Patent Publication Number: US-2012027343-A1

Title: Wavelength division multiplex optical regeneration system and wavelength division multiplex optical regeneration method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and is based upon and claims the benefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 10/516,306, filed Apr. 25, 2006, the entire contents of which is incorporated herein by reference which is the national stage of PCT/JP03/07433 filed Jun. 11, 2003, and claims the benefit of priority under 35 U.S.C. §119 from Japanese Application Nos. 2002-170429, filed Jun. 11, 2002, 2003-055645, filed Mar. 3, 2003 and 2003-125659, filed Apr. 30, 2003. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wavelength division multiplex optical regeneration system and a wavelength division multiplex optical regeneration method. 
     BACKGROUND ART 
     There is continuously increasing a transmission capacity of the communication systems, and the capacity has been significantly increasing due to a progress of an optical fiber used in a communication system. For example, the communication systems are categorized into a point-to-point trunk line network system, a metro network system and an access network system, the optical communication system has already been deployed in the trunk line network system, in addition, even in the metro network system and the access network system, an electrical communication system is going to be replaced with the optical communication system. Namely, the whole communication system is progressing to adopt the optical communication system. 
     Recently, the information capacity transmitted via one optical fiber has been dramatically increased by use of a wavelength division multiplexing system. In this system, when using a low loss band width at 0.4 bit/Hz spectrum transfer efficiency, the transmission capacity per one optical fiber becomes approximately 3.2 Tbit/s. Concretely, this capacity can be realized by 320 channels based on the 10 Gbit/s transmission speed per channel (wavelength) which is available today. 
     It is inevitable for an optical signal to deteriorate in its waveform, timing and intensity when it travels a long distance. As such, the optical signal which has traveled a certain distance is inevitably required to be regenerated. Therefore, a regenerating system is normally provided in the optical communication systems so that the degenerated signal light is recovered. This regenerating system typically comprises: a receiver unit which receives a degenerated signal light and converts it into an electrical signal; a regenerator unit which performs a necessary function such as amplifying, noise reduction, waveform reshaping, and clock signal regeneration for the electrical signal; and a transmitter unit which reconverts the regenerated electrical signal to the optical signal then sends out to the optical carrier. 
     Concretely, in the optical signal regenerating system which is inserted in the middle point of the optical fiber wherein a plurality of light signals in 320 channels travel, 320 units of receivers, regenerators and transmitters are equipped in order to accommodate the number of channels. 
     There has been a difficulty in downsizing and a disadvantage of large power consumption in the optical signal regenerating system which includes so many units. 
     Furthermore, when the optical signal regenerating system is used in a metro network system and an access network system, it is necessary to install 320 channel optical signal regenerating systems at many relay points, and consequently the whole optical communication system becomes bigger and the cost and the power consumption increase. 
     As one of the solutions for the problems, there is an approach of reducing the number of channels in the wavelength division multiplexing system and increasing its transmission speed of each channel. This is because, the transmission speed is inversely proportional to the number of channels in the wavelength division multiplexing system when maintaining the same transmission capacity. 
     However, the regenerator used in the optical signal regenerating system is made of an electric device to process the electric signals so it has a physical upper limitation in the response speed. For example, a maximum signal transmission speed processed in an electric device is 40 Gbits/s today. Further, at this speed, still 80 channels are needed. Furthermore, when an electronic device is driven at higher speed, the higher electric power is needed. Therefore, the regenerating system using an electric device has a limitation in the transmission speed of each channel and it is difficult to realize a downsizing and reduce the power consumption in the real application. 
     As another possible solution for the problems with an optical signal regenerating system using an electronic device, there is a method of using whole optical signal regenerator which processes on the optical light itself without converting it to an electric signal. The equipment of the whole optical signal regenerator typically comprises a high speed electrical modulator and a regenerator utilizing a nonlinear optical effect of the substance. 
     However, this whole optical signal regenerator uses an electric modulator, so that there is an upper limitation in the processing speed similarly to the optical signal regeneration system which performs a photo-electro conversion. Further, when regenerating an optical signal of 40 Gbits/s or faster transmission speed, the transmission speed of the optical signal is reduced by a time division process, and after regenerating the signal, a time division multiplexing is required and hence it becomes a large system. 
     Because the whole optical signal regenerator uses a nonlinear optical response (nonlinear optical effect) for regenerating an optical signal, the following problems are caused. 
     Namely, in a large capacity optical communication system, when the light of a wavelength division multiplexed signal which includes a plural signal lights with a different wave length transmits through the optical transmitting path, the polarizing condition of each optical signal becomes slightly different among each wavelength after transmitted. If the regenerating process is performed at once by a whole optical signal regenerator using a nonlinear optical effect, the intensity of the nonlinear optical effect varies depending on the polarization condition of the incident light, and consequently the wavelength division multiplexed light after regenerated may include components of the light that is not well regenerated or sometimes no components regenerated at all. 
     In the above, the problems are pointed out focusing the polarization condition of the incident transmitting light in the optical regenerator. In the below, the today&#39;s problems will be discussed focusing on the optical regeneration system. The optical regeneration means re-amplification of a light intensity, re-shaping of a waveform, and re-timing of an optical signal which is degenerated during a transmission (hereinafter, abbreviated to O3R). The optical regeneration system which has those functions can transmit a signal light to the infinite distance (long distance) of the optical fiber. 
     Regarding the infinite distance (long distance) transmission described above, it is reported by Leuthold et al. (Leuthold et al., Electron. Lett,. 38, p 890, 2002). This paper describes about 40 Gb/s, 1,000,000 km transmission using an O3R regenerator. 
     In this paper, Leuthold et al used an electronic circuit technique for the optical clock extraction required for re-timing (a technique of generating a clock pulse train synchronizing with the transmitting optical signal) and for the switching function. As such, this system cannot satisfy the transmission speed which is limited by the electrical circuit technique. For example, it is not applicable to 160 Gbits/s system which exceeds the limit of the speed of the electronic circuit. 
     Note that regarding the 160 Gbits/s system, it is reported about the optical regenerator adopting an optical switch. (Schubert et al., Electron. Lett,. 38, p 903, 2002). However, it is anticipated not to work as an O3R device because the clock extractor is not provided. 
     Summarizing the above, the O3R has not been yet realized which fundamentally utilizes an effective whole light technique. However, there are many reports about a method of re-shaping of the waveform and re-timing of the clock which are one of components in the O3R. It will be demonstrated below but limited to the technique of an optical fiber which fundamentally relates to the present invention. 
     At first, the whole optical waveform re-shaping method will be described. This can be generally categorized into two methods. First one is a method utilizing a soliton which is a combination effect made by a nonlinear optical characteristic and an anomalous dispersion of an optical fiber (Hasegawa and Tappert, Appl. Phys. Lett., 23, P 142, 1973). The waveform re-shaping device based on this technique is called as a soliton converter. Second one is a method based on the self phase modulation effect of a light pulse utilizing a nonlinear characteristic of an optical fiber (Mamyshev, ECOC &#39;98, p. 475, 1998). This is called as a Mamyshev filter originated from a name of a proponent. A method utilizing a supercontinuum light is positioned as a follow-up model. 
     As an experiment using the former system (soliton converter), the transmission of 4×40 Gb/s signal for 10,000 km is reported (Dany et al., Opt. Lett., 25, p. 793, 2000). The latter system (Mamyshev filter) is used in the 1,000,000 km transmission line in the Leuthold&#39;s system (Leuthold et al., Electron. Lett,. 38, p 890, 2002) as described above. All of them are reported from a view point of systems but has not described clearly about the performance of the device itself (only a few report such as Dany et al., ECOC &#39;01, We. P. 45, 2001). Especially, in the soliton converter, because a noise amplification by a soliton effect becomes a problem (Kubota et al., J. Opt. Soc. Am B, 16, p 2223, 1999), the designing from view point of a device performance is important as well. Namely, identifying the device performance clearly and establishing an optimum design rule of the device itself will be a future subject to be solved. 
     Next, a method of re-timing method will be described. The main stream of this technique is a complex method of an optical clock extraction and an optical switching.  FIG. 38  shows this configuration. It includes an optical clock extraction part and an optical switching part. The former (an optical clock extraction part) is to synchronize the phase of the input signal (an optical signal or an electrical signal which corresponds to a carrier signal) and the local optical light (an optical pulse train which functions as a standard clock, called an optical local oscillator (abbreviated to optical LO). Namely, an optical clock extraction is realized by synchronization between the input optical light and the local optical light. Therefore, the optical LO is required to have a variable characteristic of the repetitive frequency. 
     The latter (an optical switching part) is an optical switch such as a four wave mixing (FWM) device or a nonlinear optical loop mirror (NOLM), utilizing a nonlinear effect of an optical fiber which can realize a multiplication function in the optical region. The details of the optical clock extraction and optical switch technique will be described below. 
     In  FIG. 38 , the area enclosed by a dotted line is an optical clock extraction part which comprises an optical phase comparator, an optical LO generator and a controller circuit. The optical phase comparator senses the phase difference between the external optical signal and the optical LO, and controls the optical LO oscillating frequency (which corresponds to the repetitive frequency of the pulse train) to minimize the error. 
     Consequently, a synchronization between the external optical signal and the optical LO is realized to produce a precise time position pulse train (hereinafter, called a clock pulse train) which is synchronized with the external optical signal. By utilizing a nonlinear optical effect instead of an electronic circuit technique, a phase controller which is capable of working at speed of exceeding 160 GHz can be realized. The method of a synchronization using a phase comparator in an optical region as described above is called an optical phase locked loop (OPLL). Actually, there has been proposed a clock extractor which is carried out by a synchronization between a beat light and an external optical signal from two LDs using a NOLM (Bigo et al., U.S. Pat. No. 6,239,893 B1). 
     Remarkable point is a timing jitter (time fluctuation of a clock pulse) of the output optical clock train of the OPLL. A timing jitter means a shift of the clock pulse in the time scale. Since the jitter can be a cause of deteriorating the performance in the communication systems, its suppression is important. The timing jitter of the clock pulse train has a correlation with the OPLL operation speed and when the OPLL operation is getting faster, the jitter is reducing. Namely, increasing an operation speed of OPLL is effective for jitter reduction. 
     However, when a NOLM as described in above technique (Bigo et al., U.S. Pat. No. 6,239,893 B1) is used, the OPLL loop length becomes a longer size, and hence the band width of the OPLL operation is limited. As a result, it restricts a reduction of jitter. To solve this, shortening a fiber length of the optical nonlinear device used for the phase controller will become a key factor. By shortening the fiber length, the OPLL operation band width can be free from the restriction, Namely, it can realize a fast speed OPLL operation and generate a high quality clock pulse train with less jitter. 
     There is required an optical switch technology for the re-timing along with the clock extraction technique described above. A method of using an FWM which is a typical example of an optical switch utilizing an optical fiber nonlinear effect will be described below. When two chromatic lights with different wavelengths enter an optical fiber, newly colored optical lights different from the original color are generated if either of the incident optical lights has a sufficient power for the nonlinear effect. This is an FWM phenomenon. 
     When the clock pulse train and the optical signal enter into the optical fiber, the FWM light includes an imposed information of the input optical signal and its pulse timing is determined by the clock pulse train. Consequently, the optical signal pulse train with low jitter in which the information is imposed can be obtained. This is a principle of re-timing based on FWM. However, not only dispersion effect but also nonlinear effect give an influence to the optical pulse traveling in the optical fiber. 
     By this effect or the combined effect of those effects, the pulse waveform changes during the pulse traveling. As a result, the waveform distortion of FWM light is generated. It is effective to optimize a fiber dispersion value or the input power in order to suppress this, but the real value or the control method have not been disclosed. 
     The minimum components required for the optical regeneration system are described above. In addition, a method to improve the performance of the optical regeneration system is also important. Two components relating to the present invention are summarized below. Namely, (1) a device to convert an optical pulse waveform to a wave shape suitable to the optical switch and (2) a device to isolate a component of the optical pulse from a component of optical noise. 
     Firstly, the rectangular pulse waveform method which is effective to suppress the increased noise intensity in the optical switch will be described concerning (1) a device to convert an optical pulse waveform to a wave shape suitable to the optical switch. Typically in the optical switch, as a result of an interaction between the nonlinear optical characteristic and the dispersion, the time jitter of the input of a transmitting optical signal pulse is converted to the intensity jitter in the output signal after regenerated.  FIG. 39A  is an explanatory view. 
     Here, an optical switch for a pulse train including a jitter and a clock pulse train will be discussed. 
     The output pulse power from the optical switch correlates with a time overlapping between a transmitting pulse and a clock pulse. As a result, a change in the overlapping region between both pulses caused by a jitter is converted to an output pulse power of the optical switch. In order to suppress the amount converted from the phase jitter to the light intensity jitter, a rectangular clock pulse waveform conversion on the transmitting optical signal pulse or the extracted clock pulse is effective ( FIG. 39B ). 
     This rectangular method is categorized into a method utilizing a chromatic dispersion or a polarization mode dispersion, and a method utilizing a complex effect of nonlinear effect and a normal dispersion. The former example is a method using a fiber Bragg grating or a polarization maintaining fiber (Lee et al., OFC2001, PD30-1, 2001 and Schubert et al., Electron. Lett,. 38, p 903, 2002), and the latter example is a method using a normal dispersion fiber (the principle reported, Nakatsuka et al., Phys. Rev. Lett., 47, p. 910, 1981). 
     In the former method based on the linear function, a steepness of the rising edge and falling trail of the rectangular waveform before transforming is determined by the input pulse width. Namely, in order to obtain a steep slope of the rectangular pulse, the very narrow optical pulse is needed accordingly. Compared with that, the latter case has an advantage of ability to transform to a steep slope rectangular wave, however, the higher power of an input optical light and a longer fiber length are the mandatory required to obtain nonlinear effect and dispersion effect. 
     Secondly, a device to isolate a component of optical pulse from a component of optical noise (2) will be described. In the below, the noise reduction component is summarized. The optical pulse has a noise imposed thereon. The major component of the noise is an amplified spontaneous emission light (ASE) generated when amplifying an optical light. In general, a noise has a wider optical spectrum than an optical signal, and hence the noise components which are out of the optical signal bandwidth can be eliminated by the optical filter to some extent. 
     However, the noise components within the bandwidth of the optical signal remain. To eliminate those noise components, there is a proposed method utilizing an optical soliton characteristic, besides a method using a waveform reshaping described above. Here, attention is directed to the latter case which relates to the present invention. 
     There is reported about phenomena (soliton self frequency shift: SSFS) that the soliton shifts towards a longer wavelength side by a stimulated Raman scattering (SRS) in the optical soliton transmission (Mitschke and Mollenauer, Opt. Lett., 11, p. 659, 1986). This phenomenon also happens when the ASE noise is added to the soliton. It is proposed a method of noise reduction utilizing this characteristic which isolates the soliton from ASE noise components to filter the same in the frequency domain by using this phenomenon (Namiki et al., Provisional Publication No. 2001-109024). The configuration of the noise reduction unit is shown in  FIG. 40A . 
     It comprises an anomalous dispersion fiber (ADF) and an optical filter. An optical soliton including noise components shown in  FIG. 40B  upper part is inputted to the ADF. During the transmission, SSFS of the optical soliton component is carried out by the SRS. Further, the remarkable point is that the soliton components shift towards longer wavelength side by the SRS but the noise components stay without shifting towards the longer wavelength side. Consequently, the noise components within a bandwidth of the optical signal can be eliminated by a method of extracting the soliton components by the output optical filter after shifted. ( FIG. 40B  lower part). 
     Furthermore, this phenomenon has a function of shifting the wavelength as well, and therefore it is possible to adjust the wavelength of the optical signal to the desired wavelength by SSFS control. However, SSFS is a phenomenon which typically occurs in the femto second region, so that further improvements for the pico second soliton transmission is necessary in the SSFS and its efficiency. 
     As described above, today&#39;s status and problems about the optical regeneration systems are detailed. The present invention is aiming to solve these problems and provide simple O3R systems. 
     The purpose of the present invention is to solve the above described problems and to provide a wavelength division multiplex optical regeneration system and a wavelength division multiplexing optical regeneration method which is capable of realizing a large transmission capacity, a small footprint and a power saving. 
     DISCLOSURE OF THE INVENTION 
     To attain the above object, one embodiment of an optical regeneration system in accordance with the present invention is an optical regeneration system to regenerate a degenerated optical signal including a regenerator which comprises at least one of devices consisting of a soliton converter, a pulse roller, a Kerr shatter and a soliton purifier. 
     Another embodiment of an optical regeneration system in accordance with the present invention is an optical regeneration system wherein a polarizing converter is included either in a previous stage before the regenerator or inside the regenerator. 
     Still another embodiment of an optical regeneration system in accordance with the present invention is an optical regeneration system wherein a demultiplexer is included in a previous stage before the regenerator, or before the polarizing converter when the polarizing converter is placed before the regenerator. 
     Yet another embodiment of an optical regenerating system in accordance with the present invention is an optical regenerating system wherein a multiplexer is included in a stage after the regenerator. 
     A still further embodiment of an optical regenerating system in accordance with the present invention is an optical regenerating system wherein a phase compensator is included in a stage before the generator, before the polarizing converter when the polarizing converter is placed before the regenerator, or before the demultiplexer when the demultiplexer is placed before the polarizing converter. 
     A yet further embodiment of an optical regenerating system in accordance with the present invention is an optical regenerating system wherein a multiplexer mixing an optical signal generated by the regenerator with another optical signal is included at an output side of the regenerator. 
     Another embodiment of an optical regenerating system in accordance with the present invention is an optical regenerating system wherein the regenerators are serially connected in multi stages. 
     Still another embodiment of an optical regenerating system in accordance with the present invention is an optical regenerating system wherein an optical switch is included between the regenerators serially connected in multi stages. 
     Yet another embodiment of an optical regenerating system in accordance with the present invention is an optical regenerating system wherein an input power is adjusted before the regenerator. 
     One embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device having a soliton converter comprising an anomalous dispersion fiber (ADF) in which a fiber length thereof is up to twice of that of a soliton frequency. 
     Another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein an optical filter is included in a stage after the anomalous dispersion fiber. 
     Yet another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein an optical amplifier is included in a stage before the anomalous dispersion fiber. 
     A still further embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein a Mamyshev filter or a NOLM is included in place of the soliton converter. 
     A yet further embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein a pulse compressor is included at an input side. 
     Another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor utilizes an adiabatic compression. 
     Still another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes a dispersion decreasing fiber in which the dispersion is decreasing in a longitudinal direction of the optical fiber. 
     Yet another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes an SDPF in which the dispersion has a step like profile in a longitudinal direction of the optical fiber. 
     A still further embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes an CDPF in which the dispersion has a comb like profile in a longitudinal direction of the optical fiber. 
     A yet further embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes an optical fiber in which nonlinearity is increasing in a longitudinal direction of the optical fiber. 
     Another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes an optical fiber in which nonlinearity has a step like profile increasing in a longitudinal direction of the optical fiber. 
     Still another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes an optical fiber in which nonlinearity has a comb like profile increasing in a longitudinal direction of the optical fiber. 
     Yet another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the pulse compressor includes a Raman amplifier. 
     A still further embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein a saturable absorption substance having a saturable absorption characteristic is used in place of the soliton converter. 
     A yet further embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device which further comprises a position adjustment unit in which the saturable absorption characteristic is made variable by adjusting a position of the saturable absorption substance. 
     Another embodiment of a waveform reshaping device in accordance with the present invention is a waveform reshaping device wherein the saturable absorption characteristic has a surface distribution. 
     One embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter comprising a demultiplexer, an OPLL (Optical Phase Locked Loop), and an optical switch. 
     Another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein L Loop  is determined by satisfying the following equation: 
       Δω( L   Loop )&lt; v·X/n·L   A-B  
 
