Abstract:
A change in loading conditions of fiber amplifiers in an optical communications network causes rapid variations in the gain profile of the amplifiers due to spectral hole burning and stimulated Raman scattering. An apparatus for reducing such gain profile variations is described which monitors optical signal perturbations and reacts by adjusting pump powers of the amplifiers and, or fast variable optical attenuator according to a pre-determined function stored in the form of constants in controller&#39;s memory. The optical signal is monitored as total power, and the power of light after passing through one or more optical filters. The light detection is relatively fast, whereby the gain profile variations are compensated by fast controlled variable optical attenuator and pump power adjustment upon the change in loading conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. patent application Ser. No. 12/235,041, filed Sep. 22, 2008, and in turn claims priority to U.S. Patent Appl. No. 60/978,253, filed Oct. 8, 2007. The entireties of such patent applications are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention is related to optical fiber amplifiers, and in particular to controlling the gain profile of erbium doped fiber amplifiers (EDFA), Raman Amplifiers (RA), and hybrid EDFA-RA amplifiers. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a wavelength division multiplexing optical transmission system, various information channels are encoded into light at different wavelengths, which is combined using a multiplexor. The combined light is transmitted through an optical fiber and, or an optical fiber network to a receiver end of the optical fiber. At the receiver end, the signal is separated, or demultiplexed, back into the individual optical channels through a de-multiplexor, whereby each optical channel can be detected by an optical detector such as a photodiode, and the information can be reconstructed, channel by channel. 
         [0004]    While propagating through the optical fiber, light tends to lose power due to the losses related to the physics of how the light interacts with the optical fiber. Yet some minimal level of optical channel power is required at the receiver end in order to decode information encoded in the optical channel. In order to boost the optical signal propagating in the optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, whereby after each fiber span, the optical signal is amplified to power levels close to the original levels of the transmitter. Unfortunately, during the amplification process some amount of noise is introduced into the optical signal which effectively limits the amount of optical amplifiers a transmission link can have. 
         [0005]    Modern optical communication systems employ erbium doped fiber amplifiers (EDFAs), Raman Amplifiers (RAs) and hybrid EDFA-RAs as means to boost the optical signal power and thus to extend the communication system reach. Nowadays, optical communication systems have become more agile and reconfigurable. Reconfiguration of the optical communication system leads to variation of the signal load at the input of the amplifier. At the same time, the goal of the amplifier is to provide constant gain, which should not depend on the power or wavelength loading condition; otherwise, some channels will not have sufficient power and signal-to-noise level at the receiver end, resulting in information being lost. 
         [0006]    The control electronics of EDFAs partially solves the problem of the variable signal load. More particularly, the total optical power at the input and at the output of the amplifier is measured, and the average optical signal gain of the amplifier is calculated. The amplifier control electronic circuitry adjusts the amplifier&#39;s pump powers through a feedback loop in such a way that the measured optical gain equals to the desired or “set” optical gain and is not varied significantly in time. 
         [0007]    However, it is desired not only to have average gain of the amplifier to be constant, but also to have the gain of the individual channel constant and independent from the other channels&#39; presence or absence, that is, independent from the channel load. At the same time, due to the spectroscopy of the erbium doped fiber, namely due to the spectral hole burning (SHB) effect, the gain shape of EDFA does depend on the input load. Hence even if the average gain of an EDFA is held constant, the gain of the individual channels will vary, leading to undesirable effects, such as increased bit error rate of the transmission system. 
         [0008]    One way in which to address the problem is to check the channel powers at a location in the transmission system, using an optical channel monitor (OCM). The collected information is then used by the system control circuitry to adjust a dynamic gain equalizer (DGE) in the transmission link in such a way that the transmitted spectrum is flattened. The OCM and DGE need not necessarily be at a same location in the system. The advantage of this approach that it compensates for all gain change inducing impairments of the system, such as stimulated Raman scattering (SRS) induced tilt, not only EDFA SHB. 
