as shown in the introduction section are also namedE′ center and non bridging oxygen hole center (NBOHC) (see Section 2.2), are indeed one ofthe main causes of transparency loss due to their absorption bands peaked in visible and UVspectral range [157]. To this aim the role of fluorine doping in the response to UV pulsed laserand γ radiation of silica preforms and fibers was studied using EPR.

8.2.1 Results

Figure 8.4 shows the EPR spectra of preform samples after exposure to UV- (8×103J/cm2;panel (a)) and γ-rays (91 kGy; panel (b)); we observe that neither preforms nor fibers showEPR signals before exposure. Regardless the kind of irradiation, two identical signals areobserved in our samples. The central part of the spectra shows the typical line shape of theE′ centers, whose concentration is measured to be (5.5±0.8)×1015 cm−3 in the UV irradiatedsample, and (1.8±0.4)×1016 cm−3 in the γ irradiated one. In the spectra is also observed astructured signal extended over 10 mT, which is ascribed to the [AlO4]0 center, whose structureis shown in Figure 8.5. This defect, consisting in a substitutional Al atom [159,158], has beenextensively studied in many experimental [160, 161] and theoretical [162, 104] studies. Thecalculated concentrations of these defects are (3.9±0.7)×1015 cm−3 and (6.9±0.4)×1015 cm−3

after UV- and γ irradiation, respectively.

The presence of [AlO4]0 defect indicates the existence of extrinsic impurities in preformsamples before exposure. Though the EPR spectra do not allow to spatially resolve thedistribution of paramagnetic defects, as the sample core-cladding part is made of high pureF-doped silica, we can suppose that [AlO4]0 center are concentrated in the external layer ofnon doped silica, containing ∼10 ppm of Al impurities.

Similar irradiation treatments were performed on fiber samples; in this case, the E′ centerscould be observed only by second harmonic mode, whereas detection of [AlO4]0 EPR signalis prevented by our detection limit. Figure 8.6 shows the E′-related second-harmonic signal,both in UV (Figure 8.6(a)) and γ (Figure 8.6(b)) irradiated samples. UV irradiations wereconducted at different amounts of energy fluence, from 10 J/cm2 to 132 J/cm2. As clearlyshown in Figure 8.7, E′ concentration grows with the UV fluence up to 55 J/cm2, after that


3 4 5 . 0 3 4 5 . 33 3 8 3 4 0 3 4 2 3 4 4 3 4 6 3 4 8 3 5 0 3 5 2

3 4 5 . 0 3 4 5 . 33 3 8 3 4 0 3 4 2 3 4 4 3 4 6 3 4 8 3 5 0 3 5 2

ESR s

ignal

(arb.

units)

( a )


( b )

Figure 8.4: EPR first harmonic spectra in UV (8×103 J/cm2) (a) and γ (91 kGy) (b) irradiatedpreforms. The central enlarged zone shows the typical signal of E′ centers. Parts (a) and (b) have thesame scale.

Figure 8.5: Microscopic structure of the [AlO4]0 center. Figure adapted from ref. [158].

it saturates at a value of 8×1015 cm−3. In γ irradiated fiber, after a dose of 91 kGy, E′

concentration becomes (2.3±0.2)×1016cm−3.

8.2. EPR measurements on irradiated samples 91

3 4 5 . 5 3 4 5 . 6 3 4 5 . 7 3 4 5 . 8 3 4 5 . 90

2 0

4 0

6 0

8 0

0

2 0

4 0

( b )


1 3 25 53 71 0

U V F l u e n c e ( J / c m 2 )

ESR s

ignal

(arb.

units)

( a )

Figure 8.6: EPR second harmonic spectra in fiber samples after irradiation. (a): variation of thesignal due to E′ centers with the UV fluence on fibers.(B): E′ signal in fibers after a γ dose of 91 kGy.

8.2.2 Discussion: generation of E′ centers

The above reported results point out two aspects, common both to preform and fibers and toUV- and γ-irradiation:

1. The generation of E′ centers is due to the conversion of pre-existing precursors suchas oxygen vacancy or extrinsic Si-H or Si-F bonds. This is consistent with the growthcurve observed under UV irradiation, that manifests a saturating tendency due to theexhaustion of precursors; hence, the larger E′ concentration induced by γ radiationsuggests the presence of different kind of precursor activated by one or the other radiationsource. On the other side, the absence of NBOHC, ensured by EPR spectra within a limitof ∼1016 cm−3, and by luminescence measurements on preform within ∼1015 cm−3 [157],indicates that the radiolysis of strained Si-O bonds is an unlikely process in our F-dopedsilica samples.


0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0

0

3

6

9

[E’

] (1015

cm-3 )

F l u e n c e ( J / c m 2 )

Figure 8.7: E′ concentration as a function of the energy fluence on UV irradiated fibers.