     where 
     Δω: bit rate difference in the OPLL, 
     L Loop : loop length, 
     v: velocity of the light in the optical fiber, 
     L A-B : length of fiber between the demultiplexer and the optical switch, 
     n: refraction index of the fiber, and 
     X: arbitrary number. 
     Still another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the OPLL comprises an optical LO generator generating an optical LO, a phase comparator detecting phase difference between an external optical signal and the optical LO signal, and a controller to control a frequency of the LO signal based on the phase difference. 
     Yet another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the phase comparator includes an FWM unit generating an FWM light, an optical filter and a photo receiving device. 
     A still further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the FWM unit adopts either a high nonlinear optical fiber, a PPLN (Periodically-poled LiNO 3 ), or an SOA (Semi-conductive Optical Amplifier). 
     A yet further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the photo receiving device has a pulse roller which is placed in a front stage and monitors frequency characteristic of pulses entering to the photo receiving device. 
     Another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the LO generator has a beat light generator. 
     Still another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the beat light generator comprises at least one semiconductor laser which emits a CW light with at least two frequency components and an optical coupler which mixes the CW lights. 
     Yet another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the semiconductor lasers are driven in series. 
     A still further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein an optical fiber compressor is inserted between the beat light generator and the optical switch. 
     A yet further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the phase comparator includes a PD (Photodiode), a Loop Filter and an LD controller, and wherein the PD generates a photo current by a two photon absorption effect. 
     Another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the PD is made of a silicon avalanche photodiode (SiAPD). 
     Still another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the optical switch includes an FWM unit, an optical filter and a phase controller. 
     Yet another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the phase controller is controlled so that the phase control output does not to drift for change of an ambient temperature. 
     A still further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the phase control output is controlled by a feedback of an output pulse. 
     A yet further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the FWM unit has a relation expressed by the following equation: 
     
       
         
           
             
               Δ 
                
               
                   
               
                
               v 
             
             &gt; 
             
               
                  
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       v 
                       p 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       v 
                       s 
                     
                   
                 
                  
               
               2 
             
           
         
       
     
     where; 
     Δν: frequency delta (detuning amount) between a pump light and an optical signal, 
     Δν p : spectrum width of an input pumping pulse, and 
     Δν s : spectrum width of an input signal pulse. 
     Another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the FWM unit has a relation expressed by the following equation: 
       Δ L&gt;Δν   p +(Δν s /2)
 
     where; 
     ΔL: fiber length, 
     Δνv p : spectrum width of an input pumping pulse, and 
     Δνv s : spectrum width of an input signal pulse. 
     Still another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the fiber length L is determined by the following equation: 
     
       
         
           
             
               1 
               &lt; 
               
                 L 
                 
                   L 
                   NL 
                 
               
             
             = 
             
               γ 
                
               
                   
               
                
               
                 P 
                 0 
               
                
               L 
             
           
         
       
     
     Yet another embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter wherein the fiber length L of the FMW unit is determined by the following equation: 
     
       
         
           
             
               
                 L 
                 
                   L 
                   SOD 
                 
               
               &lt; 
               
                 1 
                 2 
               
             
             , 
             
               
                 L 
                 
                   L 
                   TOD 
                 
               
               &lt; 
               
                 1 
                 2 
               
             
           
         
       
       
         
           
             
               β 
               3 
             
             &lt; 
             
               
                 
                   1.7628 
                   3 
                 
                 2 
               
                
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     t 
                     p 
                     3 
                   
                 
                 L 
               
             
           
         
       
       
         
           
             
               β 
               3 
             
             &lt; 
             
               
                 
                   1.7628 
                   2 
                 
                 
                   4 
                    
                   π 
                 
               
                
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     t 
                     s 
                     2 
                   
                 
                 
                   L 
                    
                   
                       
                   
                    
                   Δ 
                    
                   
                       
                   
                    
                   v 
                 
               
             
           
         
       
     
     A still further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter which is designed by the following steps of: 
     a process to determine a detuning amount Δν which is a value to avoid a spectrum overlapping using the equation regarding the pumping pulse (Δt p , Δν p ) and the signal pulse (Δt s , Δν s ); 
     
       
         
           
             
               Δ 
                
               
                   
               
                
               v 
             
             &gt; 
             
               
                  
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       v 
                       p 
                     
                   
                   + 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       v 
                       s 
                     
                   
                 
                  
               
               2 
             
           
         
       
     
     a process to determine the fiber length L to obtain the FWM bandwidth exceeding 2Δν; 
     a process to determine the pumping peak power P p  which can generate an FWM without distortion in the spectrum waveform using the equation; 
     
       
         
           
             
               1 
               &lt; 
               
                 L 
                 
                   L 
                   NL 
                 
               
             
             = 
             
               γ 
                
               
                   
               
                
               
                 P 
                 0 
               
                
               L 
             
           
         
       
       
         
           
             
               γ 
                
               
                   
               
                
               
                 P 
                 P 
               
                
               L 
             
             ≤ 
             
               
                 3 
                  
                 π 
               
               2 
             
           
         
       
     
     and 
     a process to determine the third order dispersion value β 3  which is necessary to suppress a time waveform distortion of the pulse during the fiber transmission using the following equation: 
     
       
         
           
             
               β 
               3 
             
             &lt; 
             
               
                 
                   1.7628 
                   3 
                 
                 2 
               
                
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     t 
                     p 
                     3 
                   
                 
                 L 
               
             
           
         
       
       
         
           
             
               β 
               3 
             
             &lt; 
             
               
                 
                   1.7628 
                   2 
                 
                 
                   4 
                    
                   π 
                 
               
                
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     t 
                     s 
                     2 
                   
                 
                 
                   L 
                    
                   
                       
                   
                    
                   Δ 
                    
                   
                       
                   
                    
                   v 
                 
               
             
           
         
       
     
