Abstract:
An amplifier characterized by gain and output power comprises: (i) at least one gain medium; (ii) at least one pump supplying optical power into the gain medium; and (iii) a controller controlling the gain and the output power of the amplifier. The controller includes a signal compression circuit to cover a wide dynamic range for optical input and output signals, so that resolution for low optical signals is better than resolution for high optical signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly assigned pending provisional application as the parent application serial No. 60/200690, filed on Apr. 29, 2000 entitled “UNIVERSAL CONTROLLER FOR OPTICAL AMPLIFIERS” and Provisional patent application serial No. 60/196596, filed on Apr. 13, 2000 entitled “METHOD FOR CONTROLLING PERFORMANCE OF OPTICAL AMPLIFIERS”, and provisional application serial No. 60/196784 filed Apr. 13, 2000 in the name of Gerrish et al. and entitled “OPTICAL AMPILIFERS WITH A SIMPLE GAIN/OUTPUT CONTROL DEVICE” which are incorporated by reference, herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to an optical amplifier with an improved electronic controller. This controller operates in the gain or output power control mode and works over wide dynamic range of gain or optical signal powers.  
         BACKGROUND OF THE INVENTION  
         [0003]    In recent years optical amplifiers have undergone considerable transformation. Increased demand for more data transfer resulted in development of wavelength division multiplexing (WDM) technology, which allows more data to be transmitted over one fiber by increased channel count (i.e., a larger number of narrower wavelength ranges within the same predetermined wavelength window). This WDM technology suffers from unwanted effects, such as a variation in gain and output power when the input signal power is constant (for example, due to aging of the amplifier or due to stresses in the amplifier), and cross talk between different channels, for example, when the input signal is modulated at a low frequency. The low frequency is a frequency of up to 100 Hz. This low frequency modulation can be present, for example, due to the addition or dropping of some to the channels, or due to sudden loss of signal at certain wavelengths. These unwanted effects have a negative influence on the power transients (i.e., fluctuations of output optical signal power) of surviving channels, which results in a poor performance of the signal transmission, expressed in an increased bit error rate (BER). In order to minimize the unwanted output signal power fluctuation and the power transients, it is common to introduce a mechanism for controlling either the output signal power or the gain of the optical fiber amplifier. Gain is the ratio of the optical signal output power to the optical signal input power.  
           [0004]    There are several known approaches for controlling output signal power or the gain of the optical fiber amplifier. The first approach, known as the electronic feedback/feed-forward approach, utilizes electronic circuitry to control power transients caused by the change of input power, due to adding or dropping of the optical channels. More specifically, amplifier gain or power is controlled by analog tuning of the electronic components, for example by changes resistor&#39;s or capacitor&#39;s values. This approach allows the user, such as a communication company, to minimize power transients in any given optical amplifier by controlling either the amplifier gain or the amplifier output power, but not both. This approach also limits accuracy of gain control when signal power is small. Finally, this approach does not compensate for amplifier noise, such as ASE (amplified spontaneous emission).  
           [0005]    The second approach, known as the optical feedback control approach, utilizes only optical components to control power transients of the optical fiber amplifier. This approach is even less flexible than the all-electronic approach described above, because any change in power or gain control requirements requires the change in optical components.  
           [0006]    Another approach is to utilize an electronic controller and an additional monitoring channel. This monitoring channel provides a command signal based on information about the change in number of incoming channels. The gain controller, based on these commands, automatically switches between the power control and gain control modes in order to keep amplifier performance within predetermined limits. Furthermore, it is desirable that optical amplifiers operate over a wide dynamic range of input optical power, output optical power or gain while preserving control accuracy. This is especially difficult if a digital controller is utilized and when the input signal is low (less than −28 dBm).  
           [0007]    Erbium doped fiber amplifiers (EDFA) are very important components in the optical communication networks. These amplifiers are widely used to provide gain for wavelength division multiplexed optical communications. When high-speed data (over 2 Gbits/sec) is transmitted over the EDFA, the amplifier gain is nearly undisturbed by the fast signal modulation and there is no cross talk between the communication channels. However, low frequency fluctuation of input signal caused, for example, by adding or dropping one or more channels of WDM signal, can cause considerable fluctuation in the inversion levels of Er doped fibers. The negative effects of low frequency fluctuation of the input signal are multiplied if there more than one EDFA is present in the network.  