2. The drawing process does not weaken significantly the radiation toughness of the fibersamples. In fact, after γ-irradiation the ratio between the number of E′ defects infiber (Figure 8.4(b)) and in preform (Figure 8.6(b)), both irradiated with 91 kGy, is[E′]fiber/[E′]pref ∼=1.2. An analogous comparison can be made for UV irradiated samples.Considering the saturation value of E′ concentration on UV irradiated fibers, we canevaluate the ratio between this concentration and that found in preforms after an UVfluence of 8×103 J/cm2: [E′]fiber_satur/[E′]pref ∼=1.5. Hence, we can conclude that aboutthe same amount of E′ centers are generated in fiber and preforms both under UV andγ radiation.

These points qualitatively evidence the role of F-doping in governing the response toradiation of preform and fiber. In fact, irradiation of undoped silica, bulk [24] or fiber [163,164],produces the generation of the pair NBOHC and E′ center by the rupture of strained Si-O bonds, whereas the formation of NBOHC is inhibited in F-doped samples. This findingsuggests that Si-F groups reduce the presence of strained bonds, this role upholds both inbulk and fiber, thus being crucial to improve the quality of optical fibers designed for visibleand UV transmission.

Chapter 9

P-doped fibers and preforms

As shown in the introduction sections (see Sections 2.3 and 3.2.2), despite the importance ofP-doping in fiber technology, its real influence in defect generation, and consequently in lightpropagation attenuation, is still unknown. Not only the consequence of the radiation exposurein terms of attenuation, but also the structure of P-related defects in pristine P-doped a−SiO2

is still subject of study and discussion.

In this chapter we report experimental studies on phosphorous-related point defects inamorphous silica, based on photoluminescence, absorption, and electron spin resonance mea-surements carried out on P-doped canonical fibers and preforms (see Section 4.1).

9.1 Optical activity of P-related point defects

From the experimental point of view, the optical properties of diamagnetic P-relatedcenters in silica are scarcely known at the moment, since only very little data exist on opticalabsorption and luminescence of as-grown phosphosilicate glasses, expect for the basic evidencethat no strong UV absorption bands are generally induced in these materials just by P-doping [38,165].

The purpose of this section is to contribute to a better understanding of these topics, byreporting data obtained by absorption and photoluminescence measurements in the UV andvacuum UV spectral ranges on P-doped optical fiber preforms.

9.1.1 Absorption and photoluminescence analysis

Figure 9.1 shows the OA spectrum of the central zone of the canonical sample (zone 4, maxi-mum P-content '7 wt%, see Figure 4.6) in the UV and in the VUV spectral range, detected

94 9. P-doped fibers and preforms

with the JASCO V-560 and the ACTON SP-150 spectrometers respectively.

3 4 5 6 70

1 0

2 0

3 0

4 0

5 0

6 0

3 4 5 60 . 0

0 . 5

1 . 0

α (

cm-1 )

E n e r g y ( e V )

Figure 9.1: Absorption spectrum of the P-doped preform in the UV-VUV spectral range (2.5-7.5 eV).The inset shows the enlargement of the OA spectrum from 3 to 6 eV.

It is evident the presence of a strong absorption band in the VUV range centered at about6.9 eV, an absorption tail for E>7eV, and a shoulder in the UV region, due to a band centeredat about 4.8 eV (inset of Figure 9.1).

By performing a PL emission measurement at room temperature under synchrotron exci-tation at the energy corresponding to the UV band (4.8 eV), we detected a broad asymmetricluminescence signal centered at 3.0 eV, reported in Figure 9.2(a). The PLE spectrum of thissignal, measured with emission at Eem=3 eV, is reported in Figure 9.2(b) and features twocomponents: the first one is centered at 4.7 eV (with FWHM ∼0.7 eV) and the second one at6.4 eV (with FWHM ∼0.6 eV). The shape of the emission band turns out to be approximatelythe same when excited at 4.8 eV (full squares) or 6.4 eV (open circles) by synchrotron radia-tion, or upon laser excitation at 4.8 eV (solid line), as shown by the comparison of the threesignals in Figure 9.2(a).

We studied the dependence of the 3.0 eV luminescence signal intensity on P concentration,by moving the laser excitation spot across the different preform zones via a micropositioningstage. The spatial resolution of this measurement is limited by the diameter of the laserspot, partialized by an iris, that is about 0.5 mm. As shown in Figure 9.3(a), we found

9.1. Optical activity of P-related point defects 95

4 5 6 7 80 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

2 . 4 2 . 7 3 . 0 3 . 3

0

1 5

3 0

4 5 ( b )

PLE Intensity (arb. units)

E n e r g y ( e V )

E E X ( e V )

4 . 86 . 4

E n e r g y ( e V )

PL In

tensity

(arb.

units)

( a ) 4 . 8 ( l a s e r e x c . )

Figure 9.2: (a): Emission spectra measured in the central sample zone (zone 4), containing ∼7 wt%of phosphorus at 300 K, obtained exciting at 4.8 eV (full squares) and 6.4 eV (open circles) undersynchrotron radiation and at 4.8 eV under laser excitation with WT =20 ms and TD =5 ns (solid line).(b): Excitation spectrum monitored at Eem=3 eV detected at room temperature under synchrotronradiation in the range 4-8.3 eV.

that the 3.0 eV PL signal is observed only in the P-doped region of the sample, and rapidlydisappears when moving away from the center of the perform. By comparing with the spatialdependence of P concentration, we see that the luminescent region is somewhat narrower thanthe doped region. Indeed, the 3.0 eV PL is mainly localized on zones III and IV of the preform.Figure 9.3(b) shows the PL intensity on the peak of the 3.0 eV band as a function of P-content.It appears that the luminescence signal shows up only when P concentration overcomes a ∼4%threshold.


