Reading Homework 3

150 Bell Labs Technical Journal ◆ January–March 2000 Copyright 2000. Lucent Technologies Inc. All rights reserved.

IntroductionThe beginning of optical communications

depended on the development of two essential

components—the optical fiber and the semiconductor

laser. The concept of a laser was invented by

Schawlow and Townes1 in 1958 and was first realized

using a Ruby rod by Maiman2 in 1960. Following

these original inventions, a wide variety of lasers were

invented: the He-Ne laser, the CO2 laser, the dye laser,

and, most importantly from a communications point

of view, the semiconductor laser. The last was simulta-

neously invented at three different industrial labs:

IBM,3 General Electric,4,5 and MIT Lincoln Labora-

tory.6 In order for the semiconductor laser to become

commercially important, it was essential that the oper-

ating temperature be increased and the operating cur-

rents significantly decreased from these early reports.

The concept of confining electrons and holes in the

center of the optical waveguide with a double hetero-

junction device was published by Kroemer7 and also

by Alferov and Kazarinov8 in 1963 and first realized

by Hayashi et al.9 in 1970.

These early inventions were followed by an enor-

mous amount of work in refining the materials

growth of the III-V compound semiconductors

involved, defining the optimal structure of the device,

understanding possible defects and failure modes of

the device, and controlling the many properties essen-

tial for use in today’s high-speed communications sys-

tems. The end result of this activity is that lasers today

can be:

• Grown with predetermined wavelengths accu-

rate to one part in ten thousand,

• Modulated at data rates in the tens of gigabits

per second without shifting the frequency,

• Designed to withstand variations of tempera-

ture from –40 to 85°C, and

• Produced with such high yields that the laser

chip itself costs less than $10.

The laser chip is the size of a pinhead, so it takes

up very little space in a system.

The purpose of this paper is to relate the tremen-

dous progress that semiconductor lasers have made

since their original invention 37 years ago. We have

divided the progress into two main categories: materi-

als and reliability work, discussed in the next section,

and laser structures, performance, and applications,

discussed in the section following it. The final section

looks forward to new uses of semiconductor laser-like

devices in communications and what may lie ahead as

the optical communications revolution moves into its

next phase.

Materials and ReliabilityThe earliest semiconductor lasers were fabricated

of AlxGa1-xAs alloys grown as single crystals on GaAs

substrates using a liquid-phase epitaxial (LPE) tech-

nique. In this technique, liquid melts of Ga metal, con-

♦ The Lasers Behind the CommunicationsRevolutionWilliam F. Brinkman, Thomas L. Koch, David V. Lang,and Daniel P. Wilt

This paper reviews the development of the semiconductor laser—a key component ofoptical communications. It discusses the tremendous advances in materials and reliabil-ity, describes the progress in laser structures, performance, and applications, and looksforward to the role of the laser in the next phase of the telecommunications revolution.

Bell Labs Technical Journal ◆ January–March 2000 151

taining small amounts of dissolved Al, GaAs, and semi-

conductor p- and n-type dopants such as Si, Ge, Te, or

Sn, are used to grow thin layers of semiconductor to

achieve a desired laser double heterostructure. The

liquid melts are prepared for epitaxy by careful weigh-

ing the components and loading them into a multiwell

graphite boat (one well per melt) along with the crys-

tal substrate. The assembly is heated in a well-

controlled, high-temperature furnace in an inert or

reducing ambient such as He or H2 to a temperature in

the 750°C range. After baking the entire apparatus for

an extended period, the melts are cooled slightly until

they become supersaturated, and then the crystal sub-

strate is brought into contact with each melt in turn to

precipitate out the desired semiconductor crystal layers

in a process resembling that used by young scientists

to grow sugar crystals, or “rock candy.”

The AlxGa1-xAs material system only emits light in

the 600- to 900-nm “short-wavelength” range, which

was of interest in early optical communication systems.

Beginning in the late 1970s, silica optical fibers with

much improved transmission characteristics (optical

loss and dispersion) in the 1300-nm and 1550-nm

wavelength ranges were demonstrated. These fibers

required different laser materials. The most attractive

material system for this “long-wavelength” range,

which dominates the long distance communications

industry today, is the alloy In1-xGaxAsyP1-y lattice-

matched to InP substrates.10,11

The In1-xGaxAsyP1-y alloys can be grown by LPE in

a very similar fashion to AlxGa1-xAs alloys, using In

metal melts with dissolved GaAs, InAs, InP, trace

dopants, and InP substrates. The typical growth tem-

perature is somewhat lower (650°C) due to the

decomposition of P-containing compounds such as InP

at high temperatures. The great virtue of the LPE tech-

nique is the relative ease with which good layer-to-

layer interfaces with very high luminescence efficiency

can be fabricated. As early as the late 1960s, with the

relatively poor-purity source materials of the day, it

was possible to demonstrate material quality compara-

ble to that achievable today. The primary reason is that

LPE is a near-equilibrium technique that, by its nature,

tends to precipitate from the liquid melt higher-purity

material than the starting components.

As soon as semiconductor lasers were determined

to be of interest for telecommunications, the question

of their reliability came to the fore. The typical modern

communication system is targeted to operate without

interruption due to component failure, which is typi-

cally achieved through redundancy at the system level.

However, this also requires a certain level of reliability

from the component—typically, no more than 1% of

the lasers in a land-based system may fail per year,

throughout a system life of 20 to 25 years. Submarine

communication systems require about 10 times better

reliability of lasers. Reliability at this level is established

in only one way—robust design and manufacturing,

Panel 1. Abbreviations, Acronyms, and Terms

AR—antireflectionCAD—computer-aided designCBE—chemical beam epitaxyCW—continuous waveDBR—distributed Bragg reflectorDFB—distributed feedbackEA—electroabsorptionEML—EA-modulated laserFP—Fabry-PerotGSMBE—gas-source MBEHR—high reflectionITU—International Telecommunication UnionLPE—liquid-phase epitaxy/epitaxialMBE—molecular beam epitaxyMOCVD—metal-organic chemical vapor

depositionMQW—multiple quantum wellMZ—Mach-ZehnderNRZ—nonreturn to zeroOTDM—optical time division multiplexingOTU—optical terminal unitPIC—photonic integrated circuitQW—quantum wellRZ—return to zeroSAG—selective-area growthSCH—separate confinement heterostructureSOA—semiconductor optical amplifierSSC—spot-size convertedTEC—thermoelectric coolerVCSEL—vertical-cavity surface-emitting laserWDM—wavelength division multiplexingWSL—wavelength-selectable laserXBL—expanded-beam laser

152 Bell Labs Technical Journal ◆ January–March 2000

stringent screening to eliminate weak devices

(“purging”), and accelerated aging programs to defensi-

bly argue that these techniques do, in fact, work.