         [0009]    However the above approach has several disadvantages. First, because the DGE and OCM are expensive components, they are not generally installed at each amplifier node, thus they compensate several amplifiers at once, which is not optimal. Second, both OCM and DGE are comparatively slow devices, and thus the correction usually takes a few seconds. This is undesirable for agile communication systems where a typical requirement for the adjustment for a transient event such as a change of the channel load is on the order of 100 μs, which is 10,000 times shorter than for a DGE/OCM approach. 
         [0010]    To address the disadvantage of this compensating technique it has been suggested by Zhou et al. in an article entitled “Fast control of inter-channel SRS and residual EDFA transients using a multiple-wavelength forward-pumped discrete Raman amplifier”, OMN4, OFC 2007, which is incorporated herein by reference, to measure channel powers of a limited number of channels that are located at specific wavelengths, 1528.6 nm, 1544.4 nm, and 1559.6 nm in the published example. Subsequently, the Raman pump powers of the Raman amplifier are adjusted using linear feed-forward control. The work is based on RAs having 3 different wavelengths of Raman pumps. Again, similar to the aforementioned DGE/OCM approach, this compensates not only EDFA SHB, but SRS tilt as well. 
         [0011]    The main disadvantage of this technique is the requirement of the constant presence of those three channels the power of which is constantly monitored. This is a very limiting requirement for modern agile communication systems. Another potential disadvantage is the requirement to have three additional detectors. Finally, relatively good SHB compensation is possible only in the presence of three Raman pumps—the reduction of number of pumps will lead to the reduction of the amount of compensation. 
         [0012]    Further, in U.S. Pat. No. 7,359,112 by Nishihara et al. which is incorporated herein by reference, a control apparatus is described which adjusts the gain of an EDFA based on an amount of wavelengths which is calculated on the basis of optical power measured in two or three separate spectral bands by dedicated photodetectors. One disadvantage of this approach is that only one control parameter, specifically the EDFA gain, is adjusted which limits the degree to which both the SHB and SRS can be compensated. Another disadvantage stems from the fact that certain load change patterns, for example the patterns which leave the total optical power measured in a single spectral band unchanged, will not be detected by the apparatus of Nishihara et al. and therefore will not be compensated for by said apparatus. 
         [0013]    It is an object of the present invention to provide an apparatus and method for controlling a gain profile of an optical amplifier suitable for suppression of sub-millisecond scale transient variations of gain caused by changes in the amplifier load which would not require dedicated spectral channels in order to monitor the gain profile. In this context, “controlling” means stabilizing the gain profile of an optical amplifier at varying load conditions. This invention extends the technique that was suggested by Bolshtyansky et al. in an article entitled “Dynamic Compensation of Raman Tilt in a Fiber Link by EDFA during Transient Events”, JThA15, OFC 2007, where instead of measuring the actual gain change, the device measures some property of the transmitted signal, and adjusts the gain profile based on the measured property of the signal. 
       SUMMARY OF THE INVENTION 
       [0014]    The apparatus of the present invention branches off a small portion of a transmitted optical signal, splits this portion into a plurality of sub-portions, passes the sub-portions through a set of characteristic optical filters, and measures the resulting optical powers. Based on the measurements, the apparatus adjusts the pump power of an erbium doped fiber amplifier (EDFA) and, or the pump power(s) of a Raman amplifier (RA), and, or the attenuation setting of a fast variable optical attenuator, according to a pre-defined set of response functions chosen to control a gain profile of an optical amplifier, so as to lessen power variation of an amplified optical signal at varying amplifier load conditions. 
