Dispersion Compensation for Optical Fiber Systems Much of the currently embedded optical fiber was originally designed for light with a wavelength of 1.3 microns. If this fiber is to be used with tomorrows optically amplified, high-speed, long span-length lightwave system operating at 1.5 microns, the chromatic dispersion in the fiber must be compensated.

Bob Jopson and Alan Gnauck

oremost among the never-changing trends in a century of telephony expe- rience is the inexorable increase in the number of conversations that can be transported by a single long-haul transmission link. The recent history

of this trend for lightwave systems is displayed in Fig. 1. It can be seen that the bit rate achieved in laboratory hero experiments as well that of highly reliable commercial systems has been increasing exponentially. In the past 14years, the speed ofcom- mercially available systems has doubled every 2.4 years. In many areas of the world today, your long-distance conversation might travel part of its way with some 40,000 other conversations on a 2.5 Gigabit per second (Gb/s) fiber-optic line. It may not be long before this rate is quadrupled. As we shall see, the potential increase to 10 Gb/s together with changes in system architecture are spawning much interest in techniques for the compensation of chromaticdispersion. To understand this, we must first understand the present lightwave system design.

Lightwave Systems urrently, a 2.5 Gb/s long-haul fiberoptic trans- C mission link consists of a linear assemblage of

regenerator spans, each about 40 km long. The trans- mitter end of a regenerator span contains a dis- tributed-feedback laser operating at a wavelength lying in either the 1.3 pm telecommunications window of transmission fiber or the window at 1.56 pm. The NRZ (non-return-to-zero) onioff modulation is provided by direct modulation, i.e., by simply modulating the current used to drive the semiconductor laser. Direct amplitude modulation of these lasers is inherently accompanied by a large frequency modulation, or chirp, that increases with the bit rate. At multi-gigabit data rates, the chirp is typically about 0.5 nm (60-80 GHz). The receiver end of the regenerator span contains an APD (avalanche photodiode) detector followed by gain, clock recovery, and a decision circuit. The trans- mitter and receiver are joined by approximately

BOBJOPSON~S U member oftechnical s i u f l u r ~ T & ~ Bell Laboratories.

ALAN GNAUCKis U mem- ber of lechnical staff ut AT&T Bell Laboratories.

40 km of single- (transverse) mode silica fiber. After each span of fiber, the signal is retimed and reshaped before being transmitted over the next span. This regeneration has the advantage of lim- iting the accumulation of signal impairments such as optical noise, linear distortion, and nonlinearity, but it is expensive to implement and maintain.

Until recently, frequent regeneration was a neces- sity imposed by fiber attenuation and the need to convert the optical signal to an electrical signal for amplification. Now, however, an eminently practical optical amplifier has arisen: the erbium- doped fiber amplifier. This amplifier [ 11 has many properties that lend themselves to telecom- munication applications: high gain, high saturation power, a low noise figure, physical compatibility with existing components, and a slow (15 ms) time constant of the dominant nonlinearity. The band- width of the amplifier, 1.53 to 1.56 pm, is well matched to the lowest-loss telecommunications window in silica fiber. However, most national lightwave networks were designed to be used in the 1.3 pm window. Therefore the use of erbium amplifiers will impose additional constraints on system designers, particularly in the area of chro- matic dispersion.

As a direct consequence of the invention of the erbium-doped fiber amplifier, we are poised at the brink of major changes in lightwave system architecture. One change that can be clearly pre- dicted is that regenerator spanswill increase to hun- dreds of kilometers in terrestrial systems and thousands of kilometers in submarine systems. Ana- log repeaters containing erbium amplifiers will be placed every 50 to 150 km within the regenerator spans to compensate for fiber attenuation.

A less certain change is the shift from time- division-multiplexed (TDM) single-wavelength systems at ever-increasing bitrates to wavelength- division-multiplexed (WDM) systems, which attain higher throughput by increasing the number of wavelengths transmitted through a single fiber. In either case, system economics favor long regen- erator spacings and it is the long regenerator spacings

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96 0163-6804/95/$04.00 1995 0 IEEE IEEE Communications Magazine June 1995

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used in combination with high bit rates over the currently-embedded fiber network that forces sys- tem designers t o focus their attention on chro- matic dispersion.

