1

Optoelectronics

Safa O. KasapElectrical Engineering Department, University of Saskatchewan, Saskatoon, S7N 5A9, Canadae-mail: kasap@engr.usask.ca

AbstractIt is useful to view today’s optoelectronics in terms of various functional categories, mostimportant of which are the generation of light, modulation of light, transmission of lightand its connection between devices, amplification of light, switching of light, isolation oflight, filtering of light, wavelength multiplexing and demultiplexing of light, and detec-tion of light. The corresponding devices that implement these functions are light emitters(or light sources), modulators, waveguides and connectors, optical amplifiers, opticalswitches, optical isolators, optical filters, multiplexers and demultiplexers, and detectors.This introductory article reviews the principles of operation and characteristics of a num-ber of selected devices to provide a general introduction to the field of optoelectronicswith an emphasis on optical communications – a major area of application.

Keywordscoherent and incoherent sources; lasers; waveguides; optical fibers; connectors; lightmodulation; photodetectors; integrated optics; optoelectronic systems; opticalcommunications.

1 Introduction 22 Solid-state Emitters 52.1 Light-emitting Diodes 52.2 Semiconductor Laser Diodes 82.2.1 Double-heterostructure Laser Diodes 82.2.2 Quantum Well and Quantum Dot Lasers 112.2.3 Single Frequency Lasers 132.2.4 Vertical Cavity Surface Emitting Lasers (VCSELs) 14

2 Optoelectronics

3 Optical Fibers as Waveguides 153.1 Step-index Optical Fiber Fundamentals 153.2 Dispersion in Optical Fibers 193.3 Attenuation in Optical Fibers 203.4 Cables 223.5 Connectors and Splices 234 Optical Amplifiers 235 Modulators 255.1 Electro-optic Modulators 255.2 Acousto-optic Modulators 285.3 Multiple Quantum Well Modulators 296 Optical Switches, Multiplexers, and Isolators 307 Photodetectors 337.1 Fundamental Definitions and Characteristics 337.2 pin Photodiode 347.3 Avalanche Photodiode 367.4 Heterojunction Avalanche Photodiodes 378 Integrated Optics and Optoelectronics 389 Optoelectronic Systems 40

Glossary 42Acknowledgment 47Further Reading 47

1Introduction

Optoelectronics is an evolving disciplinethat describes phenomena and applica-tions using electronics and optics, orelectrons and photons; a good exampleis a laser diode that converts an electriccurrent at its input into a coherent beamof photons at its output. A major historicalmilestone in optoelectronics is the devel-opment of heterostructure laser diodes inthe early 1960s that eventually allowedsmall-scale semiconductor lasers to bemanufactured in late 1970s. Over the lasttwo decades, there have been tremendousadvances in optoelectronics driven primar-ily by the enormous growth in opticalcommunication networks. While opticalcommunications is obviously one of the

most significant uses of optoelectronics,other applications have also been devel-oping and serving other industries andmarkets. (The consumer is already famil-iar with CD and DVD players, flat-paneldisplays, digital and video cameras, in-frared binoculars, etc.) There is no cleardistinction between optoelectronics andphotonics, though the latter would encom-pass substantial optics. The term photonicsgrew from envisaging and realizing appli-cations that involve photons only withoutnecessarily relying on electronics.

It is useful to view today’s optoelectron-ics in terms of various functional cate-gories, most important of which are the (1)generation of light, (2) modulation of light,(3) transmission of light and its connectionbetween devices, (4) amplification of light,

Optoelectronics 3

(5) switching of light, (6) isolation of light,(7) filtering of light, (8) wavelength mul-tiplexing and demultiplexing of light, and(9) detection of light. The correspondingdevices that implement these functions arelight emitters (or light sources), modula-tors, waveguides and connectors, opticalamplifiers, optical switches, optical isola-tors, optical filters, multiplexers and de-multiplexers, and detectors. These devicesare shown as functional blocks in Fig. 1.An externally modulated optical commu-nications system that incorporates someof these functionalities is schematically il-lustrated in Fig. 2. In the latter example,a laser diode generates a continuous wavelight at 1550 nm with a narrow spectrum.The light is externally modulated by anelectro-optic (EO) modulator. The modu-lator modulates the light passing throughit according to the encoded informationapplied to its electrical input terminal. Theoptical signal in the form of pulses is fedinto an optical fiber for transmission to-wards its destination. The wavelength of1550 nm is preferred for long-haul com-munications because the attenuation alongthe optical fiber is least at this wavelength.Optical switches enable the optical path tobe switched at various locations as desiredby the user; for example, if there is a breakin the fiber, the optical switch in Fig. 2will switch the optical path to the back-uplink. Optical amplification is typically usedto amplify an optical signal that has been

attenuated as a result of being guided alonga transmission medium such as an opticalfiber. An optical isolator allows the opticalsignal to travel only in one direction and isuseful in preventing the amplified signalfrom traveling back to the transmitter. Anoptical coupler allows light in one fiber tobe coupled into another fiber. An opticalfilter allows either a given wavelength topass through (such as the 1550-nm wave-length) or blocks the passage of a certainwavelength (such as the 980-nm pumpwavelength of an Er-doped fiber amplifier)by reflecting it. Various optical connectors(not shown in Fig. 2) are obviously neededto connect the devices and components toeach other. Optical connectors, althoughsimple in functionality, play an undeniablyimportant role in the overall optoelectronicsystem design since various devices andcomponents have to be connected togetherefficiently and economically, and withoutfailure. For example, an inefficient connec-tion system may lead to an additional useof optical amplifiers to correct the mistake.In optical communications, typically, vari-ous devices are connected through opticalfibers and cables using connectors andsplices. The connector and splice technol-ogy have been, by and large, standardizedto facilitate the rapid and easy connectionof fibers and cables to each other.

While the above simple function-basedclassification is convenient, it is notnecessarily always sufficient. For example,

An optoelectronic system

Lightemitters

Waveguides,connectors

ModulatorsOptical

amplifiersOptical

switches

Opticalisolators

Opticalfilters

Multiplexers,demultiplexers

Detectors

Converters

Phosphors

Fig. 1 A variety of optoelectronic devices can be combined to build an optoelectronic system

4 Optoelectronics

Opt

ical

fib

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Gen

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ion

Mod

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Tra

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Enc

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info

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Dec

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Am

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Opt

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ampl

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(ED

FA)

Opt

ical

isol

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Isol

atio

npi

nph

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iode

Switc

hing

Opt

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switc

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Ele

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Las

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Bac

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1550

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980-

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Opt

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filte

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Filte

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Opt

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isol

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Opt

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Dec

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Det

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Optoelectronics 5

we can also introduce other functionalcategories such as light conversion or lightstorage; in the former, the wavelength ischanged (e.g., by second harmonic gen-eration), whereas in the second case, thelight is stored (which may involve con-version) until it is needed and then ‘‘let’’out; stimulated phosphors are good exam-ples of this, but there is a change in thewavelength. An alternative classificationscheme is also possible, for example, basedon the physical process that is exploitedin the optoelectronic device rather thanwhat the device does. (Optoelectronics en-gineers are more interested in the functionof the device than the physical processthat enables that function.) It is alsopossible to divide optoelectronics into cat-egories based on end-uses or applicationssuch as optical communications, opticalinformation storage, metrology, medicalimaging, medical diagnosis, laser surgery,holography, and so on, but not many op-toelectronics books take that approach indescribing the subject. Optoelectronics isgenerally taught as an overlap field be-tween physics, electrical engineering, andmaterials science, though each discipline

has its own slightly different description(and opinion) of the subject.

2Solid-state Emitters

2.1Light-emitting Diodes

A light-emitting diode (LED) is essentiallya pn junction diode typically made froma direct bandgap semiconductor, for ex-ample, GaAs, in which the electron–holepair (EHP) recombination results in theemission of a photon. The emitted pho-ton energy hυ is approximately equal tothe bandgap energy Eg. Figure 3(a) showsthe energy band diagram of an unbiasedpn+ junction device in which the n-side ismore heavily doped than the p-side. (Thesuperscript ‘‘+’’ on n or p indicates heavydoping.) The Fermi-level EF is uniformthrough the device, which is a require-ment of equilibrium with no applied bias.The depletion region extends mainly intothe p-side. There is a potential energy (PE)barrier eVo from Ec on the n-side to Ec

hn ≈ Eg

Eg

V

p n+

Eg

eVo

EF

p n+

Electron in conduction band

Hole in valence band(a) (b)

Ec

Ev

Ec

Ev

EF

eVo

Electron energy

+ −

Distance into device

Fig. 3 Energy band diagram of a pn (heavily n-type doped) junction. (a) No bias voltage; (b) Withforward bias V

6 Optoelectronics

on the p-side where Vo is the built-involtage. The PE barrier eVo prevents thediffusion of electrons from the n-side tothe p-side.

When a forward bias V is applied, thebuilt-in potential Vo is reduced to Vo − Vwhich then allows the electrons from then+ side to diffuse, that is, become injected,into the p-side as depicted in Fig. 3(b).The hole injection component from p-into the n+-side is much smaller thanthe electron injection component fromthe n+ to p-side. The recombination ofinjected electrons in the depletion regionand within a volume extending over theelectron diffusion length in the p-side leadsto photon emission. The phenomenon oflight emission from EHP recombinationas a result of minority carrier injection iscalled injection electroluminescence. Due tothe statistical nature of the recombinationprocess between electrons and holes, theemitted photons are in random directions;they result from spontaneous emissionprocesses. The LED structure has to be

such that the emitted photons can escapethe device without being reabsorbed bythe semiconductor material. This meansthe p-side has to be sufficiently narrow orwe have to use heterostructure devices asdiscussed below.

The external efficiency ηexternal of an LEDquantifies the efficiency of conversion ofelectrical energy into an emitted externaloptical energy. It incorporates the internalefficiency of the radiative recombinationprocess and the subsequent efficiency ofphoton extraction from the device. Theinput of electrical power into an LED issimply the product (IV) of the diode cur-rent I and the diode voltage V . If Pout isthe optical power emitted by the device,then the external efficiency is Pout/(IV),and some typical values are listed inTable 1. For indirect bandgap semicon-ductors, ηexternal is generally less than 1%,whereas for direct bandgap semiconduc-tors with the right device structure, ηexternalcan be substantial (>10%). Table 2 sum-marizes typical wavelength ranges for the

Tab. 1 Selected LED semiconductor materials, wavelengths of emission, and typical externalquantum efficiencies in commercial LEDs. Optical communication channels are at 850 nm (localnetwork), and at 1.3 and 1.55 µm (long distance). D = direct, I = Indirect bandgap. ηexternal is typicaland may vary substantially depending on the device structure

Semiconductor (active layer) D or I λ [nm] ηexternal[%]

Comment

GaAs D 870–900 10 Infrared (IR)AlxGa1−x As (0 < x < 0.4) D 640–870 3–20 Red to IRIn1−xGaxAsyP1−y (y ≈ 2.20x, 0 < x < 0.47) D 1–1.6 µm >10 LEDs in

communicationsInGaN alloys D 430–460 1–2 Blue

500–530 3–5 GreenInGaN/GaN quantum well D 450–530 >5 Blue–greenSiC I 460–470 0.02 Blue. Low efficiencyIn0.49Alx Ga0.51−x P D 590–630 1–10 Amber, green, redGaAs1−yPy (y < 0.45) D 630–870 <1 Red–IRGaAs1−yPy (y > 0.45) (N or Zn, O doping) I 560–700 <1 Red, orange, yellowGaP (Zn–O) I 700 2–3 RedGaP (N) I 565 <1 Green

Optoelectronics 7

Tab. 2 Wavelength ranges and colors as usually specified for LEDs

Color Blue Emerald green Green Yellow Amberλ (nm) λ < 500 530–564 565–579 580–587 588–594

Color Orange Red–orange Red Deep red Infraredλ (nm) 595–606 607–615 616–632 633–700 λ > 700

apparent colors of various consumer LEDs.(Color perception is a difficult topic andthe table only indicates ‘‘typical values’’as quoted by various manufacturers ofLEDs and as perceived by an averageperson.)

A junction between two differentbandgap semiconductors is called a het-erojunction. A semiconductor device struc-ture that has junctions between dif-ferent bandgap materials is called aheterostructure device. LED constructionsfor increasing the intensity of the out-put light make use of the double het-erostructure. Figure 4(a) shows a double-heterostructure (DH) device based on twojunctions between different semiconduc-tor materials with different bandgaps.In this case, the semiconductors are Al-GaAs with Eg ≈ 2 eV and GaAs withEg ≈ 1.4 eV. The double heterostructurein Fig. 4(a) has an n+p heterojunction be-tween n+-AlGaAs and p-GaAs. There isanother heterojunction between p-GaAsand p-AlGaAs. The p-GaAs region is a thinlayer, typically a fraction of a micron andis lightly doped.

A highly simplified energy band diagramfor the whole device in the absence ofan applied voltage is shown in Fig. 4(b).The Fermi-level EF is continuous throughthe whole structure. There is a PE barriereVo for electrons in the conduction band(CB) of n+-AlGaAs against diffusion intop-GaAs. There is a bandgap change at thejunction between p-GaAs and p-AlGaAswhich results in a step change �Ec in

Ec between the two CBs of p-GaAs andp-AlGaAs. This �Ec is effectively a PEbarrier that prevents any electrons in theCB of p-GaAs from passing to the CB ofp-AlGaAs. (There is also a step change�Ev in Ev but this is small and is notshown.)