         [0015]    Thus, in accordance with the invention there is provided an apparatus for controlling a gain profile G(λ) of an optical amplifier comprising an erbium doped fiber amplifier for amplifying a stream of optical signals, the apparatus comprising: 
         [0016]    a detection device arranged to receive a tapped portion of the stream of optical signals in the form of N+1 sub-portions and to provide N+1 output signals P 0  . . . P N  in dependence upon said tapped portion, wherein N is an integer positive number, the detection device comprising: N spectral filters having respective transmission functions F 1 (λ) . . . F N (λ) including at least one transmission function having two separate transmission regions; and N+1 photodetectors for producing the N+1 output signals P 0  . . . P N  in response to a light impinging thereon, wherein the first sub-portion of the tapped portion of the stream of optical signals is coupled to the first photodetector for producing the signal P 0 , and each one of remaining N of said sub-portions of the tapped portion of the stream of optical signals is coupled to one of the N spectral filters coupled to one of the remaining N photodetectors for producing the signals P i  P N ; 
         [0017]    a controller arranged to receive said signals P 0  . . . P N  from the detection device and suitably programmed to provide M control signals x 1  . . . x M  in dependence upon said signals P 0  . . . P N , wherein M is an integer positive number and x m =f m (C k , P 0  . . . P N ), wherein f m  is a pre-determined function and C k  are pre-determined constants, for each m=1 . . . M; and 
         [0018]    M spectral actuators S 1  . . . S M  arranged to receive said control signals x 1  . . . x M , respectively, and modify the gain profile G(λ) by a value ΔG(λ) according to the equation 
         [0000]    
       
         
           
             
               
                 Δ 
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                     ( 
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         [0019]    wherein A m (λ) is a fraction of said modification caused by the m th  actuator S m  upon receiving a unitary control signal by said actuator; 
         [0020]    wherein the functions f 1  . . . f M  and F 1  . . . F N  are chosen so as to stabilize the gain profile G(λ) at varying load conditions of the optical amplifier. 
         [0021]    In accordance with another aspect of the invention there is further provided a method for controlling a gain profile G(λ) of an optical amplifier comprising an erbium doped fiber amplifier for amplifying a stream of optical signals, the method comprising: 
         [0022]    splitting a tapped portion of the stream of optical signals in the form of N+1 sub-portions, wherein N is an integer positive number; 
         [0023]    measuring optical power value P 0  of the first said sub-portion; 
         [0024]    spectral filtering remaining N sub-portions through N filters having respective transmission functions F 1 (λ) . . . F N (λ) including at least one transmission function having two separate transmission regions, and measuring optical power values P 1  . . . P N  of the respective filtered sub-portions of the tapped portion; 
         [0025]    generating M control signals x 1  . . . x M  based on the formula x m =f m (C k , P 0  . . . P N ), wherein f m  is a pre-determined function and C k  are pre-determined constants, for each m=1 . . . M; 
         [0026]    applying said M control signals x 1  . . . x M  to M spectral actuators S 1  . . . S M , respectively, wherein said actuators modify the gain profile G(λ) by a value 
         [0000]    
       
         
           
             
               
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         [0000]    wherein 
         [0027]    A m (λ) is a fraction of said modification caused by the m th  actuator S m  upon receiving a unitary control signal by said actuator; 
         [0028]    wherein the functions f 1  . . . f M  and F 1  . . . F N  are chosen so as to stabilize the gain profile G(λ) at varying load conditions of the optical amplifier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Exemplary embodiments will now be described in conjunction with the drawings in which: 
           [0030]      FIG. 1  is a configuration of a prior art apparatus for transient gain deviation control in a hybrid erbium doped fiber amplifier—Raman amplifier optical amplifier; 
           [0031]      FIG. 2A-2C  are general configurations of the apparatus of the present invention for flattening a gain profile of an optical amplifier; 
           [0032]      FIG. 3  is an optical circuit of a preferred embodiment of the detection device of the present invention; 
           [0033]      FIG. 4  is an optical circuit of a preferred embodiment of the detection device for the case of only a single filter and two detectors; 
           [0034]      FIG. 5  is a calculated transmission filter function for the filter in the detection device of  FIG. 4 ; 
           [0035]      FIG. 6  is a configuration of the apparatus of the present invention showing a particular implementation of the actuators; 
           [0036]      FIG. 7  is a graph showing changes in overall amplifier gain due to Raman amplifier and erbium doped fiber amplifier pump changes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0038]    Referring to  FIG. 1 , a prior art hybrid optical amplifier  800  is shown comprising a multiplexor  802 , a first erbium doped fiber amplifier (EDFA)  808 , a span of dispersion compensated optical fiber (DCF)  810 , a second EDFA  812 , three Raman pump diode lasers  816  emitting pump light at differing pump wavelengths, a Raman pump in-coupler  818  for coupling the Raman pump to DCF  810 , and a Raman pump out-coupler  820  for removing a residual Raman pump light  822 . An incoming multi-wavelength signal  824 , carried by many optical fibers, is multiplexed by multiplexor  802  to propagate in a single optical fiber  804  coupled to amplifier  808 . After amplification by EDFAs  808  and  812  and by DCF  810  pumped with diode lasers  816 , the signal exits the amplifier as shown by an arrow  826 . 
