Abstract:
The invention relates to a regulatable optical amplifier which has at least two series-connected amplifier groups, each amplifier group having a regulating device. Connected upstream of the optical amplifier is a power monitor device for detecting changes in the input power, whose electrical output is connected both to the first regulating device and to the second regulating device. In line with the invention, the first and second regulating devices have a control line inserted between them which comprises a series circuit containing a high-pass filter, a delay and signal-shaping unit and a feed-forward control unit for generating a correction signal for the second regulating device. In this arrangement, the high-pass filter has a cut-off frequency which corresponds approximately to the cut-off frequency of the first amplifier group. The inventive control line optimizes the regulating response such that power transients in the output signal from the optical amplifier are reduced particularly after an abrupt change in the input power.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a regulatable optical amplifier, and a method for regulating an optical amplifier. 
     BACKGROUND OF THE INVENTION 
     In optical transport networks having a high range, optical fiber amplifiers whose amplifying fibers are doped with ions of an element originating from the group of rare earths are usually used for signal amplification. Fiber amplifiers doped with erbium ions (“erbium doped fiber amplifiers”, abbreviated to EDFAs) are predominantly used commercially. Such an EDFA has, besides the input for the data signal, an optical pump source, e.g. a laser diode, the output signal of which is coupled into the fiber doped with erbium ions. The optical data signal guided in the doped fiber is amplified by stimulated emission of photons. The EDFAs generally comprise a plurality of amplifier stages. Hereinafter, amplifier stage denotes in each case that part of an EDFA which contains precisely one continuous erbium doped fiber arranged between passive components. Hereinafter, amplifiers which are split into a plurality of amplifier groups are considered, where an amplifier group can comprise either a single amplifier stage or a plurality of amplifier stages. 
     In order to exhaust the capacity of optical transmission fibers, the data signals are transmitted in individual transmission channels that are often combined by means of the technique of wavelength division multiplexing (abbreviated to WDM). Transmission of WDM signals with up to 80 channels at data rates of up to 40 Gbit/s is possible nowadays by means of the WDM technique. The number of channels varies depending on capacity utilization and transport volume of the transmission system. If channels are switched in and out in the transmission system or coupled in and out at branching points, then this gives rise to abrupt changes in the aggregate signal power in the transmission system. Said changes can lead to bit errors and also to damage at the optical receivers because the latter can operate without any errors only for a limited input power range. 
     If such abrupt changes in the signal power are present at the input of an optical amplifier, then the pump power of the amplifier has to be adapted rapidly to these power fluctuations of the input signal in order to avoid large jumps in the powers of the channels that are not involved in the switching operation. The output power of an optical amplifier depends on the gain thereof. The amplifier gain is determined by the pump wavelength and pump power in addition to material parameters. Furthermore, the amplifier gain is determined by the input power upon reaching the maximum possible output power (saturation). If the gain remains constant, the power of the channels which are not involved in the switching operation does not change since they are always amplified to the same extent. Therefore, in the design of an optical fiber amplifier, it is always of importance to obtain an amplifier gain that is as constant as possible even in the event of large power jumps at the amplifier input. This is achieved by means of gain regulations. The latter are usually output power regulations in conjunction with an amplified signal, derived from the input signal, as desired value. Methods for regulating the amplifier gain or the amplifier output power are known in many cases from the prior art. Regulating devices supplemented by a control, a so-called feedforward control, are normally used. In the regulating circuit and the feedforward control chain, the optical pump forms the actuating element and the pump power accordingly corresponds to the manipulated variable. 
