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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
Bell Labs Technical Journal ◆ January–March 2000 153
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
154 Bell Labs Technical Journal ◆ January–March 2000
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
Bell Labs Technical Journal ◆ January–March 2000 155
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.
156 Bell Labs Technical Journal ◆ January–March 2000
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
Bell Labs Technical Journal ◆ January–March 2000 157
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,
158 Bell Labs Technical Journal ◆ January–March 2000
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.
Bell Labs Technical Journal ◆ January–March 2000 159
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
160 Bell Labs Technical Journal ◆ January–March 2000
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.
Bell Labs Technical Journal ◆ January–March 2000 161
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.
162 Bell Labs Technical Journal ◆ January–March 2000
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
Bell Labs Technical Journal ◆ January–March 2000 163
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.
164 Bell Labs Technical Journal ◆ January–March 2000
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
Bell Labs Technical Journal ◆ January–March 2000 165
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.
References1. A. L. Schawlow and C. H. Townes, “Infrared
and Optical Masers,” Phys. Rev., Vol. 112, 1958,p. 194.
2. T. H. Maiman, “Stimulated Optical Radiation inRuby,” Nature, Vol. 6, 1960, p. 106.
3. M. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill, Jr.,and G. Lasher, “Stimulated Emission of Radia-tion from GaAs p-n Junctions,” Appl. Phys. Lett.,Vol. 1, No. 3, Nov. 1962, pp. 62–64.
4. R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys, and R. O. Carlson, “Coherent LightEmission from GaAs Junctions,” Phys. Rev. Lett.,Vol. 9, No. 9, Nov. 1962, pp. 366–368.
5. N. Holonyak, Jr., and S. F. Bevacqua, “Coherent
SOA-MZI
Retimed clock pulses
Incoming data stream
Regenerated data stream
Delayed replica of incoming data
SOA F1
SOA F2
F1
π
0
F2
π
0
F1 – F2
π
0(Switchingwindow)
τ Time
τ
MZI – Mach-Zehnder interferometerSOA – Semiconductor optical amplifier
Packaged SOA-MZI device
Figure 6.All-optical digital regeneration by InP Mach-Zehnder interferometer.
166 Bell Labs Technical Journal ◆ January–March 2000
(Visible) Light Emission from GaAsP Junctions,”Appl. Phys. Lett., Vol. 1, No. 4, Dec. 1962, pp. 82–83.
6. T. M. Quist, R. H. Rediker, R. J. Keyes, W. E. Krag,B. Lax, A. L. McWhorter, and H. J. Zeigler,“Semiconductor Maser of GaAs,” Appl. Phys.Lett., Vol. 1, No. 4, Dec. 1962, pp. 91–92.
7. H. Kroemer, “A Proposed Class of Hetero-junction Injection Lasers,” Proc. IEEE, Vol. 51,No. 12, Dec. 1963, p. 1782–1783.
8. Zh. I. Alferov and R. F. Kazarinov, “Semicon-ductor Laser with Electrical Pumping,” U.S.S.R.Patent, author’s certificate 181,737, claim 950,840,Mar. 30, 1963.
9. I. Hayashi, M. B. Panish, P. W. Foy, and S. Sumski,“Junction Lasers Which Operate Continuouslyat Room Temperature,” Appl. Phys. Lett., Vol. 17,No. 3, Aug. 1970, pp. 109–111.
10. J. J. Hsieh, J. A. Rossi, and J. P. Donnelly,“Room-Temperature CW Operation ofGaInAsP/InP Double-Heterostructure DiodeLasers Emitting at 1.1 µm,” Appl. Phys. Lett.,Vol. 28, No. 12, June 1976, pp. 709–711.
11. M. A. Pollack, R. E. Nahory, J. C. DeWinter,and A. A. Ballman, “Liquid Phase EpitaxialIn1-xGaxAsyP1-y Lattice Matched to (100) InP over the Complete Wavelength Range 0.92 ≤ λ ≤ 1.65 µm,” Appl. Phys. Lett., Vol. 33,No. 4, Aug. 1978, pp. 314–316.
12. A.Y. Cho, “Film Deposition by Molecular BeamTechniques,” J. Vacuum Science and Technol.,Vol. 8, No. 5, Sept.–Oct. 1971, pp. S31–S38.
13. R. Dingle and C. H. Henry, “Quantum Effects inHeterostructure Lasers,” U.S. Patent 3,982,207,filed Mar. 7, 1975, issued Sept. 21, 1976.
14. R. Dingle, W. Weigmann, and C. H. Henry,“Quantum States of Confined Carriers in VeryThin AlxGa1-xAs-GaAs-AlxGa1-xAs Hetero-structures,” Phys. Rev. Lett., Vol. 33, No. 14, Sept. 1974, pp. 827–830.
15. G. C. Osbourn, “Strained-Layer Superlatticesfrom Lattice Mismatched Materials,” J. Appl.Phys., Vol. 53, No. 3, Mar. 1982, pp.1586–1589.
16. Zh. I. Alferov, V. M. Andreev, D. Z. Garbuzov,Yu. V. Zhilyaev, E. P. Morozov, E. L. Portnoi,and V. G. Trofim, “Investigation of theInfluence of the AlAs-GaAs HeterostructureParameters on the Laser Threshold Current andRealization of Continuous Emission at RoomTemperature,” Sov. Phys.—Semicond. (Engl. transl.),Vol. 4, No. 9, Mar. 1971, pp. 1573–1575; trans-lated from Fiz. Tekh. Poluprovodn., Vol. 4, No. 9,Sept. 1970, pp. 1826–1829.
