Abstract:
Disclosed herein is a method for level equalization. In this method, a first optical amplifier having a gain tilt coefficient defined as a change in gain tilt to a unit change in gain is first provided. A second optical amplifier having a gain tilt coefficient different from the gain tilt coefficient of the first optical amplifier is cascaded to the first optical amplifier. The gains of the first and second optical amplifiers are distributed so that a level tilt of light amplified by the first and second optical amplifiers becomes substantially flat. Thus, it is possible to provide a simple method for allowing automatic level equalization by providing a plurality of optical amplifiers having different gain tilt coefficients and distributing the gains of these optical amplifiers so that a level tilt of light amplified by these optical amplifiers becomes substantially flat.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method suitable for wavelength division multiplexing, and more particularly to a method, device, and system for level equalization. 
     2. Description of the Related Art 
     In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying an optical signal or signal light has been put to practical use. 
     An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and a pumping unit for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. 
     For example, an erbium doped fiber amplifier (EDFA) has been developed to amplify signal light having a wavelength band of 1.55 μm where the loss in a silica fiber is minimum. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 μm band or a 1.48 μm band, a gain band including a wavelength band of 1.55 μm can be obtained. 
     Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip. 
     As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain main signal light (WDM signal light), which is output to an optical fiber transmission line. At a receiving end, the main signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data (a main signal) is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity by a single optical fiber can be increased according to the number of WDM channels. 
     In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain of the optical amplifier which characteristic is represented by a gain deviation or gain tilt. For example, in a typical EDFA, it is known that a gain deviation is produced at wavelengths near 1.55 μm. If the gain deviations in a plurality of cascaded EDFAs are accumulated, an optical SNR (signal-to-noise ratio) in a channel included in a low-gain band is degraded. Accordingly, to allow high-quality transmission, it is preferable to flatten the wavelength characteristic of gain of an optical amplifier. 
     A long-haul repeatered transmission system including an optical fiber transmission line for transmitting WDM signal light and a plurality of optical repeaters arranged along the optical fiber transmission line is in practical use. Each optical repeater includes an EDFA for compensating for loss of the optical fiber transmission line. Further, a gain equalizing filter is adopted to equalize the wavelength characteristic of gain of the EDFA. However, it is difficult from the viewpoint of fabrication technique to make the wavelength characteristic of gain of the EDFA precisely coincide with the wavelength characteristic of loss of the gain equalizing filter, causing an equalization residual deviation. In the case of multirepeatered transmission, the equalization residual deviation is accumulated to incur an unneglible interchannel level deviation, causing a degradation in transmission quality. This is a principal factor that limits a transmittable distance and a transmittable wavelength band. 
     On the other hand, there is a possibility that the transmission line loss may change during operation of a system because of aged deterioration of the optical fiber transmission line or repair against a breakage accident of the optical fiber transmission line. To cope with this, each optical repeater performs output maintaining control such that an output level is maintained constant. For example, this control is such that the output level is monitored and the power of pump light to be supplied to the EDFA is controlled according to the result of this monitoring. However, when the power of the pump light for the EDFA is changed for the output maintaining control, the gain of the EDFA changes to result in a change in wavelength characteristic of gain. If this change in wavelength characteristic of gain is accumulated over the length of the transmission line, the transmittable distance and the transmittable wavelength band are limited as similar to the above. 
