Abstract:
To suppress S/N degradation due to optical amplification and generation of a nonlinear optical effect in a dispersion compensating fiber, and compensate for a loss due to the dispersion compensating fiber without incorporating a long fiber inside a receiver, part of an optical transmission line is constructed by a dispersion compensating fiber and at least part of the optical transmission line is designed to have a Raman gain or a distributed gain. A pump light to obtain a Raman gain or a distributed gain is transmitted from at least one of a transmitter, a receiver, and a repeater to the optical transmission line.

Description:
This application is based on patent application Ser. No. 10-318106(1998) filed on Nov. 9, 1998 in Japan, the content of which is incorporated hereinto by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical transmission system using an optical transmission line partly having a gain, in which a group velocity dispersion is not uniform with respect to a longitudinal direction. 
     2. Description of the Prior Art 
     When transmitting a high-speed, large-capacity optical signal, a deterioration of optical waveform due to a group velocity dispersion of a transmission optical fiber is a problem. To compensate for this waveform deterioration, it is effective to install a dispersion compensating fiber in a receiver, which fiber is designed to cancel the dispersion and dispersion slope of the transmission optical fiber. 
     A loss due to a dispersion compensating fiber is in general so large that cannot be neglected. To compensate for this loss, it is effective to install a lumped optical amplifier inside the receiver, or induce the distributed gain, such as a Raman gain to a dispersion compensating fiber using the optical pumping source which is installed in the receiver. 
     FIG. 6 shows the configuration of a prior art optical transmission system in which a dispersion compensating fiber is followed by an optical amplifier. In FIG. 6, numeral  1  indicates a transmitter,  2  is a receiver,  3  is an optical fiber forming an optical transmission line,  11  is an optical transmission circuit,  21  is a dispersion compensating fiber,  22  is an optical amplifier, and  23  is an optical receiver circuit. 
     Signal light received by the receiver  2 , after being dispersion compensated by the dispersion compensating fiber  21 , is amplified by the optical amplifier  22 , and received by the optical receiver circuit  23 . FIG. 6 also shows a power diagram, showing that the input power to the optical amplifier  22  is considerably reduced due to a loss of the dispersion compensating fiber  21 . 
     FIG. 7 shows the configuration of a prior art optical transmission system installed with an optical amplifier in front of the dispersion compensating fiber. In FIG. 7, numeral  1  indicates a transmitter,  2  is a receiver,  3  is an optical fiber forming an optical transmission line,  11  is an optical transmission circuit,  22  is an optical amplifier,  21  is a dispersion compensating fiber, and  23  is an optical receiver circuit. 
     Signal light received by the receiver  2  is amplified by the optical amplifier  22 , dispersion compensated by the dispersion compensating fiber  21 , and received by the optical receiver circuit  23 . FIG. 7 also shows a power diagram, in which the input power to the dispersion compensating fiber  21  is increased to more than a certain level by the optical amplifier  22 , so as to compensate for the loss of the dispersion compensating fiber  21  and prevent S/N deterioration in the optical receiver circuit  23  (K. Hagimoto et al., OAA&#39;90, Technical Digest, TUA2, 1990). 
     FIG. 8 shows the configuration of a prior art optical transmission system in which Raman gain is induced in the dispersion compensating fiber. In FIG. 8, numeral  1  indicates a transmitter,  2  is a receiver,  3  is an optical fiber forming an optical transmission line,  11  is an optical transmission circuit,  21  is a dispersion compensating fiber,  23  is an optical receiver circuit,  24  is a optical pumping source,  25  is a pumping light coupler, and  26  is an isolator. 
     Signal light received by the receiver  2  is dispersion compensated by the dispersion compensating fiber  21  and received by the optical receiver circuit  23 . The dispersion compensating fiber  21  is backward pumped using pumping light coupler  25 , and blocked by the isolator  26 . FIG. 8 also shows a power diagram, showing that loss and Raman gain in the dispersion compensating fiber  21  are balanced and the optical power is maintained at a constant value (P. B. Hansen., Elec. lett., 34, pp1136-1137, 1998). 
