Abstract:
An apparatus and method for controlling the gain in an erbium-doped fiber amplifier (EDFA) incorporated into a multi-wavelength communication system so as to amplify each of the wavelength signals. The amplifier operates near to saturation so that, if one or more of the multi-wavelength signals is removed from the transmission, the remaining channels are increasingly amplified, leading to problems with other components in the system which depend upon intensity. According to the invention, an optical signal at a wavelength that is not within any of the transmission channels is selectively fed back around the amplifier and caused to lase in a wavelength-filtered ring-laser configuration. The lasing signal governs the saturation of the amplifier such that any gain shed by a disappearing data signal is predominately used by the lasing signal, not by the remaining data signals. Thereby, the data signals do not experience gain variations dependent upon the number of data signals being amplified. In a chain of amplifiers on a long link, the wavelength of the lasing signal is chosen to lie within the gain flat band of the amplifier and is output from the equalized amplifier with the same efficiency as all the transmission signals to be received by the next amplifier. Thereby, only the first amplifier needs to be equalized.

Description:
GOVERNMENT INTEREST 
     This invention was made at least partially under government funding agreement MDA972-95-3-0027, awarded by the Advanced Projects Research Agency. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to optical fibers and optical amplifiers. In particular, it relates to gain stabilization in erbium-doped fiber amplifiers for multi-wavelength signals. 
     BACKGROUND ART 
     Modern communication networks are increasingly being implemented with optical fiber being used as the transmission medium. Fiber&#39;s early usage was concentrated on long haul transmission in which its huge bandwidth, measured in hundreds of terahertz, could be more immediately exploited. However, well made single-mode optical fiber exhibits relatively low but still finite absorption. Its useful transmission length is measured in tens to hundreds of kilometers at which point the signal amplitude has decreased so much that the signal is no longer readily detected. 
     In older systems, optical regenerators were placed periodically along a fiber transmission path. A regenerator detects the optical signal and converts it to electrical form, that is for most systems, detects the intensity of the optical envelope which corresponds to the electrical signal used to modulate the laser at the transmitting end. The regenerator then processes the electrical signal to regenerate a copy of the original modulating signal, and the copy is used to modulate another laser impressing its modulated optical signal on the next fiber link. Regenerators, however, tend to be expensive since they need to incorporate complex high-speed electronic circuitry. Furthermore, the design, construction, and operation of regenerators depend strongly on the data rate and the data format of the impressed electrical signal. If the data rate or format changes, the regenerators need to be replaced even though the fiber needs no adjustment for the changes and upgrades. 
     Erbium-doped fiber amplifiers (EDFAs) were discovered around 1987 to amplify an optical signal near the important fiber band around 1550 nm, at which wavelength the silica fiber absorption is a minimum, and they have gained nearly immediate acceptance in the telecommunications industry as a replacement for optical regenerators. In an EDFA, a silica-based fiber is doped with erbium, which forms an optically active ion having a number of excitable metastable states. A pump laser having a wavelength of, for example, 980 nm or 1490 nm, optically pumps the erbium ions until their state population is inverted. An optical signal traversing the region of inverted population will absorb energy from the excited ions and will thus grow in intensity. 
     Wavelength-division multiplexing (WDM) was being developed contemporaneously with erbium-doped fiber amplifiers. The data capacity of an optical-fiber transmission link is generally limited not by the fiber but by the electronics and opto-electronics at its two ends. In WDM, separate electrical circuits create separately modulated optical signals having different optical carrier wavelengths, and these multi-wavelength signals are combined (multiplexed) and carried on one optical fiber. At the receiving end of the fiber, the multi-wavelength signals are optically demultiplexed and thereafter separately detected. Thereby, the capacity of the fiber is increased by the number of wavelength channels. 
     Erbium-doped fiber amplifiers offer distinct advantages for WDM systems. A representative gain spectrum  10  for an EDFA is shown in FIG. 1 although the details depend upon many factors not to be discussed here. The illustrated spectrum  10  has been optimized for a relatively wide flat-gain band  12  between about 1540 and 1560 nm, but a relatively intense gain peak  14  exists around 1532 nm, corresponding to a peak in the amplified stimulated emission (ASE) at the same wavelength. 