     A yet further embodiment of a Kerr-shutter in accordance with the present invention is a Kerr-shutter which further comprises an optical LO generator, and a controller, wherein the FWM unit is commonly shared with the optical phase comparator and the optical switch. 
     One embodiment of a pulse roller in accordance with the present invention is a pulse roller having a pulse roller fiber with high nonlinear characteristic. 
     Another embodiment of a pulse roller in accordance with the present invention is a pulse roller wherein the pulse roller fiber comprises a normal dispersion increasing fiber having a characteristic in which normal dispersion is increasing in a longitudinal direction. 
     Still another embodiment of a pulse roller in accordance with the present invention is a pulse roller wherein the pulse roller fiber has a characteristic in which nonlinearity is decreasing in a longitudinal direction. 
     Yet another embodiment of a pulse roller in accordance with the present invention is a pulse roller wherein the pulse roller fiber comprises a distribution management optical fiber which is a combination of at least two fibers which have different normal dispersion and different nonlinearity characteristic in a longitudinal direction. 
     A still further embodiment of a pulse roller in accordance with the present invention is a pulse roller wherein an optical fiber whose dispersion is dominant in a longitudinal direction and an optical fiber whose nonlinearity is dominant in a longitudinal direction are arranged in the distribution management optical fiber. 
     A yet further embodiment of a pulse roller in accordance with the present invention is a pulse roller wherein the dispersion characteristic of the optical fiber in which the dispersion is dominant and the nonlinearity characteristic of the optical fiber in which the nonlinearity is dominant are arranged to form a step-like profile in the dispersion management optical fiber. 
     Another embodiment of a pulse roller in accordance with the present invention is a pulse roller wherein the dispersion characteristic of the optical fiber in which the dispersion is dominant and the nonlinearity characteristic of the optical fiber in which the nonlinearity is dominant are arranged to form a comb-like profile in the dispersion management optical fiber. 
     One embodiment of an OTDM signal generator in accordance with the present invention is an OTDM signal generator comprising the pulse roller and the optical switch. 
     One embodiment of a soliton purifier in accordance with the present invention is a soliton purifier wherein a soliton fiber is placed between two optical filters. 
     Another embodiment of a soliton purifier in accordance with the present invention is a soliton purifier wherein the gain slope (slope of gain) is controlled by a stimulated Raman scattering so that soliton wave shift is realized in the soliton fiber. 
     Still another embodiment of a soliton purifier in accordance with the present invention is a soliton purifier wherein the soliton fiber comprises a highly nonlinear fiber. 
     Yet another embodiment of a soliton purifier in accordance with the present invention is a soliton purifier which further comprises a pumping light generator for generating external pumping light, wherein a stimulated Raman scattering is generated by the external pumping light. 
     A still further embodiment of a soliton purifier in accordance with the present invention is a soliton purifier which further includes a pulse compressor at an input side. 
     A yet further embodiment of a soliton purifier in accordance with the present invention is a soliton purifier wherein a stimulated Raman scattering is generated while performing a soliton adiabatic compression. 
     One embodiment of a soliton noise controlling method in accordance with the present invention is a soliton noise controlling method of determining a maximum transmission distance at predetermined noise amplification gain based on a duty ratio (ratio of pulse period vs pulse width) and a dispersion distance during an optical nonlinear signal processing using an optical soliton train. 
     Another embodiment of a soliton noise controlling method in accordance with the present invention is a soliton noise controlling method wherein a CS-RZ pulse train is used as a modulation method. 
     One embodiment of an optical transmission system in accordance with the present invention is an optical transmission system wherein the optical regeneration systems are serially connected in multi stages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a first embodiment of the wavelength division multiplexing optical regeneration system in accordance with the present invention. 
         FIG. 2  is a schematic view of a regenerating unit  15  in the regenerating system in  FIG. 1 . 
         FIG. 3  is a schematic view of a clock regenerator  21  in the regenerating unit  15  in  FIG. 2 . 
         FIG. 4  is a schematic view of another clock regenerator  21  in the regenerating unit  15  in  FIG. 2 . 
         FIG. 5  is a schematic view of a waveform reshaping device  19  in the regenerating unit  15  in  FIG. 2 . 
         FIG. 6  is a schematic view of another waveform reshaping device  19  in the regenerating unit  15  in  FIG. 2 . 
         FIG. 7  is a schematic view of a polarization converter  13  in the regenerating system in 
         FIG. 1 . 
         FIG. 8  is a schematic view of another polarization converter  13  in the regenerating system in  FIG. 1 . 
         FIG. 9  is a schematic view of different polarization converter  13  in the regenerating system in  FIG. 1 . 
         FIG. 10  is a schematic view of one embodiment of the polarization converter  13  in  FIG. 7 . 
         FIG. 11  is a schematic view of another embodiment the polarization converter  13  in  FIG. 7 . 
         FIG. 12  is a schematic view of a different embodiment of the polarization converter  13  in  FIG. 7 . 
         FIG. 13  is a schematic view of a second embodiment of the wavelength division multiplexing optical regeneration system in accordance with the present invention. 
         FIG. 14  is a schematic view of a third embodiment of the wavelength division multiplexing optical regeneration system. 
         FIG. 15  is a schematic view of a fourth embodiment of the wavelength division multiplexing light regenerating system. 
         FIG. 16  is a schematic view of a fifth embodiment of the wavelength division multiplexing optical regeneration system. 
         FIG. 17  shows one configuration of the regenerator in accordance with the present invention. 
         FIG. 18  shows one configuration of the soliton converter in  FIG. 17 . 
         FIG. 19  is a schematic view of another embodiment of the soliton converter in  FIG. 17 . 
         FIG. 20A  is a graph showing an optical signal noise ratio OSNR of the output pulse from the soliton converter. 
         FIG. 20B  is a graph showing a soliton order number N dependency of the spectrum line width increasing ratio Δυ/Δυ in . 
         FIG. 21  shows a dependency of the output pulse self correlation wave width Δt AC  on the input power P in . The left side of the graph is an output pulse self correlation wave width Δt AC  before the output optical filter, and the right side is after the output optical filter. 
         FIG. 22  left side shows a pulse for the length of 0, 20, 40 and 80 meter after SMF transmission. The middle and right sides show the self correlation waveforms of the output pulse from the soliton converter after adjustment of P in . Further, the middle graph is a pulse before entering to the optical filter and the right side is after the optical filter. 
         FIG. 23  shows a P in  dependency of the self correlation wave width Δt AC  of the soliton output from the soliton converter and OSNR. 
         FIG. 24  shows one configuration of the Kerr-shutter. 
         FIG. 25A  is a graph showing a timing jitter characteristic when a pair of LDs are driven independently. 
         FIG. 25B  is a graph showing a timing jitter characteristic when LDs are driven in series. 
         FIG. 26A  shows one configuration of the photo receiving device. 
         FIG. 26B  shows a modification of  FIG. 25A . 
         FIG. 26C  shows a further modification of  FIG. 26A . 
         FIG. 27  shows one alternate configuration of the Kerr-shutter wherein the phase comparator and the FWM of the optical switch are integrated. 
         FIG. 28  shows one configuration of the pulse roller. 
         FIG. 29A  is a graph showing a dispersion profile of the normal dispersion increasing fiber. 
         FIG. 29B  is a graph showing a dispersion profile of the step-like dispersion profile fiber. 
         FIG. 29C  is a graph showing a dispersion profile of the comb-like dispersion profile fiber. 
         FIG. 30A  is one configuration of the pulse roller and showing an experimental transmission path for pulse reshaping to a rectangle waveform. 
         FIG. 30B  shows a dispersion profile of the pulse roller fiber in  FIG. 30A . 
         FIG. 31  shows simulation results of a pulse transmission in  FIG. 30A  and  FIG. 30B . 
         FIG. 32  is one configuration of the re-timing unit or the time division unit which consist of a pulse roller and an optical switch. 
         FIG. 33  is one configuration of the clock extractor wherein a pulse roller and an OPLL are combined. 
         FIG. 34A  and  FIG. 34B  are a graph showing a time waveform and a chirping characteristic of the output pulse from the pulse roller. 
         FIG. 35A  and  FIG. 35B  show the photo receiving device which detects a phase difference. 
         FIG. 36A  shows one configuration of the soliton purifier. 
         FIG. 36B  shows one embodiment wherein an HNLF is used in the soliton purifier. 
         FIG. 36C  shows one embodiment wherein a Raman amplifier is used in the soliton purifier. 
         FIG. 37  is an explanatory graph of a gain slope in the soliton wavelength. 
         FIG. 38  is an explanatory view of the conventional re-timing technique. 
         FIG. 39A  and  FIG. 39B  are explanatory views of the conventional pulse rectangular technique. 
         FIG. 40A  shows a configuration of the conventional noise reduction unit. 
         FIG. 40B  is a graph showing a mechanism of a noise reduction unit. 
         FIG. 41A  is a graph showing the input pulse. 
         FIG. 41B  through  FIG. 41D  are graphs showing the waveform after transformed to a rectangular waveform pulse from that in  FIG. 41A . 
         FIG. 42  shows waveforms explaining a relation between a pulse intensity and an instantaneous frequency. 
         FIG. 43  shows one configuration of the regenerator in accordance with the present invention. 
         FIG. 44  shows another configuration of the regenerator in accordance with the present invention. 
         FIG. 45  shows still another configuration of the regenerator in accordance with the present invention. 
         FIG. 46  shows one configuration of the regenerating system using the regenerator in accordance with the present invention. 
         FIG. 47  is a graph showing a relation between an input light intensity and an output light intensity of the saturable absorption substance. 
         FIG. 48  shows one configuration of the device using the saturable absorption substance. 
         FIG. 49  shows one configuration of a modification of the device of  FIG. 48 . 
         FIG. 50  shows another configuration of a modification of the device of  FIG. 48 . 
         FIG. 51A  is a graph showing an input pulse and output pulse at FWM when time is shown in a horizontal axis. 
         FIG. 51B  is a graph showing an input pulse and output pulse at FWM when frequency is shown in a horizontal axis. 
         FIG. 52  is a graph showing a relation between a bandwidth and an optical fiber length required for FWM. 
         FIG. 53  is a flow chart of a designing step to determine the optimum fiber length required for FWM. 
         FIG. 54  shows an experimental configuration after performing FMW conversion with the optimum fiber length determined by the design flow of  FIG. 53 . 
         FIG. 55  is a graph showing an input and output pulse waveform in the experiment of  FIG. 54 . 
         FIG. 56  is a graph showing a self correlation waveform at time of input and output according to the experiment of  FIG. 54 . 
         FIG. 57  shows one configuration of the waveform reshaping unit when using an FWM and SPM. 
         FIG. 58  is a graph showing a pulse waveform in the reshaping unit. 
         FIG. 59  is a graph showing a relation between the input pumping power and the output FWM power at the reshaping unit in  FIG. 57 . 
         FIG. 60A  shows one configuration to detect a phase difference wherein an optical fiber is not used in the FWM unit. 
         FIG. 60B  shows one configuration of the modified FWM unit. 
         FIG. 61  is an explanatory view of a method to change the amplitude of the rectangle pulse in time domain in the OPLL operation. 
         FIG. 62  shows one configuration of the OTDM signal generator in accordance with the present invention. 
         FIG. 63  is an explanatory view showing a relation between an amplifier gain by a Raman amplifier and a soliton spectrum. 
         FIG. 64  is a graph of a peak gain of the noise amplification for a bit rate and transmission distance obtained by a numerical calculation. 
         FIG. 65  is a graph of a spectrum of the output pulse train when setting a bit rate to be 320 GHz and a transmission distance at 1 km in  FIG. 64 . 
         FIG. 66  shows one configuration of the polarization maintaining device utilizing a nonlinear effect. 
         FIG. 67  is a graph of an input and output spectrum when the optical soliton train which has a repetitive frequency of 160 GHz and a half width of 1 ps transmitted in the 2 km distance of the HNLF. 
         FIG. 68  is a schematic constitutional view of alternate embodiment of the Kerr-shutter illustrated in  FIG. 24  and showing that the fiber length from the first demultiplexer to the optical switch is almost the same between both cases of transmitting through the OPLL part and transmitting directly. 
         FIG. 69  is a graph showing a relation between an HNLF length (distance) and a dispersion value used in  FIG. 67 . 
         FIG. 70  is a schematic constitutional view of the optical communication system wherein a plurality of regenerating systems shown in  FIG. 1  are connected in multi stages in series. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     According to the present invention, a wave length division multiplexing optical light including a degenerated optical signal is regenerated by utilizing a nonlinear optical effect which is described later. Thus, a nonlinear optical effect is firstly explained. 
     The nonlinear optical effect occurs significantly when a high intensity optical light enters into a substance which has a large nonlinear optical constant (hereinafter, called a nonlinear optical substance). Examples for the substance which has a large nonlinear optical constant are an optical fiber doped with germanium, fluorine, rare earth metal or the like (hereinafter, called a high nonlinear optical fiber), a ferroelectric substance such as LiNbO 3  and a semiconductor etc. Further, as an example for the high intensity light source, there are the optical signal itself, and besides the optical signal itself, at least one controlled lights or pump lights which are intentionally prepared to generate this effect. 
     When a nonlinear optical effect occurs, for example, the waveform or phase of the incident light entering the optical media changes. Further, a high resonance frequency, or the sum frequency or the subtraction frequency is generated which is a different frequency from the controlled light or the pump light. As the examples of the nonlinear optical effect, there are a self phase modulation (hereinafter called “SPM”), a cross phase modulation (hereinafter called “XPM”), a four wave mixing (hereinafter called “FWM”), a Raman amplification, a parametric amplification, a soliton effect, and a supercontinuum effect (hereinafter called “SC”). 
     In this phenomena, the level of the nonlinear optical effect such as an amount of the frequency modulation, an intensity of the generated higher harmonic wave is nonlinear to the intensity of the incident optical light. 
     The level of the nonlinear optical effect has a dependency on the polarizing condition. For example, assuming that an optical light with a strong intensity is not in the non-polarization condition but in polarized condition, and the non-liner optical medium shows anisotropic characteristic in the plane where the direction of the incident light of this strong intensity optical light crosses at a right angle. In this case, the level of the nonlinear optical effect has a polarization dependency on the polarization condition and the relative direction to the medium. Even if that medium has not such anisotropic characteristic, the level of the nonlinear optical effect heavily depends on the relative relation between the polarization condition of the optical signal and the condition of the controlled light or pump light when the strong intensity optical light is either a controlled light or a pump light. 
     Referring to the drawings, the embodiments of the present invention will be described below. 
       FIG. 1  shows first embodiment of the wavelength division multiplexing optical regeneration system  1  in accordance with the present invention (hereinafter called “regenerating system  1 ”). 
     The regenerating system  1  is used in the configuration installed in the wavelength division multiplexing optical communication system, for example it is inserted between the optical fiber  4  and the optical fiber  6 . 
     The regenerating system  1  recovers and regenerates a degenerated signal included in a wavelength division multiplexing light, where the degeneration is caused when a wavelength division multiplexing light transmits in the path in the wavelength division multiplexing optical communication system, to the conditions before transmission such as conditions just after emitted from the signal generator. The degeneration means that at least one selected from a group consisting of a level of intensity, a phase, a frequency or a polarization condition or combination thereof, changes and exceeds a predetermined allowable value. 
     The regenerating system  1  comprises a demultiplexer  8  and a multiplexer  10 . The demultiplexer  8  has one input port  8   a  and a plurality of output ports and the input port  8   a  is connected to the optical fiber  4 . The wavelength division multiplexing light which enters in demultiplexer  8  via the input port  8   a  from the optical fiber  4  is divided into a plurality of signal lights λ 1 , λ 2 , λ 3 , . . . λ n . Each of signal light λ 1 , λ 2 , λ 3 , λ n  is emitted from the output ports of the demultiplexers  8  which are prepared for each different wavelength. However, it is preferable to insert the dispersion compensator  300  before demultiplexing which works to compensate a chirping added by a dispersion of the transmission channel. This embodiment is shown in  FIG. 43 . One example of the dispersion compensator is a module using a fiber Bragg grating, a module using an etalon, a DCF (dispersion shifted fiber) module, a module using a prism pair or a grating or the like which is available for general use. Furthermore, the dispersion compensator  300  described above may be of a fixed type or a variable type, but a variable type is preferable because the range of chirping compensation can be broadened. 
     The multiplexer  10  has a plurality of input ports and a single output port  10   a , and the optical fiber  6  is connected to the output port  10   a . The optical signals λ 1 , λ 2 , λ 3 , . . . λ n  which enter the multiplexer  10  via each input port are multiplexed to become a wavelength division multiplexing optical light, then the light transmits to the optical fiber  6  via the output port  10   a.    
     The demultiplexer  8  and the multiplexer  10  may be constructed from an array waveguide diffraction grating, a filter type multiplexer/demultiplexer, an FBG (fiber Bragg grating) type multiplexer/demultiplexer. 
     Between the demultiplexer  8  and the multiplexer  10 , a plurality of optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n , are extended and both ends of each optical path  12   1 ,  12   2 ,  12   3 , . . .  12   n  are connected to the output ports of the demultiplexer  8  and the input ports of the multiplexer  10 . 
     For the optical path  12   1  selected at least one from the optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n , the demultiplexer  8 , the polarizing converter  13  and the optical signal regenerator  15  (hereinafter, called a regenerator  15 ) are inserted in this sequence. 
     In the regenerator system  1 , a selection which optical path from the optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n  requires a polarization converter  13  and an optical signal regenerator  15  to be inserted, is determined by taking account of the level of degeneration of the signal lights λ 1 , λ 2 , λ 3 , . . . λ n  which transmit in the respective optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n . Further, the embodiment shown in  FIG. 44  which does not include a polarization converter  13  is also possible. 
     The optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n  are constructed from an optical fiber such as a single mode fiber or a dispersion shifting fiber, an optical waveguide, or a space or combination thereof. However, the part of the optical path  12   1  extending between the polarization converter  13  and the optical signal regenerator  15  is preferably constituted of the polarization maintaining waveguide  12   a  which can keep a polarization condition of the transmitting light because of the reason to be described later. There are several examples of the polarization maintaining waveguide  12   a  such as a polarization maintaining optical fiber, a semiconductor optical waveguide and a glass made flat surface waveguide or the like. 
