Abstract:
A method, apparatus and system for controlling power transients in a Raman-amplified optical transmission system includes, in response to the detection of a power transient in an optical signal, varying the gain of at least one dispersion compensating module (DCM) in the Raman-amplified optical transmission system to correct for a change in signal power caused by the power transient.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of optical communication systems and, more specifically, to Raman-amplified optical transmission systems with transient control capabilities. 
     BACKGROUND OF THE INVENTION 
     In optical networks, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifier spans are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometer fiber-optic links. A typical amplifier span may include erbium-doped fiber amplifier components that are pumped with diode lasers. Amplifiers have also been studied that use diode-laser pumping to generate gain through stimulated Raman scattering. Optical amplifiers based on erbium-doped fibers and Raman pumping increase the strength of the optical signals being transmitted over the fiber-optic links. 
     Sometimes channels in a communications link may be abruptly added or dropped. Channels may be dropped due to an accidental fiber cut. Channels may also be added or dropped suddenly due to a network reconfiguration. When the number of channels carried by a transmission fiber span changes abruptly, the total signal power being transported over the span also changes suddenly. If a Raman amplifier span is pumped at a constant power, these sudden changes in signal power will result in transient effects in the gain of the Raman amplifier. Gain transients cause fluctuations in the power of the output signals from the amplifier. Output signals that are too weak may be difficult to detect without errors. Output signals that are too strong may give rise to nonlinear optical effects in fiber. 
     SUMMARY OF THE INVENTION 
     The invention comprises a method, apparatus and system for correcting for the effects of power transients due to the loss or addition of a channel(s) in a Raman-amplified optical transmission system. 
     In one embodiment of the present invention, a method includes in response to the detection of a power transient in an optical signal in a Raman-amplified optical transmission system, varying the gain of at least one dispersion compensating module (DCM) in the Raman-amplified optical transmission system to correct for a change in signal power due to the detected power transient. 
     In an alternate embodiment of the present invention where a Raman-amplified optical transmission system includes a plurality of optical spans, each optical span including at least one dispersion compensating module (DCM), a method includes in response to the detection of a power transient in an optical span, the gain of a respective at least one DCM in the optical span is varied to correct for a change in signal power in the optical span where a power transient is detected. 
     In another embodiment of the present invention, an apparatus includes a memory for storing program instructions and a processor for executing the instructions to configure the apparatus to perform the step of in response to the detection of a power transient in an optical signal in a Raman-amplified optical transmission system, varying a gain of at least one dispersion compensating module (DCM) to correct for a change in signal power due to the detected power transient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a high-level block diagram of a conventional Raman-amplified optical transmission system; 
         FIG. 2  depicts a high-level block diagram of a single Raman amplifier span of the Raman-amplified optical transmission system of  FIG. 1  including an embodiment of the present invention; 
         FIG. 3  depicts an embodiment of a DCM pump laser controller suitable for use in the single Raman amplifier span of  FIG. 2 . 
         FIG. 4  graphically depicts an exemplary function of a delay associated with a gain change in an amplification fiber resulting from the adjustment of the power of a Raman pump laser; and 
         FIG. 5  depicts a flow diagram of an embodiment of a method of the present invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention advantageously provides a method and system for controlling the negative effects of power transients in a Raman-amplified optical transmission system. Although an embodiment of the present invention will be described within the context of a Raman-amplified optical transmission system comprising specific components, it will be appreciated by those skilled in the art that the method of the present invention can be advantageously implemented in various other Raman-amplified optical transmission systems wherein it is desirable to control the negative effects of power transients. 
       FIG. 1  depicts a high-level block diagram of a conventional Raman-amplified optical transmission system. The Raman-amplified optical transmission system  100  of  FIG. 1  comprises a transmitter  110 , a receiver  120 , and a plurality of Raman amplifier spans  130   1 – 130   N  (collectively Raman amplifier spans  130 ). Each of the Raman amplifier spans  130  comprises a respective amplification fiber span  140   1 – 140   N  (collectively fiber spans  140 , illustratively standard transmission fiber spans) and a respective pump  150   1 – 150   N  (collectively pumps  150 , illustratively Raman pumps). Each Raman pump  150  may be either a single pump or a plurality of pumps with varied wavelengths acting collectively. The transmitter  110  transmits information to the receiver  120  over the series of Raman amplifier spans  130 . Pump light from each of the Raman pumps  150  is transmitted in the backwards direction to pump its respective fiber span  140 . Signals in the fiber spans  140  are therefore amplified by Raman gain. Although the amplification fiber spans  140  in the Raman-amplified optical transmission system  100  of  FIG. 1  are depicted as comprising standard transmission fibers, it will be appreciated by those skilled in the art that the amplification fiber spans  140  can comprise other amplification mediums such as erbium-doped fiber amplifiers, and the like. 
     The transmitter  110  may include laser diodes that each supports a channel operating at a different wavelength. As such, if one or more of these lasers is taken out of service or if new channels are added at the transmitter  110 , the total number of wavelengths being transmitted across the Raman-amplified optical transmission system  100  may change abruptly. The total number of channels in an optical transmission system may also change due to unexpected system failures such as fiber cuts, or when channels are added or dropped during system reconfigurations using add/drop terminals. 
     When the total number of channels in the Raman-amplified optical transmission system  100  changes abruptly but the powers of the Raman pumps  150  remains the same, the Raman gain in the fiber spans  140  will also change. Abrupt power changes of this sort may cause unacceptable transients in the power of individual signal channels at the output of each Raman amplifier span  130 . For example, if wavelengths (channels) are lost, the input power suddenly decreases because channels have been dropped. If the power of the Raman pumps  150  remains the same, there will be excess gain in each of the pumped fiber spans  140  and the output power per channel at the end of each of the Raman-amplifier spans  130  increases more than desired. 
     Conversely, when the input power suddenly increases due to the addition of new channels, the Raman pump is depleted more rapidly, which causes the output power per channel at the end of the pumped transmission fiber to decrease more than desired. Because these errors accumulate from Raman amplifier span to Raman amplifier span, the total error of the power for each channel can be calculated by equation (1), which follows: 
               E   t     =       ∑     i   =   1     N     ⁢           ⁢     E   i               (   1   )             
 