     It is important that the different WDM channels experience approximately equal gain at each amplifier in a multi-amplifier chain. Otherwise, the differences exponentially increase along the chain to the point that the different WDM signals cannot be treated equivalently. Placing all the WDM channels in the flat-gain band  12  reduces the differential gain problem. Eight WDM channels with equal 2 nm spacings can be placed in this band  12 , a demanding but not impossible design. This even arrangement of WDM channels is referred to as a wavelength comb. The wavelength channels, however, can be dispersed within the flat-gain band  12  with unequal spacings. Such uneven spacings affect neither the problems discussed below nor the solutions provided by the invention. 
     A further problem with the operation of EDFAs in a WDM system arises from the operational fact that not all the WDM channels are necessarily being used at the same time. Some channels may be dark. The variability of channel number is an especial problem in all-optical WDM networks in which optical signals are switched between different fibers according to the respective wavelengths of the signals. A simple exemplary WDM network, shown in FIG. 2, includes three terminal nodes  20 ,  22 ,  24 , which are linked by optical fibers through a wavelength-selective switch  26  which directs an optical signal according to its wavelength. The switch  26  can be reconfigured between selected switching states. Importantly for this invention, the fiber link  28  between the switch  26  and the third node  24  is long and contains several amplifiers  30  spaced along its length. At some times, the first node  20  may be sending one channel at λ 1  to the second node  22  and a second channel at λ 2  to the distant third node  24  while at other times, it may be sending both channels λ 1 , λ 2  to the third node  24 . The same situation obtains if the two channels originate from different nodes  20 ,  22  and are directed to a common node  24 , but both channels are not always active. In either case, dependent upon traffic, the number of channels on the long fiber  28  will vary. 
     An erbium-doped fiber amplifier shows a complex relation between the input and output powers. That is, the gain G which is the ratio of the output to input powers P OUT /P IN  depends on the magnitude of power. The linearly scaled graph of FIG. 3 plots optical output power P OUT  as a function of optical input power P IN  at a fixed pump power. In a constant gain region  32 , the two powers are proportionally related, and the gain G is constant. However, an EDFA can provide only a finite amount of optical output power, and in a saturation region  34  further increases in input power produce progressively smaller increases in output power. The graph of FIG. 3 can be replotted, as shown in FIG. 4, for the gain as a function of input power P IN . The numerical values of this (graph are measured values but are given only as representative since they depend on many factors, but the general trends are considered to be nearly universal. It is seen that gain is relatively flat at lower input power but falls off for high values of input power. The fall off is even more dramatic when gain is plotted as a function of output power. 
     It is general practice to operate an erbium-doped fiber amplifier in saturation for at least two reasons. Over some power range, the signal-to-noise ratio is better in saturation. Also, a saturated amplifier produces an output that has a relatively fixed amplitude regardless of the input power. That is, the output level is typically clamped. With clamped outputs, all the transmitters and amplifiers of an entire network do not need to be concurrently tuned to produce acceptable signal levels at the network output if the intermediate amplifiers tend to output signals of a fixed level. 
     However, the saturation effects shown in FIGS. 3 and 4 in a homogeneously broadened EDFA depend upon the total optical power, that is, the sum of the optical powers in a multi-wavelength WDM signal. The distinction between homogeneously and inhomogeneously broadened EDFAs will not be discussed, but a silica-based EDFA is homogeneously broadened, at least at room temperature. Ideally, N active WDM channels will have N times the optical power as one active WDM channel however, a saturated amplifier will amplify each of N input signals less than it would amplify one input signal because its available gain is spread between more channels. Typically, halving the number of active WDM channels will double the output power of each remaining channel, that is, a 3 dB increase. The finite amount of gain power is distributed amount the number of WDM channels being amplified. 