Beginning with AlxGa1-xAs lasers in the early

1980s, these reliability techniques were invented and

refined. Several serious reliability problems were found

with AlxGa1-xAs lasers that limited their use for many

years: first, the problem of passivating the aluminum-

containing materials to prevent erosion and failure;

and, second, the growth of extended “dark-line”

defects. These problems were not experienced to the

same extent in the In1-xGaxAsyP1-y materials. By the

early 1980s, the reliability of these long-wavelength

lasers was sufficient to deploy them widely in telecom-

munication networks (for example, in the AT&T

FT-3C system) and, by 1988, the first transatlantic sub-

marine communication cable incorporating 1.3-µm

long-wavelength lasers (TAT-8) was installed.

Although the In1-xGaxAsyP1-y materials continue to

dominate data transmission in optical fiber, beginning

in the early 1990s with the invention of the Er-doped

fiber amplifier, the AlxGa1-xAs materials system has

again become of interest in optical fiber communica-

tions. This is due to the need for short-wavelength

980-nm pump lasers to excite the Er-doped gain

medium. The reliability problems in this material sys-

tem, mentioned above, have now been largely solved

through the use of strained-layer In1-xGaxAs emitting

materials (solving the dark-line problem) and propri-

etary ultraclean passivation techniques (solving the

erosion problem).

At about the same time that the first AlxGa1-xAs

LPE double heterostructure lasers were being deve-

loped, a new method of growing epitaxial layers of

GaAs was developed by Al Cho and coworkers12 at

Bell Labs. This new technique, called molecular beam

epitaxy (MBE), made it possible to grow single-crystal

GaAs and AlxGa1-xAs layers of only a few atomic layers

thickness. In this crystal growth technique, a substrate

(for example, GaAs) is mounted in an ultrahigh vac-

uum system and heated to a temperature around

700°C. The system then directs molecular beams at the

substrate to grow a series of epitaxial layers forming a

device or a research structure. For example, beams of

Ga atoms, Al atoms, and As2 molecules are employed

to grow the AlxGa1-xAs alloy. The MBE technique has

sometimes been described as “spray-painting atoms.”

Because MBE layers could be made atomically

smooth and thin, an exciting new physics research

area developed in the 1970s to study the behavior of

electrons in layers a few nanometers thick. In 1975,

motivated by the capabilities of MBE, Dingle and

Henry13,14 at Bell Labs invented a laser structure in

which the active layer of the double heterostructure is

less than 30 nm thick and approaches the “quantum

size” of the injected carriers. They calculated and

demonstrated that such a “quantum well” (QW) laser

could have dramatically higher gain, lower threshold,

Panel 2. Chemical Nomenclature for SemiconductorLaser Materials

Al AluminumAlxGa1-xAs Aluminum gallium arsenideAs2 ArsenicAsH3 ArsineCO2 Carbon dioxideEr ErbiumFe IronGa GalliumGaAs Gallium arsenideGaN Gallium nitrideGe GermaniumH2 HydrogenHe HeliumIn IndiumInAs Indium arsenideInGaAlAs Indium gallium aluminum

arsenideIn1-xGaxAs Indium gallium arsenideIn1-xGaxAsyP1-y Indium gallium arsenide

phosphideInGaN Indium gallium nitrideInP Indium phosphideLiNbO3 Lithium niobateNe NeonP PhosphorusPH3 PhosphineSi SiliconSiGe Silicon germaniumSiO2 Silicon dioxide; silicaSn TinTe Tellurium


and lower losses than demonstrated previously. These

beneficial properties are a direct result of the fact that

the allowed energy states of the conduction and

valence bands in an ultrathin GaAs/AlxGa1-xAs double

heterostructure break into discrete sub-bands with a

stepped rather than a parabolic density of states.

Because the electrons and holes are concentrated into

the discrete energy states of the quantum well, a larger

fraction of the injected carriers can take part in the

laser action, thereby lowering the threshold current

and raising the differential optical gain per injected

carrier. In general, the best performance is obtained

when the active region of the laser is formed from sev-

eral adjacent quantum wells—that is, alternating ultra-

thin layers of lower and higher bandgap material.

Such lasers are called multiple quantum well (MQW)

lasers. As we will discuss in the next section, the per-

formance advantages of MQW lasers make this the

design of choice for nearly all of today’s lasers.

The ultrahigh-vacuum MBE technique and vari-

ants, such as gas-source MBE (GSMBE) and chemical

beam epitaxy (CBE), proved very difficult to imple-

ment for manufacturing in the In1-xGaxAsyP1-y material

system due to difficulties with handling phosphorus.

Instead, the epitaxial technique of choice for fabricat-

ing MQW lasers in this material system has proven to

be metal-organic chemical vapor deposition

(MOCVD). In this epitaxial technique, the substrate

wafer is placed in a flowing gas ambient containing

chemical precursors suitable for growth of the desired

alloy. The metal group III components are supplied as

metalorganics—for example, trimethylgallium and

trimethylindium—and the group V components are

supplied as hydrides—for example, arsine (AsH3) and

phosphine (PH3). At a sufficiently high substrate tem-

perature (for example, 630°C), these compounds

decompose on the substrate surface to grow the

desired In1-xGaxAsyP1-y alloy.

In the early days of laser development, great care

was taken to ensure that the various epitaxial layers

grown to fabricate a laser were perfectly lattice

matched to the substrate and to each other and,

hence, free of strain. It was known that strained MQW

structures of high crystalline quality could be grown

from lattice-mismatched materials if the layers were

thin enough, but strain was clearly related to early

laser reliability problems, and strained MQW lasers

were not seriously considered. In 1982, however,

Osbourn15 at Sandia pointed out that strained MQW

structures would further enhance the beneficial modi-

fications of the band structure caused by the quantum

wells. Subsequent workers have shown that strained

MQW lasers have significant performance enhance-

ments over unstrained MQW lasers. As we will discuss

in the next section, nearly all of today’s high-

performance lasers are of the strained MQW design.