When a forward bias is applied, most ofthis voltage drops between the n+-AlGaAsand p-GaAs, and reduces the PE barriereVo, just as in the normal pn junction. Thisallows electrons in the CB of n+-AlGaAs tobe injected into the CB of p-GaAs as shownin Fig. 4(c). These electrons, however, areconfined to the CB of p-GaAs since thereis a barrier �Ec between p-GaAs andp-AlGaAs. The wide bandgap p-AlGaAslayers therefore act as confining layersthat restrict injected electrons to the p-GaAs layer. The recombination of injectedelectrons and holes already present inthis p-GaAs layer results in spontaneousphoton emission. The p-GaAs layer iscalled the active layer inasmuch as thisis where light is generated. Since thebandgap Eg of AlGaAs is greater thanGaAs, the emitted photons do not getreabsorbed as they escape the active regionand can reach the surface of the device asdepicted in Fig. 4(d). Since light is also notabsorbed in p-AlGaAs, it can be reflectedto increase the light output.

The spectrum of the emitted radiationis determined by the energy spread ofthe electrons in the conduction band(CB) and the energy spread of theholes in the valence band (VB) in the

8 Optoelectronics

2 eV

2 eVeVo

Holes in VB

+−

Electrons in CB1.4 eV

No bias

Withforwardbias

Ec

EvEc

Ev

EFEF

pn+ p

∆Ec

GaAs AlGaAsAlGaAs

ppn+

~ 0.2 µm

GaAs AlGaAsAlGaAs

(a)

(b)

(c)

(d)

Fig. 4 A double-heterostructure LED. (a) adouble-heterostructure diode has two junctions that arebetween two different bandgap semiconductors (GaAs andAlGaAs); (b) a simplified energy band diagram withexaggerated features. EF must be uniform; (c) forward-biasedsimplified energy band diagram; (d) forward-biased LED.Schematic illustration of photons escaping reabsorption inthe AlGaAs layer and being emitted from the device

active region. Since the energy spreadin each case is ∼2kT (where k isthe Boltzmann constant and T is thetemperature), the spectral linewidth of theemission corresponds to a photon energyspread of a few kT. As the temperatureincreases, the linewidth �λ gets wider andthe peak emission wavelength shifts tolonger wavelengths, since the bandgap Egdecreases with temperature as apparent inFig. 5.

2.2Semiconductor Laser Diodes

2.2.1 Double-heterostructure LaserDiodesA laser diode is a semiconductor diodewhich emits coherent radiation in contrastto an LED which emits incoherent radia-tion. Laser diodes operate on the principleof stimulated emission resulting fromphoton-induced direct recombination of

Optoelectronics 9

800 900

−40°C

0

1

740

Relative spectral output power

840 880Wavelength (nm)

25°C

85°C

760

Fig. 5 The output spectrum from an AlGaAsLED. Values normalized to peak emission at25 ◦C

injected electrons and holes under forwardbias, as illustrated in Fig. 6(a). All practi-cal semiconductor laser diodes are eitherbased on the DH or on quantum wells(QWs). A highly simplified energy band di-agram of a forward-biased DH laser diodeis shown in Fig. 6(b) and (c) and is sim-ilar to the LED diagram in Fig. 4. In thiscase, the semiconductors are AlGaAs withEg ≈ 2 eV and GaAs with Eg ≈ 1.4 eV. Thep-GaAs region is a thin layer, typically 0.1 to0.2 µm, and constitutes the active layer inwhich stimulated emissions and hence op-tical amplification take place. Both p-GaAsand p-AlGaAs are heavily p-type doped andare degenerate with the Fermi level in theVB. When a sufficiently large forward biasis applied, Ec of n-AlGaAs moves above Ec

of p-GaAs which leads to a large injectionof electrons from the CB of n-AlGaAs intothe CB of p-GaAs as shown in Fig. 6(c).These electrons, however, are confined tothe CB of p-GaAs since there is a PE barrier�Ec between p-GaAs and p-AlGaAs due tothe change in the bandgap. Carrier confine-ment is the restriction of injected chargecarriers to a small volume to increase the

2 eV

Holes in VB(c)

Electrons in CB

1.4 eV

Ec

Ev

Ec

Ev

Ec

2 eVStimulatedemissions

GaAs

(b)

AlGaAsAlGaAs

pn p

(~0.1 µm)

+−

Energy

(a)

Ec

Ev

CB

VB

Density of states

Electronsin CB

Holes in VB= empty states

EFn

EFp

hno

Fig. 6 The principle of operation of thedouble-heterostructure laser diode. (a) Thedensity of states and energy distribution ofelectrons and holes in the conduction andvalence bands in the active layer, andphoton-stimulated electron and holerecombination; (b) a double-heterostructurediode has two junctions which are between twodifferent bandgap semiconductors (GaAs andAlGaAs); (c) a simplified energy band diagramunder a large forward bias. Lasing recombinationtakes place in the p-GaAs layer, the active layer

carrier concentration. A confining layer isa layer with a wider bandgap than the ac-tive layer, and adjacent to it, to confinethe injected minority carriers to the activelayer.

The p-GaAs layer is degenerately doped.Thus, the top of its VB is full of holes, or ithas all the electronic states empty above theFermi-level EFp in this layer as shown in

10 Optoelectronics

Fig. 6(a). The large forward bias injects avery large concentration of electrons fromn-AlGaAs into the CB of p-GaAs. Con-sequently, as shown in Fig. 6(a), there isa large concentration of electrons in theCB and totally empty states at the topof the VB, which means that there is apopulation inversion. An incoming photonwith an energy hυo (where h is Planck’sconstant and υo is the photon frequency)just above Eg can stimulate a conduc-tion electron in the p-GaAs layer to falldown from the CB to the VB and emita photon by stimulated emission as de-picted in Fig. 6(a). Such a transition isa photon-stimulated electron–hole recom-bination, or a lasing recombination. Thus,an avalanche of stimulated emissions inthe active layer provides an optical ampli-fication of photons with energy hυo in thislayer. The amplification depends on theextent of population inversion and henceon the diode forward current. There isa threshold current Ith below which thereis no stimulated emission and no opticalamplification as illustrated in Fig. 7. Anyemission below Ith is due to spontaneous

emission and the device operates as anLED. Figure 7 compares the output char-acteristics of a laser diode and an LED.

To construct a semiconductor laserwith a self-sustained lasing emission,we have to incorporate the active layerinto an optical cavity. The optical cavitywith reflecting ends reflects the coherentphotons back and forward, and encouragestheir constructive interference within thecavity. This leads to a build-up of high-energy electromagnetic (EM) oscillationsin the cavity. Some of this energy is tappedout by having one end of the cavity aspartially reflecting. For example, one typeof optical cavity has a dielectric mirrorat one end of the crystal and has theother crystal-end polished. Further, widerbandgap semiconductors generally havelower refractive indices, which means thatAlGaAs has a lower refractive index thanGaAs. The change in the refractive indexdefines an optical dielectric waveguidethat confines the photons to the activeregion of the optical cavity and therebyreduces photon losses and increases thephoton concentration. This increase in the

Current (mA)

0

Lig

ht p

ower

(m

W)

Laser diode (coherent radiation)

LED(incoherent radiation)

10050

10

20

Threshold current

Spontaneousemission

Stimulated emission

0

Fig. 7 Typical optical power output vs. forward current for anLED and a laser diode

Optoelectronics 11

photon concentration increases the rate ofstimulated emissions and the efficiency ofthe laser.

Buried double-heterostructure laserdiode is a good example of a double-heterostructure semiconductor laserdevice that has its active region ‘‘buried’’within the device in such a way thatit is surrounded by low refractive indexmaterials rendering the active region as awaveguide as shown in Fig. 8. Inasmuchas the active layer is surrounded by alower index AlGaAs, it behaves as adielectric waveguide and ensures that thephotons are confined to the active or theoptical gain region, which increases therate of stimulated emission and hencethe efficiency of the diode. Since theoptical power is confined to the waveguidedefined by the refractive index variation,these diodes are said to be index guided.Further, if the buried heterostructure hasthe right dimensions compared with thewavelength of the radiation, then onlythe fundamental mode can exist in thiswaveguide structure as in the case ofdielectric waveguides. This would be thecase in a single-mode laser diode.

The laser diode heterostructures basedon GaAs and AlGaAs are suitable for emis-sions around 900 nm. For operation in theoptical communication wavelengths of 1.3and 1.55 µm, typical heterostructures are

based on InP (substrate) and quarternaryalloys InGaAsP where InGaAsP alloyshave a narrower bandgap than that of InPand a greater refractive index. The compo-sition of the InGaAsP alloy is adjusted toobtain the required bandgap for the activeand confining layers.

2.2.2 Quantum Well and Quantum DotLasersA quantum well (QW) device has an ul-tra thin, typically less than 20 nm, narrowbandgap semiconductor, such as GaAs,sandwiched between two wider bandgapsemiconductors, such as AlGaAs whichis a heterostructure device as shown inFig. 9(a). Since the bandgap, Eg, changesat the interface, there are discontinuitiesin Ec and Ev at the interfaces. These dis-continuities, �Ec and �Ev, depend on thesemiconductor materials and their doping.In the case of GaAs/AlGaAs heterostruc-ture, which is shown in Fig. 9(b), �Ec isgreater than �Ev. Because of the PE bar-rier, �Ec, conduction electrons in the thinGaAs layer are confined in the z-direction.This confinement length d is so small thatwe can treat the electron as in a one-dimensional PE well in the z-direction,but as if it were free in the xy plane.The electrons in the CB of GaAs form atwo-dimensional (2-D) electron gas. Theconduction electron in the GaAs well has

Oxide insulationElectrode

n-AlGaAs

p+-AlGaAs (contacting layer)

n -GaAs (substrate)

p -GaAs (active layer)n -AlGaAs (confining layer)

p -AlGaAs (confining layer)

Electrode

Fig. 8 Schematic illustration of the cross sectional structureof a buried heterostructure laser diode

12 Optoelectronics

AlGaAs AlGaAs

GaAs

(a) (b) (c)

xy

z

d

Ec

Ev

d

E1

E2

E3

g(E)Density of states

E

BulkQW

E

∆Ec

BulkQW

∆Ev

AlGaAs AlGaAsGaAs

E ′1

Fig. 9 A quantum well (QW) device.(a) Schematic illustration of a QW structure inwhich a thin layer of GaAs is sandwichedbetween two wider bandgap semiconductors(AlGaAs); (b) the conduction electrons in the

GaAs layer are confined (by �Ec) in thez-direction to a small length d so that theirenergy is quantized; (c) the density of states of atwo-dimensional QW. The density of states isconstant at each quantized energy level

the allowed energies E1, E2, E3 . . . above Ec

and the hole energies are at E′1, E′

2 E′3 . . .

below Ev, due to the quantization in thez-direction (the energy due to electron mo-tions in the x and y directions are smalland add onto E1, E2, E3, . . .). The density ofelectronic states for the two-dimensionalelectron system is not the same as thatfor the bulk semiconductor. For a givenelectron concentration n, the density ofstates g(E), that is, the number of quan-tum states per unit energy per unit volumeis constant and does not depend on the en-ergy. The density of states for the confinedelectron and that in the bulk semiconduc-tor are shown schematically in Fig. 9(c).g(E) is constant from E1 until E2 where itincreases as a step and remains constantuntil E3, where again it increases as a stepby the same amount and at every value ofEn. Density of states in the VB behaves sim-ilarly. Since there is a finite and substantialdensity of states at E1, the electrons in theCB do not have to spread far in energy tofind states. In the bulk semiconductor, onthe other hand, the density of states at Ec iszero and increases slowly with energy (as

E1/2) which means that the electrons arespread more deeply into the CB in searchfor states. A large concentration of elec-trons can easily occur at E1 whereas thisis not the case in the bulk semiconductor.Similarly, the majority of holes in the VBwill be at E′

1 since there are sufficient statesat this energy. Under a forward bias, elec-trons are injected into the CB of the GaAslayer, which serves as the active layer. Theinjected electrons readily populate the am-ple number of states at E1, which meansthat the electron concentration at E1 in-creases rapidly with current and hencepopulation inversion occurs quickly with-out the need for a large current to bringin a great number of electrons. Stimulatedtransitions of electrons from E1 to E′

1 leadto a lasing emission. The threshold cur-rent density for population inversion andhence lasing emission is markedly reducedwhen compared to that of a DH laser, byalmost a factor of 10. Multiple quantum well(MQW) lasers have more than one QW andthe QWs form a periodic structure as inFig. 10. The smaller bandgap layers are theactive layers where electron confinement

Optoelectronics 13

Active layer Barrier layerEc

EvGaAs AlGaAs GaAs AlGaAs GaAs

Fig. 10 A multiple quantum well (MQW)structure. Electrons are injected by the forwardcurrent into active layers which are quantumwells

and lasing transition take place whereasthe wider bandgap layers are the barrierlayers. Many commercially available laserdiodes are currently MQW devices.