wherein E i  represents the error in each amplification span, N stands for the total number of amplification spans, and E t  is the total accumulated error for all of the amplification spans combined.
 
       FIG. 2  depicts a high-level block diagram of a single Raman amplifier span  130  of the Raman-amplified optical transmission system  100  of  FIG. 1 , including an embodiment of the present invention. The Raman amplifier span  130  of  FIG. 2  comprises an amplification fiber (illustratively a 100 km outside plant fiber (corresponding to the amplification fiber span  140  of  FIG. 1 ))  210 , a transmission fiber  215 , a pump (illustratively an outside plant pump laser)  220 , a pump controller (illustratively an outside plant pump laser controller)  225 , a dispersion compensating module (DCM)  230 , a DCM pump laser  235 , three taps  240   1 ,  240   2 ,  240   3  (collectively taps  240 ), a monitor  250 , and a DCM pump laser controller  260 . In the illustrative embodiment of the present invention in  FIG. 2 , the tap  240   1 , although depicted as being located within the Raman amplifier span  130 , is actually located at the end of a directly previous Raman amplifier span and is equivalent to the tap  240   3  in the illustrated Raman amplifier span  130  of  FIG. 2 . As such, it should be understood that the tap  240   1  is being depicted in  FIG. 2  for ease of understanding. In the Raman amplifier span  130 , the tap  240   1  is positioned on a transmission fiber prior to the outside plant fiber  210  (i.e., in a previous span). The tap  240   2  is located after the outside plant fiber  210  and before the DCM  230 . The tap  240   3  is located after the DMC  230 . 
       FIG. 3  depicts an embodiment of a DCM pump laser controller  260  suitable for use in the single Raman amplifier span  130  of  FIG. 2 . The DCM pump laser controller  260  of  FIG. 3  comprises a processor  310  as well as a memory  320  for storing the algorithms and control programs. The processor  310  cooperates with conventional support circuitry  330  such as power supplies, clock circuits, cache memory and the like as well as circuits that assist in executing the software routines stored in the memory  320 . As such, it is contemplated that some of the process steps discussed herein as software processes may be implemented within hardware, for example, as circuitry that cooperates with the processor  310  to perform various steps. DCM pump laser controller  260  also contains input-output circuitry  340  that forms an interface between the various functional elements communicating with the DCM pump laser controller  260 . For example, in the embodiment of  FIG. 2 , the DCM pump laser controller  260  communicates with the monitor  250  via a signal path S 1  and to DCM pump laser  235  via signal path O 1 . 
     Although the DCM pump laser controller  260  of  FIG. 3  is depicted as a general purpose computer that is programmed to perform various control functions in accordance with the present invention, the invention can be implemented in hardware, for example, as an application specified integrated circuit (ASIC). As such, the process steps described herein are intended to be broadly interpreted as being equivalently performed by software, firmware, hardware, or a combination thereof. 
     Referring back to  FIG. 2 , a signal entering the Raman amplifier span  130  was tapped (i.e., in a previous span) by the tap  240   1 . The signal from the tap  240   1  is communicated to the monitor  250 , wherein the power of the incoming signal is measured. The power measurement of the incoming signal is then fed-forward to and recorded in the DCM pump laser controller  260 . The signal then propagates through the outside plant fiber  210  wherein the signal is amplified by the outside plant pump laser  220 . The signal from the outside plant fiber  210  is tapped by the tap  240   2 . The signal from the tap  240   2  is communicated to the monitor  250 , wherein the power of the signal is measured. The power measurement of the signal is then fed-forward to and recorded in the DCM pump laser controller  260 . 
