Lasers Invention to Application (1987) / Chapter Skim
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Lasers in Communications and Information Processing
Lasers in Communications and Information Processing
Pages 45-100
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From page 45... ...
As John S Mayo of AT&T Bell Laboratories has pointed out, the transistor should be considered a killer technology because it displaced vacuum tubes (Mayo, 1985~; automatic speech recognition appears to be a new-domain technology; broadcast television ought to be viewed as occupying a niche in the information dissemination world without significantly changing the quality of information and coexisting with radio, newspapers, and other means of disseminating information.
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From page 46... ...
The introduction of lightwave systems is causing a sharp change in the rate at which channel capacity has increased over the past 100 years. OPTICAL FIBERS Free space propagation never caught on for terrestrial lightwave communications because of the potentional interruptions arising from fog, rain, and other natural phenomena.
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From page 47... ...
Initially, materials limitation due to absorption of impurities led to transmission losses of more than 100 dB/km in the 1960s. By the early 1970s, fiber losses were reduced to about 10 dB/km at 850 nm in silica fibers (see Figure 2)
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From page 48... ...
LASERS _ V / 1976 _ THEORY _ /\1983 -_1 , ~ I , 1 , 1 , 1 0.8 1.0 1.2 1.4 1.6 The year 1970 was significant for lightwave communications from two points of view. First, the optical fiber loss dropped below 20 dB/km and, second, Hayashi and Panish achieved the first continuous wave operation of a semiconductor laser at room temperature (Hayashi et al., 19701.
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From page 49... ...
Early in 1975 it became clear to many people working in the field that the region of low fiber loss was going to shift to longer wavelengths as the concentration of OH impurities in the fibers was being reduced. Further, the zero dispersion wavelength could now match with a low-loss region at 1.3 ,um (see Appendix A)
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From page 50... ...
Most have been high-capacity systems in long-distance communications networks Nearly all of the systems installed today use singlemode silica fiber and InGaAsP/InP-based semiconductor lasers operating at a wavelength of 1,300 nm. This section will describe the key parameters of these commercially available lasers and the potential for improved performance as suggested by laboratory results.
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From page 51... ...
The O-dBm output is adequate for systems operating up to several hundred megabits per second with less than 30-km repeater spacings, but only marginal for bit rates over 1 Gbit/s at 30 km or for systems (e.g., undersea cables) requiring long repeater spacings.
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From page 52... ...
. However, for 1,550-nm lasers that use the low-loss window in silica fibers, such a multifrequency spectrum is unacceptable for information systems operating faster than several tens of megabits per second because of the significant fiber dispersion at this wavelength.
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From page 53... ...
New materials are important for semiconductor lasers operating at wavelengths beyond 1,550 nm. Fiber research is currently focusing on new materials in the search for ultralow-loss fiber in the wavelength range between 2 and 5 ,um (see Appendix A)
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From page 54... ...
InAs, since InP-based materials cannot operate beyond about 1,650 nm. The best result to date for room-temperature operation of a continuous wave laser is slightly beyond 2 ,um using LPE growth in a GaSb-based system (Caneau et al., 19851.
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From page 55... ...
tin e width Although the single-longitudinal-mode, 1.5-,um lasers have dramatically improved the performance of fiber-optic communication systems, they are not without problems. Under continuous wave operation, a typical DEB (300 ,um long)
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From page 56... ...
For data rates below l Gbit/s, the chirping is of little consequence to communication systems. For the very high bit-rate systems, however, the wavelength chirping of the lasers, coupled with the dispersion of the fiber, can give a substantial penalty.
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From page 57... ...
This severely restricts the linewidth of both the transmitter and the local oscillator laser. In direct detection systems, the chirp penalty is eliminated for laser linewidths equal to or less than the information bandwidth.
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From page 58... ...
In the last few years, along with the parallel efforts in reducing the optical fiber losses and improving semiconductor laser performance, there has been an intense research and development effort on photodetectors for optical communications. Several important technological and material developments have driven and motivated this effort: (1)
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From page 59... ...
In this case, the linewidth is a million times larger than the information bandwidth, hardly an efficient apTABLE ~ Linewi~ths of Important Light Sources Used in Lightwave Communications Device Linewidth (Hz) 013 l2 108 103 Light-emitting diode Multimode laser Single-mode laser External cavity laser _~
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From page 60... ...
See Appendix C for a description of a coherent lightwave communication system approaching the theoretical limit of performance. TERABIT-KM/S EXPERIMENT In this system demonstration (Hegarty et al., 1985)
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From page 61... ...
LASERS IN COMMUNICATIONS AND INFORMATION PROCESSING 61 FIGURE 11 Wavelength division multiplexer. The lasers employed in this experiment were heteroepitaxially ridge overgrown (HRO)
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From page 62... ...
m ; at: - 31 -29 -27 - 25 -23 RECEIVED POWER (dBm) FIGURE 12 Received bit error rates for channels 5 and 9 of the wavelength division multiplex system.