         [0039]    In order to correct dynamic gain tilt caused by variations in amount and, or optical power of signals at individual wavelengths comprising incoming multi-wavelength signal  824 , a compensation circuit is implemented in the prior art amplifier  800  comprising three signal sources  828  at wavelengths λ 1 , λ 2 , and λ 3  coupled to multiplexor  802 , an output tap  830 , a demultiplexor  832  having outputs corresponding to the wavelengths λ 1 , λ 2 , and λ 3 , which are coupled to three separate photodetectors  834 , and a controller  836  arranged to receive signals from the photodetectors  834  and adjust drive currents of power supplies  814  supplying the drive currents to three Raman pump diode lasers  816 . 
         [0040]    In operation, light at three wavelengths λ 1 , λ 2 , and λ 3  is used to probe the gain profile of amplifier  800  in real time. When a transient change of the amplifier gain appears as a result of a change in the amplifier loading conditions, the ratio of optical power values of light at these three wavelengths changes which prompts the controller  836  to change the ratio of drive currents of Raman pumps accordingly, so as to reduce transient effects and flatten the gain profile of amplifier  800 . 
         [0041]      FIG. 2A  shows a preferred general configuration of the optical amplifier of the present invention with automatic control of the gain profile. The solid arrows represent optical signals and dashed arrows represent electrical or control signals. An optical amplifier  200 A comprises a tap  94 , an EDFA  5 , a gain adjuster  19 , a detection device  11 , and a control unit  533 . A small fraction of a multi-wavelength optical signal  201  is tapped off by tap  94 , while most of the signal proceeds to EDFA  5 , which amplifies the optical signal, and further to gain adjuster  19  which adjusts the gain profile of the amplifier  200 A so as to minimize differences between optical powers of signals at various wavelength comprising an output signal  202 . The specific realization of gain adjuster  19  will be considered in more detail below. Detection device  11 , which will also be described in more detail below, produces a set of electrical signals to the control unit  533  which controls EDFA  5  and gain adjuster  19 , so as to keep said differences between optical powers of signals at different wavelengths to a minimum. 
         [0042]    All possible locations of gain adjuster  19 , tap  94 , and EDFA  5  will work with respect to the present invention, but some configurations are easier to implement than others. For example, in  FIG. 2B , another preferred configuration of the amplifier of the present invention is shown. In an amplifier  200 B, gain adjuster  19  is located before EDFA  5 , and tap  94  with detection device  11  is located after EDFA  5 . Further, in  FIG. 2C , an amplifier  200 C is shown wherein gain adjuster  19  is inserted at a mid-stage of EDFA  5 , and a part of the gain adjustment function is carried by an EDFA itself. The role of gain adjuster device  19  can be performed by a dynamic gain equalizer or by a Raman amplifier. 
         [0043]    Turning now to  FIG. 3 , a detection device of the present invention is shown comprising a 1×(N+1) splitter  211 , a photodetector  212 , a set of optical filters  214 - 1  . . .  214 -N, and a set of photodetectors  213 - 1  . . .  213 -N. The first photodetector  212  measures optical power proportional to the total power of the signal  201  corning through the tap  94 . The remainder of the photodetectors  213 - 1  . . .  213 -N measure the optical power of the signal coming through splitter  211  and optical filters  214 - 1  . . .  214 -N, respectively. The transmission shapes F 1 (λ) . . . F N (λ) of these filters are selected in such a way that together with the gain adjuster  19  of  FIGS. 2A-2C  they give optimum compensation of the EDFA spectral hole burning (SHB). 