Currently, there is much interest in de-veloping quantum dot (QD) lasers. A QD isa crystal that is so tiny in all dimensionsthat the conduction electrons are confinedin the x-, y- and z-directions. The electronenergy in the QD is quantized in a similarfashion to the electron energy in a finitethree-dimensional PE well that is well de-scribed in quantum mechanics. Quantumeffects become apparent when the crystalsize is several nanometers, typically lessthan 10 nm. Electron quantization effectsassociated with tiny microcrystals of II–VIsemiconductors (such as CdS, CdSe, ZnS,etc.) in glasses are well documented. InAsquantum dots of the order of 10 nm indimension can form in a self-organizedfashion when InAs is being grown onGaAs substrates. The strain mismatch be-tween the two crystals forces the InAs tocluster together into a tiny crystal. Thepreparation of technologically useful QDsis still under intense research. QD lasershave a number of distinct theoretical ad-vantages. Compared with QW lasers, QDlasers have the lowest threshold currentdensities and have the narrowest spectralemission spectrum. Further, the threshold

current for a QD laser is expected to showvery little temperature dependence com-pared with QW lasers. An InAs QD laseremitting near 1.3 µm with a threshold cur-rent as low as 24 A cm−2 has already beendemonstrated; current extensive researchand development will undoubtedly bringQD lasers into viable commercialization inthe future.

2.2.3 Single Frequency LasersIdeally, the output spectrum from a laserdevice should be as narrow as possible,which generally means that we have to al-low only a single mode to exist and alsoreduce cavity losses from end reflectionsin a Fabry–Perot cavity-based laser. Thereare a number of device structures that op-erate with an output spectrum that has ahigh modal purity. One of the most impor-tant semiconductor single-mode lasers isthe distributed feedback (DFB) laser. In aFabry–Perot cavity laser, the crystal facesprovide the necessary optical feedback intothe cavity to build up the photon concentra-tion. In a DFB laser, as shown in Fig. 11(a),there is a corrugated layer, called the guid-ing layer, next to the active layer; radiationspreads from the active layer to the guidinglayer. These corrugations in the refrac-tive index act as optical feedback over thelength of the cavity by producing partialreflections. Thus optical feedback is dis-tributed over the cavity length. In the DFBstructure, traveling waves are reflected par-tially and periodically as they propagate.The left and right traveling waves can onlycoherently couple to set up a mode if theirfrequency is related to the corrugation pe-riodicity �, taking into account that themedium alters the wave-amplitudes viaoptical gain. The allowed DFB modes arenot exactly at Bragg wavelengths but aresymmetrically placed at about λB. The rel-ative threshold gain for higher modes is so

14 Optoelectronics

Active layer

Corrugated grating

Guiding layer

Optical power

0.1 nm

l (nm)l

Λ

lB

Ideal lasing emission

(a) (b) (c)

Fig. 11 (a) Distributed feedback (DFB) laser structure; (b) ideal lasing emission output; (c) typicaloutput spectrum from a DFB laser

large that only the lowest mode effectivelylases. A perfectly symmetric device hastwo equally spaced modes placed aroundλB as shown in Fig. 11(b). In reality, ei-ther inevitable asymmetry introduced bythe fabrication process, or asymmetry in-troduced on purpose, leads to only one ofthe modes to appear as shown in Fig. 11(c).There are various commercially availablesingle-mode DFB lasers in the market withspectral widths of ∼0.1 nm at the commu-nications channel of 1.55 µm.

2.2.4 Vertical Cavity Surface EmittingLasers (VCSELs)Vertical cavity surface emitting lasers (VC-SELs) have the optical cavity axis alongthe direction of current flow rather thanperpendicular to the current flow as inconventional laser diodes. The active re-gion length is very short compared withthe lateral dimensions so that the ra-diation emerges from the ‘‘surface’’ ofthe cavity rather than from its edge asshown in Fig. 12. The reflectors at theends of the cavity are dielectric mirrorsmade from alternating high and low refrac-tive index quarter-wave thick multilayers.Such dielectric mirrors provide a high de-gree of wavelength-selective reflectance atthe required free surface wavelength λ ifthe thicknesses of alternating layers d1

and d2 with refractive indices n1 and n2

are such that n1d1 + n2d2 = (1/2)λ which

Contact

Surface emission

−

+

Dielectricmirror

Contact

Substrate

l/4n1

Active layer

l/4n2 Dielectricmirror

Fig. 12 A simplified schematic illustration of avertical cavity surface emitting laser (VCSEL)

then leads to the constructive interferenceof all partially reflected waves at the inter-faces. Since the wave is reflected becauseof a periodic variation in the refractive in-dex as in a grating, the dielectric mirroris essentially a distributed Bragg reflector.High reflectance end mirrors are neededbecause the short cavity length reduces theoptical gain of the active layer. There maybe 20 to 30 or so layers in the dielectricmirrors to obtain the required reflectance(∼99%). The whole optical cavity looks‘‘vertical’’ if we keep the current flow thesame as in a conventional laser diode cav-ity. The active layer is generally very thin(<0.1 µm) and is likely to be a MQW for

Optoelectronics 15

improved threshold current. The requiredsemiconductor layers are grown by epitax-ial growth on a suitable substrate that istransparent in the emission wavelength.For example, a 980-nm emitting VCSELdevice has InGaAs as the active layer toprovide the 980-nm emission, and a GaAscrystal is used as substrate that is trans-parent at 980 nm. The dielectric mirrorsare then alternating layers of AlGaAs withdifferent compositions and hence differ-ent bandgaps and refractive indices. Thetop dielectric mirror is etched after all thelayers have been epitaxially grown on theGaAs substrate. The vertical cavity is gen-erally circular in its cross section so that theemitted beam has a circular cross sectionwhich is an advantage. The height of thevertical cavity may be as small as severalmicrons. Therefore, the longitudinal modeseparation is sufficiently large to allow onlyone longitudinal mode to operate. How-ever, there may be one or more lateral(transverse) modes depending on the lat-eral size of the cavity. In practice, thereis only one lateral mode (and hence onemode) in the output spectrum for cavitydiameters less than ∼8 µm. Various VC-SELs in the market have several lateralmodes but the spectral width is still only∼0.5 nm, substantially less than a conven-tional longitudinal multimode laser diode.With cavity dimensions in the micronsrange, such a laser is referred to as amicrolaser. One of the most significant ad-vantages of microlasers is that they canbe arrayed to construct a matrix emitter,which is a broad area surface emitting lasersource. Such laser arrays have importantpotential applications in optical intercon-nect and optical computing technologies.Further, such laser arrays can provide ahigher optical power than that availablefrom a single conventional laser diode.

Powers reaching a few watts have beendemonstrated using such matrix lasers.

3Optical Fibers as Waveguides

3.1Step-index Optical Fiber Fundamentals

The most important optical waveguide isprobably the optical fiber that serves as thebackbone of optical communications. It isthe most widely used waveguide ‘‘trans-porting’’ light from one point to another.An optical fiber is a thin cylindrical glassfiber, perhaps 100 microns or so in totalthickness (as thin as hair), that acts as adielectric waveguide, that is, guiding lightfrom one end of the guide to the other. Anoptical fiber has a core region of higher re-fractive index n1 and a surrounding region,cladding, of lower refractive index n2, which‘‘clads’’ the core as depicted in Fig. 13. Inthe case of a step-index fiber, the changein the refractive index from the core tothe cladding occurs as a step and typicallythis change is very small (less than 1%).The normalized refractive index difference� is defined by � = (n1 − n2)/n1 and isless than 0.01. Waveguides that have a verysmall normalized refractive difference be-tween the core and the cladding are calledweakly guiding waveguides. The core regionmust have as low optical attenuation aspossible to allow the transmission of lightthrough the optical fiber.

Light entering the fiber can only prop-agate along the fiber as certain distinctelectric field patterns across the fiber, eachof which is a distinct mode of the fiber. Eachmode of an optical fiber is a distinct trans-verse (to the waveguide axis) electric fieldpattern that can propagate and hence beguided along the waveguide. These modes

16 Optoelectronics

n

y

Cladding

Core Fiber axis

Refractive index

Fig. 13 A step-index optical fiber

Low-order modeHigh-order mode

Cladding

CoreLight pulse

t0

t

Spread, ∆t

Broadenedlight pulse

IntensityIntensity

Fundamental

Waveguide

Fig. 14 Schematic illustration of light propagation in an optical waveguide. Light pulse enteringthe waveguide breaks up into various modes which then propagate at different group velocitiesdown the guide. At the end of the guide, the modes combine to constitute the output light pulsewhich is broader than the input light pulse

are allowed traveling EM field patterns thatsatisfy Maxwell’s equations with the givenwaveguide boundary conditions. We canintuitively represent each mode as two op-positely zigzagging rays that travel alongthe guide by total internal reflection (TIR)at the core-cladding boundary as illustratedin Fig. 14. Notice that the TIR does not oc-cur exactly at the interface because the raypenetrates the cladding a little (by a dis-tance called the penetration depth δ) dueto the existence of an evanescent wave inthe cladding. Rays in Fig. 14 have to sat-isfy the TIR condition at the core-claddinginterface as well as a waveguide conditionthat leads to propagation along the guide.These zigzagging waves cannot interfere

destructively and cancel each other in thecore if they are to propagate; they haveto interfere constructively which meansthat only certain modes can exist. Thesemodes are typically labeled and identifiedby sets of integers, for example, l and mfor step-index fibers. In simple ray terms,the order (magnitudes of l and m) of amode is related to the angle of incidenceat the core-cladding boundary and the or-der increases as this angle decreases; thelowest-order mode, the ‘‘axial’’ or the fun-damental mode, has an incidence angle ofnearly 90◦.

Each mode in a step-index fiber is anallowed propagating electric field distribu-tion Elm(r, φ) that propagates with its own

Optoelectronics 17

y

zx

rElm(r,f)

E

r

E01

Core

Cladding

The electric field of thefundamental mode

(a) (b) (c)

The intensity in thefundamental mode

Cylindrical coordinates

f

Fig. 15 The electric field distribution of the fundamental mode (l = 0 and m = 1) inthe transverse plane to the fiber axis z

particular propagation constant βlm (wavenumber) along the z-axis. This is depictedin Fig. 15 for the fundamental mode, l = 0,m = 1. This mode has a cylindrically sym-metric field distribution about the fiberaxis and is the most important mode inoptical communications. Since each modehas a different propagation constant, themodes propagate with different group ve-locities along the fiber. (This is apparentin Fig. 14 where the modes have differenteffective velocities along the fiber.)

An important characteristic waveguideparameter is the V -number or normalizedfrequency, which determines the nature ofpropagation of EM waves along the guide.For a step-index fiber, it is defined by

V = (2πa/λ)[n21 − n2

2]1/2

≈ (2πa/λ)[2n21�]1/2

where a is the core radius and λ is thefree space wavelength of the radiation.When V < 2.405, then only one mode,the fundamental mode, can propagate and

the fiber is called a single-mode fiber (SMF).When V > 2.405, then the fiber becomesa multimode fiber (MMF) and the numberof modes increases sharply with V . Thecut-off wavelength λc corresponds to thatwavelength which makes V = 2.405. If thewavelength of the light is longer than λc,then the fiber operates as a single-modefiber.

The acceptance angle, or the maximumacceptance angle, αmax, as illustrated inFig. 16, is the largest possible light launchangle from the fiber axis. Light waveswithin the acceptance angle that enter thefiber become guided along the fiber core.The acceptance angle defines an acceptancecone about the fiber axis as depicted inFig. 16. The numerical aperture (NA) isanother characteristic of an optical fiberthat depends on the refractive indices ofthe core and the cladding, and measuresthe light gathering ability of the core. TheNA is defined by

NA = [n21 − n2

2]1/2

18 Optoelectronics

and αmax is given by

sin αmax = [n21 − n2

2]1/2

no= NA

no

where no is the refractive index of themedium from which light is launched intothe fiber.

2amax Optical fiber

Acceptance cone

Fig. 16 The acceptance cone and the maximumacceptance angle αmax

A graded index (GRIN) optical fiberhas a core refractive index that is gradedgradually, that is, changes continuously,towards the cladding. Figures 17(a), (b),and (c) summarize the index profilesand light propagation in single-mode andmultimode step-index fibers, and gradedindex fibers (GIFs). Typically, in a GIF, therefractive index profile is approximatelyparabolic to minimize modal dispersionto a virtually innocuous level. All differentmode rays in the GIF arrive at the sametime; compare Figs. 17(b) and (c). Theintuitive reason for this is that the velocityalong the ray path, c/n, is not constantand increases as the ray is farther awayfrom the center. A ray such as 2 that has a

n1

n2

21

3

(a)

(b)

(c)

nO

n1

21

3

n

n2

OO' O''

n2

23

n1

n2

1 n

Fig. 17 (a) Single-mode step-index fiber (SMF). There is only anaxial ray; (b) multimode step-index fiber (MMF). Ray paths aredifferent so that rays arrive at different times; (c) graded index fiber(GIF). Ray paths are different but so are the velocities along thepaths so that all the rays arrive at the same time

Optoelectronics 19

longer path than ray 1 then experiences afaster velocity during a part of its journey toenable it to catch up with ray 1. Similarly,ray 3 experiences a faster velocity than ray2 during part of its propagation to catch upwith ray 2 and so on.

3.2Dispersion in Optical Fibers

Dispersion in fiber optics is the spread intime �τ , known as temporal broadening,of a very short optical pulse as it propagatesalong the fiber as schematically depictedin Fig. 14. The temporal broadening is dueto the different propagation characteristicsof different wavelength components oflight that are coupled into the fiber andpropagate along the guide. The group delaytime τ is the time it takes for a veryshort light pulse of a particular modeof given wavelength to travel from theinput to the output of the fiber; a ‘‘delaytime’’ from the fiber’s input to output.The temporal spread in τ , �τ is calleddispersion. Owing to the dispersion effect,there is an upper limit for the rate atwhich we can transmit light pulses alonga fiber, that is, there is a maximumbandwidth and maximum bit-rate. Eachmode in the waveguide has a differentpropagation constant so that at the endof the fiber, the modes arrive at differenttimes and constitute an output pulse that isbroadened in time as illustrated in Fig. 14.Modal dispersion increases with the fiberlength and the normalized refractive indexdifference �. It is zero in an SMF thatallows only one mode, the fundamentalmode, to propagate. Multimode step-indexfibers suffer from modal dispersion, whichlimits their use to local area networks.

