WDM system equalization with EDFA optical amplifiers

An in-line repeater implemented within a Wavelength Division Multiplexed (WDM) optical fiber communication system is shown that includes an attenuator. The communication system includes a WDM multiplexer that transmits a WDM signal including a series of WDM channels, via the in-line repeater, to a demultiplexer that separates the WDM channels into a series of output signals. Within the in-line repeater, the attenuator is coupled between two stages of a single amplifier in order to aid in the power equalization of the WDM channels at the demultiplexer. The addition of the attenuator results in the powers and OSNR of the individual WDM channels becoming closer in value, hence reducing the time and the dynamic range of transmitter power adjustments required for traditional methods to equalize the channels' powers.

FIELD OF THE INVENTION
 This invention relates generally to Wavelength Division Multiplexed (WDM)
 systems and more specifically to equalization within a WDM system using
 Erbium-Doped Fiber Amplifier (EDFA) optical amplifiers.
 BACKGROUND OF THE INVENTION
 The use of erbium-doped fiber amplifier technology is increasing within
 optical fiber communication systems in a wide range of applications in
 which weak optical signals require amplification. These applications
 include, but are not limited to, preamplifiers, postamplifiers, and
 in-line repeaters in optical fiber communication systems.
 Also within current optical fiber technology, there is a growing
 requirement to increase the capacity of the existing communication
 systems. According to current technology, an increase in capacity can be
 achieved by increasing the bit rate and/or by adding wavelength division
 multiplexed (WDM) channels. As a result of the need for more capacity, the
 use of WDM channels and further, an increased number of such channels are
 becoming increasingly popular.
 FIG. 1 illustrates a typical unidirectional optical fiber communication
 system in which first and second optical fibers 102,104 couple a
 wavelength division multiplexer 106 at a first location to a wavelength
 division demultiplexer 108 at a second location which is remote from the
 first location. The multiplexer 106 is used to wavelength division
 multiplex a series of channels (.lambda.1-.lambda.N) and the demultiplexer
 108 is used to subsequently demultiplex the WDM channels. As depicted
 within FIG. 1, coupled between the multiplexer 106 and the first fiber 102
 is an EDFA postamplifier 110 and coupled between the second fiber 104 and
 the demultiplexer 108 is an EDFA preamplifier 112. Further, as depicted
 within FIG. 1, coupled between the fibers 102,104 is an in-line repeater
 which comprises an EDFA optical amplifier 114. One skilled in the art
 would understand that further line-repeaters could also be utilized in
 such an implementation. This setup is a well understood unidirectional
 optical fiber communication system.
 One major problem in such an implementation as disclosed in FIG. 1 is the
 non-uniform wavelength dependent gain profile of the EDFA amplifier 114
 within the in-line repeater and further within any other EDFA optical
 fiber amplifiers that may be included between the multiplexer 106 and the
 demultiplexer 108 such as the post/preamplifiers 110,112. These problems,
 inherent to the currently utilized EDFA optical fiber amplifiers, result
 in each channel within a particular WDM system having a different optical
 gain and a different resulting Optical Signal to Noise Ratio (OSNR).
 Hence, some channels could have a relatively low OSNR and low received
 power which, in turn, could result in an excessively high bit error rate.
 Considerable efforts are being expended in order to equalize the received
 powers and OSNRs of the individual WDM channels at the demultiplexer 108
 and therefore ensure that all channels have corresponding OSNRs that are
 above a predetermined allowable threshold level. One technique to equalize
 the received powers between the channels (.lambda.1-.lambda.N) is to add
 Variable Optical Attenuators (VOAs) for each channel directly after the
 demultiplexer 108, so that, within a certain range, the received powers
 can be adjusted to a common value. Although effective in reducing the
 difference in received powers, the implementation of these VOAs does not
 reduce the differences between OSNRs of the individual channels
 (.lambda.1-.lambda.N).
