Optical amplifier, optical amplifying repeater and transmission apparatus of wavelength division multiplexed signal light using the same

Abstract

Flatness of a gain (optical output level) is maintained even when a wavelength division multiplex signal light (WDM signal light) is amplified in an optical amplification utilizing Raman amplification function. The optical amplifier comprises: two (first and second) pumping light sources for Raman amplification for outputting first and second pumping lights; the wavelength multiplexer for Raman amplification for allowing the first and second pumping lights to be incident on optical fiber transmission paths, where the WDM signal light propagates, in the direction opposite to the propagation direction of the signal light. The signal lights are Raman amplified by the both pumping lights. A wavelength of the first pumping light is set such that the gain of the Raman amplified signal light declines in the right direction, and, on the other side, the wavelength of the second pumping light is set such that the gain declines in the left direction. Gradients for showing a relation between the wavelength and the gain in Raman amplification by the first and second pumping lights are made to be in opposite directions from each other. Optical output levels and the wavelengths of the first and second pumping lights are set such that the gains for the signal lights included in the Raman amplified WDM signal light become substantially equal to each other.

What is claimed is:  
     
         1 . An optical amplifier, comprising: 
 a first pumping light source for Raman amplification, which outputs a first pumping light having a first wavelength;    a second pumping light source for Raman amplification, which outputs a second pumping light having a second wavelength; and    a wavelength multiplexer for Raman amplification, which allows said first and second pumping lights to be incident on an optical fiber transmission path where a signal light propagates,    wherein said signal light is Raman amplified in said optical fiber transmission path by said first and second pumping lights.    
     
     
         2 . The optical amplifier according to  claim 1 , 
 wherein said wavelength multiplexer for Raman amplification allows said first and second pumping lights to be incident on said optical fiber transmission path in an opposite direction to a propagation direction of said signal light.    
     
     
         3 . The optical amplifier according to  claim 1 , 
 wherein said wavelength multiplexer for Raman amplification allows said first and second pumping lights to be incident on said optical fiber transmission path in the same direction as the propagation direction of said signal light.    
     
     
         4 . The optical amplifier according to  claim 1 , 
 wherein said signal light is a wavelength division multiplexed signal light (a WDM signal light), in which the signal lights having a plurality of wavelengths different from each other are wavelength division multiplexed.    
     
     
         5 . The optical amplifier according to  claim 4 , 
 wherein said first wavelength is the wavelength to allow said signal light to be Raman amplified such that the wavelength having a gain by Raman amplification as a peak becomes shorter than the wavelength of the signal light included in said WDM signal light, and    said second wavelength is the wavelength to allow said signal light to be Raman amplified such that the wavelength having the gain by Raman amplification as a peak becomes longer than the wavelength of the signal light included in said WDM signal light.    
     
     
         6 . The optical amplifier according to  claim 5 , 
 wherein a gradient showing a relation between the wavelength and the gain in Raman amplification by said first pumping light and a gradient showing the relation between the wavelength and the gain in Raman amplification by said second pumping light are in opposite directions from each other, and    an optical output level and the wavelength of said first and second pumping lights are respectively set such that the gains for signal lights included in said WDM signal light, on which Raman amplification was performed by said first and second pumping lights, become substantially equal to each other.    
     
     
         7 . The optical amplifier according to  claim 4 , 
 wherein said first pumping light source for Raman amplification comprises:    a first laser diode for oscillating a first light: and    a first wavelength selective optical reflector for transmitting said light of a first wavelength in a specified ratio and reflecting a residual light, which is arranged in a previous step of said first laser diode, and    said second pumping light source for Raman amplification comprises: a second laser diode for oscillating a second light: and    a second wavelength selective optical reflector for transmitting said light of a second wavelength in a specified ratio and reflecting a residual light, which is arranged in a previous step of said second laser diode.    
     
     
         8 . The optical amplifier according to  claim 4 , 
 wherein said first pumping light source for Raman amplification comprises:    a first laser diode for oscillating a first light; and    a first optical filter for selectively transmitting said light of a first wavelength, which is arranged in a previous step of said first laser diode, and    said second pumping light source for Raman amplification comprises:    a second laser diode for oscillating a second light; and    a second optical filter for selectively transmitting said light of a second wavelength, which is arranged in a previous step of said second laser diode.    
     
     
         9 . The optical amplifier according to  claim 4 , 
 wherein said wavelength multiplexer for Raman amplification comprises:    a first pumping light multiplexer for Raman amplification, which transmits said signal light and allows said first pumping light to be incident on an optical fiber, the multiplexer being arranged by connecting to said optical fiber connected to said optical fiber transmission path; and    a second pumping light multiplexer for Raman amplification, which transmits said signal light and allows said second pumping light to be incident on the optical fiber, the multiplexer being arranged by connecting to said optical fiber connected to said optical fiber transmission path.    
     