- 2 - 1 0 1 20 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

1 . 2

1 . 4

1 2 3 4 5 6 7 80

2

4

6

8

1 0

1 2

1 4

2

4

6

8

P L I n t e n s i t y

1

23

1

23

PL In

tensity

at 3

eV (a

rb. un

its)

D i s t a n c e f r o m c e n t e r ( m m )

4( a )

PL I

ntens

ity at

3 eV (

arb. u

nits) ( b )

P c o n t e n t ( w t % )

P - c o n t e n t

P content (wt%)

Figure 9.3: (a): PL intensity at 3 eV measured at room temperature under laser excitation at 4.8 eVwith WT = 20 ms and TD =5 ns (full circles, left vertical scale) and phosphorus content (open circles,right vertical scale) as a function of the distance from the preform center. Vertical lines refer to thedifferent sample zones (from 1 to 4) as listed in Table 4.1 and in Figure 4.6. (b): PL intensity at 3.0 eVas a function of the phosphorus content.

We performed time-resolved emission measurements (Figure 9.4) on the 3.0 eV band, byacquiring at room temperature several emission spectra upon laser excitation, withWT = 500 µsand TD going from 1 µs to 25 ms. These data allow to study the decay kinetics of the PLsignal at several spectral positions within the emission band. Since the decay turns out tobe single-exponential at any fixed emission energy, the lifetimes were obtained by a fittingprocedure with a single exponential function of time-resolved PL data at several spectral po-sitions, as shown in Figure 9.5, where full lines represent the best fit curves. We found a slightdispersion of the lifetime inside the emission band: τ varies from 6.9 ms at Eem = 2.7 eV to


5.1 ms at Eem = 3.2 eV. Such a dispersion of the lifetime corresponds to a progressive red shiftof the emission peak of Figure 9.4 during the decay, due to the low energy tail of the signaldecaying slower than the right tail.

Figure 9.4: Time resolved PL spectra measured under laser excitation at 4.8 eV with WT = 500 µsand TD from 1 µs to 25 ms.

We also studied the dependence on temperature of the luminescence signal measured byexciting at 4.8 eV in the central sample region. Figure 9.6 shows the PL spectra under laserexcitation at 4.8 eV at different temperatures. Notwithstanding a certain degree of scatteringof data points, from this investigation we can clearly see that the emission intensity excitedat 4.8 eV is poorly dependent on temperature in the range 10-300 K. (inset of Figure 9.6);also the peak position and width do not depend significantly on temperature. The lifetimeτ measured on the peak, at 3 eV, is independent of temperature as well within experimentalaccuracy, as shown in Figure 9.7 (a), where several decay curves at various temperatures arereported. Performing the same investigation on the left tail of the band, a small variation ofthe τ value with the temperature appears (full circles and triangles in Figure 9.7(b)). Thismay suggest the existence of another small component centered at energies < 2.8 eV. Thissecondary effect needs a specific investigation but it will not be analyzed in this Thesis. It isworth noting, however, that the presence of this component may contribute to the observedasymmetry of the emission band.


0 5 1 0 1 5 2 0 2 5 3 0 3 5e - 6

e - 4

e - 2

e 0

e 2

e 4

e 6 E e x = 4 . 8 e VT = 3 0 0 K

E e m = 2 . 7 e Vτ = 6 . 9 m sE e m = 2 . 9 e Vτ = 5 . 7 m sE e m = 3 . 0 e Vτ = 5 . 2 m sE e m = 3 . 2 e Vτ = 5 . 1 m s

PL In

tensity

(arb.

units)

D e c a y T i m e ( m s )

Figure 9.5: Decay curves detected at room temperature at different emission energies (Eem) underlaser excitation at Eex=4.8 eV (WT = 500 µs and TD from 1 µs to 25 ms). For viewing purposes,the initial values of the decay curves are arbitrarily scaled. Full lines represent the best fit curves by asingle exponential equation.

2 . 1 2 . 4 2 . 7 3 . 0 3 . 3 3 . 60

1 0

2 0

3 0

4 0

5 0

0 1 0 0 2 0 0 3 0 0 0

1 5

3 0 3 0 0 K 1 6 0 K 4 0 K

PL In

tensity

(arb.

units)

E n e r g y ( e V )

T ( K )

Figure 9.6: PL bands under laser excitation at 4.8 eV in the central sample zone detected at varioustemperatures. The inset shows the temperature dependence of the integrated PL intensity.