     Signal delays are unavoidable in the overall arrangements of optical amplifiers. During signal amplification, in EDFAs in particular, delays of the optical signal occur just as a result of the propagation time in the optical fiber. Said delays amount to approximately 0.3 to 0.6 μs. Furthermore, delays also arise as a result of the physical operation of amplification. When a pump source of 980 nm is used, the electrons of the doping element erbium, during the pumping operation, are initially raised to a first, higher atomic energy level, from which they first relax in a non-radioactive transition to a metastable intermediate level before falling back to the atomic ground level with emission of photons. In addition to these delays of the optical signal, delays of the electrical signal also occur within the regulating device due to the individual structural elements thereof. These include for example delays during the detection and optoelectrical conversion of the input and output signals, delays at the actuators of the pump device and during the signal processing, which can be effected in analog or digital fashion. All these factors adversely influence the regulating behavior, that is to say that the dynamic properties of the regulating device do not lead to an optimum system response. Thus, during the transition recovery time of the regulating device, undesirable transients occur in the amplifier gain, which are manifested in the form of overshoots or undershoots in the output power of the amplifier and in undesirable gain changes. 
     If a plurality of single-stage amplifiers are cascade-connected in order to obtain higher ranges, then an amplifier cascade arises. Overshoots and undershoots in the output power of the amplifier can accumulate in this case. Small deviations in the gain of individual amplifiers lead to large deviations in the gain at the output of the amplifier cascade. In addition, the abovementioned optical and electrical signal delays make it more difficult to exactly regulate the amplifier gain at the output of the amplifier cascade. Considerable delays of the optical signal of an order of magnitude of 100 μs occur due to the signal propagation time if dispersion compensating fibers (abbreviated to DCFs) are connected between individual amplifier stages. 
     An earlier German patent application bearing the application number 10 2004 052 883.7 discloses a solution for the compensation of gain fluctuations of a multistage optical amplifier. In the event of a power jump in the input power, the pump power of the first amplifier stage is adapted, the change in the input power that is to be expected at a downstream second amplifier stage is determined and a new pump power for the second pump device is calculated depending on this. In this case, the new pump power is set at the beginning of a predetermined lead time before the arrival of the power jump at the input of the second amplifier stage. One disadvantage of this solution is that the effect of the regulation commences prematurely, and that the gain deviations produced as a result, although they are very small, have a disadvantageous effect in an amplifier cascade. Moreover, the lead time is dependent on the ratio of the optical powers at the input and at the output of the amplifier stages and on the regulator setting. The regulating behavior can be optimized with difficulty under these preconditions. 
     SUMMARY OF THE INVENTION 
     The present invention provides an optical amplifier having a plurality of amplifier stages or groups and having a regulating behavior that is as optimal as possible, such that power transients in the output signal of the optical amplifier, in particular after an abrupt change in the input power, are reduced as far as possible. Furthermore, a corresponding method for regulating an optical amplifier is to be specified. 
     In one embodiment of the invention, improves the feedforward control of an optical amplifier, having at least two series-connected amplifier groups, by inserting an additional control chain between the regulating devices of the individual amplifier groups. The error that arises as a result of the inherent delays in the regulating device of the first amplifier group is compensated for. The control chain according to the invention receives the same input signal as the regulating device of the first amplifier group. It has a series circuit preferably containing a high-pass filter, a delay and signal-shaping unit and a feedforward units. The high-pass filter has a cut-off frequency approximately corresponding to the cut-off frequency of the first amplifier group. A control signal that simulates the temporal error of the actuating signal of the first group is formed in the delay and signal-shaping unit. In the delay and signal-shaping unit, the control signal is temporally superposed with a temporally delayed copy of the signal, the delay duration corresponding to that of the regulation of the first amplifier group. A correction signal for the actuating signal of the second regulating device with the appropriate amplitude is subsequently generated in the feedforward unit. The dynamic response of the control and regulation process is optimized in this way. An optimum setting of the pump power for the second amplifier group is advantageously achieved, which has the effect that transients are prevented at the output of the optical amplifier. Optimum setting of the pump power is taken here to mean that the abrupt change in the power of the optical signal and the correspondingly adapted pump power signal arrive at the same point in time at that location of the amplifier at which the pump signal is coupled into the transmission fiber. The dynamic response of the totality of the regulating devices is coordinated with said point in time by means of the control chain according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in more detail with reference to the figures: 
       In the figures: 
         FIG. 1  shows a block diagram of an amplifier group with a connected regulating device. 