     For relaxation of these limits, it is necessary to repair the interchannel level deviation at suitable intervals in the transmission line and thereby reduce the interchannel level deviation at a receiving end. For example, a system in practical use employs a method such that a gain equalizing filter for compensating for the accumulation of an equalization residual deviation is inserted in the transmission line at every tens of repeaters. In this method, however, the wavelength characteristics of gain of tens of repeaters are measured at a site where an optical repeater and an optical fiber each provided as a product are connected, and a suitable one is selected from a plurality of gain equalizing filters according to the result of this measuring. Thus, this method is very complex to perform. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a simple method, device, and system for allowing automatic level equalization. Other objects of the present invention will become apparent from the following description. 
     In accordance with a first aspect of the present invention, there is provided a method for level equalization. First, a first optical amplifier having a gain tilt coefficient defined as a change in gain tilt to a unit change in gain is provided. Second, a second optical amplifier having a gain tilt coefficient different from the gain tilt coefficient of the first optical amplifier is cascaded to the first optical amplifier. Then, the gains of the first and second optical amplifiers are distributed so that a level tilt of light amplified by the first and second optical amplifiers becomes substantially flat. 
     Thus, it is possible to provide a simple method for allowing automatic level equalization by providing a plurality of optical amplifiers having different gain tilt coefficients and distributing the gains of these optical amplifiers so that a level tilt of light amplified by these optical amplifiers becomes substantially flat. 
     In accordance with a second aspect of the present invention, there is provided a device comprising first and second optical amplifiers and a control unit. The first optical amplifier has a gain tilt coefficient defined as a change in gain tilt to a unit change in gain. The second optical amplifier is cascaded to the first optical amplifier and has a gain tilt coefficient different from the gain tilt coefficient of the first optical amplifier. The control unit distributes the gains of the first and second optical amplifiers so that a level tilt of light amplified by the first and second optical amplifiers becomes substantially flat. 
     Thus, it is possible to provide a simple device for allowing automatic level equalization by cascading a plurality of optical amplifiers having different gain tilt coefficients and distributing the gains of these optical amplifiers so that a level tilt of light amplified by these optical amplifiers becomes flat. 
     In accordance with a third aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths, and a plurality of optical repeaters arranged along the optical fiber transmission line. At least one of the plurality of optical repeaters includes the device according to the present invention. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a preferred embodiment of the system according to the present invention; 
     FIGS. 2A,  2 B, and  2 C are graphs for illustrating the operation of a gain equalizer in an optical repeater shown in FIG. 1; 
     FIG. 3 is a graph for illustrating a level deviation and a level tilt; 
     FIG. 4 is a graph for illustrating changes in gain tilt of an EDFA (erbium doped fiber amplifier) according to gain; 
     FIG. 5 is a block diagram showing a preferred embodiment of the optical repeater to which the present invention is applied; 
     FIGS. 6A,  6 B, and  6 C are diagrams for illustrating the principle of operation of a tilt monitor shown in FIG. 5; 
     FIGS. 7A,  7 B,  7 C, and  7 D are diagrams showing an example of the operation of the optical repeater shown in FIG. 5; 
     FIGS. 8A,  8 B,  8 C, and  8 D are diagrams showing another example of the operation of the optical repeater shown in FIG. 5; 
     FIGS. 9A,  9 B,  9 C, and  9 D are diagrams showing still another example of the operation of the optical repeater shown in FIG. 5; 
     FIG. 10 is a graph showing a distribution of the gains of an EDF 1  and an EDF 2 ; and 
     FIG. 11 is a block diagram showing another preferred embodiment of the optical repeater to which the present invention is applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. The same reference numerals in similar drawings denote like or similar parts. 
     FIG. 1 is a block diagram showing a preferred embodiment of the system according to the present invention. This system includes a first terminal device  2  as a transmitting end, a second terminal device  4  as a receiving end, an optical fiber transmission line  6  placed between the terminal devices  2  and  4 , and a plurality of optical repeaters arranged along the optical fiber transmission line  6 . The plurality of optical repeaters include a plurality of optical repeaters  8  to which the prior art is applied and at least one optical repeater  10  to which the present invention is applied. 
     The first terminal device  2  includes a plurality of optical transmitters (TX)  12  for outputting optical signals having different wavelengths and an optical multiplexer  14  for wavelength division multiplexing the optical signals output from the optical transmitters  12  and launching resultant WDM signal light into the optical fiber transmission line  6 . 