     Such traffic dependence of the intensities of optical signals places severe constraints on an optical network because many of its optically active components enjoy relatively small operating margins. Even the transmission fiber may experience non-linear effects at undesirably high signal powers. The problem is worsened in all-optical networks that are more complexly connected than that of FIG. 2, such as the type disclosed by Brackett et al. in “A Scalable Multi wavelength Multihop Optical Network: A Proposal for Research on All-Optical Networks,  Journal of Lightwave Technology,  vol. 11, 1993, pp. 736-753. In this case, different signals on one link may have passed through distinctly different paths before being combined on the long fiber link containing one or more amplifiers. Assuming that the combining element does not perform signal equalization, if the signal amplitudes are not tightly controlled throughout the network (that is, the gain made constant for all signals at all amplifiers), multiple WDM signals on one link may have significantly different intensities. 
     Thus, it is greatly desired to stabilize the gain of erbium-doped fiber amplifiers or whatever other kinds of amplifiers are used in an optical network so that their gain is relatively independent of the number of channels being amplified. 
     It would be possible to monitor the number of channels passing through an amplifier and to accordingly adjust its gain, for example, by changing the pumping power. However, either local monitoring or overall control by the network is difficult and adds undesired cost and complexity to otherwise simple amplifiers. In point-to-point WDM links, the solution is simpler since the channels are available in electronic form at the transmitting end. An extra signal may then be imposed at a wavelength not otherwise used with its intensity level depending on the number of vacant channels. This solution is workable for point-to-point at the cost of some complexity, but becomes less feasible for all-optical networks in which electronic versions of the signals are not readily available at the switching nodes. 
     Fake et al. have disclosed an automatic gain compensation technique in “Optically stabilized EDFA for in-band WDM systems,”  Technical Digest, Optical Fiber Conference  &#39;95, paper TuP3, pp. 79, 80. They have placed the amplifying fiber between two gratings that selectively reflect at a wavelength outside the band of the data signals but within the spontaneous emission band of the EDFA. The gratings multiply reflect the extra signal through the amplifying medium and cause it to lase, thus consuming any extra gain provided by changing numbers of channels. Fake et al.&#39;s approach is relatively inflexible since the interference filter can&#39;t be tuned and an attenuator cannot be placed in the feedback loop for the purpose of controlling the feedback intensity. 
     Okamura in “Automatic Optical Loss Compensation with Erbium-Doped Fiber Amplifier,”  Journal of Lightwave Technology,  vol. 10, 1992, pp. 1110-1116 have disclosed a technique for automatic gain control so as to stabilize output power. Okamura, in one embodiment, use a feedback loop around the EDFA having a narrow, out-of-band wavelength band to cause lasing at that wavelength. In an experimental circuit, he uses a modulated attenuator in the feedback loop. Zirngibl has disclosed a similar feedback structure in “Gain control in erbium-doped fibre amplifiers by an all-optical feedback loop,”  Electronics Letters,  vol. 27, 1991, pp. 560, 561. 
     Longer transmission links may require that multiple amplifiers be placed along the length of the link in a chain arrangement. The same problem of the amplifier gain depending upon the number of active channels occurs at each of the sequentially arranged amplifiers even though the number of active channels does not vary along the amplifier chain. Indeed, for non-saturated operation of the amplifiers, the gain variation is multiplicative over the number of amplifiers since, as the number of active channels varies, the fiber loss remains constant but the amplifier gain changes so that the gain variations accumulate. 
     Accordingly, it is desired to provide an erbium-doped fiber amplifier whose gain is independent of the number of active channels. It is also desired to apply such an amplifier to a multi-amplifier chain in a cost-effective design. 
     SUMMARY OF THE INVENTION 
     The invention may be summarized as a gain-compensated optical amplifier, particularly an erbium-doped fiber amplifier in a telecommunications network that is particularly useful for compensating for a varying number of channels being amplified in the gain-saturated amplifier. A feedback loop is created around the amplifier that passes only a signal having a selected wavelength other than the intended data signals. Preferably for a single amplifier, the couplers to and from the loop are wavelength-selective. An attenuator is placed in the loop and is set so that the feedback signal lases. Thereby, the lasing feedback signal consumes most of the excess gain of the saturated amplifier, so that the channel signals experience nearly the same gain regardless of the number of such channels that are active. In a multi-amplifier chain, only the first amplifier requires the feedback loop if the feedback signal is in the flatband of the amplifier and is output to the next amplifier with the same efficiency as the signal wavelengths. That is, the output coupler should be wavelength independent although the input coupler may advantageously be made wavelength selective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of gain spectrum of a representative erbium-doped fiber amplifier. 
