Abstract:
This specification describes technologies relating to controlling optical amplifiers. In one implementation, an optical amplifier is provided. The optical amplifier includes a light amplifying medium for receiving an input optical signal and outputting an output amplified signal; a first measuring block for measuring a change in power of the input signal; a pump laser for supplying pump light to the light amplifying medium; and an electronic control for controlling the power of the pump light in response to the measured change in power of input signal to provide an output amplified signal having a substantially constant power for one or more changes in the power of the input signal.

Description:
BACKGROUND 
     The present disclosure relates to optical fiber amplifiers. 
     Conventional optical fiber communications typically use dense wavelength division multiplexing (DWDM). DWDM allows a plurality of light streams having distinct and finely spaced wavelengths to propagate together, e.g., in a single-mode fiber. DWDM therefore provides an increased bandwidth for an optical fiber network. 
     DWDM implementations typically include the use of DWDM filters. DWDM filters can combine (e.g., multiplex) a plurality of separate light streams having finely spaced wavelengths into a single-mode fiber. DWDM filters can also separate (e.g., demultiplex) a combined light stream (e.g., a multiplexed signal) exiting, for example from a fiber, into a plurality of separate light streams each having one or more distinct, spaced wavelengths. 
     Conventional optical networks using DWDM typically include optical amplifiers capable of amplifying multiple light streams simultaneously. Rare earth doped fiber optical amplifiers, e.g., erbium doped fiber amplifiers (EDFA&#39;s), are commonly used in DWDM networks, although other types of optical amplifiers can also be used. When a multiplexed optical signal propagates through an EDFA, each light stream is amplified by a particular amount independently without interaction among the propagating light streams. 
     An erbium doped fiber (EDF) is a form of a single-mode fiber, having a core that is heavily doped with the rare earth element erbium. Conventional EDFA&#39;s include a pump laser. The pump laser provides a pump light to the erbium doped fiber to provide amplification. For example, when pump light at 980 nm or 1480 nm from a pump laser is transmitted into an EDF, erbium atoms absorb the pump light, pushing the erbium atoms into excited states. When stimulated by light streams, for example an input optical signal having particular wavelengths, (e.g., in a C-band (1528-1570 nm) or an L-band (1570-1620 nm)), the excited atoms return to a ground or lower state by stimulated emission. The stimulated emission has the same wavelength as that of the stimulating light (e.g., if the stimulating light has a wavelength of 1528 nm, the stimulated emission will also have a wavelength of 1528 nm). Therefore, the optical signal is effectively amplified as it is propagating through the EDF. Furthermore, the EDF typically amplifies all received light streams regardless of wavelength. 
     The power of the output amplified signal (i.e., output power, P out ) is a function of both the power of the input signal (i.e., input power, P in ) and the power of the pump light (i.e., pump power, P pump ). The output power and the input power are related by a gain G. The gain G is the ratio of the output power to the input power, or:
 
 P   out   =G×P   in ,  (1)
 
where gain G is proportional (linearly or non-linearly) to the pump power P pump .
 