     However, as shown in FIG.  6  and FIG. 7, in the configuration where the optical amplifier  22  is disposed in the receiver  2 , the signal quality is deteriorated for the reason described below. That is, when the optical amplifier  22  is placed after the dispersion compensating fiber  21  (FIG.  6 ), due to a loss by the dispersion compensating fiber  21  in addition to a loss L of the optical fiber  3 , SIN of the main signal light is degraded at the output of the optical amplifier  22 , resulting in a degradation of the sensitivity of the optical receiver circuit  23 . 
     When the optical amplifier  22  is placed in front of the dispersion compensating fiber  21  (FIG.  7 ), since the dispersion compensating fiber  21  is smaller in core diameter than an ordinary fiber, it is necessary to limit the output level of the optical amplifier  22  to a level at which a nonlinear optical effect induced in the dispersion compensating fiber  21  can be neglected, thus limiting a loss compensation range of the dispersion compensating fiber  21 . 
     On the other hand, when Raman gain is induced in the dispersion compensating fiber  21  (FIG.  8 ), waveform degradation of the main signal caused by a nonlinear optical effect, which restrict the system performance of the configuration shown in FIG. 7, can be relaxed. However, S/N of main signal light received by the optical receiver circuit  23  is determined by the loss L of the optical fiber transmission line  3 , which cannot be improved any further. To utilize a Raman gain, the dispersion compensating fiber  21  is required to have a length of several tens of km, which increases propagation delay inside the receiver  2 , resulting in an increased delay in the optical transmission system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an optical transmission system which suppresses S/N degradation caused by the optical amplification and nonlinear optical effect in the dispersion compensating fiber and compensates for a loss due to the dispersion compensating fiber without incorporating a long fiber inside the receiver. 
     In accordance with the present invention, the re is provided an optical transmission system in which part of an optical transmission line is constructed by a dispersion compensating fiber and designed so that at least part of the optical transmission line has a distributed gain. However, to suppress a nonlinear optical effect, the dispersion compensating fiber is not disposed at immediately after a transmitter except for a case where the entire optical transmission line has a gain. Pump light to induce a distributed gain is transmitted to the optical transmission line from at least one of a transmitter, a receiver, and a repeater. 
     With such a configuration, since part or entire of the optical transmission line has no loss, a loss L in a transmission section becomes small compared to the prior art configuration shown in FIGS. 7 and 8, optical power received by the optical receiver circuit can be sufficiently high even when launched power to the dispersion compensating fiber is restricted. That is, while performing dispersion compensation by the dispersion compensating fiber and suppressing a nonlinear optical effect, its loss can be compensated for. Further, since the dispersion compensating fiber comprises part of the optical transmission line, it is not necessary to incorporate a long dispersion compensating fiber inside the receiver. 
     In the optical transmission system according to the present invention, the optical transmission line is constructed by transmission optical waveguides and dispersion compensating optical waveguides, and part or all of them are induced a distributed gain, thereby compensating for signal light loss due to dispersion compensating optical waveguides while suppressing degradation of signal quality to a minimum. 
     That is, by reducing loss of the optical transmission line, it becomes possible to simultaneously achieve an increase in optical power received by the optical receiver circuit and a limitation of launched power to the optical transmission line, of which the former contributing to improvement of S/N of received signal, and the latter contributing to suppression of waveform degradation caused by nonlinear optical effects such as self-phase modulation. 
     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a basic configuration of the optical transmission system according to the present invention; 
     FIG. 2 is a block diagram showing a first embodiment of the optical transmission system according to the present invention; 
     FIG. 3 is a block diagram showing a second embodiment of the optical transmission system according to the present invention; 
     FIG. 4 is a block diagram showing a third embodiment of the optical transmission system according to the present invention; 
     FIG. 5 is a block diagram showing a fourth embodiment of the optical transmission system according to the present invention; 
     FIG. 6 is a block diagram showing configuration of a prior art optical transmission system; 
     FIG. 7 is a block diagram showing configuration of a prior art optical transmission system; and 
     FIG. 8 is a block diagram showing configuration of a prior art optical transmission system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a basic configuration of the optical transmission system according to the present invention. 
     In FIG. 1, a transmitter  1  and a receiver  2  are connected through an optical fiber  3  and a dispersion compensating distributed fiber amplifier  4  constituting an optical transmission line. However, the optical fiber  3  is disposed at the transmitter side and the dispersion compensating distributed fiber amplifier  4  is disposed at the receiver side. The dispersion compensating distributed fiber amplifier  4  is an optical fiber having a distributed gain with a function to compensate for a group velocity dispersion of the optical fiber  3 . 