Chromatic Dispersion hromatic dispersion, or more precisely group- C velocity dispersion, in lightwave systems is

caused by a variation in the group velocity in a fiber with changes in optical frequency. Since it can cause pulse spreading in a lightwave signal. chro- matic dispersion can impair system performance. This is shown schematically in Fig. 2a. An isolated mark, if for no reason other than its modulation. contains a spectrum of wavelengths. As it travers- es the fiber. the shorter wavelength components of the pulse (shown in blue) travel faster than the longer wavelength components (shown in red). Thus the pulse broadens as it travels down the fiber and by the time i t reaches the receiver. it may have spread over several bit periods and be a source of errors. The measure of chromatic dispersion used by the lightwave community is D. in units of psinmikm, which is the amount of broadening in picoseconds that would occur in a pulse with a bandwidth of one nanometer while propagating through one kilometer of fiber. D is given by

D=--=- d 1 dP dA i v g d u o

where A, rs, p and o are the wavelength, group velocity, propagation constant and angular frequency, respectively. Conventional fiber, so-called because i t was the first widely-deployed single-mode fiber,

contain5 a step-index waveguide with a zero in the dispersion near 1.31 pm. I n the 1.56 pm window, L) is about 17 psinmikm. The slope (dDldA) o f D is O.08 psinmikm.

Chromatic dispersion places a limit on the maximum distance a signal can be transmitted without regenerating the original digital signal. This distance. the dispersion limit o r dispersion length. can be est imated by determining the transmission distance at which a pulse has broad- ened by one bit interval. The estimated disper- sion limit for a signal of bitrate B and width AA. is given by 1-1) = I/(BDAA). For the state-of-the-art commercial systems, we can use 7.5 Gbis. 17 ps,nni,km. and 0.5 nm for E, D , and AA. respec- tively. to estimate a dispersion limit of 37 km. To significantly increase either the bit rate or the regen- erator span lengths in these directly-modulated systems. one must turn to low-chirp modulation of the optical signal. This can be obtained by using an external modulator. In this case. the optical handwidth of an NRZ signal in frequency units can be approximately 1.2 times the bit rate B. Converting to units of wavelength, we find, for externally-modulated systems. t ha t Lo = 6100/(B2)km (Gb/s)2 for conventional fiber at 1.56 pm. Thus a 1.56 pm externally modulated system using conventional fiber has an estimated dispersion limit of nearly 1000 km at a bit rate of 2.5 Gbis. At IO Gbis, this estimated limit is reduced to 61 km. close to the 70 km observed in practice for a 1 dB penalty. One way to increase the dis- persion limit is to reduce the dispersion of the fiber. Currently, the overwhelming advantages of erbium amplifiers and the low fiber loss at 1 .56 pmvirtuallyruleoutoperation nearthe 1.31 pmdis- persion zero of conventional fiber, but it is possi- ble t o use dispersion-shifted fiber (DSF). This fiber is designed such that the waveguide contri- bution to the chromatic dispersion shifts the dis- persion zero to some desired wavelength near the 1.56 pm window.

While the use of DSF can enhance the perfor- mance of completely new systems greatly, systems must often be designed that use the existing con- ventional fiber network. Many nations through- out the world invested in lightwave long haul networks during the late 80s and early 90s. By now. about 70 million kilometers of fiber have been deployed. At first, these networkscontained conventional fiber almost exclusively. Now, some DSF is starting to be deployed, but the national networks still pre- dominantly contain conventional fiber. Since these networksare anenormousinvestment, there isstrong incentive to design systems that use conventional fiber.

Dispersion Compensation he reason for activity in the arca o f chromatic T dispersion compensation is now clear. Decign-

ers of terrestrial lightwave systems wish to use erbium amplifiers, which forces operation to the 1.56 pm transmission window and allows many- fold increases in the regenerator span lengths: they are constrained to use the embedded fiber base which often contains conventional fiber with a dispersion of 17 psikmlnm in the 1.56 pm win- dow; and finally, as the cost of 10 Gb/s electronics and electrooptics comes down, there will be a

-m*> .

National networks still mostly contain conventional fiber; providing a strong incentive to design systems that use conven tiona I fibel:

IEEE Communications Magazine Junc 1995 97

- Through transmitter- based compensation techniques, the onset of chromatic- dispersion impairments can be delayed. Unfortunate-

cannot be delayed forevel:

&! they

desire to operate at higher bit rates. In order to meet all these requirements, it is necessary to either eliminate or mitigate the impairments caused by chromatic dispersion.