 A technique that is utilized to reduce the difference in received powers
 and OSNRs between the WDM channels at the demultiplexer 108 is disclosed
 in U.S. Pat. No. 5,225,922 entitled "Optical Transmission System
 Equalizer" by Chraplyvy et al, issued on Jul. 6, 1993 and assigned to AT&T
 Bell Laboratories of Murray Hill, NJ. With this technique, a controller
 detects the power of the optical signals of each individual channel at
 each amplifier with use of a series of power detectors and subsequently
 adjusts the transmission power corresponding to each of the channels at
 the multiplexer 106 with use of a series of transmission power adjusters.
 The controller, input with the detected powers, operates to adjust the
 transmission power for each channel in order to compensate for the
 non-uniform gain problems caused by the optical fiber amplifiers. Hence,
 any channels with a low OSNR will have their corresponding transmission
 power increased while any channels with a high OSNR will have their
 transmission power reduced. Eventually, this feedback technique will
 equalize the power corresponding to the received optical signals on all
 the channels, ensuring that all channels have satisfactory OSNRs and also
 limiting unnecessary transmission power.
 There are a number of key problems with this technique for equalizing the
 OSNRs corresponding to the individual WDM channels. For one, this feedback
 technique typically requires numerous iterations, and therefore a
 considerable amount of time, to complete. This is especially true as the
 number of channels increase. Secondly, this technique must allow for the
 transmission power for the individual WDM channels to be adjustable over a
 large dynamic range. As the dynamic range increases, the complexity and
 cost of the transmission power adjusters required within the multiplexer
 106 also increase.
 It can be seen that the unidirectional system of FIG. 1 can be expanded to
 a typical bidirectional optical fiber communication system as depicted in
 FIG. 2. This system comprises first and second optical fibers 202,204
 coupled between first and second WDM couplers 206,208, each coupler
 operating as a red and blue band splitter. Further coupled to the first
 WDM coupler 206 is a blue band signal multiplexer 210 and a red band
 signal demultiplexer 212, while further coupled to the second WDM coupler
 208 is a red band signal multiplexer 214 and a blue band signal
 demultiplexer 216. The multiplexers 210,214 are used, similar to that for
 the multiplexer 106 within FIG. 1, to wavelength division multiplex a
 series of respective channels (.lambda.b1-.lambda.N,
 .lambda.r1-.lambda.rN) and the demultiplexers 212,216 are used to
 subsequently demultiplex the channels.
 As depicted within FIG. 2, coupled between the first WDM coupler 206 and
 the first fiber 202 is a blue post/red pre amplifier 218 and coupled
 between the second fiber 204 and the second WDM coupler 208 is a blue
 pre/red post amplifier 220. Further, as depicted in FIG. 2, coupled
 between the fibers 202,204 is a bidirectional in-line repeater 222. It can
 be seen from FIG. 2 that there is a blue and red transmission path which
 respectively traverse blue multiplexer 210, WDM coupler 206, blue
 postamplifier 218, fiber 202, repeater 222, fiber 204, blue preamplifier
 220, WDM coupler 208, and blue demultiplexer 216; and traverse red
 mulitplexer 214, WDM coupler 208, red postamplifier 220, fiber 204,
 repeater 222, fiber 202, red preamplifier 218, WDM coupler 206, and red
 demultiplexer 212. One skilled in the art would understand that the key
 differentiating feature between the red and blue paths is the transmission
 wavelengths of the corresponding WDM channels, those being in one sample
 case between 1528 to 1542 nm for the blue path and 1547 to 1561 nm for the
 red path.
 One skilled in the art would understand that the bidirectional repeater 222
 of FIG. 2 has similar problems as discussed herein above with respect to
 the unidirectional repeater 114, hence requiring an equalization technique
 to be implemented in the bidirectional system. The complexity of such an
 equalization technique in a bidirectional WDM system increases compared
 with that in a unidirectional WDM system.