     FIG. 2 is a network diagram of an example of a wavelength-division multiplexed (WDM) optical network. 
     FIG. 3 is a graph showing the power saturation effect in a fiber amplifier. 
     FIG. 4 is a graph, similar to that of FIG. 3, but replotted to show gain as a function of input power and also showing actual experimental data. 
     FIG. 5 is an optical circuit diagram for a first embodiment of the invention using wavelength-selective couplers to the feedback loop. 
     FIGS. 6 through 8 are schematical graphs showing the effect on output power for the circuit of FIG. 5 when the number of wavelength channels is varied. 
     FIG. 9 is an optical circuit diagram of a second embodiment of the invention using wavelength-independent couplers to the feedback loop and a filter in the loop. 
     FIG. 10 is an optical power spectrum measured for the optical circuit of FIG.  9 . 
     FIGS. 11 and 12 are optical spectra showing the wavelength assignments for the WDM system utilizing the invention. 
     FIG. 13 is an optical network diagram of a first embodiment of particularly advantageous use of the invention involving a chain of amplifiers. 
     FIG. 14 is an optical network diagram of a second embodiment similar to the first embodiment of FIG.  13 . 
     FIGS. 15 and 16 are optical network diagrams illustrating possible placements of optical isolators. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides automatic compensation at an optical amplifier for a changing number of WDM channels being amplified. As mentioned previously, there may be N WDM channels, N typically being 8 at the present stage of technology, but the number is expected to increase to 16 and possibly beyond as WDM technology advances. Importantly, however, for an N-channel system, at any point in time, it is possible that fewer than N channels are filled with optical power. Any change in the number of active channels tends to cause gain variations in the saturated amplifier as the number of channels changes. 
     According to the invention, as shown in FIG. 5, an erbium-doped fiber amplifier (EDFA)  50  is placed along an optical fiber  52  carrying multiple WDM channels in the wavelength band of 1540 to 1560 nm. The EDFA  50  is represented as a system complete with pumping source. Often these commercial units include isolators which prevent backward propagation through the amplifier  50  and may also include a filter to eliminate the ASE radiation at its peak in the neighborhood of 1532 nm. 
     In a first embodiment of the invention, the EDFA  50  is separated from the transmission fiber by two optical couplers  54 ,  56  which couple a 10 nm passband of optical wavelengths near 1532 nm between the EDFA  50  and a feedback loop comprising a feedback fiber  58  including a variable attenuator  60 . The couplers  54 ,  56  are bidirectional so that the same 10 nm band is passed from the feedback loop in either direction to the EDFA  50 , but they substantially block this band from either the input or output side of the transmission fiber  52 . 
     The result is that within the narrow passband a significant portion of the optical energy arising from spontaneous emission is fed back through the EDFA  50  to be amplified therein. Let the gain of the EDFA  50  within the passband be represented by G, the loss introduced by the variable attenuator be represented by L A , and the coupling loss introduced by the two couplers  54 ,  56 , fiber splices, and other losses be represented by L C . If the gain within the feedback loop exceeds the losses 
     
       
           G&gt;L   A   +L   C   (1) 
       
     
     for a wavelength λ L  within the passband, optical radiation at this wavelength λ L  will be increasingly amplified as it repetitively passes around the loop, and radiation at the wavelength of maximum net gain will begin to lase and to consume a substantial portion of the available excess gain from the EDFA  50 . Of course, the lasing wavelength λ L  cannot be one of the signal wavelengths, but it can be either outside of the wavelength comb of the WDM signal or can be within the comb but between two adjacent WDM wavelengths. The peak ASE wavelength  14  around 1532 nm shown in FIG. 1 provides a ready source of optical radiation to be amplified. However, other portions of the optical gain spectrum  10  can also be used, including unused wavelengths within the flat band portion  12 . The attenuator  60  is set so that the signal at the lasing wavelength λ L  is barely lasing in the condition of all data signals being present. This condition guarantees that the lasing signal uses most of the available excess gain but does not significantly reduce the available gain when all transmission channels are present. Once the variable attenuator  60  is adjusted to just barely permit lasing, it is not normally readjusted during normal operation of the fiber amplifier  50 . 