         SUMMARY OF THE INVENTION.  
         [0008]    According to one aspect of the present invention an amplifier characterized by gain and output power comprises: (i) at least one gain medium; (ii) at least one pump supplying optical power into the gain medium; (iii) a controller controlling the gain and the output power of the amplifier. The controller includes a signal compression circuit to cover a wide dynamic range for optical input and output signals, so that resolution for low optical signals is better than resolution for high optical signals.  
           [0009]    According to one embodiment of the present invention the controller utilizes a logarithmic circuit. According to another embodiment of the present invention the controller utilizes an electronic gain switch circuit.  
           [0010]    According to one embodiment of the present invention an amplifier characterized by gain and output power comprises: (i) at least one gain medium; (ii) a pump supplying optical power into said gain medium; (iii) a controller controlling said gain and said output power of said amplifier. This controller includes an electronic gain switch to cover a wide dynamic range for optical input and output signals, so that resolution for low optical signals is better than resolution for high optical signals.  
           [0011]    It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic diagram of an optical amplifier, which includes an electronic controller.  
         [0013]    [0013]FIG. 2 is a schematic diagram of exemplary optical amplifier of FIG. 1.  
         [0014]    [0014]FIG. 3 is a schematic diagram of another exemplary optical amplifier.  
         [0015]    FIGS.  4 A- 4 D illustrate the behavior of the amplifier of FIG. 1 during transient regime, when channels are added a dropped.  
         [0016]    FIGS.  5 A- 5 D illustrate the affect of the electronic controller in eliminating cross talk.  
         [0017]    [0017]FIG. 6 is a schematic simulation block diagram of an optical amplifier operating in gain control mode.  
         [0018]    FIGS.  7 A- 7 D illustrate the closed-loop performance of the amplifier with automatic gain control mode.  
         [0019]    [0019]FIG. 8 is a schematic simulation block diagram of an optical amplifier operating in electronic power control mode.  
         [0020]    FIGS.  9 A- 9 D illustrate the closed-loop performance of the amplifier with automatic power control mode. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The negative effects of low frequency fluctuation of input signal to an EDFA can cause cross talk between channels and decrease signal to noise ratio of the amplifier. Therefore, it is crucial to remove or minimize low-frequency fluctuation effects before they affect the amplifier performance. The exemplary amplifiers of the present invention provide suppression of fast (about 50 μseconds or faster) power and gain transients, as well as pump temperature control and re-configurable signal processing. In addition, in order to be able to control gain or power transients, it is preferable that the amplifier&#39;s controller accommodates other communication system requirements and has the ability to perform some additional control function (for example, pump laser temperature control, coil temperature control). Furthermore, the controller may also provide monitoring, self-testing, and other diagnostic functions, for example alarm processing. A remote operator can interact with the controller by remotely issuing the commands and monitoring the performance of the amplifier. These commands and monitoring can be implemented via serial interface, parallel interface, or Ethernet. As stated above, according to the present invention the optical amplifier includes an electronic controller. This electronic controller preferably provides all of the necessary functions by digital and analog electronics and software. The exemplary electronic controller disclosed herein is a two-processor unit embedded system. However, a single processor unit may also be utilized. The electronic controller described in this section is flexible enough to accommodate multiple optical amplifier designs.  