 Hence, an improvement in both unidirectional and bidirectional optical
 fiber communication systems is required that equalizes the OSNRs of the
 WDM channels in a more efficient manner. Preferably this improvement would
 reduce the number of iterations required and the dynamic range of the
 transmission power adjusters. As well, this improvement would preferably
 not require a significant redesign of the amplifier system, but possibly
 could take advantage of advancements in two-stage optical fiber amplifier
 technology to allow for a reduced implementation cost.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to overcome at least one of the
 disadvantages of the prior art and, in an exemplary embodiment, to provide
 a system and method by which the equalization of WDM channels is performed
 more efficiently.
 The present invention, in one broad aspect, is a method of reducing the
 difference in gain experienced by Wavelength Division Multiplexed (WDM)
 channels that are input to an amplifier. This method includes the step of
 adjusting the amplifier's input signal power relative to the amplifier's
 pump power. Preferably, this method further includes the step of adjusting
 the transmit power of the WDM channels to aid in an equalization of the
 WDM channels.
 The present invention, in a second broad aspect, is an amplifying apparatus
 that includes an attenuator and an amplifier. The attenuator receives
 signals including WDM channels and outputs attenuated versions of the
 received signals. The amplifier then receives the attenuated signals,
 amplifies them such that the WDM channels are amplified by different
 amounts, and outputs amplified signals. The attenuator adjusts the
 amplifier's input signal power relative to the amplifier's pump power to
 reduce the difference in gain experienced by the individual WDM channels.
 In preferred embodiments, the amplifying apparatus includes amplifying
 modules which each consist of a coupler, a first amplifier having an input
 attached to the coupler, and a second amplifier having an output coupled
 to the coupler. These two amplifying modules are coupled together in these
 preferred embodiments such that the output from the first amplifier within
 one module is coupled to the input of the second amplifier within the
 other module, with an attenuator coupled in between.
 The present invention, in a third broad aspect, is a WDM optical fiber
 communication system that includes at least one of the amplifying
 apparatuses of the first broad aspect. This communication system further
 includes a WDM multiplexer that combines a plurality of input signals into
 a WDM signal, a demultiplexer that separates the WDM signal into a
 plurality of output signals, and a number of optical fibers that connect
 the amplifying apparatus between the multiplexer and demultiplexer.
 In preferred embodiments, the WDM optical fiber communication system is
 expanded to include the amplifying apparatus according to the above
 described preferable embodiment. In this expanded system, there is a
 multiplexer and demultiplexer for each direction with couplers combining
 the channels of the two directions for transmission on the optical fibers.
 Other aspects and features of the present invention will become apparent to
 those ordinarily skilled in the art upon review of the following
 description of specific embodiments of the invention in conjunction with
 the accompanying figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Although a preferred embodiment of the present invention is described in
 detail herein below for a specific optical fiber communication system, it
 should be understood that the scope of the present invention is not meant
 to be limited to such an embodiment. In particular, despite the present
 invention being conceived of to compensate for problems with EDFA
 amplifiers, it can be seen that the present invention could further be
 used with amplifiers that have similar characteristics.
 A preferred embodiment of the present invention is now described with
 reference to FIG. 3. In this figure, a bidirectional optical fiber
 communication system is depicted which is modified from the bidirectional
 communication system of FIG. 2. In FIG. 3, the communication system
 comprises a blue band signal multiplexer 302 and a red band signal
 demultiplexer 304 coupled to a blue post/red pre amplifier module 306; a
 blue band signal demultiplexer 308 and a red band signal multiplexer 310
 coupled to a blue pre/red post amplifier module 312; and an in-line
 repeater 314 coupled to the blue post/red pre and blue pre/red post
 amplifier modules 306,312 via first and second optical fibers 316,318
 respectively.
 The blue post/red pre amplifier module 306 comprises a blue EDFA
 postamplifier 320 coupled to the output of the blue band signal
 multiplexer 302, a red EDFA preamplifier 322 coupled to the input of the
 red band signal demultiplexer 304, and a first WDM coupler 324 coupled to
 the first fiber 316, the blue postamplifier 320, and the red preamplifier
 322. Similarly, the blue pre/red post amplifier module 312 comprises a
 blue EDFA preamplifier 326 coupled to the input of the blue band signal
 demultiplexer 308, a red EDFA postamplifier 328 coupled to the output of
 the red band signal multiplexer 310, and a second WDM coupler 330 coupled
 to the second fiber 318, the blue preamplifier 326, and the red
 postamplifier 328.
