Risø-R-1306(EN)

A Polarisation Coupled Diode Laser System for Photodynamic Therapy Per Malm

Risø National Laboratory, Roskilde November 2002

Abstract This master thesis deals with the description and specifications of a novel diode laser system for (interstitial) photodynamic therapy.

Two 635nm broad area diode lasers are coupled to external cavities created by an end mirror. Spatial filtering is introduced in each of the two external cavities to improve the spatial characteristics of the diodes. This improves the M2 value of the output from the diodes considerably. A large portion of the output power of the freely running diode is in this way gathered in an output lobe with good beam quality.

By the use of a polarisation beam splitter the output of two diode lasers is efficiently combined while retaining their good M2 value. The output of the two diode lasers is coupled into an optical fiber with a 200µm core. It is shown that a coupling into an even thinner fiber is possible.

ISBN 87-550-2976-0 ISBN 87-550-2977-9 (Internet) ISSN 0106-2840 Print: Pitney Bowes Management Services Danmark A/S, 2002

Risø-R-1306(EN) 3

Contents

List of symbols and Abbreviations 4

Preface 6

Svensk sammanfattande motivering 7

1 Introduction 9 1.1 PDT 9 1.2 Motivation 11 1.3 Scope of thesis 11 1.4 Organisation of the rapport 12

2 Diode lasers 12 2.1 Introduction 12 2.2 Principle for Diode Lasers 13 2.3 Diode laser structures 20 2.4 Other aspects of diode lasers 25

3 The polarisation coupled laser system 26 3.1 The rail system 27 3.2 The lasers 28 3.3 The collimating lenses 28 3.4 External feedback 30 3.5 Polarisation coupling 31 3.6 The fiber coupling 32

4 Experiments and Results 34 4.1 Measurement procedures 34 4.2 Results of all measurements 36

5 Conclusions 44 5.1 Perspectives 45

Clinical trials 46

References 48

Appendices 50

4 Risø-R-1306(EN)

List of symbols and Abbreviations Symbols

A Proportionality constant spontaneous emission [ s-1] B12 Proportionality constant stimulated absorption [s-1] B21 Proportionality constant stimulated emission [s-1] c Speed of light [m/s] D(E) Density of states [-] Ec Conduction band energy [eV] Ev Valence band energy [eV] Eg Band gap energy [eV] EF Fermi-level energy [eV] g Gain coefficient [m-1] gmodal Modal gain [m-1] = h/2π,

h

Plank’s constant

[Js]

I, I0 Intensity and Current [W/m2], [A] k Wave number [-] k Boltzmann’s constant [J/K] L Cavity length [m] me Electron effective mass [] mh Hole effective mass [] ml Longitudinal mode number [] M2 Beam quality [-] n Index of refraction [-] nm Spatial mode number [-] R1, R2 Reflectivity [-] R12 Transition rate for stimulated absorption [s-1m-3] R21 Transition rate for stimulated emission [s-1m-3] Rsp Transition rate for spontaneous absorption [s-1m-3] T Temperature [K] w0 Beam waist [m] Å Ångström [10-10 m] α Absorption coefficient [m-1] αi Intrinsic absorption [m-1]

FSRf∆ Free spectral range [s-1] Γ Confinement factor [-] λ Centre wavelength [m] λ0 Vacuum Wavelength [m] θ Angle made by ray marginal with optical axis [degrees] ω = 2πf Angular frequency [ s-1]

Risø-R-1306(EN) 5

Abbreviations ALA δ-aminolevulinic acid PpIX Protoporphyrin IX PDT Photodynamic Therapy ALA-PDT PDT with ALA induced photosensitizer as used at Lund

University Hospital BAL Broad Area Laser BAR Diode laser bar PBS Polarisation Beam Splitter NA Numerical Aperture Ga Gallium As Arsenide Al Aluminium In Indium P Fosfor TEC Thermal electric cooler

6 Risø-R-1306(EN)

Preface The present master thesis describes the work carried out in the Optical and Fluid Dynamics Department at Risø National Laboratory, during the period from September to December 2001. The work was supervised by

Eva Samsøe, Ph. d. Student in the Optics and Fluid Dynamics department, Risø National Laboratory Dr. Peter E. Anderssen, Senior Scientist in the Optics and Fluid Dynamics department, Risø National Laboratory Prof. Stefan Andersson-Engels, Lund University Medical Laser Centre

I would like to thank Prof. Stefan Andersson-Engels for introducing me to medical optics and giving me the opportunity to do this work and also for helping me to write this report. My thanks go also to Dr. Peter E. Anderssen, who has taught me that there is something worth knowing about everything. He has been a great support throughout my work. My semester at Risø was a leap straight into a foreign country. Yet I have felt very much welcomed and at home. My thanks to Heidi and Lone for helping me finding my way in the administration (and breakfast). Thanks go to Birgitte, Carsten, Mingjun and Paul Michael for theoretical discussions and help. Thanks to Bjarne and Søren for readily manufacturing any possible device. Thanks to Thomas Martini for making me run faster. Thanks to you all and also Bo, Lian, Morten, Niels, Steven and many, many more, including Dorrit and Kurt, for making my time at Risø enjoyable. Finally, my greatest thanks go to Eva Samsøe who has with never-tiring efforts supported my throughout my work. She has helped me out, regardless whether it has been difficult physics or keeping me from swallow my own tongue while learning to pronounce Danish phrases. She has been a great morale support and kept me in good spirits at all times. My experience at Risø would not have been the same without her.

Per Malm March 2002

Risø-R-1306(EN) 7

Svensk sammanfattande motivering

Lasersystemet som presenteras i detta arbete är ett diodlasersystem. Det är avsett att användas för cancerbehandling med fotodynamisk terapi (PDT). Behandlingen går till så att man tillför kroppen ett visst ämne som samlas i cancerceller. Ämnet kan sedan fås att döda cancercellerna genom att man belyser det.

Fotodynamisk terapi För fotodynamisk terapi (PDT) krävs tre komponenter: ljus, syre och ett tumörsökande ämne. Det tumörsökande ämnet tillförs kroppen och kallas sensibiliserare eller sensitizer. Ljus kan tillföras på flera sätt, laserljus är ett. Syre finns naturligt i vävnaden. Det finns flera typer av tumörsökande ämnen. På Lunds Universitetssjukhus används en behandlingsform som kallas ALA-PDT. Genom att tillföra kroppen ALA, som omvandlas till PpIX i cancerceller, och sedan lysa med ljus vid 635nm på tumörer kan man selektivt döda cancerceller. Ibland kan man behöva behandla organ eller inre delar kroppen och då behöver man leda ljuset genom optiska fibrer. Då är lasrar att föredra, eftersom laserljus går att leda in i fibrer.

Diodlasrar En laser är en ljuskälla som sänder ut ljus skapat genom stimulerad emission av fotoner. I en optisk resonator förstärks strålningen medan en (liten) del släpps ut. Genom detta skapas koherent ljus med ett mycket smalt våglängdspektra. Lite förenklat kan man säga att en laser sänder ut ljus med en mycket tydlig “färg” och att det svänger i takt, likadant hela tiden. En diodlaser är en halvledare, ofta i materialet Galliumarsenid, GaAs (se fig. 2.5). En diodlaser påminner mycket om en vanlig lysdiod, man leder ström genom den och den lyser, ungefär som en cykellampa. Det som är speciellt med en diodlaser är att den är liten, billig och effektiv, men jämfört med andra lasrar är ljuset från den mer divergent samt har ofta en sämre spatiell koherens. Det sista innebär att ljuset inte kan fokuseras ner till en liten punkt och kopplas in i en tunn fibrer. Att en diodlaser är liten och billig gör att den passar bra i en klinisk miljö. I detta arbetet används en typ av diodlasrar som kallas Broad Area Laser med rött ljus vid 635nm De är ganska enkla i sin konstruktion men ger mycket ljus. I gengäld är ljuset från dem mycket svårt att fiberkoppla. Ett sätt att förbättra koherensen hos ljuset och därigenom fiberkopplingen, beskrivs i detta arbete.

ALA-PDT På Lunds Universitetssjukhus används en behandlingsform som kallas ALA-PDT. ALA, δ-aminolevulinsyra, är en kroppsegen aminosyra som ingår i hemcykeln. I hemcykeln ombildas ALA via flera steg till hem, som sedan används vid tillverkning av hemoglobin. Steget innan hem i hemcykeln är Protoporfyrin IX (PpIX) (se fig 1.1). Detta (PpIX) är sensibiliseraren som används i ALA-PDT. Det är två processer som sörjer för att sensibiliseraren återfinns i cancerceller. För det första är hemcykeln mer aktiv i bl.a. celler som delar sig, d.v.s. cancerceller. Mer ALA kommer att tas upp i cancerceller än normal vävnad. För det andra är hemcykeln förändrad i cancerceller jämfört med normala celler. Stegen från ALA fram till sensibiliseraren PpIX sker snabbare medan det sista steget till hem sker långsammare. Detta gör att sensibiliseraren PpIX ackumuleras i tumörer. ALA kan tillföras kroppen som kräm, dryck eller i blodet. Efter några timmar har ALA:n ansamlats i tumören och omvandlats till PpIX. Sensibiliseraren PpIX absorberar ljus vid bland annat våglängden 635nm. Då den gör det kommer den i ett högre energitillstånd (exciterat). Den extra energin kan överföras till omgivande syre som då kommer i ett mycket reaktivt tillstånd. Det reaktiva syret oxiderar omgivande vävnad och orsakar skador som dödar cancercellen (se fig 1.2). Sensibiliseraren återvänder till sitt grundtillstånd efter att den överfört överskottsenergin till syre och kan sedan upprepa cykeln.