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From page 63... ...
However, with special edge-emitting LEDs with small spot size, as much as 30 ,uW of power has been coupled into a single-mode fiber, and a transmission distance of 35 km at 180 Mbit/s has been demonstrated. Conventional semiconductor lasers emit light in a few (3-10)
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From page 64... ...
The solid line is for conventional step index fiber. Note the zero dispersion region near 1.3 ,um, which permits the use of multimode laser sources without sacrificing bit rate or distance between repeaters.
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From page 65... ...
With the advent of these narrow linewidth lasers, the improvement in capacity, transmission distance, and bit rate of fiber communication systems was dramatic. System experiments over 103 km at 420 Mbit/s, 130 km at 2 Gbit/s, and 117 km at 4 Gbit/s were demonstrated in rapid succession.
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From page 66... ...
In particular, the highly focusable intensity that is obtainable with lasers makes nonlinear optics possible for use in optical logic and switching applications, and the generation of picosecond optical pulses with mode-locked semiconductor lasers makes high bit rates feasible in optical systems. OPTICAL SWITCHING Optical switching serves the function of routing a signal into various alternative paths.
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From page 67... ...
At this time, we do not know what kind of cycle times might be used or what data rates in each channel might be necessary. Optical crossbars have been constructed using electro-optical materials such as lithium niobate in waveguide structures (Schmidt and Kaminow, 1974; Schmidt and Kogelnik, 1976; Alferness, 19811.
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From page 68... ...
Optical fibers have been suggested as a means of storing optical information. However, the speed of light and the rate at which photons are lost to dissipative processes would seem to militate against the storage
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From page 69... ...
A method to counteract the loss is required. Many kilometers of single-mode optical fiber can be coiled onto small spools a few centimeters in diameter without severe loss due to bending (J.
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From page 70... ...
) , and index variations (optical logic etalons)
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From page 71... ...
When light is incident on the device, the induced photocurrent reduces the voltage across the quantum well, which shifts the resonance back in the direction of the unbiased excitonic absorption. This in turn reduces the absorption, which increases the intensity, and the device becomes optically bistable.
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From page 72... ...
In normal operation, the control beam wavelength is shorter than the probe beam wavelength in order to provide the third port. Were such devices the only ones available for optical logic, the total number of sequential logic operations would be limited, because the probe wavelength would continue to shift down monotonically in each operation.
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From page 73... ...
However, various approaches have been suggested. One that is particularly interesting is the use of optically bistable elements PROBE INPUT 1 ~~g \1 INPUT 2 ~ NONLINEAR E TALON FIGURE 21 Schematic of an optical logic etalon.
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From page 74... ...
A weak signal beam falls on the elements and switches the devices into high output mode, thus providing net differential gain. This scheme removes the coherence between the signal and amplified signal beams, but provides the desirable modal isolation and saturable output characteristics for regenerative systems.
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From page 75... ...
But even in these areas, we can see that lasers, together with a number of significant achievements in related fields, have caused a revolution in communications. In information processing, the impact of laser technology is not yet visible, but if the past is any guide, information processing also will see its capabilities significantly enhanced through a variety of laser applications, many of which are still in the conceptual stage.
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From page 76... ...
IEEE l. Quantum Electron.
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From page 77... ...
At present, the silica fiber losses have been reduced to minimum levels predicted by Rayleigh scattering, which, because of its A-4 dependence, suggests that operation at longer wavelengths can yield even lower losses than the smallest loss seen in Figure 2. To exploit this possibility, a considerable amount of work is in progress to determine the best material systems that do not have intrinsic loss limits imposed by molecular vibrational frequencies of the constituents of the materials APPENDIX A: CHARACTERISTICS OF OPTICAL FIBERS
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From page 78... ...
Silica fibers, as mentioned above, are already at or near their theoretical minimum loss limits of approximately 0.1-0.2 dB/km at 1.55 ,um. Fluoride materials, for example, should show minimum loss approaching 10-2 dB/km at about 2.3 ,um.
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From page 79... ...
This dispersion in group velocity has little or no effect on low-bit-rate transmission. However, at high bit rates, the group velocity difference for frequencies comprising the Fourier component of the light pulse leads to a pulse broadening that limits the maximum fiber span between repeaters.
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From page 80... ...
However, in practical situations, the lasers used for lightwave communication themselves possess spectral widths arising from multilongitudinal or other nonoptimal operation. For a typical multimode laser operating in the 1.3- or 1.55-,um region, linewidths as broad as 70-100 A are not unusual, corresponding to laser oscillation on three longitudinal modes of the optical cavity of the semiconductor laser.
