CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Provisional application No. 61/378,721, filed Aug. 31, 2010 which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to optical amplifiers, and in particular to reducing or compensating transient variations of gain in optical amplifiers. 
     BACKGROUND OF THE INVENTION 
     In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of an optical fiber comprising a transmission link of a WDM fiberoptic network. At a receiver end of the transmission link, the optical channels are separated, whereby each optical channel can be detected by an optical receiver. 
     While propagating through an optical fiber, light loses power. This power loss is well understood and is related to the physics of propagation of light in the fiber. Yet some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an 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, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end. 
     An erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical amplifiers employed in fiberoptic networks. A single EDFA module can amplify about a hundred optical channels at a time, thus providing significant cost savings. One of the main components of an EDFA is a length of an active optical fiber having a core doped with ions of a rare earth element erbium. The erbium doped fiber (EDF) is optically pumped by using a suitable pump, such as a diode laser, so as to create a population inversion between energy states of the erbium ions comprising a gain medium of the EDF. Once the population inversion is created, the gain medium begins to amplify an optical signal propagating along the core of the EDF. The gain medium is characterized by a wavelength-dependent gain coefficient. During the amplification process, the optical power of the pump is absorbed by the gain medium, which simultaneously amplifies all the optical channels present in the optical signal. The amplification coefficient of a particular channel depends on the input optical power and on the optical power of the pump. When the number of optical channels changes suddenly, for example, due to adding, dropping, or routing of some of the optical channels, the input optical power changes stepwise, which results in a change of the gain coefficient of the gain medium of the EDF. The gain coefficient change impacts output optical power of the rest of the optical channels. 
     Most optical amplifiers of the prior art have a gain stabilization circuit that reacts to changes of input optical power by changing the pump optical power. For example, in U.S. Pat. No. 6,989,923 by Stentz, an apparatus for automatically controlling gain of an optical amplifier is disclosed. The apparatus of Stentz generates a first control signal from a feed-forward control circuit and a second control signal from a feedback control circuit. The optical power of the pump is adjusted in accordance with both control signals. Similarly, in U.S. Pat. Nos. 6,246,514 and 6,522,460 by Bonnedal et al., the feed-forward and feedback controls are combined, and in addition, pilot tones are used to measure the amplifier gain. In one embodiment, an EDFA controller of Bonnedal et al. calculates the number of channels present in the input optical signal and adjusts the optical power of the pump accordingly. 
     Disadvantageously, the accuracy of transient control, that is, the degree to which transient fluctuations of optical channel power may be suppressed, is limited by the temporal dependence of the EDF optical gain. Even when the optical power of the pump increases instantaneously, the EDF optical gain does not. There is a certain delay of the EDF gain growth following the pump increase, which is related to the rate of populating an excited meta-stable energy level  4 I 11/2  of erbium ions. Similarly, when the power of the pump decreases, or when the input optical power increases, the gain does not decrease instantaneously. The gain in fact decreases at a rate of change of the population inversion in the EDF gain medium. As a result, a transient change of the gain coefficient and, consequently, a transient change of output optical power is produced. 
     These transient fluctuations of optical power of a signal can grow in magnitude as the signal propagates along a transmission link containing many EDFAs, which can ultimately lead to a loss of information and even to a loss of network stability and/or to damage of optical receivers. To avoid stability loss or damage to network components, it is imperative that transient changes of optical power of propagating signals be kept below a certain acceptable level. 
     A method for adaptively controlling an optical gain in an EDFA has been described in U.S. Pat. No. 6,894,832 by Aweya et al. In the method of Aweya et al., the temporal behavior of the optical gain in the EDF is approximated by using a so-called reference model. The reference model of Aweya et al. is a dynamic model used for computing a reference value of the output optical power corresponding to the input optical power and target optical gain of the EDFA. The reference value is compared to a measured value of the EDFA output optical power, and the optical power of the pump is adjusted so as to bring the measured value of the EDFA output optical power to the computed reference value. An adaptation mechanism is described for adjusting a ratio between the signal from the reference model, the feedback signal, and the feed-forward signal, wherein all three signals are used to adjust the optical power of the EDFA pump. 