As we have noted, a major concern with the

invention of strained quantum-well devices was relia-

bility. It had been shown in early AlxGa1-xAs reliability

work that high mechanical stress dramatically accele-

rated the growth of defects and device failure. Thus there

was great concern that strained MQW lasers would be

unreliable. It was known that if the strained layer was

thin enough and the strain was small enough, the

layer would grow as a homogeneously distorted but

perfectly crystalline layer without the dislocations

commonly found in thicker strained layers. Fortuitously,

it was found that under these conditions, the reliability

of strained MQW lasers was often better than that of

their unstrained predecessors. Perhaps even more

amazing, the introduction of strain into the AlxGa1-xAs

materials system in the form of an InyGa1-yAs strained

QW layer seems to suppress the dark-line defect

growth mechanism in these laser devices. This is

important for 980-nm pump lasers for Er-doped fiber

amplifiers.

An additional advantage of MOCVD growth is the

conformal coverage of etched features, such as the laser

active stripe, waveguides, and gratings. This makes pos-

sible the complex structures discussed in the next sec-

tion. Two MOCVD innovations are particularly

noteworthy—the development of semi-insulating

Fe-doped InP blocking layers for high-speed lasers and

selective-area growth (SAG) for complex integrated

structures. The selective-area growth feature of

MOCVD makes possible the integrated distributed feed-

back laser and the electroabsorption-modulated laser

discussed in the next section. The principle of selective-

area growth is that, under the right conditions,

MOCVD growth will take place only on a substrate of


similar composition to the growth constituents (such

as InP) and not on a very dissimilar material (such as

SiO2). Thus, for example, if one grows InGaAsP on an

InP substrate covered by a patterned SiO2 layer, the

molecules falling on the SiO2 regions will not react but

will move to the adjacent InP, causing faster and

thicker growth of InGaAsP on the InP near the edges

of the SiO2. When the layers being grown are the

alternating InP and InGaAsP layers of a MQW struc-

ture, the quantum wells are of different composition,

strain, and thickness near the SiO2 compared to

those far from the SiO2. Thus, regions of different

bandgap can be grown on the same substrate during

the same MOCVD growth run. In the case of the

electroabsorption-modulated laser, the SiO2 is

patterned to give adjacent MQW regions appropriate

for the laser and the modulator, respectively. Such a

complex process is only practical in manufacturing due

to the extremely high level of control and repro-

ducibility of MOCVD as well as the excellent CAD

tools developed to model the SAG process.

Semiconductor Laser Structures, Performance, andApplications

The materials advances described in the preceding

section have provided a rich foundation for semicon-

ductor laser designs of ever-increasing sophistication

and complexity. The resulting improvements in laser

performance have been instrumental to the rapid

advances in both span lengths and raw capacity that

can be achieved with optical fiber transmission. This

section will recount the evolution in laser designs that

have become critical enablers for many new regimes

of optical fiber communications system design.

Basic Laser StructuresEarly research and development in laser designs

focused primarily on improvements in basic laser oper-

ating parameters that were essential to prove the viabil-

ity of semiconductor lasers as a communications light

source. These improvements included reductions in

threshold current Ith, improvements in differential

quantum efficiency hd, and closely related improve-

ments in device reliability. These laser characteristics

were clearly paced by the rapid improvements in mate-

rial quality and semiconductor microfabrication tech-

nologies as outlined in the previous section, but they

were also governed by elements of design that rapidly

matured into highly sophisticated, topologically rich

laser structures. It is this three-dimensional processing

and design environment which most markedly distin-

guishes the manufacture of semiconductor lasers from

that of other microelectronic components and circuits.

Fundamental to efficient laser operation are the

simultaneous confinement of light in a low-loss res-

onator and the confinement of a population inversion

inside the resonator to provide optical gain. For edge-

emitting semiconductor lasers, in contrast to vertical-

cavity surface-emitting lasers to be described briefly

later, the optical resonator consists of a low-loss dielec-

tric optical waveguide terminated by cleaved crystal

facets serving as feedback mirrors. The population

inversion is generated by injection of a high-density

nonequilibrium electron-hole plasma using a p-n

junction in forward bias.

Optical waveguides in semiconductor lasers con-

sist of a waveguide core comprising epitaxially grown

layers with a composition chosen to have an average

index of refraction higher than that of the surrounding

epitaxially grown layers above and below the core,

which compose the waveguide cladding. It was a fun-

damental observation by Kroemer7 and by Alferov

and Kazarinov8 as early as 1963 that a heterostructure

sandwich formed by placing a narrow bandgap layer

between higher bandgap layers provided not just the

aforementioned optical waveguide, but also served to

confine the electron-hole population inversion in the

narrow bandgap waveguide core, exactly as required

for efficient laser operation. This optical and electrical

confinement, while operative only in one dimension,

nevertheless ultimately led to the demonstration of

room-temperature continuous wave (CW) opera-

tion.9,16 This one-dimensional confinement was read-

ily modified to stripe geometry and gain-guided

designs simply by limiting the lateral extent of the

electrical excitation, limiting current injection by either

narrow electrical contacts (typically 5 to 20 µm) or

proton implantation to render all but a protected cen-

tral stripe region highly resistive. Typical chip lengths

are today as they were then—in the range of 200 to

600 µm between the cleaved facet mirrors. AlGaAs


lasers of this generation were used in the first com-

mercial deployment of 0.82-µm lasers in multimode

fiber at 45 Mb/s on the eastern coast of the United

States in 1981.