         [0044]    Once the powers P 1  . . . P N  at photodetectors  213 - 1  . . .  213 -N are measured, the controller generates a vector of numbers x=x 1  . . . x M , where M is the amount of independently adjustable parameters of gain adjuster  19  in  FIGS. 2A-2C . These parameters may correspond to individual pump powers and, or variable optical attenuator (VOA) settings. The vector x is passed to gain adjuster  19  and to EDFA  5  of  FIGS. 2A-2C . It is assumed that the overall gain change due to this adjustment, that is, the gain change between input  201  and output  202  of  FIGS. 2A-2C , is the following: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0045]    where each A m (λ) is the gain modification by a single “actuator”, that is, by the element of the gain adjuster  19  that is controlled by one of the component of the vector x. In other words, A m (λ) is a fraction of the gain modification caused by a m th  actuator upon receiving a unitary control signal by said actuator. In equation (1), the gain modifications are expressed in dB units. 
         [0046]    In the preferred embodiment the controller calculates vector x using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
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                     m 
                   
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                         , 
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         [0047]    Here, C m,n  are some constant coefficients obtained during system design, P n  is the power measured at n-th detector in linear units such as in milliwatt, and P 0  is the power measured at detector  212  of  FIG. 3 , that is the total, or unfiltered, power. The total number of detectors is N+1. 
         [0048]    Even though equation (2) gives very good results for SHB compensation, other formulas can be used for x i  calculation. The most generic formula is x m =f m (C k , P 0  . . . P 0  . . . P N ), wherein f m  is a pre-determined function and C k  are some pre-determined constants. 
         [0049]    During system design one needs to optimize the coefficients C m,n  together with filter shapes F 1 (λ) . . . F N (λ) in such a way that the overall gain change is minimal for different loading conditions. This can be done via simulation when optimization procedure runs through randomly generated signal loading conditions while adjusting coefficients C m,n  and filter shapes F 1 (λ) . . . F N (λ). Upon each adjustment, the optimization procedure calculates resulting gain change and, out of all filter shapes and coefficients C m,n  tried, it chooses the ones corresponding to the minimal perturbation of the original gain profile. The calculated coefficients C m,n  are then stored in the memory of control unit  533  to generate vector x. Since coefficients C m,n  are pre-calculated, the response time of the control unit  533  can be in sub-microsecond domain which is fast enough to compensate for most transients caused by changes of loading conditions of amplifiers  200 A- 200 C of  FIGS. 2A-2C . 
         [0050]    The apparatus of present invention will work using different numbers of detectors and actuators. While increasing the number of detectors and actuators generally improves the degree of achieved gain profile flatness of amplifiers  200 A- 200 C of  FIGS. 2A-2C , an optimal number of detectors and actuators exists which is capable of adequately compensating for both SHB and stimulated Raman scattering tilt. A simulation has shown that, surprisingly, only one filter, two detectors, and three or four actuators are sufficient to compensate for these effects. 
         [0051]    In case of optimization involving more than one filter, the transmission functions of the filters may have common regions of non-zero transmission. Thus, the different filters are not just different bandpass filters used to obtain optical powers in different areas of the spectrum of multi-wavelength optical signal to be amplified, as it is in the case of, for example, an apparatus of U.S. Pat. No. 7,359,112. Advantageously, the spectral shapes F 1 (λ) . . . F N (λ) of the filters of the present invention are optimized using the abovementioned optimization procedure, so as to ensure that the filters  214 - 1  . . .  214 -N filter out signals which are most representative of transient perturbations of the amplifier gain profile caused by spectral variations in optical signal  201  of  FIG. 3 . 
         [0052]    Further, tap 94 and 1×(N+1) splitter  211  of the detection device of  FIG. 3  can be replaced by any combination of taps and splitters tapping a portion of signal  201  in the form of N+1 sub-portions, one sub-portion being coupled to detector  212  and remaining N sub-portions each being coupled to one of filters  214 - 1  . . .  214 -N coupled to detectors  213 - 1  . . .  213 -N, respectively. Any such modification would result in an operational apparatus and, therefore, is a part of the present invention. 