     Disadvantageously, the method of Aweya et al. is computation intensive, which can lengthen the response time of a corresponding control apparatus. Transient variations of the output optical channel power can occur in a sub-microsecond time domain. Given the amount of the computations required to implement the method of Aweya et al. for controlling the optical gain of an EDFA, the sub-microsecond response time may be difficult to achieve in combination with the required degree of transient suppression. 
     An apparatus and a method for controlling gain in an optical amplifier by accounting for transient changes of energy levels population in the EDF gain medium has been described by Lelic in U.S. Pat. No. 6,900,934. The method of Lelic involves real-time tracking of the population of an excited energy state of erbium by measuring a residual pump power, that is, by measuring the optical power of the pump light which has not been absorbed in the EDF. Disadvantageously, the apparatus of Lelic comprises a complicated digital processor, as well as an optical tap, an optical filter, and a photodetector dedicated to measuring the residual pump optical power. 
     Another known approach to reducing transient variations of gain of an optical amplifier consists in stabilizing overall power of the input optical signal before it reaches the optical amplifier. For example, Cordina in U.S. Pat. No. 6,668,137 discloses a power control apparatus for stabilizing overall optical power of a signal propagating in an optical fiber. Referring to  FIG. 1 , a power control apparatus  100  of Cordina includes an optical tap  102 , a photodiode  104 , an optical delay element  106 , a variable optical attenuator (VOA)  108 , and a controller  110 . In operation, the controller  110  receives a signal from the photodetector  104  and adjusts the VOA  108  to keep the output optical power constant. The delay element  106  is required to compensate for a finite response time of the VOA  108  and the controller  110 . The Cordina apparatus  100  enables power transients to be detected in time for a “pre-emptive” control, to suppress transients which are too fast to be suppressed by the above described conventional control of optical amplifiers. 
     Lundquist et al. in U.S. Pat. No. 7,483,205 discloses a similar apparatus. The apparatus of Lundquist et al. includes a variable optical attenuator, an optical power sensor disposed upstream of the variable optical attenuator, and a control loop configured to stabilize optical power after the variable optical attenuator. The control electronics and the variable optical attenuator of the Lundquist apparatus must be very fast to be able to react to sub-microsecond input optical power changes, which increases the cost of the Lundquist apparatus. 
     Furthermore, a common drawback of optical power stabilizers of Cordina and Lundquist et al. is that optical power stabilizers of Cordina and Lundquist et al. achieve power stabilization by attenuating the optical signal before the amplifier, thus lowering achievable optical signal-to-noise ratio. The signal-to-noise ratio is lowered because the amplified spontaneous emission (ASE) noise of the optical amplifier is not attenuated by the variable optical attenuator, while the incoming optical signal is. 
     In view of the foregoing, there is a need to provide an apparatus and a method for reducing optical transients, that would overcome the above described prior-art shortcomings of slow response time, complexity of the control circuitry, and varying per-channel optical power. The present invention allows one to considerably reduce transient variations of output power of optical amplifiers by using a simple control circuit, which can be easily added to existing optical amplifiers of different designs. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the drawbacks of the prior art by providing a circuit that simulates a response of an active optical fiber to a rapid change of input loading conditions. The circuit is connected to an optical tap/photodetector disposed upstream of the optical fiber, and to a variable optical attenuator disposed downstream of the active optical fiber. The circuit provides a control signal for the variable optical attenuator. The control signal mimics the expected optical amplifier gain variation. As a result, the transient variation of gain of the optical amplifier is suppressed, or at least significantly reduced, by the variable optical attenuator. 
     A control circuit of the invention can be added to existing optical amplifiers already having some form of feed-forward and/or feedback control, for improving achievable suppression of optical transients. Advantageously, the input optical tap and photodetector used for feed-forward control or optical power management can be shared with the control circuit of the invention. The variable optical attenuator can also be shared when present in an optical amplifier. For example, multistage optical amplifiers often have a mid-stage variable optical attenuator that can be shared with a control circuit of the invention. 