 The in-line repeater 314 comprises in-line blue pre/red post and in-line
 blue post/red pre amplifier modules 332,334 coupled to the first and
 second fibers 316,318 respectively and further coupled together. The
 in-line blue pre/red post amplifier module 332 comprises a third WDM
 coupler 336 coupled to the first fiber 316, and in-line blue pre and
 in-line red post EDFA amplifiers 338,340 coupled to the third WDM coupler
 336. The in-line blue post/red pre amplifier module 334 similarly
 comprises a fourth WDM coupler 342 coupled to the second fiber 318, and
 in-line blue post and in-line red pre EDFA amplifiers 344,346 coupled to
 the fourth WDM coupler 342. According to a preferred embodiment, an
 in-line blue attenuator 348 and an in-line blue Dispersion Compensation
 Module (DCM) 350 are coupled in series between the in-line blue pre and
 in-line blue post amplifiers 338,344 which can be seen as first and second
 amplifying stages of a two-stage amplifier. As well, an in-line red
 attenuator 352 and an in-line red DCM 354 are coupled in series between
 the in-line red pre and in-line red post amplifiers 346,340 which can also
 be considered together as a two-stage amplifier. The implementation of the
 attenuators 348,352 are described in more detail herein below.
 Therefore, the blue communication path for transmitting blue channels
 (.lambda.b1-.lambda.bN) from the blue band signal multiplexer 302 to the
 blue band signal demultiplexer 308 consists of, in the preferred
 embodiment depicted in FIG. 3, the blue preamplifier 320, the first WDM
 coupler 324, the fiber 316, the third WDM coupler 336, the in-line blue
 preamplifier 338, the in-line blue attenuator 348, the in-line blue DCM
 350, the in-line blue postamplifier 344, the fourth WDM coupler 342, the
 second fiber 318, the second WDM coupler 330, and the blue postamplifier
 326. The red communication path for transmitting red channels
 (.lambda.r1-.lambda.rN) from the red band signal multiplexer 310 to the
 red band signal demultiplexer 304 consists of similar components but in
 the opposite direction.
 In one alternative embodiment, an Optical Add/Drop Multiplexer (OADM) can
 be added between the in-line blue pre and post amplifiers 338,344 and/or
 between the in-line red pre and post amplifiers 346,340. In yet another
 alternative embodiment, at least one of the in-line blue and red DCMs
 350,354 can be removed, hence leaving only the in-line blue and red
 attenuators 348,352 between the in-line amplifier modules 332,334. One
 skilled in the art would further understand that although FIG. 3 only
 depicts one in-line repeater it would be understood that in practical
 applications a plurality of in-line repeaters similar to the in-line
 repeater 314 may be implemented within the fibers 316,318.
 The key to the present invention is the addition of the attenuators 348,352
 within the in-line repeater 314. As will be described herein below, the
 addition and adjustment of these attenuators 348,352 can result in a more
 time efficient equalization process by compensating for the non-uniform
 gain ripple within the in-line EDFA amplifiers. As well, the addition and
 adjustment of the attenuators 348,352 can also reduce the transmission
 power adjustment range required when utilizing the known equalization
 technique described previously, which can result in an overall cost
 reduction by simplifying the transmission power adjusters. The addition
 and adjustment of the attenuators preferably causes the OSNRs of
 individual channels to become closer in value as will be described herein
 below in detail, hence reducing the required number of iterations of
 transmission power adjustments to equalize the output powers of the
 individual channels and the effective dynamic range needed for the
 adjustments.
 In a preferred embodiment, the attenuators 348,352 are well known
 components that can be manually or automatically adjusted. In other
 embodiments, the desired values for the attenuators are determined or
 estimated prior to their implementation and therefore no adjustment of the
 attenuators is performed. It is further contemplated that an exemplary
 embodiment of the present invention would utilize tunable attenuators with
 an RS232 or Ethernet interface that are controlled by a remote controller.
