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Appendix D
Technology Status of Optical Telecommunications
LASERS
A key element of all long-haul optical communication systems is the semi-
conductor laser. The desirable characteristics of the semiconductor laser are
determined in large measure by the characteristics of the optical fiber and the
lightwave system architecture. Future lightwave systems are likely to contain
a large number of closely spaced channels operating in the 1.5- to 1.6-micron
wavelength band of low loss. In addition, high-bit-rate systems (> 1 Gbit/s)
require narrow-line, single-frequency lasers to offset the chromatic dispersion
of the fiber.
Two types of lasers have been extensively investigated for obtaining single-
wavelength emission. They are the external cavity laser and the distributed
feedback (DFI3) laser. External cavity lasers that have been extensively inves-
tigated include the cleaved-coupled cavity laser (C3 laser); graded-index,
external cavity laser; fiber external cavity laser; the silicon chip Bragg reflector
(SCBR) laser; and InP-based, external Bragg reflector lasers. The linewidth
of the semiconductor laser is determined by the fluctuations in the phase and
intensity of the photon field in the laser cavity. These depend markedly on the
cavity length. Continuous wave linewidths on the order of a few kilohertz (kHz)
(necessary for coherent applications) have been obtained for external cavity
lasers compared to those of many megahertz (MHz) for DEB lasers. It is likely
that some form of external cavity laser will be the laser of choice for the next
generation of high-data-rate, coherent transmission systems.
84
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APPENDIX D
85
Among key desirable features of such external cavity lasers would be the
fabrication of a truly compact, robust laser. Multielectrode lasers capable of
wavelength tuning over the gain spectrum of the laser will become important
for closely spaced wavelength WDM applications. Even though tuning features
have been demonstrated in the laboratory, we do not to date have a stable,
compact, widely tunable, single-frequency semiconductor laser suitable for
practical commercial use. Further research in this area is warranted. Another
important problem is the behavior of the laser under modulation conditions.
"Chirping" (change of frequency during a pulse interval) of the laser under
direct modulation is an important limitation as one goes beyond about
2 Gbits/s. The "chirping" behavior of semiconductor lasers is determined in
part by the internal structure of the laser. Buried heterostructure (BH) lasers
have generally low chirp and are favored, despite their complexity in manufac-
ture, for high-bit-rate systems. As one goes to the 10-Gbits/s regime, it is likely
that external modulation willbe necessary for high-bit-rate, error-free transmis-
sion. The use of external modulators, however, results in additional power loss,
and the system designers would need high-power (~50 mW) lasers for practical
high-bit-rate systems. In applications where optical interconnection distances
are relatively short (e.g., internal equipment interconnects), and laser output
power is therefore not critical, lasers with very low-threshold currents ~ < 1 mA)
based on single-quantum well structures represent an important emerging
technology.
FIBER AND CABLES
The two main characteristics of a fiber waveguide are its attenuation and
bandwidth. Attenuation in present silica materials has been reduced to almost
the theoretical lower limit. Research is currently under way to fabricate fibers
using fluoride glasses, which may have attenuations a hundred times lower than
present fibers. There are enormous problems to overcome in making practical
waveguides from these new materials, but the research warrants continuation
because the potential benefits are immense. Conceivably, transoceanic cables
could be constructed without underwater repeaters, as an example of one
benefit.
The maximum data rate (bandwidth) of an optical signal that can be sup-
ported in a fiber is presently limited by fiber material and waveguide dispersion,
interacting with the spectral width of the optical sources. With emerging single-
frequency sources, this shows no sign of becoming a limiting factor. Further,
carefully tailored waveguide designs have been used to make fibers that can
provide for flexibility in the choice of light source (the dispersion-shifted and the
dispersion-flattened fibers, for example). This fiber design work is an important
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APPENDIX D
research effort that should proceed simultaneously with other activities to
facilitate alternative system designs.
Beyond attenuation and bandwidth are a number of environmental
requirements defining fiber degradation in use. These requirements have
become increasingly important as the desire has evolved to both reduce fiber
protection and submit fibers to more hostile environments. This has led to
complex materials studies aimed at the improvement of materials properties.
One such large effort is in hermetic coatings for strength. A second involves
studies of atomic defects generated (or activated) by hydrogen diffusion into
the glass, leading to optical absorption at system wavelengths being used. While
this phenomenon is generally understood and empirical results show that the
attenuation increase is usually negligible, there remains concern about adverse
environments with high hydrogen content or high temperature. Continuing
research to understand this problem at the atomic level will help provide
reassurance to fiber users and will assist manufacturers in expanding the
environmental durability.