Waveguide dispersion is the dispersionthat arises because of the wavelengthdependence of the V -number, which

determines the propagation constant in-side the guide. As the radiation fed intothe core has a finite range of wavelengths,�λ, the V -number is not constant, whichleads to fundamental modes with differ-ent wavelengths that propagate at differentvelocities. Waveguide dispersion resultsfrom the guiding properties of the dielec-tric structure, and it has nothing to do withthe frequency (or wavelength) dependenceof the refractive index.

Material dispersions occur as a result ofthe variation of the refractive index n1 ofthe core glass with the wavelength of lightcoupled into the fiber. The propagationvelocity of the guided wave along the fibercore depends on n1 which in turn dependson the wavelength. Its manifestation is theresult of no practical light source beingperfectly monochromatic in that there arewaves in the guide with various free spacewavelengths, that is, a range of λ values.

Profile dispersion is the broadening ofa propagating optical pulse in a fiberas a result of the group velocity of thefundamental mode also depending onthe refractive index difference �, that is,� = �(λ). If � changes with wavelength,then different wavelengths from the sourcewould have different group velocities andexperience different group delays leadingto pulse broadening.

Chromatic dispersion is the time spreadof a propagating optical pulse in an opticalguide due to the wavelength dependence ofall the guide properties. It includes waveg-uide, material, and profile dispersionsadded together. Chromatic dispersion isnormally quoted as dispersion in picosec-ond per nanometer of input light linewidthand per kilometer of fiber, for example,10 ps nm−1 km−1. For an input laser oflinewidth 0.1 nm and for a fiber of 100 km,chromatic dispersion is 100 ps.

20 Optoelectronics

Polarization mode dispersion arises whenthe fiber is not perfectly symmetric and ho-mogeneous, that is, the refractive index isnot isotropic. When the refractive index de-pends on the direction of the electric field,the propagation constant of a given modedepends on its polarization. Polarizationmode dispersion depends on the extent ofanisotropy in the refractive index, which iskept minimum by various manufacturingprocedures (such as rotating the fiber dur-ing drawing, etc.). Typically, polarizationmode dispersion is less than a fraction ofa picosecond per kilometer of fiber. How-ever, this type of dispersion does not scalelinearly with fiber length L; in fact, thedispersion scales roughly as L1/2 and thepolarization mode dispersion in currentfibers is typically 0.05–0.5 ps km−1/2.

Self-phase modulation dispersion is thetemporal spreading of a propagating lightpulse due to the dependence of therefractive index on the light intensity; aconsequence of the nonlinear propertiesof glasses at high fields or large lightintensities. When the propagating lightpulse is very intense, the small changes inthe index can lead to this type of dispersion.

In most applications, optical fiberscarry digital information and what is ofsignificance is the maximum rate at whichbits can be sent, the maximum bit-rateB. The latter quantity depends inverselyon the dispersion �τ of the fiber. Thedispersion itself depends on the linewidth�λ of the input light, the length of thefiber L, and the mechanisms that arecausing the dispersion as summarizedabove. To achieve high bit rates, thedispersion has to be minimized. In thelate eighties and early nineties, there wasa drive to shrink �λ by using narrowlaser linewidths, and to minimize theinherent dispersion in the fiber by tryingdifferent waveguide geometries (index

profiles) and by accurately controllingthe manufacturing processes. The recenttrend however, has been the controlof dispersion by dispersion-compensatingfibers. The fiber for use is produced withsome chromatic dispersion, for example,+10 ps nm−1 km−1 and dispersion isallowed to build upto say, �τ1 over acertain length, L1. Then, dispersion �τ1

is ‘‘cancelled’’ by using a second fiber,called compensating fiber, that has negativedispersion characteristics with respect tothe first, that is, modes that travel fast in thefirst fiber are slow in the second fiber. Theintroduction of dense wavelength divisionmultiplexing (DWDM) has led to certainrequirements not only on the magnitudeof dispersion but also on its dependenceon the wavelength (‘‘dispersion slope’’).Consequently, dispersion management infibers is one of the most active areas ofcurrent research development.

The dispersion shifted fiber is a fiber inwhich the chromatic dispersion character-istic (dispersion vs wavelength) has beenshifted to longer wavelengths by adjustingthe waveguide dispersion by appropriatelychanging the waveguide geometry or therefractive index profile. Zero dispersionshifted fiber has its chromatic dispersionzero at around 1550 nm. Nonzero disper-sion shifted fiber is designed for use withWDM (wavelength division multiplexing)and has its chromatic dispersion zero out-side the Er-doped amplifier band, 1525 to1620 nm.

3.3Attenuation in Optical Fibers

As light propagates through an opticalfiber, it becomes attenuated by a number ofprocesses that depend on the wavelength oflight. Attenuation is the decrease in the op-tical power of a traveling wave, or a guided

Optoelectronics 21

wave in a dielectric waveguide, in thedirection of propagation due to scatteringand absorption. Scattering is a processby which the energy from a propagatingEM wave is redirected as secondary EMwaves in various directions away from theoriginal direction of propagation. Rayleighscattering of waves in a medium ariseswhenever there are small inhomogeneousregions in which the refractive index is dif-ferent from that of the medium (which hassome average refractive index). This meansa local change in the relative permittivityand polarizability. The result is that thesmall inhomogeneous region acts like asmall dielectric particle and scatters thepropagating wave in different directions.In the case of optical fibers, dielectric in-homogeneities arise from fluctuations inthe relative permittivity that is part of theintrinsic glass structure. As the fiber isdrawn by freezing a liquid-like flow, ran-dom thermodynamic fluctuations in thecomposition and structure that occur inthe liquid state become frozen into thesolid structure. Consequently, the glassfiber has small fluctuations in the relative

permittivity, which leads to Rayleigh scat-tering. Nothing can be done to eliminateRayleigh scattering in glasses, as it is partof their intrinsic structure. Rayleigh scat-tering decreases as λ4 and depends on theextent of ‘‘structural randomness’’ in theglass structure.

In conventional fibers, there is a markedattenuation peak centered at around1.4 µm as apparent in Fig. 18. This at-tenuation region arises from the presenceof hydroxyl ions as impurities in the glassstructure inasmuch as it is difficult to re-move all traces of hydroxyl (water) productsduring fiber production. Further, hydro-gen atoms can easily diffuse into the glassstructure at high temperatures during pro-duction, which leads to the formation ofhydrogen bonds in the silica structure andOH− ions. Absorption of energy is mainlyby the stretching vibrations of the OH−bonds within the silica structure that hasa fundamental resonance in the infraredregion (beyond 2.7 µm) but overtonesor harmonics at shorter wavelengths (orhigher frequencies). The first overtone ataround 1.4 µm is the most significant. The

0.2

0.4

0.6

01280 1320 1360 1400 1440 1480 1520 1560 1600

O E S C L

Atte

nuat

ion

(dB

km

−1)

Band

Wavelength l (nm)

Standard single-mode fiber

Low-water peak fiber

Rayleigh scatteringOH− absorption peak

1640

Fig. 18 Attenuation in a standard single-mode optical fiber and also in a newlydeveloped low-water peak fiber. (Lucent; Lindstrom, A. (2002), Optical fiber: where is itheaded?, Photonics Spectra, Vol. 36. Pittsfield, MA: Laurin Publishing Company,pp. 68–72)

22 Optoelectronics

OH− peak at 1.4 µm has prevented the useof the E-band in optical communications.However, recent developments in fibermanufacturing have almost totally elim-inated the water peak as apparent inthe attenuation versus wavelength char-acteristic of a low-water peak fiber, newlydeveloped by Lucent, in Fig. 18.

Bending losses arise whenever a fiberis bent. Microbending loss occurs typicallywhenever the radius of curvature of thebend is microscopically sharp, typicallyless than 0.1 to 1 mm. It is due to a sharplocal bending of the fiber, for example,due to a ‘‘kink’’ in the fiber that mayoccur during cabling, which changes theguide geometry and refractive index profilelocally, and which therefore leads to someof the light energy being radiated awayfrom the guiding direction. The bendingloss increases exponentially as the radiusof curvature decreases, that is, as thebend becomes sharper. Macrobending lossis the attenuation in the fiber when thefiber’s direction is bent from a straightline over macroscopic distances, typicallywhen the radius of curvature is greaterthan ∼1 mm, for example, when the fiber

cable is curved around a corner. Bendinglosses are normally ignored when the bendradius is more than 100 mm or so.

3.4Cables

Optical fibers are always cabled in use,that is, they are surrounded by othermaterial for protection. A bare fiber caneasily ‘‘crack’’ and break when it is bent orstretched. The fracture is usually a brittlefracture that initiates at a flaw, such asa microcrack, on the fiber surface. It istherefore essential to protect the fiber bycabling it, which makes it easier to handle.As much as possible, the cable preventsthe fiber from experiencing crushing andtensile forces that would otherwise damagethe fiber. The cable also protects the fiberfrom heat, cold, moisture, and variousspecies of chemicals that may be present inthe atmosphere. An optical cable can carryone to hundreds of fibers. Usually, thefibers are placed in a cushioned or bufferedtube inside the cable as depicted in Fig. 19.There are various types of cables in themarket, each with its own particular designand structure that depends on its use, for

Optical fiber

Central member

Buffer tube

Yarn antibucklingstrength member

PVC or PE sheath

Binding tape

Fig. 19 A schematic sketch (not to scale) of a typical indoorstranded loose tube cable carrying many individual fibers

Optoelectronics 23

FiberCable

Ferruleholds the fiber

Connector body

Bend relief bootCoupler

Alignment sleeve

Fig. 20 A typical connector

example, indoor, outdoor, underground,or submarine applications.

3.5Connectors and Splices

In many optoelectronic applications, wehave to connect optical fibers or cables.Connectors are primarily mechanical de-vices that temporarily bring fiber endstogether to couple light from one to theother; an example is shown in Fig. 20. Theconnection is temporary because the con-nector can be ‘‘unplugged.’’ The connectormay contain an optical component such asa lens, or lenses, to couple the light fromone fiber to the other.

A splice is a permanent connectionbetween two fibers, usually achieved byaligning the fibers and then fusing the twoglasses to each other at a high temperaturein an electrical arc between two electrodes.Whenever two fibers are to be connectedpermanently, for example, when layingdown additional fiber or repairing a brokenfiber, they are always spliced. Splicing isachieved by an instrument called the fusionsplicer.

One of the most important character-istics of connectors and splices is theirattenuation, that is, how much light is lostthrough the connection. There are vari-ous reasons for the attenuation, the most

significant are misalignment of fibers,mismatch of core areas, mismatch in theNAs, dead space between fibers, fiber-endreflections, extraneous materials such asdirt or grease on the fibers, and so on.Splices normally have much lower attenu-ation than connectors and are thereforepreferred whenever the connection canremain permanently. An important char-acteristic of a connector is its insertion loss,that is, its attenuation. Typically a splicewill have an insertion loss less than 0.1 dB.On the other hand, mechanical connectorstend to have substantially higher lossesthat typically range from 0.2 dB to 1 dB,depending on the type of connector. AnST (straight tip) connector for an SMFwould have a loss of about 0.40 dB.

4Optical Amplifiers

An Optical amplifier is an amplifierthat amplifies an optical signal, thatis, light. A common type of opticalamplifier is an erbium ion doped opticalfiber amplifier (EDFA), in which erbiumions in the glass are pumped to anexcited state by using a laser diode (oran LED) at 980 nm. This pumping leadsto a population inversion of Er-ions thatcorresponds to a narrow wavelength range

24 Optoelectronics

of around 1550 nm, which is the long-haulcommunications wavelength. Figure 21illustrates schematically how EDFAs areused in communications.

An optical semiconductor amplifier is asemiconductor laser structure that can beused as an optical amplifier that amplifieslight waves passing through its active re-gion. Light to be amplified is made to enterthe active region of the laser semiconduc-tor diode and the active region is pumpedwith a sufficiently large diode current justas in a normal laser diode. The wavelengthof radiation to be amplified must be withinthe optical gain bandwidth of the laser.Such a device would not be a laser os-cillator, emitting lasing emission withoutan input, but an optical amplifier with in-put and output ports for light entry andexit. In the traveling wave semiconductorlaser amplifier, as in Fig. 22(a), the ends of

the optical cavity have antireflection (AR)coatings so that the optical cavity doesnot act as an efficient optical resonator, acondition for laser oscillations. Light, forexample, from an optical fiber is coupledinto the active region of the laser struc-ture. As the radiation propagates throughthe active layer, optically guided by thislayer, it becomes amplified by the inducedstimulated emissions, and leaves the op-tical cavity with a higher intensity. TheFabry–Perot laser amplifier, as shown inFig. 22(b), is similar to the conventionallaser oscillator, but is operated below thethreshold current for lasing oscillations;the active region has an optical gain butnot sufficient to sustain a self-lasing out-put. Light passing through such an activeregion will be amplified by stimulatedemissions, but because of the presence ofan optical resonator, there will be multiple

Signal in Signal outSplice

Er3+-dopedfiber (10−20 m)

Wavelength-selectivecoupler

Pump laser diode l = 980 nm

Splice

l = 1550 nm l = 1550 nm

Termination

Opticalisolator

Opticalisolator

Fig. 21 Er-doped optical fiber amplifiers are widely used for optical amplification

Pump current

Active region

AR = Antireflectioncoating

AR

Signal in Signal out

Traveling wave amplifier

Partial mirror Partial mirror

Fabry−Perot amplifier(a) (b)

Fig. 22 Simplified schematic illustrations of two types of laser amplifiers

Optoelectronics 25

internal reflections. These multiple re-flections lead to the gain being highestat the resonant frequencies of the cavitywithin the optical gain bandwidth. Opticalfrequencies around the cavity-resonant fre-quencies will experience higher gain thanthose away from resonant frequencies. Al-though the Fabry–Perot laser amplifiercan have a higher gain than the travelingwave amplifier, it is less stable.