     In some implementations, a given optical amplifier can be configured to maintain a constant gain during operation. For example, an input signal of an EDFA may initially have 20 separate channels or light streams. Later, the input signal may have 18 channels because two channels are dropped before the input signal enters the EDFA (e.g., using a demultiplexing DWDM filter). If the EDFA is supplied with the same pump power, the output power per channel will increase since a smaller number of channels share the same pumped energy. Similarly, if two channels are added such that the input signal now has 22 channels (e.g., using a multiplexing DWDM filter), without changing the pump power, the output power per channel will decrease. To keep the output power per channel constant, the pump power P pump , can be adjusted. Consequently, the total output power P out  varies as the total input power P in  varies, but gain G (e.g., as calculated using Eq. (1)) is constant. 
     However, in some other implementations, a given optical amplifier is configured to maintain a constant output power P out . For example, a DWDM signal can be demultiplexed such that an input signal to an EDFA has only one channel. This single channel input signal is amplified by the EDFA, and the amplified output signal from the EDFA is fed to a detector. 
     The output power level should match the sensitivity and the dynamic range of the detector. If the output power is too low, it cannot be properly detected. Additionally, if the output power is too high, it can cause damage to the detector. When the input power varies, for example, as a result of channels being added or dropped, e.g., in a stage in the network prior to the current EDFA, the output power should be maintained in order to provide the same performance. An abrupt change in the input power can also be caused, for example, by network reconfigurations, failures or recovery from failures. Thus, the gain G varies as the total input power P in  varies, but the total output power P out  in Eq. (1) is constant. In order to keep the output power P out  constant, the pump power P pump  is adjusted, typically using a detected P out . 
     In another example, in some DWDM systems, an input signal to an EDFA can have more than one channel. The output amplified signal from the EDFA is fed as an input signal to the next stage in the network. The total power of the input signal to the next stage needs to be at a level predetermined for that stage. Therefore, the EDFA again has to provide a constant output power P out  instead of a constant gain G. 
     Either a constant gain or a constant output power can be provided by properly controlling the pump power. However, an EDFA has a finite response time to the pump power change. Consequently, transient spikes can occur in the output power that can include a power overshoot or undershoot or both. 
     SUMMARY 
     This specification describes technologies relating to controlling optical amplifiers. 
     In general, one aspect of the subject matter described in this specification can be embodied in an optical amplifier including a light amplifying medium for receiving an input optical signal and outputting an output amplified signal; a first measuring block for measuring a change in power of the input signal; a pump laser for supplying pump light to the light amplifying medium; and an electronic control for controlling the power of the pump light in response to the measured change in power of input signal to provide an output amplified signal having a substantially constant power following a transient period for one or more changes in the power of the input signal. Other embodiments of this aspect include systems and methods. 
     These and other embodiments can optionally include one or more of the following features. The optical amplifier can further include a second measuring block for measuring the level of power of the output signal, where the electronic control additionally controls the power of the pump light in response to the measured level of power of the output signal to provide the output amplified signal having the substantially constant power. The light amplifying medium can be one or more erbium doped fibers. The input signal can include one or more channels, each channel having one or more distinct wavelengths. The first measuring block can include a first photo-detector, where the first photo-detector receives a tapped portion of the input optical signal. 
     In general, one aspect of the subject matter described in this specification can be embodied in a method of controlling the output power of an optical amplifier including the steps of measuring change in input power and, in response to the measured change in input power, controlling the optical amplifier such that a power of an output amplified signal is substantially constant for one or more changes in the input power. Other embodiments of this aspect include systems and apparatus. 
     These and other embodiments can optionally include one or more of the following features. Controlling the optical amplifier can include controlling a power of a pump light supplied to a pump laser such that the power of the output amplified signal is returned to substantially a power prior to the change in input power. Controlling the optical amplifier can include decreasing the power of the pump light when the measured change in input power is positive and larger than a specified transient threshold. Controlling the optical amplifier can include increasing the power of the pump light when the measured change in input power is negative and an absolute value of the change is larger than a specified transient threshold. Controlling the optical amplifier further includes providing a short transient time for returning the output amplified signal to substantially a same power as before the change in input power. 