8 Risø-R-1306(EN)

Risø-R-1306(EN) 9

1 Introduction Photodynamic therapy (PDT) is described in this chapter. PDT is a treatment where a tumour selective substance is given the body. The substance accumulates in cancer cells and when it is irradiated by light, it is made to selectively kill cancer cells. After the PDT part follows a motivation for the work with improving diode lasers and then the scope of the thesis is outlined.

1.1 PDT PDT requires three components, a photosensitizer, oxygen and light. Oxygen is naturally present in the body. The photosensitizer is administrated to the body and accumulated in the tumour tissue. The light may come from different sources and have different spectral characteristics. The light must match an absorption peak/band of the photosensitizer. In this thesis ALA-PDT as used at Lund University Hospital is considered. ALA, δ-aminolevulinic acid, is an amino acid naturally present in the body. It is active in one of the early stages of the haem cycle. ALA is here through subsequent steps transformed to protoporphyrin IX (PpIX), the stage before heam in the cycle. PpIX is in turn, trough the incorporation of Fe2+ transformed into haem [1] p28, fig 1.1. PpIX is the effective photosensitizer in the PDT-treatment.

Fig 1.1 Picture of haem cycle (adopted from [1]). The steps in the haem cycle leading from ALA to Protoporphyrin IX Two processes are responsible for the selective accumulation of PpIX in cancer cells: they have a more active haem cycle and the cycle is altered. Although the haem cycle is active in all mammalian, nucleated cells it is especially expressed in liver cells and cells undergoing dividing, as cancer cells. This means that ALA supplied to the body will be absorbed at a higher

10 Risø-R-1306(EN)

rate in cancer cells. By supplying an excess of ALA, the natural regulation is overridden and transformation of ALA in the cycle will go on despite the negative feedback in the system [1] p29. Furthermore, the enzymatic functions in cancer cells are changed from the normal. The stages leading to PpIX is done quicker than normal while the transformation from PpIX to haem is slower. This leads to an accumulation of PpIX in cancer cells. ALA-PDT gives the possibility for both diagnostics and treatment. If PpIX is illuminated with light at 405nm it will give rise to fluorescence that can be distinguished from the auto fluorescence of the tissue. This makes a detection of cancer tissue possible [1,3]. When illuminated with light around 635nm, PpIX is excited from its singlet ground state, S0, to the first excited singlet state, S1. From here it can decay to the lowest triplet state T1 through a spin-forbidden crossing. The low energy separation makes this possible. The further decay down to the singlet ground state is unlikely, and the triplet state has a long lifetime (milliseconds). The long lifetime of this transition makes it possible for the photosensitizer to interact with surrounding molecules. Oxygen has a triplet ground state. When the excited PpIX interacts with oxygen the excess energy may be transferred to the oxygen molecules, and oxygen is excited to a singlet state. Oxygen in a singlet state is very reactive and will oxidise tissue surrounding it, making damage. By this cancer cells is selectively destroyed.

Fig 1.2 Picture of reaction scheme for PpIX (adopted from [2]). The photosensitizer is excited from its ground state S0 to the excited state S1 by a photon. It then relaxes to the meta-stable triplet state T1 trough the spin-forbidden intersystem crossing IX. The photosensitizer returns to its ground state by transferring the excess energy to a oxygen molecule in its triplet ground state. The oxygen is excited to a reactive singlet state. After the transition of excess energy to oxygen the photosensitizer is returned to its ground state and may be excited again. The energy in the triplet state of PpIX may also be transferred to another molecule by electron transfer or hydrogen abstraction. This also returns PpIX to its ground state. The singlet oxygen may also oxidize the photosensitizer and render it useless. This is referred to as photobleaching or photodegradation. [1,3] ALA may be administrated orally or superficially (or intravenously). Superficially it penetrates the cancer tissue thanks to the changed epidermis of

Risø-R-1306(EN) 11

the tumours. In normal epidermis ALA would not pass due to the protective properties of skin [1]. In this way shallow tumours may be treated. For thicker lesions, ALA needs to be administrated systematic. This is also required for treatment of inner organs or for example vocal folds. Often endoscopes with optical fibers may be used to deliver the light. When larger tumours needs to be treated optical fibers are used to deliver the light. They are inserted into the tissue. This treatment is refereed to as interstitial PDT or I-PDT.

1.2 Motivation Starting with sunlight in ancient times, a long variety of light sources have been used for medical treatment. That is also the case for PDT. Flash lamps, filtered and unfiltered, fluorescent tubes etc. and of course lasers have been used. Many sources loose a lot of efficiency because they produce wavelengths that are not useful for the photosensitizer. Lasers have the ability to precisely hit the right wavelength range. In this way they do not loose any efficiency as other light sources do. This prevents unnecessary heating of the tissue. While gas, dye or solid state lasers have the possibility to give a lot of output power at the desired wavelength, they are expensive, large and requires a lot of maintenance, and therefore are not suited to large-scale clinical use. For example, Nd:YAG pumped dye lasers have been used in Lund but that system was very bulky and heavy. It weighed 400 kg, needed a lot of cooling water and 3-phase current. As replacement for these, diode lasers are now appearing. Diode lasers are small, cheap and well suited for a clinical environment. Just recently diodes at the right wavelength for ALA-PDT, 635nm, have become commercially available, but the output power is still too low and their beam properties make them difficult to couple into fibers. Today diode laser bars at 635nm that give a few watts output through 600 µm or 400 µm fibers are available. These are, however, expensive and coupling to fiber optics is inefficient. A desirable fiber size for I-PDT would be a core size of 50 µm - 200 µm. The small fiber size is to decrease the damage to tissue made when inserting fibers. Otherwise there is a risk of spreading cancer materiel in the body. A system that splits the light into 3 or 6 fibers has been developed in Lund [4]. The 6 fibers are used to deliver the light evenly in the tissue. A too high local intensity risks burning the tissue. At the same time a high power is desired to reduce treatment time. A group at Risø National Laboratory in Denmark work with improving the spatial profile of diode lasers. This is done by the use of spatial filtering and optical feedback. By improving the spatial profile of the beam, it is possible to focus the beam into thinner optical fibers. One approach is to work with broad area lasers (BAL). These lasers are cheap and have a long lifetime and a high output power. In this thesis work with 635nm BAL’s is described.

1.3 Scope of thesis The aim of this master project is to develop a new diode laser system for interstitial photodynamic therapy in Lund. Broad area diode lasers were used. The work has been carried out at Risø National Laboratory in collaboration with Lund University Medical Laser Centre during one semester.

12 Risø-R-1306(EN)

A fiber-coupled system is demonstrated. It features spatially filtered mirror feedback from two individual diode lasers where the outputs are combined using a polarisation beam splitter. Each diode laser was placed in an external coupled cavity created by a mirror that directed a part of the output back into the diode. The output was spatially filtered in the far-field of the slow axis to select feedback for only one spatial mode of the diode. In this way a large portion of the output power could be gathered in an output lobe. By combining the two laser beams with a polarisation beam splitter, the power was doubled. A total power exceeding 250mW could be coupled through a 200µm fiber with a coupling efficiency of around 93%. It is also shown that this may be done in a thinner fiber.

1.4 Organisation of the rapport This chapter, the first, holds an introduction to PDT and the mechanism of it is described. A motivation for my work with diode lasers is given. The scope of the thesis and the organisation of the report are presented. The following chapter, 2, is a general description of laser diodes and theoretical presentation; the principles of laser action in a semiconductor material, guiding mechanisms and different types of laser diodes are described. Chapter 3 describes the components of the system constructed. Their functions and my reason for using them are also given. This is the necessary background for chapter 4. In chapter 4 measurement procedures, first, and then all the results are presented. My experiences from the lab are conveyed in this chapter. The conclusions and further outlook of work in the area is found in chapter 5. After that a description of the preliminary clinical trials is provided and then appendix and references follows. A list of symbols and abbreviations is found in the beginning of the thesis.

2 Diode lasers This chapter is to a large extent based on reference [5]. It is marked when other references is used. The basic principles of semiconductors and diode lasers are reviewed. Throughout this chapter and the rest of the chapters, both diode and laser will often be used in the meaning of diode laser.

2.1 Introduction The first diode laser was demonstrated in 1962 [6]. Despite the early appearance, diode lasers have for long only yielded low output powers. Diode laser are small, cheap in manufacturing and have a high efficiency in input power to light conversion. Their disadvantage is their often poor beam quality.

Risø-R-1306(EN) 13

2.2 Principle for Diode Lasers

Principles for semiconductors

A semiconductor can consist of several compounds, mainly the III, IV and V-compounds. The III, IV and V refer to the number of valence electrons of the atoms, 3, 4 or 5. The valence electrons are used to form covalent bondings [7] p72 with surrounding atoms. This means that they share electrons to fill their (in this case) four outer orbitals, two electrons in each orbital. Silicon as IV-compound is the most used for semiconductors. For diode lasers however, GaAs, a III-V-compound is commonly used. In a semiconductor material the discrete energy levels of the single atoms becomes split and smeared into broad energy bands because of the overlapping orbitals [5,7] A semiconductor with equal amounts of Ga and As or pure silicon is characterised by its filled orbitals. In this case the uppermost energy band, called the conduction band, is completely empty and the energy band below the conduction band, called the valence band, is completely filled with electrons. The semiconductor is called undoped. If there are extra electrons in the conduction band they can be regarded as moving freely, as an electron gas. The same is valid for extra holes in the valence band. However, the band structure is made more complex by the impact of momentum on the energy structure. The energy of a free electron is dependent on the wave number of the state it is occupying. In the nearly-free carrier approximation this is taken account for. The energy as a function of wave number, k, can be seen in equation (1). This gives the parabolic shape of the bands depicted in fig 1. The energies of the band edges:

(1)

The energy is measured from the top of the valence band. Eg is the bandgap energy, meaning the minimum separation between valence band and conduction band. me and mh is the effective mass for electrons and holes respectively [5,7]. is Plank’s constant divided with 2π.

egc m

kEkE2

)(22

+= , h

v mkkE

2)(

22

−= .