0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 04 . 5

5 . 0

5 . 5

6 . 0

6 . 5

7 . 0

0 5 1 0 1 5 2 0 2 5 3 01 0 - 2

1 0 - 1

1 0 0

1 0 1

1 0 2

1 0 3

( b )

τ (m

s)

P e a k p o s i t i o n 2 . 7 e V 2 . 9 e V 3 . 0 e V 3 . 2 e V

T ( K )

PL In

tensity

(arb.

units)

E E X = 4 . 8 e VE E M = 3 e V

D e c a y T i m e ( m s )

T = 1 0 Kτ = 5 . 1 m sT = 7 0 Kτ = 5 . 3 m sT = 1 3 0 Kτ = 5 . 3 m sT = 3 0 0 Kτ = 5 . 2 m s( a )

Figure 9.7: (a): Decay curves detected at Eem=3 eV at different temperatures under laser excitationat Eex=4.8 eV (WT = 500 µs and TD from 1 µs to 25 ms). For viewing purposes, the initial values ofthe decay curves are arbitrarily scaled. Full lines represent the best fit curves by a single exponentialequation. (b): Temperature lifetimes dependency at different energetic positions inside the 3.0 eV-PLband at Eex = 4.8 eV under laser excitation.

Additionally, we performed EPR measurements both on pristine and X-ray irradiatedpreform samples aiming to explore the local arrangements of P atoms in the SiO2 matrix, aswell as the radiochemical processes activated by X irradiation. The pristine samples did notshow any detectable paramagnetic defects. After a total deposited 10 keV-X-dose of 20 kGy,at a dose rate = 0.1 kGy/s, the EPR spectrum shows a structured signal extended over 10 mT,and shown in Figure 9.8-(a). The measurement was performed with a modulation amplitudeof 0.1mT and a non-saturating microwave power of 0.19 mW. By comparison with literature,most of the observed signal can be ascribed to POHC-type defects [40]. Specifically, the


2 5 0 3 0 0 3 5 0 4 0 0- 2

- 1

0

1

23 3 8 3 4 0 3 4 2 3 4 4 3 4 6 3 4 8 3 5 0- 3

- 2

- 1

0

1

ESR s

ignal

(arb.

units)


( 2 )

1 1 2 m T ( b )

( a )

( 1 )

Figure 9.8: Electron spin resonance spectrum detected in P-doped preform after a total depositedX-dose of 20 kGy. The signal in panel (a) was acquired by using a modulation amplitude of 0.1 mTand a 0.19 mW power. (1) and (2) indicate characteristic features of r-POHC and l-POHC respectively.The spectrum in panel (b) was acquired by using a modulation amplitude of 0.5 mT and a 1.9 mWpower. We verified both values of power to be below the saturation threshold of the respective signals.

negative peaks at 341.9 mT and 346.5 mT (marked as (1)) are associated to r-POHC, whilethe two additional peaks at 342.8 mT and 347.3 mT (marked as (2)) are characteristic featuresof l-POHC. The positive portions for B<341.5 mT and for 345.5mT<B<346.2mT are due toa combination of l-POHC and r-POHC. Both signals are actually hyperfine doublets with∼5mT splitting, due to the interaction with the 31P nucleus (100% natural abundance). Itis worth noting that the observation of l-POHC contrasts with previous suggestions of thiscenter being metastable at room temperature [40]. Finally, the central part of the spectrum

9.2. Discussion: luminescent P-defects structure 101

also shows an additional structure around 344 mT that is similar to a signal attributed tothe so called Si(E′)(P) center [40], a type of Si(E′) center with phosphorus next-nearest-neighbors (see section 3.2.2). Since the contribution of the latter defect to the overall signal isminor, the overall concentration of POHCs can be estimated in good approximation by doubleintegration of the whole spectrum in Figure 9.8-(a) and comparison with a reference sample.In this way we get: [POHC]=(1.2±0.1)×1017 cm−3. aBy scanning a wider magnetic field range(Figure 9.8-(b)), we observe a hyperfine doublet with 112 mT separation which is consistentwith the spectral features of the P2 defect, i.e. a 4-fold coordinated P trapping an unpairedelectron [40]. The substantially larger hyperfine splitting as compared to POHC is due to themuch stronger localization of the unpaired electron on the P nucleus. This measurement wascarried out with a modulation amplitude of 0.5 mT and a non-saturating microwave power of1.9 mW. The concentration of P2 centers can be estimated to be [P2]=(1.4±0.1)×1017 cm−3

and thus is consistent with [POHC] within experimental error. Finally, by PL measurements inthis sample we observed a 42% reduction of the intensity of the native PL band at 3.0 eV. b Thesame measurements were performed on another sample irradiated with a lower (2 kGy) dose,where we found similar results: the emission intensity was reduced of 30%, while from EPRmeasurements we get: [POHC]=[P2]=(8.5±0.8)×1016 cm−3. In neither sample we observedthe characteristic hyperfine doublets of P1 or P4 centers [40,111]. This suggests that in thesematerials the 2-fold and 3-fold coordinated configurations of P impurities are either presentin much lower concentration or are much less sensitive to radiation than the tetrahedrical[(O-)3P=O]0 and [(O-)2P(-O)2]+ configurations acting as precursors for POHC and P2.