The function of cabling is primarily to protect the fiber from mechanical
stress, both lateral and longitudinal. The latter is conceptually the most
straightforward and is accomplished by adding strength members to the cable,
as well as by incorporating excess fiber length so that some cable elongation
can occur before the fibers are strained. Progress in this area has been
adequate and continues as higher-specific-strength materials become available.
Lateral stresses on the fibers are more complex and actually are generated
during the cabling process. These minute cabling stresses result in "micro-
bends" that increase the fiber attenuation. Advances in cabling techniques and
the increased use of single-mode fibers have diminished the importance of this
problem at present attenuation levels. However, increased understanding is
helpful in advancing present technology and will become essential if the lower-
attenuation fluoride fibers are to become commercially available. Microbends
from cabling have an added dimension of complexity when multimode fibers are
used, since the mode stripping (removal of higher-order propagating modes)
they cause also increases the fiber bandwidth (reduces pulse spreading because
of the smaller spread of propagation speeds amongst the modes that are not
stripped).
System designers desire a length-bandwidth relation that optimizes fiber
performance. Since pulse spreading in multimode fiber depends on a large
number of fiber and excitation parameters, so far it has not been possible to
provide a satisfactory analytical expression for the length-bandwidth relation.
While the importance of the multimode fiber length-bandwidth relationship has
diminished with increasing single-mode fiber usage, multimode fibers are being
considered for future applications, which would again increase the importance
of research in this area.
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APPENDIX D
87
Aside from these specific technical areas, research and development in
cable technology is gradually advancing with corresponding progress in cabled
fiber cost reduction. This cost-performance improvement work should continue
to be supported.
PASSIVE COMPONENTS
The cost and performance of passive components are major impediments
to the advance of a number of applications of optical communications, e.g.,
local area networks. An exception to this may be fibers and cables that have
benefited from a great deal of research and development over the years. The
other components (discussed here) have not received as much attention and
are not improving at the same rate as fibers and cables. For example, almost
all the commercial passive components of today were designed and built in the
laboratory over 10 years ago--the incorporation of new ideas has been rare.
The obstacles these components present can be illustrated by the performance
of Remountable connectors, perhaps the most commonly used device, which
typically can have insertion losses of up to about 1 dB. The power loss a system
can tolerate is typically 20 to 30 dB. Therefore at the 1-dB-per-connector loss
level, no more than about 20 connectors can be incorporated between a source
and detector. Contrasting this with the negligible loss of present coaxial con-
nectors shows that major design compromises have to be made in optical fiber
system design.
The basic reason for this difficulty is the small cross-sectional size of the
optical beam--which is, at the same time, a major advantage in terms of
miniaturizing complex systems. There is a lack of sufficiently precise, three-
dimensional forming techniques for manufacture of optical fiber system
components. The two-dimensional solution, photolithography, has been used
to some advantage when the third dimension is small; but it is generally
inadequate. The forming problem has led to cost-performance trade-offs in
component insertion loss. This problem becomes particularly vexing with the
increasing incorporation of single-mode (i.e., small-core) fibers that require
tighter tolerances. It is generally conceded that systems of the future will utilize
single-mode fibers, so that this problem will assume increasing importance.
These general considerations, and some more specific ones, will become
more evident in the following discussion of individual components.
Connectors
Single-f~ber connectors are classified into two types, contact or buttjoint
and expanded beam. The expanded-beam connector is more stringent in
angular alignment tolerances, whereas the contact method is more stringent in
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APPENDIX D
lateral positional tolerances. The practical compromise between these two
needs further investigation, including the necessity for optically polishing fiber
ends. Single-fiber connectors typically cost $10 to $100 for multimode fiber
and $20 to $200 for single-mode fiber, depending on performance. In general,
assembly in the field needs simplification.
Multif~ber connectors generally accommodate only a few fibers and employ
some type of circular geometry. As in all connectors, draftspersons also have
difficulty assembling these in an adverse environment.
Array connectivity seeks the advantage of easily joining cables with large
numbers of fibers. Presently, a flat grooved plate holding the array of fibers is
the basic element. The difficulty of obtaining uniform results for all fibers is
apparent, and no attempt is made to fabricate such connectors in the field.
Instead, cables are terminated in the factory, with attendant lack of flexibility
in deployment.