     The method can further include measuring a power of the output amplified signal; and using both the power of the output amplified signal with the change in the input signal to control the power of pump light. Controlling the optical amplifier can include decreasing the power of the pump light when a measured power of the output amplified signal is larger than a calculated sum of a target output power and a specified tolerance value. Controlling the optical amplifier can include increasing the power of the pump light when a measured power of the output amplified signal is less than a calculated difference between a target output power and a specified tolerance value. The change in input power can be measured including calculating a difference between two detected input power levels separated by a specified time interval. The method can further include using the pump laser to provide an amplifying light to a light amplifying medium, the light amplifying medium including one or more erbium doped fibers. The input power can be a total power of an input signal that includes one or more channels where each channel includes one or more distinct wavelengths. 
     In general, one aspect of the subject matter described in this specification can be embodied in methods including the steps of monitoring an input signal received at an optical amplifier; detecting a change in a power of the input signal; and using the detected change in the input signal to control the optical amplifier such that a power of an output amplified optical signal remains substantially constant following a specified transient period. Other embodiments of this aspect include systems and apparatus. 
     These and other embodiments can optionally include one or more of the following features. Controlling the optical amplifier can include decreasing the power of a pump light when the measured change in input power is positive and larger than a specified transient threshold. Controlling the optical amplifier can include increasing the power of a pump light when the measured change in input power is negative and an absolute value of the change is larger than a specified transient threshold. 
     The method can further include monitoring the power of the output amplified optical signal; and using the detected change in the power of the input signal and monitored power of the output amplified signal to control the optical amplifier. Controlling the optical amplifier can include decreasing a power of a pump light when a measured power of the output amplified signal is larger than a calculated sum of a target output power and a specified tolerance value. Controlling the optical amplifier can include increasing a power of a pump light when a measured power of the output amplified signal is less than a calculated difference between a target output power and a specified tolerance value. 
     Particular embodiments of the subject matter described in this specification can be implemented to realize one or more of the following advantages. The output power of an optical amplifier can be maintaining at a predetermined level regardless of changes in input power. The output power can be maintained while resulting in a relatively small overshoot and/or undershoot and short transient time. A bit error rate and transient time can be reduced relative to a conventional optical amplifier. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a conventional optical amplifier. 
         FIG. 2  shows a flow diagram of a conventional output power control process for the optical amplifier of  FIG. 1 . 
         FIG. 3(   a ) shows an example display of a conventionally controlled output power of an optical amplifier when the input power suddenly increases by 5 dB. 
         FIG. 3(   b ) shows an example display of a conventionally controlled output power of an optical amplifier when the input power suddenly decreases by 5 dB. 
         FIG. 4  shows a block diagram of an example optical amplifier for providing a constant-power output signal for changes in input power. 
         FIG. 5(   a ) shows the first part of a flow diagram of an example output power control process in response to the change in input power. 
         FIG. 5(   b ) shows the second part of a flow diagram of an example output power control process in response to the level of output power. 
         FIG. 6(   a ) shows an example display of a controlled output power of an optical amplifier in response to a 5 dB increase in input power. 
         FIG. 6(   b ) shows an example display of a controlled output power of an optical amplifier in response to a 5 dB decrease in input power. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a conventional optical amplifier  100 . The conventional output-power-controlled optical amplifier  100  includes an EDFA  22 , a photo-detector  30 , a pump laser  32 , and an electronic control  34 . 
     An input optical signal  20  that can include one or more channels is input to the EDFA  22 . An output amplified optical signal  26  from the EDFA  22  is tapped by a tap  24  to the photo-detector  30 . The photo-detector  30 , using a tapped optical signal  28 , measures the total output power P out  of the optical amplifier  100 . The tap  24  can be, for example, a fused fiber coupler. In some implementations, the tap  24  directs substantially 1% to 5% of the output amplified signal to the photo-detector  30 . The majority of the output amplified signal  26  passes through the tap  24  and is fed to a signal detector (not shown) or to a next stage in the network (not shown). The electronic control  34  controls the power of the pump laser  32 . 
       FIG. 2  shows a flow diagram  200  of a conventional output power control process for an optical amplifier (e.g., optical amplifier  100 ). The process  200  can be performed by a controller, for example, by the electronic control  34  of  FIG. 1 . At step  202  a target output power P target  and a tolerance T are set. For example, an input can be received providing instructions to the controller to change the existing P target  and/or T. 
     At step  204  the photo-detector  30  measures the output power P out . At step  206  the measured output power P out  is compared to the sum of the target output power P target  and tolerance T. If P out &gt;P target +T, the power of a pump laser (e.g., pump laser  32 ) is decreased to decrease the power of pump light P pump  (step  210 ), and the process is iterated back to step  206  again. In some implementations, the decreasing step of the pump power P pump  is determined by the value of [P out −(P target +T)]. In some other implementations, the decreasing step can be a fixed value. 