9.2 Discussion: luminescent P-defects structure

Data in Figures 9.2-9.7 demonstrate the existence in as-grown P-doped silica of an emissionsignal peaked at 3.0 eV and featuring two excitation channels at 4.8 eV and 6.4 eV. The lowestenergy excitation is detectable as well as a weak absorption band. The spatial dependence ofthe signal intensity in Figure 9.3 strongly suggests this emission to be associated to a P-relateddefect. The fact that the spatial extension of the luminescent region is narrower than thatof the P-doped zone implies that the concentration of the specific center responsible for thisemission is not strictly linearly correlated with the overall P concentration (see Figure 9.3(b)).

aThis estimate is based on the assumption that P-related paramagnetic centers are uniformly distributed inall the P-doped volume. If one assumes instead, following the results of Figure 9.3-(b), that the paramagneticcenters are located in zones III and IV only (as the luminescence signal is), a correction factor must be applied,leading to a higher estimate: (3.2±0.3)×1017 cm−3. Under this hypothesis, all the other concentration valuesreported later on must be scaled for the same factor.

bThe bleaching of the PL signal is accompanied by the appearance of new absorption signals which will bediscussed in the next section. It is worth noting that the induced absorption at 4.8 eV (∼0.1 optical density)is not sufficiently intense to compromise the measurement of luminescence intensity.


Specifically, data in Figure 9.3 suggest the emitting center to be formed in detectable concen-trations only when the overall P content overcomes a threshold of about 4%.

The main spectroscopic features of this PL signal are its long lifetime in the ms range andits weak dependence on temperature. The first result suggests the emission to originate froma spin-forbidden transition from an excited triplet state (T1). At the same time, the tempera-ture independence implies the absence of efficient non-radiative decay channels from T1 to theground state, so that the ∼6 ms lifetime has to be interpreted as a purely radiative decay life-time. Finally, the dispersion of the radiative decay lifetime within the emission band suggestsstrong inhomogeneity effects to affect the overall width of the band, as recently pointed out forother defects embedded in an amorphous matrix [166]. Data analysis aimed to quantitativelyestimate the degree of inhomogeneity of the band is in progress, being complicated by thepossible presence of another weaker signal on the left side of the band.

The first excitation peak at 4.8 eV detected by the PLE measurement is likely to berelated to the 4.8 eV band found in the OA spectrum of the defect (Figure 9.1). However, itcan be argued that the 3.0 eV luminescent band cannot be the inverse transition of the 4.8 eVabsorption.

To demonstrate this assertion, we evaluate the oscillator strength f of the 4.8 eV peak asif the 3.0 eV was its inverse transition. Using Smakula’s equation [50] we evaluate the productNf , where N is the defects concentration :

Nf = n

(E0

Eeff

)2

αmaxΓ∆( mec

2π2e2~

)≈ n

(E0

Eeff

)2

αmaxΓ∆× 9.111× 1015[eV −1cm−2] (9.1)

n is the refractive index of our samples, (Eeff/E0)2 is the effective field correction [50], αmax

is the amplitude and ∆ is the full with at half maximum (FWHM) of OA band, Γ is a numericcoefficient which depends on the bandshape and me, c, ~ have their usual meaning.If we consider that Γ ≈ 1.0645 for a gaussian shape, and n(E0/Eeff )

2 for silica is close tounity throughout the IR to near-UV spectral range, eq. 9.1 becomes:

Nf ≈ αmax∆× 9.699× 1015[eV −1cm−2] (9.2)

To evaluate the product αmax∆, we performed a gaussian fit of the OA peak at ∼4.8 eV of theinset of Figure 9.1, imposing the same FWHM of the 4.8eV PLE band (0.7 eV) (Figure 9.2(b)),and finding αmax = 0.10 cm−1. From this calculation we obtain:

αmax∆ ' 0.07eV cm−1 (9.3)

Nf = 6.79× 1014cm−3 (9.4)

The oscillator strength is related to the radiative decay time τ of the inverse emission by [50]:

f ≈ 1

(~ω)2τ

(E0

Eeff

)21

n× 2.305× 10−8 ≈ 1

(~ω)2τ× 1

n2× 2.305× 10−8 = 1.18× 10−7 (9.5)

9.2. Discussion: luminescent P-defects structure 103

where n = 1.466 and ~ω represents the zero phonon line energy position, roughly half waybetween absorption and emission: ~ω = 3.7 eV.From eq. 9.4 and 9.5, with τ =6 ms, we finally obtain:

N ' 6× 1021cm−3 (9.6)

This concentration value is higher than the maximum phosphorus concentration in our samples(7.09 wt% in zone 4 corresponds to [P] = 3.0×1021 cm−3). Hence, we consider really improbablefor the 3.0 eV PL band to be the inverse transition of the 4.8 eV OA. On the contrary, assuminga non-degenerate singlet ground state S0 for the defect, the 4.8 eV band must arise from anallowed S0 →S1 transition. Also, the second PLE peak at 6.4 eV, whose intensity is comparableto the 4.8 eV component, must be associated to a S0 →S2 transition to an upper excitedstate. Given the absence of any other emission signals upon 4.8 eV or 6.4 eV excitations,the decay from these two excited levels (S1, S2) to the emitting 3.0 eV state (T1) must occurby a very efficient intersystem crossing (ISC) process active at all temperatures. Based onthese considerations, we can propose for the P-related luminescent defect the level scheme inFigure 9.9.