         FIG. 2  shows a block diagram of a regulatable optical amplifier with two amplifier groups and the control chain according to the invention. 
         FIG. 3  shows a comparison of the time profiles of the optical and electrical signals involved in the control and regulation process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Firstly the regulating device of an individual EDFA stage will be explained in more detail with reference to  FIG. 1 . A corresponding block diagram is indicated in  FIG. 1 . From the input port  1 , an optical WDM signal  1  is fed to an amplifier stage S_A, the signal preferably having a wavelength range around 1550 nm. The amplifier stage S_A comprises an erbium-ion-doped amplifying fiber EDF_A and a wavelength-selective coupler WDM_A, via which a pump signal  15  from a pump source LD_A is fed to the amplifying fiber EDF_A. The pump source can be for example a laser diode having the emission wavelength of 980 nm or 1480 nm. A power monitor device M 1  for the input signal is connected to the input of the amplifier stage S_A (port  1 ), the device comprising for example a coupler K 1  with a monitor photodiode PD 1  connected downstream. The photocurrent emitted at the electrical output of the power monitor device M 1  is fed as electrical input signal  10  to the regulating device RE_A. A further power monitor device M 4  for the output signal  4  is arranged upstream of the output port  4  of the overall arrangement. Said device likewise comprises a coupler K 4  with a connected photodiode PD 4 . The power monitor device M 4  serves for detecting the regulated variable (=output power). The photocurrent supplied by the monitor device M 4  accordingly corresponds to the actual value of the regulated variable. The desired value of the regulated variable is generated with the aid of the power monitor device M 1 . The electrical input signal is firstly fed to a scaling unit G 14 . This can be for example an electrical amplifier. The scaling unit G 14  has the function of simulating the gain of the EDFA stage. The gain is predetermined in this case. Consequently, a signal having a desired output power  11 , to which regulation is to be effected, is present at the output of the scaling unit G 14 . Said desired output power  11  serves, on the one hand, as desired value of the regulated variable and, on the other hand, as reference signal for the feedforward control unit FF_A. The signal  11  is therefore fed to a branching location V 1 , which is connected to the feedforward control unit FF_A, on the one hand, and to a first mixing location SUB_A, on the other hand. The first mixing location SUB_A has an input for the desired value of the output power  11  and an input for the actual value of the output power  19 . In order that the two values are present at the mixing location at the same time, a delay element VZ(f_A) is inserted in the path of the desired value signal. In the delay element VZ(f_A), the desired value signal is delayed by the time period Δt f     —     A . Said time period corresponds to the delay experienced by the optical WDM signal upon passing through the EDFA stage with the associated conducting fibers. Said delay is identified as a block having the designation Δt f     —     A , with a dashed line in the signal path of the optical signal. 
     At the first mixing location SUB_A, the desired and actual values of the output power are compared with one another and the difference signal  12 , also referred to as regulation deviation  12 , is fed to the regulating or correction unit C_A. The latter calculates a positive or negative correction value for the manipulated variable depending on the regulation deviation  12 . The manipulated variable or the actuating signal of the regulating unit is the pump current  16 , which is linked approximately linearly with the output power of the pump source  18 . In order to accelerate the regulating process and in order to be able to hold the desired gain value more precisely after a change in the input power, the regulation is supplemented by the feedforward control. In the feedforward control unit FF_A, the pump power for the pump source which is estimated to be necessary in order to maintain a constant gain is calculated on the basis of a predetermined model. The correction value  13  calculated by the regulating unit C_A is therefore modified at a further mixing location ADD_A by virtue of the estimated value  15  predetermined by the feedforward control unit FF_A being added to the signal  13 . This value  16  for the manipulated variable resulting from the feedforward control and the regulating loop is then fed to the pump source LD_A. In order to indicate the delays that occur during the signal processing, a block having the designation Δt C     —     A  with a dashed line has been inserted at this location. Hereinafter it should always be taken into consideration that the blocks with a dashed line are not structural elements of the regulation device or other functional blocks, rather the blocks with a dashed line are only intended to highlight the time behavior of the signal respectively considered. On account of the identification of the signal delays by the box Δt C     —     A , it becomes clear that the pump signal  18  will always reach the EDFA in a delayed manner. Moreover, the box Δt C     —     A  is intended to illustrate in summary not only the signal delays in the electronic regulating device but also the delay that acts in the same way in the optical path and in the physical pump operation already described. 