     If P out  is not greater than P target +T, then the process goes to step  208 . At step  208 , if P out &lt;P target −T, the power of the pump laser is increased by an incremental step value to increase the power of pump light P pump  and the process returns to step  208 . In some implementations, the incremental step of the pump power P pump  is determined by the value of [(P target −T)−P out ]. In some other implementations, the incremental step can be a fixed value. 
     When the output power, P out  is no longer less than P target −T, the process returns to step  204  measuring a new output power P out . 
     Because there is a delay typically from 1-100 μs from the moment the pump light is injected into the EDF to the moment stimulated emission occurs to amplify the signal light, the optical amplifier will overshoot or undershoot the output power for some period of transient time. 
       FIG. 3(   a ) shows an example display  300  of a conventionally controlled output power of an optical amplifier (e.g., optical amplifier  100 ) calculated in the event the input power suddenly increases by 5 dB. An overshoot peak is shown of substantially 2.3 dB, and the transient time is substantially 2500 μs. The transient time is the time from when the output power begins to change to when the output power returns to a stable target level (e.g., 0 dB relative power). 
       FIG. 3(   b ) shows another example display  302  of the same conventionally controlled optical amplifier calculated in the event the input power suddenly decreases by 5 dB. The undershoot value is about −3 dB and the transient time is also about 2500 μs. To accommodate the overshoot and undershoot without any resulting damage or other problems, a detector or a network requires a large power margin, which may substantially increase in cost. Furthermore, the long transient time can increase the bit-error-rate (BER) of the optical network. 
       FIG. 4  shows a block diagram of an optical amplifier  400  for providing a constant-power output signal for changes in input power. The optical amplifier  400  includes a light amplifying medium  406  within an EDFA  416 , a first measuring block  426  for measuring the level of the input power, and a second measuring block  428  for measuring the level of the output power. The first measuring block  426  includes a first tap  404  and a first photo-detector  414 . The second measuring block  428  includes a second tap  408  and a second photo-detector  422 . The optical amplifier  400  also includes a pump laser  418  for supplying pump energy to the light amplifying medium  406 , and an electronic control  424  for controlling the power of the pump light. 
     In operation, an input optical signal  402  including one or more channels (e.g., each having one or more distinct wavelengths) is tapped by the first tap  404  to provide a first tapped optical signal  412  to the first photo-detector  414 . The first photo-detector  414 , using the first tapped optical signal  412 , measures the total input power P in (t). 
     The majority of the input signal  402  is passed through the first tap  404  and is incident to the light amplifying medium  406  in EDFA  416  (e.g., EDFA  416  can be similar to EDFA  22  of  FIG. 1 ). The light amplifying medium  406  can include one or more erbium doped fibers. The EDFA  416  can also include other optical elements, for example, couplers, filters, variable optical attenuators, isolators, and other components. 
     The pump laser  418  supplies the light amplifying medium  406  in the EDFA  416  with optical energy to amplify the input optical signal  402 . An output amplified optical signal  410  amplified by the light amplifying medium  406  and leaving the EDFA  416  is tapped by the second tap  408  to provide a second tapped optical signal  420  to the second photo-detector  422 . The second photo-detector  422 , using the second tapped optical signal  420 , measures the total output power P out  from the EDFA  416 . The majority of the output amplified signal  410  is passed through the second tap  408  and is forwarded to a signal detector (not shown) or output to a next stage in the optical network (not shown). 
     The first and second taps  404  and  408  can be, for example, fused fiber couplers. In some implementations, each tap directs substantially 1% to 5% of the optical signal to the respective photo-detector. 
     The first and second photo-detectors  414  and  422 , send outputs P in (t) and P out (t), respectively, to the electronic control  424 . The electronic control  424  controls the pump power P pump (t) in response to the detected values of P in (t) and P out (t) as disclosed below. 
       FIG. 5(   a ) shows a first part of a flow diagram  500  of an example output power control process for the optical amplifier  400  in response to a change in input power. The process  500  can be performed by an electronic control, for example, electronic control  424  as part of optical amplifier  400  of  FIG. 4 . 
     At step  502  a specific target output power P target  and a tolerance T are specified. Additionally, at step  504  an input transient threshold δ and input sampling time interval Δt are also specified. For example, an input can be received that instructs the electronic control to change one or both of the existing P target  and T values. Similarly, another input can be received that instructs an electronic control to change one or both of the existing δ and Δt values. In some implementations, the input sampling time interval Δt can have values ranging from 0.1-10 μs. 
     At step  506  the input power P in (t) is measured, for example, using a photo-detector (e.g., first photo-detector  414 ). At step  508  the detected input power P in (t) is compared to a previous input power P in (t−Δt). A change in input power, α, is computed as:
 