     The intended operation of the invention is schematically illustrated in the output power spectra of FIGS. 6 through 8. The spectrum of FIG. 6 is the output of a conventional erbium-doped fiber amplifier without the inventive feedback. It amplifies a first and a second WDM signal  62 ,  64 . There is a strong ASE signal  66  at the ASE peak, but in a well designed system its level is kept below the levels of the WDM signals  62 ,  64 . The spectrum of FIG. 7 is the output of the inventive erbium-doped fiber amplifier with the ASE signal  66  lasing to a much higher intensity level. The overall gain of the amplifier is adjusted so that two WDM signals  62 ,  64  are amplified to their previous levels. The spectrum of FIG. 8 shows that for the invention, when the second WDM signal  64  becomes inactive, the gain of the first WDM signal  62  is not significantly increased because most of the saturated amplifier gain is consumed by the lasing signal  66 . 
     Another embodiment of invention is illustrated in the schematic of FIG.  9  and includes in the feedback loop both the variable attenuator  60  and a narrow-passband filter  90  at the wavelength desired to be lased, for example, the ASE peak near 1532 nm. Two relatively wavelength-insensitive couplers  92 ,  94  couple a fraction of the total bandwidth power into the feedback loop. Their wavelength insensitivity should extend over the entire wavelength range of the data signals in order to not perturb the gain flatness of the EDFA. The lasing wavelength λ L  is determined by the filter  90 . The structure of the equalized amplifier of FIG. 9 resembles that of Zirngibl, ibid. The operation of the embodiment of FIG. 9 is similar to that of the embodiment of FIG.  5 . The couplers  92 ,  94  couple a fraction of all the wavelengths in the feedback loop  58 , but only the selected lasing wavelength is allowed to pass the feedback filter  90 . The signal at this selected wavelength, even though it arises from noise, lases if the variable attenuator  60  is set for a loss below the lasing threshold of Eqn. (1). 
     The fractional feedback through the wavelength-insensitive amplifiers is less than 50% (−3 dB) and preferably 10% (−10 dB) or less. The amount of desired feedback is determined by the intended equalized gain of the amplifier. If the gain is to be fixed to 18 dB, then the feedback loop and its associated couplers and attenuators must provide slightly less than 18 dB of loss at the lasing wavelength. Some of this loss is provided by the connectors and by the two couplers. Experimentally, it has been found that an added loss of 12 dB to 15 dB is sufficient to keep the fed back wavelength just barely lasing when all signals are present. These latter values are consistent with the values presented immediately above. However, if the intrinsic losses of the connectors and couplers can be reduced, then more signal energy can be immediately output with an increase in the required controlled loss in the feedback loop. 
     Although the embodiment of FIG. 9 shows a narrow bandpass filter  90 , if the lasing wavelength is selected to be outside the wavelength comb, a low-pass or high-pass filter could be substituted that would block the wavelength comb and pass a wavelength of sufficiently high net gain that its signal would lase. The same comments apply to the wavelength-selective couplers  54 ,  56  of FIG.  5 . It should also be noted that some fraction of the data signals can be circulated through the feedback loop as long as their net gain is less than unity. That is, the loop must be gain/loss selective to the lasing wavelength of the WDM wavelengths. 
     The embodiment of FIG. 9 has the disadvantage that some of the amplified data signal is sacrificed in the feedback loop, where it is then blocked. This loss of amplified data signal is disadvantageous because wide-band amplified stimulated emission is present at the WDM wavelengths and intentionally discarding some of the amplified signal reduces the gain available from the EDFA, thereby reducing the signal-to-noise ratio. On the contrary, the embodiment of FIG. 5 has that advantage that it does not sacrifice amplifier gain, insofar as the wavelength-selective couplers are efficient, and thus minimizes any reduction in signal gain incurred by the feedback circuit. 