     The delay Δt C     —     A  caused by the regulation device and amplifier group can be compensated for in a multistage amplifier by utilizing the propagation time of the optical signal in the course of propagation in a dispersion compensating fiber DCF arranged between two stages. These signal propagation time delays can be up to 100 μs and can be utilized during the signal processing of the electrical signals in the regulating device. 
     Under this assumption the control and regulation process is optimized by optimally adapting the propagation time of the electrical signals within the regulation system (or here within the regulating devices) to the propagation time of the optical signal in the transmission system. A signal analysis is carried out in order to improve the dynamic behavior within the regulating devices. Since an EDFA is a nonlinear system, only small signal disturbances at a defined operating point are considered, i.e. for a fixed gain value or a predetermined input and output power. In this case, the system can be linearized and linear systems theory is applicable. In this case of small signal analysis, therefore, only small changes in the input power are assumed below. These are achieved for example by modulating an electrical signal having a specific frequency onto the optical WDM signal. The electrical input signal impressed in this way can be composed of a sine function or of a plurality of such periodic functions having different frequencies. In this way it is possible to produce a step function such as would be present optically when channels are switched in or out. The response of a regulating block to the input signal (such as here the step function) is suitable for fully describing the time behavior of the regulating block as well as those of the entire regulating circuit, provided that linear behavior is ensured. The time function which describes the temporal profile of the signal at the output of a regulating block as a response to an abrupt change in the input signal is called step response or transition function. In the frequency domain, the Fourier transform of the temporal signal profile at the output of the system results from the product of the Fourier transform of the temporal signal profile at the input of the system and a transfer function of the system. If the frequency of the electrical input signal or, in the case of a plurality of frequencies, the spectrum of the input signal at the input of the spectrum is altered, then the time behavior of the regulation system can be checked by recording the electrical spectrum at the output of the EDFA or the regulating device. Conclusions about the magnitude and phase of the input signal can be drawn using a vector network analyzer. 
     If the transfer function of an EDFA is determined by means of the small signal analysis, then it is evident that the transfer function, for an input signal amplified at saturation in an EDFA, represents a high-pass filter. The cut-off frequency of the high-pass filter is proportional to the output power of the EDFA. If a variable optical attenuator (abbreviated to VOA) is inserted within an amplifier group comprising a plurality of EDFA stages, for example, then the cut-off frequency is increased by the attenuation factor. This means that the effect of an amplifier group on an abrupt input signal can be simulated by means of a high-pass filter. To put it more precisely, the high-pass filter simulates the system response of the pump path of the first amplifier group, which was modified by the conversion from the optical path to the pump path of the second amplifier group. This system response is similar to the frequency response of the optical path. If the delays which occur both in the optical path and in the regulating devices are furthermore balanced, then it is possible to achieve an ideal step-response function at the output of the optical amplifier. According to the invention, for this purpose an additional control chain is inserted between the regulating devices of a first amplifier group and a second amplifier group, said additional control chain preferably comprising a high-pass filter, a delay and signal-shaping unit and a scaling unit for the control signal. Details can be gathered from the exemplary embodiment below. Even without the high-pass filter, the introduction of the control chain already results in an improvement in the dynamic behavior, although a much more precise compliance with a constant gain is obtained with a precisely dimensioned high-pass filter. 