     The incident light λ 1  which enters the polarization converter  13  is converted by the polarization converter  13  to the predetermined polarized condition suitable for processing at a regenerator  15  such as converting to a linear polarization, and after that it emits from the polarization converter  13 . 
     At the time when it enters to the polarization converter  13 , the optical signal λ 1  holds a certain polarizing condition which is determined by the original wavelength and the distance, its type, or other conditions of the transmission path from the generator to the polarization converter  13 . Namely, the optical signal λ 1  holds its polarization condition corresponding to the chromatic dispersion of the light path and the polarization mode dispersion (including those by an optical elastic effect) or the like. 
     The polarization converter  13  performs a polarization conversion on the optical signal λ 1  which may take all possible polarizing condition responding to such a various and uncertain element, and then converts its polarizing condition into a desired polarizing condition. Namely, the polarization converter  13  emits a desired polarized light independently of the polarizing condition of the input optical signal at a time when it enters it, so that it has functions of converting a light in any arbitrary state of the polarization to a desired polarization condition. 
     The optical signal λ 1  which is emitted from the polarization converter  13  transmits through the polarization maintaining waveguide  12   a  and is inputted to the regenerator  15 . 
     In this situation, the optical signal transmitting through the polarization maintaining waveguide  12   a  can maintain its polarizing condition during a transmission. Thus, the optical signal λ 1  which is emitted from the polarization converter  13  transmits into the regenerator  15  maintaining its polarizing condition. 
     More concretely, when the optical signal λ 1  after a polarization conversion is made to a linear polarization, and when a polarization maintaining optical fiber is used as a polarization maintaining waveguide  12   a , it is possible to maintain the polarizing condition of the optical signal λ 1  between the polarization converter  13  and the regenerator  15 , by means of adjusting a polarization plane of the optical signal to align to the same direction as the primarily plane of the polarization maintaining optical fiber, i.e. a phase lead axis or phase delay axis. 
     However, if the polarizing condition wave is maintained even without using a polarization maintaining waveguide  12   a , a standard waveguide is acceptable. For example, by shortening the distance of the light path  12   1  between the polarization converter  13  and the regenerator  15 , it can maintain the polarizing condition or control the change of the optical signal λ 1  between them. 
     Furthermore, even in the case that the light path  12   1  between the polarization converter  13  and the regenerator  15  is not a polarization maintaining waveguide  12   a , the optical signal λ 1  in the desired polarizing condition can be inputted into the regenerator  15 , by taking account of the birefrengency and setting a polarization conversion of the polarization converter  13 , if a birefrengency of the light path  12   1  is known. 
     The regenerator  15  processes on the optical signal λ 1  using nonlinear effect and regenerates the optical signal λ 1 . 
     The optical signal λ 1  which emits from the regenerator  15  and transmits in the light path  12   1  enters into the multiplexer  10  through the input port. On the other hand, to the multiplexer  10 , the optical signals λ 2 , λ 3 , . . . λ n  which transmit in the respective optical paths  12   2 ,  12   3 , . . .  12   n  enter through separated input ports, respectively. The multiplexer  10  mixes the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  to make it a wavelength division multiplexing light, and then emits the same from the output port  10   a.    
     As shown in  FIG. 70 , an optical communication system may be constructed by connecting a plurality of the regenerating systems  1  described above in multi stages in series at an arbitrary distance. In the real application of the optical communication system, the regenerating system  1  is arranged in the manner as shown in  FIG. 70 . Namely, since the optical signal degeneration is caused by a characteristic of the optical fiber  4  or the optical fiber  6  or the transmission speed, the regenerating system may be arranged in the place where its regenerating is needed. Especially, in case that the optical signal is transmitted between the long distant stations, the optical communication system as shown in  FIG. 70  is effective. 
     There will be described a method for regenerating a wavelength division multiplexing light using the regenerating system  1  (hereinafter referred to Method-A) with reference to  FIG. 1 . 
     The Method-A comprises the steps of a wave demultiplexing process, a polarization converting process and a regenerating process. At first, in the demultiplexing process, the demultiplexer  8  separates a wavelength division multiplexing light into a plurality of optical signals λ 1 , λ 2 , λ 3 , . . . λ n  for each wavelength. 
     Then, in the polarization conversion process, the polarization converter  13  processes on at least one optical signal selected from a plurality of optical signal λ 1 , λ 2 , λ 3 , . . . λ n , which has been separated by the demultiplexing process. This polarization conversion converts the polarizing condition of the optical signal λ 1  to the best condition for realizing the desired nonlinear optical effect at the regenerator  15  or to the most efficient condition of the polarization for regenerating the optical light at the regenerator  15 . 
     In the regenerating process, the regenerator  15  performs a regeneration utilizing a nonlinear optical effect on the optical signal λ i  which has been applied a polarization conversion. 
     The regenerating system  1  and the method-A have the following functions. 
     A plurality of optical signals with different wavelength contained in the wavelength division multiplexing light, are changed in polarizing conditions based on the respective wavelengths, after the wavelength division multiplexing light emits from the signal generator of the communication system and transmits through the communication system. Therefore, when a plurality of optical signals with different wavelengths are regenerated as a whole by the regenerator using a nonlinear optical effect, the levels of the regeneration of the optical signal varies in respective wavelengths. This means that one optical signal with a certain wavelength can be excellently regenerated but another light with a different wavelength may not be regenerated. 
     As such, it is necessary to take into account the following considerations in the regenerating system  1  and the method-A: a polarizing condition of the optical signal has a wavelength dependency, and a level of the regeneration of the optical signal using a nonlinear optical effect is heavily depends on the relationship between a polarizing condition of the optical signal and a polarizing condition of the controlled light or the pump light. Thus, the wavelength division multiplexing light is separated into a plurality of optical signals with different wavelengths and the respective optical signals are treated each other as the independent light. After that, the polarizing condition of the optical signal which needs regeneration performed by the regenerator  15  is changed to the most suitable polarizing condition for the regeneration before entering to the optical light regenerator  15 . 
     In this way, the regenerating process  1  and the method-A can reproduce the optical signal which needs truly the same regeneration as the condition just after emitted from the signal generator, or the condition of the optical signal before degenerated. As a result, the waveform of the wavelength division multiplexing light including the regenerated optical signal can be regenerated in a good condition. 
     Furthermore, in the regenerating system  1  and the method-A, since the optical signal is regenerated without transforming into an electrical signal, the transmitting capacity is not restricted by electronic devices or the like. Therefore, by using the regenerating system  1  and the method-A, the transmission speed of each wavelength (channel) can be increased to exceed 40 Gbit/s, and hence the number of channels of the wavelength division multiplexing light can be decreased. This results in that the regenerator  1  is constructed from a less number of regenerators  15  than the conventional case, and further downsizing and power saving is possible. 
     The details of the regenerator  15  will be described below. 
     The regenerator  15  includes an amplifier  17 , a waveform reshaping device  19 , a clock regenerator  21  and a noise reduction unit  23  as shown in  FIG. 2 . 
     Corresponding to the required function of the regenerating system  1 , the regenerator  15  consists of at least one selected from a group of an amplifier  17 , a waveform reshaping device  19 , a clock regenerator  21  and a noise reduction unit  23 . 
     The selected devices process a regeneration utilizing nonlinear effect. Further, an arrangement of the amplifier  17 , the waveform reshaping device  19 , the clock regenerator  21  and the noise reduction unit  23  is not restricted but flexible so as to be changed as required. For example, the clock regenerator  21  may be placed before the waveform reshaping device  19 , 
     In this configuration, the amplifier  17  amplifies the attenuated optical signal, the waveform reshaping device  19  reshapes the waveform of the optical signal, the clock regenerator  21  regenerates the clock of the optical signal, and the noise reduction unit  23  eliminates the noise included in the optical signal. 
     Further, even if the optical signal is amplified by the amplifier  17  which includes a natural emitting light (non polarized light) as a noise, the noise included in the optical signal can be reduced to one half by passing the polarizer. Namely, a combination of the amplifier  17  and the polarizer can be used as a noise reduction unit  23 . 
     There are several types of amplifier  17  such as an erbium doped fiber type amplifier, a Raman amplifier, a semiconductor amplifier and a parametric amplifier or the like. 
     In the Raman amplifier, the Raman gain depends on the relative relation of conditions between the incident light and the pumping light. In the regenerating system  1 , the polarizing converter  13  converts the optical signal in advance to the suitable condition of polarization to obtain the required Raman gain, so that the Raman amplifier can amplify the optical signal stably. 
     Similarly to the Raman amplifier, both of the semiconductor amplifier and the parametric amplifier have a polarization dependency in the gain, respectively. In the regenerating system  1 , the polarizing converter  13  converts the optical signal in advance to the suitable condition of polarization to obtain the required gain, so that the semiconductor amplifier and the parametric amplifier can amplify the optical signal stably. 
     In the clock regenerator  21 , the XPM or FWM are used as a nonlinear optical effect, for example. 
     As the former (utilizing an XPM) clock regenerator  21 , there are some examples as shown in  FIG. 3 . The clock regenerator  21  comprises: a controlled light generator  25  which emits a controlled light in a pulse shape; a phase synchronizing means  31  which synchronizes a phase of the optical signal with the controlled light by the optical delay means  27  and the control unit  29 ; a high nonlinear type fiber  35  generating an XPM which is inserted in the optical path via an optical coupler  33  wherein the controlled pulse light and the optical signal transmit; WDM couplers  37 ,  39  which cause the controlled light to enter the high nonlinear optical fiber  35  or emit from that. Furthermore, it is preferable for the clock regenerator  21  to have an isolator  41  which protect the returning light from the nonlinear type fiber  35  and the optical filter  43 . 
     As the latter case (using an FWM) clock regenerator  21 , one configuration is shown in  FIG. 4 . The clock regenerator  21  comprises: a pump light emitter  45  which emits a pump light in a pulse shape; a phase synchronizing means  31  which synchronizes a phase of the optical signal with the pump light; a nonlinear optical medium  49  where an FWM is generated when the optical signal and the pump light enters thereto via the multiplexer  47  (a WDM coupler, a 3 dB coupler or the like). In this clock regenerator  21 , the optical signal entering to the nonlinear optical medium  49  is outputted along with the light to which a wavelength conversion is implemented by an FMW, so that the optical signal without the wavelength conversion and the pump light are isolated by the optical filter or the WDM coupler  51  from the optical signal with the wavelength converted. 
     In the clock regenerator  21  as shown in  FIG. 3  and  FIG. 4 , a clock regeneration process for the optical signal is carried out by using an XPM or an FWM in the highly nonlinear fiber  35  or the nonlinear optical medium  49 . In this clock regeneration process, when a gain of the nonlinear effect is so small, it is loosing a performance in the clock regeneration for the optical signal. Therefore, it is necessary to maintain a desired relative relation between the conditions of the optical signal and the controlled light or the pump light, and maintain the level of the nonlinear effect as required. 
     Concretely, if the polarizing condition of the optical signal is in a linear polarization when entering to the clock regenerator  21 , the polarizing condition of the optical signal is maintained so that the plane of the polarization of the controlled light or the pump light aligns with the plane of the polarization of the optical signal so as to be in parallel. By this alignment, the clock regenerator  21  can regenerate a clock of the optical signal at a desired level. 
     In the waveform reshaping device  19 , a soliton effect, an SPM or an SC is used as a nonlinear optical effect. 
     The former (using a soliton effect) type waveform reshaping device  19  comprises, for example as shown in  FIG. 5 , an amplifier  17   a , a nonlinear optical medium  53  and an optical filter (band pass filter)  55 . For the nonlinear optical medium  53 , a highly nonlinear fiber and a semiconductor element are listed. For example, the amplifier  17   a  is placed inside the waveform reshaping device  19  as illustrated in  FIG. 5 . However, in case that the amplifier  17  is placed as shown in  FIG. 2  and if the intensity of the optical signal entering to the waveform reshaping device  19  can be increased at a sufficient level by only this amplifier  17 , the amplifier  17   a  is not required to be added. Eventually, the amplifier  17   a  in the waveform reshaping device  19  may be added only when required. 
     In the waveform reshaping device  19  shown in  FIG. 5 , the optical signal with the intensity increased by the amplifier  17   a  enters into the nonlinear optical medium  53 , and a noise component in time region included in the optical signal is eliminated by the soliton effect generated in the medium  53 , and thus the optical signal waveform is regenerated accordingly. 
     Shown in  FIG. 6  is an example for the latter (using an SPM or an SC) type waveform reshaping device  19 . The waveform reshaping device  19  comprises an amplifier  17   a , a nonlinear optical medium  56 , an optical filter  55  and a waveform converter  57 . Similarly, because of the same reason as  FIG. 5 , the amplifier  17   a  may be added when required. 
     In the waveform reshaping device  19  of  FIG. 6 , the optical signal with the intensity increased by the amplifier  17   a  enters the nonlinear optical medium  53 , and a noise component in time region included in the optical signal is eliminated by the SPM or SC generated in the medium  53 , thus the wave band width of only the optical signal with the noise component in time region eliminated is broadened. 
     The optical signal with bandwidth broadened enters the optical filter  55 , and the only optical signal which has a predetermined wavelength can pass through the optical filter  55 , thus the reshaping of the waveform of the optical signal is carried out. The optical signal with the waveform reshaped enters the wavelength converter unit  57  and the wavelength is converted therein. 
     The wavelength conversion by the wavelength converter unit  57  is to cause the bandwidth of the optical signal which emitted from the waveform reshaping device  19  to return to the original bandwidth before broadened by the medium  56 . Therefore, if it is not necessary to return the bandwidth back to the original, the wavelength converter unit  57  may not be added. 
     In the wavelength converter unit  57 , as a nonlinear optical effect, an FWM, an XPM, SPM or the like are used. Among these, the application using an FWM or an XPM has a basically identical configuration to the clock regenerator  21  explained above. Consequently, the wavelength converter  57  can be functioned as the clock regenerator  21 . Thus in the waveform re-shaping device  19  of  FIG. 6 , there is no need to place the clock regenerator  21  at the output side. 
     Furthermore, when the wavelength converter unit  57  adopts an SPM, it may consist of an amplifier, a nonlinear optical fiber and an optical filter. 
     As described above, the waveform reshaping process by the waveform reshaping device  19  utilizes a nonlinear optical effect such as an SPM, an XPM, an FWM, a soliton, or a supercontinuum in the medium  53 ,  56 ,  35  and  49 . A XPM or an FWM is generated in the waveform reshaping process using an XPM and FWM, by means of entering both of the controlled light or pump light which is emitted from the light generator  25  or  45  and the optical signal light into the same medium  35  and  49 . 
     In the process of the waveform reshaping, it may not reshape well the waveform of the optical signal when the gain of the nonlinear optical effect is too large or too small. Therefore, it is necessary to maintain the predetermined relative relations between the polarization condition of the optical signal and the controlled light or the pump light and maintain the level of the nonlinear effect to the required level. 
     The noise reduction unit  23 , like the waveform reshaping device  19 , may be constructed by a nonlinear optical medium and an optical filter. The noise reduction unit  23  separates a signal component from the noise component after the wave bandwidth of the signal component is broadened, using a nonlinear optical effect such as an SPM or an SC. 
     For the generator of the controlled light or the pump light used in the waveform reshaping device  19 , the clock regenerator  21  and the noise reduction unit  23 , it is preferable to use one from a group of a light pulse source using a comb like dispersion arrangement (comb like dispersion profile), a light pulse source compressed by a fiber type compressor unit, a super continuum light source, a soliton pulse light source and a fiber ring laser. This is because, the light source can generate a high repetitive pulse with narrow time width of the pulse, and further the pulse time width and repetitive pulse pattern can be optimally set, considering the frequency of the optical signal or the like. 
     Referring to  FIG. 7 , the polarizing converter  13  will be detailed. 
     The polarizing converter  13  is expected to perform a polarizing conversion from an arbitrary polarizing condition into a desired polarizing condition as described above. Such a polarizing converter  13  has at least a polarization conversion unit  71  which process a polarization conversion on the incident light (optical signal) and obtain the desired polarizing conditions. 
     Concretely, if the controlled light or the pump light is in a linear polarization, the condition of the optical signal is controlled by the polarizing converter  13  so that it becomes a linear polarization and aligns its polarization plane with the controlled light or the pump light so as to be in parallel. 
     The polarizing converter  13  senses the polarization condition of the optical signal entering the polarization conversion unit  71  and performs a feed-forward control of the polarization conversion unit  71  based on the sensing output as illustrated in  FIG. 7 . Alternatively, the polarizing converter  13  senses the polarization condition of the optical signal exiting the polarization conversion unit  71  and performs a feed-back control of the polarization conversion unit  71  based on the sensing output as illustrated in  FIG. 8 . Therefore, the polarizing converter  13  is preferably equipped with the sensing unit  73  and the control unit  75  as shown in  FIGS. 7 and 8 , because the optical signal is surely polarization-converted to the desired polarization condition. 
     Concretely, as a sensing unit  73 , a polarization analyzer which senses both of the polarization condition and the intensity, or a power meter which senses only the intensity of the optical signal after divided by the optical divider  77  are listed. The control unit may be constructed by a computer unit or the like. 
     Next, referring to  FIG. 9 , there is shown one embodiment of the polarizing converter which converts the incident optical signal to a linear polarization light, in which neither a sensing unit nor a control unit is included. 
     The polarization conversion unit  71  comprises a polarization splitter  79  which splits the input optical signal into two polarized lights in which the polarization planes are crossing at right angle each other and a multiplexer  81  which mixes the two polarized lights. Two light paths extend between the polarization splitter  79  and the multiplexer  81 , and a  212  wavelength plate  83  is inserted in only one optical path. 