     The power measurement of the tap  240   1  (located in the previous span and communicated to this span by conventional means) is compared to the power measurement of the tap  240   2  in the DCM pump laser controller  260  to determine the gain experienced by the signal in the outside plant fiber  210 . The determined gain is compared to an expected amplification gain for the outside plant fiber  210  stored within the DCM pump laser controller  260  to determine a change in the gain of the outside plant fiber  210  (if any) caused by a loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 . 
     Optionally, the gain change of the outside plant fiber  210  can be determined by measuring the power of the signal entering the DCM  230  (i.e., via the tap  240   2 ) and comparing the power of the measured input signal to an expected power for a signal entering the DCM  230  stored in the DCM pump laser controller  260 . The DCM pump laser controller  260  can then estimate the gain that the input signal would experience in the outside plant  210 . The estimated gain is then compared to an expected amplification gain for the outside plant fiber  210  stored within the DCM pump laser controller  260  to determine a change in the gain of the outside plant fiber  210  (if any) caused by a loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 . 
     The gain in the outside plant fiber  210  is determined periodically to check for a gain change. It will be appreciated by those skilled in the art that the time interval for checking for a gain change in the outside plant fiber  210  is system dependent and can be set to any interval desired by a user, within system capabilities. 
     The signal from the outside plant fiber  210  then propagates through the DCM  230 , where it is amplified by the DCM pump laser  260 . Any gain change in the DCM  230  caused by the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100  can be estimated by measuring the power of the propagating signal entering the DCM  130  (i.e., via the tap  240   2 ) and comparing the power of the measured propagating signal to an expected power for an input signal to the DCM  230  stored in the DCM pump laser controller  260 . Optionally, the gain of the DCM  230  can be calculated by comparing the power measurement of the tap  240   2  to the power measurement of the tap  240   3  as described below. 
     After propagating through the DCM  230 , the output signal propagates through the transmission fiber  215  and is tapped by the tap  240   3 . The signal from the tap  240   3  is communicated to the monitor  250  wherein the power of the output signal is measured. The power measurement of the output signal is then fed-forward to and recorded in the DCM pump laser controller  260 . The power measurement of the tap  240   2  is compared to the power measurement of the tap  240   3  to determine the gain experienced by the signal in the DCM  230 . The determined amplification gain is compared to an expected amplification gain for the DCM  230  stored within the DCM pump laser controller  260  to calculate a gain change (if any) of the DCM  230  caused by the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 . Additionally, the signal from the tap  240   3  can be used to measure the delay in the DCM  230  for other system implementations that require information regarding the delay of the DCM  230 . The transmission fiber  215  is primarily used to couple the output signal from the DCM  230  to an amplification fiber (outside plant fiber) in a next Raman amplifier span. 
     In accordance with the present invention, the desired result in each Raman amplifier span of the Raman-amplified optical transmission system  100  is that the power of an input signal coming into the span is equal to the power of the output signal exiting the span. That is, the gains and the losses of the particular Raman amplifier span are balanced. The desired result is depicted, implementing the power measurements of the taps  240 , in equation (2), which follows: 
                 P   ⁡     (       λ   i     ,     tap   ⁢           ⁢     240   3         )         P   ⁡     (       λ   i     ,     tap   ⁢           ⁢     240   1         )         =   1.           (   2   )             
 
