Abstract:
Methods and apparatus for amplifying a signal are provided. In one aspect, a method is provided for amplifying an optical signal using an EDFA and includes amplifying an input signal using an EDFA producing an amplified output signal; measuring a pump residual power component of the amplified output signal, and using the measured pump residual power component to adjust a performance of the EDFA.

Description:
The present invention relates generally to optical technology. 
   BACKGROUND OF THE INVENTION 
   Optical fiber amplifiers are commonly used in communication systems. Examples of optical fiber amplifiers include Erbium Doped Fiber Amplifiers (“EDFA”) and other type of Rare Earth Doped Fiber Amplifiers. The optical fiber amplifiers are usually pumped by one or more light emitter diodes (LEDs) or lasers.  FIG. 1  illustrates a configuration for an EDFA  100  that is optically pumped by a light source, such as, a laser. 
     FIG. 1  illustrates a forward pumping scheme. With a forward pumping scheme, the optical signal to be amplified and the pumping light travel in a same direction. A first pump light Pl received from pump laser  102  is transmitted to a wave division multiplexing (WDM) coupler  108  as pumping light. An input optical signal Si, after passing through a tap  104  and an isolator  106  is coupled to WDM coupler  108 . WDM coupler  108  combines the input optical signal Si and the pumping light Pl and provides an output to EDFA  100 . Input optical signal Si is amplified in EDFA  100 , and becomes output light So. Output light So is coupled to a tap  112  whose primary output is an amplified version of the input signal Si. The tap ports of tap  104  and tap  112  are coupled to detectors  110  and  114  whose outputs are coupled to a feedback circuit, e.g., PCB  120 . Using the input power measured at detector  110  and the output power as detected at detector  114 , the gain for EDFA  100  can be adjusted. 
   In the example shown in  FIG. 1 , two photodiodes (detectors  110  and  114 ) at respective input and output ports of EDFA  100  are used to detect input power Pin and output power Pout, respectively. The gain of EDFA  100  can be controlled by the pump power associated with the pumping light P 1 . More specifically, detector  110  measures the total input power Pin associated with the signal Si provided to the EDFA. Detector  114  detects the total output power Ptol of EDFA  100 , which contains signal power Ps, amplified spontaneous emission (ASE) power Pase and pump residual power Ppr. In this example, signal gain is Pout/Pin=(Ptol−Pase−Ppr)/Pin. Conventional systems assume a correction factor for ASE power and pump residual power in order to produce accurate gain control, for example, in systems that require constant gain. If the wavelength of input signal is known, the correction factor can be determined accurately, thus the gain control is accurate. But if the wavelength of input signal is unknown, there exist problems in certain applications. More specifically different wavelength signals produce different ASE power and pump residual power for a given gain setting. When EDFA  100  is used in applications such as line-amplifiers and boosters, the output signal power, in general, is much larger than the ASE power and pump residual power. Accordingly, a near constant correction factor can be used to provide accurate gain control. But for EDFA applications such as preamplifiers with small input power (for example: −38 dBm) and smaller output power (for example: −14 dBm), the ASE power at the output port of the EDFA is larger than the output signal power. In these applications, the use of a constant correction factor will not produce accurate gain control. Further, the amount of pump residual signal Ppr depends on numerous factors including the power of the input signal, the wavelength of the input signal, and the performance of the amplifier and the WDM splitters. Again, a constant correction factor will not produce accurate gain control as the EDFA is used in different applications. What is desirable is an amplifier that automatically compensates for the pump residual and ASE in the output signal no matter the application. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention provides an integrated optical fiber amplification system. The integrated optical fiber amplification system includes an optical amplifier comprising an optical signal input for receiving an input optical signal to be amplified; a pumping laser input for receiving a pumping laser input signal for use in amplifying the input optical signal; means for measuring a power of the pumping laser input signal; a combiner for combining the pumping laser input signal and the input optical signal; an EDFA having an input coupled to the output of the combiner and an output coupled to a splitter, the splitter dividing out a portion of the signal output from the EDFA and attributable to a pump residual power of the pumping laser after amplification by the EDFA; means for measuring the pump residual power; and feedback means for adjusting a current of the pumping laser using the residual power and the pumping laser input signal power. 