A myriad of techniques for compensa t ing chromatic dispersion have been demonstrated or proposed. Before examining them, it is instructive to calculate t h e amount of pulse broadening occurring in a typical high-speed system. Consider a 1.56 pm, 10 Gbis system with regenerators sep- arated by 300 km of conventional fiber. With a fiber dispersion of 17 psinmikm, the net disper- sion in a regenerator span is 5100 psinm. If chirp- free modulation is employed, then an isolated mark requires 0.1 nm of optical bandwidth and it will broaden by 510 ps during passage through the 300 km regenerator span. Thus the pulse will occupy 5 or 6 bit slots at the end of the span, with the blue spectral components of the pulse being some 10 cm of fiber ahead of the red spectral com- ponents. (Despite our exclusive interest in the infrared region of the optical spectrum, we will use blue to mean the shorter-wavelength spectral components of a signal and red to denote the longer-wavelength spectral components of the signal.) This large separation means that if optical components are employed to compensate the dis- persion, they will have to be large. Such a device must provide 15 cm (in air) of differential path length between the spectral components of the signal and it must operate over a bandwidth of at least 0.1 nm centered on the signal wavelength. Even allowing for multi-passing, the smallest of these components will have dimensions measured in centimeters and, as we shall see, some of them will be many kilometers long. If a directly modu- lated 10 Gb/s signal is transmitted over the 300 km regenerator span, the pulse will occupy a half meter or 26 bit slots at the end of the span! An optical device used to compensate dispersion for this signal must provide 77 cm (in air) of differential path length over a bandwidth of 0.5 nm.

Spectral Shaping at the Transmitter

t is possible t o provide only a sample of t h e I many methods used to compensate chromatic dispersion. It is not surprising, given the importance of the problem and the presence of three subsys- tems in a lightwave regenerator span, that attempts have been made to attack dispersion in the trans- mitter, in the fiber and in the receiver. We first describe modifications to the transmitter. Most transmitter techniques modify the spectrum of the data stream, often by pre-chirping. This is illustrated in Fig. 2b. The spreading of the bit into adjacent slots can be delayed by arranging for the light in the leading edge of the bit to be of longer-than-average wavelength and that in the trailing edge of the bit to be of shorter-than-aver- age wavelength. By pre-chirping the pulse in this manner, long-wavelength light in the leading edge must pass through t h e en t i re pulse slot before it starts impinging on the trailing pulse and causing errors. Light in the trailing edge will behave similarly.

A variety of techniques have been used to pre- chirp the transmitted signal. One simple method for

generating a chirped bit stream is to add phase modulation through the use of an unbalanced Mach-Zehnder amplitude modulator. This tech- nique has demonstrated modest compensation of dispersion. Another method is to frequency-mod- d a t e (FM) the laser to provide chirp in the opti- cal signal entering an external modulator. In fact, one of the more successful system demonstra- tions of pre-chirping, referred to as dispersion- supported transmission [2], has used FM alone. In this scheme, illustrated in Fig. 2c, the transmit- ter generates an F M signal with the modulation tailored to the span length so that the dispersion in the span converts the FM at the transmitter to amplitude modulation at the receiver. With this technique, span lengths in excess of 200 km have been achieved at 10 Gb/s with a 6 d B receiver- sensitivity penalty compared to baseline. Draw- backs t o d ispers ion-suppor ted transmission include having to tailor the laser drive current to the span length, the requirement for a laser with good broadband FM response, and the need to decode a three-level optical signal at the receiver. A third technique takes advantage of the trans- mission-fiber nonlinearity. At modest power levels of several milliwatts, self-phase modulation pro- duces a shift to longer wavelength at the beginning of an optical pulse, and a shift to shorter wave- length a t the trailing edge. This effect has been used in conjunction with an unbalanced Mach- Zehnder amplitude modulator to obtain 200 km transmission at 10 Gb/s with a penalty relative to baseline of less than 2 dB [3].