     Characterizing the above equation as two separate gain components implementing the power measurements of all three of the taps  240  in the Raman amplifier span  130  of  FIG. 2 , equation (3) and equation (4) are written as follows: 
                 P   ⁡     (       λ   i     ,     tap   ⁢           ⁢     240   2         )         P   ⁡     (       λ   i     ,     tap   ⁢           ⁢     240   1         )         =     g     1   ⁢   i               (   3   )                   P   ⁡     (       λ   i     ,     tap   ⁢           ⁢     240   3         )         P   ⁡     (       λ   i     ,     tap   ⁢           ⁢     240   2         )         =     g     2   ⁢   i               (   4   )             
 
wherein g 1i  represents the gains or losses in the outside plant fiber  210  and g 2i  represents the gains or losses in the DCM  230 . As such, the product of g 1i  and g 2i  should equal one (g 1i *g 2i =1) for all wavelengths.
 
     In one embodiment of the present invention, after a change in the gain of the outside plant fiber  210  is detected, an adjustment to the pump power of the outside plant pump laser  220  is made to alter the gain of the outside plant fiber  210  to correct for a gain change in the outside plant fiber  210  due to the loss or addition of channels (transient event) in the Raman-amplified optical transmission system  100 . Additionally, an adjustment to the pump power of the DCM pump laser  235  is made to alter the gain in the DCM  230  to correct for a time delay associated with adjusting the pump power of the outside plant pump laser  220  to alter the gain of the outside plant fiber  210  and to correct for a gain change in the DCM  230  due to the loss or addition of channels (transient event) in the Raman-amplified optical transmission system  100 . The delay associated with adjusting the power of the outside plant pump laser  220  to alter the gain in the outside plant fiber  210  is attributed to the amount of time that it takes the photons traveling from the outside plant pump laser  220  to propagate through the outside plant fiber  210  before achieving the desired (altered) gain. 
       FIG. 4  graphically depicts an exemplary function of a time delay associated with a gain change in the outside plant fiber  210  resulting from the adjustment of the power of the outside plant pump laser  220 . In  FIG. 4 , t o  depicts the point in time that the outside plant pump laser  220  was adjusted; t 1  depicts the point in time that the desired (corrected) gain in the outside plant fiber  210  is achieved; and Δt depicts the amount of time between the adjustment of the outside plant pump laser  220  and when the desired gain is achieved in the outside plant fiber  210 . A delay time Δt is dependent upon the effective length of an amplification fiber and can be calculated from equation (5), which follows: 
               Δ   ⁢           ⁢   t     =         L   eff     ×     n   ⁡     (     λ   ⁢           ⁢   p     )         c             (   5   )               
wherein n(λp) is the refractive index of the fiber at the corresponding Raman pump wavelength, c is the speed of light in a vacuum, and L eff  is the effective length of the amplification fiber. Equation (5) above is merely a rearrangement of the Rate×Time=Distance formula.
 
     In addition, the effective length L eff  of a fiber can be calculated using equation (6), which follows: 
               L   eff     =         1   α     ⁡     [     1   -     exp   ⁡     (       -   α     ⁢           ⁢   L     )         ]       .             (   6   )             
 
In equation (6) above, α represents the attenuation of the fiber and L represents the actual length of the fiber.
 