14 Risø-R-1306(EN)

Figure 2.1 The parabolic wave number-energy relation in the nearly-free electron approximation (adopted from [5]). Fig 2.1 shows the parabolic band structure of a semiconductor with a direct bandgap. This means that the bottom of the conduction band is situated precisely above the top of the valence band. This is the most favourable for diode lasers. Electrons can recombine without

changing wave number. Also semiconductors with an indirect bandgap exist, meaning the bottom of the conduction band and top of the valence band is not on top of each other but shifted along the k-axis. Transitions in such materials involve phonons (heat), and are not suitable for lasers. This is because the probability for a process involving both a phonon and a photon is less than for one involving just a photon. Unwanted heat might also be generated. In thermal equilibrium the electron population is given by the density of states and the Fermi function. The Fermi function f (E,T), express the probability of a state being occupied:

(2)

The Fermi-level energy, EF, is dependent on the material. In an undoped semiconductor it is situated in the middle between the conduction and valence band edges. At the Fermi-level energy the probability for a state to be occupied by an electron is one half (if there is any). The density of states for the conduction band, Dc and valence band, Dv, for a semiconductor bulk material are given by [8]:

(3a)

(3b)

Here E is measured from the top of the valence band, as in fig 2.1. The other

1exp

1),(+

−=

TkEE

TEf

B

F

( ) ( )

( ) ( ) 212

3

22

212

3

22

22

1

22

1

EmED

EEmED

hv

ge

c

−

=

−

=

π

π

Risø-R-1306(EN) 15

symbols are as in equation (1). The actual electron distribution is given by the product of the Fermi function and the density of states. An undoped semiconductor with no impurities is an isolator. This is because of the filled orbitals. A semiconductor becomes doped by the introduction of a III or V- compound in a VI-compound semiconductor, as silica, or by making one of the constituents more abundant, as for GaAs. If there is more of the group-V material, meaning more electrons, the material is said to be n-doped. It is p-doped if there are additional holes. The extra electrons (or holes) are free to move in the material. The doping moves the Fermi-level energy up (n-doped) or down (p-doped).

Principles of semiconductor lasers

A simple laser requires two things: a resonating cavity to sustain the optical field, and a gain medium that amplifies the optical field. This is true for a gas laser. A diode laser is different in that respect that it also needs a confinement of the optical field. The gain in the lasing material is also too great for the resonator to have a high feedback; most of the optical field is coupled out directly.

The gain - electronic transitions Three types of radiative band-to-band transitions may occur. These are stimulated absorption, stimulated emission, and spontaneous emission. The last, that at first seems the least complicated, is due to the fact that an electron in a higher energy state after some time relaxes to a lower energy state. This leads to the emission of a photon with an energy corresponding to the energy difference between the two states. In a semiconductor this would correspond to a relaxation from the conduction band to the valence band. The probability for this to happen, i.e. the proportionality constant for spontaneous emission denoted A, is the inverse average lifetime of an excited electron [9] p259. In a semiconductor the transition rate of spontaneous emission per volume is [5]

(4)

Here Dc(E) and Dv(E) is the density of states for the two bands and f(E,T) the Fermi function. 1-f(E,T) is consequently the probability for holes. The subnotation 1 and 2 refers to the ground (valence) and excited (conduction) states, (as in fig 2.1). The stimulated absorption is the transition of an electron from the valence band to the conducting band under absorption of a photon. The energy of the photon must be equal to or higher than the bandgap to be absorbed. When an excited electron exists, a photon can cause the electron to recombine while sending out a new photon. This can happen if there is an unoccupied state in the material at a lower energy. The state needs to be as much lower in energy as the energy of the photon that stimulated the recombination. If this recombination happens a new photon with the same energy, phase and direction as the one that caused the recombination, is emitted. This process is called

)],(1)[(),()( 1122 TEfEDTEfEADRsp −=

16 Risø-R-1306(EN)

stimulated emission. With )( ωρ expressing the density of photons as a function of energy and notations otherwise as above, the transition rates for these stimulated transitions are:

(5)

B12 and B21 are the proportionality constants for stimulated absorption and emission respectively. It can be shown that the probability for stimulated absorption and emission are the same, B12=B21 [5]. The process of stimulated emission is the one responsible for the laser action in a laser, Light Amplification by Stimulated Emission of Radiation. For laser action to occur the optical gain from stimulated emission must be greater than the loss due to absorption. This requires more excited electrons than non-excited, a condition called population inversion. In thermal equilibrium the amount of excited electrons can never exceed the amount in the ground state. (This would correspond to a temperature below 0°K.) Pumping is required for population inversion to be achieved. In a gas laser this is usually done in a way described by the 4-level-cheme, see for example [8] p7. In a semiconductor laser it is obtained with an electric current over a forward-biased diode, hence the name diode laser. The region that is pumped so that population inversion is achieved is called the active region. In a forward-biased double heterostructure diode laser the active layer consists of an undoped layer sandwiched between p-doped and n-doped material, se fig 2.2. All state of the art diodes are of this type. In the active region of a diode laser, the thermal equilibrium is no longer the case. Here, the Fermi-level is no longer described by one function value but two, one for the conduction band and one for the valence band.

(6)

( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( )

21 21 2 2 1 1

12 12 1 1 2 2

, 1 ,

, 1 ,

R B D E f E T D E f E T

R B D E f E T D E f E T

ρ ω

ρ ω

= − = −

1exp

1),(+

−=

TkEE

TEf

B

FC

C

,

1exp

1),(+

−=

TkEE

TEf

B

FV

V

Risø-R-1306(EN) 17

Fig 2.2 Picture of the energy structure in a double heterostructure junction (adopted from [5]). The electrons arrive from the n-doped side and the holes from the p-doped. They recombine in the undoped active layer where population inversion is achieved. The Fermi-level is split in the active layer due to the inversion. qU is the voltage applied over the junction. To determine the condition for laser action in a semiconductor material the quotient between stimulated absorption, R12, and emission, R21, is considered.

(7)

R12/R21 needs to be less than one for population inversion and laser action. See page 6 [5] for a more thorough derivation. In the case of thermal equilibrium where EFc = EFv = EF, this equation is reduced to the normal Boltzman relation between energy levels [9] p256. EFc - EFv needs to be greater than ω for the R12/R21-ratio to be less than one. ω also needs to be greater than the bandgap energy; gFF EEE

vc>>− ω .

The Resonator Mere gain is not enough for laser action; a resonator to sustain the optical field is also required. The resonator locks the laser to certain modes and helps to maintain the population inversion. The gain threshold, the gain required to maintain a population inversion in a cavity is investigated here. The resonator of a diode laser is confined by the cleaved edges of the semiconductor crystal. The reflectivity of the cleaved edges of the diode is around 30% and one side is often made highly reflective while the other is anti-reflection coated. The high reflectivity of the one side is to get light just in one direction and the anti-reflection is to reduce the optical field in the diode. The high amplification would otherwise lead to a too high optical field that destroys the diode. The resonator works as a Fabry-Perot resonator, promoting wavelengths that satisfies

( )

−−=

TkEE

RR

B

FF VCω

exp21

12

18 Risø-R-1306(EN)

(8)

where λ0 is the vacuum wavelength, n is the refractive index of the semiconductor material, L the length of the cavity and ml the longitudinal mode number. The frequency separation between different longitudinal modes in a cavity is called the free spectral range,

(9)

where c is the speed of light in vacuum [10] p137. At each roundtrip the amplitude of the wave travelling in the resonator is decreased due to power coupled out through the mirrors and losses inside the resonator. The gain must be high enough to overcome these losses for laser action to take place. When light travels through a medium absorption occurs. The exponential decrease in the intensity of the light can be described trough the Beers-Lambert’s law [9] p110. A plane optical wave with initial intensity I0 decreases exponentially as zeIzI α−= 0)( where α is the absorption coefficient. If the medium provides gain then g = -α and the wave is amplified. In a double-heterostructure diode laser the active region constitutes a layer of thickness d where gain is provided. The confinement factor Γ is defined as the part of the optical wave that overlaps with the active region. The modal gain is defined as the part of the optical wave that is amplified, gmodal = Γg. For a wave travelling in the laser medium the absorption coefficient α can be described as α = αi - Γg, where αi refers to the intrinsic absorption, i.e. scattering at crystal defects and absorption by free carriers. The intensity of the optical wave after a roundtrip in the cavity is given by

))(2exp()( 210 LgRRIzI iα−Γ= where L is the length of the cavity and R1 and R2 is the reflectivity of the end facets. Consequently the modal gain needs to be high enough to overcome the losses for laser action. At threshold, the gain is just high enough to compensate for the losses. By setting I(z) =I(0), the threshold gain can then be written as

(10)

The actual current to get the required gain varies between different diodes. The width of the active layer and the confinement of the optical field is affecting the current. In for example quantum well diode lasers the threshold current is low because of the tiny active layer even though the confinement factor is low.