Through transmitter-based compensation techniques, the onset of chromatic-dispersion impairments can be delayed. Unfortunately, they cannot Se delayed forever. This is because the modulation requires a minimum bandwidth for the optical signal and because the light from a bit is initially confined to a single bit period. Once the outlying spectral components in the pulse disperse o n e clock per iod , they will s ta r t to cause errors. Thus prechirping techniques have extended the dispersion limit by a factor of three and this is probably about as far as they will go. The limitations of the technique have been over- come in two ways. O n e method is a proposal to widen the transmitted pulses to 1.5 clock periods, chirp them, multiplex them into a single (over- lapping) bit stream, and then use dispersion t o narrow the pulses into a single bit period [4]. T h e second method is to filter the modulating signal to reduce its bandwidth. In one imple- mentation, this results in the generation of a narrower bandwidth duobinary optical signal, which can propagate further before dispersion causes problems [5]. Conventional duobinary transmission involves a three-level optical signal which increases signal-to-noise requirements and requires decoding at the receiver. A recent duobinary proposal uses a Mach-Zehnder modu- lator transmitter to produce a conventional binary signal, but with phase reversals in the electric field [6]. Although there is a limit to the dispersion that can be compensated by transmitter tech- niques, they may become very important at lOGbis, because they are easily implemented a n d also because 200 km of dispersion compensation is sufficient for a large fraction of the worlds point- to-point links.

98 IEEE Communications Magazine June 1995

Receiver

L1 L2 m . 0 . Receiver

bl

Receiver

d)

Figure 2 . Spectral evolution of isolated marks for various dispersion-compensation techniques. The color variation represents the spectral variation of the pulses with blue indicating shorter wavelength light. red indicating longer wavelength light, and purple representing a white spectrum: a ) no dispersion compensa- tion; b ) pre-chirped pulses; c ) dispersion-supported transmission using frequency modulation; d ) negative dispersion at the end of a span; e ) mid-system spectral inversion.

Data Recovery at the Receiver eceiver-based compensation techniques can R be used with e i the r cohe ren t -de tec t ion

receivers or direct-detection receivers. A coherent receiver mixes the incoming signal with a local oscillator, thereby shifting any phase and ampli- tude fluctuations on the optical carrier to a carrier at an electronic frequency. Then linear dispersion compensation can be performed on the electronic carrier.

One 8 Gb/s demonstration [7] used heterodyne detection and a 31.5 cm-long microwave-stripline dispersion compensator operating over an IF band of 6 to 18 GHz. It compensated 188 km of conventional fiber.

In direct-detection receivers, the optical signal is simply coupled to a photodiode and converted to a current proportional to the square of the optical electric field. This detection process loses information about the phase of the dispersed sig- nal so i t is not surprising that linear equalization of a dispersed signal cannot compensate much dispersion. However, it has been recognized that nonlinear equalization can be of benefit. The sig- nal distortion caused by chromatic dispersion is con- stant and predictable and although an individual bit may be spread over many bit periods, the detected waveform is changed by its presence. Two methods of processing the detected waveform have been proposed and modeled [8]. In one method, the threshold of the decision circuit isvaried, depend- ing on preceding bits. In another, the decision about a given bit is made by analyzing the analog waveform for a band of clock periods surrounding

the bit in question. Modeling suggests that these techniques, particularly the latter one, will be able to compensate many dispersion lengths of fiber but confirmation awaits experimental verifi- cation.

In contrast to single-bit transmitter-based tech- niques, receiver-based techniques for dispersion compensation can, in principle, compensate many dispersion lengths of fiber. This results from the availability of many bit-slots of received signal for processing. However, network designers are reluctant to employ coherent detection in long- haulsystems,so the heterodyne technique has a dim future. Since 1) the direct-detection compensator requires logic operating at the bit-rate, 2 ) the complexity of the logic increases as 2 where N is the number of bits over which a pulse has been broadened, and 3) the required signal-to-noise ratio is proportional to N , the direct-detection compensator will probably be limited to compen- sation of several dispersion lengths of fiber.

The Optical Regime n contrast to the electrical processing used in I transmitter- and receiver-based compensation

techniques, those operating on the fiber span employ optical processing. Figure 2d illustrates the use of optical compensation at the end of a regenerator span. After the pulse has been broad- ened by the positive dispersion in a span, it passes through an equal amount of negative dispersion and the original pulse shape is restored before detec- tion. Figure 3a shows the pulse broadening expe- rienced by a 1.5 pm, 10 Gb/s, externally-modulated

- In contrast to singZe- bit transmitter- based techniques, receiver- based techniques for dkprswn compensa- tion can, in principle, compensate many dispersion lengths of fiber:

IEEE Communications Magazine June 1995 99

- The suitability of chirped fiber gratings for compen- sation is advancing more rapidly than that of any other compensa- tion technique.