         [0053]    Turning now to  FIG. 4 , an optical circuit of a detection device is shown having a 1×2 splitter  2110 , a filter  214 , and two detectors  212  and  213 . Similarly, tap 94 and 1×2 splitter  2110  of  FIG. 4  can be replaced, for example, by two taps, not shown, the first tap, not shown, being coupled to detector  212 , and the second tap, not shown, being coupled to filter  214  coupled to detector  213 . Upon such modification, or any other similar modification, the apparatus will still perform its intended function and, therefore, any such modification is a part of the present invention. 
         [0054]    The filter transmission function F 1 (λ) of filter  214  of  FIG. 4 , obtained through the abovementioned optimization procedure, is shown in  FIG. 5 . The filter transmission function of  FIG. 5  has a transmission peak reaching a maximum transmission at a wavelength of 1532+−2 nm, an attenuation peak reaching a minimum transmission at a wavelength of 1541+−2 nm, and an intermediate transmission of between 10% and 30% of the maximum transmission minus minimum transmission within a 1550+−5 nm wavelength band. An apparent drop below zero in  FIG. 5  at 1541+−1 nm is a result of optimization, and, in a real filter, the transmission in this region can be taken equal to zero or, alternatively, the whole curve can be shrunk to fit between 0% and 100% transmission, Both methods were found to give adequate results. 
         [0055]    Turning now to  FIG. 6 , another preferred embodiment of an amplifier  600  of the present invention is shown comprising an EDFA  84  working together with a distributed Raman amplifier comprising Raman pumps  53 , a WDM combiner  51  and a transmission fiber  500 . The actuators are the Raman pumps  53  and erbium doped fiber average inversion of EDFA  84 . The average inversion adjustment is performed by varying EDFA pump powers as is symbolically shown with an arrow  130   b . When EDFA pumps are adjusted, the average EDFA gain is measured via a detector  104  and detectors in a detection device  11  having two detectors and one filter, not shown. Detection device  11  receives an optical signal from a tap  91  located before EDFA  84 , and passes corresponding electrical signals to a control unit  535  through a line  113 , and detector  104  receives a fraction of an output optical signal tapped by an output tap  92  and passes corresponding electrical signal to control unit  535  through a line  114 . The measured gain is then held constant by control unit  535  via adjustment of a VOA  85  through a line  135   b  using (configuration-specific) values of C m,n  or C k  stored in its memory. Generally, VOA  85  can be positioned anywhere in amplifier  600 , including before or after erbium doped fiber coils, not shown. Also, there can be more than one VOA, in this case any of the VOA or all of them can be adjusted. The pump powers of Raman pumps  53  are adjusted by control unit  535  through a line  115 . 
         [0056]    An example of the actuator functions A m (λ) is shown in  FIG. 7 . Functions  61  and  62  are the gain changes due to Raman pump change (two Raman pumps in this example) and a function  63  is the change due to average inversion adjustments, before gain of EDFA  84  of  FIG. 6  is adjusted by VOA  85  of same Figure. Using the average inversion actuator reduces the need of having more than two Raman pump actuators for good SHB compensation. 
         [0057]    It should be noted that even though a distributed counter-propagation Raman amplifier topology is described in the preferred embodiment of  FIG. 6 , the present invention is not limited to this particular topology; other topologies can be used, such as co-pumping or discrete Raman amplifier located anywhere near or within EDFA. 
         [0058]    Simulations over 520 randomly generated cases have shown that actuator functions shown in  FIG. 7  together with the filter function shown in  FIG. 5  and with optimized coefficients C m,n  of equation (2) can reduce the gain change due to SHB by a factor of 2 on average. For further reduction of the SHB induced changes one needs to increase the number of filters and detectors in detection device  11 . The increase of the number of Raman pumps also helps with the SHB compensation but the improvements are minor in case of a single filter  214 , however the improvements will be more significant together with larger number of filters.