     
         10 . The optical amplifier according to  claim 4 , 
 wherein said wavelength multiplexer for Raman amplification comprises:    a pumping light multiplexer for Raman amplification, which multiplexes said first and second pumping lights and outputs a multiplexed pumping light; and    a pumping light multiplexer for Raman amplification, which transmits said signal light and allows said multiplexed pumping light to be incident on an optical fiber, the multiplexer being arranged by connecting to said optical fiber connected to said optical fiber transmission path.    
     
     
         11 . The optical amplifier according to  claim 4 , 
 wherein said wavelength multiplexer for Raman amplification comprises:    a polarization combiner for Raman amplification for performing a polarization combination on said first and second pumping lights and outputs the multiplexed pumping light, where said first and second pumping lights respectively output from said first and second pumping light sources for Raman amplification are perpendicular to each other, that is, in a polarization state; and    a pumping light multiplexer for Raman amplification, which transmits said signal light and allows said multiplexed pumping light to be incident on an optical fiber, the multiplexer being arranged by connecting to said optical fiber connected to said optical fiber transmission path.    
     
     
         12 . The optical amplifier according to  claim 4 , further comprising: 
 an optical splitter for splitting a part of said WDM signal light and outputting a split WDM signal light;    signal light extracting means for extracting two signal lights, which have wavelengths different from each other, from each signal light included in said split WDM signal light;    an optical receiver for detecting an optical output level of each of the two signal lights extracted by said signal light extracting means; and    a pumping light output control circuit for controlling any one of the optical output levels and the wavelengths of said first and second pumping lights for Raman amplification according to an amount of difference between said two optical output levels.    
     
     
         13 . The optical amplifier according to  claim 4 , further comprising: 
 an amplification medium for amplifying said signal light;    a third pumping light source for outputting a third pumping light to allow said amplification medium to be in an excited state; and    a wavelength multiplexer for allowing said third pumping light to be incident on said amplification medium.    
     
     
         14 . The optical amplifier according to  claim 13 , 
 wherein said wavelength multiplexer is arranged on a side from which the signal light is input with respect to said amplification medium, and    said third pumping light is allowed to be incident on the amplification medium in the same direction as the propagation direction of said signal light.    
     
     
         15 . The optical amplifier according to  claim 13 , 
 wherein said wavelength multiplexer is arranged on a side to which the signal light is output from said amplification medium, and    said third pumping light is allowed to be incident on the amplification medium in an opposite direction to the propagation direction of said signal light.    
     
     
         16 . The optical amplifier according to  claim 4 , further comprising: 
 an optical isolator for selectively transmitting only light propagating to the same direction as said signal light and obstructs light propagating in the opposite direction, the optical isolator being arranged on a side of said wavelength multiplexer to which said signal light for Raman amplification is output.    
     
     
         17 . The optical amplifier according to  claim 13 , further comprising: 
 an optical splitter for splitting a part of said WDM signal light and outputting a split WDM signal light;    signal light extracting means for extracting two signal lights, which have the wavelengths different from each other, from each signal light included in said split WDM signal light;    an optical receiver for detecting the optical output level of each of the two signal lights extracted by said signal light extracting means; and    a pumping light output control circuit for controlling any one of the optical output levels and the wavelengths of said first and second pumping lights for Raman amplification according to the amount of difference between said two optical output levels.    
     
     
         18 . The optical amplifier according to  claim 17 , 
 wherein said optical splitter is arranged on a side from which said WDM signal light is input to said wavelength multiplexer for Raman amplification.    
     
     
         19 . The optical amplifier according to  claim 17 , 
 wherein said optical splitter is arranged on a side to which said WDM signal light is output from said wavelength multiplexer for Raman amplification.    
     
     
         20 . An optical amplification repeater, comprising: 
 an optical amplifier for a down signal, which is arranged in an optical fiber transmission path for the down signal; and    an optical amplifier for an up signal, which is arranged in an optical fiber transmission path for the up signal,    wherein said optical amplifier for the down signal includes the optical amplifier according to any one of  claims 4  to  12 , and    said optical amplifier for the up signal includes the optical amplifier according to any one of  claims 4  to  12 .    
     