     When the optical signal enters the polarization conversion unit  71 , one from polarized lights split by the polarization splitter  79  enters the multiplexer  81  while the polarized plane is kept parallel to the plane of the paper as illustrated by an arrow. Another polarized light which has a polarized plane in perpendicular to the surface of the paper is rotated by 90° after passing through the λ/2 wavelength plate  83 , then enters the multiplexer  81 . Consequently, at the time when it enters the multiplexer  81 , both polarized planes of the polarized lights are in the same direction after transmitting in the two separate paths, and hence the optical signal mixed from the two polarized lights by the multiplexer  81  is always made to a linear polarized light. 
       FIG. 10  shows the polarizing converter which converts the incident optical signal to a linear polarized light, and comprises a sensing unit  73  and a control unit  75  which are used to control the polarization conversion unit  71  by a feedback control. 
     The polarization conversion unit  71  comprises a polarization splitter  79  which splits the incident optical signal into two polarized lights in which the polarization planes are in perpendicular to each other, and a multiplexer  81  which mixes the two polarized lights. The two light paths extend between the polarization splitter  79  and the multiplexer  81 , and a λ/2 wavelength plate  83  is inserted in one of the optical paths, while an optical delaying means  85  which allows to vary the optical path length is inserted in the other path. 
     Furthermore an optical light distributor  77 , which splits the incident optical signal into two lights at predetermined ratio of the intensity, is placed at the output side of the multiplexer  81 . Further, a power meter to receive and measure the intensity of the optical signal is placed as a sensing unit  73  at another output side of the optical light distributor  77 . The control unit  75  consisting of a computer or the like is electrically connected to the sensing unit  73  in order to control the intensity of the optical signal based on the sensing result by the sensing unit  73 . On the hand, the control unit  75  is electrically connected to the optical delaying means  85  as well so that the sensing results inputted from the sensing unit  73 , that is the intensity of the optical signal by the sensing unit  73 , is maximized by adjusting the optical path of another path by the optical delaying means  85 . 
     Furthermore,  FIG. 11  shows a polarization conversion unit which converts the input optical signal to an arbitrary polarized signal light including a linear polarization light, and includes the optical light distributor  77 , the sensing unit  73  and the control unit  75  which are needed to feedback control the polarization conversion unit  71 . 
     The polarization conversion unit  71  of the polarization converter comprises: a λ/4 wavelength plate  89 , a λ/2 wavelength plate  91  and a λ/4 wavelength plate  93  which are arranged in this sequence on the line extending from the collimator  86  to the collimator  87 ; and a rotating means  95 ,  97 ,  99  which are attached to each of the wavelength plates and which rotate them around each axis of the optical axis of the wavelength plate. Namely, in this polarization conversion unit  71 , the delaying phase axis and the lead phase axis of the wavelength plate  89 ,  91 ,  93  in the polarization condition of the optical signal can be adjusted by rotating each of the wavelength plates  89 ,  91 ,  93  with the rotating means  95 ,  97 ,  99 . 
     The sensing unit  73  consists of a polarizer analyzer which is capable of obtaining one of polarization conditions, for example, a stokes parameter of the optical signal, and a control unit  75  controls the rotation angle of each rotating means  95 ,  97 , 99  so that the stokes parameter becomes a desired value. Consequently, by using the polarization converter as shown in  FIG. 11 , the optical light which has an arbitrary polarization condition can be converted to the desired polarization condition. 
     In the polarization converter as shown in  FIG. 11 , when the polarization conversion unit  71  converts the optical signal to a linear polarization light, it is preferable to place a linear polarizer  101  at an output side of the conversion unit  71 , and for the sensing unit  75 , a power meter to measure the intensity of the optical signal is preferably used. 
     The reason is that, when the optical signal λ 1  passes a linear polarizer  101 , the level of polarizing is enhanced so that the output of regeneration by the regenerator  15  can be further stabilized. In addition, as described later, when the optical amplifier  105  is placed at the input side (refer to  FIG. 13 ) of the polarizing converter  13 , the noise amplified by the optical amplifier  105 , that is a spontaneous emission light in non-polarized which is added to the optical signal λ 1 , is eliminated so that a signal-noise ratio of the optical signal λ 1  can be increased. 
     The polarization conversion unit  71  may be constructed by using at least one birefringence material arranged in place of the wavelength plates  89 ,  91 ,  93 . In this case, the birefringence material is controlled by the control unit  75  so that the optical power of the optical signal after passing through the polarizer  101  is maximized at the sensing unit  73 . Furthermore, it is preferable to use a polarization maintaining waveguide  12   a  (refer to  FIG. 1 ) between the polarizer  101  and the regenerator  15  (refer to  FIG. 1 ), which can maintain the polarization condition. The primary axis of the polarizer  101  is adjusted so as to be able to maintain the polarization condition in the polarization maintaining waveguide  12   a.    
     When using the polarizing converter with the polarizer  101  equipped, a fluctuation of the control appears as a variation of the output light intensity. In contrast, it is also possible to make an incident angle at which the polarization condition can be maintained in the polarization maintaining waveguide  12   a  without using the polarizer  101 . In this case, the fluctuation of the control appears as an increase of the extinction ratio. Namely, the polarizing converter using the polarizer  101  is effective in the nonlinear signal processing in the case that a tolerance of the optical power variation is allowed to be larger than a tolerance of the variation of the extinction ratio. 
       FIG. 13  shows a second embodiment of the wavelength division multiplexing optical regeneration system  103  (hereinafter, called a regenerating system  103 ) in accordance with the present invention. 
     The regenerating system  103  is the identical configuration to the regenerating system  1  except that: an optical amplifier  105  is inserted in the optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n , extending from the demultiplexer  8  to the polarizing converter  13 ; and a variable dispersion compensator  107  is inserted in the polarization maintaining waveguide  12   a ,  12   b ,  12   c  . . .  12   n  extending from the polarizing converter  13  to the optical signal regenerating system  1  (refer to  FIG. 1 ). 
     The optical amplifier  105  is placed between the demultiplexer  8  and the polarizing converter  13  and receives the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  transmitted in the optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n  from the demultiplexer  8 . The optical amplifier  105  amplifies the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  to be at predetermined intensity and sends out to the optical paths  12   1 ,  12   2 ,  12   3 , . . .  12   n . 
     Namely, the optical amplifier  105  amplifies the incident optical signals λ 1 , λ 2 , λ 3 , . . . λ n  as required and sends out. By this means, even if the intensity of the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  after transmitting through the wavelength division multiplexing optical regeneration system is largely attenuated, it is possible to recover and compensate for the attenuation. 
     However, the position for the optical amplifier  105  is not limited to the position between the demultiplexer  8  and the polarizing converter  13 , but also any position between the demultiplexer  8  and the regenerator  15  is acceptable. Alternatively, it is acceptable to place the optical amplifier  105  at the side of the input port  8   a  of the demultiplexer  8  and amplifies the optical signals λ 1 , λ 2 , λ 3 , . . . λ n , which are in the condition as involved in the wavelength division multiplexing light. 
     The gain of optical signal amplification of the optical amplifier  105  may be controlled by feedback or feed-forward signal from the sensing unit and the control unit in the polarizing converter  13 . 
     The variable dispersion compensator  107  is placed between the polarizing converter  13  and the regenerator  15 , and the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  emitted out from the polarizing converter  13  transmit in the polarization maintaining waveguide  12   a ,  12   b ,  12   c  . . .  12   n  and enter the variable dispersion compensator  107 . The variable dispersion compensator  107  compensates the accumulated waveform distortion of the optical signal caused by a wavelength dispersion through transmitting in the optical path, and then sends out to the polarization maintaining waveguide  12   a ,  12   b ,  12   c  . . .  12   n.    
     Note that, the position for the variable dispersion compensator  107  is not limited to the specific position, so that the position between the polarizing converter  13  and the regenerator  15 , between the demultiplexer  8  and the optical amplifier  105 , between the optical amplifier  105  and the polarizing converter  13 , or between the regenerator  15  and the demultiplexer  8  is acceptable. 
       FIG. 14  is a schematic view of the third embodiment of the wavelength division multiplexing optical regeneration system  109  (hereinafter, called a regenerating system  109 ) in accordance with the present invention. 
     The regenerating system  109  is the identical configuration to the regenerating system  103  shown in  FIG. 13  except that the polarization mode dispersion compensator  110  is inserted in the polarization maintaining waveguide  12   a ,  12   b ,  12   c  . . .  12   n  extending from the variable dispersion compensator  107  to the regenerating system  15 . 
     This polarization mode dispersion compensator  110  is a device to compensate the polarization mode dispersion which is a phenomenon that the birefringency existing randomly and locally in the transmitting path makes the difference in the transition speed among polarization mode to make a distortion in the time waveform of the optical signal. 
     Consequently, by this regenerating system  109 , a distortion of the waveform by the polarization mode dispersion in the transmitting path can be compensated so that the transmission distance of the optical signal or the wavelength division multiplexing light after sent out from the regenerating system  109  can be further extended. 
       FIG. 15  shows a fourth embodiment of the wavelength division multiplexing light regenerating system  111  (hereinafter, called a regenerating system  111 ) in accordance with the present invention. 
     The regenerating system  111  is different from the regenerating system  103  in that the multiplexer is an interleaver  112  which has a polarizing interleaver function, and in addition, the polarizing converter  114  is placed at an output side of the polarizing interleaver  112  to convert the polarizing condition of the wavelength division multiplexing light sent out from the polarizing interleaver  112 . 
     The polarizing interleaver  112  has an interleave function to mix the optical signals in the condition of polarization in such manner that each of neighboring optical signals in wavelength crosses at right angle, when mixing a plurality of the optical signals of different wavelength λ 1 , λ 2 , λ 3 , . . . λ n . 
     Therefore, when the wavelength division multiplexing light after mixed by the polarizing interleaver  112  transmits in the same optical path, it is possible to suppress a degeneration of the optical signal caused by the interaction between the neighboring lights in wavelength i.e., neighboring channels. 
     The polarizing converter  114  performs a polarizing conversion on the wavelength division multiplexing lights sent out from the output port  112   a  of the polarizing interleaver  112  so that the effect of the polarizing mode dispersion on the wavelength division multiplexing lights is minimized in the optical path (optical fiber  6 ) extending between the polarizing converter  114  and the next wavelength division multiplexing regeneration system, or the receiver. 
     Therefore, according to the regenerating system  111 , it is possible not only to regenerate the incident light of the wavelength division multiplexing light entering to the regenerating system  111 , but also to adjust the condition of the wavelength division multiplexing light being suitable in the polarizing condition for the transmitting optical path after sent out from the regenerating system  111 . For example, the regenerating system  111  can suppress a degeneration of the wavelength division multiplexing light until reaching to the next wavelength division multiplexing regenerating system or the receiver unit. 
     In this regenerating system  111 , the optical amplifier  105  is not inserted between the demultiplexer  8  and the polarizing converter  13 , but inserted in the polarization maintaining waveguide  12   a ,  12   b ,  12   c  . . .  12   n  extending from the polarizing converter  13  to the regenerator  15 , and the regenerating system  111  controls the optical amplifier  105  by the feed-forwarding based on the intensity of the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  detected by the sensor of the polarizing converter  13 . 
     In this configuration, the intensity of the incident optical signals λ 1 , λ 2 , λ 3 , . . . λ n  to the regenerator  15  can be maintained constant, and consequently the regeneration of the optical signals λ 1 , λ 2 , λ 3 , . . . λ n  by the regenerator  15  can be further stabilized. 
       FIG. 16  shows a fifth embodiment of the wavelength division multiplexing optical regeneration system  116  (hereinafter, called a regenerating system  116 ) in accordance with the present invention. 
     This regenerating system  116  includes a plurality of subsystems  116   a ,  116   b , . . . which are connected each other. 
     The subsystems  116   a ,  116   b , . . . are the identical configuration to the regenerating system  111  except that: an optical switch  118   a ,  118   b , . . . and an optical switch  120   a ,  120   b , . . . are connected closely to both ends of the optical paths  12   1  that is the side of the demultiplexer  8  and the multiplexer  112 , respectively. In  FIG. 16 , in order to avoid complexity of lines, it is omitted to draw optical paths  12   2 ,  12   3 , . . .  12   n , in each subsystem  116   a ,  116   b , . . . . 
     In the regenerating system  116  as illustrated in  FIG. 16 , the optical switches  118   a  and  118   b  are connected through optical path  122  and the optical switches  120   a  and  120   b  are connected through an optical path  124 . 
     As a result, the regenerating subsystems  116   a ,  116   b , . . . can process a regenerating on not only the optical signal which is demultiplexed light at own demultiplexer  8  itself, but also the optical signals which are transmitted from other point of the wavelength division multiplexing optical communication systems, because at least one optical switch is inserted in the optical paths  12   1  or the polarization maintaining waveguide  12   a.    
     By connecting each subsystem  116   a ,  116   b , . . . via the optical paths  122  and  124 , the regenerating subsystems  116  can work as a router in the whole system. 
     Furthermore, the optical switches  118   a ,  118   b , . . . or the optical switches  120   a ,  120   b  are preferable to be a semiconductor switch. This is because, it can avoid larger dimensions of the regenerating systems  116  and increase the switching speed to switch the optical path between the subsystems  116   a ,  116   b , . . . . 
     More specifically, the semiconductor switch is an optical switch which is made from a semiconductor material by applying a micro machining technology such as an anisotropic etching or a sacrificing etching, and it is one of Micro-Electro-Mechanical-Systems (hereinafter, called MEMS). 
     The present invention is not limited to the embodiments described above, but it is possible to change the configuration. For example, the regenerator  15  in  FIG. 2  may be fabricated on a semiconductor substrate as an integrated circuit, and thus the regenerator  15  can be integrated into one package with further miniaturized. 
     In the regenerator  15 , since a waveform re-shaping unit, the noise suppression unit and the clock regenerator have the same function when utilizing nonlinear effect, one or two of these can be omitted to decrease the number of components of the regenerator  15 . 
     The details of the regenerator of the present invention will be further detailed below (refer to  FIG. 1 ). 
       FIG. 17  is a conceptional view to show one embodiment of the regenerator  200  in accordance with the present invention. The regenerator  200  consists of a soliton converter  202 , a pulse roller  204 , a Kerr-shutter  206  and a soliton purifier  208  which are arranged in this sequence. As a regenerator system  1 , the polarizing converter  13  is preferably arranged to be placed before the regenerator  200  as illustrated in  FIG. 1 , but as far as the polarization wave is maintained, the system configuration without using the polarizing converter  13  may be acceptable as shown in  FIG. 44 . 
     At first, the waveform of the optical signal  210  which has been degenerated while transmitting is reshaped and the residue of the chirping is eliminated by the soliton converter  202 . Then, the optical signal is sent to the pulse roller  204 , where the reshaped optical signal is transformed to a rectangular waveform as suitable to next stage of the Kerr-shutter  206 . After the preprocessing as described above, using the Kerr-shutter  206 , the waveform of the optical signal is reshaped and also the clock time is regenerated. Finally, in order to improve the quality of the regenerated light, the components excluding a soliton, for example, a spontaneous emission light generated when amplifying an optical light can be eliminated by the soliton purifier  208 . Since the function of each component in the regenerator  200  follows its nonlinear optical characteristic, it generally shows a polarization dependency. Therefore it is preferable to place the polarizing controller  302  and the polarizer  304  at the input side of each component  202 ,  204 ,  206 ,  208  as required, as shown in  FIG. 45 . But if the change of polarization condition in the regenerator  200  is so small, those are not required. As illustrated in  FIG. 46 , it may be acceptable to construct a system using at least one of the components  202 ,  204 ,  206 ,  208  in the regenerator  200 . For example, it can be constructed using a device A, a soliton converter which is disposed as our device, and device B as illustrated in  FIG. 46 . Based on the purpose, the our device may be selected appropriately from the component  202 ,  204 ,  206 ,  208  described above, and the corresponding device A and device B are disposed. 
     Each component  202 ,  204 ,  206 ,  208  in the regenerator  200  will be described below. 
     The first stage of the regenerator  200  is a soliton converter  202 . The related technology to a soliton converter are reported in Dany et al., Opt. Lett., 25, p. 793, 2000 or Matsumoto et al., IEEE, Photon, Technol, Lett., 14, p. 319, 2002. 
       FIG. 18  shows one embodiment of the soliton converter  202 . The soliton converter  202  comprises an anomalous dispersion fiber (ADF)  212  and an optical band pass filter (BPF)  214 . The incident optical signal is converted to a soliton like pulse train by the interaction between the anomalous dispersion effect and the nonlinear effect along with transmission of the anomalous dispersion fiber. Furthermore, by passing the optical filter  214 , an optical limiter function works and a noise reduction is also carried out. If the input optical power for the ADF  212  is insufficient to generate a soliton effect, the optical amplifier  216  and the optical filter  218  which eliminates the spontaneous emission light may be placed in a stage before the ADF  212  as shown in  FIG. 18 . 
     In the configuration of the soliton converter  202  as shown in  FIG. 18 , an input noise amplification by a soliton effect during a soliton transmission occasionally causes a problem. (Kubota et al., J. Opt. Soc. Am, B, 16, p. 2223, 1999). The reason is that an amplitude/phase noise is amplified by a parametric gain generated by the interaction between the anomalous dispersion effect and the nonlinear effect during a soliton transmission with a noise included. 
     To suppress this, it is preferable to shorten the fiber length of ADF  212  in the soliton converter  202 . When the ADF  212  fiber length is shortened, the distance in which a noise and a soliton transmit is decreased, and this directly works for suppressing the interaction. Therefore, the shortening of the ADF  212  length is effective for the reduction of the noise amplification with the reshaping of the waveform. Further, details of the fiber length of the ADF  212  will be described later. 
     On the contrary, the nonlinear effect is essential to realize a complete reshaping of the waveform. The soliton converter  202  needs a certain fiber length which is a minimum length (nonlinear length) to generate nonlinear effect. The published paper (Deny et al., Opt. Lett., 25, p. 793, 2000) discloses that a soliton converter needs a certain fiber length exceeding a soliton frequency Z 0 . Z 0  is given by the following equation: 
     