     The transmitter  1  comprises an optical transmission circuit  11  for transmitting a signal light, a optical pumping source  12  for outputting pump light for forward pumping excites the dispersion compensating distributed fiber amplifier  4 , and a pumping light coupler  13  for coupling the pump light with the signal and launching these coupled lights to the optical transmission line. The receiver  2  comprises an optical receiver circuit  23  for receiving the signal light, a optical pumping source  24  for outputting a pump light for backward pumping excites the dispersion compensating distributed fiber amplifier  4 , a pumping light coupler  25  for launching the pump light in a direction reverse to the signal light. 
     With the above construction, pump lights are launched from the front side and the rear side of the dispersion compensating distributed fiber amplifier  4 , signal light transmitted in the optical fiber  3  and the dispersion compensating distributed fiber amplifier  4  is dispersion compensated and amplified before it is received by the receiver  2 . However, pumping is not always necessary to be performed bidirectionally, and it is possible to omit one of the two directions. Further, the optical fiber  3  may be provided with a distributed gain. In this case, pump light outputted from the transmitter  1  is mainly used for pumping the optical fiber  3 . 
     FIG. 2 shows a first embodiment of the optical transmission system according to the present invention. Here, a configuration is shown in which pumping light is launched from the receiver side to the dispersion compensating distributed fiber amplifier, and the configuration of the transmitter is omitted. In the following, detailed configuration of each part will be described comparing with the basic configuration of FIG.  1 . 
     For the optical fiber  3  and the dispersion compensating distributed fiber amplifier  4 , a single mode fiber  31  and a dispersion compensating fiber  41  using silica as a base material are used. The dispersion compensating fiber  41  is the reverse in the sign of the dispersion and dispersion slope, and it is designed so that the dispersion and the dispersion slope are zero in the vicinity of 1.5 μm as the entire optical transmission line. 
     Signal light of wavelength 1.5 μm outputted from the transmission station (out of the figure) is launched to the single mode fiber  31  through an isolator  5 . Signal light outputted from the single mode fiber  31  is launched to the dispersion compensating fiber  41 . 
     The optical pumping source has a configuration to make polarization multiplexing and outputting to eliminate polarization dependence of Raman gain of the dispersion compensating fiber  41 . Here, laser light outputted from a first semiconductor laser (LD  1 )  24   a  and a second semiconductor laser (LD  2 )  24   b  is polarization multiplexed by a polarization multiplexing circuit  24   c  and outputted. The pumping light coupler uses a WDM (wavelength division multiplexing) coupler  25   a . Polarization multiplexed pump light is launched to the dispersion compensating fiber  41  in the reverse direction to the signal light through the WDM coupler  25   a . An optical circulator may be used instead of the WDM coupler  25   a.    
     A carrier frequency difference between pump light and signal light is set to be equal to a Raman shift (about 9.5 THz) of silica. Signal light is Raman amplified by the dispersion compensating fiber  41  and received by the receiver  2 , and received by the optical receiver circuit  23  through the WDM coupler  25   a  and an optical bandpass filter (BPF)  26 . Raman gain may also be induced in the single mode fiber  31 , when the pump light intensity is sufficiently high. 
     The optical bandpass filter  26  is used to block Stokes wave spontaneously emitted in the dispersion compensating fiber  41 . The isolator  5  is disposed mainly to prevent pump light from mixing to the transmitter. 
     FIG. 3 shows a second embodiment of the optical transmission system according to the present invention. Here, a configuration is shown in which pump light is launched from the receiver side to the dispersion compensating distributed fiber amplifier, and the configuration of the transmitter is omitted. 
     The present embodiment is characterized by using an optical filter  6  which transmits the signal light wavelength and reflects the pump light wavelength, disposed between the single mode fiber  31  and the dispersion compensating fiber  41  in place of the isolator  5  in the first embodiment. Other configuration is the same as in the first embodiment. Pump light reaching the optical filter  6  is reflected there, and launched again to the dispersion compensating fiber  41 . This enables efficient pumping. By providing a wavelength dependence to reflectivity of pump light or transmissivity of signal light, it is possible to equalizing the wavelength dependence of Raman gain. 