0

2lL mn

λ= ,

nLcf FSR 2

=∆ ,

+=Γ

21

1ln21

RRLg ith α

Risø-R-1306(EN) 19

Wavelengths The gain spectrum of diode lasers is quite broad. At threshold only wavelengths corresponding to the bandgap in the semiconductor material is amplified. As the pumping gets stronger and the number of carriers is increased, electrons occupy higher energies (and holes lower) in the band and shorter wavelengths is amplified, se fig 2.4 (b). Several wavelengths separated by the free spectral range of the Fabry-Perot resonator may experience lasing. Changing the energy structure in the active region may alter the wavelength of a diode laser. Different doping, strain or quantum wells can be used [5,11]p19. In GaAs, Al, a group III-compound, may be incorporated. AlAs has the same lattice constant as GaAs. This means that GaAs may be combined with AlxGa1-

xAs for any x, without strain or lattice mismatch. By using different amount of Al the bandgap may be varied. Incorporating atoms with different radius may induce strain. This will alter the band edges in similar ways as above. There are many different materials and ways of doing this and in this way the band structure can be modified. The size of the active layer may be decreased. When the thickness of the active region of the diode laser is as low as 50-100 Å, it is called a quantum well. Here the number of atoms involved is significantly lower than the case before and the energy levels no longer appear as smeared bands. The small size of the potential energy well for carrier confinement (se section 3.1) makes the energy levels of a quantum well effective, se fig 2.3. The exponential decreasing Fermi-function makes sure that only the lowest energy level in the quantum well is populated at moderate pumping and carrier concentration. This gives a small energetic width, fig 2.4 (a). At strong pumping higher energy levels in the quantum well also provide gain.

E

Fig 2.3 Structure of the quantum well energy states.

20 Risø-R-1306(EN)

(a) (b) Fig 2.4 Picture of gain spectrum for quantum wells (a) and normal bulk (b) active region (adopted from [5]). N is the carrier concentration. In the bulk material (b) the gain is experiencing a gradual broadening with increasing carrier concentration. In the quantum well (a) the gain has a small energetic width until the carriers begin to populate higher energy state. The smaller active region of the quantum well allows low threshold current and the band-filling effects (the broadening of spectrum) are smaller. By exchanging some atoms in the quantum well, biaxial strain can be achieved to tune the wavelength. However, the product of the strain and the thickness of the quantum well layer must not be too great or non-radiative dislocations will occur.

2.3 Diode laser structures

2.3.1 Structure

V

Output

Electrons

Holes

Intensity Profile

xy

z

AlGaInPGaInP (p-type)

GaAs (p-type)

GaInP (n-type)GaAs (n-type)

Metallization

Ind

ex-

gui

de

d

Transversedirection

Longitudinal direction

Figure 2.5 The basic structure of a broad area diode laser. The transverse index guided direction is along the y-axis and the lateral current-confined direction is along the x-axis.

Gallium Arsenide is the most common material for diode lasers. With the active region surrounded by layers of, for example, Ga0,7Al0,3As, a double heterostructure laser is achieved, se fig. 2.6(a). The surrounding material has a lower index of refraction that makes the active region act as an optical

Risø-R-1306(EN) 21

waveguide. It also has a higher bandgap that makes sure that the carriers recombine in the active region.

Ga

As (p

-typ

e)

Ga

AlAs

(p-ty

pe)

1-y

y

Ga

AlAs

(n-ty

pe)

1-z

z

Ga

As (n

-typ

e)

Ga

AlAs

1-x

x

Position y (a)

(b) (c) (d) Figure 2.6 Transverse y-position versus: (b, c, d adopted from [5])

a Composition of active and cladding layer b energy structure

c index of refraction structure d Optical wave

2.3.2 Types of beam confinement The beam confinement may be considered in the transverse and lateral direction individually. Different mechanisms of guiding are referred to as gain guiding, index guiding and guiding using buried heterostructure and proton implant [5]. The types of confinement used in the transverse direction are usually the same for all types of diodes.

22 Risø-R-1306(EN)

Gain guiding As mentioned above, the region where carriers, i.e. holes and electrons, recombine may be limited using materials with different bandgap and doping. By using materials that are p-doped on the positive side (anode) of the diode and n-doped on the negative (cathode) side, the carriers can travel without risk of recombination to the active region. The bandgap difference of the active region and the cladding then confines the carriers to the active region in the transverse direction, se fig 2.6 (b). In the lateral direction the gain guiding is achieved by using current confinement. This is done by only inserting the current in a thin stripe on top of the diode. Gain is then only available in a region under the stripe, se fig 2.5.

Index guiding A high gain in the active region leads to a lowering of the index of refraction. This, called anti-guiding effects, makes the optical wave spread into the surrounding material with loss, and also creates uneven distributions of the intensity on the output facets [12] for the transverse direction. This ruins the single-mode profile and lead to local higher intensities that may cause facet destruction. By differentiating the index of refraction of the cladding material from the active region’s it is possible to create an optical waveguide. This may be done with the same material that gives the transverse gain profile, se figure 2c. This confines the optical field to the active region and helps to counteract the anti-guiding effects.

Buried heterostructure and Proton implants The confinement in the lateral direction may be improved by introducing a buried heterostructure. This is done by a manufacturing method where portions of the diode is drilled out, leaving only a profile along the longitudinal axis of the laser that is to form the active layer and waveguide. By subsequent regrowth of the wafer using material of lower index of refraction and layers that block the current, a waveguide and current confinement is produced [5] p17, 21. Another way of making lateral current confinement is by introducing proton implants. Protons are inserted in parts of the crystal to block the current.

2.3.3 Different types of diode lasers The need for diodes with higher power has lead to the development of various diode types. Here the broad area laser, the laser diode array and the laser diode bar are reviewed.

Risø-R-1306(EN) 23

Broad area laser

V

Output

Electrons

HolesTransverse Lateral Intensity Profile

x

y

z

AlGaInPGaInP (p-type)

GaAs (p-type)

GaInP (n-type)GaAs (n-type)

Metallization

Ind

ex-g

uid

ed

Figure 2.7 Structure of the BAL. This is the composition of the diodes used in this work. The broad area laser (BAL) is a high power semiconductor diode laser, typically with a double heterostructure as shown in fig. 2.7. The carries are injected using a broad metal stripe on top of the diode. The field in broad area lasers is strongly confined in the transverse direction, simply because the tiny dimension (≈<1µm) of the wave-guiding active layer only supports the fundamental, nearly Gaussian, mode. The lateral direction, however, is significantly wider (≥≈100µm). The wider the lateral direction; the more optical power can be extracted without damaging the end facets. The width is limited due to lateral lasing; the diode may start lasing in the direction orthogonal to the desired. The broad lateral dimension permits several spatial modes to oscillate in the lateral direction on the expense of spatial coherence. Due to the non-linear interaction between the optical field and the refractive index, broad area lasers suffer from self-induced filamentation, which leads to a periodic field distribution in the lateral direction of the end facet [12]. The field consists of a superposition of the fundamental and higher order modes, which all together can be expressed as superposition of sinusoidal functions. The fundamental mode constitutes a half period of a sinusoidal, while higher-order modes results in the observed variation in optical field. The fundamental mode results in a single lobe in the far-field in the straightforward direction, originally refered to as the in-phase mode. Each higher order sinusoidal mode leads to the formation of a double lobe distribution in the far-field (the absolute square of the Fourier transform of a sine), symmetrically distributed around the straightforward direction, see fig 2.8.

24 Risø-R-1306(EN)

m+1

m

Mode #Near-field

x [ m]µ

Far-field

θ [deg]0

| |Fourier Transform 2

Sinusoidaldistribution

Twin-lobeshape

Lateral intensity distribution

Fig 2.8 The spatial Fourier transform. The far field of the freely running diode consists of a number of superimposed dual lobes. There are more complex models of this including the Super mode theory and the Perturbed Broad Area Model [15]. The description presented above is a part of the latter model.

Laser diode arrays The BAL has a large emitting area that permits a high total energy, but a poor beam profile. Narrow-stripe diode lasers with apertures 3-4 µm wide, has single mode operation but low output power. To increase power but avoid lateral lasing, arrays of narrow-stripe diodes are produced. Laser Diode Arrays (LDA) are produced from a large number of laser elements on a row in one crystal, a so-called monolithic array. The active regions are 5-10 µm wide and have a separation of similar size [16] p22. The coupling between the elements is crucial for the beam profile. Basic types of phase-locked arrays include leaky wave coupled, evanescent wave coupled, Y-junction coupled and diffraction coupled [17].

Laser diode bars To further increase power, arrays of diodes can be placed after each other on the same wafer. The array width is in the range 100-300 µm and separation about the same[16] p23. They are called laser diode bars (LDB’s) and have sizes of some centimetres in the lateral direction. These may be stacked together to form 2-dimensional arrays. The density is limited due to the problem of heat removal and the brightness of the system is not increased because of the incoherent sources.