       
         
           
             
               
                 
                   
                     z 
                     0 
                   
                   = 
                   
                     
                       π 
                       2 
                     
                      
                     
                       
                         T 
                         0 
                         2 
                       
                       
                          
                         
                           β 
                           2 
                         
                          
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     wherein T 0  is a full-width at half maximum of the point where the intensity is a 1/e of the peak value of the input pulse, and β 2  is a dispersion value of a soliton converter. In order to demonstrate experimentally the fiber length dependency of the performance of the soliton converter  202 , an optical signal noise ratio (OSNR) of the output pulse from the soliton converter and a dependency of the input power P in  at maximized spectrum width using three different types of fiber length normalized by the soliton frequency were measured. The results are shown in  FIG. 20A  and  FIG. 20B . The soliton order N is on the horizontal axis. N is a square root of the normalized P in  value carried by a basic soliton excitation power and it is given by the following equation: 
     
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                       γ 
                        
                       
                           
                       
                        
                       
                         P 
                         0 
                       
                        
                       
                         T 
                         0 
                         2 
                       
                     
                     
                        
                       
                         β 
                         2 
                       
                        
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     wherein γ is a fiber nonlinear coefficient. 
     As indicated in  FIG. 20A  and  FIG. 20B , the OSNR degenerates as N is increasing in all the soliton converter independently of the normalized fiber length: z. However, when ADF  212  fiber length is at around second order of the soliton frequency, its degeneration is small. Also it is indicated that it is not only this soliton converter that increases the spectrum width (Δυ/Δυ in ). From the above experimental result, the soliton converter whose normalized fiber length: z is less than 2, is effective to suppress the noise amplification. When z=2 or less, the relation between N and OSNR or the relation between N and Δυ/Δυ in  shows the same result as shown in the upper side of  FIG. 20A  and  FIG. 20B , so that it is not illustrated in the figures. 
     As the soliton converter  202  is placed at the first stage of the regenerator  200 , the input pulse may have a chirping by the residue of the dispersion. The optical signal has a problem that the pulse width is broadened when converted into a pulse if it has a chirping. Therefore, in the soliton converter  202 , the performance to a chirping included in the input pulse becomes important. 
     To investigate this, the pulse having (before the soliton converter) a chirping added during a transmission in the single mode fiber (SMF) is inputted to the soliton converter  202  and the output pulse waveform is measured. The self correlation waveform width dependency on the input power P in  is shown in  FIG. 21 . The left side in  FIG. 21  shows a pulse width before the output optical filter and the right side in  FIG. 21  shows a pulse width after the output optical filter. 
     As understood from  FIG. 21 , when the amount of input chirping increases (SMF length is increased) as increase of P in , it is shown that Δt AC  has a tendency of decreasing regardless the amount of input chirping (i.e., length of SMF). Namely, according to the interpretation of the experimental results, it demonstrates that the input chirping dependency can be compensated by the P in  adjustment.  FIG. 22  shows the results which are directly confirmed by the experiment. 
     The left side in  FIG. 22  shows an input pulse, the middle part in  FIG. 22  shows an output pulse waveform from the soliton converter after the P in  adjustment, and the right side in  FIG. 22  shows an output pulse waveform from the output filter. It is thus shown that there can be obtained the same pulse waveforms as the output pulse waveforms by the P in  adjustment even when pulse trains having different characteristics are inputted into the soliton converter, respectively, as shown in the left side of  FIG. 22 . One of the specific example of the P in  adjustment is a method to control the optical power by placing a variable attenuator at the front stage of the soliton converter. 
     The dependency of the self correlation wave width Δt AC  of the soliton output and OSNR on the P in  in the short length soliton converter of this embodiment is shown in  FIG. 23 . As described above, OSNR degeneration is suppressed to a certain extent compared to the long type. On the other hand, at the value of P in  when Δt AC  is in the minimum condition, the OSNR degeneration is remarkable. 
     In other words, there is a trade-off between a pulse compression and an increase in noise. A method for realizing a compatible method of a noise suppression and a pulse compression is a combination of the noise suppression and the pulse compression which is one of the embodiments of the present invention ( FIG. 19 ). In this configuration, the pulse compressor is placed before the waveform reshaping device. In this case, the waveform reshaping device includes a Mamyshev filter or a NOLM as well as the soliton converter as described above. In general, the higher the input pulse peak power is, the higher performance it is expected for the waveform reshaping device which is based on the fiber nonlinear effect. The present invention intends to utilize this fact. It compresses the pulse train in advance and amplifies the peak power, then forwards to the waveform reshaping device. 
     Especially, the compressor based on an adiabatic compression method which has an advantage of noise immunity is suitable to this application. For this application, there is a method utilizing a fiber whose dispersion values decreases along with the fiber longitudinal direction (a dispersion decreasing fiber), or a fiber whose gain is gradually increasing (a Raman amplifier fiber). It is proposed to construct a fiber having a step-like dispersion profile (SDPF) by connecting a few different types of fiber which simulates the fiber in the former case, or a fiber having a comb-like dispersion profile (CDPF). Especially, a CDPF which is constructed from two types of fiber is preferable because of the easiness of manufacturing. 
     In addition, the adiabatic compression utilizes the characteristic that the optical soliton is a stable pulse in which the dispersion effect and the nonlinear effect are balanced, and when the effect of either one increases or decreases, the another effect follows so that the optical pulse parameters automatically changes. Here, the dispersion effect is inverse proportion to the squared number of a fiber dispersion and a pulse width, and the nonlinear effect is proportional to the fiber nonlinear constant and the pulse power. In brief, in the condition of nonlinear effect is constant, when the fiber dispersion decreases, the square of the inverse number of a pulse width is needed to increase to satisfy it, and as a result, the pulse width becomes smaller. 
     Based on the above principle, in order to realize the pulse adiabatic compression, besides a method of making a fiber whose dispersion values decrease along with the fiber longitudinal direction, there are considered a method using a distributed Raman amplifier and a method increasing the nonlinearity along with the fiber longitudinal direction. In the former method, as the pulse power is increased by a distributed Raman amplifier while transmitting, the nonlinear effect increases, so that the pulse width becomes smaller as the dispersion effect follows it in the fiber in which the dispersion value and nonlinear constant is fixed. 
     In the latter case, in the fiber whose dispersion value is constant and nonlinear effect increases in the longitudinal direction (even if the pulse power decreases by the fiber loss, the fiber nonlinearity constant sufficiently increases so that when a product of the power and the nonlinear constant increase in the longitudinal direction, and the nonlinear effect increases accordingly), the dispersion effect has to increase, thus the pulse width becomes smaller in order to follow the increase of the nonlinear effect. 
     The method to increase the nonlinear constant can be realized by, besides continuously increasing in the fiber longitudinal direction, varying in a step-like manner, or arranging the nonlinear constant in a comb-shape to approximate it. 
     The fiber type waveform reshaping device is described as above, but it is not limited to the fiber type to realize the function of the waveform reshaping device. Generally, a device in which the input/output characteristic has a threshold and saturation characteristic (referred to as a saturable absorber), has a function of waveform reshaping. Herein below, the saturable absorber will be explained. 
     Devices each having a saturable absorbing characteristic is essential in the application of a whole optical regenerating relay (O3R) or a mode synchronized laser. Conventionally, there has been utilized an optical fiber nonlinearity for the saturable absorber, however, it has been proposed to directly utilize a saturable absorbing characteristic of a material such as; a semiconductor saturable absorber mirror (SESAM) (S. Tsuda, W. H. Knox, E. A. de Spuza, W. Y. Jan, and J. E. Cunningham, “Low-loss intracavity AlAsAlGaAs saturable Bragg reflector for femtosecond mode locking in solid-sate lasers,” Opt. Lett., Vol. 20, No. 12 pp. 1406-1408, Jun. 15, 1995), and a carbon nanotube (CNT) (S. Y. Set, H. Yaguchi, Y. Tanaka, M. Jablonski, Y Sakakibara, A. Rozhin, M. Tokumoto, H. Kataura, Y. Achiba, K. Kikuchi, “Mode-locked fiber lasers based on a saturable absorber incorporating carbon nanotubes” Postdeadline papers, OFC2003, PD 44, 2003). However, when using the above-mentioned saturable absorber of the SESAM, the saturable absorbing characteristics of the material have to be optimized to obtain the desired characteristics, and thus it is difficult to maintain stable the conditions when manufacturing the material and to reproduce the material. 
     The saturable absorbing characteristic in case of using SESAM is determined by a composition in the neighbouring area of the saturable absorbing layer, the absorbing spectrum, the thickness and the relative locations of the Bragg mirror and the saturable absorbing layer. In particular, the total optical intensity density has a distribution on the depth direction of the semiconductor, because the reflection light at the Bragg mirror forms a standing wave with the incident light. The relative relation between the saturable absorbing layer and the distribution of the optical intensity density is one of elements giving a different saturable absorbing characteristic to the same intensity of the incident light. In case of the CNT, a band gap and a thickness are primary design parameters. In any cases, it is difficult to realize a desired saturable absorbing characteristic precisely, and there are problems such as a yield or a limitation in the characteristic in the O3R or the mode synchronized lasers when constructing systems 
     Since, in the mode synchronized lasers, the oscillation pulse characteristic largely depends on the saturable absorbing characteristic, the obtained pulse characteristic is determined by a selection of the saturable absorber. This is not only worsening the yield of the mode synchronized laser, but also limiting the characteristic of the oscillating pulse. The saturable absorbing characteristic required for oscillating pulses is fundamentally different from the saturable absorbing characteristic required for the shorter pulse operation and higher energy operation, and consequently the pulse characteristic under normal operation is problematically sacrificed to oscillate the pulse. 
     In order to solve such problem, as one of the embodiments of the present invention, the saturable absorber described above is changed to a variable characteristic saturable absorber. If the saturable absorbing characteristic is variable, there can be added a process to adjust the characteristic realizing a desired performance in the manufacturing, thus improving the yield. If it is possible to adjust the saturable absorbing characteristic after installing the system, it allows a flexible adjustment when the operating conditions are needed to change due to external turbulence or change of system parameters. 
     In the mode synchronized lasers, if the saturable absorbing characteristic is variable, the saturable absorbing characteristic can be adjusted so as to obtain a desired pulse characteristic from the saturable absorbing characteristic at the time when oscillating the pulse, while keeping a pulse operation. Consequently it makes possible to coexist the pulse oscillation with the short pulse/high energy operation which has been difficult. 
     The contents mentioned above will be further explained referring to the drawing. FIG.  47  shows a typical saturable absorbing characteristic. The horizontal axis is an intensity of the input light and the vertical axis is an intensity of the output light. It is remarked that in the region of lower intensity of the input light, there is a threshold, and in the region of higher intensity of the input light, there is a saturated characteristic, which is different from a linear characteristic. Being variable of the saturable absorbing characteristic means that the curve is adjustable. Further detailed, this threshold saturation characteristic can be made variable to a certain intensity of the input light. Namely, the curve in  FIG. 47  can be extended or shortened or a gradient angle thereof can be changed, as shown in the figure. 
     Generally, if the optical intensity density of the incident light entering the saturable absorber is variable, the saturable absorbing characteristic can be adjusted.  FIG. 48  shows its configuration. During the process starting from collecting the light which is emitted out from the optical fiber  306  to reconnect to another optical fiber  308 , the optical intensity density of the incident light entering the saturable absorber  310  varies depending on the position where the saturable absorber  310  is placed.  FIG. 48  shows a transmissive type, and  FIG. 49  shows a reflective configuration. In the reflective type, the same result can be obtained by moving the optical system so that the beam waste changes. 
     It is also possible to add a surface distribution to the saturable absorbing characteristic of the SESAM or CNT. For example, in case of the SESAM, it can fabricate so as to continuously change the relative position on the surface between the Bragg mirror and the saturable absorbing layer by utilizing a distributed layer thickness in the surface. Alternatively, instead of the relative position between the Bragg mirror and the saturable absorbing layer, it is possible to have a distributed saturable absorbing characteristic in the surface by changing the composition or the thickness of the saturable absorbing layer. In the CNT, it is also possible to have a distributed saturable absorbing characteristic in the surface by adding a distributed inner thickness of the surface or changing a band gap in the surface by making a temperature gradient when manufacturing or changing the CNT construction. 
     By placing the saturable absorber  310  which has an inner surface distribution of the saturable absorbing characteristic in the optical path so as to freely move between the optical fibers  306  and  308  connected in the space as shown in  FIG. 50 , a device having a variable saturable absorbing characteristic can be realized. The movable direction is anyone of vertical, lateral, diagonal directions and inclined direction to the optical axis. In any direction, the length of the optical path running through the saturable absorber  310  is allowed to vary. The moving directions of the saturable absorber for both of the transmissive type as  FIG. 48  and the reflective type as  FIG. 49  can be the same configuration. 
     When applying the present invention to the mode synchronized laser, by shifting the curve in  FIG. 47  gradually upwards from the saturable absorbing characteristic to start a pulse oscillation, the pulse width and the energy can be made gradually narrower and larger, respectively. This corresponds to move the saturable absorber  310  gradually towards the lens side  316  from the center position  312  in  FIG. 48 . In  FIG. 48 , although the saturable absorber  310  moves from the center  312  to the lens side  316 , the same results may be obtained even when moved from the center  312  to the lens side  316 . 
     When moving the saturable absorber  310  in this manner, the intensity density of the incident light becomes smaller, so that this case corresponds to expand the curve in  FIG. 47  vertically and horizontally. In the configuration as  FIG. 50 , when the saturable absorber  310  is moved towards the position where the saturable absorbing characteristic is getting weaker, the same results may be obtained. The weak saturable absorbing characteristic corresponds to a thinner film layer of the CNT, if taking the CNT as an example. The variability can be used for adjusting the saturable absorbing characteristic in the manufacturing process and can be also applicable as a controllable device during the system operation. 
     Next, there is shown in  FIG. 24  one of the embodiments of the Kerr-shutter  206  as a timing regenerator. The embodiment comprises an OPLL (optical phase locked loop)  220  and an optical switch  222 . The OPLL  220  comprises a phase comparator  224 , an optical LO generator  226  and a controller  228 . 
     In the OPLL  220 , the phase comparator  224  senses a phase difference between the external optical signal and the optical LO signal light (hereinafter, called an optical LO), and the controller  228  controls a repetitive frequency of the optical LO based on the phase difference. As a result, the optical clock pulse train which is synchronized with the external optical signal can be generated. Thus, the optical signal with time reformed can be generated by switching the optical clock pulse train and the external optical signal in optical domain. The details of respective parts of the present invention will be described below. 
     As described above, the OPLL  220  comprises a phase comparator  224 , an optical LO generator  226  and a controller  228 . The phase comparator  224  essentially comprises an FWM unit which is made of an optical fiber, an optical filter  232  and a photo receiving unit  234 . The FWM unit  230  is made of an optical element such as a nonlinear optical fiber, a PPLN (Periodically-poled LiNO 3 ), SOA (semi-conductive optical amplifier) or the like which performs an FWM. 
     In the OPLL  220 , by entering an external optical signal and an optical LO to the FWM unit  230 , an FWM light is newly generated. The FWM light is filtered by the optical filter  232  and sensed by the photo receiving unit  234  which is made of a photodiode (PD). 
     The photo receiving unit  234  and the control part  228  detect the electrical signal as a phase difference signal, and control the optical LO oscillation frequency (corresponding to repetitive frequency) based on this value. As a result, the clock pulse train synchronized with the external optical signal is generated. In order to enhance the nonlinear characteristic sufficiently in the FWM unit (generating an FWM at high efficiency), the optical amplifier  236  such as an EDFA or a semiconductor amplifier, and the optical filter (not shown) may be placed before the FWM unit  230  as required, as shown in  FIG. 24 . Also, the optical element which performs an FWM such as PPLN, or SOA or the like may be placed in the FWM unit in order to sense the phase difference signal. In this configuration, the length of OPLL loop is further shortened and the OPLL bandwidth is broadened. 
     In addition, an intensity variation of a second-harmonic generation (SHG) can be used for sensing a phase difference between the external optical signal and the optical LO, replacing the FWM by a PPLN, or a nonlinear optical crystal. Further, by causing to enter the PD which has no sensitivity to the wavelength of the input signal light but is sensitive to the shorter wavelength light, the phase difference can be converted to the photo electric current by a two photon absorption effect in the PD. 
     On the other hand, the optical LO generator  226  consists of the beat light generators  238 . Concretely, one or more semiconductor lasers which have two or more frequency components are preferable. In  FIG. 24 , the configuration constructed by two laser diodes (LD) is shown as an example. CW light emitted from the beat light generators  238  are mixed by the optical coupler, then a beat light is generated. The beat frequency can be controlled by adjusting the driving current and the temperature of at least one of the beat light generators  238 . 
     The beat light is inputted to the phase comparator  224  as an optical LO and at the same time, and it is also inputted to the optical switch part  222  as a clock signal. If necessary, it may reshape the waveform of the clock signal to fit the optical switch part  222  by converting the beat light to the soliton train by the pulse compressor  239  before entering the optical switch part  222 . 
     The most important point during the clock extraction process is a reduction of the timing jitter of the output clock pulse train. Namely, it is the preciseness in the time position of the pulse. The present embodiment adopts the following three methods to decrease the timing jitter. 
     The first method is a shortening of the OPLL  220  loop length. The timing jitter is correlated with the OPLL  220  loop length and the shortening of the loop length (shorter fiber length) is effective to suppress the jitter. Especially in this embodiment, since the phase comparator  224  fundamentally consists of the FWM unit, it is easier to shorten the loop length than the conventional NOLM type. As a result, in this case, it is possible to generate a clock pulse train with a suppressed jitter. A highly-nonlinear fiber (HNLF) that is more than five times of the nonlinear coefficient of the typical transmission fiber may be adopted as the optical fiber of FWM unit  230  in the phase comparator  224  to realize the short loop length (fiber length). 
     In the second method, the fiber length L A-B  [m] is adjusted to about the same length as the fiber length L A-C  [m] in  FIG. 68 . The L A-B  is a fiber length when directly connecting the demultiplexer A to the optical switch B, and the L A-C  is a fiber length when connecting the demultiplexer A via the clock extractor, the fiber compressor to the optical switch C. τ A-B  is a time that the separated optical signal at the demultiplexer A travels to the optical switch B, and τ A-C  is a time that the separated optical signal at the demultiplexer A travels via the clock extractor, the fiber compressor and reaches to the optical switch C. They are expressed as τ A-B =(L A-B /V) [s], and τ A-C =(L A-C /v) [s], wherein v [m/s] is a velocity of a light traveling in a fiber. The length of the filer compressor in  FIG. 68  is a few km distance. Therefore, if a fiber of the same length as L A-C  [m] is not added between the demultiplexer A and the optical switch B, |τ A-B −τ A-C | is derived to a few μs which is a time difference that the optical signal reaches to the optical switch. The upper limit of the bit rate difference between the optical signal and the optical LO referring to the time difference is shown as: 
       Δω&lt;|δ B+δL|×|τ   A-B −τ A-C |
 
     wherein 
     δB [Hz]: a signal bit rate line width, and 
     δL [Hz]: a frequency line width of LO output light. 
     Therefore, by changing the |τ A-B −τ A-C | to a smaller value (in other words, by changing the difference of the fiber length |L A-B −L A-C | to a smaller value), the bit rate difference Δω becomes smaller and hence the timing jitter during optical switching can be reduced. 
     The third method is an LD pair driving circuit. In the optical LO generator  226 , it can compensate the LD wavelength fluctuation caused by a driving current source by a method of driving two LDs in series. Actually, the measured results of the noise suppression by the method of LD series driving is shown in  FIG. 25 .  FIG. 25A  shows a timing jitter characteristic when driving the LD pair independently and  FIG. 25B  shows a timing jitter characteristic when driving the LD pair in series. It is demonstrated that the jitter when using a series driving is reduced by one-tenth compared to the independent driving. The noise reduction by the series driving is realized by keeping a change to be equal among the two LD driving currents. Instead of the LD pair driving circuit, a current mirror circuit can make the same result. 
     As shown in  FIG. 24 , generally the OPLL  220  is separated from the optical switch  222  and the pulse compressor  239  in the optical LO generator  226  has a length L(m). τ 1  is defined as a time that the incident light travels through the phase controller  246  to the optical switch  222  after the incident light is divided at the first demultiplexer  241  (the demultiplexer in  FIG. 24  where the incident optical signal is divided into the directions of the OPLL and the optical switch). Further, τ 2  is defined as a time that the synchronized signal reaches to the optical switch  222  after synchronized at the OPLL  220 . Then, τ 1  and τ 2  show different values, and time lag is expressed by the following equation: 
         T   lag =|τ 2 −τ 1   |=n·L/c  
 
     wherein n: refraction index of a fiber and c: velocity of a light. The input signal light frequency ω1(t−τ 1 ) and the beat light frequency ω2(t−τ 2 ) which is emitted from the optical LO generator  226  after clock extraction are different in general. The phase difference Δφ between the input signal light at the optical switch  222  and the synchronizing signal generated at the optical LO generator  226  is expressed as follows: 
       Δφ=2π(ω2( t−τ   2 )−ω1( t−τ   1 ))· T   tag   =Δω·T   lag .
 