Major advances throughout the 1970s focused on

the refinement of designs and fabrication techniques

that extended this optical and electrical confinement

more efficiently to the lateral plane as well. This

required defining a full low-loss two-dimensional rib

waveguide and also providing for the confinement of

the highly mobile injected minority carriers efficiently

inside the micron-scale core of this waveguide. Figure 1

shows an example of a buried heterostructure laser,

one of the designs that first emerged in 197417 and

achieved the required optical and electrical lateral con-

finement by the epitaxial regrowth of lateral cladding

layers around a mesa etched through the active layer

stack. A long-wavelength version of this design was

developed in 1980.18 This design, along with numer-

ous variants resulting from a proliferation of etching

and crystal growth techniques, also provides for reverse-

biased junctions in the lateral cladding to block current

leakage paths and force the current into the excitation-

confining active region. As a result, this design and its

descendants have provided for many of the most effi-

cient and highest-performance lasers, routinely achiev-

ing threshold currents of Ith < 10 mA and efficiencies of

hd ~ 0.4 W/A. Another laser design that continues to

enjoy widespread use, especially in high-power lasers

required for pumping optical amplifiers, is the ridge-

waveguide laser. Here the active layer is not laterally

restricted, but the upper cladding is etched to form a

ridge-shaped dielectric loading above and along the

length of the active core, thereby providing weak stripe

optical waveguiding. The ridge upper cladding also

serves to laterally restrict current injection into the

active layer beneath the ridge. The tight layer thickness

control and uniformity required to achieve good perfor-

mance at high yield using this design were practical

once advanced MBE and MOCVD crystal growth

became prevalent, and this design does offer a simplified

fabrication and epitaxial growth sequence. Both the

buried heterostructure and ridge-waveguide lasers have

clearly demonstrated reliability exceeding the require-

ments for deployment even in undersea applications.

Another important activity in the 1970s, as noted

earlier in the “Materials and Reliability” section, was

the exploration of quantum well lasers. Research in

Au

AuZn

p-InGaAsP

TiPt

SiO2

n-InGaAsP

N-InP

P-InP

1.5 µm

Laser schematic(end view)

0.2 µm

N-InP

P-InP

n-InGaAsP active

N-InP

AuSnEtched mirror photomicrograph

Figure 1.1.3-µm InGaAsP buried heterostructure laser.


quantum well lasers initially focused on improved

materials and interface quality, driven by the theoreti-

cal promise of higher performance. This work soon

reached a state of high sophistication with the intro-

duction of separate confinement heterostructure

(SCH) designs. The SCH designs effectively separated

the engineering of the quantum wells, providing for

the desirable effects of quantum-size carrier confine-

ment and gain enhancements, from the engineering of

the optical confinement or waveguiding. Very high

performance ensued, ultimately allowing a single

quantum well to provide adequate gain within a well-

engineered, low-loss waveguide to promote high-

efficiency, ultra-low threshold operation.19

Direct ModulationA critical feature of semiconductor lasers for com-

munications is their ability to directly modulate the

intensity of the light output simply by modulating the

drive current. This feature was studied early on, where

it was determined20 that lasers can be directly modu-

lated up to frequencies in the neighborhood of the

relaxation oscillation frequency fRO given by

, (1)

where νg is the group velocity of the light in the wave-

guide mode, g´ is the “differential gain” given by the

rate of change of optical gain (in cm-1) per unit change

in excited carrier density, S is the average photon den-

sity inside the active layer, and τph is the photon lifetime

of the optical cavity as determined by optical losses and

output coupling. With little explicit engineering, speeds

up to about 1 GHz were readily achieved, allowing

early laser designs to be deployed for systems operating

at speeds up to 622 Mb/s. Throughout the 1980s, work

in both the AlGaAs system21 and the InGaAsP sys-

tem22 showed that speed is typically limited by the

electrical parasitic capacitance, series resistance, and

bonding wire inductances. Chief among these was the

capacitance of the reverse-biased lateral current block-

ing layers in buried heterostructure lasers. Designs

were introduced employing thick, low-capacitance

semi-insulating Fe-doped InP layers in the lateral

blocking structure to reduce this capacitance, often

accompanied by longitudinal trenches to electrically

isolate an approximately 10-µm wide region contain-

ing the buried active stripe. With the addition of thick

dielectrics under the bonding pads, sharp reductions in

the capacitance down to the range of several pico-

farads have proven practical, allowing for commercial

devices to be modulated at speeds up to 10 Gb/s.

The ability to reach speeds in this range also bene-

fits from the factors determining fRO in Equation (1)

above. Achieving high speed strictly by increasing the

photon density S often requires impractically high out-

put powers. This has the added disadvantage of the

resulting need for excessively high modulation current

swings to turn the laser on and off, placing difficult

demands on the driver integrated circuits employed in

transmitters. Increases in differential gain that result

from MQW active layers, as discussed in the “Materials

and Reliability” section, provide for high speeds

(10 Gb/s) at reasonable drive currents (< 100 mA).

Spectral Characteristics: DFB Lasers and DispersiveTransmission

The lasers described above are termed Fabry-Perot

(FP) lasers since the longitudinal optical resonator

structure comprises a waveguide terminated on each

end by cleaved facet mirrors, similar to the Fabry-Perot

etalon. The resonances of such an optical cavity are

equally spaced in frequency ν by ∆ν = c/2nL, where c is

the speed of light, n is the group index of refraction of

the waveguide (typically ~ 3.8), and L is the chip

length between facets (typically ~ 300 µm). Since the

optical gain spectrum provided by the electrical excita-

tion is quite broad (typically ~ 30 nm), the output of a

typical FP laser, especially when the device is kept

from reaching equilibrium by direct modulation, con-

sists of a small number of longitudinal modes spaced in

wavelength by about 1 nm. At a wavelength of 1.3 µm,

near the chromatic dispersion zero of conventional

single-mode fiber, even this spectral width can permit

transmission to distances of 40 km at speeds of 1.7 Gb/s

without excessively restrictive tolerances on the center

wavelength of laser operation. However, speeds of

2.5 Gb/s and higher make the wavelength tolerance

around the fiber dispersion zero prohibitive, due to

both variance in fiber dispersion zero and variance in

laser manufacture. This provided one incentive to

design lasers that restrict their operation to a single

fg S

ROg

ph

= ⋅′ ⋅1

2πν

τ


longitudinal mode of the laser cavity, providing a dra-

matic reduction in laser spectral width.

A more important incentive came from the signifi-

cantly lower fiber loss at 1.5 µm, with values below

0.2 dB/km allowing loss-limited spans of 100 km or

more, and the resulting savings that could result from

increasing the span length between regenerators

beyond 40 km. It was also attractive to provide

increased capacity by offering coarse wavelength divi-

sion multiplexing (WDM) with channels at both 1.3 µm

and 1.5 µm simultaneously on one fiber. However, the

high dispersion value of D = 17 ps/nm-km for conven-

tional fiber at 1.5 µm prohibited the use of FP lasers.