         [0022]    [0022]FIG. 1 illustrates an exemplary EDFA  100 . This amplifier  100  includes separate optical section  110  and electronic controller  120 . FIGS. 2 and 3 show, in more detail, exemplary optical section  10  of the EDFA  100  of FIG. 1. FIG. 2 illustrates an optical section  110  that utilizes a single pump laser. FIG. 3 illustrates an alternative optical section  110  that utilizes two pump lasers. The amount of the optical power, delivered by the EDFA  100  is determined by the electronic gain (or the output power) control section of the electronic controller  120 . In this EDFA  100  the actual amount of input and output optical power is monitored by appropriate photo-diodes  130 . Signals from the photo-diodes  130  are conditioned by the transimpedance amplifiers  140  and converted to a digital form by A/D (Analog to Digital) converters  150 . Driver circuit  145  are situated between the transimpedance amplifiers  140  and the A/D (Analog to Digital) converters  150 . These driver circuits  145  include logarithmic amplification circuits or, alternatively, electronic gain switch circuits. These circuits are described in detail further down in this specification. The electronic controller  120  includes two main sections—automatic gain/power/temperature control (AGC) embedded system section  160  and communication/alarm processing (CAP) embedded system section  170 .  
         [0023]    The AGC section  160  provides high-speed gain/power control algorithms. Its main function is sensing of the input and output optical signals, for example by photo diodes and utilization of this information to provide high-speed control of the pump laser diode. Secondary functions include pump laser diode temperature control and communications implemented by another processor  180 .  
         [0024]    This AGC section  160  relies on a fast fixed-point arithmetic DSP processor  190 , such as Motorola 56311™ processor that has built in a co-processing unit. The AGC section  160  performs all classical feedback loop control tasks such as, for example, reading input and output optical power, calculating gain, comparing gain with its desired value and calculating the control signal to control the laser pump. It may also control the coil temperature, the pump laser temperature to improve gain spectrum. The DSP processor  190  is programmed to work as a multi-rate sampling unit, because of two different control speeds for the gain/power transients and pump temperature. An optional (second) pump laser  192  may be controlled at a different speed from the speed of the principal pump laser  194 . Therefore, the second pump may be controlled either by the AGC section  160  or the CAP section  170 . One and two pump laser control configurations are illustrated in FIGS. 2 and 3, respectively.  
         [0025]    The CAP section&#39;s primary responsibility is to maintain the link from the external user to the optical amplifier  100 . In addition, CAP section  170  maintains communications with the AGC section  160 .  
         [0026]    While capable of performing the tasks of the CAP section  160 , the AGC section  170  is optimized to efficiently complete a high-speed digital control loop. This is accomplished by the analog-to-digital conversion of the appropriate input values (input and output optical power for this case), conducting the proper digital filtering algorithm with optimized software and creating the output control signal (digital-to-analog conversion driving a laser diode pump signal for this case). Other options for the output control signal includes, but are not limited to, PWM (pulse width modulation) for the pump temperature control.  
         [0027]    A challenge that must be overcome in designing the digital controller  120  is problem created by the dynamic range of the input and output signals as well as the desired amplifier gain range. More specifically, the DSP processor  190  has a limited processing speed. The A/D converter has limited number of bits of resolution. Therefore, the controller  120  should provide a balance between the controller sampling speed and the number of bits of resolution of the A/D (analog to digital) converter. The electronic controller  120  of this embodiment may utilize at least two alternative methods to overcome this problem. These methods are signal compression, which utilizes logarithmic amplifiers and electronic gain (range) switching method.  
       Logarithmic Amplification/Compression  
       [0028]    The driver circuits  145  may include, for example, a logarithmic amplification (compression) circuit  195 A (not shown). The Logarithmic amplification (compression) circuit  195 A performs a logarithmic calculation (such as log10, for example) of electrical input or output signals, corresponding to input and output optical powers, with analog electronics. In this way, the wide range of input or output optical powers is compressed and can be represented by a limited number of bits of A/D converter. This compressed electrical signal provided by the logarithmic amplification circuit is sampled and converted to the digital domain. At this point, this digital signal can be processed directly, or decompressed by performing an anti-log function with the digital-processing algorithm. Furthermore, when the electrical signal (corresponding to optical power) is transformed by a logarithmic function, input or output signals corresponding to low optical power can be represented by a larger number of bits, improving the resolution of these signals and therefore, improving the accuracy of gain/output power control.  