   Aspects of the invention can include one or more of the following features. The optical amplifier can comprise a pumping laser having a first frequency and coupled to the pumping laser input. The means for measuring a power can be a photodiode. The optical amplifier can further comprise a gain flattening filter coupled to an output of the splitter for receiving and filtering a remainder signal attributable to an amplified input signal received from the splitter and providing a flattened output signal. The optical amplifier can further comprise a variable optical attenuator coupled to the output of the gain-flattening filter (GFF) for variably adjusting a received signal to achieve constant power output. The combiner can be a wave division multiplexing (WDM) combiner. The WDM combiner can combine an input signal of substantially 1550 NM with a pumping laser input of substantially 980 NM. The splitter can be a wave division multiplexing (WDM) splitter. The WDM splitter can split an output of the EDFA into a first signal having a first frequency and a second signal having a second frequency, where the first signal has a frequency that is substantially 1550 NM and the second frequency is substantially 980 NM. The WDM splitter can split an output of the EDFA into a first signal having a first frequency and associated with an amplified version of the input signal and a second signal having a second frequency and associated with the pumping laser signal. 
   In another aspect, an optical amplifier is provided that includes an optical signal input for receiving an input optical signal to be amplified; a pumping source input for receiving a pumping source input signal for use in amplifying the input optical signal; a combiner for combining the pumping source input signal and the input optical signal; an EDFA having an input coupled to the output of the combiner and an output coupled to a splitter, the splitter dividing out a portion of the signal output from the EDFA and attributable to a pump residual of the pumping laser after amplification by the EDFA; and error correction means for measuring the pump residual and adjusting the pumping input signal provided by the pumping source. 
   Aspects of the invention can include one or more of the following advantages. The optical amplifier can comprise a pumping laser having a first frequency and coupled to the pumping source input. The error correction means can include a photodiode for measuring a power of a pump residual. A gain flattening filter (GFF) can be coupled to an output of the splitter for receiving and filtering a remainder signal attributable to an amplified input signal received from the splitter and providing a flattened output signal. The optical amplifier can further comprise a variable optical attenuator coupled to the output of the GFF for variably adjusting a received signal to achieve constant power output. The combiner can be a wave division multiplexing (WDM) combiner. The WDM combiner can combine an input signal of substantially 1550 NM with a pumping source input of substantially 980 NM. The splitter can be a wave division multiplexing (WDM) splitter. The WDM splitter can split an output of the EDFA into a first signal having a first frequency and a second signal having a second frequency, where the first signal can have a frequency that is substantially 1550 NM and the second frequency can be substantially 980 NM. The WDM splitter can split an output of the EDFA into a first signal having a first frequency and associated with an amplified version of the input signal and a second signal having a second frequency and associated with the pumping source signal. 
   In another aspect, an optical amplifier is provided including an optical signal input for receiving an input optical signal to be amplified; a pumping source input for receiving a pumping source input signal for use in amplifying the input optical signal; an EDFA operable to use the pumping source input signal to amplify the input optical signal producing an output optical signal; and an error correction controller for measuring the pump residual and adjusting the pumping input signal provided by the pumping source. 
   In another aspect, a method is provided for amplifying an optical signal using an EDFA and includes amplifying an input signal using an EDFA producing an amplified output signal; measuring a pump residual power component of the amplified output signal, and using the measured pump residual power component to adjust a performance of the EDFA. 
   Aspects of the invention can include one or more of the following advantages. An integrated optical fiber amplification system is provided that automatically compensates for pump residual power when determining gain. The proposed amplification system can automatically compensate for the error introduced due to the pump residual. The proposed amplification system can be used for constant gain applications without requiring reconfiguration to compensate for errors. The proposed amplification system can automatically compensate for error correction introduced when signal inputs of different levels are amplified. 