     When the tolerance of the phase difference Δφ is 0.05 (rad), it results in Δω&lt;0.05 (n·c)/(2πL) (Hz). For example, when L=1000 (m), n=1.5 and a frequency difference Δω&lt; about 500 Hz, the phase difference is acceptable. In this way, depending on the length L of the pulse compressor  239 , the tolerance of the frequency line width of the output beat light from the optical LO generator  226  is determined. 
     Next, referring to  FIG. 24  and  FIG. 68 , the optimization of the loop length is described. In this case, the loop length means the optical fiber length in the optical clock extraction unit as shown in  FIG. 68 . As described above, the loop length L Loop , is preferably determined so as to satisfy the following equation: 
       Δω( L   Loop )&lt; v·X/n·L   A-B  
 
     wherein 
     Δω: bit rate difference, 
     L Loop : loop length, 
     v: velocity of the light in the optical fiber, 
     L A-B : length of the fiber connected between the demultiplexer A and the optical switch B, 
     n: refraction index of the optical fiber, and 
     X: arbitrary number. 
     Namely, in the present invention, the L Loop  becomes small because a highly nonlinear fiber is used as the FWM unit  230 . Consequently, Δω becomes small, thus reducing the timing jitter. 
     The optical switch  222  comprises an FWM unit  242 , an optical switch  244  and a phase controller  246  (refer to  FIG. 24 ). This is identical to the phase comparator  224  except the phase controller  246 . 
     In the optical switch  222 , firstly an optical signal and a clock pulse train are inputted into the FWM unit  242 . Then, the timing reshaped optical signal is generated as an FWM light at the output port of the FWM unit  242 . The light wave is separated from the input light components by the optical filter  244  and outputted. 
     The clock pulse and the optical signal are synchronized (with the same frequency) by the OPLL  220 , but the phases need to be adjusted. To this end, the phases are adjusted by the phase controller  246  consisting of the variable delay line (DL) which is inserted in at least one optical path of either the optical signal or the clock pulse. 
     Regarding the delay line in the phase controller  246 , either a half fixed or full variable type is acceptable. If the phase controller  246  is a half fixed type, the temperature of the whole system needs to be controlled constant so that the amount of adjustment necessary for the ambient temperature change would not vary. If the phase controller  246  is a variable type, the amount of adjustment for phase control is determined based on the electrical signal obtained by sensing a par of the output pulse. 
     The fiber type FWM unit  242  will be described below. In this fiber type FWM unit, there are occasions where a waveform distortion occurs due to the effect of the dispersion and nonlinear during a transmission of the light in the optical fiber. To suppress this, it is important to select a suitable wavelength (frequency) arrangement and a fiber type to fit the input pulse condition. The method of suppression will be described in sequence from (1) through (4), as described below. A frequency arrangement and a designing of the fiber will be reviewed, where abbreviated as follows (refer to  FIG. 51 ). 
     Δt p : a time width of the input pumping pulse, 
     Δν p : a spectrum width of the input pumping pulse, 
     P p : a peak power of the input pumping pulse, 
     Δt s : a time width of the input signal pulse, 
     Δν s : a spectrum width of the input signal pulse, and 
     P s : a peak power of the input signal pulse. 
     (1) In the FWM, it is necessary to maintain sufficient frequency distance Δν (detuning amount) between the pumping light and the signal light in order to avoid overlapping among spectrum components of the three wave lights. The condition is given by the flowing equation: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     v 
                   
                   &gt; 
                   
                     
                        
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             v 
                             p 
                           
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             v 
                             s 
                           
                         
                       
                        
                     
                     2 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     (2) In the wavelength conversion to Δν given from the above equation, the band width of at least 2Δν is needed. On the other hand, the possible causes to limit the band width of the fiber in the FWM are (a) un-matching phase due to wavelength dispersion, (b) polarizing dispersion, and (c) coherence degeneration. In case of the cause (a), it can be avoided by frequency arrangement to satisfy the phase matching condition, that is by adjusting the pumping wavelength to be coincide with the fiber zero dispersion wavelength. Generally, the cause (c) affects less influence than the cause (b). Consequently, in the typical FWM, the limitation in the band width due to the polarizing dispersion is dominant. For the limitation in the band width, a fiber polarizing holding (S. Watanabe et. al., ECOC97, PD7, 1998) is effective, but there is a problem in which it is difficult to manufacture. Another method of broadening a bandwidth is a shortening of the fiber length (O. Aso, et. al., EL, vol. 36, p 709, 2000). It is shown in  FIG. 52 . Based on the relation between the fiber length and the bandwidth shown in  FIG. 52 , the fiber length ΔL which can obtain the required bandwidth for Δν is prepared. 
     (3) To obtain sufficient nonlinear effect to generate an FWM, it is necessary for the fiber length L to be the same as L NL  which is a nonlinear length for the input pumping pulse. This is defined by the following equation: 
     
       
         
           
             
               
                 
                   
                     1 
                     &lt; 
                     
                       L 
                       
                         L 
                         NL 
                       
                     
                   
                   = 
                   
                     γ 
                      
                     
                         
                     
                      
                     
                       P 
                       0 
                     
                      
                     L 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     However there is an upper limit in the L value to obtain the FWM bandwidth as described above. Therefore, to satisfy the above equation, the increase of γP 0  value becomes important. On the other hand, in the high power pulse transmission in the optical fiber, there is a problem in which a spectrum waveform distortion is caused by a self phase modulation (SPM) due to the nonlinear effect. To suppress this, the upper limit of γP p L is set to be up to 3π/2 which is a nonlinear phase shift where the pumping pulse spectrum starts to have two peaks by the SPM. (Authored by Agrawal, Yoshioka Book Co.). 
     
       
         
           
             
               
                 
                   
                     γ 
                      
                     
                         
                     
                      
                     
                       P 
                       P 
                     
                      
                     L 
                   
                   ≤ 
                   
                     
                       3 
                        
                       π 
                     
                     2 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     From the above two equations, the range of γP p  is identified to perform the FMW without the spectrum distortion. 
     (4) In the FWM transmission, there is an occasion where a time waveform distortion occurs by the dispersion effect on the input pumping pulse and the input optical signal pulse. To suppress this, it is necessary to reduce the second and third dispersion effects on the pulses. The condition is given by the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       L 
                       
                         L 
                         SOD 
                       
                     
                     &lt; 
                     
                       1 
                       2 
                     
                   
                   , 
                   
                     
                       L 
                       
                         L 
                         TOD 
                       
                     
                     &lt; 
                     
                       1 
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     Under the condition that wave length (frequency) arrangement satisfies the phase matching, the second dispersion effect in the wave length of the input pumping pulse is small. Consequently, the third dispersion effect becomes dominant for the pumping pulse. The condition to suppress this is given by the following equation: 
     
       
         
           
             
               
                 
                   
                     β 
                     3 
                   
                   &lt; 
                   
                     
                       
                         1.7628 
                         3 
                       
                       2 
                     
                      
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           t 
                           p 
                           3 
                         
                       
                       L 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     The second dispersion effect in the wave length of the input signal optical pulse is given by 2πβ 3 Δν, and the condition suppressing this is given by the following equation: 
     
       
         
           
             
               
                 
                   
                     β 
                     3 
                   
                   &lt; 
                   
                     
                       
                         1.7628 
                         2 
                       
                       
                         4 
                          
                         π 
                       
                     
                      
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           t 
                           s 
                           2 
                         
                       
                       
                         L 
                          
                         
                             
                         
                          
                         Δ 
                          
                         
                             
                         
                          