While a number of structures were examined in

the early 1980s for achieving single longitudinal mode

operation in semiconductor lasers, the distributed

feedback (DFB) laser emerged as the clear choice for

widespread manufacture and deployment. First

demonstrated by Kogelnik and Shank in dye lasers,23

this laser design replaced the cleaved facet mirror with

an optical feedback from a corrugated waveguide grat-

ing. Instead of relying on a discrete mirror reflection

with no spectral selectivity, this corrugation provides a

multitude of tiny sub-reflections from each corruga-

tion period that are phased properly for a large cumu-

lative net reflection only near the Bragg wavelength

λB given by λB = 2nΛg, where n is the phase refractive

index of the waveguide mode and Λg is the spatial

period of the corrugation. A typical value of Λg for 1.5-µm

operation is 0.23 µm, requiring ultraviolet laser inter-

ference to lithographically pattern the mask features

for etching the waveguide corrugation. In today’s DFB

lasers, the corrugation is typically etched into a surface

with buried quantum wells and then planarized with a

burying epitaxial growth to form buried rectangular

islands, as shown in Figure 2.

To avoid FP laser operation, the output facet of the

DFB laser is typically antireflection (AR) coated while

the other end is high-reflection (HR) coated to avoid

wasting power. DFB lasers routinely provide highly

single longitudinal mode operation, with other modes

rejected by values of 30 dB or more, and the advent of

1.5-µm DFB lasers quickly led to demonstrations of

transmission of rates as high as 4 Gb/s over distances

of 100 km.24 DFB lasers have also been critical to the

optical transmission of analog signals in the cable tele-

vision industry, where the linearity of the light-current

relationship and reduced dispersive distortion are

extremely critical to signal fidelity. Here, highly opti-

mized laser designs have been implemented where

analog distortion metrics, such as composite second-

and third-order distortions, are kept at maximum val-

ues of –63 dB and –67 dB below the carrier.

In the digital applications, further increases in

transmission data rates using directly modulated DFB

lasers were limited by both loss and remaining disper-

sion impairments related to laser chirp—the dynamic

spectral broadening that occurs during direct modula-

tion, even for a single longitudinal mode DFB laser.

The reasons for this chirp were understood largely due

to the theoretical work of Henry on laser linewidth.

Early measurements of semiconductor laser linewidths

revealed serious discrepancies from the well-known

Schawlow-Townes linewidth formula for lasers. Henry

was the first to realize the impact on laser dynamics

that results from the fundamental difference of band-

to-band gain in a semiconductor compared to typical

isolated atomic or molecular laser transitions.25 In the

semiconductor, small increases in gain with increasing

excitation are inherently accompanied by reductions

in absorption at shorter wavelengths that are actually

larger in magnitude than the gain increase at the gain

peak. The peak in the differential gain change is thus

inherently shifted to shorter wavelength than the gain

peak, which is most often the lasing wavelength.

Examination of the famous causal Kramers-Kronig

relations between real and imaginary indexes of

refraction then requires that a decrease in the real

index of refraction occur at the gain peak when gain is

increased. This negative change is further enhanced by

contributions resulting from the mobile carrier

“plasma” index of refraction, both producing negative

changes in index with increases in gain.

Henry’s analysis25 led to the widespread introduc-

tion of the “alpha factor,” with α defined as the differ-

ential real index change per unit carrier density divided

by the differential imaginary index (gain) change per

unit carrier density. Thus materials and structures with

high differential gain will also have small α factors.

Typical bulk active layers have values of α close to 6,


but MQW structures have values as small as α = 2. This

can be further reduced by forcing the laser to operate—

using the DFB corrugation, for example—at wave-

lengths that are blue-shifted relative to the gain peak,

further increasing the differential gain.

The importance of Henry’s α factor permeates

nearly all aspects of semiconductor laser dynamics and

noise, since it represents a fundamental amplitude-

phase coupling in the gain medium. Henry’s original

work illustrated that linewidths were increased by a

large factor of (1 + α2) above the Schawlow-Townes

linewidth, since the saturated gain medium dynami-

cally stabilizes spontaneous-emission-induced intensity

noise with gain fluctuations and, hence, laser fre-

quency fluctuations. Since changes in feedback into

lasers alter the threshold gain requirement and thus

the lasing frequency through α, the dynamics of feed-

back instabilities and injection locking are also gov-

erned by α. It was also shown that chirp is very simply

related, through the Henry α factor, to the optical

power excursions of direct modulation that arise

deterministically from injection-current-induced non-

Antireflectioncoating

Light

n-Contact

p-ContactHigh-reflection

coating

p-InGaAs

p-InP

n-InPsubstrate

Currentblockinglayers

InGaAsPMQW layers

DFB grating

DFB – Distributed feedbackMQW – Multiple quantum well

Figure 2.Dense WDM DFB laser.


equilibrium gain excursions.26 This related chirp to

a fundamental material parameter, promoted the

development of MQW lasers, and illustrated the desir-

ability of eliminating unnecessary waveform excur-

sions such as relaxation oscillations. By using highly

optimized MQW designs with very low α values,

today’s commercial DFB lasers can achieve transmis-

sion in conventional fiber over distances in excess of

200 km at 2.5 Gb/s with negligible dispersion penalty.

Lasers for Low-Cost ApplicationsA number of applications for lasers are extremely

cost sensitive, and some reductions in performance

can be traded against cost, operational simplicity, and

power consumption. Since the thermoelectric coolers

used in traditional long-haul laser modules consume

substantial power and space and also add to cost,

uncooled lasers, which are free to swing in tempera-

ture with the ambient, are desirable. This requires high

performance over a typical range of –40 to 85 °C—a daunting requirement when laser thresholds typi-

cally increase a factor of two for every 40°C increase in

temperature. This has required great care in optimizing

both blocking structures to avoid increases in leakage

currents and, especially, the detailed doping and layer

sequence of the MQW active region to provide ample

gain at the highest operating temperature.27 Today’s

uncooled lasers can achieve lasing in the laboratory to

temperatures as high as 130°C and commercially meet

customer requirements up to module case tempera-

tures of 85°C.