Figure 9.9: Electronic levels scheme of the diamagnetic P-related defect supposed to be at the originof the observed optical activity. Solid arrows indicate the radiative transitions in absorption and lumi-nescence. Dashed arrows indicate the Inter System Crossing (ISC) non-radiative transitions from S1 orS2 to T1.

Contrastingly, the 6.9 eV absorption band is due to another diamagnetic P-related defect,unrelated to the emitting center. As a matter of fact, the 6.4 eV peak in the PLE signal occursin a spectral region where the absorption is almost zero (compare Figure 9.1 and Figure 9.2),


while the excitation of the sample at 6.9 eV did not result in any measurable emission at anytemperature. The 6.4 eV OA band which, based on PLE data, one may expect to find in theOA spectrum with comparable intensity to the 4.8 eV, is likely to be buried under the intense6.9 eV peak.

Indications on the microscopic structure of the luminescent P-related defect can be drawnfrom the explorative EPR measurements described in the previous section. Several evidencesin literature lead to the assumption that in phosphosilicate glass most of P atoms are arrangedas [(O−)3P = O]0 tetrahedra [115,114,116,113]. The other possible defective arrangements ofP in silica have been proposed based on EPR data, and are expected to be present only in minorconcentrations. EPR data on each of the two irradiated preforms reveal POHC and P2 defectsinduced by X-rays in about the same concentration. This finding is consistent with a schemein which the formation of the two paramagnetic defects is correlated, and occurs by trappingon Si-substitutional P centers [(O−)2P (−O)2]

+ of the electron made available by ionizationof [(O−)3P = O]0 c. Similar correlated POHC-P2 formation under laser irradiation has beenobserved in a recent study on P-doped SiO2 glass [38]. In this context, the concurrent obser-vation of a partial bleaching of the 3.0 eV emission band upon irradiation, leads to tentativelyidentify the luminescent center with one of the two precursors, i.e. the most likely microscopicstructure of the emitting diamagnetic defect is either [(O−)3P = O]0 or [(O−)2P (−O)2]

+.A 42% reduction of the PL accompanied by the formation of (1.2±0.1)×1017 cm−3 param-agnetic defects implies a concentration of (2.8±0.3)×1017 cm−3 for the luminescent center inthe unirradiated sample d. Comparing with [P]=3.0×1021 cm−3 in zone 4, we see that the the[(O−)3P = O]0 model is unlikely, since literature data strongly suggest that this structureshould be present in concentrations much higher than 1017cm−3, and possibly close to thetotal P content. This conclusion is consistent also with the lack of a strict linear correlationbetween the overall concentration of P and that of the luminescent defect (Figure 9.3(b)).Hence, present data suggest the diamagnetic Si-substitutional P impurity, [(O−)2P (−O)2]

+,as a tentative microscopic model of the luminescent center. Our model of the emitting de-fect and of the conversion process activated by X radiation is represented in Figure 9.10. Bycomparing the concentration inferred above with the intensity of the 4.8 eV band, one canestimate the oscillator strength f∼2×10−3 of the S0->S1 transition. Computational studiesmay help now to find out if these optical properties are consistent with those expected for aSi-substitutional P impurity.

cThe observation of both l-POHC and r-POHC, according to Griscom’s model, may slightly modify thisinterpretation by suggesting that a significant fraction of ionized P sites feature two doubly-bonded non-bridging O atoms instead of one.

dThis line of reasoning still holds if one uses the higher estimate of the concentration of paramagneticdefects obtained by assuming them to be present only in zones III and IV

9.3. Conclusions 105

P

o

o

o

o P

o

o

o

o+

o

o

o

oP o

o

o+

o

P

POHC P2

LuminescentCenter

e-

X

Figure 9.10: Graphical representation of the photochemical processes activated by X-rays in the P-doped preform sample, and of the model proposed (upper right structure) for the luminescent defectresponsible for the 3.0 eV emission band.

9.3 Conclusions

By luminescence measurements on step-index P-doped SiO2 fiber preforms, carried out underexcitation by laser and synchrotron UV and VUV light, we detect an optical activity consistingin a long-lived (∼6 ms radiative lifetime) and temperature-independent luminescence emissionpeaked at 3.0 eV, featuring two excitation bands centered at 4.8 eV and 6.4 eV. The 4.8 eVtransition can be also revealed as a weak band in the optical absorption spectrum of thepreform. The spatial profile of the PL intensity on the preform is consistent with that ofP doping, thus allowing the attribution of this optical activity to a P-related point defect.The detailed study of the spectroscopic features of the defect allows to propose a scheme of itselectronic transitions, comprising two singlet (S1, S2) and one triplet (T1) excited levels, wherethe long-lived emission is due to the spin-forbidden transition to the ground state from theexcited triplet populated by very efficient non-radiative decay from S1 and S2. PL and EPRmeasurements on X-irradiated samples allow to propose a microscopic model for the defect


consisting in a 4-coordinated diamagnetic P impurity substitutional to Si atoms in the SiO2

matrix.