       FIG. 2  illustrates a block diagram of an optical amplifier comprising two amplifier groups G_A and G_B in this exemplary embodiment. The upper path contains the optical components and corresponds to the optical path of the optical WDM signal. The connected regulating devices contain the electronic functional units and paths. They can be realized either in analog or digital fashion, for example by means of a digital processor unit (abbreviated to DSP). The amplifier groups in this exemplary embodiment can be constructed from one or more amplifier stages according to  FIG. 1 . Furthermore, they can contain more than one pump laser diode as pump device, or else just one laser diode that pumps a plurality of amplifier stages. In this case, the functional block G_A or G_B is intended to contain, besides a number of erbium doped fibers, all the passive optical components such as couplers, isolators, VOAs and a pump device designed in any desired manner according to the prior art. A power monitor device M 1  (likewise according to  FIG. 1 ) is connected downstream of the input port  1  of the amplifier arrangement shown in  FIG. 2 , the WDM signal  1  being fed to the amplifier group G_A via the optical output of said power monitor device and the optoelectrically converted input signal  10  being fed to a first regulating device RE_A via the electrical output of said power monitor device. Arranged downstream of the amplifier group G_A is a further monitor device M 4  for detecting the output power. The electrical output signal from M 4  corresponds to the actual value of the regulated variable and is fed to the regulating device RE_A. The regulating device RE_A is constructed analogously to the regulating device described in  FIG. 1  and supplies as actuating signal a pump current  14  for the pump device contained within the amplifier group G_A. The dashed box Δt f     —     A  is depicted for identifying delays of the optical signal which result through fiber leads within the amplifier group A. The dashed box Δt C     —     A  is indicated for identifying the inherent delays of the electrical signal within the regulating device RE_A and the delays in connection with the pump operation of the amplifier G_A. The time period Δt C     —     A  is also referred to as reaction time of the regulation RE_A. 
     The output-side monitor device M 4  of the first amplifier group G_A can be followed for example by a dispersion compensating fiber DCF, which brings about a delay of the optical signal of Δt DCF . In this exemplary embodiment, the second amplifier group G_B has only one output-side power monitor device M 6 . This amplifier group, too, can include as required a plurality of erbium fibers with one or a plurality of associated pump devices. The delays of the optical signal due to the erbium doped fiber and the fiber leads are indicated here by the dashed box Δt f     —     B . The pump power for the amplifier group G_B is adapted by means of the regulating device RE_B. The latter is designed in principle like the regulating device RE_A of the first group. The regulating device RE_B receives the input signal from the first monitor device M 1  upstream of the first amplifier group G_A. Since the optical signal experiences the delays Δt f     —     A +Δt DCF  in the upper path from port  1  to port  5 , the electrical signals must also be adapted temporally in the regulating device RE_B in order to act at the same time as the optical signal. 
     The optoelectrically converted input signal  30  tapped off at the branching point V 0  downstream of the monitor device M 1  is firstly fed to the scaling unit G 16  of the regulating device RE_D, where it is multiplied by a gain factor corresponding to the entire optical path from port  1  to port  6  including the amplifier group G_B. The signal  31  scaled in this way is subsequently fed to the branching location V 3 . A first output of said branching location passes the signal  32  via a first delay element VZ 1  to the feedforward control unit FF_B. In said delay element VZ 1 , the signal is delayed by the time period Δt f     —     A +Δt DCF −Δt C     —     B . If, by way of example, further elements are added to this path, then the delay time in VZ 1  should be correspondingly reduced by the delay of these elements. What is achieved in this way is that the control signal of the feedforward control unit FF_B is already generated before the reaction time of the second regulation Δt C     —     B . A second output of the branching location V 3  leads via a second delay element VZ 2  to the mixing location SUB_B of the regulating device RE_B, where the desired value  35  is compared with the actual value  39  and the regulation deviation  36  is output to the regulating unit C_B. In this case, the delay element VZ 2  was set in such a way that the signal of the desired value  33  is delayed by a time period Δt f     —     A +Δt DCF +Δt f     —     B  because the signal of the actual value  39  has also undergone precisely these delays and a simultaneous substation thus takes place. The regulating unit C_B is followed by an adder ADD_B, in which the control signal  34  of the feedforward control unit FF_B and the correction signal  37  generated in the regulating unit are added together. 