     In the single Raman amplifier span  130  of  FIG. 2 , a typical communications grade fiber was used (i.e., a SMF, TrueWave, LEAF fiber) as the outside plant fiber  210 . The length of the outside plant fiber  210  is typically 100 km and the attenuation for such typical fibers is approximately 0.21 dB/km. Inputting these values for the attenuation and the actual length, respectively, in the equation (6) above, the effective length L eff  of the outside plant fiber  210  is calculated as 20 km. Inputting this value for the effective length L eff  in the equation (5) above, the delay time Δt op  for the outside plant fiber  210  is calculated as 10 −4  seconds or 100 μs. The delay time Δt op  is calculated by and recorded in the DCM pump laser controller  260 . 
     As such, the gain change in the outside plant fiber  210  due to an adjustment of the pump power of the outside plant pump laser  220  must be considered as a function of time. This change in gain as a function of time (shape from t 0  to t 1  in  FIG. 4 ) is calculable (as described above) from the determined delay time Δt op  associated with the outside plant fiber  210  and the determined gain change (described above) in the outside plant fiber  210 . This function (shape from t 0  to t 1 ) is considered by the inventors as f 1 (t). The function f 1 (t) is determined by and recorded in the DCM pump laser controller  260 . That is, the parameters for the outside plant, fiber  210 , such as the values for the actual length and attenuation of the outside plant fiber,  210 , are stored in the DCM pump laser controller  260 . Utilizing equations (5) and (6) above, the DCM pump laser controller  260  then calculates the delay time Δt op  for the outside plant fiber  210 . The DCM pump laser controller  260  then awaits for information from the tap  240   2 , to determine the amount of gain change in the outside plant fiber  210 , to calculate the function f 1 (t). The value of the delay function f 1 (t) is stored in the DCM pump laser controller  260 . 
     As described above, an adjustment to the pump power of the DCM pump laser  235  is made to change the gain in the DCM  230  to compensate for f 1 (t) and to correct for a gain change in the DCM  230  due to the loss or addition of a channel(s) (transient event) in the Raman-amplified optical transmission system  100 . As previously disclosed, the gain change in the DCM  230  can be estimated by measuring the power of the propagating signal entering the DCM  130  (i.e., via the tap  240   2 ) and comparing the power of the measured propagating signal to an expected power for an input signal to the DCM  230  stored in the DCM pump laser controller  260 . This gain change in the DCM  230 , due to the loss or addition of channels in the Raman-amplified optical transmission system  100 , is considered by the inventors as Δg 2DCM . 
     As such, the DCM pump laser controller  260  needs to calculate a function f 3 (t) to correct for f 1 (t) and Δg 2DCM , such that f 3 (t) is utilized by the DCM pump laser controller  260  to adjust the power of the DCM pump laser  235  to alter the gain in the DCM  230 . Similar to the case of the outside plant fiber  210  above, though, adjusting the pump power of the DCM pump laser  235  does not instantaneously change the gain in the DCM  230  to the desired gain. As such, a delay time associated with adjusting the power of the DCM pump laser  235  to alter the gain of the DCM  230  to correct for the gain change Δg 2DCM  in the DCM  230  must also be considered. 
     A DCM typically has a much shorter fiber length than an amplification fiber in a transmission system. For example, the DCM  230  of  FIG. 2  has a much shorter overall actual length than the outside plant fiber  210 . The length of the DCM  230  used in the in the Raman amplifier span  130  of  FIG. 2  is, in this case 10 km, but varies with the length of the outside plant fiber  210 . As such, a delay associated with changing the pump power of the DCM pump laser  235  to compensate for a change in gain in the DCM  230  due to the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100  will be significantly shorter. For example, inputting the actual length L of the DCM  130  into the equation (6) above, the effective length L eff  of the DCM  130  is calculated to be 3 km. To calculate for a delay time Δt DCM  associated with adjusting the pump power of the DCM pump laser  235  to change the gain in the DCM  230 , the determined effective length L eff  is input into the equation (5) above. Inputting 3 km for the effective length L eff  of the DCM  230  in the equation (5) above, the delay time Δt DCM  associated with adjusting the pump power of the DCM pump laser  235  to change the gain in the DCM  230  is calculated to be 15 μs. Knowing the delay time Δt DCM  and the amount of gain change Δg 2DCM  in the DCM  230 , a delay function is calculated to represent the delay time associated with adjusting the pump power of the DCM pump laser  235  to change the gain in the DCM  230 . This function is considered by the inventors as f 2 (t). The value of f 2 (t) is stored in the DCM pump laser controller  260 . 
     Because f 1 (t), Δg 2DCM  and f 2 (t) can be modeled based on measurements of a propagating signal taken at the tap  240   1  (from the previous span) and the tap  240   2  and because the signal from the tap  240   2  is fed-forward to the DCM pump laser controller  260 , f 1 (t), Δg 2DCM  and f 2 (t) are determined so the correction can be performed at the DCM  230 . That is, there is time for the DCM pump laser controller  260  to determine a function f 3 (t) to correct for f 1 (t), Δg 2DCM  and f 2 (t) because the information from the tap  240   2  is fed-forward to the DCM pump laser controller  260 . 
     Recalling that the product of g 1i  and g 2i  should equal one (g 1i *g 2i =1) for all wavelengths, the function f 3 (t) is calculated by the DCM pump laser controller  260  using equation (7), as follows:
 