     Preferably, the control circuit includes a logarithmic amplifier connected to a high-pass electrical filter. By adding serially coupled logarithmic amplifier and high-pass filter to an existing optical amplifier already having an input optical tap, a photodetector, and a variable optical attenuator, the transient performance of the optical amplifier can be improved at little cost. When the input optical tap, the photodetector, or the variable optical attenuator are absent in the existing optical amplifier, the required components can be provided as a part of the controller apparatus. 
     In accordance with the invention there is provided a controller for reducing a transient variation of gain of an optical amplifier including 
     an optical tap having an input port and first and second output ports, for receiving an input optical signal at the input port, splitting the input optical signal into first and second optical signals, and directing the first and the second optical signals to the first and the second output ports, respectively, 
     a photodetector coupled to the first output port, for detecting the first optical signal, 
     a length of an active optical fiber coupled to the second output port, for amplifying the second optical signal when the active optical fiber is pumped by an optical pump source, and 
     a variable optical attenuator coupled to the active optical fiber downstream thereof, for attenuating the amplified second optical signal in dependence upon a control signal applied to the variable optical attenuator, so as to produce the output optical signal, 
     wherein the controller is coupled to the photodetector and to the variable optical attenuator, 
     wherein the controller comprises a control circuit adapted to generate, based on the detected first optical signal, the control signal for the variable optical attenuator, having a magnitude and a temporal profile corresponding to a magnitude and a temporal profile of a transient change of optical gain of the active optical fiber caused by a transient change of optical power of the second optical signal, thereby reducing the transient variation of gain of the optical amplifier. 
     In accordance with another aspect of the invention there is further provided a method for reducing a transient variation of gain of an optical amplifier, comprising 
     (a) splitting an input optical signal into first and second optical signals and detecting the first optical signal using a photodetector; 
     (b) amplifying the second optical signal in a length of an active optical fiber pumped by an optical pump source; 
     (c) generating, based on the detected first optical signal, a control signal for a variable optical attenuator, the control signal having a magnitude and a temporal profile corresponding to a magnitude and a temporal profile of a transient change of optical gain of the active optical fiber caused by a transient change of optical power of the second optical signal; and 
     (d) applying the control signal to a variable optical attenuator disposed downstream of the active optical fiber, so as to compensate for the transient variation of gain of the optical amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings in which: 
         FIG. 1  is a block diagram of a prior-art transient control apparatus; 
         FIG. 2  is a block diagram of an optical amplifier of the invention, including a control circuit for reducing a transient variation of optical gain; 
         FIG. 3  are time traces of various electrical signals and the output optical signal in the optical amplifier of  FIG. 2 ; 
         FIG. 4  is a block diagram of the optical amplifier of  FIG. 2 , including an electrical schematic of the control circuitry; 
         FIG. 5  is a block diagram of a method for reducing a transient variation of gain of the optical amplifier of  FIGS. 2 and 3 ; and 
         FIGS. 6A and 6B  are block diagrams of optical amplifier configurations usable with a transient control apparatus of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Referring to  FIG. 2 , an optical amplifier  200  includes first and second optical taps  202  and  207 , respectively, coupled to first and second photodetectors  204  and  209 , respectively, a length of active optical fiber  206  pumped by a pump laser diode  208 , a logarithmic preamplifier  224 , and a feedback controller  215 . The active optical fiber  206  is preferably an erbium-doped optical fiber (EDF). Other active optical fibers, including other rare earth doped active fibers, can be used. In operation, the first optical tap  202  splits off a small fraction  203 , e.g. 1% to 10%, of an input optical signal  214  for detection at the first photodetector  204 . A major fraction  205  of the input optical signal  214  is amplified by the active optical fiber  206 . A small fraction of the amplified light is split off by the second optical tap  207  for detection at the second photodetector  209 . The logarithmic preamplifier  224  amplifies the photocurrent generated by the second photodetector  209 . The pump laser diode  208  is powered by a pump current I PUMP , which is supplied by the feedback controller  215  in dependence upon an output signal  225  of the preamplifier  224 , so as to stabilize the optical power at the output of the active fiber  206 . The feedback control methods of optical amplifiers are well known in the art. The feedback methods can include, for example, a proportional integral-differential (PID) control method. While the feedback controller  215  stabilizes the average output optical power, fast optical transients are nonetheless generated upon a stepwise increase or decrease of the input optical power. The generation of the optical transients is related to dynamics of population inversion in the active optical fiber  206 . 