     Each optical repeater  8  includes an optical amplifier  16  for amplifying the WDM signal light transmitted by the optical fiber transmission line  6 , a feedback loop  18  for feedback controlling the optical amplifier  16  so that an output level from the optical amplifier  16  or the optical repeater  8  is maintained constant, and a gain equalizer (GEQ)  20  having a wavelength characteristic of loss compensating for the wavelength characteristic of gain of the optical amplifier  16 . The configuration and operation of the optical repeater  10  to which the present invention is applied will be hereinafter described. 
     The second terminal device  4  includes an optical demultiplexer  22  for separating the WDM signal light transmitted by the optical fiber transmission line  6  into individual optical signals and a plurality of optical receivers (RX)  24  for receiving the optical signals output from the optical demultiplexer  22 . 
     The operation of the gain equalizer  20  in each optical repeater  8  shown in FIG. 1 will now be described with reference to FIGS. 2A,  2 B, and  2 C. FIG. 2A is a graph showing the wavelength characteristic of gain of the optical amplifier  16 , FIG. 2B is a graph showing the wavelength characteristic of loss (transmittance) of the gain equalizer  20 , and FIG. 2C is a graph showing a wavelength characteristic of gain in the optical repeater  8  as a whole. In the case that the optical amplifier  16  has a wavelength characteristic of gain as shown in FIG. 2A, the gain equalizer  20  having a characteristic substantially opposite to the characteristic shown in FIG. 2A is used as shown in FIG. 2B, thereby making the wavelength characteristic of gain of the optical repeater  8  substantially flat as shown in FIG.  2 C. 
     FIG. 3 is a graph for illustrating a level deviation and a level tilt between channels of WDM signal light. In the system shown in FIG. 1, each optical repeater  8  employs the gain equalizer  20 , so that the wavelength characteristic of gain in each optical repeater  8  is made substantially flat as described above with reference to FIGS. 2A to  2 C. However, providing a gain equalizer  20  having a wavelength characteristic of loss precisely compensating for the wavelength characteristic of gain of the optical amplifier  16  is difficult from the viewpoint of fabrication technique. As a result, the use of the plural optical repeaters  8  causes accumulation of equalization residual error, incurring an interchannel level deviation. As shown in FIG. 3, this interchannel level deviation forms a curve with respect to wavelength, and it can be linearly approximated to a level tilt with respect to wavelength. Accordingly, by making the level tilt substantially flat, the interchannel level deviation can be greatly improved. 
     FIG. 4 is a graph for illustrating changes in gain tilt of an EDFA according to gain. Changes in wavelength characteristic of gain of an EDFA with changes in gain appear as changes in gain tilt with respect to wavelength. In the case that a gain equalizing filter is used, the gain tilt is flat for a normal gain, and becomes downward to the right with respect to wavelength when increasing the gain, whereas becomes upward to the right with respect to wavelength when decreasing the gain. The condition where the gain is higher at longer wavelengths is defined as a positive gain tilt, whereas the condition where the gain is lower at longer wavelengths is defined as a negative gain tilt. Accordingly, when the gain is increased as compared with the normal gain, the gain tilt is negative, whereas when the gain is decreased as compared with the normal gain, the gain tilt is positive. A change in gain tilt to a unit change in gain (tilt [dB/nm]/gain [dB]) is defined as a gain tilt coefficient [dB/dB/nm]. The gain tilt coefficient can be set by adjusting the length of an EDF or the doping concentration of Er. 
     The object of the present invention can be achieved by combining a plurality of optical amplifiers having different gain tilt coefficients to configure the optical repeater  10  shown in FIG.  1 . This will now be described more specifically. 
     FIG. 5 is a block diagram showing a preferred embodiment of the optical repeater to which the present invention is applied. The optical repeater to which the present invention is applied can be used as the optical repeater  10  shown in FIG.  1 . 
     The optical repeater shown in FIG. 5 has an input port  26  for receiving WDM signal light, an output port  28  for outputting amplified WDM signal light, and a first optical amplifier  30  and a second optical amplifier  32  cascaded between the input port  26  and the output port  28 . 
     The first optical amplifier  30  includes an erbium doped fiber  34  (which will be hereinafter referred to also as an “EDF 1 ”) as an optical amplifying medium, and a laser diode (LD) or LD module  36  as a pumping source for supplying pump light to the doped fiber  34 . The first optical amplifier  30  further includes a WDM coupler  38  provided downstream of the doped fiber  34  in respect of a propagation direction of WDM signal light. The pump light output from the laser diode  36  is supplied through the WDM coupler  38  to the doped fiber  34 . 