[ g   1i   +f   1 ( t )][ g   2i   +Δg   2DCM   +f   2 ( t )+ f   3 ( t )]=1  (7)
 
where g 1i  and g 2i  are the original gains of the outside plant fiber  210  and the DCM  230 , respectively (before any transient event), f 1 (t) is the delay function associated with an adjustment of the power of the outside plant pump laser  220  to alter the gain of the outside plant fiber  210 , Δg 2DCM  is the gain change in the DCM  230  due to the loss or addition of a channel(s) (transient event) in the Raman-amplified optical transmission system  100 , f 2 (t) is the delay function associated with an adjustment of the power of the DCM pump laser  235  to alter the gain of the DCM  230 , and f 3 (t) is a function to be calculated by the DCM pump laser controller  260  to adjust the power of the DCM pump laser  235  to adjust the gain in the DCM  230  to correct for the functions f 1 (t) and f 2 (t), and to compensate for the gain change Δg 2DCM  in the DCM  230  due to the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 . Solving for f 3 (t): 
                 f   3     ⁡     (   t   )       =       1       g     1   ⁢   i       +       f   1     ⁡     (   t   )           -     g     2   ⁢   i       -     Δ   ⁢           ⁢     g     2   ⁢   DCM         -         f   2     ⁡     (   t   )       .               (   8   )             
 
     The function f 3 (t) is calculated by the DCM pump laser controller  260  to adjust the pump power of the DCM pump laser  235  to alter the gain in the DCM  230  to compensate for f 1 (t) and f 2 (t), and to compensate for the gain change Δg 2DCM  in the DCM  230  due to a loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 . 
     Although the single Raman amplifier span  130  of the Raman-amplified optical transmission system  100  of  FIG. 1  was depicted as comprising a single amplification fiber, it will be appreciated by those skilled in the art that the methods of the present invention can be implemented in Raman-amplified optical transmission systems comprising amplification spans comprising a plurality of amplification fibers and other amplification mediums in a single span. 
     In an alternate embodiment of the present invention, a gain change in the outside plant fiber  210  and in the DCM  230  due to the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100  is compensated by adjusting only the pump power of the DCM pump laser  235  to alter the gain in the DCM  230 . 
     As described above for the first embodiment, because adjusting the pump power of the DCM pump laser  235  does not instantaneously change the gain in the DCM  230  to a desired gain, the change in gain in the DCM  230  due to an adjustment of the pump power of the DCM pump laser  235  must again be considered as a function of time. Referring to  FIG. 2 , because the properties of the DCM  230  do not change, the delay time Δt DCM  associated with the adjustment of the DCM pump laser  235  to alter the gain in the DCM  230  remains the same (15 μs). Again, a delay associated with adjusting the pump power of the DCM pump laser  235  to alter the gain in the DCM  230  is attributed to the amount of time that it takes the photons traveling from the DCM pump laser  235  to propagate through the DCM  230  before achieving the desired (corrected) gain. Knowing the amount of gain change desired in the DCM  230  to correct for a gain change in the outside plant fiber  210  and the DCM  230  due to a loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 , a delay function f 4 (t) can be calculated to account for the delay time Δt DCM  associated with the adjustment of the DCM pump laser  235  to alter the gain in the DCM  230  to correct for a gain change in the outside plant fiber  210  and the DCM  230 . Because the signals from the tap  240   2  is fed-forward to the DCM pump laser controller  260 , the amount of gain change required in the DCM  230  to correct for a gain change in the outside plant fiber  210  and the DCM  230  due to a loss or addition of a channel(s) in the Raman-amplified optical transmission system  100  is known by the DCM  230 . As such, the delay function f 4 (t) can be modeled based on the measurements of a propagating signal taken at the taps  240   1  and  240   2  and calculated by the DCM laser controller  260  before the propagating signal reaches the DCM  230 . 
     In addition to correcting for f 4 (t), the DCM  230  must also correct for a gain change Δg 2OP  in the outside plant fiber  210  and a gain change Δg 2DCM  in the DCM  230  due to a loss or addition of a channel(s) in the Raman-amplified optical transmission system  100 . As such, a function f 5 (t) is calculated by the DCM pump laser controller  260  to correct for the delay function f 4 (t) and to correct for both, the gain change Δg 2OP  in the outside plant fiber  210  and the gain change Δg 2DCM  in the DCM  230  due to the loss or addition of channels in the Raman-amplified optical transmission system  100 . Because f 5 (t) can be modeled based on measurements of a propagating signal taken at the taps  240   1  (from the previous span) and  240   2  and because the signals from the tap  240   2  is fed-forward to the DCM pump laser controller  260 , the correction function f 5 (t) is determined so a correction can be performed at the DCM  230 . Recalling that the product of g 1i  and g 2i  should equal one (g 1i *g 2 =1) for all wavelengths, the function f 5 (t) is determined by the DCM pump laser controller  260  as follows:
 