     FIG. 4 shows a third embodiment of the optical transmission system according to the present invention. Here, a configuration is shown in which pump light is launched from the receiver side to the dispersion compensating distributed fiber amplifier, and the configuration of the transmitter is omitted. 
     The present embodiment is characterized by using an erbium doped dispersion compensating fiber  42  which is doped with erbium in the core and used in place of the dispersion compensating fiber  41  in the first embodiment. The erbium doped dispersion compensating fiber  42  is the reverse in the sign of dispersion and dispersion slope compared to the single mode fiber  31 , and is designed so that the dispersion and the dispersion slope are zero in the vicinity of 1.5 μcm as the entire optical transmission line. 
     The pump light outputted from the optical pumping source  24  is launched through the WDM coupler  25   a  to the erbium doped dispersion compensating fiber  42  in the reverse direction to the signal light. Since the erbium doped fiber amplifier has no polarization dependence, a polarization multiplexing circuit in the pumping optical source  24  is not necessarily required. The pump light wavelength is in the vicinity of 1480 nm or 980 nm. An optical circulator may be used in place of the WDM coupler  25   a.    
     The signal light is amplified by the erbium doped dispersion compensating fiber  42 , launched to the receiver  2 , and received by the optical receiver circuit  23  through the WDM coupler  25   a  and the optical bandpass filter  26 . The optical bandpass filter  26  is used to block amplified spontaneous emission (ASE) generated in the erbium doped dispersion compensating fiber  42 . 
     FIG. 5 shows a fourth embodiment of the optical transmission system according to the present invention. 
     The present embodiment is characterized in that the optical transmission line is divided into a plurality n of transmission sections, and each transmission section is provided with a optical pumping source for pumping the single mode fiber  31  and the dispersion compensating fiber  41 . Transmission sections # 1  to #n are quite the same in configuration, each transmission section is required to have a net gain of 0 dB. 
     The present embodiment has a configuration in which laser light outputted from two semiconductor lasers  71  and  72  as optical pumping sources is polarization multiplexed by a polarization multiplexing circuit  73  and outputted. Further, semiconductor lasers  71   a  and  72   a  and a polarization multiplexing circuit  73   a  are provided as optical pumping sources for forward pumping the single mode fiber  31 , and semiconductor lasers  71   b  and  72   b  and a polarization multiplexing circuit  73   b  as optical pumping sources for backward pumping excites the dispersion compensating fiber  41 . 
     Signal light of wavelength 1.5 μm outputted from the transmission station (out of the figure) is launched to the single mode fiber  31  of the transmission section # 1  through the isolator  5 . The pump light for forward pumping excites the single mode fiber  31  is polarization multiplexed and launched to the single mode fiber  31  in the same direction as the signal light through the WDM coupler  74 . The signal light outputted from the single mode fiber  31  is launched to the dispersion compensating fiber  41 . The pump light for backward pumping excites the dispersion compensating fiber  41  is polarization multiplexed and launched to the dispersion compensating fiber  41  in the reverse direction to the signal light through a duplexer  75 . An optical circulator may be used in place of the duplexer. Further, both of the forward pumping and backward pumping are not always necessary, and one of them may be omitted. 
     The carrier frequency difference between the pump light and signal light is set equal to a Raman shift (about 9.5 THz) of silica. The signal light is Raman amplified by the single mode fiber  31  and the dispersion compensating fiber  41 , and outputted to the transmission section # 2  through the duplexer  75 . In the following, in the respective transmission sections, dispersion compensation to the signal light and Raman amplification for compensating for the loss thereof are performed by the same configuration. 
     The signal light outputted from the transmission section #n is received by the optical receiver circuit  23  through the optical bandpass filter  26  in the receiver  2 . The optical bandpass filter  26  is used to block Stokes wave spontaneously emitted in the single mode fiber  31  and the dispersion compensating fiber  41 . Back scattering light and backward pump light generated in the transmission section # 1  are blocked by the isolator  5 . Back scattering light generated in the transmission sections # 2  to #n is terminated by a terminator  76  through duplexers  75  of the respective transmission sections, and will never back flow to other transmission sections. 
     In the present embodiment, the erbium doped dispersion compensating fiber  42  as shown in the third embodiment may be used in each transmission section, so that non-polarization multiplexed pump light is launched. 
     The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the invention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.