 The remote controller makes it unnecessary for a human operator to
 physically change the components in order to adjust the attenuation
 values.
 The theoretical principles of the preferred embodiments of the present
 invention will now be described. An analytical model of an EDFA amplifier
 can be considered as follows:
 ##EQU1##
 where G(.lambda.) is the fiber amplifier gain in dB at wavelength .lambda.;
 L is the erbium-doped fiber length; .alpha.(.lambda.) and g(.lambda.) are
 the absorption and gain coefficients of the erbium-doped fiber
 respectively; n.sub.t is the total density of erbium ions which consists
 of both ground and metastable states; and &lt;.eta..sub.2 &gt; is the
 metastable population average along the length of the fiber amplifier.
 Further, the theoretical calculation to determine &lt;.eta..sub.2 &gt; can
 be performed as follows:
 ##EQU2##
 where .eta..sub.2 (z) is the local metastable population; .tau. is the
 lifetime of the metastable level; h is the Planck's constant; .nu..sub.k
 is the frequency of optical beam k; I.sub.k is the light intensity of the
 beam k; and .sigma..sub.ak and .sigma..sub.ek are absorption and emission
 cross-section spectra for optical beam k, respectively. One skilled in the
 art would understand that formula (2) indicates that &lt;.eta..sub.2 &gt;
 is determined by the pump power of the particular fiber amplifier used and
 the signal power level input to the fiber amplifier. As the input signal
 power decreases and/or the pump power increases, &lt;.eta..sub.2 &gt;
 increases. It can be shown that the maximum &lt;.eta..sub.2 &gt; value is
 n.sub.t while the minimum &lt;.eta..sub.2 &gt; value is zero.
 A signal gain difference .DELTA.G between wavelengths .lambda.1 and
 .lambda.2 can be determined using formula (1) as follows:
 ##EQU3##
 Assuming that .lambda.1 &lt;.lambda.2, it is typical in a normal
 erbium-doped fiber spectra for .DELTA.(.lambda.1)&gt;.alpha.(.lambda.2)
 and g(.lambda.1)&gt;g(.lambda.2). Therefore, if the pump power is strong
 enough and the input signal power is very low, &lt;.eta..sub.2 &gt; will
 be approximately equal to n.sub.t and the gain difference .DELTA.G will
 be:
EQU .DELTA.G=G(.lambda.1)-G(.lambda.2).apprxeq.4.
 34*L*[g(.lambda.1)-g(.lambda.2)]&gt;0 (4)
 On the other hand, if the pump power is too weak and the input signal is
 too strong, &lt;.eta..sub.2 &gt; will be approximately equal to zero and
 the gain difference .DELTA.G will be:
EQU .DELTA.G=G(.lambda.1)-G(.lambda.2).apprxeq.4.
 34*L*{-[.alpha.(.lambda.1)-.alpha.(.lambda.2)]}&lt;0 (5)
 Therefore, if the pump power and the input signal power are carefully
 controlled, &lt;.eta..sub.2 &gt; can be tuned such that .DELTA.G will be a
 particular value. In the case of a one-stage amplifier, &lt;.eta..sub.2
 &gt; should be tuned such that .DELTA.G becomes approximately zero.
 In a two-stage amplifier such as the in-line repeater 314 depicted in FIG.