A Raman fiber amplifier is a fiber am-plifier that is based on the stimulatedRaman scattering effect. In Raman scat-tering, a light wave of frequency υP isscattered from the molecular vibrations ina medium as a result of which its frequencyis downshifted to υS. This is an inelasticinteraction and depends on the nonlineardielectric behavior of the medium. Thechange in the photon energy from hυPto hυS excites the molecular vibrations inthe interaction region by a phonon en-ergy, that is, hυR where υR is the Ramanshift frequency, a resonance frequency as-sociated with molecular oscillations of themedium. The photon hυS is called theStokes photon, and the downshift in the fre-quency is called the Stokes shift. For silica,υR is not actually a single frequency buta range of frequencies, a ‘‘band,’’ with apeak at about 13 to 14 THz. Suppose thatwe send a pump laser beam at frequencyυP into a fiber to excite the molecular vi-brations, and suppose that the pump beamis very intense. (In practice, this would besomething like 1 W fed into an SMF). Ifthe relatively weak incoming signal hasa frequency υS, then it can stimulate theStokes shift and hence extract energy fromthe pump beam to add to itself. The resultis an optical amplification of the signalat υS. Since the discovery of the EDFA,the interest in Raman fiber amplifiers hassomewhat waned due to long fiber lengthsand high pump powers that are required

to achieve acceptable optical gains. One oftheir advantages is that they have a wideoptical bandwidth, and unlike EDFA theyare not restricted to the 1550-nm range.

5Modulators

It is possible to modulate the intensityof light directly by modulating the drivingcurrent of a light emitter such as an LEDor a laser diode. In direct modulation, theintensity of light generated is controlledby an electrical signal. Direct modulationis widely used in optical communica-tions. There are certain drawbacks todirect modulation. It is not possible to ob-tain high-speed modulation, typically morethan ∼10 GHz. The reason is that the mod-ulation is limited by the relaxation timeassociated with the charge carriers andphotons. Secondly, modulation introduceschirping in which the frequency of the lightpulse varies over the duration of the signal.

External modulation involves modulat-ing a characteristic of light, such asintensity, phase, frequency, or direction,as the light passes through the modula-tor. There are various types of modulatorsbased on the physical effect utilized, mostimportant of which are the electro-optic(EO), acousto-optic (AO), magneto-optic, andmultiple quantum well (MQW) basedelectro-absorption (EA) modulators.

5.1Electro-optic Modulators

Electro-optic effects refer to changes inthe refractive index of a material inducedby the application of an external electricfield, which therefore ‘‘modulates’’ theoptical properties; the applied field is notthe electric field of any light wave, but a

26 Optoelectronics

separate external field. We can apply suchan external field by placing electrodes onopposite faces of a crystal and connectingthese electrodes to a battery. The presenceof such a field distorts the electron motionin the atoms or molecules of the substance,or distorts the crystal structure resultingin changes in the optical properties. Forexample, an applied external field cancause an optically isotropic crystal suchas GaAs to become birefringent. In thiscase, the field induces principal axes andan optic axis. Typically, changes in therefractive index are small. The frequencyof the applied field has to be such thatthe field appears static over the time scaleit takes for the medium to change itsproperties, that is, to respond, as well as forany light to cross the substance. The EOeffects are classified according to first andsecond order effects. If we were to take therefractive index n to be a function of theapplied electric field E, that is, n = n(E),we can of course expand this as a Taylorseries in E. The new refractive index n′would be

n′ = n + a1E + a2E2 + · · ·

where the coefficients a1 and a2 are calledthe linear EO effect and second order EOeffect coefficients respectively. Althoughwe would expect even higher terms in theexpansion of the above equation, theseare generally very small and their effectsnegligible within most highest practicalfields. The change in n due to the first Eterm is called the Pockels effect. The changein n due to the second E2 term is calledthe Kerr effect, and the coefficient a2 isgenerally written as λK where K is calledthe Kerr coefficient. Thus, the two effectsare summarized by

�n = a1E

and�n = a2E2 = (λK)E2

All materials exhibit the Kerr effect. Itmay be thought that we will always findsome (nonzero) values for a1 for all ma-terials but this is not true and onlycertain crystalline materials exhibit thePockels effect. Thus a1 = 0 for all non-crystalline materials (such as glasses andliquids). Similarly, if the crystal struc-ture has a center of symmetry, thena1 is again zero. Only crystals that arenoncentrosymmetric exhibit the Pockels ef-fect. For example, an NaCl crystal (cen-trosymmetric) exhibits no Pockels effectbut a GaAs crystal (noncentrosymmetric)does.

One of the simplest examples of Pock-els effect in an optoelectronic applicationis the polarization modulator shown inFig. 23, where an embedded waveguidehas been fabricated by implanting anLiNbO3 substrate with Ti atoms, which in-creases the refractive index. Two coplanarstrip electrodes run along the waveguideand enable the application of a transversefield Ea to the light propagation directionz. The external modulating voltage V(t) isapplied between the coplanar drive elec-trodes, and by virtue of the Pockels effect,induces a change �n in the refractive indexand hence, a voltage-dependent phase shiftthrough the device. We can represent lightpropagation along the guide in terms oftwo orthogonal modes, Ex along x and Ey

along y. These two modes experience sym-metrically opposite phase changes. Thephase shift �φ between the Ex and Ey

polarized waves is proportional to the re-fractive index change, hence to the appliedfield. However, the applied field is notuniform between the electrodes, and fur-ther, not all applied field lines lie insidethe waveguide. The EO effect takes place

Optoelectronics 27

V(t)Coplanar strip electrodes

EO Substratez

y

x

Polarizedinputlight

Waveguide

Ea

Cross sectionLiNbO3

LiNbO3

d

L

Thin bufferlayer

Fig. 23 Integrated transverse Pockels cell phasemodulator in which a waveguide is diffused intoan electro-optic (EO) substrate. Coplanar stripelectrodes apply a transverse field Ea through thewaveguide. The substrate is an x-cut LiNbO3 and

typically there is a thin dielectric buffer layer(e.g., ∼200-nm-thick SiO2) between the surfaceelectrodes and the substrate to separate theelectrodes away from the waveguide

over the spatial overlap region betweenthe applied field and the optical fields.Nonetheless, �φ is proportional to V/d(apparent applied field) and the length Lof the channel (Fig. 23) through the rel-evant Pockels coefficient r. The device isa phase modulator; �φ is modulated byV(t). For example, if the voltage inducesa phase change of π/2, then a linearlypolarized light at an angle 45◦ to thex-axis can be converted into a circularlypolarized light as depicted in Fig. 23. Thevoltage that is necessary to induce a half-wave phase shift (�φ = π ) is called thehalf-wave voltage, Vλ/2. At λ = 1.5 µm, foran x-cut LiNbO3 modulator as in Fig. 23with d ≈ 10 µm, Vλ/2L ≈ 35 V·cm. For ex-ample, a modulator with L = 2 cm has ahalf-wave voltage Vλ/2 = 17.5 V. By com-parison, for a z-cut LiNbO3 plate, that is,for light propagation along the y-directionand Ea along z, the relevant Pockels coef-ficients are much greater, which leads toVλ/2L ≈ 5 V·cm.

The Mach–Zehnder modulator is alithium niobate–based EO device whoselight transmittance is controlled by anapplied external voltage. It uses the Pockelseffect of LiNbO3 and the interferenceof two waves that have a relative phase

difference, induced by an applied externalvoltage. It converts the induced phase shiftby an applied voltage to an amplitudevariation by using an interferometer, adevice that interferes two waves of thesame frequency but different phase. Asshown in Fig. 24, the device has implantedsingle-mode waveguides in an LiNbO3

(or other EO) substrate in the geometryshown in Fig. 24. The waveguide at theinput branches out at C to two armsA and B and these arms are latercombined at D to constitute the output.The splitting at C and combining at Dinvolve simple Y-junction waveguides. Inthe ideal case, the power is equally splitat C so that the field is scaled by a factor√

2 going into each arm. The structureacts as an interferometer because thetwo waves traveling through the armsA and B interfere at the output port Dand the output amplitude depends on thephase difference (optical path difference)between the A and B branches. Two back-to-back identical phase modulators enablethe phase changes in A and B to bemodulated. Notice that the applied field inbranch A is in the opposite direction to thatin branch B. The refractive index changesare therefore opposite, which means that

28 Optoelectronics

V(t)

LiNbO3EO Substrate

A

BIn

OutC

DA

B

Waveguide

Electrode

Fig. 24 An integrated Mach–Zehnder optical intensity modulator. The input light is splitat C into two coherent waves A and B, which are phase shifted by the applied voltage, andthen the two are combined again at D at the output

the phase changes in arms A and B arealso opposite. For example, if the appliedvoltage induces a phase change of π/2 inarm A, this will be −π/2 in arm B sothat A and B would be out of phase byπ . These two waves will then interferedestructively and cancel each other atD. The output intensity would then bezero. Since the applied voltage controlsthe phase difference between the twointerfering waves A and B at the output,this voltage also controls the output lightintensity, though the relationship is notlinear. It is apparent that the relative phasedifference between the two waves A and Bis therefore doubled with respect to a phasechange φ in a single arm. We can predictthe output intensity by adding waves Aand B at D. The power transfer is zerowhen the phase difference φ is π/2, asexpected. In practice, the Y-junction lossesand uneven splitting result in a less-than-ideal performance; A and B do not totallycancel out when φ = π/2.

There are of course other types of EOmodulators based on the Pockels effect.There are Kerr effect modulators as well.For further reading, the reader is referredto the references.

5.2Acousto-optic Modulators

Acousto-optic modulators are based on thephotoelastic effect in which an inducedstrain (S) in a crystal changes its refractiveindex n. The strain changes the densityof the crystal and distorts the bonds (andhence the electron orbits), which leads to achange in the refractive index n. The strainand refractive index relationship is quitecomplicated, because we must considerthe effect of a strain S along one directionin the crystal on the induced change in nfor a particular light propagation directionand some specific polarization. To a firstorder, we can take the photoeleastic effectto be linear; the induced change �nis proportional to the magnitude of theinduced strain S.

We can generate traveling acoustic orultrasonic waves on the surface of apiezoelectric crystal (such as LiNbO3)by attaching interdigital electrodes ontoits surface, as shown in Fig. 25, andapplying a modulating voltage at radiofrequencies (RF). The piezoelectric effect isthe phenomenon of generation of strain ina crystal by the application of an externalelectric field. The modulating voltage at

Optoelectronics 29

Interdigitally electrodedtransducerModulating RF voltage

Piezoelectriccrystal

Acousticwave fronts

Induced diffractiongrating

Incidentlight

Diffracted light

Through light

Acoustic absorber

2qq

Fig. 25 Traveling acoustic waves create a harmonic variation in therefractive index, and thereby create a diffraction grating that diffracts theincident beam through an angle 2θ

electrodes will therefore generate a surfaceacoustic wave (SAW) via the piezoelectriceffect. These acoustic waves propagateby rarefactions and compressions of thecrystal surface region which lead to aperiodic variation in the density andhence a periodic variation in the refractiveindex in synchronization with the acousticwave amplitude. The result is that akind of ‘‘diffraction grating’’ is set upnear the surface of the crystal by theacoustic wave. This acoustically induceddiffraction grating diffracts the opticalbeam and allows the angle and intensityof the diffraction to be modulated by theamplitude and frequency of the applied RFsignal. There is a change in the frequencyof the diffracted beam by an amountthat corresponds to the phonon (acoustic)frequency.

5.3Multiple Quantum Well Modulators

Electro-absorption is the induced absorp-tion of light in a device as a result of

applying (or changing) an electric fieldwithin the device. There are fundamen-tally three types of EA processes. In theFranz–Keldysh process, a strong appliedfield modifies the photon-assisted proba-bility of an electron tunneling from the VBto the CB and thus corresponds to an ef-fective reduction in the ‘‘bandgap energy.’’It was first observed for CdS in which theabsorption edge was observed to shift tolower energies with the applied field, thatis, photon absorption shifts to longer wave-lengths with the applied field. The effectis normally quite small but nonethelessobservable. In this type of modulation,typically, the wavelength is chosen to beslightly smaller than the bandgap wave-length so that absorption is negligible.When a field is applied, absorption is en-hanced by the Franz–Keldysh effect.

In free carrier absorption, the concentra-tion of free carriers N in a given bandis changed (modulated), for example, byan applied voltage, to change the extentof photon absorption. The absorption co-efficient is proportional to N and to the

30 Optoelectronics

light wavelength λ raised to some power,typically 2 to 3.