     In order to compensate for the reaction time of the first regulating device, the actuating signal  38  present at the output of the adder ADD_B is adapted both temporally and in terms of amplitude by means of a further correction signal  27 . For this purpose, a further control chain SK is used, which uses as input signal the desired value signal  11  output by the scaling unit G 14 . For this purpose, within the regulating device RE_A, in contrast to  FIG. 1 , a second branching point V 2  is provided downstream of the first branching point V 1 , from which second branching point the input signal  11  previously scaled in G 14  is fed to the control chain SK according to the invention. 
     The electrical signal  20  is firstly fed to a high-pass filter HP. Through the filtering of the input signal by means of a high-pass filter having the same cut-off frequency as the amplifier group G_A, the feedforward signal is shaped in such a way that the optical output signal of group G_B has the same shape as if the input signal of group A were present at the input of group G_B. A feedforward control signal which generates no overshoots in the output signal can then be used within group G_B. In this exemplary embodiment, the high-pass filter HP is followed by a delay and signal-shaping unit DY. The latter can also be arranged upstream of the high-pass filter HP. Within said delay and signal-shaping unit, the signal firstly experiences a delay corresponding to the time period Δt f     —     A +Δt DCF −ΔC   —     B , that is to say that the electrical signal is delayed by the time required by the optical signal for passing through the amplifier group G_A and the DCF. The reaction time or inherent delay time of the amplifier group G_B is subtracted from this delay in order that the feedforward signal can be generated in a timely manner. The delay element VZSK is provided for setting the delay Δt f     —     A +Δt DCF −ΔC   —     B . The delay element VZSK is followed by a branching point V 4 , the first output of which is connected directly to an adder ADD, while the second output is connected to the adder via an interposed delay element VZ (C_A). In this way, a copy  24  of the electrical signal  23  is generated, the copy being delayed in VZ(C_A) by the reaction time or the inherent delay time Δt C     —     A  of the regulating device RE_A of the first amplifier group V_A. The undelayed signal  23  and the delayed signal  25  are added together in the adder ADD, with the result that a signal pulse of the time period Δt C     —A   , is present at the output of the delay and signal-shaping unit DY. If the transfer functions at the output of the blocks HP and DY are considered, then the transfer function at the output of the high-pass filter represents a simulation of the amplifier group G_A, while the transfer function at the output of the delay and signal-shaping unit DY represents the error generated by the inherent delay of the amplifier group G_A. A correction signal is then generated in the feedforward control unit FF 2  connected to DY, the correlation signal compensating for the error of group G_A. The correction signal  27  thus represents an additional feedforward control signal and is added to the actuating signal  38  of the conventional regulating device of group G_B in a mixing location ADD_FF. The resulting actuating signal  40 , which is provided for setting the pump power in amplifier group V_B, is set temporally such that the inherent delay of the regulating device of amplifier group V_B, which is indicated by the dashed box Δt C     —     B  in the drawing, is no longer significant and has been compensated for. The dashed box Δt C     —     B  has been inserted at this point only in order to draw attention summarily to the delays occurring in RE_B and the pump device of G_B, but has no effect on the signal  40  at this point. 