     The embodiment of FIG. 5 uses two wavelength-selective couplers  54 ,  56  while the embodiment of FIG. 9 uses two wavelength-insensitive couplers  92 ,  94 . These approaches can be combined, with one coupler being wavelength-selective and the other being wavelength-insensitive. If the input coupler  54  is wavelength-selective, the intensity loss of the input signal is minimized and the bandpass filter  90  can be removed from the feedback loop. 
     Many types of wavelength-selective couplers are available which can be easily used with optical fibers. A first type is a GRIN rod lens having an interference filter on one side that passes a narrow band to or from the feedback loop and reflects the rest between an input or output fiber and the fiber amplifier. In practice, the interference filter is placed between two GRIN rod lens. A second type is an acousto-optical tunable filter utilizing a photoelastic material such as LiNbO 3  which is mechanically modulated by an RF signal applied to interdigitated electrodes overlying the optical waveguide. The frequency of the RF signal corresponds to the wavelength of the optical signal that is affected. Cheung et al. describe such acousto-optical filters as applied to optical fibers in U.S. Pat. No. 5,002,349. A third type is a liquid-crystal switch that selectively deflects different optical wavelengths, as described by Patel et al. In U.S. Pat. Nos. 5,414,540 and 5,414,541. Other functionally similar wavelength-selective couplers are available. 
     EXPERIMENT 
     The configuration of FIG. 5 has been experimentally tested. The wavelength-selective couplers  54 ,  56  were combination of GRIN rod lenses and an interference filter. The interference filter reflected wavelengths of greater than 1535 nm while passing wavelengths of less than 1532 nm. These were configured so that all wavelengths went through the EDFA, only the lower wavelengths went through the feedback loop, and only the higher wavelengths passed to or from the external ports. 
     In one case, the EDFA  50  was a commercial unit, Model X1706 available from AT&amp;T, which includes an isolator to prevent backward propagation so that only the forward propagating noise signals are allowed to lase. With this EDFA, the ASE signal at 1532 nm was observed to lase. In another case, the EDFA  50  was Model X1712 also available from AT&amp;T, which includes a filter at the peak ASE wavelength. With this EDFA, lasing occurred at 1539 nm, which is assumed to be the wavelength of maximum total gain, that is, gain less loss including filter loss. 
     Two signals of wavelengths λ 1 , λ 2  (1542 nm and 1552 nm) were selectively applied to an erbium-doped fiber amplifier operating in the saturated regime. The power of the signal with the first wavelength λ 1  was about ten times (+10 dB) that of the signal with second wavelength λ 2  so that adding or removing the first signal λ 1  is a more severe condition for the gain experienced by the second signal λ 2  than the worst possible signal-power variation in an 8-wavelength WDM system. The power spectra of FIG. 10 shows the spectrally resolved optical power output by the gain-stabilized EDFA. Spectrum  80  was measured for the weaker signal of the second wavelength λ 2  only with no feed back. Spectrum  82  was measured for signals of both wavelengths λ 1 , λ 2  with the feedback signal lasing at 1532 nm. Spectrum  84  was measured for the weaker signal of wavelength λ 2  only with no signal at wavelength λ 1  but with the feedback signal at 1532 nm. The lasing signal observed at the output is less than the data signals because the wavelength-selective couplers discriminate against the lasing wavelength on their in line outputs. It is noted that the feedback signal at 1532 nm is measured outside of the loop so that its observed intensity is about 100 times less (−20 dB) than its intensity within the loop for the WDM configuration used in the tests. 
     This experiment shows that in the presence of the 1532 nm feedback signal the presence or absence of the much stronger optical signal at λ 1  has minimal effect on the amplification of the weaker signal at λ 2 , measured in fractions of a dB. Thus, the invention is effective in reducing the effect of channels being added or removed. 