In the confined Stark effect, the appliedelectric field modifies the energy levelsin a QW, for example, the energy levelcorresponding to E1 in Fig. 9(b) is alteredby the field. The field reduces the energylevels by an amount proportional to thesquare of the applied field. A MQWpin device has MQWs in its intrinsiclayer between the p and n layers asshown in Fig. 26. Without any appliedbias, light with photon energy just lessthan the QW exciton excitation energy,for example, from E′

1 to E1 in Fig. 9(b),will not be significantly absorbed. Whena field is applied, the energy levelsare lowered and the incident photonenergy is now sufficient to excite anelectron–hole pair in the QWs. The relativetransmission decreases with reverse biasVr applied to the pin device. Such MQW pindevices are usually not very useful in thetransmission mode because typically thesubstrate material is such that it absorbsthe light (for example, GaAs/AlGaAsMQW pin would be grown on a GaAssubstrate that would absorb the radiationthat excites the QWs; see Fig. 9). Thus, a

Vr

p ni

Vr

Relative transmission

100 %

00

MQW

E

Fig. 26 A schematic illustration of an MQWmodulator based on the quantum confined Starkeffect. The i-region has MQWs. The transmittedlight intensity can be modulated by the appliedreverse bias Vr to the pin device, because theelectric field Ez modifies the absorption spectrum

‘‘mirror’’ would be needed to reflect thelight back before it reaches the substrate;such devices have indeed been fabricated.

6Optical Switches, Multiplexers, andIsolators

The optical modulators discussed inSect. 5 can also be used to switch light,that is, change the light output of the mod-ulator from ‘‘transmission’’ to almost ‘‘notransmission.’’ Such an action would con-stitute a simple optical switch because thepassage of light through the device is either‘‘on’’ or ‘‘off,’’ depending on an indepen-dent control signal. In general, one candistinguish between two types of opticalswitching operations. In one type of op-tical switching, the action of the deviceon light corresponds to either one of thedigital logical states, ‘‘0’’ or ‘‘1.’’ Such op-tical devices would constitute the buildingblocks of an optical digital signal proces-sor, an optical computer. In another typeof optical switching, the switch redirectsthe optical signal from one optical path toanother. The latter is the type of opticalswitching that is widely encountered inoptoelectronic systems. A multiplexer is adevice that combines two or more signalsinto a single output (two or more inputsare switched into a single output).

Many optoelectronic systems rely exten-sively on optical switches that can switchthe signal from one optical path to an-other. Figure 27(a) illustrates how a simple4 × 4 optical cross-connect (optical crossbar)switch can switch the light from any inputfiber to any output fiber depending on theelectrical control signal. (Practical cross-connect switches have more input–outputconnections.) The output of each inputfiber is converted to an electrical signal (via

Optoelectronics 31

Control signals (electrical)(a)

(b)

(c)

Input fibers Output fibers

Detector array LED array

Electro-optic directional coupler

VA A′

B B ′

Waveguides

B

C

A

Tilting mirror

Current carryingconductor

Possible MEMS optical switches

Fig. 27 (a) A cross-connect switch or crossbar switch; (b) anelectro-optic directional coupler; (c) MEMS optical switch

photodetectors) and then a programmedlogic circuit drives LEDs (or laser diodes)that emit light into the appropriate outputfibers. This type of switch is limited by theelectronic speed with which the detectorcan respond, and the light emitters can bedriven; depending on the semiconductorsused, typical speeds are less than a fewGb s−1.

Electro-optic directional couplers provide aconvenient and fast means of switching(coupling) an optical signal from one

waveguide (AA′) to another (BB′) by theapplication of an electrical signal V asdepicted in Fig. 27(b); light at the input intoguide A is switched to B′ at the output. Anumber of these EO directional couplersconnected together can obviously switchan optical signal between various opticalpaths.

Opto-mechanical switches rely on a me-chanical motion that moves one guide toline up with another guide and therebychange the optical path; these switches are

32 Optoelectronics

limited by the speed of mechanical motion.The mechanical motion may be imple-mented by an EM relay for example, inwhich case the speed would be of the orderof milliseconds. By having tiny electrome-chanical structures, however, it is possibleto construct fast and reliable switches.The tiny electromechanical structuresare called micro-electro-mechanical systems(MEMS) that are fabricated by vari-ous semiconductor fabrication techniquesthat use special lithographic processes.Figure 27(c) illustrates how an MEMS op-tical switch can tilt a mirror and therebydeflect an optical signal from one guide toanother. Such MEMS-based switches arefast (microseconds) and reliable.

An important operation that is thecornerstone of today’s optical communica-tions is the multiplexing and demultiplex-ing of optical signals. In WDM, N opticalsignals (each one a channel) at variouswavelengths λ1, λ2, . . . λN , are multiplexedinto a single fiber. At the destination, ademultiplexer has to separate out the Nchannels based on their wavelengths. Themultiplexing procedure may be as simple

as focusing the N channels into the inputof a single fiber, but the demultiplexingprocedure needs a ‘‘demultiplexing de-vice’’ that will deflect or diffract light byan angle based on its wavelength. Prism-or diffraction grating–based demultiplex-ers, as shown in Fig. 28, are available; inprinciple, they can also work as multiplex-ers, multiplexing N different wavelengthchannels into a single fiber.

Optical isolators are useful devices thatallow light to pass in one direction and notin the opposite direction. For example, asensitive laser diode connected to a fibercan be isolated from reflected or scatteredlight traveling in the fiber towards thelaser by using an optical isolator. Manycommercial optical isolators are based onusing a simple polarizer and a Faradayrotator, a device that utilizes the Faradayeffect. In the latter effect, the optical fieldE, or the polarization of light passingthrough a medium is rotated by an appliedmagnetic field B placed in the medium.The rotation θ of polarization dependsonly on the direction of B. The directionof light propagation in a Faraday rotator

l1, l2, l3, ... , lN

l1, l2, l3, ... , lN

l1

l2

lN

l3

Optical demultiplexer

l1

l2

lN

l3

Diffraction grating

Prism

Fig. 28 Demultiplexers based on using a prism and adiffraction grating

Optoelectronics 33

(a medium in a magnetic field) does notchange the absolute sense of rotation ofθ . If we reflect the light to pass throughthe medium again, the overall rotationincreases to 2θ , the polarization rotation inthe reverse direction does not cancel that inthe forward direction but adds to it. (Thiseffect is distinctly different from opticalactivity.) Suppose that we use a polarizerand a Faraday rotator in series betweenthe laser diode in the above example andthe fiber. The forward going light from thelaser will first pass through the polarizer,become polarized, and it will then passthough the rotator once. Its polarizationwould be rotated by θ . But any reflectedwave will pass through the rotator twiceand thus have its polarization rotated by 2θ .It then cannot pass through the polarizerand return to the laser.

7Photodetectors

7.1Fundamental Definitions andCharacteristics

Photodetectors convert an incident lightsignal into an electrical signal such asvoltage or current. In many photodetectorssuch as photoconductors and photodiodes,this conversion is typically achieved by thecreation of free EHPs by the absorption ofphotons, that is, the creation of electronsin the CB and holes in the VB. Thefree carriers that are photogenerated driftin the detector due to the presenceof an applied field (E) and therebygenerate an external current, called thephotocurrent Iph, and eventually the carriersare collected by the battery. The basicprinciple of photodetector operation isshown in Fig. 29 where the detector is a

simple photoconductor with electrodes. Ifthe absorbed photon generates one EHP asin Fig. 29, and both carriers are collected,then the total charge Q collected in theexternal circuit is e, the electronic charge.

Two most important photodetectors arethe pin and the avalanche photodiode (APD),which are described below. There are anumber of important detector quantitiesthat characterize the photodetector perfor-mance.

Internal quantum efficiency (QE) of aphotodetector is the number of free EHPsphotogenerated per absorbed photon; thisis not per incident photon on the device.Inasmuch as internal QE is defined interms of per absorbed photon, it is greaterthan external QE, which is defined in termsof per incident photon; not all incidentphotons are absorbed.

The QE η of a detector, or external QE,is defined as the number of free EHPscollected per incident photon. Not all theincident photons are absorbed to create freeEHPs that can be collected and used to giverise to a photocurrent. The efficiency of theconversion process of received photons tofree EHPs is measured by the above QEdefinition. The measured photocurrent Iphin the external circuit is due to the flow ofelectrons per second to the terminals ofthe photodiode. The number of electrons

DetectorR

Iph

V

Vout

e–h+

LightOptical power = Po

Current-voltageamplifer

Fig. 29 A schematic diagram illustrating thebasic principle of operation of a photodetector

34 Optoelectronics

collected per second is Iph/e. If Po is theincident optical power, then the numberof photons arriving per second is Po/hυ.Then the QE η can also be defined by

η = Iph/e

Po/hυ

where h is the Planck’s constant, e isthe electronic charge, and υ is the fre-quency of light. Not all of the absorbedphotons may photogenerate free EHPsthat can be collected. Some EHPs maydisappear by recombination without con-tributing to the photocurrent or becomeimmediately trapped. Further, if the semi-conductor thickness for absorbing the lightis comparable with the penetration depth(1/α, where α is the absorption coefficientof the material), then not all the photonswill be absorbed. The device QE is there-fore always less than unity. It depends onthe absorption coefficient of the semicon-ductor at the wavelength of interest andon the structure of the device. QE canbe increased by reducing the reflectionsat the semiconductor surface, increasingabsorption within the depletion layer, andpreventing the recombination or trappingof carriers before they are collected.

The responsivity is the photocurrentgenerated by a photodetector per unitincident optical power. It depends on theQE and the wavelength of the incidentradiation. The responsivity R of a detectorcharacterizes its performance in termsof the photocurrent generated (Iph) perincident optical power (Po) at a givenwavelength by

R = ηe

hυ

where η is the QE that depends on the lightwavelength λ, and υ is the light frequency.The responsivity therefore clearly dependson the wavelength. R is also called the

spectral responsivity or radiant sensitivity.The R versus λ characteristics representthe spectral response of the photodiodeand is generally provided by the manufac-turer. Ideally, with a QE of 100% (η = 1),R should increase with λ upto λg as de-picted in Fig. 30. In practice, QE limits theresponsivity to lie below the ideal photodi-ode line with upper and lower wavelengthlimits as shown for two typical Si photodi-odes in Fig. 30. The QE of a well-designedSi photodiode in the wavelength range 700to 900 nm can be close to 90 to 95%.

7.2pin Photodiode

Figure 31(a) shows the schematic diagramof a silicon pin photodiode that has a thinheavily doped p-type Si (p+; superscript‘‘+’’ indicates heavy doping), a wideintrinsic region (i), and a heavily dopedn-type region (n+). The intrinsic layer hasa much smaller doping than both p+ and

0

0.1

0.2

0.3

0.4

0.5

0.6

200 400 600 800 1000 1200

Wavelength (nm)

A

B

Res

pons

ivity

(A

/W)

0

0.7

0.8

0.9

lg

Ideal photodiodeQE = 100% (h = 1)

Fig. 30 Responsivity (R) versus wavelength (λ)for an ideal photodiode with QE = 100% (η = 1)and for two typical commercial Si pinphotodiodes with different structures; A isdesigned to be sensitive in the UV region andhas a quartz window to pass the UV radiation

Optoelectronics 35

n+ regions and is much wider than theseregions, typically 5 to 50 µm, dependingon the particular application. Figures 31(b)and (c) show the net space charge densityand the internal field across the wholedevice. There is a uniform built-in fieldEo in the i-Si layer from the exposedpositive donor ions in the n+-region toexposed negative acceptor ions in the p+-region as illustrated in Fig. 31(c). The pindetector is normally reverse biased witha voltage Vr as shown in Fig. 31(d). Theapplied field drops across the resistive i-region and increases the built-in field toE = Eo + Vr/W where W is the widthof the i-region. When a photon with anenergy greater than the bandgap energyEg is incident, it becomes absorbed inthe i-region (p+-region is very thin) tophotogenerate a free EHP, that is, anelectron in the CB and a hole in theVB. Usually, the energy of the photonis such that photogeneration takes placein the i-layer. The field E in the i-layerthen separates the EHP and drifts themin opposite directions until they reach theneutral regions as indicated in Fig. 31(d).Drifting carriers generate a photocurrentIph, in the external circuit that providesthe electrical signal. The pin photodiodestructure shown in Fig. 31(a) is, of course,idealized. In reality, the i-Si layer willhave some small doping. For example,if the sandwiched layer is lightly n-typedoped, it is labeled as a υ-layer and thestructure is p+ υn+. The sandwiched υ-layer becomes a depletion layer with asmall concentration of exposed positivedonors. The field then is not entirelyuniform across the photodiode. The fieldis maximum at the p+υ junction anddecreases slightly across υ-Si to reach then+ side. As an approximation, we canstill consider the υ-Si layer as an i-Silayer.

p+

i-Si n+

SiO2Electrode

(a)

(b)

(c)

(d)

rnet

–eNa

eNd

x

x

E(x)

R

+−

Eo

E

h+e−

Iph

hn > Eg

W

Vr

Vout

Electrode

Fig. 31 (a) The schematic structure of anidealized pin photodiode; (b) the net spacecharge density across the photodiode; (c) thebuilt-in field across the diode; (d) the pinphotodiode in photodetection is reverse biased

The pin photodiodes have a number ofdistinct advantages over the ordinary pnjunction photodiodes. Having a wider de-pletion region, they have relatively betterquantum efficiencies. The depletion layercapacitance is much smaller than the pnjunction depletion layer capacitance and

36 Optoelectronics

relatively independent of the reverse bias.Consequently, the RC time constant asso-ciated with the depletion layer capacitanceC is small if the external resistance R issmall as shown in Fig. 31(d). Further, ahigher reverse bias voltage can be appliedto a pin diode than to a pn junction withoutbreakdown. The speed of a pin photodiodeis normally limited by the transit time ofthe slowest photogenerated charge carri-ers. The speed improves with reverse biasuntil the carrier drift velocities saturate atthe highest fields. The pin photodiode isprobably one of the most popular photode-tectors used in optoelectronic applicationsdue to its fast speed and good responsivity.