 3, &lt;.eta..sub.2 &gt; of the second stage amplifier should be tuned such
 that the .DELTA.G of the second stage amplifier compensates for the
 .DELTA.G of the first stage amplifier. In particular, if the above
 equations are modelling one stage of a two-stage amplifier, it can be seen
 that the gain differences at wavelengths .lambda..sub.1 and .lambda..sub.2
 for the first and second stage amplifiers are respectively .DELTA.G.sub.1
 (.lambda..sub.1, .lambda..sub.2) and .DELTA.G.sub.2 (.lambda..sub.1,
 .lambda..sub.2). .DELTA.G.sub.1 (.lambda..sub.1, .lambda..sub.2) is
 determined by the pump power of the first stage amplifier and by the
 output signal power at the previous amplifier and the intervening fiber
 span loss. .DELTA.G.sub.2 (.lambda..sub.1, .lambda..sub.2), on the other
 hand, can preferably be controlled by an attenuator preceding the second
 stage amplifier. When this attenuator is carefully controlled, the total
 gain difference .DELTA.G.sub.total (.lambda..sub.1, .lambda..sub.2) of the
 two-stage amplifier, that being .DELTA.G.sub.1
 (.lambda..sub.1,.lambda..sub.2)+.DELTA.G.sub.2 (.lambda..sub.1,
 .lambda..sub.2), can be set to approximately zero.
 Referring back to the blue communication path within FIG. 3, the input
 signal power to the in-line blue postamplifier 344 is preferably
 controlled with the addition of the in-line blue attenuator 348 so that
 the total gain differences .DELTA.G between different wavelengths caused
 by the in-line blue pre/post amplifiers 338,344 is minimized. A similar
 minimization is preferably performed for the red communication path.
 Graphical experimental results generated with the operation of a WDM
 optical fiber communication system according to one embodiment of the
 present invention are now shown to illustrate the advantages of the
 present invention with reference to FIGS. 4a to 4d. The experimental
 communication system utilized was a 5 span OC-192 Non-Dispersion Shifted
 Fiber (NDSF) fiber 16 wavelength (8.lambda. in blue and 8.lambda. in red)
 system with an average span loss of 23 dB. Since the typical loss of NDSF
 fiber is 0.22 dB/km, the 23 dB loss of each span indicates that each span
 is over 100 km. In this system, 4 in-line repeaters similar to the in-line
 repeater 314 were used that each included a two stage amplifier and a
 tunable attenuator for each band. For simplicity, only the experimental
 results of the blue band are described herein below.
 Within each of FIGS. 4a to 4d, there is a horizontal axis 402 that
 represents the range of wavelengths, in units of nm, corresponding to the
 channels within the WDM communication system and a vertical axis 404 that
 represents the amplitude of the power, in units of dBm, at the
 corresponding wavelengths. FIG. 4a depicts a power spectrum illustrating
 the transmission power for 8 WDM channels directly after the multiplexing
 stage in the transmitter. It can be seen that these transmission powers
 are at approximately equal power levels.
 FIGS. 4b to 4d depict power spectra illustrating the power of the
 transmitted signals directly prior to the demultiplexing stage in the
 receiver assuming no equalization adjustment has been made. The key
 difference between the spectra of the individual figures is the level of
 attenuation that is added by the attenuators between the two amplifier
 stages within the in-line repeaters.
 In FIG. 4b, each of the attenuators add 11 dB of attenuation to the signal.
 It is apparent that there is a serious ripple in the peak powers for the
 channels due to the non-uniform nature of the amplifiers within the
 pre/post amplifiers and the amplifiers within the in-line repeaters. In
 the case shown in FIG. 4b, the OSNR values for the channels range from
 20.3 to 29.2 dB. It is noted that the known setup in which there is no
 attenuation added would result in an even wider distribution range of OSNR
 values for the channels. In FIG. 4c, the attenuation is increased to 15 dB
 at each attenuator. This results in a range of OSNR values for the
 channels from 22.9 to 28.6 dB. In FIG. 4d, the attenuation is increased
 once again to 19 dB. This results in a range of OSNR values for the
 channels from 23.2 to 27.3 dB. Therefore, by increasing the attenuation
 from 11 dB to 19 dB the minimum OSNR value is increased from 20.3 to 23.2
 dB and the maximum OSNR difference is decreased from 8.9 to 4.1 dB. At the
 same time, the amplifier gain ripple is also reduced and the channel power
 difference becomes smaller.