     One advantage of doped fiber amplifiers is their slow gain dynamics so that inter-bit interference is not a problem. However, the slow gain dynamics in the presence of strong feedback and varying signal intensities have the capability of inducing oscillations following an intensity change. 
     Time-resolved gain has been measured in the gain-stabilized EDFA when a channel is abruptly dropped. A characteristic oscillation period of about 20 μs in the intensity of the remaining signal was observed, but the oscillations damped out after about 100 μs. In another experiment, the channel was not abruptly dropped, but was gradually dropped over a time period greater than the characteristic oscillation time. In the test, the channel-dropping time was about 40 μs. No oscillations in the gain of the remaining channel were observed. Thus, it is preferable that the optical network be controlled such that channels are added or dropped with finite transition times. The control algorithm can be implemented with this constraint. Many unplanned changes, such as laser failures, cable cuts, and loss of a power supply, have natural transition times that are acceptably long on this time scale. However, if the oscillations at transition do not produce higher excursions than can be tolerated, then transition times do not need to be additionally controlled. 
     Both the experiments reported above used the ASE (amplified stimulated emission) radiation around 1532 nm as the source of radiation that is caused to lase in the feedback loop since this wavelength corresponds to the peak ASE. A typical spectrum is shown in FIG. 11 in which 8 WDM channels  100  are evenly spread over the flat gain band of the erbium-doped fiber amplifier in a wavelength comb (although such even spacing is not required) and an ASE signal  102  at the ASE peak is fed back and caused to lase. According to the invention, the number of active WDM channels  100  can change without significantly affecting the gain of the remaining WDM channels  100 . This design has the advantage of fully using the flat-gain band of the EDFA and separating the strong lasing signal  102  from the weaker WDM signals. Also, less gain is required to lase the ASE signal since it is already fairly strong. However, the invention can use other wavelength allocations. The principal constraint is that the lasing signal  102  must be located at a wavelength not used by the WDM signals. As shown by the gain spectrum of FIG. 1, the ASE spectrum is relatively wide and any wavelength within the spectrum that avoids the WDM channels can be caused to lase in the feedback loop. 
     An advantageous wavelength allocation, though technically demanding, is illustrated in the spectrum of FIG.  12 . Again, the eight WDM channels  100  occupy the flat-gain band of the EDFA. However, a lasing signal  104  is placed within the WDM comb at a wavelength between two WDM channels  100 . This design has the advantage, to be justified later, of the lasing signal  104  also occupying the flat-gain band of the EDFA. The design of FIG. 12 has the disadvantage that the strong lasing signal  104  is separated in wavelength from the adjacent WDM channels  100  by only half the normal WDM inter-channel spacing. A first alternative design avoids this problem by placing the lasing signal either closely above or closely below the WDM wavelength comb. A second alternative design evenly spaces all the signals but assigns one of the middle channels to the lasing signal. In all these spectral assignments, other considerations may favor uneven wavelength spacing of the signals. 
     An advantage of the feedback structure of FIG. 9 using wavelength-independent couplers  92 ,  94  is that the lasing signal is coupled onto the output fiber  52  at an intensity that compensates for any missing WDM channels, that is, at the same intensity relative to the “real channels” that it has within the EDFA. As a result, if a transmission link is long enough to require multiple doped fiber amplifiers, it is only necessary to equalize the gain at the first amplifier if the lasing wavelength is within the flat-gain band. As illustrated in the circuit diagram of FIG. 13, three erbium-doped fiber amplifiers  110 ,  112 ,  114  are arranged serially along an optical fiber  116  forming a transmission link. A multiplexer  118  combines multiple WDM signals on the transmitting end, and a demultiplexer  120  separates at least some of the WDM signals on the receiving end. The number of WDM channels entering the link from the multiplexer  118  can vary over time, but along the link the number of channels remains the same. That is, channels are not added or dropped along the link but only possibly at the ends  118 ,  120  of the link. 