     In accordance with the invention, a control circuit  250  and a variable optical attenuator  210  are provided for suppressing the optical transients. The function of the control circuit  250  is to generate, based on the tapped signal  203  detected by the photodetector  204 , a control signal  217  to control the attenuation level of the variable optical attenuator  210 , to dynamically attenuate light amplified by the active optical fiber  206 , so as to counterbalance and suppress the optical transients. To achieve transients suppression, the control signal  217  is generated to have a magnitude and a temporal profile corresponding to a magnitude and a temporal profile of a transient change of gain of the optical amplifier  200 , caused by a transient change of optical power of the main part  205  of the optical signal  214  applied to the active optical fiber  206 . 
     Preferably, the control circuit  250  includes a logarithmic amplifier  220  and a high-pass filter  222 , serially coupled as shown in  FIG. 2 . It has been discovered that the serially connected logarithmic amplifier  220  and the high-pass filter  22  provide the control signal  217  closely resembling the transient change of gain of the optical amplifier  200 . Conveniently, the logarithmic amplifier  220  also serves as a preamplifier for the first photodiode  204  at the same time. The controller  212  can also be constructed to include a microprocessor programmed to generate the control signal  217 , although the latter solution is more costly. 
     Adjusting the variable optical attenuator  210  disposed downstream from the active optical fiber  206  results in a more quick and efficient transient suppression, than adjusting the pump current I PUMP  as is commonly done in the prior art. Accordingly, very fast transients, in sub-microsecond time domain, can be suppressed, provided that the logarithmic amplifier  220  and the variable optical attenuator  210  have a fast (for example, sub-microsecond) response time. 
     Referring to  FIG. 3 , a stepwise increase of optical power of the input optical signal  214  due to adding a new optical channel to the signal  214  is detected by the first photodetector  204 , causing a stepwise increase  300  of an output signal  221  of the logarithmic amplifier  220 . At the same time, the stepwise increase of input optical power causes a variation of the optical power of the amplified optical signal, which is detected by the second photodetector  209 . This causes the controller  215  to adjust the pump current I PUMP , so as to reduce the output power back to the original level; as a result, the output signal  225  of the logarithmic preamplifier  224  will show a transient variation  302 . 
     The first electrical signal  221  is filtered by the high-pass filter  222 , producing a pulse  312  in the control signal  217 . A logarithm of the linear attenuation value of the variable optical attenuator  210  is preferably proportional to the magnitude of the control signal  217 . In other words, the attenuation in dB units provided by the variable optical attenuator  210  is preferably proportional to the magnitude of the control signal  217 . Because the magnitude and the temporal profile of the control signal  217  corresponds to the magnitude and the temporal profile of the transient variation  302  of the optical power of the signal amplified by the active optical fiber  206 , the transient  302  of the amplified signal is compensated for, or suppressed. An output transient  304  of the output optical signal  216  is considerably reduced in magnitude. 
     The control circuit  250  can be added to existing optical amplifiers to suppress optical transients. If an existing optical amplifier does not have the input tap  202  or the variable optical attenuator  210 , these can be provided as a part of the controller apparatus for reducing optical transients. Erbium doped optical amplifiers (EDFAs) used in optical communications frequently include input/output taps and/or variable optical attenuators for traditional feedback/feed-forward control. Using the control circuit  250  in addition to the traditional feed-forward/feedback control circuits allows one to significantly reduce optical transients at a moderate cost. 
     Turning to  FIG. 4 , an exemplary implementation of the optical amplifier  200  and the control circuit  250  is shown. The feedback controller  215  of  FIG. 2  is represented by a proportional integral (PI) controller  402  connected to a pump current source  404 . The high-pass filter  222  includes a 33 nF condenser and a 10 k resistor. A repeater  406  and a current source  408  generate the control signal  217  for the variable optical attenuator  210 . A TEC  410  is used to stabilize the temperature of the pump diode  208 . The logarithmic amplifiers  220  and  224  can be based on MAX4206 and AD8304 logarithmic amplifiers. Circles  412  denote spans of optical fiber. 