       Dynamic Range Switching  
       [0029]    The other option for increasing dynamic range performance involves direct control of the value of the analog input signal that enters the A/D converter. To increase the resolution of the conversion the A/D converter needs to see a higher analog signal (higher voltage) representing optical signal power when optical input signal is small. Thus, section  160  includes A/D converter, a TZA (trans-impedance amplifier) that converts electrical current values, provided to it by a photo-detector, to voltage, and also includes A/D drivers which convert voltage provided by a TZA (trans-impedance amplifier) to a different voltage range suitable as input to the A/D converter. When this approach is utilized, the A/D driver circuit  145  includes a gain switch  195 B. When the optical input signal is small (for example, less than −25 dBm), this gain switch  195 B multiplies (for example by a factor of 16) and thus increases the analog signal provided to the A/D converter, which will increase resolution (conversion accuracy) for the low input signals. Thus, in normal operating mode, when the optical signals are low or not very large, the gain switch of A/D drivers would be set to its maximum value, allowing for the best signal-to-noise ratio.  
         [0030]    If either of the input signals to the drivers  145  (i.e., the signals corresponding to the optical input and output powers, respectively) becomes very large, an overflow condition will result. The digital value sampled by the AGC section  160  will be at it&#39;s maximum, and will not represent the actual value corresponding to the power of the optical signal. The AGC section  160  automatically recognizes that the input signal is in an overflow condition (i.e. the input value being at maximum for a pre-defined, minimum period of time). At this point, the electronic gain of the saturated A/D driver can be reduced with the proper synchronization with the digital processing algorithm. With this gain reduced, the analog input signal to the A/D converter would no longer be in an overflow condition and normal control processing continues. It is preferable that a hysteresis is used to avoid unnecessary, oscillatory gain switching, caused by noise. This noise may be introduced by fluctuations in the optical signal power or can be introduced by electronic circuits.  
         [0031]    Another novel feature of this amplifier involves the usage of fast electronic A/D converters  197  designed for use in a different industry (in this case, the audio industry). The multiple A/D converters  197  are pre-packaged in a single chip, take very little space and are more reliable than if multiple separate A/D chips are used. Therefore, by utilizing a single chip A/D converters  197  we achieve reduced cost and maximum product lifetime. These single chip A/D converters  197  enable simultaneous sampling of electrical signals representing temperature of the pump lasers. This simultaneous sampling allows the AGC section  160  to control the laser diode temperature with minimal processing time. Once the signals representing the input and output optical powers and temperature of the pump lasers are sampled, the digital nature of the AGC section  160  allows easy implementation of different control modes. For example, this controller  120  utilizes two alternative control modes: (i) optical output power control and (ii) optical gain control mode. The control modes are selected by the operator, via remote commands, for example. Only one control mode is used at the time and the switch from one mode to another mode is be made by operator&#39;s command.  
         [0032]    With the controller  120  operating in optical output power control mode, the AGC section  160  must produce the proper control signal to ensure the optical output power of the EDFA is held at a constant or near a constant desired value.  
         [0033]    The amplifier is required to perform well in suppression of output power transients. FIG. 4A shows oscilloscope trace illustrating the transient behavior of amplifier total input and out put powers without the presence of the electronic gain controller. It illustrates the worst-case scenario, where in addition to one existing channel in the amplifier new 32 channels are introduced to the amplifier at t≈5.8 msec. More specifically, curve # 1  of the FIG. 4A shows that a change in total optical input power occurs at about t≈5.8 msec (increased signal). Curve # 2  of the FIG. 4A represents total optical output power. More specifically, this curve shows a large transient spike, at time t≈5.8 msec, corresponding to the change in the input optical power.  
         [0034]    [0034]FIG. 4B represents the behavior of the same amplifier with the functional electronic gain controller. More specifically, it shows that the increase in optical output power is now proportional to the increase in optical input power and the absence of the transient output power spike. It is noted, the gain was held at 20 dB.  
         [0035]    [0035]FIG. 4C illustrates transient behavior of amplifier without the electronic gain controller. It illustrates the worst-case scenario, where the 32 of 33 channels are dropped at time t≈5.8 msec and one channel is left in the amplifier. More specifically, curve # 1  of the FIG. 4A shows that a change in total optical input power occurs at about t≈5.8 msec (decreased signal power). Curve # 2  of the FIG. 4C represents total optical output power. More specifically, this curve shows a large negative transient spike, at time t≈5.8 msec, corresponding to the change in the input optical power.  