     The second optical amplifier  32  includes an erbium doped fiber  40  (which will be hereinafter referred to also as an “EDF 2 ”) as an optical amplifying medium, and a laser diode or LD module  42  as a pumping source for supplying pump light to the doped fiber  40 . 
     The second optical amplifier  32  further includes a WDM coupler  44  provided downstream of the doped fiber  40  in respect of a propagation direction of WDM signal light. The pump light output from the laser diode  42  is supplied through the WDM coupler  44  to the doped fiber  40 . 
     An optical isolator  46  is connected between the optical amplifiers  30  and  32  to make it possible to independently control the optical amplifiers  30  and  32  and to prevent undesirable oscillation in each of the optical amplifiers  30  and  32 . Accordingly, the WDM signal light input to the input port  26  is amplified by the first optical amplifier  30 , passed through the optical isolator  46  in the forward direction, and supplied to the second optical amplifier  32 . The WDM signal light further amplified by the second optical amplifier  32  is passed through a gain equalizing filter  48  and a tilt monitor  50  to reach the output port  28 . The gain equalizing filter  48  functions to gain equalize at least one of the optical amplifiers  30  and  32  so as to cancel a level deviation of the light amplified by the optical amplifiers  30  and  32 . The tilt monitor  50  functions to detect a level tilt of the light amplified by the optical amplifiers  30  and  32 . 
     The tilt monitor  50  includes a beam splitter  52  for branching off a monitor beam from the light amplified by the optical amplifiers  30  and  32 , an optical filter  54  to which the monitor beam is supplied, and a photodetector  56  to which light passed through the optical filter  54  is supplied. The optical filter  54  is used for weighting, and its transmittance increases or decreases according to wavelength in a given band. FIGS. 6A to  6 C illustrate the principle of operation of the tilt monitor  50  shown in FIG.  5 . It is assumed that the optical filter  54  has a characteristic such that its transmittance linearly increases with an increase in wavelength. In the case that light having a flat gain tilt is input as shown in FIG. 6A, a photocurrent (PD current) flowing in the photodetector  56  has a normal value. In the case that light having a positive gain tilt is input as shown in FIG. 6B, the PD current has a value higher than the normal value. In the case that light having a negative gain tilt is input as shown in FIG. 6C, the PD current has a value lower than the normal value. 
     In this manner, the level tilt of optical input and the PD current are in 1:1 correspondence in the tilt monitor  50  employing the optical filter  54  and the photodetector  56 , so that the level tilt can be detected with a simple configuration. 
     A control unit is provided to distribute the gains of the optical amplifiers  30  and  32  so that the level tilt of light amplified by the optical amplifiers  30  and  32  becomes substantially flat. In this preferred embodiment, the control unit includes the tilt monitor  50  and a controller  58 . The controller  58  includes an I/O port  60  for inputting an output signal from the tilt monitor  50  after analog/digital conversion of the output signal and outputting a control signal for deciding the power of pump light to be output from each of the laser diodes  36  and  42  after digital/analog conversion of the control signal, a CPU (central processing unit)  62  for performing a computation according to a predetermined rule on gain distribution or the like, a ROM (read-only memory)  64  previously storing a program, data, etc. required for the computation, a RAM (random access memory)  66  for temporarily storing the results of the computation, etc., and a data bus  68  for mutually connecting the I/O port  60 , the CPU  62 , the ROM  64 , and the RAM  66 . 
     The operation of the controller  58  in the optical repeater shown in FIG. 5 will now be described by using actual numerical examples. The initial conditions are such that the gain in the optical repeater as a whole is preset to 10 dB (constant value), the wavelength bandwidth is present to 10 nm, and the normal gain and the gain tilt coefficient of each of the EDF 1  and the EDF 2  are preset as shown in Table 1. Further, it is assumed that the wavelength characteristics of gain of the EDF 1  and the EDF 2  are smoothened by the gain equalizing filter. The gain tilt coefficient shows a gain tilt in the wavelength bandwidth of 10 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 EDF1 
                 EDF2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Normal Gain 
                   5 dB 
                   5 dB 
               