     Referring now to  FIG. 5  with a further reference to  FIG. 2 , a method  500  for reducing a transient variation of gain of the optical amplifier  200  includes: 
     a step  502  of splitting the input optical signal  214  into first and second optical signals  203  and  205 , respectively; 
     a step  504  of detecting the tapped (first) portion  203  of the optical signal  214  using the photodetector  204 ; 
     a step  506  of amplifying the remaining (second) portion  205  of the optical signal  214  in the active optical fiber  206  pumped by the pump laser diode  208 ; 
     a step  508  of generating, based on the detected tapped portion  203 , the control signal  217  for the variable optical attenuator  210 , the control signal  217  having the magnitude and the temporal profile  312  corresponding to the magnitude and the temporal profile  302  of a transient change of optical gain of the active optical fiber  206  caused by the transient change  300  of optical power of the second portion  205  of the input optical signal  214 ; and 
     a step  510  of applying the control signal  217  to the variable optical attenuator  210 , so as to compensate for the transient variation of gain of the active optical fiber  206 . 
     In one embodiment, the step  508  of generating the control signal  217  includes a step of generating the electrical signal  221  proportional to a logarithm of optical power of the tapped optical signal  203 , followed by a step of high pass filtering the electrical signal  221  to obtain the control signal  217 . 
     Referring now to  FIGS. 6A and 6B , the control circuit  250  of the invention is usable with a variety of existing optical amplifier types. Turning to  FIG. 6A , a two-stage optical amplifier  600 A is shown. The two-stage optical amplifier  600 A includes input and output taps/photodetectors (T/PD)  604 , first and second stage optical amplifiers  606 , the variable optical attenuator  210  disposed in between the first and the second stage optical amplifiers  606 , and a controller  602 A for pumping the active optical fibers of the first and the second stage optical amplifiers  606  and providing feed-forward and/or feedback control according to methods of the prior art. To improve the transient suppression, the control circuit  250  of the invention is connected to the input tap/photodetector  604  and the variable optical attenuator  210 . Preferably, the control circuit  250  includes the logarithmic amplifier  220  and the high-pass filter  222 , shown in  FIG. 2 . 
     Turning now to  FIG. 6B , a single-stage optical amplifier  600 B is shown having one input tap/photodetector  604  and a controller  602 B for feed-forward control and pumping the active optical fiber of the single-stage optical amplifier  606 . To improve the transient suppression, the control circuit  250  of the invention and the variable optical attenuator  210 , disposed downstream form the stage optical amplifier  606 , are added to the single-stage optical amplifier  600 B. The control circuit  250  is connected to the input tap/photodetector  604  and the variable optical attenuator  210 . 
     For any of the optical amplifiers  200 ,  600 A, or  600 B of  FIGS. 2 ,  6 A, and  6 B, the amplification coefficients of the logarithmic amplifier  220  and the RC constant of the high-pass filter  222  should be selected in dependence upon simulated dynamics of transient response. Such simulations are well known to a person skilled in the art. The simulations must include not only the dynamics of the gain transients in the stage optical amplifiers  606 , but also the reaction of the existing feed-forward and/or feedback controls of the optical amplifiers  600 A and  600 B, as the case may be. The reaction time of the variable optical attenuator  210  preferably must be much smaller than the transient response time, or at least, the reaction time of the variable optical attenuator  210  has to be included in the simulations. For each of the optical amplifiers  200 ,  600 A, and  600 B, the inclusion of the control circuit  250  of the invention will provide an improvement of transient performance. 
     For best results, a controller of the invention may be added to each optical amplifier in an optical communication link. Alternatively, a controller of the invention may be provided only for the final amplifier in the link, to protect photodetectors of the receivers from damage.
A controller for reducing a transient variation of gain of an optical amplifier is disclosed. The controller includes a control circuit adapted to generate an electrical signal mimicking optical gain transient variation upon an abrupt change in input loading conditions. The electrical signal is applied to a variable optical attenuator disposed downstream of the active optical fiber of the optical amplifier. The control circuit can be realized in a variety of ways, but preferably it includes a logarithmic amplifier and a high-pass filter sequentially connected. The logarithmic amplifier is connected to an input tap/photodetector, and the high-pass filter is connected to the variable optical attenuator.