Connectors with low reflection coefficients are increasingly important in
high-data-rate and coherent communications systems because reflections
returned to the lasers used in those applications can cause instabilities in the
laser output that are harmful to system performance. Reflections at connectors
placed in series along a fiber can also cause fluctuations in the output power at
the end of the fiber due to interference effects.
Wavelength-Division Multiplexing (WDM) Components
for Filtering, Multiplexing, and Demultiplex~ng
Wavelength-division multiplexing (WDM), optical communication in the
wavelength multiplex mode, allows modulated radiation from several laser
sources of clearly distinct wavelengths to be transmitted simultaneously over a
single fiber. Spectrally selective optical multiplexers or de-multiplexers are
used at the start and end of the transmission route to ensure low-loss combina-
tion and separation of light of the various wavelengths. WDM technology is a
key to the utilization of the full bandwidth capability of optical fibers.
Commercial communication systems in operation today utilize wavelengths that
are widely separated (0.8 microns, 1.3 microns, and 1.5 microns), but future
systems are expected to utilize single frequency lasers in the 1.5- to 1.6-micron
low-loss band, spaced a nanometer or less apart in wavelength.
Integrated optics is expected to play an important role in developing the
necessary active and passive WDM components required for high-capacity
WDM. These will require arrays of stable, single-frequency lasers of well-
defined wavelength, low-loss waveguides, narrow-band gratings, filters, and
multi-wavelength photodetectors.
Realization of practical WDM components for closely spaced wavelength
applications hinges greatly on the development of new materials technology.
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89
Vapor-phase growth techniques for fabrication of arrays of InP-based lasers
and detectors of high uniformity need to be developed. New materials
combinations with large electrical field effects for switching of optical signals
in waveguides need to be explored. The possibility of using the mature silicon
materials and processing technology for fabrication of low-loss waveguides and
gratings on a silicon chip is particularly attractive.
Fiber Backplane and Other Optical Interconnects
As data rates increase, interconnections inside equipment become
increasingly difficult to implement with conventional copper traces on circuit
boards and backplanes, twisted pairs, and coaxial cables. Recently very short-
distance f~ber-optic interconnects have been used in place of copper wires and
cables. However, there is research in progress to implement both the photonic
equivalent of printed-circuit interconnects and also optical free-space intercon-
nects.
The challenges associated with optical circuit-board and backplane traces
are twofold. First, appropriate materials and processing technologies must be
created to form light-guiding regions on circuit boards and backplanes of
sufficiently low loss, of sufficient dimensional tolerance, and sufficient reliability
--all at an acceptable cost. Second, optoelectronic interfaces are needed to
economically couple light into these light-guiding traces and to remove the light
at the other end (or possibly at several places along a light-guiding trace). It is
desirable that these light-launching and light-receiving optoelectronic interfaces
be integrated into electronic circuits, so that packaged electronic circuits with
these optoelectronic interfaces can be fabricated as components that can be
mounted directly on circuit boards containing the optical traces they will access.
Free-space interconnects offer the possibility that arrays of optical
transmitters can be connected to arrays of optical receivers with relatively
simple and rugged lenses to define the optical paths. Such free-space optical
interfaces offer certain advantages over fiber and optical traces on circuit
boards, such as propagation delays that can be identical for a large number of
connections that are not exactly parallel. Free-space interconnects eliminate
the need for connectors and may improve reliability of interconnections.
Realization of practical free-space interconnects awaits the development of
arrays of reliable optoelectronic transmitter and receiver modules and ap-
propriate lens and physical design technologies to achieve the desired align-
ments. Research in interconnect topologies is also needed to obtain a synergy
between the capabilities of free-space interconnects and interconnection
applications that can use those capabilities.
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APPENDIX D
Other Passive Components
As higher data rates and coherent techniques are employed in f~ber-optic
systems, the lasers used to meet the requirements of these applications are
increasingly sensitive to reflections that cause various instabilities. Optical
isolators are needed to reduce reflection effects. Miniature opto-isolators are
being employed in high-performance laser packages today. Further improve-
ments in packaging and integration are desirable to reduce cost and increase
reliability.
Optical directional couplers with low insertion losses and predictable
coupling ratios are needed for removing and adding light in passive bus
configurations. Although various directional coupler designs have been
demonstrated and manufactured (e.g., fused tapered couplers), improvements
in cost, performance, and reliability are still needed for many applications.
Similar remarks apply to star couplers, which are used in networking configura-
tions that are alternatives to passive bus configurations.
Some couplers are incorporated in flat substrates and therefore suffer from
geometrical mismatch in going from fibers to rectangular waveguides. Planar
structures by themselves do not have a bright future but will become very
important when combined with active devices in the emerging technology of
optoelectronic integrated circuits.