               
                   
                 Gain Tilt 
                 0.5 dB/dB/10 nm 
                 0.1 dB/dB/10 nm 
               
               
                   
                 Coefficient 
               
               
                   
                   
               
             
          
         
       
     
     FIGS. 7A to  7 D and Table 2 show an example of the operation of the optical repeater shown in FIG.  5 . In this example, the input signal light has a flat level tilt. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Input Signal 
                 EDF1 
                 EDF2 
                 Repeater 
                 Output 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Gain 
                   
                 5 dB 
                 5 dB 
                 10 dB 
                   
               
               
                 Tilt 
                 0 dB/10 nm 
                 0 dB/ 
                 0 dB/ 
                 0 dB/ 
                 0 dB/ 
               
               
                   
                   
                 10 nm 
                 10 nm 
                 10 nm 
                 10 nm 
               
               
                   
               
             
          
         
       
     
     FIGS. 8A to  8 D and Table 3 show another example of the operation of the optical repeater shown in FIG.  5 . In this example, the input signal light has a level tilt of +2 dB. The gain of the EDF 1  is set higher than the normal gain, and the gain of the EDF 2  is set lower than the normal gain, thereby obtaining a characteristic such that the gain tilt in the optical repeater as a whole is reverse in sign to the level tilt of the input signal light. As a result, the level tilt of the output signal light can be made substantially flat. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Input Signal 
                 EDF1 
                 EDF2 
                 Repeater 
                 Output 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Gain 
                   
                 10 dB 
                 0 dB 
                 10 dB 
                   
               
               
                 Tilt 
                 +2 dB/ 
                 −2.5 dB/ 
                 +0.5 dB/ 
                 −2 dB/ 
                 0 dB/ 
               
               
                   
                 10 nm 
                 10 nm 
                 10 nm 
                 10 nm 
                 10 nm 
               
               
                   
               
             
          
         
       
     
     FIGS. 9A to  9 D and Table 4 show still another example of the operation of the optical repeater shown in FIG.  5 . In this example, the input signal light has a level tilt of −1 dB. The gain of the EDF 1  is set lower than the normal gain, and the gain of the EDF 2  is set higher than the normal gain, thereby obtaining a characteristic such that the gain tilt in the optical repeater as a whole is reverse in sign to the level tilt of the input signal light. As a result, the level tilt of the output signal light can be made substantially flat. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Input Signal 
                 EDF1 
                 EDF2 
                 Repeater 
                 Output 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Gain 
                   
                 2.5 dB 
                 7.5 dB 
                 10 dB 
                   
               
               
                 Tilt 
                 −1 dB/ 
                 +1.25 dB/ 
                 −0.25 dB/ 
                 +1 dB/ 
                 0 dB/ 
               
               
                   
                 10 nm 
                 10 nm 
                 10 nm 
                 10 nm 
                 10 nm 
               
               
                   
               
             
          
         
       
     
     Letting G1 and G2 denote the gain of the EDF 1  and the gain of the EDF 2 , respectively, the following simultaneous equations are given under the conditions shown in Table 1. 
     