signal over a 600-km span of conventional fiber, in the absence of dispersion compensation. Figure 3b shows the pulse broadening that would occur if compensating negative dispersion were added at the beginning of the span (dashed), the end of the span (solid), o r distributed every 100 km along the span (dotted).The latter design isan attrac- tive architecture, not onlybecause it can reduce non- linearity impairments, but also because it allows the dispersion-compensation components to be included in the optical-amplifier repeaters. With proper design, the optical attenuation of the com- pensation components can then be mitigated without an increase in the component count.

lnterferometers any optical techniques for dispersion com- M pensation have been demonstrated. They

can be classified into three categories: interfer- ometers, negative-dispersion fibers, and phase-con- jugation techniques. Interferometers compensate dispersion by providing wavelength-dependent pathsof different lengthsfor different spectral com- poncntsof the signal. One promising interferometric device is a silica-on-silicon planar circuit containing cascaded Mach-Zehnder interferometers. In a Mach- Zehnder interferometer, the light is split into two, generally unequal, length paths and then recombined, in this case, by a 2 x 2 combiner. The distribution of light in the two output ports will depend on the relative phase delay provided by the two arms. In the planar dispcrsion-compensa- tion circuit, these intcrferometersare cascaded with the path lengths adjusted so that the bluc light travels mostly in the longer arms while the red light travels mostly in the shorter arms. A 5-cm by 8-cm circuit containing five cascaded intcrferom- eters provided a dispersion of 836 psinm over a IO-GHz bandwidth with a loss o f 3 5 dB [Y]. This is sufficient t o compensate 50 km of conventional fiber. The device is polarization-dependent and suffers from the disadvantage of a narrow band- width of operation and a relatively limited com- pens at i o n cap a b i 1 it y. de f i c i e n c i e s co m m o n to m any in t e r fe r o m e t r i c devices. How ever , t he phase in one arm of each interferomctcr is thermally tunable, so the device should be broadly tunable. In addition, the dispersion of a cascaded Mach- Zehndcr device is periodic. Thus, by matching the periodicity of the cascaded Mach-Zehnder device t o the channel spacing in a W D M system, all channels can be simultaneously compensated.

The interferometric device that is currently attracting the most attention is the chirped fiber Bragg grating. Its principle of operation is easily understood. A grating is written down the length o f an optical fiber by periodically changing the fiber refractive index. Light in thcfiberwith awavelength of twice the grating period is reflected. In adispersion compensator, the grating period is reduced lin- early down the length of the device. The blue light is thereforc reflected at a point farther into the device than the red light and is thus delayed relative to the red light. These devices can be quite compact. A 5 cm long grating, in principle, can compensate the 300 km, 1 0 Gbis, externally modulated system used in the example above. Long (multi-cm) gratings are difficult t o make since sub-micron tolerances must be maintained

over the length of the device. Fiber grating com- pensators have the disadvantage that the compen- sated signal is retro-reflected, so that an optical circulator must be employed to separate the input from the output.Theyalsohave anarrowbandwidth of operation, and unlike other interferometric devices, the dispersion does not have a useful periodicity. However, the gratings offer low loss to non-resonant light passing through them, so multiwavelength operation can be obtained by putting several gratings in series down the fiber, each centered on a different wavelength. Recently. chirped gratings as long as 5 cm have been madc. One of these provided -2700 psinm of dispersion over a bandwidth of 18 GHz. I t could compensate 160 km of conventional fiber[l0]. The suitability of chirped fiber gratings for compensation is advancing more rapidly than that of any other compensation technique. It is possible that in the future, they will be the method of choice.

Negative-Dispersion fiber he method ofchoice at the present time issingle- T mode negative-dispersion fiber. Negative dis-