17. T. J. Tasukada, “GaAs-Ga1-xAlxAs Buried-Heterostructure Injection Lasers,” J. Appl. Phys.,
Vol. 45, No. 11, 1974, pp. 4899-4906.18. M. Hirao, S. Tsuji, K. Mizushi, A. Doi, and
M. Nakamura, “Long-Wavelength IndiumGallium Arsenide Phosphide/Indium PhosphideLasers for Optical Fiber CommunicationSystems, “J. Optical Commun., Vol. 1, No. 1,1980, pp. 10–14.
19. W. T. Tsang, “Extremely Low Threshold(AlGa)As Graded-Index Waveguide Separate-Confinement Heterostructure Lasers Grown byMolecular Beam Epitaxy,” Appl. Phys. Lett., Vol. 40, No. 3, Feb. 1982, pp. 217–219.
20. T. Ikegami and Y. Suematsu, “Resonance-LikeCharacteristics of the Direct Modulation of aJunction Laser,” Proc. IEEE, Vol. 55, 1967, p. 122.
21. K. Y. Lau, C. H. Harder, and A. Yariv, “UltimateFrequency Response of GaAs Injection Lasers,”Optics Commun., Vol. 36, No. 6, 1981, pp. 472–474.
22. J. E. Bowers, “High-Speed Semiconductor LaserDesign and Performance,” Solid State Electron.,Vol. 30, No. 1, Jan. 1987, pp. 1–11.
23. H. Kogelnik and C. V. Shank, “StimulatedEmission in a Periodic Structure,” Appl. Phys.Lett., Vol. 18, No. 4, Feb. 1971, pp.152–154.
24. A. H. Gnauck, B. L. Kasper, R. A. Linke,R. W. Dawson, T. L. Koch, T. J. Bridges,E. G. Burkhardt, R. T. Yen, D. P. Wilt,J. C. Campbell, K. T. Nelson, and L. G. Cohen,“4 Gb/s Transmission over 103 km of OpticalFiber Using a Novel Electronic Multiplexer/Demultiplexer,” Optical Fiber Commun. Conf. (OFC ’85), San Diego, Calif., Feb. 11–13, 1985,paper PD2.
25. C. H. Henry, “Theory of the Linewidth of Semi-conductor Lasers,” IEEE J. Quantum Electron.,Vol. QE-18, No. 2, Feb. 1982, pp. 259–264.
26. T. L. Koch and J. E. Bowers, “Nature of Wave-length Chirping in Directly Modulated Semicon-ductor Lasers,” Electron. Lett., Vol. 20, No. 25–26,Dec. 1984, pp. 1038–1040.
27. K. Kojima, “High-Power, High-Efficiency, HighlyUniform 1.3-µm InGaAsP/InP Strained MQWLasers,” Optical Fiber Commun. Conf. (OFC ’95)Tech. Digest, San Diego, Calif., Feb. 26–Mar. 3,1995, paper ThG3, pp. 253–254.
28. T. L. Koch, U. Koren, G. Eisenstein, M. G. Young,M. Oron, C. R. Giles, and B. I. Miller, “TaperedWaveguide InGaAs/InGaAsP Multiple QuantumWell Lasers,” IEEE Photon. Technol. Lett., Vol. 2,No. 2, Feb. 1990, pp. 88–90.
29. H. Soda, K. Iga, C. Kitahara, and Y. Suematsu,“GaInAsP/InP Surface Emitting Injection Lasers,”Jpn. J. Appl. Phys., Vol. 18, No. 12, Dec. 1979,pp. 2329–2330.
Bell Labs Technical Journal ◆ January–March 2000 167
30. J. L. Jewell, A. Scherer, S. L. McCall, Y. H. Lee,S. Walker, J. P. Harbison, and L. T. Florez,“Low-Threshold Electrically Pumped Vertical-Cavity Surface-Emitting Microlasers,” Electron.Lett., Vol. 25, No. 17, Aug. 1989, pp. 1123–1124.
31. H. Soda, M. Furutsu, K. Sato, M. Matsuda, andH. Ishikawa, “5 Gb/s Modulation Characteristicsof Optical Intensity Modulator MonolithicallyIntegrated with DFB Laser,” Electron. Lett., Vol. 25, No. 5, Mar. 1989, pp. 334–335.
32. J. E. Johnson, T. Tanbun-Ek, Y. K. Chen,D. A. Fishman, R. A. Logan, P. A. Morton,S. N. G. Chu, A. Tate, A. M. Sargent, P. F. Sciortino, Jr., and K. W. Wecht, “Low-Chirp Integrated EA Modulator/DFB LaserGrown by Selective-Area MOVPE,” Proc. 14th
IEEE Intl. Semicond. Laser Conf., Maui, Hawaii,Sept. 19–23, 1994, paper M4.7, pp. 41–42.
33. B. Mikkelsen, G. Raybon, R.-J. Essiambre,K. Dreyer, Y. Su, L. E. Nelson, J. E. Johnson,G. Shtengel, A. Bond, D. G. Moodie, and A. D. Ellis, “160-Gb/s Single-Channel Trans-mission over 300-km Nonzero-Dispersion Fiberwith Semiconductor-Based Transmitter andDemultiplexer,” European Conf. on OpticalCommun. (ECOC ’99) Postdeadline Paper Digest,Nice, France, Sept. 26–30, 1999, paper PD2-3,pp. 28–29.
(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. ◆