PHOTONIC SWITCHES
A number of technologies have been demonstrated for switching an optical
signal between two or more outgoing paths. These include mechanical devices
that physically move fibers or that physically move lenses or mirrors directing
an optical beam; optoelectronic devices where an applied voltage across two or
more electrodes causes a field within an electro-optic material, which in turn
changes the coupling of waveguides within the material or otherwise modifies
the optical characteristics of an optical circuit within the material; electrically,
acoustically, or optically controlled gratings created within a material to cause
diffraction of an optical beam; and electrically or optically controlled non-linear
optical devices.
Of the variety of optical switching devices demonstrated or proposed, some
are more practical than others, and some have near-term applications (e.g.,
simple mechanical switches for remotely controlled optical cross connects).
However, optical switching devices are in general larger and more power
consuming than their electronic counterparts; and many of these devices have
numerous practical limitations such as temperature sensitivity, polarization
dependence, wavelength dependence, requirements for high voltages, and high
loss. Materials improvements and device-design improvements are the two key
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APPENDIX D
91
dimensions of current research on these devices. While these device limitations
are being actively addressed, much systems research is also needed to achieve
large-scale application of optical switching devices in systems as a replacement
for electrical-to-optical conversion accompanied by electronic switching.
OPTICAL AMPLIFIERS
Optical amplifiers are potentially important building blocks of all optical
communication systems. In present optical communication systems the
amplification function is accomplished by converting the optical signal to
electronic form (detection), amplifying the electronic signal with an electronic
amplifier, and then reconverting the amplified electronic signal to optical form.
There are two main types of optical amplifiers: (1) f~ber-based amplifiers, and
(2) semiconductor laser-based amplifiers. The main uses of optical amplifiers
are in (1) pre-amplifier applications where amplification of low-level signals is
performed and there is no intentional loss between the output of the amplifier
and the receiver and (23 in-line applications where relatively large optical signals
are amplified and loss is expected between the output of the amplifier and the
receiver. The former are likely to be important in high-bit-rate (>2 Gbits/s)
systems if good APDs do not exist. In-line amplifiers are believed to be useful
in both long haul (to compensate for fiber losses), in the local loop (to
compensate for split-off and coupling losses), and in optical switching to
compensate for losses in the switches.
Over the last few years, there has been considerable worldwide activity in
developing amplifiers with large available gain, low insertion loss, low noise,
large bandwidth, and saturation output power. To date, no practical semicon-
ductor laser amplifier has been developed. The main potential advantages are
the ease of manufacturing, high gain, and the tunability of the bandpass used for
noise filtering and channel selection. However, semiconductor laser amplifiers
have polarization-dependent gain that needs to be controlled through devel-
opment of better optical isolators.
Fiber amplifiers, especially Raman amplifiers, suffer from the high pump
power required for amplification. Research needs to be performed on special
fibers with low loss and high Raman cross-section as well as special dopants
for optically pumped fiber amplifiers. This is a promising field that needs
increased attention.
INTEGRATION AND PACKAGING
The interfacing of optical components with electronic ones is a key element
for all future information transmission systems where one envisages the merger
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APPENDIX D
of optical signal processing with purely electronic media such as high-speed
computers. One can imagine that in the interest of low cost and circuit
simplicity, the terminal sources and receivers in the optical link may take on
electronic processing involved with the communication link. One can fabricate,
for example, a heterojunction bipolar transistor driver and a laser on a single
chip or a pin photodiode and a field-effect transistor on the same chip. More
complex integrated devices involving arrays of lasers, detectors, amplifiers,
transistors, and modulators can be imagined. One of the main motivations for
optoelectronic integration besides cost is performance. As speed increases, the
interconnection of integrated circuits and subsystems becomes more critical and
cans ot be easily implemented with technologies available today. Compound
semiconductor-based transistors are intrinsically faster than Si ones, and the
monolithic approach provides significant additional improvements through
reduction of undesirable parasitics associated with packaging discrete devices.
A key stumbling block in the exploitation of optoelectronic integrated
devices has been the materials and processing technology. With high levels of
integration, large-area compound semiconductor substrates of exceptional
quality (low defect and dislocation density) and a vapor-phase crystal growth
technique for growing uniform, epitaxial layers on the surface are required.