       
           G 1+ G 2=10 
       
     
     
       
         −1×(level tilt of optical input)=0.5×( G 1−5)+0.1×( G 2−5) 
       
     
     Referring to FIG. 10, there is shown a distribution of the gains of the EDF 1  and the EDF 2  according to the level tilt of optical input. The graph shown in FIG. 10 is given by the solution of the simultaneous equations mentioned above. In this manner, by cascading the EDF 1  and the EDF 2  having different gain tilt coefficients to thereby set a gain distribution to be determined uniquely according to the level tilt of optical input given, the level tilt of optical output can be made substantially flat. 
     For example, in the optical repeater shown in FIG. 5, the controller  58  can feedback control the power of pump light to be output from each of the laser diodes  36  and  42  so that the level tilt detected by the tilt monitor  50  becomes substantially flat. Alternatively, the tilt monitor  50  may be provided on the input side to perform feedforward control such that the above-mentioned simultaneous equations are solved by the CPU  62  to set the power of pump light to be output from each of the laser diodes  36  and  42  so that the gains of the EDF 1  and the EDF 2  are properly distributed. 
     FIG. 11 is a block diagram showing another preferred embodiment of the optical repeater to which the present invention is applied. This preferred embodiment is similar to the preferred embodiment shown in FIG. 5 in the point that the first optical amplifier  30 , the optical isolator  46 , the second optical amplifier  32 , and the gain equalizing filter  48  are cascaded between the input port  26  and the output port  28 . The preferred embodiment shown in FIG. 11 employs a first feedback loop for controlling the gain of the optical amplifier  30  so that the level tilt of light amplified by the optical amplifiers  30  and  32  becomes substantially flat, and a second feedback loop for controlling the gain of the optical amplifier  32  so that the output level from the optical amplifier  32  becomes substantially constant. 
     The first feedback loop includes a beam splitter  70  and a tilt sensor  72  each for detecting the level tilt of light amplified by the optical amplifiers  30  and  32 , and a control circuit  74  for adjusting the power of pump light to be output from the laser diode  36  so that the level tilt detected above becomes substantially flat. 
     The second feedback loop includes a beam splitter  76  and a photodetector  78  each for detecting the output level from the optical amplifier  32 , and a control circuit  80  for adjusting the power of pump light to be output from the laser diode  42 . 
     The beam splitter  70  and the tilt sensor  72  may be configured like the tilt monitor  50  shown in FIG.  5 . 
     Also according to this preferred embodiment, the level tilt of optical output can be made substantially flat irrespective of the level tilt of optical input in accordance with the above-mentioned principle. 
     Preferably, the time constant of the first feedback loop is set larger than the time constant of the second feedback loop. In this case, when signal light having a level tilt is input, the output level is first controlled to a constant value by the second feedback loop, and the level tilt is next compensated. Accordingly, the level tilt can be made substantially flat in the condition where the output level is always constant. 
     In this preferred embodiment, the EDF 1  and the EDF 2  are used as an optical amplifying medium. Accordingly, by setting the wavelength of pump light to a 0.98-μm band (0.96 to 1.00 μm) or a 1.48-μm band (1.46 to 1.50 μm), a gain band including 1.55 μm can be obtained. 
     In each preferred embodiment mentioned above, the WDM couplers  38  and  44  are provided downstream of the EDF 1  and the EDF 2 , respectively. Accordingly, WDM signal light and pump light propagate in opposite directions in each of the EDF 1  and the EDF 2  (backward pumping). Alternatively, the WDM couplers  38  and  44  may be provided upstream of the EDF 1  and the EDF 2 , respectively, to make WDM signal light and pump light propagate in the same direction in each of the EDF 1  and the EDF 2  (forward pumping). Further, the forward pumping and the backward pumping may be combined to perform bidirectional pumping. However, to make it possible to independently control the pump lights in the optical amplifiers  30  and  32  by using the optical isolator  46 , the backward pumping is suitably adopted. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.