α= P   in ( t )− P   in ( t−Δt ).  (2)
 
     At step  510  the change in input power α is compared to the input transient threshold δ. If the absolute value of α is greater than δ, |α|&gt;δ, then the process continues to step  512 . At step  512  a determination is made as to whether the change in input power is greater than zero, α&gt;0. If the change in input power is greater than zero, at step  514  the power of a pump laser (e.g., pump laser  418 ) is decreased to decrease the power of pump light P pump  as follows:
 
 P   pump ( t )= P   pump ( t−Δt )− A,   (3)
 
where A has a value calculated as a function of α, and the process returns to step  506  where a new input power, i.e., new P in (t) is measured. The previous P in (t) then becomes the new P in (t−Δt) used in determining whether a change in input power has occurred.
 
     If the change in input power is not greater than zero at step  512 , then at step  516  the power of the pump laser is increased to increase the power of pump light P pump  as follows:
 
 P   pump ( t )= P   pump ( t−Δt )+ B,   (4)
 
where B has a value calculated as a function of α, and the process returns to step  506  where a new input power, i.e., new P in (t) is measured. The previous P in (t) then becomes a new P in (t−Δt) used in determining whether a change in input power has occurred.
 
     In some implementations, the values of A and B provide decreasing and increasing incremental steps and are pre-calculated and stored in a lookup table as shown in Table 1 below, where α1, α2, α3, . . . , A1, A2, A3, . . . , B1, B2, B3, . . . , are positive numbers. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 α 
                 A 
                 α 
                 B 
               
               
                   
                   
               
             
             
               
                   
                 α1 
                 A1 
                 −α1 
                 B1 
               
               
                   
                 α2 
                 A2 
                 −α2 
                 B2 
               
               
                   
                 α3 
                 A3 
                 −α3 
                 B3 
               
               
                   
                 .  
                 .  
                 .  
                 .  
               
               
                   
                 .  
                 .  
                 .  
                 .  
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                   
               
             
          
         
       
     
     In some implementations, if α=α1 then A=A1, if α=α2 then A=A2, and so on. Similarly, if α=−α1 then B=B1, if α=−α2 then B=B2, and so on. If α1&lt;α&lt;α2, then A can be interpolated from A1 and A2, and so on. Similarly, if −α2&lt;α&lt;−α1, then B can be interpolated from B2 and B1, and so on. 
     At step  510 , if the determination of whether (|α|&gt;δ) results in “no”, the process goes to step  518 , which is shown in  FIG. 5(   b ). 
       FIG. 5(   b ) shows a second part of the flow diagram  500  of an example output power control process for an optical amplifier in response to the level of output power. At step  518  the output power P out (t) from the optical amplifier is measured, for example, using a second photo-detector (e.g., second photo-detector  422 ). At step  520  the measured output power P out (t) is compared to the sum of the target output power P target  target and tolerance T. If P out (t)&gt;P target +T, at step  524  the power of the pump laser is decreased to decrease the power of pump light P pump  and the process is returned to step  520  to again compare the output power with the sum of the target output power and tolerance value. 
     In some implementations, the decreasing step of the pump power resulting at step  524  is determined by β value, where β is given by:
 