 The spectra of FIGS. 4b to 4d illustrate how changing the attenuation
 within the in-line repeaters can reduce the range of the OSNR values
 within the channels by raising the OSNR of the weakest channel and
 lowering the OSNR of the strongest channel. Since a communication system
 essentially fails if any one of the channels has an OSNR below an
 acceptable threshold, the increase of the OSNR of the weakest channel by
 changing of the attenuation between the two amplifier stages can be seen
 to reduce the possibility of such a failure; though there are numerous
 other advantages as will be described herein below.
 In the past, OSNR equalization was typically realized by changing the
 transmitter power for each of the individual WDM channels. According to
 the preferred embodiments described above, in-line repeaters are
 implemented such that their attenuators can be adjusted, hence allowing a
 new second degree of freedom in the equalization process. It is noted, as
 described previously, that a determination could be made prior to the
 implementation of the attenuators to determine which attenuators of fixed
 value to utilize within the in-line repeaters. The key to the preferred
 embodiments is that the optical amplifier gain ripple becomes smaller due
 to the attenuation between the two stages of the amplifier, since this
 will result in a quicker equalization process that utilizes a smaller
 transmission power dynamic range compared to the transmission power adjust
 equalization method by itself.
 With use of a preferred embodiment of the present invention, the
 equalization of the OSNR values for the channels can be done more quickly
 and at a potentially lower cost. To illustrate these advantages, a further
 experiment was carried out to compare the equalization method using only
 transmitter power adjustments (Method A) and the equalization method of
 the preferred embodiments using both transmitter power adjustments and
 in-line attenuation adjustments (Method B). The experimental communication
 system utilized was identical to that described previously with 8.lambda.
 for the blue band.
 Although Method B was simply an experimental method in which the power
 adjustments and in-line attenuation adjustments were carried out by a
 human operator, it is contemplated that the method could be carried out
 automatically without human intervention. As indicated hereinabove,
 tunable attenuators would be controlled by a remote controller. Also, the
 same remote controller could be used to control the transmission power of
 the input signals. As would be readily understood by a person skilled in
 the art, the control feedback loop would take the form illustrated in
 chain-dotted lines in FIG. 3. Specifically, power detectors 680 located at
 the outputs of blue band demaltiplexer 308 are connected to a controller
 682 which in turn is connected to the tunable attenuator 348 and to power
 adjustors 684 on the inputs of the blue band multiplexer 302. A similar
 control arrangement (not shown) would be connected on the red band side.
 Summaries of the results can be seen in Table 1 below as well as in FIG. 5
 described in detail herein below.
 TABLE 1
 Adjust. Maximum Tx OSNR OSNR
 Time Power Adjust. Minimum Average .DELTA.OSNR
 Method (min) (dB) (dB) (dB) (dB)
 A 12 -9.1 25.5 25.9 0.6
 B 6 -4.9 26.2 26.4 0.6
 From the experiment, it was found that the equalization method of Method A
 required 3 iterations of transmission power adjustments. Each iteration
 takes approximately 30 seconds for each channel's transmission power
 adjustment. Hence, it can be seen that the equalization process would take
 (3*number of channels*time for one iteration) approximately 12 minutes as
 depicted in Table 1. In the case that Method B, which is consistent with
 the preferred embodiments of the present invention, is utilized, the
 equalization was found to require a single transmission power adjustment
 iteration and a single adjustment of the attenuation within the in-line
 repeaters. Each transmission power and attenuation adjustment takes
 approximately 30 seconds. Hence, since there are three in-line repeaters,
 the equalization process would take ((1*number of channels+1*number of
 in-line repeaters)*time for one iteration) approximately 6 minutes as
 depicted within Table 1. This reduced time between equalization methods
 becomes more significant as the communication system increases in the
 number of channels. Hence, in the future, the implementation of
 attenuation adjustment according to preferred embodiments of the present
 invention will allow for significantly reduced equalization times.
 The implementation of the preferred embodiments of the present invention
 can also reduce costs in a WDM communication system due to the reduced
 dynamic range required to adjust the transmission power for each of the
 WDM channels. This can be seen in Table 1 for the experiment performed as
 the maximum transmission power adjustment is reduced from 9.1 dB with the
 well-known Method A to 4.9 dB with the Method B of the preferred
 embodiments. This is further illustrated in detail with reference to FIG.