   In an implementation using an EDFA, the invention can include one or more of the following advantages. By keeping the ratio of pump residual to injected pump power in the amplifier cavity constant, the spectral gain profile of the EDFA is conserved notwithstanding the input-channel distribution injected in the EDFA. Accuracy of gain control only depends on output spectral profile of the EDFA, and can be 1 dB of flatness without care of the wavelength and power of the input signal. The detected parameters in the implementation shown are input power, pump power and pump residual power. These parameters are independent of the ASE and can be detected accurately. The gain control accuracy only depends on the flatness of the output optical spectrum because the ratio only depends on the flatness. Accordingly, the effect of ASE on gain control accuracy can be eliminated. Other advantages will be readily apparent from the attached figures and the description below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an Erbium Doped Fiber Amplifier. 
       FIG. 2  illustrates an integrated amplification system for an optical signal. 
       FIG. 3  illustrates a flow chart for an operation of the electrical controller of the integrated amplification system of FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates to an improvement in optical technology. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the invention will be readily apparent to those skilled in the art and the generic principals herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principals and features described herein. 
   The present invention will be described in terms of an integrated amplifier having specific components having specific configurations. Similarly, the present invention will be described in terms of components having specific relationships, such as interconnections between components. However, one of ordinary skill in the art will readily recognize that the devices and systems described can include other components having similar properties, other configurations, and other relationships between components. 
     FIG. 2  illustrates an integrated amplification system  200  for optical signals. The integrated amplification system shown is configured for a forward pumping application. The integrated amplification system includes isolators  204  and  214 , wave division multiplexers (WDM)  206  and  210 , amplifier  208  and pump source  228 . In addition, various monitoring and control components are provided including photo detectors  220 ,  224 , and  226 , taps  202  and  222  and electrical controller  230 . In the implementation shown, a gain flattening filter  212  is provided as well. The operation of the gain flattening filter  212  is described in greater detail below. 
   Integrated amplification system  200  includes a first port for receiving an input signal Si and a second port for providing an output signal So. The input signal is provided as an input to tap  202 . In one implementation, tap  202  is a 1550 nm tap that includes a primary output and a tap output. The primary output of tap  202  is coupled to an input of isolator  204 . The tap output from tap  202  is provided as an input to photo detector  220 , whose output is provided as an input to electrical controller  230 . Photo detector  220  can be a photodiode and measures the input power of input signal Si. 
   The output of isolator  204  is coupled to an input of WDM  206 , whose second input is provided by pump source  228 . Pump source  228  can be a pump laser, a light emitting diode or other source. Pump source  228  receives a control signal from electrical controller  230  for varying the performance of the pump source. The control signal can be a signal for increasing or decreasing the pump current. The output of pump source  228  is provided as an input to tap  222 . The primary output of tap  222  is coupled to the second input of WDM  206 . The tap output from tap  222  is provided as an input to photo detector  224 , whose output is provided as an input to electrical controller  230 . Photo detector  224  can be a photodiode and measures the injected pumping signal power. 
   WDM  206  is a combiner and operates to combine the input signal Si and the injected pumping signal provided from the pumping source  228  and provides an output signal to amplifier  208 . In one implementation, WDM  206  combines an input signal, at for example 1550 nm, with an injected pumping signal, at for example 980 nm. Amplifier  208  can be an EDFA. 
   The output from amplifier  208  is provided as an input to WDM  210 . WDM  210  is configured to isolate the pump residual signal from the amplified output signal (and any noise, i.e., ASE) and includes two output ports. In one example, WDM  210  splits the signal received from amplifier  208  into two components, for example, a first component at 980 nm reflecting the pump residual signal and a second component at 1550 nm reflecting the amplified output signal and a noise component (ASE). The first output port of WDM  210  (the residual output) is coupled to an input of photo detector  226 , whose output is provided as an input to electrical controller  230 . Photo detector  226  can be a photodiode and measures the pump residual power of the pumping signal after use by amplifier  208 . In the implementation shown, no direct measurement is made of the output power of the amplified signal So. 
   The second output of WDM  210  provides an amplified signal with ASE (but without residual pumping power) to an isolator  214 . The output of isolator  214  is coupled to a second port of the amplification system  200  providing an output signal So. In the implementation shown, one output of WDM  210  is coupled to an input of a gain-flattening filter  212 , whose output is in turn coupled to an input of isolator  214 . The gain flattening filter  212  can be used in implementations that require a particular degree of flatness in the performance of the amplification system, for example within 1 db of flatness over a given spectrum. 