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From page 81... ...
experimental demonstration of single-longitudinal versus multilongitudinal laser source injection into a fiber 35 km long. The pulse width is 1 ns at input, corresponding to a data rate of approximately 500 Mbit/s.
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From page 82... ...
The breakup of the output pulse into three distinguishable components reflects the laser operation on three longitudinal modes, each one of which has a slightly different group velocity.
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From page 83... ...
Nonlinear phenomena such as soliton propagation have to be invoked to compensate for the unacceptable pulse broadening that will accompany the longer repeater spans promised by ever-decreasing fiber losses. Finally, another way of measuring the figure of merit of a lightwave system is to calculate the product of the data rate (bits per second)
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From page 84... ...
P = 1.2 W Output pulse width narrows to the same as that for input pulse corresponding to the fundamental soliton propagation.
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From page 85... ...
Such amplification can take advantage of Raman gain in silica fibers when laser radiation at high frequency and relatively high power is injected (see Figure A-71. The Raman gain overcomes the distributed loss of the fiber, as well as local loss due to insertion of the directional coupler into the fiber path.
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From page 86... ...
KUMAR N PATEL 1 1 TABLE A- ~ Design Parameters for a Sing~eChanne~ So~iton-Based Lightwave System Parameter Value Input power In-line amplifier separation Pulse width Data rate Total distance 3.0 mW 50 km 22.6 ps 4.4 Gbit/s 6,600 km The in-line optical amplification avoids the multitude of complexities associated with the conventional repeaters that rely on optical-to-electronic-to-optical signal conversions.
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From page 87... ...
front-end amplifier, have found wide use in fiber-optic receivers at data rates up to 400 Mbit/s. Experimental tests and theoretical evaluations have indeed shown that in this bit rate range, the use of state-of-the-art InP/GaInAs avalanche photodiodes (APDs)
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From page 88... ...
This sensitivity is—42 dB/m at A = 1.3 ,um for a bit rate of 500 Mbit/s (Forrest, 1985~. HETEROJUNCTION AVALANCHE PHOTODIODES Simple homojunction GaO.47InO.53As photodiodes cannot be operated as low-noise avalanche detectors because the dark current becomes prohibitively large because of Zener tunneling across the band gap at voltages at which impact ionization sets in (Pearsall and Pollack, 1985~.
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From page 89... ...
in the pulse response, making it impossible to use these devices at high bit rates. Researchers at the AT&T Bell Laboratories recently solved this problem when they introduced an intermediate quaternary GaInAsP grading layer between the absorbing GaInAs layer and the InP avalanche region (Pearsall and Pollack, 1985)
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From page 91... ...
to the concept of a solid-state photomultiplier. In 1981 a group at Bell Laboratories showed that in an MBE-grown AlGaAs/GaAs quantum well APD the a/,8 ratio is enhanced by a factor of 4 over the bulk value for c~l,B GaAs (Capasso, 19851.
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From page 93... ...
Clearly, the staircase detector has the potential for achieving unprecedented receiver sensitivities at both high and low bit rates, provided one can minimize the dark current of the device. Another approach to the solid-state photomultiplier is based on a recently discovered avalanche multiplication mechanism (impact ionization across the band-edge discontinuity (Capasso et al., 19861.
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From page 95... ...
LASERS IN COMMUNICATIONS AND INFORMATION PROCESSING 95 FIGURE B-6 Band diagram of solid-state photomultiplier based on impact ionization across the band-edge discontinuity of carriers stored in the wells.
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From page 96... ...
The best results so far obtained at bit rates of 1 Gbit/s are 1 or 2 dB lower in sensitivity than the best PIN-FET receivers (Chen et al., 1984~. Extensive theoretical analyses at the AT&T Bell Laboratories have shown that in the above wavelength range and at bit rates ranging from 500 Mbit/s to 2 Gbit/s, the photoconductor can, at best, match the performance of a PIN in a receiver but never do better than an APD (Forrest, 1985~.
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From page 98... ...
. As the local oscillator power is increased, the receiver sensitivity approaches the fundamental shot noise limit.
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From page 99... ...
_ _ ~ /~ _ _ ~ ~ _ = i~ = ~ _ _ 4r -25 - 20 -15 -10 -5 0 LOCAL OSCILLATOR POWER (dBm) SHOT NOISE LIMIT Dl RECT DET ECT 10N FIGURE C-1 Sensitivity improvement with coherent detection at a bit rate of 150 Mbit/s.
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From page 100... ...
Part of the discrepancy is accounted for by the thermal noise of the receiver, and by the less-than-unity quantum efficiency of the photodetectors. In spite of the deviation from the ideal shot noise limit, the measured receiver sensitivities are the best reported for the respective data rate and are about six times better than the best reported direct detection sensitivities.
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