β=| P   out ( t )− P   target |.  (5)
 
     A look up table similar to that shown in Table 1 can be used to identify the value of the decreasing step. In some implementations, the decreasing step is a fixed value regardless of β. 
     At step  520  if P out (t) is not greater than P target +T, then at step  522  a determination is made as to whether P out (t)&lt;P target −T. If P out (t)&lt;P target −T, then at step  526  the power of the pump laser is increased to increase the power of pump light P pump  and the process returns to step  522  to determine again whether P out (t) is still less than P target −T. 
     In some implementations, the incremental step of the pump power is determined by the value of β in Eq. (5), and a table similar to Table 1 can be provided. In some implementations, the incremental step is a fixed value regardless of β. If P out (t) is not less than (P target −T) then process returns to step  518  measuring new output power, i.e., a new P out (t). 
     Because the change in input power is detected before an input signal enters an EDFA and the pump power is adjusted accordingly when the change is detected, the generated overshoot or undershoot at the output power can be significantly reduced. Furthermore, the transient at the power output can be substantially minimized, which in turn can reduce the BER of the network. 
       FIG. 6(   a ) shows an example display  600  of a controlled output power of an optical amplifier in response to the change in input power using an optical amplifier (e.g., optical amplifier  400  of  FIG. 4) . The display  600  shows the controlled output power level calculated in the event the input power suddenly increases (e.g., by 5 dB). An overshoot peak is shown of substantially 0.6 dB. Additionally, the display  600  shows a transient time of less than or equal to 500 μs. 
       FIG. 6(   b ) shows another example display  602  of the same optical amplifier calculated in response to a decrease in input power (e.g., a sudden decrease by 5 dB). An undershoot valley is shown of substantially −0.6 dB. Additionally, the display  602  shows a transient time less than or equal to 500 μs. Compared with  FIGS. 3(   a ) and ( b ), the overshoot and undershoot are suppressed by substantially 1.7 dB (1.5×) and 2.4 dB (1.7×), respectively. Table 2 below shows a comparison of output power obtained by a conventional control process (e.g., as shown in  FIGS. 1-3) , the optical amplifier and control process shown in  FIGS. 4-6 , and an optical amplifier having no control process. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
                 No 
                 Conventional Output  
                 Output Power Control Based 
               
               
                   
                 Output 
                 Power Control 
                 on Change in Input Power and 
               
               
                   
                 Power 
                 No output power 
                 Level of Output Power 
               
               
                   
                 Control 
                 change after 
                 No output power change after 
               
               
                 Input 
                 Output 
                 transient (2500 μs) 
                 transient (500 μs) 
               
             
          
           
               
                 Power 
                 Power 
                 Transient 
                 Transient 
                 Transient 
                 Transient 
               
               
                 Change 
                 Change 
                 Overshoot 
                 Undershoot 
                 Overshoot 
                 Undershoot 
               
               
                   
               
               
                 +5 dB 
                 +5 dB 
                 +2.3 dB 
                 — 
                 +0.6 dB 
                 — 
               
               
                 −5 dB 
                 −5 dB 
                 — 
                 −3 dB 
                 — 
                 −0.6 dB 
               
               
                   
               
             
          
         
       
     
     The overshoot and undershoot produced by the disclosed control process based on the change in input power and the level of output power are smaller than that of conventional process. Thus, to accommodate the overshoot and undershoot, a smaller power margin is required, which may substantially reduce a cost of the system. Furthermore, the transient time is substantially shortened from 2500 μs to 500 μs, which can reduce the bit-error-rate (BER) of the network. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.