We point out that the acquired data will be used for future multiscale simulation besidesgiving important contribution for the study of all silica P-doped glasses.

Conclusions

In this Thesis we have reported an experimental study on three types of canonical fibersand their related preforms doped with germanium, fluorine and phosphorus.

Results on Ge-doped samples could be grouped in two main points: i) the effect of thedrawing process and ii) the influence of the radiation exposure

i) We have checked the germanium lone pair center (GLPC) distribution in fibers andpreforms using confocal microscopy luminescence technique. We demonstrated that theGLPC radial distribution is different in fibers and preforms. This difference can changethe photosensitivity of the fiber regarding the preform sensitivity. This effect could bedue to a mechanical stress introduced by the drawing process.

ii) Different paramagnetic defect species, like GeE′, Ge(1) and Ge(2), were induced by ra-diation exposure (X-10 keV and γ) and they were revealed by electron paramagneticresonance measurements. Their concentration was studied as a function of the irradi-ation dose. The comparison with the optical absorption spectra points out the mainrole of Ge(1) on the optical transmission loss of fibers in the UV region (below 5 eV)allowing their localization in the central part of canonical samples containing an higherconcentration of GeO2.

Regarding the study on F-doped samples, in this Thesis we focused the attention on onemain aspect: the influence of the radiation exposure on the generation of paramagnetic species.Multiple step preforms and fibers, doped with different F concentration, were studied to findout the effects of UV- and γ-irradiation. Regardless the samples, EPR spectra evidence thatthe E′ center is the main defect induced by the conversion of precursors, whereas the absenceof EPR signal of NBOHC rules out the occurrence of radiolysis of strained Si − O bonds.These results prove the effectiveness of Si− F group in reducing the generation of defects insilica, also after drawing, thus improving the optical transmission of F-doped core silica fibers.

Phosphorus doped samples in this study have been the subject of specific results, con-sidering the lack of peculiar literature. P-related point defects in amorphous silica, based onphotoluminescence, absorption, and electron paramagnetic resonance measurements has been

108 Conclusions

investigated. By photoluminescence measurements excited by laser or synchrotron light wedetected an emission band peaked at 3.0eV with a lifetime in the range of ms. The excitationspectrum of the 3.0 eV emission consists of two transitions peaked at 4.8eV and 6.4eV, theformer giving rise also to a measurable absorption band. We attribute this optical activityto a P-related point defect embedded in SiO2, based on the spatial correlation between theemission intensity and the P doping level. A detailed spectroscopical investigation allows us topropose a scheme of the electronic levels of this P-related defect, in which the 4.8eV and 6.4eVexcitation channels arise from transitions from the ground to two excited singlet states, whilethe long-lived 3.0eV emission is associated to a spin-forbidden transition from an excited tripletto the ground state. Finally, electron spin resonance measurements on X-irradiated sampleslead us to propose a tentative microscopic model of the defect as a diamagnetic 4-coordinatedP impurity substitutional to a Si atom.

Results on Ge-doped samples form the main part of our experimental results, entering awide scientific literature field. Otherwise, results on F-doped and P-doped fibers and preformsare more innovative from the experimental and technological points of view, so interestingquestions remain open and could be investigated by further experiments.

List of related papers

1. G. Origlio, S. Girard, F. Messina, M. Cannas, A. Boukenter, R. Boscaino and Y. Ouer-dane, 10 keV X-ray irradiation effects on phosphorus-doped fibers and preforms: electronspin resonance and optical studies, submitted for publication in IEEE Transactions onNuclear Science

2. G.Origlio, F. Messina, M. Cannas, R. Boscaino, S. Girard, A. Boukenter and Y. Ouer-dane, Optical properties of phosphorus-related point defects in silica fiber preforms, Phys.Rev. B., in press

3. G. Origlio, M. Cannas, S. Girard, R. Boscaino, A. Boukenter, , and Y. Ouerdane, Influ-ence of the drawing process on the defect generation in multistep index germanium-dopedoptical fibers, Optics Lett. 34, 2282 (2009)

4. G. Origlio, S. Girard, M. Cannas, Y. Ouerdane, R. Boscaino, and A. Boukenter, Para-magnetic germanium-related centers induced by energetic radiation in optical fibers andpreforms, J. Non-Cryst. Solids 355, 1054 (2009)

5. S. Girard, C. Marcandella, G. Origlio, Y. Ouerdane, A. Boukenter, and J-P. Meunier,Radiation-induced defects in fluorine-doped silica-based optical fibers: Influence of a pre-loading with H2, J. Non-Cryst. Solids 355, 1089 (2009)

6. G. Origlio, S. Girard, M. Cannas, A. Boukenter, R. Boscaino, and Y. Ouerdane. Opticaland photonic material hardness for energetic environments. 9me Colloque sur les SourcesCohérentes et Incohérentes UV, VUV et X (UVX), S. Jacquemot, A. Klisnick and T.Ruchon Eds. 127, EDS Sciences.