7.3Avalanche Photodiode

Avalanche photodiodes (APDs) are widelyused in various optoelectronic applica-tions, particularly optical communica-tions, due to their high speed and internalgain. A simplified schematic diagram of anSi reach-through APD is shown Fig. 32(a)The n+-side is thin and it is the side that isilluminated through a window. There arethree p-type layers of different doping lev-els next to the n+-layer to suitably modifythe field distribution across the diode. Thefirst is a thin p-type layer and the second isa thick slightly p-type doped (almost intrin-sic) π -layer, and the third is a heavily dopedp+-layer. The diode is reverse biased to in-crease the fields in the depletion regions.The net space charge distribution acrossthe diode due to exposed dopant ions isshown in Fig. 32(b). Under zero bias, thedepletion layer in the p-region (betweenn+p) does not normally extend across thislayer. But when a sufficient reverse biasis applied, the depletion region in the p-layer widens to reach-through to the π -layer(and hence the name reach-through). The

field extends from the exposed positivelycharged donors in the thin depletion layerin n+ side, all the way to the exposednegatively charged acceptors in the thindepletion layer in p+-side.

The variation in the field across the diodeis shown in Fig. 32(c). The field lines startat positive ions and end at negative ionsthat exist through the p, π , and p+ layers.This means that E is maximum at the

p+n+

SiO2Electrode

(a)

(b)

(c)

rnet

x

x

E(x)

R

−+

+

−

E

p

hn > Eg

p

Iph

e−

h+

Absorptionregion

Avalancheregion

Electrode

Fig. 32 (a) A schematic illustration of thestructure of an avalanche photodiode (APD)biased for avalanche gain; (b) the net spacecharge density across the photodiode; (c) thefield across the diode and the identification ofabsorption and multiplication regions

Optoelectronics 37

n+p junction, and then decreases slowlythrough the p-layer. Through the π -layer,it decreases only slightly as the net spacecharge density here is small. The fieldvanishes at the end of the narrow depletionlayer in the p+ side.

The absorption of photons and hencephotogeneration takes place mainly in thelong π -layer. The nearly uniform fieldhere separates the EHPs and drifts themat velocities near saturation towards then+- and p+-sides respectively. When thedrifting electrons reach the p-layer, they ex-perience even greater fields and thereforeacquire sufficient kinetic energy (greaterthan Eg) to impact-ionize some of the Sicovalent bonds and release EHPs. Thesegenerated EHPs themselves can also beaccelerated by the high fields in this re-gion to sufficiently large kinetic energiesto further cause impact ionization andrelease more EHPs, which leads to anavalanche of impact ionization processes.Thus, from a single electron entering thep-layer, one can generate a large num-ber of EHPs, all of which contribute tothe observed photocurrent. The photodi-ode possesses an internal gain mechanismin that a single photon absorption leadsto the generation of a large number ofEHPs. The photocurrent in the APD,in the presence of avalanche multiplica-tion, therefore corresponds to an effectiveQE in excess of unity. The reason forkeeping the photogeneration within theπ -region and reasonably separate fromthe avalanche p-region is that avalanchemultiplication is a statistical process andhence leads to carrier generation fluctu-ations which lead to excess noise in theavalanche multiplied photocurrent. Thisis minimized if impact ionization is re-stricted to the carrier with the highestimpact ionization efficiency, which in Siis the electron.

7.4Heterojunction Avalanche Photodiodes

III–V compound-based heterojunctionAPDs have been developed for use at thecommunications wavelengths of 1.3 µmand 1.55 µm. As in the reach-through SiAPD, the absorption or photogenerationregion is separated from the avalancheor multiplication region which allowsthe multiplication to be initiated by onetype of carrier. The separate absorptionand multiplication (SAM) APD is typicallya heterostructure, as shown in Fig. 33,(e.g., InGaAs-InP) with different bandgapmaterials to separate absorption and mul-tiplication. InP has a wider bandgap thanInGaAs and the p- and n-type doping ofInP is indicated by capital letters, P and N.The main depletion layer is between P+-InP and N-InP layers, and is within theN-InP. This is where the field is greatestand therefore it is in this N-InP layer whereavalanche multiplication takes place. Withsufficient reverse bias, the depletion layerin the n-InGaAs reaches through to theN-InP layer. The field in the depletionlayer in n-InGaAs is not as great as thatin N-InP. Although the long wavelengthphotons are incident onto the InP side,they are not absorbed by InP since thephoton energy is less than the bandgap en-ergy of InP (Eg = 1.35 eV). Photons passthrough the InP layer and become ab-sorbed in the n-InGaAs layers. The fieldin the n-InGaAs layer drifts the holesto the multiplication region where im-pact ionization multiplies the carriers. Thereal device is more complicated than thissimple description. Photogenerated holesdrifting from n-InGaAs to N-InP becometrapped at the interface because there isa sharp increase in the bandgap and asharp change �Ev in Ev (VB edge) be-tween the two semiconductors, and holes

38 Optoelectronics

N n

Electrode

x

E(x)

R

hn

Iph

Absorptionregion

Avalancheregion

InP InGaAs

h+

e−E

InP

P+ n+

Vr

Vout

+−

Fig. 33 Simplified schematic diagram of a separate absorption andmultiplication (SAM) APD using a heterostructure based onInGaAs-InP. P and N refer to p- and n-type wider bandgapsemiconductor

cannot easily surmount the PE barrier�Ev. This problem is overcome by usingthin layers of n-type InGaAsP with in-termediate bandgaps to provide a gradedtransition from InGaAs to InP. Effectively,�Ev is broken up into multiple steps.These devices are called separate absorp-tion, grading, and multiplication (SAGM)APDs. Both the InP layers are grown epi-taxially on an InP substrate. The substrateitself is not directly used to make thePN junction in order to prevent crystaldefects (for example, dislocations) in thesubstrate appearing in the multiplicationregion and hence deteriorating the deviceperformance.

8Integrated Optics and Optoelectronics

Integrated circuits that revolutionized theelectronics industry are based on incor-porating millions of transistor switchesinto a single monolithic device called theintegrated circuit (IC). A monolithic cir-cuit has all the devices manufactured onthe same crystal whereas a hybrid circuithas devices that are not integrated into asingle crystal but typically placed and in-terconnected on a separate substrate. Inan IC, a single crystal of silicon is used asthe substrate and the devices are incorpo-rated onto this substrate using the planarfabrication technology. The counterpart

Optoelectronics 39

of this technology in optoelectronics isIntegrated Optics in which two or moreoptical and/or optoelectronic functionali-ties are integrated onto a single substrateto form an integrated optical or optoelec-tronic circuit; a planar assembly of devicesthat forms an integrated optic device oran optoelectronic integrated circuit (OEIC),which is a more common term in op-toelectronic engineering. Figure 23 showsone of the simplest integrated optoelec-tronic devices, an integrated Pockels cellphase modulator. There is a single electro-optic substrate, LiNbO3, onto which awaveguide has been implanted and copla-nar strip electrodes have been placed toapply an electric field and hence mod-ify the refractive index of the waveguide.There are two functionalities: waveguid-ing and the electro-optic effect. This istherefore an integrated transverse Pockelsphase modulator. (The adjective ‘‘inte-grated’’ sometimes is also used with adevice that has a single functionality if thedevice has been fabricated using the planartechnology on single substrates that canaccommodate further devices.). Figure 24shows an integrated Mach–Zehnder opti-cal intensity modulator. In the latter casetwo waveguides have been embedded andthere are three electrodes. It is clear thatin principle, with the right choice of thesubstrate material, one should be ableto achieve sophisticated integrated opticdevices that can carry out quite com-plicated operations, analogous to ICs inthe sixties. To date, however, the com-mercial integrated optoelectronic deviceshave integrated only a small number ofdevices. It should be apparent that sinceintegrated optical circuits handle photonsthese photons have to be coupled intothe integrated optic device, guided andmanipulated, from one device to anotherwithin the monolithic assembly. Thus, a

key element in an integrated optical circuitis the integrated optical waveguide. Thereare various possible waveguide structuresone can use depending on the required ap-plication. At present there are essentiallytwo important classes of substrates thatare commonly used in integrated opto-electronics. The LiNbO3 (as in Figures 23and 24) substrates are used for variouselectro-optic based functionalities. OEICsthat use lasers and photodetectors arebased on III–V semiconductors such asGaAs or InP as substrates. III–V com-pounds (Sect. 2) are the materials of choicefor fabricating LEDs and semiconductorlaser diodes as well as detectors (Sect. 7)for optical communications.

Integrated optoelectronic circuits have anumber of distinct advantages over thosecircuits that are based on connecting dis-crete optoelectronic devices. The reductionin RC time constants in integrated opto-electronic circuits leads to an increase inthe electrical bandwidth, and thus to ahigher modulation frequency. The smallsize of the integrated optoelectronic cir-cuit implies smaller lengths and widthsthat translate to smaller external voltagesfor applying the necessary electric fieldfor electro-optic effects. When properly de-signed and fabricated, the final integratedoptoelectronic device is free from align-ment problems. As in the case of ICs,integrated optical and optoelectronic cir-cuits offer cost-effective mass productionof reliable devices.

The goal of developing commercial in-tegrated optical and optoelectronic circuitstook an enormous importance with thedevelopment and commercialization of af-fordable small semiconductor lasers. Overthe last two decades, a variety of integratedoptic devices have been demonstrated anddeveloped some examples of which areintegrated optical switches, multiplexers

40 Optoelectronics

and demultiplexers, modulators, erbium-doped waveguide amplifier arrays, DFBlaser arrays, tunable lasers, multiwave-length lasers and so on. The rapid growthof optical communications in the last twodecades has demanded ever-increasinglevels of integration. For example, it ispossible to fabricate an OEIC that func-tions as a multiwavelength transmitterbased on integrating an array of lasersthat can be modulated, a coupler and anoptical amplifier. Such an OEIC is ob-viously very useful as a transmitter forwavelength division multiplexed (WDM)optical communications.

9Optoelectronic Systems

Combinations of various optoelectronicdevices can lead to the implementationof quite impressive and sophisticated op-toelectronic systems. Typically, an opto-electronic system may contain not onlyconventional optoelectronic devices (e.g.,lasers, modulators, fiber, photodetectors)but also purely optical components (e.g.,polarizers, filters, etc.) and purely elec-tronic devices (e.g., amplifiers, encoders,A/D, D/A converters, power supplies, etc).

A simplified block diagram of a WDMoptical communication system is shown inFig. 34. N optical information channels aregenerated by modulating N light waves,each with a different wavelength fromλ1 to λN . The N optical channels arethen multiplexed into a single fiber fortransmission. Optical amplifiers are usedat various points along the transmissionmedium to amplify the attenuated signal.If a particular information channel, forexample, the third channel at λ3, needsto be dropped and another informationchannel (at the same wavelength) needs

to be added, this can be done by using awavelength add/drop multiplexer. The newchannel is labeled as λ3′ . At the destination,the optical signal has to be separated intoits constituent N channels by a wavelengthdemultiplexer. Each channel is then fedinto a receiver, which like the transmittercan be quite complicated in terms ofelectronics.

An examination of the WDM systemin Fig. 34 illustrates a number of im-portant basic functions: (1) generation ofdifferent wavelengths of light each with anarrow spectrum to avoid any overlap inwavelengths; (2) modulation of light with-out wavelength distortion, that is, withoutchirping (variation in the frequency of lightdue to modulation); (3) efficient couplingof different wavelengths into a single trans-mission medium; (4) optical amplificationof all the wavelengths by an amount thatcompensates for attenuation in the trans-mission medium, which depends on thewavelength; (5) dropping and adding chan-nels when necessary during transmission;(6) demultiplexing the wavelengths into in-dividual channels at the receiving end; and(7) detecting the signal in each channel.To achieve an acceptable bandwidth, weneed to dispersion-manage the fiber (usedispersion compensation fibers); and to re-duce cross talk and unwanted signals, wehave to use optical filters to block or passthe required wavelengths. We need variousoptical components to connect the devicestogether and implement the whole system.