Since laser packaging usually represents the

largest component in final module cost, significant

work in recent years has been aimed at laser designs

with low-divergence output beams that may permit

passively aligned module assembly or, at least, simplifi-

cation of the packaging optics. Such lasers, termed

expanded-beam lasers (XBLs) or spot-size converted (SSC)

lasers, were shown in 1990 to be realizable based on

tapered waveguide extensions monolithically added to

the InP chip.28 Today, XBL laser designs have been

realized with performance nearly equivalent to that of

standard laser designs but with alignment tolerances

reduced by a factor of about 3. Such designs are

expected to be enablers for cost-reduced automated

packaging assembly, and it is intriguing that low-cost

applications may effectively employ a complex, inte-

grated laser structure that would have been unimagin-

able in the 1980s. This stems from the cost savings of

complex processes performed at batch or wafer level,

compared to complexity at the individual module

assembly level.

In the mid-1980s, a radically different approach to

laser design was demonstrated with good performance

based on a vertical-cavity configuration—the vertical-

cavity surface-emitting laser (VCSEL). First introduced

by Soda et al. in the 1970s,29 practical VCSEL opera-

tion awaited the advent of high-quality MQW gain

media and Bragg reflector mirrors as employed by

Jewell et al. in 1989.30 A high-performance VCSEL

structure of today, shown in Figure 3, illustrates the

basic design elements. Light in a VCSEL propagates

normal to the plane of the wafer and the planar epitax-

ial layers, with feedback provided by layered epitaxial

Bragg mirrors above and below the thin MQW gain

region. Since the gain available per pass is typically

below 1%, the mirrors are required to have reflectivi-

ties in excess of 99%. However, with low enough

losses, such structures can still achieve efficient lasing,

operating in regimes more analogous to low-gain gas

lasers. VCSELs have proven practical at wavelengths of

0.85 µm, using the AlGaAs material system on GaAs

substrates. While commercially practical designs offer-

ing a single spatial mode have not been widely intro-

duced, their application in short-distance data links

using multimode fiber is becoming increasingly com-

mon. The extension of VCSEL designs into the 1.3-µm

and 1.5-µm bands has proven difficult due to the diffi-

culty of achieving epitaxial high-reflectivity Bragg mir-

rors and the poorer temperature performance of the

gain in the InGaAsP material system.

Higher-Functionality ModulesThe extended transmission spans of the 1990s are

achieved with the addition of Er-doped fiber ampli-

fiers, which have been responsible for a revolution in

optical transmission system design. In particular, it

became practical to cascade amplified spans to provide

for unregenerated transmission over dispersion-limited

distances of 600 km and more in conventional fiber at

1.5 µm. However, this required spectrally pure sources

with chirp still smaller than that achievable with a


directly modulated DFB laser. These will be discussed

below as higher-functionality modules.

The obvious answer to this problem was the use of

external modulation, whereby the laser is operated

CW and its output is gated on and off through a mod-

ulator such as a LiNbO3 traveling-wave Mach-Zehnder

modulator. However, it was observed in the late 1980s

that electroabsorption (EA) modulators have design

features very similar to those of semiconductor lasers

and thus offer the potential to form a photonic inte-

grated circuit (PIC) in which both the laser and the EA

modulator are simultaneously fabricated on one InP

substrate. This design offered the promise of reduc-

tions in cost and power consumption compared with

LiNbO3 solutions, and its reduced size also allowed

nominally the same footprint as that of the prevalent

DFB packages of the day.

Electroabsorption modulators operate on a quan-

tum tunneling principle, termed the Franz-Keldysh effect

in bulk materials and the quantum-confined Stark effect

in quantum wells. Here, light is propagated through a

layer selected so that the light has a photon energy

lower than the bandgap, or onset of absorption, of the

layer. The application of a sufficiently large electric

field across this layer results in a voltage drop—generated in the very short distances accessible by

quantum mechanically tunneling—that is sufficient to

effectively reduce the transition energy and allow

absorption for the lower-energy lasing photons. Thus

the application of a field across the modulator causes

the light to be extinguished and converted to photo-

current drawn from the modulator.

The integration of a DFB laser and an EA modula-

tor was first demonstrated by locally etching away the

laser gain layers and regrowing a new, higher-bandgap

waveguide layer for the modulator.31 The electric field

is generated by reverse-biasing the p-n junction that is

fabricated in the same steps used for the forward-

biased laser p-n junction. However, the development

of advanced epitaxial techniques such as selective-area

N-ohmiccontact

P-ohmiccontact

Deposited top Bragg mirror

Index guide

Shallow implantedaperture

P+-layer

P/π-layerActive layer

Epitaxiallygrownbottom

Bragg mirror

Undopedsubstrate

N-layer

Laser emission

Figure 3.VCSEL cross section.


epitaxy, described in the “Materials and Reliability”

section, allowed for a simpler process. Here, suitable

masking during epitaxy allows for a change in thick-

ness of the waveguide core quantum wells along the

length of the device. As shown in Figure 4, this can

produce a structure in which the thinner quantum

wells in the modulator region have a higher effective

bandgap due to the shift in the quantum ground state

from the thinner quantum well.

This technique has been used for high-volume

manufacture of the electroabsorption-modulated laser

(EML) for deployment in long-haul, optically ampli-

fied systems.32 These devices routinely provide 2.5-Gb/s

sources with peak wavelength excursions of about

0.1 Å, or frequency excursions of about 1 GHz, result-

ing in only a small frequency-modulation contribution

to the inherent bandwidth of the digital intensity

encoding of the optical signal. For this reason, these

sources can transmit over optically amplified distances

in excess of 600 km in conventional fiber—close to the

fundamental limits imposed by the dispersion of a

pure intensity-encoded waveform. Achieving this level

of spectral purity also required great care to eliminate

electrical crosstalk between the modulator drive and

the laser bias, as well as extraordinary suppression of

output facet reflections that would provide time-

varying, destabilizing feedback into the laser.