 5.
 FIG. 5 is a graphical illustration that compares the Methods A and B in
 terms of the OSNR and total transmission power adjustment required at each
 wavelength of the 8.lambda. blue band. Within FIG. 5, there is a
 horizontal axis 502 that represents the range of wavelengths, in units of
 nm, corresponding to the channels within the WDM communication system, a
 first vertical axis 504 that represents the OSNR, in units of dB, at the
 corresponding wavelengths, and a second vertical axis 506 that represents
 the total transmission power adjustment, in units of dB, at the
 corresponding wavelengths. Also depicted on FIG. 5 are lines 508,510 which
 represent the total transmission power adjustment required at particular
 wavelengths for Method A and B respectively and lines 512,514 which
 represent the OSNR at particular wavelengths for Method A and B
 respectively.
 As can be seen from the lines 508,510, Method B, which is the equalization
 method according to the preferred embodiments, requires significantly less
 transmission power adjustment in terms of dB for each channel than does
 Method A which has previously been implemented. Hence, unlike within the
 well known equalization setups, the transmitter does not require complex
 and expensive modifications to adjust for large ranges of OSNRs
 corresponding to the WDM channels at the receiver. At the same time, the
 lines 512,514 show that the resulting OSNR for the individual channels
 using the two methods are close, but Method B of the preferred embodiments
 slightly improves the OSNRs for each wavelength in this experimental
 system.
 Although a preferred embodiment of the present invention is described with
 reference to FIG. 3 in which a bidirectional optical fiber communication
 system is implemented with use of amplifier modules 306,312,332,334, the
 scope of the present invention should not be limited to such an
 implementation. There are numerous alternative embodiments that can be
 contemplated in which an attenuator is used to aid in the equalization of
 WDM channels within an optical fiber communication system that utilizes
 non-uniform optical amplifiers.
 For instance, the unidirectional communication system depicted in FIG. 1
 could be expanded into an embodiment of the present invention as shown in
 FIG. 6. In this system, similar to FIG. 1, the multiplexer 106 is coupled
 to the demultiplexer 108 via the optical fibers 102,104 and the post/pre
 amplifiers 110,112. However, instead of the in-line repeater simply
 comprising a single EDFA amplifier 110, the in-line repeater 602 depicted
 in FIG. 6 comprises first and second stage EDFA amplifiers 604,606 and an
 in-line attenuator 608 coupled between the amplifiers 604,606. The
 operation of the optical fiber communication system of FIG. 6 can be seen
 to have some of the same benefits to that of the preferred embodiment
 depicted in FIG. 3.
 Further alternative embodiments of the present invention are contemplated
 that allow for a plurality of attenuators to be implemented such that the
 signal power of each individual WDM channel can be adjusted independently.
 Preferably, these alternative embodiments replace the attenuators
 348,354,608 depicted within FIGS. 3 and 6 with corresponding attenuation
 apparatuses. FIG. 7 illustrates an attenuation apparatus 700 which could
 replace each of the attenuators 348, 354 and 608. Each attenuation
 apparatus 700, according to these preferable alternatives, comprises a
 demultiplexer 701 that separates the individual WDM channels; a plurality
 of attenuators 702, each attenuating a corresponding WDM channel; and a
 multiplexer 703 that combines the attenuated WDM channels. This
 alternative embodiment can allow for the signal power input to an EDFA
 amplifier for each WDM channel to be independently adjusted relative to
 the amplifier's pump power. This modification can further reduce the
 difference in the channels' gains caused by the non-uniform nature of the
 EDFA amplifiers.
 Persons skilled in the art will appreciate that there are yet more
 alternative implementations and modifications possible to implement an
 attenuator within an optical fiber communication system, and that the
 above implementation is only an illustration of this embodiment of the
 invention. The scope of the invention, therefore, is only to be limited by
 the claims appended hereto.