[ g   1i   +Δg   2OP   []g   2i   +Δg   2DCM   +f   4 ( t )+ f   5 ( t )]=1  (9)
 
where g 1i  and g 2i  are the original gains of the outside plant fiber  210  and the DCM  230 , respectively, Δg 2OP  and Δg 2DCM  are the gain changes in the outside plant fiber  210  and the DCM  230 , respectively, due to the loss or addition of a channel(s) (transient event) in the Raman-amplified optical transmission system  100 , f 4 (t) is the delay function associated with an adjustment of the power of the DCM pump laser  235  to alter the gain of the DCM  230 , and f 5 (t) is a function to be calculated and utilized by the DCM pump laser controller  260  to adjust the power of the DCM pump laser  235  to alter the gain in the DCM  230  to correct for the gain changes in the outside plant fiber  210  and the DCM  230  caused by the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100  while accounting for the delay function f 4 (t) associated with an adjustment of the power of the DCM pump laser  235  to alter the gain of the DCM  230 . Solving for f 5 (t): 
                 f   5     ⁡     (   t   )       =       1       g     1   ⁢   i       +     Δ   ⁢           ⁢     g     2   ⁢   OP             -     g     2   ⁢   i       -     Δ   ⁢           ⁢     g     2   ⁢   DCM         -         f   4     ⁡     (   t   )       .               (   10   )             
 
     Again, the function f 5 (t) is calculated and utilized by the DCM pump laser controller  260  to adjust the pump power of the DCM pump laser  235  to alter the gain in the DCM  230  to correct for the gain changes in the outside plant fiber  210  and the DCM  230  caused by the loss or addition of a channel(s) in the Raman-amplified optical transmission system  100  while accounting for the delay function f 4 (t) associated with an adjustment of the power of the DCM pump laser  235  to alter the gain of the DCM  230 . 
       FIG. 5  depicts a flow diagram of an embodiment of a method  500  of the present invention. The method  500  is entered at step  502  when a gain change is detected in an amplifier of an amplification span of a Raman-amplified optical transmission system. For example, the power of a propagating signal in an amplification span is measured after an outside plant fiber by a monitor. Information of the measured signal powers of the propagating signal is communicated to a DCM pump laser controller. The DCM pump laser controller calculates the gain in the outside plant fiber and compares the calculated gain to a stored expected gain for the outside plant fiber to determine a gain change in the outside plant fiber. The method  500  then proceeds to step  504 . 
     At step  504 , the method  500  calculates a gain change in a DCM. For example, the gain change in the DCM  230  can be estimated by measuring the power of the propagating signal entering the DCM  130  (i.e., via the tap  240   2 ) and comparing the power of the measured propagating signal to an expected power for an input signal to the DCM  230  stored in the DCM pump laser controller  260 . The method  500  then proceeds to step  506 . 
     At step  506 , the method  500  calculates a function to control the pump power of a DCM pump laser to compensate for the detected gain changes of step  502  and step  504 . For example, the DCM pump laser controller may use the measured signal power information to calculate the appropriate pump power for the DCM pump laser as described in connection with equations 1–10. The DCM pump laser controller may use feed-forward control techniques, feedback control techniques, hybrid control techniques, or any other suitable control techniques to calculate the pump power for the DCM pump laser. 
     At step  508 , the method  500  controls the DCM pump according to the function calculated at step  506 . For example, the DCM pump laser controller adjusts the power of the DCM pump laser to the values calculated in step  506 . The method  500  is then exited. 
     While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.