In addition to optically amplified spans, the 1990s

also witnessed the rampant deployment of dense

WDM transmission systems, providing cost-effective,

upgradable capacity while maintaining the benefits of

optical amplifiers capable of boosting an entire wave-

length channel set in one device. In the mid-1990s,

the International Telecommunication Union (ITU)

accelerated the acceptance of WDM by providing for a

set of standardized channel wavelengths evenly spaced

AR – AntireflectionDFB – Distributed feedbackEA – ElectroabsorptionHR – High-reflection

MOCVD – Metal-organic chemical vapor depositionMQW – Multiple quantum wellSCH – Separate confinement heterostructure

DFB lasersection

EA modulatorsection p-InGaAs/InP cap

HR

Selective-areaMOCVD-grown

MQW-SCHInGaAsPgrating

n-InPsubstrate

Fe:InPblocking

AR

Figure 4.Integrated DFB laser/EA modulator by SAG.


in frequency above and below 193.1 THz in 100-GHz

increments.

The succession from initial deployments of 8 chan-

nels at 200-GHz channel spacing (1.6-nm spacing at

1.5 µm) to 16 channels at 100 GHz and 80 channels at

50 GHz has required sources with extreme wavelength

stability in addition to all the spectral attributes

described above. It remains a remarkable feature of

DFB lasers that the “gain clamping” at the threshold

value also fixes the operating index of refraction of the

excited optical waveguide and, hence, the operating

wavelength of the laser. This made the DFB laser, or

the DFB-based EML discussed above, the universal

ideal source for WDM systems. Reliability screening

similar to that already used for standard operation

readily yielded lasers that could maintain their operat-

ing wavelengths within ± 0.1 nm over system life, pro-

vided that the laser temperature drifts were kept

below a few tenths of a degree centigrade using

thermoelectric coolers (TECs) in the laser modules.

The latter requirement stems from the fact that the

operating lasing wavelength increases in a DFB laser at

a rate of approximately 0.1 nm/°C.

As the channel spacing in systems has narrowed

to current values of 50 GHz (0.4 nm), systems design-

ers have increasingly resorted to external optical

wavelength references to specify the desired ITU

wavelength channel. These references, typically

etalons, narrow-band thin-film filters, or fiber Bragg

grating filters, are used with photodetectors in electri-

cal servo loops, adjusting laser wavelength with tem-

perature using the TEC in the laser module. Very

recent work has seen the inclusion of the reference

inside the laser module, reinforcing the trend toward

ever-increasing functionality from the same module

through monolithic and hybrid integration. Using

etalons with cyclically repeating transmission reso-

nances, such modules are capable of locking on a

number of different successive ITU channels, forming

the basis for the first reliable multichannel WDM mod-

ules that offer channel selection to the end user.

This wavelength-selectable laser (WSL) feature is

particularly interesting to WDM network operators

who must inventory optical terminal units (OTUs) for

populated channels in the system. In addition to spar-

ing for failures in the field, this requires difficult vendor

supply logistics to ensure timely delivery and deploy-

ment of required channels in specified geographic

areas. A simpler solution would be universal OTUs that

can be assigned any ITU channel dynamically under

software control. Ultimately this functionality may be

used for dynamic routing and bandwidth allocation in

flexible add-drop elements in a WDM network.

Considerable work has gone into tunable lasers for

this WSL functionality. While laser temperature tun-

ing is a well accepted and reliable method, the

dynamic range is limited to about 3 nm for reasonable

temperature swings. Alternative structures include

arrays of DFB lasers and a variety of tunable distrib-

uted Bragg reflector (DBR) lasers. In the array sources,

a DFB array is fabricated with wavelengths spanning

the desired tuning range. To access a particular ITU

channel, the DFB with its wavelength closest to that

channel is activated and temperature tuned to the

exact value. This is accompanied by a servo loop simi-

lar to that described above.

The tunable DBR laser uses a single laser that

includes a tunable Bragg filter to change wave-

lengths, as shown in Figure 5. In contrast to the DFB

laser, in which the grating feedback is continuously

located along the gain medium, the DBR laser is fun-

damentally a two-mirror laser cavity. One mirror is

the cleaved facet, while the other mirror is a trans-

parent (higher bandgap) Bragg reflector waveguide

containing the corrugated grating. This provides a

wavelength-selective narrow-band mirror that selects

a single longitudinal mode for operation, rejecting

adjacent modes to levels of 40 dB or better, as in the

DFB laser. However, current injection into this higher

bandgap Bragg reflector section changes its index of

refraction and thus moves the center wavelength of

this narrow-band mirror to shorter wavelengths. This

in turn selects longitudinal modes at shorter wave-

lengths, thereby providing a WSL function in a single

resonator. Such lasers have also been fabricated incor-

porating both integrated EA modulators for low-chirp

information encoding and integrated semiconductor

optical amplifiers as power boosters for improved

transmission. Wavelength coverage of about 8 nm is

readily achievable with this design, providing 20 ITU


Gainsection

DBRtuning mirror

Opticalamplifier Detector Modulator

0

–10

–20

–30

–40

–50

–60

–701545 1550 1555

Wavelength (nm)

Inte

nsi

ty

HR coating(back facet)

AR coating(front facet)

AR – AntireflectionDBR – Distributed Bragg reflectorHR – High-reflection

Figure 5.Wide-band wavelength-selectable laser.


channels at 50-GHz spacing from one module with the

incorporation of a suitable reference etalon and servo

control for the module.

Future AdvancesAdvances in semiconductor lasers have clearly been

paced by the maturation of the underlying materials

and processing technologies. In fields outside of

telecommunications, the 1990s have seen exciting basic

materials advances in the GaN/InGaN system that

resulted in practical lasers emitting in the blue and ultra-

violet regions of the spectrum. For long-wavelength

materials, there has been continued research and even

commercialization of the InGaAlAs/InP system, where

advantageous band offsets have shown improvements

in both high-temperature laser performance and

quantum-well modulator performance. The remark-

able achievements in the infrared quantum cascade

lasers have shown how artificially engineered transi-

tions can form the basis for fundamentally new semi-

conductor laser designs, and explorations are

beginning to assess the potential of these structures in

the 1.5-µm band. Speculation and research continue

on the potential merits of reduced-dimensionality

quantum confinement in quantum-wire or quantum-

dot lasers, but performance from these materials has

not yet been truly competitive with quantum-well and

bulk active-layer devices.