         [0036]    [0036]FIG. 4D represents the behavior of the same amplifier with the functional electronic gain controller. More specifically, it shows that the decrease in optical output power is now proportional to the decrease in optical input power and the absence of the transient output power spike.  
         [0037]    [0037]FIG. 5A illustrates the effect of crosstalk on surviving channel(s). More specifically, the vertical axis response to the optical power, and the horizontal axis represents time t. Curve # 1  represents the total input power at any one time. Curve # 2  represents total output power, and curve # 3  represents output power of the surviving channel. From the time period of t=0 to t≈5.8 msec only one signal input power is present (λ c =1533.47 nm). At time t≈5.8 msec additional 32 channels were added (1528 nm≦λ ci ≦1565 nm). Thus, Curve # 1  of FIG. 5A illustrates that the total input optical power went up at time t≈5.8 msec. Curve # 2  corresponds to the total output optical power. Curve # 2  shows a large transient spike at time t≈5.8 msec. Curve # 3  corresponds to the output power of the original dropped channel. Curve # 3  shows that the output power of the original channel dropped when the other channels were added. This is the evidence of undesirable crosstalk between the channels.  
         [0038]    [0038]FIG. 5B illustrates the behavior of the same amplifier with the functional electronic gain control. This figure illustrates that the power of the original channel did not change with addition of other channels. Thus, the controller successfully eliminated crosstalk while preserving the gain of the amplifier at 20 dB.  
         [0039]    [0039]FIGS. 5C and 5D are similar to FIGS. 5A and 5D, but illustrate the behavior of the amplifier when the channels are dropped. FIG. 5C illustrates the effect of crosstalk on surviving channel(s). More specifically, the vertical axis corresponds to the optical power, and the horizontal axis represents time t. Curve # 1  represents the total input power at any one time. Curve # 2  represents total output power, and curve # 3  represents output power of the surviving channel. From the time period of t=0 to t≈5.8 msec a total of 33 channels was present. At time t≈5.8 msec 32 channels were dropped. Thus, curve # 1  of FIG. 5A illustrates that the total input optical power went down at time t≈5.8 msec. Curve # 2  corresponds to the total output optical power. Curve # 2  shows a large negative transient spike at time t≈5.8 msec. Curve # 3  corresponds to the output power of the original dropped channel. Curve # 3  shows that the output power of the original channel increased when the other channels were dropped. This is also the evidence of undesirable crosstalk between the channels.  
         [0040]    [0040]FIG. 5D illustrates the behavior of the same amplifier with the functional electronic gain control. This figure illustrates that the power of the original channel did not change with drop of other channels. Thus, the controller successfully eliminated crosstalk while preserving the gain of the amplifier at 20 dB.  
         [0041]    The controller  120  described in a previous section can be significantly simplified if the amount of signal monitoring, remote commands, and alarm processing is reduced to a minimum. In this case the controller  120  can be a single processor embedded system. It is required that this single processor embedded system includes a high-speed (equal to or greater than 150 MHz clock rate) processor, such as a fast DSP processor. An example of this processor is a Motorola processor 5631™. The controller  120  preferably has at least two control feedback loops—a fast control loop utilized for automatic gain control and a slow loop for pump laser temperature control, or for rear earth doped fiber temperature control, for example. Minimum processing speed in the fast loop cannot be lower than 0.5 MHz rate, while the slow loop can operate at around 1 Hz to 10 kHz. It is preferred that the fast loop operates in the range of 500 kHz to 10 MHz.  
         [0042]    If the high-speed transients suppression is less important than steady-state control of gain or output power and, the amplifier module is required to monitor and process high volume of low speed signals such as remote commands and alarms a slower embedded system, with floating-point arithmetic capability can be used. A typical processor would include a Motorola Power PC™ running an “off the shelf” embedded operating system such as VX Works™. The low frequency characteristics of the signals being processed allow implementation of more computationally demanding control/processing algorithms, such as adaptive control. This system can take care of system adjusting set points, remote commands, alarm processing, gain and temperature control, control parameters tuning, etc.  