persion is gcncrally achieved by guiding the mode very weakly so that small increases in wavelength are accompanied by relatively large changes in mode size. This puts more of the mode into the cladding where the lower ref'ractivc indexprovidesan increase in the speed o f propagation. Unfortunately, as the guiding is reduced, the fiber attenuation and bending loss increase. The optimal singly-cladded fiber designs (designs containing one core and one cladding region) have a dispersion of around -100 ps/nm/km and an attenuation of 0.35 dB/km. In these fibers, the slope in the wavelength depen- dence of the dispersion has the same sign as that of conventional fiber (positive), so the fibers will cancel dispersion completely only at one wave- length. However, here, in contrast to the interfer- ometric devices, the compensation mismatch changes weakly with wavelength, so the constraints imposed by this incomplete compensation are not very severe. A 10 Gbis externally-modulated, 30 nm- wide WDM system will have a dispersion limit of about 700 km when compensated with this fiber. By adding more cladding layers, a fiber designer can obtain better control over the mode-expan- sion-induced changes in the propagation constant a n d achieve b roadband compensa t ion (i .e. , design a fiber with a negative slope to its disper- sion vs. wavelength curve). The figure of merit (FOM) for compensating fibers is the negative of the dispersion dividcd by the at tenuat ion (in dB1km) a n d usually is expressed in units of ps/nm/dB. The best reported FOM'sare slightly less than 300 ps/nm/dB. Dispersion-compensating fiber offers the advantages of low polarization dependence, broadband compensation, and pas- sivity. It has the disadvantage that a significant fraction (114 to lib) of the span length is needed to compensate a span. A 300 km span will require around 50 km of compensating fiber with an atten- dant loss of 18 dB. Despite this disadvantage, this fiber is already commcrcially available and is likely to be the first compensation technique to be widely deployed.

Another fiher dispersion-compensating tech- nique uses a higher-order ( L P , , ) fiber mode [ I 11.

100 IEEE Communications Magazine June 1995

This mode can provide much more negative dis- persion per given length than the fundamental (LPol) mode and it can compensate a broad band- width as well. Fiber that compensates 35 times its own length has been demonstrated. This tech- nique requires mode conversion between the fun- damental mode and the L P ~ I mode. Although this has been demonstrated to have high efficiency, i t is abamer to field deployment. The higher-order-mode technique currently suffers a higher loss than the

.. fundamental-mode technique. Demonstrated figures of merit arearound 100. In principle? the LP, I mode should not exhibit high loss, so it remains to be seen whether the two-mode technique will become practical.

Spectral Inversion I I the methods of dispersion compensation A discussed above require varying amounts of

compensation, depending on the length of the span. Mid-system spectral inversion (MSSI) is unique in that a single component compensates any span, regardless of length. This technique i s illustrated in Fig. 2e. The signal is dispersed in the first half of the span, resulting in distorted pulse shapes with the blue light leading the red light. This dispersed signal is then phase conjugated. (For practical rea- sons, it usually is also frequency shifted, but the frequency shift is not needed for dispersion com- pensation.) The phase conjugation reverses o r inverts the optical spectrum of the signal so that red becomes blue and blue becomes red. As shown in Fig. 2e, the shape of a pulse remains the same, but now the leading edge is rcd and the trailing edge is blue. Now the dispersion in the second half of the span reshapes the pulse so that, if the dispersion before the phase conjugator matches the dispersion after it, the original pulse shape will be restored at the end of the span. The change in pulse broadening is shown in Fig. 3c. It can be seen that the phase conjugator near the center of the span reverses the broadening and that this allows the broadening to be zero at the receiver. Methods for phase conjugating signals in system demon- strations of MSSI include both degenerate and non-degenerate four-wave mixing behvecn the sig- nal and a high-power pump or pumps using either a semiconductor,laser. a semiconductor amplifier or dispersion-shifted fiber as the nonlinear clement. Polarization-independent phase conjugation has been used to compensate 560-km of conventional fiber in a two-channel WDM system operating at 10 Gbis [12]. It can be seen from Fig. 3c that the optimal position for the phase conjugator is not the physical center of the span. This is a consequence of t h e frequency shift tha t accompanies t h e phase conjugation. Since the signal has a different (in Fig. 3c, longer) wavelength in the second half of the system than in the first half, the dispersion in the fiber is different and it therefore requires dif- ferent fiber lengths to balance the dispersions o n either side of the conjugator. Figure 3d shows the results of not balancing the dispersions before and after the conjugator. The dispersion in the second section over- compensates the conjugated dis- persion in the first section when the conjugator is too close to the transmitter and under-compensates i t when the conjugator is too close to the receiver. The tolerable error in the placement of the conjugator

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-1 L Figure 3. Pulse broudening experienced by a 1.5 pm 10 Gbls. externally modulated signal over U 600 k m spun of conivntional fiber for vurious disper- sion compensution techniques: U ) no competisution; b) negutiw dispersion ut the beginning (dushed) or end (solid) of u spun or distributed twy 100 km (dotted); c ) mid-gstetn spectrul inversion; d ) spectral inver~ion not ut the middle o f u SJS- tem: e) mid-system .spectrul inversion in u multi-wuvelength system. The hori- zontal. durk blue lines si7ow the toleruble umount ofhroudening for u IO dbls system.