     In order to illustrate the method of operation of the individual functional blocks of the control chain SK according to the invention,  FIG. 3  indicates some time profiles of the electrical signal and of the optical signal at different locations within the regulating device and within the optical path of the amplifier arrangement. Curve K_ 1  represents the power of the optical WDM signal  1  at the input (port  1 ) of amplifier group V_A. This is a step function that is intended to represent the omission of channels. The power jump takes place at the instant t=0. Curve K_ 15  represents the output signal  15  of the feedforward control unit FF_A. The actuating signal at the output of the adder ADD_A would also look like K_ 15 . Curve K_ 16  represents the effective actuating signal, or in other words the effect of the pump signal. It becomes clear on the basis of curve K_ 16  that the pump signal acts too late by the time period Δt C     —     A . On account of this reaction time Δt C     —     A  of the amplifier group G_A, overshoots or undershoots occur in the optical output power. Curve K_ 4  represents such an overshoot in the optical power at the output of group A (port  4 ). Curve K_ 21  shows the electrical signal  21  at the output of the high-pass filter. This is the step response of the regulating block HP. 
     The following sets of curves show the signal profiles at a delayed instant t=Δt f     —     A +Δt DCF  in comparison with t=0. Thus the curve K_ 5  shows the profile of the optical signal  5  at the input of amplifier group G_B. The power jump in the optical signal power arrives at the amplifier group GB in a manner delayed by the propagation time Δt f     —     A +Δt DCF  (=propagation time through amplifier group G_A+propagation time through DCF). The curve K_ 33  shows the electrical signal  33  at the output of the delay element VZ 1  within the regulating device RE_B of the second amplifier group G_B. The effect of the feedforward control unit FF_B is indicated in curve profile K_ 34 . The signal  34  would act in a manner delayed by the reaction time Δt C     —     B  if the control chain SK according to the invention were not inserted. 
     Within the control chain SK, the electrical signal does not undergo said reaction time Δt C     —     B . The signal propagation time of the electrical signal is only adapted to that of the optical signal at the beginning of the amplifier group G_B. For this purpose, it has to pass through the delay element VZSK. The electrical signal  21  at the output of VZSK is represented in the curve profile K_ 21 . Furthermore, curve profile K_ 26  indicates the electrical signal  26  at the output of the delay and signal-shaping unit DY. A signal pulse having the length Δt C     —     A , has been produced by the superposition of the signal  23  with a copy  25  time-delayed by the reaction time Δt C     —     A , of RE_A. The signal  26  is fed to the feedforward control unit FF 2 , where the amplitude is adapted, and the control signal  27  is subsequently added to the regulating signal  38  in the adder ADD_FF. The output signal  40  of the regulating device RE_B is illustrated in the curve profile K_ 40 . K_ 40  shows the effective effect of the control chain SK according to the invention. The associated pump signal that takes effect in the amplifier group G_B is the subject of curve profile K_PB. Curve profile K_PB arises as a result of the addition of K_ 40  with K_ 34 . It should be noted that the time periods Δt C     —     A  and Δt C     —     B  are identical in this exemplary embodiment. The effect of the pump signal becomes clear on the basis of curve profile K_ 6 . This concerns the output power of the optical signal  6  at the output port  6  of the amplifier group G_B. It becomes clear that during the reaction time interval Δt C     —     A , the pump power must first be abruptly decreased in order to reduce the amplifier gain and to counteract the overshoot, and the pump power must subsequently be increased again in order to raise the amplifier gain again. It becomes clear on the basis of the curve profiles K_ 5 , K_PB and K_ 6  that the time behavior of the individual regulating blocks of the regulating device RE_B is optimally designed. An optimal feedforward control has been achieved for the entire amplifier arrangement. 
     A further advantage of the design of the feedforward control according to the invention is that the magnitude of the correction signal output by the feedforward control unit FF 2  is independent of the operating point of the regulating devices RE_A or RE_B. The magnitude of the correction signal essentially depends on the scaling—effected in the component G 14 —of the input signal tapped off from the monitor device M 1 . The correction signal generated in FF 2  is thus independent of the output power of the individual amplifier groups G_A and G_B. 
     Further possibilities for realization can be formed if the feedforward control of the amplifier group G_A is set to be weaker or stronger or is switched off entirely. The signal  25  would have to be attenuated by the corresponding factor by the VZ (C_A), or this delay element could be obviated if FF_A were not present.