Risø-R-1306(EN) 25

2.4 Other aspects of diode lasers

Lifetime & Failure mechanisms

Diode lasers have a limited lifetime. Their efficiency decreases gradually over time but catastrophic failures can also occur. Bulk degradation, facet failure and problems with the soldering is here briefly discussed. Bulk degradation is due to imperfections in the crystal of the diode laser. This includes point defects such as interstitial (extra) or missing atoms, line and plane defects, where planes are displaced in some way so that the crystal lattice is disturbed. All of these give non-radiative traps, locations that absorb light and dissipate it as heat [6] p311. During operation these imperfections might move and create larger dark areas. The origin of the dislocations could include defects from contamination during growth of the crystal, migration of defects from the substrate crystal, lattice mismatches between different layers of the diode or mechanical stress. The facets are normally cleaved along a crystal plane. Oxidation and heating of the crystal rapidly leads to a high absorption of light in the facet, leading to a quick ageing and a point where the facet melts. To prevent this facets are coated, extending the lifetime considerably. Diode lasers are mounted in a way to remove heat from the diode. In high power diodes the thermal shifts from high temperatures during operation followed by cooling to room temperature may cause stress to the soldering. This may lead to degradation of the heat removal and thermal destruction of the diode [6] p315. Metal from the soldering may also in some cases migrate into the crystal, interfering with the active parts of the diode. All of this usually leads to catastrophic failures.

Microoptics

An approach used to improve the bad spatial properties of diode lasers is the use of microoptics. This includes microlenses or microfibers glued directly on the diodes. A way of describing the output of a device is through output power per unit area per unit solid angle. This is called radiance or brightness. Following [18] p125 there are two theorems that govern an optical system. The first is the one stating that the radiance produced by an imaging system cannot exceed the radiance of the original light source. The second one states that mutually incoherent, but identical, light sources cannot be combined to a higher radiance than the individually strongest source. This is using passive optics, i.e. no feedback. If coherence can be achieved, the radiance of the sources can, in principle, be added up to the total sum. Another way is to use sources with different properties as polarisation or wavelength. The beams can in that case be combined using passive optics. In the transverse direction cylindrical fibers can be used to collimate the light. A cylindrical fiber suffer from aberrations and a better lens can be made by

26 Risø-R-1306(EN)

making a macroscopic better shaped fiber that is then drawn out while retaining its shape. This can create a lens with a high numerical aperture. Numerical aperture, NA, is defined as NA = n0 sin θ, where n0 is the index or refraction and θ the maximal acceptance angel. NA is defined again in section 3.6. Other methods are Gradient Index, or GRIN lenses, materials with a continuous variation of index of refraction [18]. Lithography and etching methods can be used to create diffractive lenses. These can be made lightweight, very accurate and closely packed, an advantage in diode laser arrays [18].

4.3 Present and future utilisation The utilisation of diode lasers is growing as cheaper and better diodes become available. Diode lasers are already used to a big extent in tele-com and with better control of the spatial properties free-space tele-com becomes available. Material processing often works fine with laser beams far from single-mode, which makes high power bars a fine option. With the lower prices and smaller dimensions diode lasers can be used more in different medical applications. High-power diode laser bars have a potential for replacing the flash lamp in solid-state laser pumping. The beam quality is not very crucial in that application and the energy to light conversion is very good.

3 The polarisation coupled laser system Below is a scheme of the system set-up and below it a picture of the system built. In this chapter the various components will be described and their functions explained. The components include two sets of the following: a diode laser, two collimating lenses (L1 and L2) and a mirror providing partial feedback. These are mounted on rails, on for each set. Then there is a half-wave plate for one of the laser sets, a polarisation beam splitter (PBS) and a fiber coupler with an achromatic focusing lens (L6). In some cases two beam expanding lenses (L4 and L5) were also present.

Risø-R-1306(EN) 27

Fig 3.1 Scheme of the set-up. L1 and L2 is two collimating lenses, a plane glass plate makes a reflection for beam diagnostics. L4 and L5 are beam-expanding lenses and L6 is an achromatic focusing lens.

Fig 3.2 Picture of the system.

3.1 The rail system The rail system used needs to be mentioned with a few words. The idea of it is that optical components easily may be placed on a rail and aligned along an optical axis. Once aligned, components may be removed and put back without losing their alignment. The rails are also free to be moved around. Two rails with a length of 30 cm were used. Special sleighs that can be fastened on the rails are used for the optics. They are meant for mounting optics centred 65mm above the optical table. The rails can then by various means be fastened onto the optical table. In this case one of the rails is placed on a 5-axis aligner

Far-field plane

To beam diagnostics

Far-field planeTo beam diagnostics

The PolarizationBeam Splitter

Mirror

Half-waveplate

Single mode fiber aligner

50 m fiberµ

28 Risø-R-1306(EN)

while the other one is placed on bases with additional plates for a good height alignment.

3.2 The lasers The two diode lasers used are broad area lasers (HPD1302-TO3-TEC), BAL’s, as described in section 2.4. They have an active quantum well region of AlGaInP and cladding layers of GaInP grown on a GaAs substrate. In the transverse direction they have index guiding and in the lateral current confinement. The diodes were mounted in a house with cooling. The cooling was made with a Peltier element connected to a thermal electric cooler, TEC, from Thorlabs. This regulates the resistance over the element and in that way the temperature. The mounting on the sleighs and the diode mounting in the house allowed a small degree of height and angle adjustment of the diodes. The lasers had their Gaussian, highly divergent axis, also called fast axis, in the same plane as the table. Perpendicular to this was the direction with the dual lobe profile, the slow axis. The diodes are polarised in the plane parallel to the table (horizontal), i.e. in the direction of the fast axis.

3.3 The collimating lenses The first collimating lens was mounted directly on the laser house. It was an anti-reflection coated planar-aspherical lens with a focal length of fxy=4,5mm and a large numerical aperture of 0.55. The high NA aspherical lens ensures that a large fraction of the highly divergent light is transmitted. It was placed very close to the diode in order to catch as much of the light as possibly. The lens had some degrees of freedom. It could be translated vertical, horizontal and longitudinal. This lens collimated the fast axis and at the same time made a Fourier transform of the near-field of the slow axis. This Fourier transform created a far-field plane at a distance from the lens equal to the focal length of the lens. The second lens only affects the slow axis. This lens made an image of this far-field and also, almost but not fully, collimated the slow axis. The reason it is not fully collimated is that it would place the image of the far-field at an infinite distance. The lens was a planar-cylindrical anti-reflection coated lens with a focal length of 40mm from Linos Photonics. The lens was placed in a holder on a sleigh, providing x-, y- and z-translation.

Risø-R-1306(EN) 29

L , f1 1 L , f2

f1f1

s1 s2

BAL

NF FF Image of FF Fig 3.3: The 2-lens configuration The first aspherical lens Fourier transforms the near-field to a far-field. The second planar-cylindrical lens makes an image of this far-field. The filter is then placed in that image. The position of the image of the far-field was calculated using the lens-makers formula:

(11)

f is the focal length of the second planar cylindrical lens and the distances s1 and s2 can be seen in figure 3.3. A configuration with 3 collimating lenses instead of 2 was also carried out. In that case a 10mm planar-cylindrical lens were placed close to the aspherical lens. This created an imaginary far-field plane created before the lens. This allowed the 40mm lens to make an image of the far-field closer to the laser and a shorter total length of the set-up. The calculations needed for finding the image of the far-field is a little trickier than above. With d denoting the distance between lens 2 and 3 and otherwise using the notations as found in figure 3.4 below, the location was calculated as:

(12)

1

12

21

11111−

−=⇒+=

sfs

ssf

( )0111112

1

122

212

<

−=⇒+=

−

ssf

sssf

334

23

11)(

sfs

dss

−=

+−=

30 Risø-R-1306(EN)

L , f2 2L , f1 1

f1f1

s1

BAL

NF FF Image of FFVirtuel image of FF

S3 S4

s2

L , f3 3

Fig 3.4: The 3-lens configuration The first aspherical lens Fourier transforms the near-field to a far-field. The second lens, a planar-cylindrical, makes a virtual image of the far-field. The third lens, also a planar-cylindrical, in turn, images this virtual image. The filter is placed in that final image. Both lens arrangements gives a collimated fast axis and a slightly divergent slow axis.

3.4 External feedback The most central part of the laser system is the spatial filter with feedback mirror. This central component combines two components in one. As described in chapter 2, the slow axis far-field profile of the diode consists of a number of superimposed twin lobes. The spatial filter selects one of these lobes. The lobe is then feed back to the diode laser using the mirror, se fig 3.5. The filter is a thin mirror stripe created from a half mirror partly covered with a black metal plate, se fig 3.6. Moving the plate can change the width of the stripe. At the same time the mirror is providing feedback. The mirror with the stripe can be adjusted to select one lobe and reflect it back into the diode. The feedback filters are designed, developed and constructed at Risø.

x

y z L1 L2 Mode number m

SF

M

Output lobe

To opticalfiber+ m

- m

ExternalfeedbackFar-field

planeNear-field

plane Fig 3.5 Picture of the lobes and the feedback.

Risø-R-1306(EN) 31

Fig 3.6 Picture of the filters. The spatial filter feedback mirror creates an external cavity that is coupled to the laser cavity. This makes the diode operate in the chosen mode and the main part of the energy is collected in the twin-lobes of that mode, se fig 3.5. One lobe is feed back to the diode while the other, amplified, is coupled out. Intently a lobe in the edge of the profile is chosen. A lobe of mode number m = 10 or above has been shown to provide the best amplification, [15]. Also, if the lobe is outside the profile of the freely running laser there is no naturally closely laying mode competing with the one chosen by the mirror filter. This might possibly improve and stabilise the single-mode performance of the feedback. Above (or below) the mirror, opposite to the plate, the amplified output beam is free to propagate. In a perfect situation it is a single mode Gaussian but it often contain several other modes to some extent. On the backside of the filter are two metal plates that can be moved vertically to filter even the output beam.