       EXAMPLE  
       [0043]    In order to illustrate transient control capabilities of the controller  120  simulations have been done for both for the gain and output power control modes of the amplifier. A classical proportional plus integral (PI) control algorithm is used in both cases. FIG. 6 shows a simulation block diagram corresponding to the amplifier operating in gain control mode. This simulation block diagram depicts the optical section  110  and the electronic controller  120 .  
         [0044]    The optical section  110  includes EDFA  220  and pump laser(s)  221 . At least two optical input signals  222  and  224  are provided to EDFA  220  from the input port  210 . The power provided by the signal  222  is variable. Thus, signal  222  represents adding and dropping channels. The signal  224  has a constant value and represents the surviving channels entering the optical amplifier. The optical section  110  of the amplifier amplifies the signals  222  and  224  and provides an amplified output signal  226 . A small part  226 ′ of the output signal  226  is provided to the controller  120 . A taped input signal  228  is proportional to the total input power provided by lines  222  and  224 . This taped signal  228  is also provided to the controller  120 .  
         [0045]    According to FIG. 6, the electronic controller  120  includes a gain calculation block  230 , analog-to-digital converter (AD) block  240 , gain setpoint block  250 , gain error calculation block  260 , PI control block  270 , digital-to-analog converter (DA) block  280  and pump drive circuit block  290 . The gain calculation block  230  calculates the amplifier gain  232  based on the powers of the taped output and input signals  226  and  228 , respectively. The AD block  240  converts analog value of the gain provided to block  240  by signal  232  to a digital form. This digital value of calculated gain  242  is compared in the gain error calculation block  260  with the gain set point value provided by the gain setpoint block  250 . The calculated gain error  262  enters the PI control block  270 . This block  270  calculates and provides the control signal  272  for the DA converter  280 . This control signal  272  is in a digital form and it is converted into an analog form by the DA converter block  280 . The output  282  of the DA converter block  280  enters the pump drive circuit  290 . This pump drive circuit  290  controls the pump laser  221 . The pump laser  221  provides the amplifier section  110  with necessary optical power, which keeps the amplifier gain  228  equal to the gain set point value  252 .  
         [0046]    FIGS.  7 A- 7 D illustrate the closed-loop performance of the amplifier with automatic gain control mode. (By closed loop we mean that the control algorithm is in place and provides feedback control.) FIGS. 7A, 7B,  7 C illustrate the behavior of the input signal P in , gain set point Gsp and pump power Pp, respectively. More specifically, FIG. 7A illustrates that input signal power drops from −5.23 dBm (0.3 mW) to −8.24 dBm (−0.15 mW) at time t=4 msec. FIG. 7B illustrates a change in the gain setpoint. More specifically, gain setpoint Gsp (desired value of gain) changes from 20 dB to 17 dB at time t= 6  msec. FIG. 7C illustrates that pump laser power drops when the input power drops because the amplifier is in the gain control mode, i.e. the total output optical power is proportional to the total input power. Drop in the pump power is not instant but goes through transient period of 250 microseconds. FIG. 7C also illustrates that the optical power of laser pump drops when the gain setpoint is set at a lower value. Thus, at time t=6 ms, when the gain setpoint drops (see FIG. 7B) and input power does not change, the laser pump power drops in order to decrease the optical output power of the amplifier. FIG. 7D illustrates the effect of gain control on the gain of this amplifier. More specifically, this graph shows that at time t=4 msec, when the input signal power drops, the amplifier gain will go throught a short transient regime (in time period from 4 msec to 4.5 msec). During this period the pump laser will change its value to bring the amplifier gain back to its setpoint value of 20 dB. On the other hand, when the gain setpoint drops from 20 dB to 17 dB (at time t=6 msec) the pump control signal (FIG. 7C) will start to change in order to decrease the actual amplifier gain. After transient period of 0.75 msec, the amplifier gain will be equal to the new set point value o 17 dB.  