     
         21 . The optical amplification repeater according to  claim 20 , 
 wherein said first pumping light source for Raman amplification, which is provided in said optical amplifier for the down signal, and said first pumping light source for Raman amplification, which is provided in the optical amplifier for the up signal, are a common first pumping light source for Raman amplification, and    said second pumping light source for Raman amplification, which is provided in said optical amplifier for the down signal, and said second pumping light source for Raman amplification, which is provided in said optical amplifier for the up signal, are a common second pumping light source for Raman amplification,    the optical amplification repeater comprising:    an optical splitting coupler for coupling and splitting first and second pumping lights for Raman amplification output from said common first and said common second pumping light sources for Raman amplification and respectively outputting the split pumping lights to said wavelength multiplexer for Raman amplification, which is provided in said optical amplifier for the down signal, and in said wavelength multiplexer for Raman amplification, which is provided in said optical amplifier for the up signal.    
     
     
         22 . An optical amplification repeater, comprising: 
 an optical amplifier for a down signal, which is arranged in an optical fiber transmission path for the down signal; and    an optical amplifier for an up signal, which is arranged in an optical fiber transmission path for the up signal,    wherein said optical amplifier for the down signal includes the optical amplifier according to  claim 13 , and    said optical amplifier for the up signal includes the optical amplifier according to  claim 13 .    
     
     
         23 . The optical amplification repeater according to  claim 22 , 
 wherein said first pumping light source for Raman amplification, which is provided in said optical amplifier for the down signal, and said first pumping light source for Raman amplification, which is provided in the optical amplifier for the up signal, are a common first pumping light source for Raman amplification, and    said second pumping light source for Raman amplification, which is provided in said optical amplifier for the down signal, and said second pumping light source for Raman amplification, which is provided in said optical amplifier for the up signal, are a common second pumping light source for Raman amplification,    the optical amplification repeater comprising:    an optical coupler for coupling and splitting first and second pumping lights for Raman amplification output from said common first and said common second pumping light sources for Raman amplification and respectively outputting the split pumping lights to said wavelength multiplexer for Raman amplification, which is provided in said optical amplifier for the down signal, and said wavelength multiplexer for Raman amplification, which is provided in said optical amplifier for the up signal.    
     
     
         24 . The optical amplification repeater according to  claim 23 , 
 wherein said third pumping light source provided in said optical amplifier for the down signal and said third pumping light source provided in said optical amplifier for the up signal are a common third pumping light source,    the optical amplification repeater comprising:    the optical coupler for coupling and splitting the third pumping lights output from said third common pumping light source and respectively outputting the split pumping lights to said wavelength multiplexer provided in said optical amplifier for the down signal and said wavelength multiplexer provided in said optical amplifier for the up signal.    
     
     
         25 . The optical amplification repeater according to  claim 23 , 
 wherein said optical amplifier for the down signal and said optical amplifier for the up signal further comprises respectively fourth pumping light sources, each of which outputs a fourth pumping light to make said amplification medium in an excited state,    said third pumping light source provided in said optical amplifier for the down signal and said third pumping light source provided in said optical amplifier for the up signal are a common third pumping light source, and    said fourth pumping light source provided in said optical amplifier for the down signal and said fourth pumping light source provided in said optical amplifier for the up signal are a common fourth pumping light source,    the optical amplification repeater comprising:    the optical coupler for coupling and splitting the third pumping light output from said third common pumping light source and the fourth pumping light output from said common fourth pumping light source and respectively outputting the split pumping lights to said wavelength multiplexer provided in said optical amplifier for the down signal and said wavelength multiplexer provided in said optical amplifier for the up signal.    
     
     
         26 . A transmission apparatus of wavelength division multiplexed signal light, comprising: 
 an optical transmission terminal station for wavelength division multiplexing a plurality of signal lights having the wavelengths different from each other and for sending out a wavelength division multiplexed signal light (a WDM signal light);    an optical fiber transmission path for propagating said WDM signal light;    an optical receiving terminal station for receiving said WDM signal light; and    the optical amplifier according to  claim 4 , which is arranged in the midway of said optical fiber transmission path to repeat and amplify said WDM signal light.    
     
     
         27 . A transmission apparatus of wavelength division multiplexed signal light, comprising: 
 an optical transmission terminal station for wavelength division multiplexing a plurality of down signal lights having the wavelengths different from each other and for sending out a down wavelength division multiplexed signal light (a WDM signal light);    an optical fiber transmission path for the down signal light for propagating said down WDM signal light;    an optical receiving terminal station for the down signal light for receiving said down WDM signal light;    an optical transmission terminal station for wavelength division multiplexing a plurality of up signal lights having the wavelengths different from each other and for sending out an up WDM signal light;    an optical fiber transmission path for the up signal light for propagating said up WDM signal light;    an optical receiving terminal station for the up signal light for receiving said up WDM signal light; and    the optical amplifiers according to  claim 13 , which are arranged midway of said up and down optical fiber transmission paths to repeat and amplify said up and down WDM signal lights.