7. S. Girard, Y. Ouerdane, G. Origlio, C. Marcandella, A. Boukenter, N. Richard, J. Baggio,P. Paillet, M. Cannas, J. Bisutti, J-P. Meunier, and R. Boscaino, Radiation effects onsilica-based preforms and optical fibers - I: Experimental study with canonical samples,IEEE Transactions on Nuclear Science 55, 3473 (2008)

8. S. Girard, N. Richard, Y. Ouerdane, G. Origlio, A. Boukenter, L. Martin-Samos, P.Paillet, J-P. Meunier, J. Baggio, M. Cannas, and R. Boscaino, Radiation effects on silica-

110 List of related papers

based preforms and optical fibers - II: Coupling ab initio simulations and experiments,IEEE Transactions on Nuclear Science 55, 6 (2008)

9. G. Origlio, A. Boukenter, S. Girard, N. Richard, M. Cannas, R. Boscaino, and Y. Ouer-dane, Irradiation induced defects in fluorine doped silica, Nucl. Instrum. Methods B,266, 2918 (2008)

10. G. Origlio, S. Girard, R. Boscaino, A. Boukenter, M. Cannas, and Y. Ouerdane, Opticaland photonic material hardness for energetic environments, 9me Colloque sur les SourcesCohérentes et Incohérentes UV, VUV et X (UVX), (2008)

11. M. Cannas and G. Origlio, Ultraviolet optical properties of silica controlled by hydrogentrapping at Ge-related defects Phys. Rev. B, 75, 233201 (2007)

List of communications to congresses

1. G. Origlio, S. Girard, F. Messina, M. Cannas, A. Boukenter, R. Boscaino and Y. Ouer-dane. 10-keV X-ray irradiation effects on phosphorus doped fibers and preforms: elec-tron spin resonance and optical studies. 10th European Conference on Radiation Effectson Components and Systems (RADECS). Bruges, Belgium, September 2009.

2. G. Origlio, S. Girard, A. Boukenter, C. Marcandella, M. Cannas and Y. OuerdaneInfluence du rayonnement X sur des fibres et préformes canoniques dopées au phosphore.Journées Nationales d’Optique Guidée et Horizons de l’Optique (JNOG), Lille, France,July 2009.

3. G. Origlio, S. Girard, M. Cannas, A. Boukenter, R. Boscaino, and Y. Ouerdane. Dur-cissement de matériaux pour l’optique et la photonique destinés à l’utilisation dans unenvironnement énergétique. 9me Colloque sur les Sources Cohérentes et IncohérentesUV, VUV et X (UVX), Dourdan, France, October 2008.

4. G. Origlio, S. Girard, M. Cannas, Y. Ouerdane, R. Boscaino, and A. Boukenter. Para-magnetic germanium-related centers induced by energetic radiation in silica based de-vices. The 7th symposium SiO2 , advanced dielectrics and related devices, Saint-Etienne,France, July 2008.

5. S. Girard, C. Marcandella, G. Origlio, Y. Ouerdane, A. Boukenter, and J-P. Meunier.Radiation-induced defects in fluorine-doped silica-based optical fibers: Influence of theH2-loading. The 7th symposium SiO2 , advanced dielectrics and related devices, Saint-Etienne, France, July 2008.

6. . Girard, N. Richard, Y. Ouerdane , G. Origlio, A. Boukenter, L. Martin-Samos, P.Paillet, J.-P. Meunier, M. Cannas, R. Boscaino. Radiation-induced effects in silica-based glasses: experimental and theoretical results, The 7th symposium SiO2 , advanceddielectrics and related devices, Saint-Etienne, France, July 2008.

7. S. Girard, Y. Ouerdane, G. Origlio, C. Marcandella, A. Boukenter, N. Richard, J. Baggio,P. Paillet, M. Cannas, J. Bisutti, J-P. Meunier, and R. Boscaino. Radiation effects onsilica-based preforms and optical fibers - I: Experimental study with canonical samples.

112 List of communications to congresses

The Nuclear and Space Radiation Effects Conference (NSREC), Tucson, Arizona, July2008.

8. S. Girard, N. Richard, Y. Ouerdane, G. Origlio, A. Boukenter, L. Martin-Samos, P.Paillet, J-P. Meunier, J. Baggio, M. Cannas, R. Boscaino. Radiation Effects on Silica-Based Preforms and Optical Fibers - II: Coupling Ab initio Simulations and Experiments,The Nuclear and Space Radiation Effects Conference (NSREC), Tucson, Arizona, July2008.

9. G. Origlio, A. Boukenter, M. Cannas, S. Girard, R. Boscaino, and Y. Ouerdane. Photo-luminescence properties of point defects in Ge-doped fibers and preforms. The 15th Inter-national Conference on Luminescence and Optical Spectroscopy of Condensed Matter(ICL), Lyon, France, July 2008.

10. G. Origlio, A. Boukenter, S. Girard, N. Richard, M. Cannas, R. Boscaino, and Y. Ouer-dane. Irradiation induced defects in fluorine doped silica. Radiation Effects in Insulators(REI), Caen, France, August 2007.

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