     A feedback loop including wavelength-independent couplers  92 ,  94  is provided only for the first amplifier  110 . The portion of the fiber  116  between the first and second amplifiers  110 ,  112  carries both the active WDM channels and lasing signal required for the first amplifier  110  to maintain the proper gain for the remaining WDM channels. If the second amplifier  112  has the same characteristics as the first amplifier  110  and amplifies the WDM signals to the same level, then its gain is properly equalized if the equalizing signal (the laser signal from the first amplifier  110 ) is within the flat-gain band of the EDFA. The second amplifier  110  also amplifies the equalizing signal so that the same distribution of intensities is incident on the third amplifier  114  and any other amplifier in the serial link. Within the flat-gain band, each amplifier in the chain amplifies the WDM and equalizing signals to the same relative intensities. 
     Only the first amplifier  110  is a stabilized-gain amplifier. The downstream amplifiers  112 ,  114  are preferably operated in saturation, that is, fixed output per channel. As a result, if added loss is introduced into the link by damage to the fiber, connector going bad, etc., the saturated operation can to some extent compensate for this new loss. 
     A more preferred embodiment of the first-stage amplifier, as illustrated in FIG. 14, includes the wavelength-selective coupler  54  on the input side and the wavelength-insensitive coupler  94  on the output side. The wavelength-selective input coupler  54  eliminates the need for the bandpass filter  90 , but the wavelength-independent output coupler couples out all signals with the same efficiency, thus guaranteeing that the feedback signal will have the same intensity relative to the data signals on the output as in the fiber amplifier  50 . 
     Although the variable optical attenuator in the feedback loop of the examples was mechanically adjusted, other types of attenuators can be utilized. Indeed, an attenuator can be placed in the amplifier portion of the loop if it can selectively attenuate the selected lasing wavelength over the WDM wavelengths, e.g., a variable wavelength-selective coupler which could be functionally incorporated into the coupler  54 ,  56 , even though this is a complicated configuration. 
     The absence of feedback loops in subsequent amplifiers simplifies the design and reduces the cost of long transmission links, and provides the further advantage that all amplifiers are operated in saturation with fixed output per channel rather than fixed total gain. 
     Commercially available erbium-doped fiber amplifiers typically include isolators to prevent backward propagation through the amplifier and perhaps also include an ASE filter in line with the amplifier to suppress the ASE peak. With the invention, these components need to be reconsidered. Insofar as wavelength-selective couplers are used at the peak ASE wavelength, the need for backward isolators is reduced since the selected wavelengths of the ASE radiation is mostly coupled into the feedback loop rather than backwardly to the transmitter. Nonetheless, there will be leakage in the backward direction. Insofar as backward isolators are required, they can be inserted at different places. 
     As illustrated FIG. 15, an isolator  130  can be placed outside of the feedback loop  132  on the input side to block backwardly propagating light. This configuration allows feedback to circulate in both directions around the loop (unless other means are used to block the counter-clockwise feedback), and the isolator  130  thus protects the transmitter from spurious signals from the amplifier  132 , including counter-clockwise rotating lasing light leaking from the loop. While this configuration has the advantage that loop loss is minimized, it is disadvantageous for a multi-stage amplifier string, as in FIG. 13, because only the clockwise rotating laser light exits the loop  132  on the output side so that the ratio of signal and laser intensities is different in the amplifier  50  and on the output. 
     Alternatively, as illustrated in FIG. 16, the isolator  130  can be placed within the loop  132  on the input side of the amplifier  50 . This configuration prevents feedback in the counter-clockwise direction and thus is consistent with the multi-stage amplifier strings of FIGS. 13 and 14 since it assures the same relative intensities of signal and laser light inside and outside the loop  132 . 
     Similarly, the need for ASE filters with the stabilized amplifier of the invention appears to be reduced since most ASE radiation is restricted to the feedback loop, especially for wavelength-selective coupling with a passband at the peak ASE wavelength. 
     Although the invention is particularly useful for erbium-doped fiber amplifiers because of the present degree of commercialization, the invention can be applied to fiber amplifiers doped with other active species. Indeed, the invention is applicable to any optical amplifier that operates in the saturation region and carries uncertain number of wavelength-multiplexed signals. 
     The invention thus provides an economical and automatic equalization mechanism for wavelength-division multiplexed optical communication systems.