         [0047]    [0047]FIG. 8 shows a simulation block diagram corresponding to the amplifier operating in the power control mode. This simulation block diagram depicts the optical section  110  and the electronic controller  120 .  
         [0048]    The optical section  110  includes EDFA  220  and pump laser(s)  221 . At least two optical input signals  222  and  224  are provided to EDFA  220  from the input port  210 .  
         [0049]    The power provided by the signal  222  is variable. Thus, signal  222  represents adding and dropping channels. The signal  224  has a constant value and it represents surviving channels entering the optical amplifier section  110 . Signal  226 , as before, represents the output power of the amplifier.  
         [0050]    The AD block  240  converts analog value of the output power (provided to block  240  by the output signal portion  226 ′) to a digital form. This digital value of output power  242 ′ is compared in the power error calculation block  260  with the output power set point value  244 , provided by the output power setpoint block  243 . The calculated power error  262 ′ enters the PI control block  270 . This block  270  determines the control signal  272 ′ for the pump laser  221 . This control signal  272 ′ is in a digital form and it is converted into an analog form by the DA converter block  280 . The output of this converter block  280  enters the pump laser  221 . The pump laser  221  provides the optical section  110  with necessary optical pump power, which keeps the total output power  226  of the amplifier  100  equal to the output power set point value  244 .  
         [0051]    FIGS.  9 A- 9 D illustrate the closed-loop performance of the amplifier with automatic power control mode. FIGS. 9A, 9B,  9 C illustrate the behavior of the input signal P in , output power set point (i.e. desired amount of output power Po and, pump power signal P p , respectively. More specifically, FIG. 9A illustrates that input signal power dropped from −2.22 dB (approx 0.6 mW) to approx. −4 dBm (0.4 mW) at t=4 msec. FIG. 9B illustrates that the output power setpoint P o,sp  increased from 13 dBm 20 mW (approx 20 mW) to 16 dBm (40 mW) at time t=2.5 msec. FIG. 9C illustrates the resultant changes in pump optical power, P p . More specifically, FIG. 9C illustrates that at time t=2.5 ms when the output power setpoint  244  increases, the pump power  294  also increases in order to drive the output power  226  of the amplifier to its new set point value  244  of 40 mW. The pump power increases again at t=4 msec, when input signal  222  drops. This increase in pump power at t=4 msec is needed because it needs to make up for lost power in the amplifier due to the input signal drop. FIG. 9D corresponds to the actual output power  226  provided by this amplifier. Note that after the output setpoint changed (FIG. 9B) the output of the amplifier starts to increase from 13 dBm to 16 dBm. The transient time (the time needed for output power to change from 13 to 16 dBm) is approximately 250 microseconds. On the other hand, when the total optical input signal power (FIG. 9A) drops from 0.6 to 0.4 mW, the amplifier output power  226  will first go down, and than will go back to its original setpoint value of 16 dBm (40 mW). In this case the transition time is close to 0.5 seconds.  
         [0052]    The exemplary amplifier  100  of the present invention has a controller  120  with a capability of output power and gain transient suppression and temperature control of the laser pump diode. Control requirements, such as set point and control mode are remotely sent to the amplifier  100  via standard interface (serial, parallel or Ethernet). Monitoring signal and alarms from the amplifier are received by using the same interface. Two embedded processors allow high level of flexibility in the choice of control and processing algorithms. The technique of signal compression and dynamic range switching makes this device suitable for use in systems with very wide dynamic range of signals. This particular amplifier  100  utilizes two independently controlled pump lasers. The second pump is used to boost the signal power. The controller  120  may also utilize more sophisticated control algorithms, which can cope with some long-term effects, such as parameter changes caused by the optical component aging.  
         [0053]    Accordingly, it will be apparent to those skilled in the art that various modifications and adaptations can be made to the present invention without departing from the spirit and scope of the invention. For example, the controller  120  can be utilized with non-erbium-doped fiber amplifiers, for example, Tm-doped amplifiers. It may also be utilized in amplification systems that include Raman amplifiers, or planar waveguide amplifiers. It is intended that the present invention covers the modifications and adaptations of this invention as defined by the appended claims and their equivalents.