IEEE Communications Magazine June 1995 101

- The adoption

of any particular technique for use in a high-speed network will depend on the constraints imposed by the, as yet, undefined network architecture.

can be determined by geometric construction on Fig. 3d, to be plus or minus one half the disper- sion limit, e.g., 2 3 5 km for 10 Gbis. Figure 3e shows the broadening that would occur in various wavelength channelsof a WDM system unless each channel is conjugated individually at its optimal location. The location of the conjugator can be perfect for only one wavelength. Channels having a longer than optimal wavelength in the first sec- tion will experience more broadening than the optimal wavelength in the first section. After con- jugation, theywill have ashorter than optimalwave- length and will experience less broadening than the conjugated optimal wavelength. Thus, chan- nels having longer than optimal wavelength will be undercompensated and those with shorter than optimal wavelength will be overcompensated. This limits the regeneration span length of an MSSI-compensated, 10 Gbis, 30-nm-wide WDM system t o about 1000 km. O n e disadvantage of MSSI is its complexity. It requires one or two sin- gle-frequency pump lasers, several narrow-band optical filters and, if DSF is used as a conjugator, at least one high-power optical amplifier. It offers the advantage that it can be implemented with com- mercially-available components and that once assem- bled, it can compensate any span length.

The Next Step ne of the next steps in chromatic dispersion 0 compensation will be the commercialization

of the more promising techniques. This has already taken place for negative-dispersion fiber. Disper- sion compensation will start to be incorporated into other components such as optical amplifiers. The use of multiple gain stages in amplifier modules will allow the compensation to be placed between the gain stages where it will neither materially affect the signal-to-noise ratio nor the output power of the amplifier module. The desired dis- tribution of dispersion compensation in a high- speed link is currently being studied. The viability of many of the techniques discussed here depends on the results of these studies.

Given the large variety of compensation tech- niques with vastly different capabilities and prop- erties, it is not surprising that dispersion compen- sation is attracting the attention of an Internation- al Telecommunications Union-Telecommunication (ITU-T)Standardization Sector standards commit- tee(Question25 ofworkingparty 1514). Atthisearly stage, they are investigating dispersion-compensat- ing fiber and some transmitter-based dispersion compensation techniques. As other techniques mature, they will probably be considered as well.

Summary ispersion compensation will be required in D optically amplified, long-haul 10-Gbis systems

using conventional fiber. Many compensation techniques have been demonstrated and they exhibit avariety of different and often complimentary properties. Transmitter compensation techniques are the most easily implemented but provide a limited amount of compensation. The most commercially advanced technique is negative-dispersion fiber. Chirped Bragg gratings a r e advancing rapidly, bu t will always b e h a m p e r e d by their narrow bandwidth. The adoption of any particular technique for use in a high-speed network will depend on the constraints imposed by the, as yet, undefined network architecture.

References 11 I E. Desurvire. "Erbium-Doped Fiber Amplifiers." (New York: John

Wiley & Sons, 1994). 121 B. Wedding, B. Franz, and B. Junginger, "lO-Gb/s optical transmis-

sion u p t o 253 k m via standard single-mode f iber using the method of dispersion-supported transmission." J. Lightwave Techno/., 12. 1994. pp 1720-1707.

[31 B. F . Jsrgensen. "Unrepeatered transmission at 10 Gbit/s over 204 k m standard fiber," ECOC '94 Technical Digest, Firenze, 1994, pp 685-688.

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Biographies BoeJo~so~ receiveda B.S. in physicrfrom theUniversityofCalifornia. Davis and a Ph.D. in physics f rom Harvard University. Since 1983 he has beenamemberoftechnical staff atAT&TBell Laboratories. Holmdel. New Jersey, where he has devoted his energy to semiconductor and fiber optical amplifiers and to long-haul lightwave systems.

ALAN GNAUCK received an M.S. in electrical engineering f rom Rutgers University in 1986. He worked on laser chemistry at Exxon Research and Engineering f rom 1976 t o 1982. Since 1982 he has worked on multi-gigabit electronics and lightwave systems at AT&T Bell Labora- tories, Holmdel. New Jersey.

102 IEEE Communications Magazine June 1995