   Operation 
   An input signal Si is provided as an input to tap  202 . Tap  202  taps off a small portion of the input signal to allow for the measurement of the input power of the input signal. The remainder of the signal is provided as an input to isolator  204 . Isolator  204  prevents signals from flowing back out the input port. The input signal is provided from the output of the isolator  204  as an input to WDM  206  where it is combined with the injected pumping signal from pump source  228 . The injected pumping signal provided by pump source  228  is measured using tap  222  and photo detector  224  and provided as an input to electrical controller  230 . The combined injected pumping and input signals are provided to amplifier  208  whose output produces an amplified signal having two components, an amplified version of the input signal (and ASE) and a pump residual signal. The pump residual signal is taped off using WDM  210  and measured using photo detector  226 . The amplified version of the input signal (and ASE) is isolated by isolator  214  (providing isolation from signals entering the output port) and provided as an output signal So. Electrical controller  230  can be used to adjust the performance of the pumping source in accordance with the data derived from measurements associated with the input power, the injected pumping power and the pump residual power. 
   Feedback Operation 
   Referring to  FIGS. 2 and 3 , a method  300  is shown for adjusting the pumping source using one implementation of the invention. 
   In step  302 , the input power of the input signal is detected, for example using photo detector  220 . In step  304 , a ratio R is defined of the residual pump power and the injected pumping power of the pumping signal for a given application. The ratio R is a design feature typically associated with the performance of a given EDFA. In this way, the ratio R can be seen as an expected ratio that reflects the designed performance of the amplifier. When an EDFA is designed and built, the ratio R has been assumed. In general, a fitting linear equation can be used to calculate the ratio when different input power is considered. Calibration may be required to fit the linear equation. In one implementation, R is a design parameter for special gain. For example, when an EDFA&#39;s gain is designed as G (the gain desired), the ratio of residue power to initial pumping power should be R, which is set as a reference. As will be discussed below, if the measured real ratio RR is not equal to R (or within a tolerance), the pump power will be adjusted until the measured ratio is within the tolerance. 
   In step  306 , the power for the pumping source (i.e., pumping source  228 ) is turned on. In step  308 , the real ratio of the injected pumping power Pp and the pump residual Ppr is measured. In the example shown, a measurement is made using photo detectors  224  and  226  of the injected pumping power Pp and the pump residual Ppr, respectively. The real ratio RR of pump residual to injected pump power is RR=Ppr/Pp. 
   In step  310 , a check is made to determine if the difference of the real ratio RR and the ratio R is less than a tolerance value (e.g., using a absolute value of the difference and a positive variable for the tolerance amount). If not, the process proceeds to step  312 . If the difference in step  310  is less than the tolerance level, then the process ends at step  318 . The tolerance level can be preset and stored in the electrical controller  230 . 
   If the difference is not less than the tolerance, the process continues at step  312  where a check is made to determine which is greater, the ratio R or the real ratio RR. If the real ratio is greater than the ratio R, then the process continues at step  314  where the pump source current is decreased (e.g., electrical controller  230  adjusts or sends a control signal to the pumping source  228  to decrease the pumping current). Alternatively, if the real ratio is less than the ratio R, then the process continues at step  316  where the pump source current is increased (e.g., electrical controller  230  adjusts or sends a control signal to the pumping source  228  to increase the pumping current). After each of steps  314  and  316 , control passes to step  308  where the real ratio RR is again determined based on the new pumping conditions. 
   A method and system has been disclosed for providing an integrated amplification system. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. For example, for an Erbium Doped Fiber Amplifier EDFA, the input optical signal can have a wavelength of 1550 nm, and the pump light can have a wavelength of 980 nm. The pump light signal can also have a wavelength of 1480 nm or a few other wavelengths. Though only a forward pumping implementation is shown, the invention has applicability to backward pumping applications. In the backward pumping applications, the pump residual is isolated from the amplified signal and used in adjusting the performance of the amplification system. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.