Recent progress with hybrid MOCHA and MBE techniques suggests that this
may be close at hand. However, because of the many conflicting processing
requirements for optical and electronic devices, the ability to grow patterned
structures in situ in a multichamber MBE machine up ultimately be extremely
important if one is to exploit the full benefits of optoelectronic integrated
devices. Major emphasis should be placed on developing further the materials
and processing technology based on hybrid MBE/chemical-vapor deposition
multiwafer, multichamber machines.
In addition to the integration of optoelectronic devices and electronic
devices, the integration of the optoelectronic device with a fiber into an
appropriate package is an area ripe for development. In many applications
what is desired by the system or subsystem manufacturer is an optoelectronic
module consisting of some electronics, an optical emitter or detector (or
possibly both), and an attached pigtail consisting of an appropriately protected
optical fiber. Today, because of relative low-volume production, many of the
assembly operations for optoelectronic modules are done by manual procedures
or at least require considerable human intervention. In the future, as higher-
volume applications for optoelectronic modules emerge, automated assembly
and testing technologies will be essential to compete in these markets. This
involves careful design of the components (including locational tolerances)
within the assembly in order to facilitate automated assembly, and also
development of appropriate robotic manufacturing equipment with the
necessary tolerances.
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93
RECEIVER SUBSYSTEMS
The key elements of a receiver subsystem are the optical detector and the
low-noise amplifier that couples the optical detector to conventional electronics.
In telecommunications applications two types of optical detectors are typically
used. These are the PIN photodiode and the avalanche photodiode (APD).
Both types of devices can be fabricated from silicon material for 800- to 900-nm
wavelength systems. Both types of detectors can also be fabricated from
indium-gallium arsenide phosphorus material compositions for 1300- and
1500-nm wavelength systems. At these longer wavelengths, however, the APD
is just beginning to emerge from the research laboratory as a practical device.
Leakage current (dark current) is often critical to the overall performance of the
receiver, and careful material processing is needed to keep leakage current low
in the longer-wavelength devices. In the APD, precise control of the composi-
tion and thickness of a sequence of sequentially grown layers of material is also
critical.
To construct a receiver with high sensitivity and large dynamic range (ability
to accommodate a wide range of optical signal levels without overload) as well
as high bandwidth requires careful circuit design and the use of advanced
electronic integrated-circuit technology.
The United States is well positioned in terms of both detector technology
and low-noise mnplif~er technology, although recently the Japanese have been
more aggressive in the development of InGaAsP APDs (which were fast
demonstrated in U.S. research laboratories).
What is ripe for development is the fabrication of low-cost InGaAsP APDs
and the incorporation of these devices into low-cost modules with attached fiber
pigtails and with integral low-noise preamplifiers.
An area meriting continued research emphasis is the monolithic or hybrid
integration of very low-capacitance photodiodes and very low-front-end
capacitance preamplifiers in order to achieve the ultimate in low-noise
performance, and thus high sensitivity. The United States is very competitive
In this area.
LONG-WAVELENGTH AVALANCHE
PHOTODECTECTORS
For high-bit-rate, high-performance, 1.3- to 1.6-micron communication
applications, APDs are the detectors of choice because their internal carrier
multiplication process allows weaker light signals to be properly detected.
APDs currently in development are based on the InP/InGaAs(P) material
system and are of the heterojunction type with a separate absorption region
(usually the InGaAs layer) and a separate multiplication region. The pn junction
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APPENDIX D
is in the wide bandgap InP layer to avoid excessive tunneling dark currents.
Hence the name SAM APD is given to this structure. The response speed of
SAM APDs is limited by trapping of holes at the heterojunction interface. This
can be greatly improved by placing a graded InGaAsP layer between the
absorbing and multiplication regions. Such separate absorption and graded
multiplication (SAGM) APDs have exceptional response speeds and high
sensitivity. For example, with a receiver operating at 8 Gbits/s, at a wavelength
of 1.5 microns, and using a GaAs PET front-end amplifier, a sensitivity of -26
dbm has been obtained for a 10-~° bit error rate. TypicalAPDs of this type yield
receiver sensitivities that are 5 to 10 db better than those achieved with
non-multiplying PIN detectors.
The sensitivity of a receiver employing an APD is determined by the relative
impact ionization rates of electrons and holes and by dark current. Recently,
several advanced APDs have been proposed that rely on superlattice band-
structure engineering to modi~the relative impact ionization rates. SuchAPDs
are in early stages of research and require exceptional control of materials both
in terms of doping as well as composition for their practical realization. They,
however, hold the promise for achieving receivers with a performance dictated
by the laws of quantum mechanics.
Representative terms from entire chapter:
semiconductor laser