Channel density is an important quantityin designing WDM systems. It is usuallymeasured in terms of frequency spacingbetween two neighboring optical channels.If the frequency spacing �υ is less than200 GHz, then WDM is called dense wave-length division multiplexing and is denotedas DWDM. At present, DWDM standstypically at 100 GHz separated channels,

Optoelectronics 41

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42 Optoelectronics

which is equivalent to a wavelength separa-tion of 0.8 nm. DWDM imposes stringentrequirements on lasers and modulatorsused for generating the optical signals. Itis not possible to tolerate even slight shiftsin the optical signal frequency when chan-nels are spaced closely. As the channelspacing becomes narrower as in DWDM,one also encounters various other prob-lems not present previously. For example,any nonlinearity in a component carryingthe channels can produce intermodulationbetween channels, an undesirable effect.Thus, the total optical power must bekept below the onset of nonlinearity inthe optical amplifiers within the WDMsystem. Further, the nonlinearity of op-tical fiber at high optical powers leadsto a phenomenon called four-photon mix-ing. Consider three optical signals at υ1,υ2 = υ1 + 100 GHz, υ3 = υ1 + 200 GHz.The nonlinearity in the optical fiber mixessignals to generate a new signal at υ4 =υ1 + υ2 − υ3 = υ1 − 100 GHz, on top ofthe channel below υ1. Four-photon mix-ing thus adds noise to the communicationsystem.

There are limits to how much opticalpower can be efficiently transmitted ina fiber. The optical power transmittedthrough a fiber does not increase linearlywith the input power when the latteris sufficiently high to cause stimulatedBrillouin scattering (SBS). Interaction ofthe photons with acoustics phonons inthe fiber generates a periodic variationin the refractive index, which in turnreflects some of the power back. Theeffect increases as the input light powerincreases and the spectral width of theinput light becomes narrower. The onsetof SBS depends not only on the fibertype and core diameter, but also on thelinewidth �λ of the laser output spectrum.SBS is enhanced as the laser linewidth

�λ is narrowed or the light pulse islengthened. For example, for a directlymodulated laser diode emitting at 1550 nminto an SMF, the onset of SBS is expectedto occur at power levels greater than 20 to30 mW. In DWDM systems with externallymodulated lasers, that is, narrower �λ, theonset of SBS can be as low as ∼10 mW.SBS is an important limiting factor intransmitting high power signals in WDMsystems.

The WDM example in Fig. 34 is one ofmany examples on optoelectronic systems.The complexity of such systems dependsnot only on whether individual optoelec-tronic devices can efficiently implementthe required function, but also on the avail-ability of various optical components thatare needed to properly interconnect thedevices. Optoelectronics today is an inter-disciplinary activity. Advances at the devicelevel are enabling systems that wouldhave been thought impossible a coupleof decades ago (DWDM being an excellentexample).

Glossary

Acceptance Angle: The largest possibleangle with respect to the fiber axis forlaunching light into the fiber.

Acousto-optic (AO) Modulator: A photoe-lastic effect–based device that can deflectlight when a suitable radio frequency sig-nal is applied to it.

Avalanche Photodiode (APD): Photodetec-tor that has internal gain due to theimpact ionization multiplication of pho-togenerated carriers so that the quan-tum efficiency is normally more thanunity.

Optoelectronics 43

Chromatic Dispersion: The temporalspread of a propagating optical pulse inan optical guide due to the wavelengthdependence of all the guide properties. Itincludes waveguide, material, and profiledispersions added together.

Cladding: A medium of lower refractive in-dex surrounding a core of higher refractiveindex that guides light; cladding confinesthe light to the core.

Compensating Fiber: A length of fiberthat is connected (spliced) to a transmis-sion fiber whose chromatic dispersion isto be reduced. Compensating fiber hasdispersion characteristics that nearly can-cel the dispersion in the transmissionfiber.

Connector: A convenient mechanical de-vice that is attached to the end of a fiberthat connects to another such device andthereby connects fibers together.

Core: The central region of an optical fiberthat has a higher refractive index than thesurrounding cladding region, and guideslight along the fiber.

Demultiplexer (DMUX): A device that isable to separate a multiplexed signal intoits original component channels.

Dense Wavelength Division Multiplexing(DWDM): The transmission of manychannels of information that are closelyspaced in wavelength along the same fiber.The channels are transmitted at wave-lengths λ1, λ2, . . ., λN , which are separatedin frequency by 200 GHz, or wavelengthsseparated by 1.6 nm if the wavelengths arein the 1550-nm window.

Dispersion: Dispersion in optical fiberrefers to the spread in time, known astemporal broadening, of an infinitesimally

thin optical pulse as it propagates alongthe fiber.

Dispersion Shifted Fiber: A fiber whosewavelength for zero chromatic dispersionhas been shifted to the 1550-nm region;not useful in WDM optical systems.

Distributed Feedback (DFB) Laser: A ‘‘sin-gle frequency’’ laser, that is, a laser witha very narrow output spectrum, in whichthe optical feedback in the cavity occursdistributively over the cavity length due toa periodic corrugation in the cavity; suchcorrugation forms a periodic change in therefractive index and hence forms a grating.

Double Heterostructure (DH): A structurein which a semiconductor layer is sand-wiched by two wider bandgap semicon-ductor layers. There are two heterojunc-tions since the narrow bandgap semi-conductor makes junctions with its twowider bandgap neighbors, hence the namedouble heterostructure. DH junctions areuseful in confining injected minority car-riers in the narrow bandgap layer.

Electro-absorption: The absorption of pho-tons with energies slightly less than thebandgap energy as a result of the applica-tion of an electric field.

Electro-optic Effect: Changes in the refrac-tive index of a material induced by theapplication of an external electric field,which therefore ‘‘modulates’’ the opticalproperties.

Electro-optic Modulator: A device thatmodulates a characteristic of light such asits intensity or phase in response to an elec-trical signal by virtue of the electro-opticeffect.

Erbium Doped Optical Fiber Amplifier(EDFA): An optical amplifier made from

44 Optoelectronics

a glass fiber whose core has been dopedwith Er3+ ions; the latter are pumped toan excited state by using a laser diode (oran LED) at 980 or 1480 nm.

Faraday Effect: A physical phenomenonin which the plane of polarization of apropagating light wave in a medium isrotated by placing a magnetic field into thatmedium. The amount of rotation dependson the material and is proportional to theapplied magnetic field and the length ofthe medium.

Franz–Keldysh Effect: Electric field in-duced absorption of light with photonenergies slightly less than the bandgap ina semiconductor due to the modificationof the energy band edge by the appliedfield.

Graded Index (GRIN) Optical Fiber: A fiberthat has a core refractive index that isgraded gradually, changes continuously,towards the cladding. Typically in a gradedindex fiber, the refractive index profileis approximately parabolic to minimizemodal dispersion.

Injection Electroluminescence: Light emis-sion by radiative recombination of anelectron and a hole that have been injectedby an electrical current into a semiconduc-tor device such as an LED.

Integrated Optics or Optoelectronics: Amonolithic optical or optoelectronic circuitin which two or more optical and/or op-toelectronic functionalities are integratedplanarly into a single substrate to form anintegrated optic (or optoelectronic) device.

Kerr Effect: A second order effect exhibitedby all materials in which the change inthe refractive index is proportional to thesquare of the applied electric field.

Laser Diode: A semiconductor diode, usu-ally a heterostructure device, that emitscoherent radiation, a lasing output, whenthe current through it exceeds the thresh-old current to achieve the necessarypopulation inversion for stimulated emis-sion.

Light-emitting Diode (LED): Usually a pnjunction diode type device that emitslight by the spontaneous emission ofphotons through injection electrolumines-cence when it is forward biased.

Mach–Zehnder Modulator: A lithium nio-bate device whose light transmittance iscontrolled by an applied external voltage; itis based on interfering two waves passingthrough the crystal in such a way that theirphase difference can be controlled by thePockels effect.

Modal Dispersion: Dispersion due to thepropagation of an optical signal alonga multimode fiber as different modesof radiation with each having its ownpropagation velocity. These modes ar-rive at different delay times and causethe optical pulse to broaden. Modaldispersion is absent in single-modefibers.

Modulator: A device that changes a charac-teristic of light, such as its intensity, phase,polarization, or direction, passing throughit in response to an electrical input signal.

Multimode Fiber (MMF): A fiber in whichlight propagates as many modes.

Multiple Quantum Well (MQW): A super-lattice that is formed by many quan-tum wells; many alternating thin layersof narrow bandgap semiconductors sep-arated by higher bandgap semiconduc-tors.

Optoelectronics 45

Multiplexer (MUX): A device where two ormore signals (channels) are combined intoa single output.

Nonzero Dispersion Shifted Fiber: A fiberwhose wavelength of zero chromatic dis-persion has been shifted to be just outsidethe Er-doped fiber amplifier region (1525to 1620 nm).

Numerical Aperture (NA): A characteristicparameter of an optical fiber that dependson the refractive indices of the core andthe cladding, and measures light gatheringability of the core.

Optical Amplifier: An amplifier that ampli-fies an optical signal, namely, light.

Optical Fiber: A thin glass–based cylindri-cal optical waveguide whose core regionhas a higher refractive index and therebyguides light. Signals in optical commu-nications are transmitted through opticalfibers.

Optical Fiber Mode: A distinct electric fieldpattern that is allowed to exist in thefiber and propagate along the fiber withoutattenuation if the fiber is lossless.

Optical Isolator: A device that allows lightto pass in one direction and not in theopposite direction.

Optical Semiconductor Amplifier: A semi-conductor laser structure that can beused as an optical amplifier that ampli-fies light waves passing through its activeregion.

Optical Switch: A device that can beactuated by an external signal to switchan optical signal from one optical pathto another, or can be actuated to eithertransmit or not transmit light.

Photodetector: An optoelectronic devicethat converts an incident optical signalto an electrical signal, usually a photocur-rent.

pin Photodiode: A photodetector that hasthe structure, p+/intrinsic/n+ with arelatively thick intrinsic (depletion) regionwhere the electric field is relatively uniformand where photogeneration takes placefor efficient charge collection and highquantum efficiency.

Pockels Effect: An effect exhibited bynoncentrosymmetric crystals in which thechange �n in the refractive index is linearlyproportional to the applied electric field E;if �n is positive for a field in a certaindirection, reversing the field results in anegative �n.

Quantum Dot (QD): A crystal that isso small (for example, a crystal that isseveral nanometers) that the electronsand holes in the crystal are confined inthree dimensions and exhibit quantumeffects such as quantized energies in alldirections.

Quantum Efficiency: A quantity that de-scribes the efficiency of conversion inthe photodetection process from collectedphotons to detectable charge carriers;number of free electron–hole pairs col-lected per incident photon.

Quantum Well (QW): A heterostructurethat has an ultra thin, typically lessthan 20 nm, narrow bandgap semiconduc-tor, such as GaAs, sandwiched betweentwo wider bandgap semiconductors, suchas AlGaAs. The electrons and holesin the thin narrow bandgap layer ex-hibit quantum effects along the di-rection perpendicular to the layers (z-direction) and have quantized energies

46 Optoelectronics

similar to an electron in a finite one-dimensional potential energy well; energyis not quantized for motion in the xyplane.

Raman Fiber Amplifier: Is a fiber basedoptical amplifier in which energy is trans-ferred from a powerful pump radiation fedinto a fiber to a weak propagating signalas a result of nonlinear frequency mixing;the result is an optical amplification. Thepump frequency is higher than the sig-nal frequency by a phonon frequency ofthe medium.

Responsivity, Radiant Sensitivity: A charac-teristic property of a photodetector thatindicates how responsive the detector isat a certain wavelength by measuring thephotocurrent generated per incident opti-cal power.

Separate Absorption and Multiplication(SAM) APD: A heterostructure avalanchephotodetector with different bandgap ma-terials to separate absorption and multi-plication processes to reduce the excessavalanche multiplication noise.

Single-mode Fiber (SMF): A fiber in whichonly the fundamental mode can propagate;the fiber has a V -number, V < 2.405 at theoperating wavelength.

Single-mode Waveguide: A waveguide thatcan carry only the fundamental mode(lowest mode) of radiation within thewavelength range of interest, that is,wavelengths longer than the critical cut-offwavelength.

Splice: A permanent connection betweentwo fibers achieved by aligning the fibersand then fusing the two glasses to eachother at a high temperature in an electricalarc between two electrodes.

Threshold Current: The minimum currentthat must be passed through a laser diodeto obtain coherent output radiation thatexceeds the spontaneous radiation output.More strictly, it is the minimum currentfor self-sustaining lasing oscillations in thelaser diode.

Vertical Cavity Surface Emitting Laser(VCSEL): A small semiconductor laser thathas the optical cavity axis along the direc-tion of current flow rather than perpendic-ular to the current flow as in conventionallaser diodes. The active region length isvery short compared with the lateral di-mensions so that the radiation emergesfrom the ‘‘surface’’ of the cavity ratherthan from its edge.

V-number: A dimensionless quantity thatis a characteristic of a dielectric waveguidewhich determines the nature of prop-agation of EM waves along the guide.For a step-index fiber, it is defined byV = (2πa/λ)[n2

1 − n22]1/2, where a is the

core radius, λ is the free space wave-length of the radiation to be guided,and n1 and n2 are the refractive indicesof the core and cladding respectively. IfV < 2.405, only the fundamental modecan propagate.

Wavelength Add/Drop Multiplexer (WADM)or Optical Add/Drop Multiplexer (OADM):A multiplexer that is able to selectivelytake out, that is, remove, a signal at aspecified wavelength λs from wavelengthdivision multiplexed signals propagatingalong a fiber, and then add new or differentinformation at the same wavelength λs intothe fiber, in the same direction.

Wavelength Division Multiplexing (WDM):Multiplexing of different channels ofinformation along the same fiber atdifferent wavelengths.

Optoelectronics 47

Acknowledgment

The author is grateful to NSERC andTRLabs (Edmonton) for financial supportof his projects on photonic devices, andthanks his colleague Chris Haugen atTRLabs for many enjoyable discussions onthe subject.

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