3.5 Polarisation coupling The polarisation beam splitter (PBS) is another important part of the system. It allows us to double the output power provided by one laser. According to the radiance theorem the brightness of incoherent light sources cannot be combined to a higher total brightness than the individually strongest [18]. The diode lasers, however, are planar polarised. Using different polarisation, this allow us to combine the two laser beams with added brightness. The half-wave plate changes the polarisation of one of the diodes by 90° and the PBS reflects the beam with one polarisation and lets the other pass straight through, which makes overlapping possible. One of the two lasers was mounted on a five-axis aligner for making good overlapping possible.

32 Risø-R-1306(EN)

3.6 The fiber coupling

The beam expanding lenses - focus matching The output beam from the filtered laser is not symmetrical in shape; it has a rectangular or elliptical shape. This is also the case after the PBS. For a good focusing of the beam into a fiber, an achromat lens is used to minimise the aberrations. In order to get a good (small) focus, the beam should preferably cover the whole lens. This can be understood from the approximate expression for the beam waist, w0 of a Gaussian after a lens [19] p476:

(13)

λ denotes wavelength, f focal length of lens and w1 the beam size before the lens. There is also another aspect of the focusing; the two axes of the beam(s) do not have the same divergence. The fast axis can be perfect collimated but the imaging required for the filtering of the slow axis makes that axis slightly divergent, se fig 3.3, 3.4. This means that after a focusing lens the focus of the two axes do not coincide and consequently there is not the smallest spot size possible. Ideally the beam expanding should both match the beam size of the two axes and the divergence of them to each other. In the case where beam expanding were present, a 10mm and a 40mm planar-cylindrical lens was used. This was to expand the slow axis 4 times. The 10mm lens was placed first and the 40mm lens at a distance of 50mm after the first. This is the sum of the focal length of the lenses. This may be understood by studying the geometrical ray propagation. Unfortunately the 10mm lens was not anti-reflection coated and a lot of power was lost here. This is the reason why it was not always used.

The NA for the fibers Numerical Aperture The numerical aperture, NA, measures the ability of a lens or a fiber to collect light. It is defined as

(13)

where n0 is the index of refraction of the surrounding media and θ the maximum acceptance angle.

01

fwwλ

π≅

θsin0nNA =

Risø-R-1306(EN) 33

Planar-spherical achromat lenses were used for focusing into fibers. When choosing the focal length, the numerical aperture of the fibers were considered. Used fibers were 50-/100- and 200-my fibers from Thorlabs with NA=0.22 and SMA-connectors. For example, with a fiber with a NA of 0.22, a lens with a focal length of 20mm was chosen. It was based on the following calculations for a beam with a (1/e2) width of 8mm.

( ) mmNA

rrf

fr

nNA

2022.04

arcsintantan

tan

22.0sin0

≈≈==

⇒=

==

θ

θ

θ

n0 denotes the refractive index of the surrounding media and is put to 1. x is half the beam width and f the sought focal length of the lens.

The fiber aligner The aligner includes an adapter for SMA connectors. The achromate lens could be mounted on the aligner, fixed perpendicular to the fiber end and the fiber could then be translated in the three dimensions and rotated along its axis. This made a fast and precise fiber alignment possible.

θx

f

Fig 3.7 Illustration of achromat focus concerning the NA of fiber.

34 Risø-R-1306(EN)

4 Experiments and Results The previous chapter concern the build-up of the laser system. This one will show the results along the work of building the system as well as different practical aspects of the process. First the different measurement procedures used are described and then actual conduct.

4.1 Measurement procedures

Far-Fields recordings

The far-field pattern of the slow axis was obtained by putting a power meter on a lateral translator. A slit, 5mm wide, was made in black hard paper and put in front of the power meter. Then the meter was translated about 5mm between each measurement point. The distance between the power meter and laser was about 1,5 meters. The far-field was recorded at 3 or 4 different drive currents for each laser tested. The results in the graphs is shown in intensity as a function of degrees and smoothed using a spline function in the software Origin.

Power-Current measurements

When measuring the characteristics of the freely running laser the power meter was placed as close to the laser as possible. The physical appearance of both the laser mount and the power meter made this distance quite large. A lot of light did not reach the detector because of the high divergence of the light. The power was measured at currents from 0 mA to Imax and from this the threshold current could be determined. The maximum power measured did not match the power specified by the manufacturer. An integrating sphere would probably be required to catch all light. The threshold current could be determined despite that. When measuring power at other positions in the system the power meter was simply put in the beam and a value recorded. These recordings were probably a little more reliable.

M2-measurements

There is obviously a need for a measurement of the quality of a laser beam in some way. There exist several ways to characterise beams. One is the M2-faktor, another the concept of diffraction-limited beams. In this work only M2 will be used. The M2 tells how much a beam resembles a Gaussian, a value of 1 as the best, and otherwise higher. M2 is defined as the ratio between the divergence of the considered beam and a Gaussian beam with the same beam waist. Equally, if the two beams have the same divergence, M2 is given by the ratio of the beam waists.

Risø-R-1306(EN) 35

(15)

(a) (b)

Fig 3.8 Divergence and beam waist. Adopted from [20]. The M2 of a beam may be drawn from a beam in two ways. Either comparing the divergence of it with a Gaussian (a) with the same focus or the beam waist of it compared with a Gaussian with the same divergence (b). M2 could also be defined as the product of the variance of the beam, σx0, and variance of the spatial frequency co-ordinates of the beam, σx, compared to a Gaussian [8] p 481:

(16)

When using measurements far from the beam waist, M2 can easier be calculated as

(17)

Here θd is the divergence angle, ω0 the beam waist radius and λ the wavelength. This relation works only for small values of M2 [21]. To obtain the M2, the beam was focused with a planar-spherical 100mm lens and the beam diameter was measured at several positions after the lens. The beam waist was determined and then the angle of divergence was calculated. Finally M2 was calculated using D0θfullπ/4λ. Here D0 = 2 ω0 is the beam diameter and θfull the full divergence angle and is equal to equation (17). A corresponding M2 value at 50% may also be calculated by using a factor 1.72 in equation (17):

(18)

gaussian

beamMθθ

=2 or gaussian

beam

WW

M0

02 =

( )( ) ( )

x

x

xsx

Gsx

sxM σσπσσσσ

00

02 4== .

λπωθ 02 dM = .

2,50% 0,50%2 1.7dM

θ πωλ

=

36 Risø-R-1306(EN)

For a true Gaussian the relation in beam width between the 50% height and the 13.5% height is 1.72. The reason for the 50% measurement is to avoid any fluctuations from higher order modes in the flanks of the beams. These may be removed by having an aperture but that would lower the power. Errors in this measurements include errors when reading the fluctuating value of the beam diameter with the beam profile analyser (≈5%), the uncertainty of the longitudinal position of the beam analyser (≈5%), accidental tilting of the analyser and also the determination of divergence angle through numerical fitting using the least square method.

Numerical Aperture

The numerical aperture of the fibers was measured to compare with the data of the manufactures. Light was simply coupled into one end of the fiber while the other end was directed at a piece of paper. The distance from fiber tip to paper (l) was measured as well as the diameter (d) of the spot. The numerical aperture, NA, was then calculated as:

ldnNA 2sin0 == θ

θd

l

Fig 4.2 Illustration of NA-measurements

4.2 Results of all measurements Some of the first measurements were done at both 21.5° and 15° C. This was because earlier 21.5° was the lowest temperature that could be used for the red diodes. After it was assured that 15° was fine, 15° was mainly used although one of the diodes sometimes showed better characteristics at 18° C and some of the measurements was done at that temperature. In the end both diodes were used at 15° C because of the higher output power.

Risø-R-1306(EN) 37

Characteristics of Lasers

Far-field The far-field patterns of diode #4290A, #4290B, #4680A and #4680B can be seen below. #4290A and #4680B are the two diodes used in the final system. Initially work was conducted with diode #4290A and #4290B. They are cut from the same wafer, side by side, and it were thought that they would resemble each other and produce matching output beams. The far-field patterns of these did, however, not look alike, as can be seen in fig. 4.3. The reason #4290B was not used was partly that it was mounted tilted in the laser house and partly (possibly because of the first) that it did not produce very good results. The far-field of #4680A can also be seen below to compare with #4680B. These are also from the same wafer.

-3 -2 -1 0 1 2 3 40

10

20

30

40

50

60

70

80

17/9-01

at 21.5o C (11590 ohm)

1.6 Ith (550mA) 1.4 Ith (465mA) 1.2 Ith (400mA) 1.1 Ith (370mA)

Inte

nsity

[arb

. uni

ts]

Angle [deg]

-3 -2 -1 0 1 2 3 40

20

40

60

80

100

120

Inte

nsity

[arb

. uni

ts]

Angle [deg]

Intensity profile in the far-field for the#4290A HPD (635,0 nm). d=1,3 m Detector resolution 5 mm. File: farfield21.opj

at 15.0o C (14920 ohm)

1.8 Ith (550mA) 1.5 Ith (465mA) 1.3 Ith (400mA) 1.2 Ith (370mA)

-3 -2 -1 0 1 2 3 40

102030405060708090

100110120130

at 21.5o C (11590 ohm)

18/9-01

1.6 Ith (550mA) 1.5 Ith (510mA) 1.4 Ith (465mA) 1.2 Ith (400mA)

Inte

nsity

[arb

. uni

ts]

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Intensity profile in the far-field for the#4290B HPD (634,7 nm). d=1,3 mDetector resolution 5 mm. File: farfield.opj

at 15.0o C (14920 ohm)

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Far-field of #4680A at 15.0o C011031

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Fig 4.3 Comparison of far-fields of diodes #4290A, #4290B, #4680A and #4680B. The ones with the same number are cut from the same wafer.