Critically important materials advances continue

to stem from improved epitaxial reactor design leading

to larger wafers and higher levels of uniformity. The

first decade in the new millenium can be expected to

see a migration to 3- and 4-inch reactors, achieving

thickness uniformity at the 0.5% level and photolumi-

nescence (composition) uniformity at approximately

the 1-nm level. These advances will enable designers

to implement high-performance designs with high

yield and ultimately eliminate much of the testing and

yielding that takes place between manufacturing steps.

Process advances already nearing completion

include the development of new automation equip-

ment specific to optoelectronic chip handling, includ-

ing automated cleaving techniques, automated AR/HR

coating processes, and even fully automated chip test-

ing and sorting operations. Other advances are

expected in the area of process modeling for etching,

doping, specialized epitaxial growth steps, and associ-

ated wafer-level characterization tools. One example—selective-area growth—has already been mentioned in

the “Materials and Reliability” section.

In the area of higher performance, we expect to

see EML modules in the next few years with speeds

reaching 40 Gb/s and beyond. There are early indica-

tions that a return-to-zero (RZ) or pulse transmission

format may emerge, in addition to today’s common

nonreturn-to-zero (NRZ) format, suggesting that new

integrated modulator configurations may become

desirable. For pump lasers, we expect powers reaching

the 1-watt level. We expect continued pressure in the

area of WDM, with advances in manufacturing allow-

ing on-demand fulfillment of transmitter orders at

arbitrary ITU channels. We expect to see WSL trans-

mitter assemblies or chip designs that dynamically

access 80 or more ITU channels.

In the area of higher or new functionality, there is

an especially provocative body of research under way

in all-optical signal processing and regeneration. While

SiGe and InP electronics are rising to the challenge of

terminal equipment for even the latest generation of

40-Gb/s transmission, past experience suggests that

the required functions at 160 Gb/s are likely to be in

commercial demand before electronics have advanced

to the required maturity. Using the concept of optical

time division multiplexing (OTDM), researchers have

already demonstrated transmission of 160 Gb/s over

300 km of fiber.33 These demonstrations exploit the

ultrafast gating capabilities of fast EA modulators and

all-optical nonlinear switching phenomena in semi-

conductor optical amplifier (SOA)-based devices to

perform functions of multiplexing/demultiplexing,

optical clock recovery, and digital optical regeneration.

Figure 6 illustrates one such configuration, show-

ing how SOA-based Mach-Zehnder (MZ) inter-

ferometers can be used with short RZ pulses to

perform digital optical regeneration. A pulse from the

data stream saturates one arm of the interferometer

and, due to the same amplitude-phase coupling dis-

cussed earlier for the Henry α factor, causes a phase

change that unbalances the interferometer momen-

tarily, allowing a retimed, clean clock pulse to be


transmitted. A time-delayed replica of the data pulse

then feeds the other side of the MZ interferometer,

rebalancing to prevent further transmission.

Demultiplexing can be accomplished using similar

configurations, switching the role of the clock and the

data streams and using a divided-down clock.

These are only today’s examples of all-optical

functionality, and the coming years show great

promise in providing a wealth of new integrated con-

figurations employing lasers, SOAs, and EA modula-

tors for practical ultrafast OTDM. The same all-optical

functionality may even be useful for logical manipula-

tion of packet headers to allow for routing functions in

all-optical ultrafast networks.

The proliferation of WDM, the potential of ultra-

fast OTDM, and continued pressure on advanced low-

cost packaging technologies provide a huge range of

challenges and exciting research for the coming

decade. The potential for impact in these areas sug-

gests that advances in semiconductor lasers and related

components are likely to sustain their role as defining

elements in the progress of optical communications.

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(Manuscript approved May 2000)

WILLIAM F. BRINKMAN is vice president of research atBell Labs in Murray Hill, New Jersey. Hereceived B.S. and Ph.D degrees in physicsfrom the University of Missouri in Columbiaand subsequently spent one year as aNational Science Foundation Postdoctoral

Fellow at Oxford University in England. Early in hiscareer, Dr. Brinkman worked on theories of condensedmatter, which included the theory of spin fluctuationsin metals and other highly correlated Fermi liquids.This work resulted in a new approach to highly cor-related liquids in terms of almost localized liquids.Later, he explained the superfluid phases of one of theisotopes of helium and many properties of these exoticstates of matter. He co-developed the theoreticalexplanation of the existence of electron-hole liquids insemiconductors. His theoretical work on liquid crystalsand incommensurate systems contributed to the theo-retical understanding of condensed matter. Dr. Brinkmanis a fellow of both the American Association for theAdvancement of Science and the American PhysicalSociety. He chaired the National Academy of SciencesPhysics Survey and the organization’s Solid State

Committee. He served on the Council of the NationalAcademy of Sciences and is a member of the AmericanAcademy of Arts and Sciences. For his own researchand his research leadership, Dr. Brinkman received the1994 George E. Pake Prize.

THOMAS L. KOCH is head of the Lightwave DevicesResearch Department at Bell Labs in Holmdel,New Jersey, and Chief Technical Officer forOptoelectronic Products at Lucent Techno-logies. He holds an A.B. in physics fromPrinceton University in Princeton, New Jersey,

and a Ph.D. in applied physics from the California Instituteof Technology in Pasadena. Dr. Koch received theDistinguished Lecturer Award and the William StreiferAward for Scientific Achievement from IEEE/LEOS andis a Bell Labs Fellow as well as a fellow of both the IEEEand the OSA.

DAVID V. LANG, adjunct director of Materials PhysicsResearch at Bell Labs in Murray Hill, NewJersey, is responsible for materials researchon III-V semiconductors and dielectrics usedin microelectronics and optoelectronics. Heholds a B.A. in physics from Concordia

College in Moorhead, Minnesota, and a Ph.D, also inphysics, from the University of Wisconsin in Madison.Dr. Lang received the IEEE Morris E. Leeds Award and isa fellow of the American Physical Society.

DANIEL P. WILT is director of Telecommunications Product Development for Lucent’s Micro-electronics Group in Breinigsville, Pennsylvania.He received an A.B. in mathematics andphysics from the University of SouthernCalifornia in Los Angeles and a Ph.D. in

applied physics from the California Institute ofTechnology in Pasadena. Dr. Wilt, a Bell Labs Fellow,is responsible for development of optoelectronic components for the telecommunications market. ◆

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