38 Risø-R-1306(EN)

Power-current of lasers Power-current characteristics were measured for all diodes. Because of the inefficient light collection described earlier, only the threshold currents are presented. The threshold current for diodes #4290A and B is lower at 15° C than at 21.5° C. (Used diodes in bold.) Table 1. Threshold currents for diodes #4290A, #4290B, #4680A and #4680B.

#4290A #4290B #4680A #4680B 15° C 310mA 310mA 300mA 290mA 21.5° C 340mA 340mA Not measured Not measured

Collimation

The collimation is very crucial for a successful filtering. The operation of the lenses is described in chapter 3. The handling of the first lens is in theory quite simple; align it so that the fast axis is constant in width or just slightly divergent. The planar-cylindrical lens(es) may be manoeuvred so that the slow axis is shown in different shapes on a distant screen. There are two positions where the size of the spot is the smallest. One is where the far-field is imaged on the wall. Then, by moving the planar-cylindrical lens a little closer to the laser, another minimum is reached. In this position the image of the far-field is positioned as described in section 3.3. No theory will be given to support this. It is rather a knowledge gained through experience. The cylindrical lens(es) needs to be tilted at the same angle as the lateral direction of the diode. Especially for the 3-lens collimation the precision of this is very important because there is 4 components involved (diode, 2 lenses and filter).

Filtering and Spatial improvement

External feedback using spatial filtering as described in chapter 3 was provided for the diodes. This procedure will in short be referred to as filtering. Figures of the spatial profiles with and without filtering illustrate the spatial improvements. These profiles were recorded with a beam profile analyser on a reflection from a plane glass plate inserted before the filter (the spatial feedback filter). Filtering with 3 collimating lenses as well as 2 was performed. In fig 4.4 filterings on diode #4290A with 2 and 3 lenses is compared. The improvements are about the same in both cases. The 3-lens set-up was shorter than the 2-lens set-up. However, it was much more delicate to align. The position (and angle) of the lenses and filter was more crucial and consequently it was harder to get good a good filtering. The easier handling and more stable system and finally, the lack of anti-reflection coated 10mm lenses, led to a choice of 2 lenses for collimation.

Risø-R-1306(EN) 39

(a) (b) Fig 4.4 Comparison between 3 and 2 collimating lenses. (a) Three collimating lenses (2 cylindrical) and (b) two collimating lens (1 cylindrical). The non-matching far-field profile of the freely running laser is due to that the beam profile analyser was not placed at the same (or precise far-field) distance. The spatial profiles were not the final judge over the beams. That was the M2 value of the output beams after the filter. For diode #4290A the M2 value, for 13.5% and 50% respectively, can be seen to improve from 9.3 and 16.0 before the filter to 4.0 and 2.4 after, fig 4.5.

Fig 4.5 M2-values for the unfiltered and filtered diode #4290A. For diode #4680B the M2 value, for 13.5% and 50% respectively, can be seen to improve from 6.9 and 11.8 to 3.9 and 1.1, fig 4.6.

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θfull, 50% = 15,5 mrad, D0, 50% = 308 µm

M250% = πD0θfull 1,72/ 4λ = 16,0

θfull, 13.5% = 16,3 mrad, D0, 13.5% = 462 µm

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M250% = πD0θfull 1.72/ 4λ = 2,4

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40 Risø-R-1306(EN)

Fig 4.6 M2-values for the unfiltered and filtered diode #4680A.

Current-Power of filtered diodes

The current-power relation for the filtered output beams was also examined. The absolute power measured here is a little more relevant than the one measured for the uncollimated diodes. The interesting part is that the threshold current is now lower than before. For diode #4290A at 15° C the threshold current is just slightly lower but for #4680B at 15° C the threshold is seen to drop from 290 to 260mA. The filtered feedback has given the provided an increased stability to the resonator. Table 2. Threshold currents for diodes #4290A and #4680B with spatial filtered mirror feedback.

#4290A #4680B 15° C 300mA 260mA

0 50 100 150 200500

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θfull, 50% = 4,7 mrad, D0, 50% = 704 µm

M250% = πD0θfull 1,72/ 4λ = 11,8

θfull, 13.5% = 5,6 mrad, D0, 13.5% = 988 µm

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Risø-R-1306(EN) 41

The combined beams Using a mirror, a half-wave plate and a PBS the output beams of the two lasers were combined. A M2 value of 4.7 and 2.1, for 13.5% and 50% respectively, was measured, fig 4.7. After the PBS the beam was focused with an achromat and the shape of the focus studied with the beam analyser. A lens with 40mm focal length was used for the slow axis and a 50mm for the fast axis focus. By blocking one of the lasers at a time the shapes in focus and the shape were the two axes had the same size was studied, fig 4.8. This was then done for the combined beams, fig 4.9.

0 20 40 60 80 100

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200400600800

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M2-values for polarisation coupled filtered output of #4290A T=18oC I=550mA (1.8 Ith) and #4680B T=15oC I=500mA (1.7 Ith) 011126

θfull, 50% = 14.2 mrad, D0, 50% = 42 µm

M250% = πD0θfull (1.7)2/ 4λ = 2.1

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Fig 4.7 M2-values for the combined beams of diode #4290A and #4680B.

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focus of fast axis

Fig 4.8 The shape of focus of diodes #4290A and #4680B. The shape of the two axes in focus of the fast axis and slow axis. Also the shape where the two axes has the same size.

42 Risø-R-1306(EN)

In fig. 4.8 and 4.9 it can be seen clearly that the divergence of the two axes is not the same. The smallest beam sizes in the two axes do not overlap. As it is in this situation, the smallest beam size is around 180 µm in diameter. A fiber with a diameter of 200 µm is also what gives the best percentage of light transmitted in the fiber coupling. It is evident that with a good placement of the beam-expanding lenses, the focuses of the two axes can be made to coincide. Since the width of the fast axis focus is 14.8 µm and the slow axis focus 58 µm it should be no problem to get a good coupling in a 100 µm fiber. Even a 50 µm may be possible. Unfortunately, because of the time restraints on this work, this has not been properly tried.

Power

The power was measured at different positions throughout the system to se how much was lost in different components. The value for measurement position 1 is taken after the first collimating lens. The power of the uncollimated diodes is given in the data sheet for the diodes. However, that value could not be recorded with the instruments used for the other measurements and the value is not really comparable with the other values. This is because, as mentioned earlier, not all light could be collected. The value for measurement position 5 is not recorded at the same time as the other but from other measurements. The power at 5 has a linear dependence on position 4. The numerical aperture of the used fibers caused confusement along the whole work. The NA for the 200µm fiber was in one measurement found to be 45,5/162 ≈ 0,28. At Marselisborg Hospital, Århus, the NA was at a later time found to be 0.23. The given value of 0.39 seems unlikely. The fiber could have been damaged at some point.

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Fast axis fokus

Fig 4.9 The shape of focus of the combined beams of diodes #4290A and #4680B. The shape of the two axes in focus of the fast axis and slow axis and also the shape where the two axes have the same size are shown. This is the effective smallest beam waist, the smallest beam spot to be matched to a fiber.

Risø-R-1306(EN) 43

Far-field plane

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Fig 4.10 Power transmitted trough system. L1 and L2 are collimating lenses and L6 is an achromatic lens mounted on the fiber aligner. Power of #4290A at 550mA 15°C from data sheet: 238mW #4290A 1 2 3 4 5* 155mW 149mW 118mW 116mW (127mW) % of 1 96% 76 % 75 % 70 % % of 2 79 % 78 % 73 % % of 3 98 % 92 % % of 4 94 % I = 550mA, T = 18°C, * I = 570mA, T = 15°C and 135mW at position 4 (interpolated values). Power of #4680B at 500mA 15°C from data sheet: 212mW #4680B 1 2 3 4 5* 197mW 195mW 130mW 129mW (154mW) % of 1 99 % 66 % 65 % 60 % % of 2 67 % 66 % 61 % % of 3 99 % 91 % % of 4 92 % I = 500mA, T = 15°C, * I = 530mA, T = 15°C and 167mW at position 4 (interpolated values). (The big difference in power from the data sheet and measurement position 1 of diode #4290A is possible due to its higher divergence.)

Wavelengths

The spectra of both diodes, filtered and unfiltered, were recorded. It can clearly be seen that the filtering makes the spectra narrow down, fig 4.11.

44 Risø-R-1306(EN)

Fig 4.11 Spectra for unfiltered and filtered diodes #4290A and #4680B

To the left, fig 4.12, the spectra of the two filtered laser beams can be seen. The diode running at lower temperature is seen to be running at a slightly lower wavelength.

5 Conclusions A summary of diode laser theory was presented in chapter 2. A laser system delivering fiber coupled light at 635nm was constructed. A number of practical solutions were evaluated. A combined beam with a M2 of 4.7 and 2.1 for the 13.5% and 50% levels, respectively, was measured. The laser light was efficiently coupled into a 200 µm fiber and it is shown that the step down to a 50 µm fiber may be done. BAL’s with higher output will become available at the market giving the possibilities of more coupled power. With a little better filtering than was achieved in this work an even lower M2 value may be reached. With a better focus matching and beam expanding than the time allowed for this work, a smaller focus will be possible. The system was brought to Marselisborg Hospital in Århus and used to treat a superficial lesion in a clinical environment, see Clinical trials. In a relatively cheep and efficient way red laser light in the range of several

633 634 635 636 637 638

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Wavelengths measured of polarisation coupled lasersMirror feedback appliedFibercoupled

Fig 4.12 Spectra for the beams of diodes #4290A and #4680B

Risø-R-1306(EN) 45

100mW can be focused to a small spot or coupled into a thin fiber. This could be used in medical application, for printing or imaging display systems.

5.1 Perspectives For the treatment of superficial lesions there are today many different light sources to choose from. A spatially filtered feedback BAL system is probably not a good alternative, because of the losses involved in the filtering. But for interstitial-PDT it has bigger potential. In Lund the fiber splitting system that splits the light into 3 of 6 different fibers is used. Why the effort of squeezing Watts of light into one fiber just to split it again? Instead one or two BAL’s could be coupled into every one of the 6 fibers. This would be a highly effective system and would also give the possibility to adjust the power for every individual fiber. Similar works with LDA’s and BAR’s may also be a way.

46 Risø-R-1306(EN)

Clinical trials This is a review of the clinical PDT trial done at Marselisborg Hospital, Århus, Denmark, 21-22th February 2002. Eva Samsøe and Per Malm were handling the laser system. Henrik Sølvsten was the conducting physician. Lesion and Patient: Lesion type: The treated lesion was of the type Superficial Basal Cell Carcinoma (S-BCC). Size of lesion: 1cm in diameter, i.e. a treatment radius of 1,2cm (⇒ area = 4,5cm2). Patient: 37 year-old, man, 3 S-BCCs on back/shoulder area. Two was treated with scrape and burning (scrape, burn, scrape, burn - typical treatment efficiency of 90%), while the third was treated with ALA-PDT and our laser. Follow up: In one month. The lesion will be studied with the eye. If there is nothing to see, it will be another three months before the next follow up. If there is something suspicious a biopsy is taken. The following treatment parameters was used: LIGHT DOSE (LD): 60 J/cm2 FLUENCE RATE (FLR): 100mW/cm2 TREATMENT TIME (t): 10 min. DOSE [J/cm2] = FLUENCE RATE [W/cm2] x TREATMENT TIME [s] A scheme for treated area versus treatment time can be found below. Size of lesions, laser power and treatment time: The lesion treated had a diameter of 1cm. A safety margin of 7mm was used around the lesion. This gave an area with a radius of 12mm to be treated. Our laser system gave 250mW trough a 200µm fiber. With our system we could not reach the treatment time of 10 min at a fluence rate of 100mW/cm2. A fluence rate of 56mW/cm2 was used under 18min. Treatment procedure:

1) Biopsy is taken 2) ALA is applied as creme and left for 4 hours 3) PDT treatment with 55.6mW/cm2 for 18min (60J/cm2).

Risø-R-1306(EN) 47

Treatment parameters for ALA-PDT with 635nm polarisation coupled laser using a 200µµµµm fiber with NA=0,23.

Dose: 60 J/cm2 Fluence rate (FLR): 56-100mW/cm2

Area (cm2)

Radius (cm)

Distance (cm)

Power (mW)

FLR (mW/cm2)

Time (min)

0,8 0,5 2,1 80 100 10 1 0,56 2,3 100 100 10 1,5 0,69 2,9 150 100 10 2 0,80 3,3 200 100 10 2,5 0,89 3,7 250 100 10 3 0,98 4,1 250 (300) 83,3 12 3,5 1,06 4,4 250 (350) 71,4 14 4 1,13 4,7 250 (400) 62,5 16 4,5 1,20 5,0 250 (450) 55,6 18

If the dose is to be increased to 90J/cm2, the treatment time should be increased by 50% (10->15, 12->18, 14->21, 16->24, 18->27)

48 Risø-R-1306(EN)

References

1. I. Wang, Photodynamic therapy and laser-based diagnostic studies of malignant tumours. 1999. Lund, Sweden, Lund University. Ref Type: Thesis/Dissertation

2. E. Samsoe, A New Diode Laser System for Photodynamic Therapy. 2001. Roskilde, Denmark, Risø National Laboratory. 7-17-2000. Ref Type: Thesis/Dissertation

3. C. af Klinteberg, On the use of light for the characterization and treatment of malignant tumours. LRAP-245. 1999. Lund, Sweden, Lund Institute of Technology. 1999. Ref Type: Thesis/Dissertation

4. N. Ohlsson and O. Rylow, Development of a multifibre system for interstitial photodynamic therapy of malignant tumours. LRAP-240. 1998. Lund, Sweden, Lund Institute of Technology. Lund Reports on Atomic Physics. Ref Type: Thesis/Dissertation

5. P. Unger, "Introduction to Power Diode Lasers," in High-Power Diode Lasers, R. Diehl, Ed., pp. 1-53, Springer Verlag, Heidelberg, Germany (2000).

6. D. R. Scifres and H. H. Kung, "High-power diode laser arrays and their reliability," in Diode Laser Arrays, D. et. a. Botez, Ed., pp. 294-335, Cambridge University Press, (1994).

7. C. Kittel, Introduction to Solid State Physics, John Wiley & Sons, Inc., USA (1996).

8. O. Svelto, Principles of Lasers , Plenum, (1998). 9. G. Jönsson, Atomfysikens grunder del ett, Teach Support, Lund, Sweden

(1998). 10. G. Jönsson, Atomfysikens grunder del två, Teach Support, Lund,Sweden

(1999). 11. J. J. Coleman, "Strained layer quantum well heterostructure laser arrays,"

in Diode Laser Arrays, D. Botez, Ed., pp. 336-367, Cambridge University Press, (1994).

12. D. Mehuys, R. J. Lang, M. Mittelstein, J. Salzman, and A. Yariv, "Self-Stabilized Nonlinear Lateral Modes of Broad Area Lasers," IEEE J. Quant. Electr. QE-23, 1909-1920 (1987).

13. J. K. Butler, D. E. Ackley, and D. Botez, "Coupled-mode analysis of phase-locked injection laser arrays," Appl. Phys. Lett. 44:293, (1984).

14. E. Kapon, J. Katz, and A. Yariv, "Mode analysis of phase-locked arrays of semiconductor lasers," Opt. Lett. 10:125, (1984).

15. J. M. Verdiell and R. Frey, "A Broad-Area Mode-Coupling Model for Multiple-Stripe Semiconductor Lasers," IEEE J. Quant. Electr. 26, 260-279 (1990).

16. M. B. H. Løbel, Improvement of the Coherence Characteristics of Laser Diode Arrays Using Photorefractive Phase Conjugation. 1998. Roskilde, Denmark, Risø National Laboratory. Ref Type: Thesis/Dissertation

17. D. Botez, "Monolithic phase-locked semiconductor laser arrays," in Diode laser arrays, D. Botez, Ed., pp. 1-71, Cambridge University Press, (1994).

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18. J. R. Leger, "Microoptical components applied to incoherent and coherent laser arrays," in Diode Laser Arrays, D. et. a. Botez, Ed., pp. 123-179, Cambridge University Press, (1994).

19. F. L. Pedrotti and S. J. L. S. Pedrotti, Introduction to Optics, Prentice Hall, Inc., Upper Saddle River, New Jersey (1996)

20. "ModeMaster PCTM - M2 Beam Propagation Analyzer", User Manual, Coherent Auburn Division, Coherent inc. (2001).

21. K-J. Boller, B. Beier, R. Wallenstein , "Properties and Frequency Conversion of High-Brightness Diode-Laser Systems," in High-Power Diode Lasers, R. Diehl, Ed., pp. 225-235, Springer Verlag, Heidelberg, Germany (2000).

50 Risø-R-1306(EN)

Appendices

The instruments used • Lasers – (HPD1302-TO3-TEC), BAL’s with active quantum well region of

AlGaInP and cladding layers of GaInP grown on a GaAs substrate. • Power supplies from Thorlabs: LDC 2000 TEC 2000 • Power meter: Melles Griot Universal Optical Power Meter 13PDC

001/IEEE Ophir LaserStar • Spectrometer: Ophir WaveStar CCD Spectrometer • Beam Profile Analyser: Melles Griot Super BeamalyzerTM 13SKP003 • Lenses: collimating lenses

first: f4.5 planar-aspherical NA=0.55 from Thorlabs (second: f10 planar-cylindrical) third: f40 planar-cylindrical from Linos Photonics

beam-expanding lenses:

first: f5mm planar-asperical second f20mm planar-asperical

Fiber coupling:

planar-achromat f20 • Fibers: NA=0.39; 50-, 100- and 200-µm core diameter from Thorlabs

FT-200-UMT • Fiber aligner: 5-axis aligner from New Focus • Polarisation Beam Splitter (PBS)

Risø-R-1306(EN) 51

Bibliographic Data Sheet Risø-R-1306(EN) Title and authors A polarisation coupled diode laser system for photodynamic therapy Per Malm ISBN ISSN 87-550-2976-0 87-550-2977-9 (Internet) 0106-2840 Department or group Date OFD 2002-11-04 Groups own reg. number(s) Project/contract No(s) Sponsorship Pages Tables Illustrations References 51 2 29 21 Abstract (max. 2000 characters) This master thesis deals with the description and specifications of a novel diode laser system for (interstitial) photodynamic therapy.

Two 635nm broad area diode lasers are coupled to external cavities created by an end mirror. Spatial filtering is introduced in each of the two external cavities to improve the spatial characteristics of the diodes. This improves the M2 value of the output from the diodes considerably. A large portion of the output power of the freely running diode is in this way gathered in an output lobe with good beam quality.

By the use of a polarisation beam splitter the output of two diode lasers is efficiently combined while retaining their good M2 value. The output of the two diode lasers is coupled into an optical fiber with a 200µm core. It is shown that a coupling into an even thinner fiber is possible. Descriptors INIS/EDB