                         v 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     From the equations described above in the sections (1) through (4), the fiber can be designed necessary for the FMW without time waveform distortion. The design procedure is summarized in the flow chart in  FIG. 53 . 
     Next, the experimental results will be described in which the wavelength conversion of the 160 GHz pico second pulse train is performed, using the FWM unit having the fiber designed following the procedures in  FIG. 53 . The block diagram used for the experiment is shown in  FIG. 54 . In the configuration of  FIG. 54 , a 1556 nm 160 GHz 2 ps pulse train and a 1540 nm CW light are mixed at 3 dB coupler, then it enters the low slope HNLF of 0.2 km length. The FWM components generated is extracted by the two stage BPFs (1572 nm of transmission center wavelength and 4.5 nm of full width half maximum). Further, the γ and the dispersion slope of HNLF used in this case are 24.1/W/km and 0.014 ps/nm2/km, respectively. These values are determined by the procedure as shown in  FIG. 53 . The HNLF input and output spectrum waveform are shown in  FIG. 55  at the condition where the power of the input pulse and output pulse are 19 dBm and 13 dBm, respectively. It shows the FWM components are generated near the 1572 nm and 1524 nm in the output. The self correlation waveform of the FWM pulse is shown in  FIG. 56 . For comparison purpose, the self correlation waveform of the input pulse is also shown. It shows that wavelength conversion has been performed without a distortion in time waveform of the pulse, although the width has been increased by 10% by means of the two stages of BPF used for FWM component extraction. After passing through the two stages of BPF, the FWM power becomes −2 dBm. 
     In  FIG. 54 , two BPFs are connected but one BPF is acceptable as long as it can eliminate the components of the input pulse and CW sufficiently. Furthermore, multiple connection like three stage, four stage . . . , n stage configuration are acceptable. In case of  FIG. 54  and  FIG. 55 , the BPF with 1572 nm of transmission center wavelength is used and extracted only near 1572 nm wavelength pulse generated in the longer wave length side as an output pulse by the FWM. If using a BPF with 1524 nm of transmission center wavelength (not shown), it is obviously possible to extract the FWM components generated in the shorter wavelength side. 
     In this embodiment, it is focused that a career suppressed pulse whose phases are in reversed with the neighboring pulses, so-called CS-RZ pulse (Carrier Suppressed Return-to-Zero), is used as an input pulse. Firstly, the relationship of the phase between neighboring pulses in the pulse train generated by FWM will be studied. When the input CW light functions as an FWM pump, the FWM converted light generated in the shorter wavelength side (in this case, a component of near 1524 nm length) is a CS-RZ pulse train whose phases are in reversed with the neighboring pulses. On the other hand, when the input pulse train functions as an FWM pump, the FWM converted light generated in the longer wavelength side (in this case, a component of near 1572 nm length) is an RZ pulse train whose phases are in the same phase as the neighboring pulses. By utilizing the characteristic, it has been made possible to convert the CS-RZ pulse train to the RZ pulse train. If it is necessary to keep the same phase relation between the neighboring pulses, it is carried out by not using the input pulse as an FWM pumping. 
     In the foregoing, the optical switch and the waveform converter using the FWM are described. Another embodiment using the FWM phenomenon is a device which can realize a waveform reshaping function. The configuration is shown in  FIG. 57  and the phenomenon is shown in  FIG. 58 . The elements per se are almost identical to the configuration of the embodiment in  FIG. 54 . The different points lie in that the input pumping power is strengthened and the spectrum bandwidth broadened by the SPM is positively utilized. Along with this, the generated FWM pulse spectrum is also broadened. In addition, by increasing the input pumping power of the input optical signal, the effect of the cross phase conversion in the nonlinear effect results in further broadening of the FWM pulse spectrum. 
     In that, when the pumping power is low, the FWM pulse spectrum width is maintained about the same as that of the pump pulse or the signal optical pulse. When the pumping power is high, the FWM pulse spectrum width is broadened. Consequently, the broadened spectrum is cut off by arranging the BPF having an optimized FWHM, and hence it can realize the nonlinear loss. Then, the relation between the input pumping power and the output FWM power becomes as shown in  FIG. 59 , thus the threshold and saturation characteristic are realized. As a result, the FWM device can make an optical switch which has a waveform reshaping function and the O3R can be realized by using this. 
     Referring to  FIG. 26A  through  FIG. 26C , a configuration of the electrical circuit in the photo receiving unit  234  (refer to  FIG. 24 ) which senses the phase error signal will be described.  FIG. 26A  shows one configuration of the photo receiving unit  234 . The photo receiving unit  234  comprises a photodiode (PD), a loop filter and an LD controller. The LD controller is a device to control the LD driving current and the temperature. 
     The photo receiving unit  234  in  FIG. 26A  receives the optical output from the phase comparator  224  by a PD, and then extracts its low frequency signal components by a loop filter. By means of controlling the LD driving current based on the output of the electric signal, the wavelength emitted from the LD of the beat light generator, that is a beat frequency, is controlled. 
       FIG. 26B  shows a modified embodiment of  FIG. 26A , and it comprises two PDs, a calculator, a loop filter, and an LD controller. In  FIG. 26B , two PDs detect the output light from the phase comparator as well as the optical power of the input optical signal pulse, and then the difference between them is forwarded to the loop filter. Thus, the extinction ratio of a change of phase difference signal between the optical signal pulse and the optical LO is improved. The configuration of the photo receiving unit according to  FIG. 26B  can obtain higher sensibility. 
       FIG. 26C  shows another modified embodiment of  FIG. 26A , and it comprises a PD, a multiplier, a loop filter, an adder, an LD controller, and a reference signal generator. After the PD detects the output of the optical switch and multiplied with a reference signal, then it extracts low frequency components in the electrical signal by the loop filter. Then it controls the LD based on the difference between its output and the reference signal. The configuration of the photo receiving unit according to  FIG. 26C  can obtain the maximum point of the optical output power at the phase comparator in an effective way. 
       FIG. 60A  shows a modified embodiment of the phase comparator  224  and shows one example to detect a phase difference where an optical fiber is not used in the FWM unit. As an FWM light generator  500 , the SOLN and the PPLN can be used. Replacing an FWM light generator, a PPLN and a nonlinear optical crystal can be used as an SHG generator. Furthermore, as shown in  FIG. 60B , it is also possible to detect by a two photon absorption replacing the PD with an SiPD (Silicon photodiode). 
     Furthermore, it is also possible to integrate the phase comparator  224  in the optical timing reforming system and the FWM fiber in the optical switch. The configuration is shown in  FIG. 27 . It consists of the FWM unit  250  wherein the optical phase comparator and the optical switch are shared, and the optical LO generator  252 . Each element is identical to that of  FIG. 23  as described above. The input optical signal and the optical LO are inputted to the FWM unit  250  and the generating FWM light is extracted by the optical filter  254 . The FWM light is inputted to the optical LO frequency controller as an input phase error signal and also it is output as a clock pulse train. 
     As described above, there have been explained the components which are the minimum necessities for the optical regenerator system. The regenerator system  200  (refer to  FIG. 17 ) according to the present invention comprises a pulse roller and a soliton purifier in order to improve the performance. Some examples will be described below. 
     One embodiment of the pulse roller  204  is shown in  FIG. 28 . It is a configuration in which the optical signal is inputted to the pulse roller fiber  260 . The input optical signal is reformed to a rectangular waveform by the interaction of the normal dispersion effect and nonlinear effect in the pulse roller fiber  260 . The nonlinear effect broadens the pulse spectrum band width and in addition, the normal dispersion effect reshapes the pulse time waveform to a rectangular form. 
     Now, a method of reshaping to a rectangular waveform will be described referring to  FIG. 41A  through  FIG. 41D  and  FIG. 42 . When a pulse as illustrated in  FIG. 41A  transmits through a normal dispersion fiber, it changes to a rectangular waveform pulse. The interaction between the normal dispersion effect and nonlinear effect (Kerr effect) is essential to cause it. Namely, the up-chirping as illustrated in the lower part in  FIG. 42  occurs by the nonlinear effect, and the lower frequency components transmits at a faster speed and the higher frequency components transmits at a slower speed by the normal dispersion effect. This results in a waveform distortion as illustrated in  FIG. 41B . 
     In more details as illustrated in  FIG. 42 , the pulse is linearly broadened in the area between T 1  and T 2 , which are a point of inflection of the pulse intensity, by the linear up-chirping according to the normal dispersion effect. T 0  is a center of the pulse. Since it is in a down-chirping between T X &lt;T 1  and T X &gt;T 2  (T X  is an arbitrary time), the closer to the top of the pulse (|T| is larger), the slower the transmission speed is, and the closer to the rear end (|T| is larger), the faster the transmission speed is. More specifically, the energy is concentrated on the portions T 1  and T 2 , and the pulse intensity comes to change rapidly for the time axis. The steepness is proportional to the peak value of the instantaneous frequency at T 1  and T 2 . 
     The present invention uses a normal dispersion increasing fiber (NDIF) as a pulse roller fiber  260  to generate a rectangular phenomenon at a high efficiency. The dispersion profile of the NDIF is shown in  FIG. 29A . The vertical axis of  FIG. 29A  through  FIG. 29C  show a negative dispersion value [ps/nm/km] in all the cases, and when it moves to the top side, the dispersion value approaches a “0” value. 
     As shown in  FIG. 29A , because of lower dispersion value in the input side of the NDIF, the nonlinear effect relatively becomes dominant. Because of higher dispersion value in the output side of the NDIF, the normal dispersion effect relatively dominates. Hence, the pulse is sufficiently broadened in the spectrum width in the NDIF input side, and the pulse time waveform is made to a rectangular form at a high efficiency by strengthening the normal dispersion effect in the output side of the NDIF. 
     As a result, a length of the pulse roller fiber  260  (optical fiber: NDIF) can be shortened. In a quantitative expression using a nonlinear length L NL  and a dispersion length L D , it is designed to satisfy L D &gt;&gt;L NL  at the input side, and L D &lt;&lt;L NL  at the output side, respectively. Only, the dispersion length is expressed as L D =T 0   2 /|β2|, where T 0  and β 2  show an input pulse time width and a dispersion value, respectively. Furthermore it satisfies L NL =1/γP 0 , where γ and P 0  is a nonlinear coefficient and an input peak power, respectively. 
     By adopting an NDIF, a complete flat rectangular waveform pulse in the top part can be made as shown in  FIG. 41C . This is because, in the region of L D &gt;&gt;L NL , the pulse can be converted to the super Gaussian pulse with up-chirping, and then, in the region of L D &lt;&lt;L NL , the pulse can be broadened linearly by the normal dispersion effect. It is verified by theoretical calculation of the fact that the top of the waveforms changed to a complete flat shape when the super Gaussian pulse with up-chirping transmits through the linear transmission path under the nonlinear effect. The principle of the NDIF will be described in detail below. 
     Conventionally, when using a pulse roller which has a constant dispersion value, there has been required a large amount of input power to generate a nonlinear effect and a certain amount of dispersion value to generate a dispersion effect. The reason is that when a dispersion value is small, even the chirping is generated by nonlinear effect, and the dispersion effect is so small that no waveform is changed to a rectangular shape. When dispersion effect is so high, the components including the up-chirping generated by the nonlinear effect rapidly distribute, and since the peak power is reduced, the pulse transmits without accumulation of the chirping. 
     As a result, the rising of the pulse is slow as illustrated in  FIG. 41B , and the top part is substantially the same as the rising waveform as illustrated in  FIG. 41A  which is an upwardly convex shape. The rising steepness of the pulse depends on the high frequency components included in the spectrum, and therefore the fact that the rising is dull corresponds to the fact that the high frequency components have not been generated by a sufficient nonlinear effect. 
     From the foregoing, in order to effectively make the pulse to be rectangular waveform, it is identified that the up-chirping is accumulated to the sufficient level under the condition of less influence from the dispersion, then the pulse waveform is converted by the large dispersion effect. This situation can be realized by using an optical fiber whose normal dispersion increases in a longitudinal direction. 
     More specifically, it may be designed so that the nonlinear distance corresponding to the input power of the pulse at the input end of the optical fiber is to be sufficiently shorter than that of the dispersion distance corresponding to the input pulse width and dispersion value. As a result, the nonlinear effect becomes dominant than the dispersion effect so that the sufficiently large chirping is accumulated in the initial period of the transmission. 
     Furthermore, at the output end, it may be designed so that the dispersion distance is shorter than the nonlinear distance, and the dispersion effect becomes dominant. Since the high frequency components are already generated, the rising of the pulse becomes steep. That pulse can be simulated by a super Gaussian function with a large up-chirping. It is verified by a simple calculation that such a pulse becomes the shape of a function featuring from an upward concave to a downward concave in the area of the pulse center as shown in  FIG. 41C , when transmitting in the fiber with the normal dispersion effect dominant. It is possible to make a rectangular pulse whose top part is approximately flat shape as shown in  FIG. 41D  by causing the pulse to output during the transition from the status as shown in  FIG. 41B  to the status as shown in  FIG. 41C . 
     As described above, to realize the situation where a nonlinear effect is dominant at the input end of the fiber and a dispersion effect is dominant at the output end, there is a method for decreasing the nonlinear constant in a longitudinal direction with the constant dispersion, and in addition, there is another method for increasing a (normal) dispersion in a longitudinal direction. In case of decreasing the nonlinear constant, it is preferable to control the nonlinear constant in such a manner that the characteristic is varied so as to make a step like profile, or to make comb like profile except continuously decreasing it in a longitudinal direction. 
     Next, there will be described another embodiment of the pulse roller  204 .  FIG. 29B  and  FIG. 29C  are another embodiment of  FIG. 29A . The embodiment comprises a pulse roller  204  wherein the NDIF (refer to  FIG. 29A ) is replaced by a dispersion management optical fiber  260  consisting of combined at least two different types of optical fibers whose dispersion/nonlinear coefficients are different. 
     The pulse roller  204  having one of the fibers as shown in  FIG. 29B  and  FIG. 29C  shows the same function and effect as the pulse roller fiber  260  as shown in  FIG. 29A . Namely, both types of the pulse roller  204  have a characteristic that the nonlinear effect is dominant at the portion of the input end and the dispersion effect is dominant at the portion of the output end. 
     In the pulse roller  204  as shown in  FIG. 29B  and  FIG. 29C , the pulse roller fiber  260  is made of a combination wherein the optical fiber whose nonlinear effect is dominant and the optical fiber whose dispersion effect is dominant are arranged in series. 
       FIG. 30A  shows an experimental configuration of reshaping a pulse to a rectangular waveform wherein the pulse roller fiber  260  is made of a pulse roller fiber  204  which consists of two types of optical fibers. A 160 GHz repetitive 2 ps pulse train is inputted to the pulse roller fiber  260 . The dispersion profile of the pulse roller fiber  260  is illustrated in  FIG. 30B . In  FIG. 30B , the horizontal axis is a length of an optical fiber and the vertical axis is a dispersion. The pulse roller fiber  260  illustrated in  FIG. 30B  comprises an HNLF and an NDF, and the length of the HNLF is 200 m, and the dispersion D=−0.7 ps/nm/km, as well as the length of the NDF is 80 m, and the dispersion D=−7.5 ps/nm/km. 
     This case adopts a highly nonlinear fiber(HNLF) in which the nonlinear effect is dominant. The HNLF is an optical fiber wherein the nonlinear constant is larger than that of a fiber used in a communication channel. By using the HNLF, the length of the optical fiber can be shortened. In addition, when using the HNLF, the length can be shortened compared to the standard fiber, even if the NDIF as shown in  FIG. 29A  through  FIG. 29C  is not used and the dispersion is constant. 
     Next, the simulation results of a 2 ps pulse transmission in the pulse roller fiber of  FIG. 30B  is shown in  FIG. 31 . A solid line shows a real time waveform of the output pulse, and a dotted line shows the corresponding self correlation waveform thereto. P 0  is an input peak power. It is shown that the output pulse waveform is changed to a rectangular shape at P 0 =23.5 dBm as shown by a solid line. The self correlation waveform (dotted line) is a triangle shape. 
     Represented by white circles is an experiment result for the self correlation waveform. Because this experiment uses a 160 GHz pulse train, it is shown that there is a wide separation between white circles and a dotted line in the area where the trailing edges of pulses are overlapping, while both pulses are coincident in the area of the pulse center. Therefore, from the experiment result, it can be considered that the waveform is made to a rectangular shape at P 0 =23.5 dBm in the same manner as the simulation result. 
     By placing the pulse roller fiber  204  before the optical switch (refer to  FIG. 24 ), it can realize a high performance in a timing reforming device and a time division device. With reference to  FIG. 24 , the pulse roller fiber is placed between the first coupler and the optical switch, or between the LO generator and the optical switch.  FIG. 32  shows one of the embodiments. 
     In the configuration in  FIG. 32 , the pulse roller  204  is placed before the optical switch  222 , and reshapes the external signal light or the clock pulse to a rectangular waveform, then an optical switching is carried out by the optical switch (refer to  FIG. 24 ). In this case, the configuration of the optical switch  222  is not limited to an FWM type as shown in  FIG. 24 , and may comprise a NOLM type or an SOA (semi-conductive optical amplifier) type. 
     Because one group of pulses is made to a rectangular waveform, it can suppress a conversion to an intensity jitter from a fluctuation in a relative time positions which are caused by a timing jitter of the external optical signal. The principle thereof is described as shown above ( FIG. 39 ). The width of flat part of the rectangular pulse is equivalent to the range of the suppression. 
     Now, one embodiment of the clock extraction device in which the pulse roller  204  and the OPLL  220  are combined will be described below. The configuration is shown in  FIG. 33 . It comprises a pulse roller  204  and a clock extraction device in  FIG. 24 . 
     The optical signal pulse is made to a rectangular waveform by the pulse roller  204  and the rectangular waveform light and the optical LO enter the FWM unit. Then the generated FWM light is converted to an electrical signal at the photo receiving device. One of the remarkable points of the present invention is that the output pulse characteristic from the pulse roller is utilized in a method of detecting the FWM light as an error signal. 
     In the optical switch as shown in  FIG. 24 , the setting of the peak values of the input signal light and the optical LO is important. It is necessary to shift both peak timing positions in order to detect the phase difference between the input signal and the optical LO by means of the average power changes of the output from the optical switch. 
     In the optical switch as shown in  FIG. 24 , when using the photo receiving device in  FIG. 26 , it is necessary to adjust both time positions of the peak points of the signal light from the optical switch and the optical LO to be the same, since the average light intensity is used for sensing the phase. Hence, the optical switch in  FIG. 24  needs to be operated independently of the OPLL. 
     In a method of sensing a phase of the output light from the optical LO which is applied a frequency modulation, the peak value of the input signal light is generally coincide with the optical LO in its average, however, its time difference of the peak points between each pulse varies depending on the modulated frequency. In addition, because a frequency modulation is applied to the output light from the optical LO, there is an occasion where the jitter is accumulated at multiple times whenever the optical regeneration is carried out. 
     In contrast, the peak values coincide at the photo receiving device as shown in  FIG. 35A  and  FIG. 35B , although it is an average value processing when sensing a phase by means of a spectrum of an FWM light. Therefore, it can effect functions of both of the optical switch and the OPLL at the same time. 
     Typical time waveforms of the output pulse and the chirping (a change in time of the instantaneous frequency) characteristic of the pulse roller are illustrated in the top part of  FIG. 34A  and  FIG. 34B , respectively. In general, a linear chirping is accumulated on the pulse being made to a rectangular waveform by the interaction of a normal dispersion and nonlinear effect. The FWM light generated from this pulse and the conversion limit pulse (a center part of  FIG. 34A  and  FIG. 34B ) also has a linear chirping. The FWM pulse characteristic is shown in the bottom part of  FIG. 34A  and  FIG. 34B . 
     Because the rectangular pulse has a linear chirping, the time difference between the input pulses is converted to the frequency difference of the output FWM light. Consequently, by monitoring the frequency difference of the output FWM light, the phase difference between the input pulses can be detected. The photo receiving device to sense the phase difference from the FWM light is illustrated in  FIG. 35A  and  FIG. 35B . 
     The photo receiving device comprises a demultiplexer  262 , at least two optical filters  264  (three in  FIG. 35A ) and an optical power comparator  266  for each filter output as illustrated in  FIG. 35A . An AWG (arrayed waveguide) may be used in place of the demultiplexer  262  and plurality of the optical filters  264 . 
     At the photo receiving device, FWM light is divided into three waves by the demultiplexer  262 , and each wave is inputted into the optical filters  264  which has different transmission frequency. The difference between the filter output powers corresponds to the frequency difference of FWM light or time difference between input pulses of the optical phase comparator. Therefore, the time difference between input pulses can be detected by monitoring the power difference, and the frequency of the optical LO is controlled based thereon. Further,  FIG. 35B  is another embodiment of the photo receiving device in  FIG. 35A  which comprises a grating and a PD. The photo receiving device in  FIG. 35B  has the same function as that in  FIG. 35A . 
     As one of the applications of the OPLL operation using a rectangular pulse, the OPLL operation is also possible of changing a pulse amplitude in time as shown in  FIG. 61  and sensing the shift of time position as a shift of intensity. As shown in  FIG. 61 , another method of changing a rectangular pulse amplitude in time is a synchronization modulation technique in which the phase at gain peak is slightly shifted from the center of the rectangular pulse. 
     As described in  FIG. 32 , the combination of the pulse roller  204  and the optical switch  222  can be applicable not only to a regenerator as described in  FIG. 17 , but also to an OTDM signal generator having a high quality repetitive characteristic. 
       FIG. 62  shows a whole view of the OTDM signal generator. The pulse light source  604  with repetitive frequency of N×f [GHz] has a superior repetitive characteristic and no information is carried on. In contrast, the pulse light source  606  has an f [GHz] repetitive frequency, and the time width is compressed so that an optical time division into N channel can be made, furthermore it is assumed that f [Gbit/s] data modulation is to be performed. All of the pulse light source  604  and the N units of pulse light sources  606  are assumed to be synchronized with the generated electrical clock signal. In  FIG. 62 , the electrical clock signal  602  is used for synchronizing the pulse light source 604  and the pulse light source  606 , however it is not limited to this, for example, and a beat signal light can be used. 
     The information signals generated at the N units of pulse light sources  606  are multiplexed of time division by the optical time division multiplexer  608 . The combination of the pulse light source  606  and the optical time division multiplexer  608  has been used conventionally, however its repetitive frequency characteristic is not in high quality all the case, for example, it includes a timing jitter. The signal which is multiplexed by the optical time division multiplexer  608  is reshaped to a rectangular waveform by the rectangular unit  610 . 
     The optical signal generated at the pulse light source  604  is adjusted in its time position with the output signal from the rectangular unit  610  by using a delay line  612 , then multiplexed by the multiplexer  614 , and then the only logical signal of the OTDM is copied onto the optical pulse which is generated by the pulse light source generator  604 , and as a result, the high quality pulse which is the same level of the repetitive frequency characteristic as the pulse generated by the pulse light source generator  604  can be obtained. 
     The delay line  612  may be placed either before or after the rectangular unit  610 , or disposed within the optical time division multiplexer  608 . Furthermore, in order to improve the efficiency of the rectangular unit  610 , a soliton converter may be placed before it. 
     Next, the soliton purifier  208  as a noise reduction device in this embodiment will be shown in  FIG. 36A . The soliton fiber  272  is placed between the two optical filters  270  and  274 . 
     The operation of the soliton purifier  208  will be described briefly. Firstly, noise components outside the signal bandwidth are eliminated by the BPF  270 , and then the signal spectrum exceeding the BPF bandwidth is shifted in frequency domain by utilizing a Raman self frequency shift phenomenon in the soliton fiber  272 . At that time the noise components existing in the original signal bandwidth is not frequency-shifted to be able to separate the soliton from the noise. At last, the noise components outside the signal bandwidth is eliminated again by the BPF  274 . Thus, the noise of the signal is remarkably reduced. 
     The optical signal pulse entering the soliton purifier  208  is performed a noise elimination, and the noise components outside the optical signal bandwidth are eliminated by the input side optical filter  270 . After that, it is inputted to the soliton fiber  272  which has an anomalous dispersion. In the soliton fiber  272 , it is important that the soliton is controlled to allow a sensing of a slope of the gain by the stimulated Raman scattering (hereinafter called a gain slope) in the region within the bandwidth. As a result of the existence of the gain slope, the soliton is wavelength-shifted. This phenomenon is known as a Raman self frequency shifting (Mitschke et al., Opt. Lett., vol. 11, p. 659 (1986) and Gordon, Opt. Lett., vol. 11, p. 662 (1986)). The soliton with wavelength shifted is extracted by the output side optical filter  274 . 
     The distinctive characteristic of the present invention is that the gain slope is controlled to effectively realize the wavelength shift of soliton. This embodiment adopts the following two methods. 
     The first method is to utilize an HNLF as a soliton fiber ( FIG. 36B ). When a soliton transmits to the fiber, the soliton itself becomes a pump and generate a Raman gain in the lower frequency side. By sensing the gain slope, the center frequency of the soliton shifts towards the lower frequency side thereof. This phenomenon is a soliton self-frequency shift (SSFS). The typical Raman gain of HNLF is shown in  FIG. 37A . For the comparison purpose, the Raman gain of SMF is shown as well. 
     Because the HNLF has a larger gain slope than SMF, even the fiber length is short, the SSFS effect can be strengthened. Therefore, by using an HNLF, the fiber length can be shortened, as a result the fiber loss is reduced, and the interaction between solitons can be suppressed. 
     The second method is a Raman amplification utilizing an external pumping light ( FIG. 36C ). In  FIG. 36C , the pumping light generator is placed before and after. The position of the pumping light generator is acceptable in either before or after side or both sides. 
     The soliton purifier in this configuration works to control the SSFS effect through the amplification adjustment by the external pumping light.  FIG. 63  shows a relationship between the amplified gain by the Raman amplifier and the soliton spectrum in a frequency domain. When a pumping light frequency is expressed by νp, the Raman amplifier gain is in a peak at νp-13 THz approximately. On the other hand, the condition where the gain slope of for the frequency is maximized at frequency of νp−10 THz approximately, however, the SSFS effect can be maximized by setting the center frequency of the soliton Δν to this value. Furthermore, when the amount of frequency shift by the SSFS is expressed as Δν, the soliton frequency shift can be limited by setting to satisfy ν−Δν=νp−13 THz. 
     It is known that a soliton self frequency shift for the unit transmission distance is inversely proportional to the biquadrate of the pulse width. The frequency shift can be more effectively realized by compressing a pulse after inserting the pulse compressor in a position before the soliton purifier  208 . 
     In case of performing a stimulated Raman amplification at front and rear sides as illustrated in  FIG. 36C , by means of designing in such a manner that the optical power is gradually increasing along with a transmission distance so that a soliton adiabatic compression occurs, the pulse is compressed during the transmission, and hence the frequency shift can be done efficiently. 
     When studying a whole optical nonlinear signal processing using an optical soliton train having a good repetitive characteristic, it may be a problem that a noise near the signal bandwidth is amplified.  FIG. 67  shows an experimental result of the input and output spectrum wherein the optical soliton which has a 160 GHz repetitive frequency and 1 ps full width half maximum travels a  2  km HNLF. As is clear from  FIG. 67 , the noise near the center frequency is amplified. The dispersion used continuously decreases from about 3.5 ps/nm/km to about 1.8 ps/nm/km, and the nonlinearity is at 21.8 w −1 km −1  and the loss is at 1 dB/km. In  FIG. 69 , the straight line (a) is approximated in linear from the measured dispersion values and hence the relation derived is expressed as an equation of D=3.5−0.9Z where a dispersion value is D [ps/nm/km] and a distance is Z [km]. On the other hand, the curved line (b) is the function expressed as D=3.25×10 −0.1Z , where a ratio of decrease of D is 1 dB/km. It is estimated from  FIG. 69  that the fiber dispersion almost linearly decreases for the length, but because the distance is short, it can be approximated by an exponential function, in this case, the decreasing ratio is 1 dB/km which is coincident with 1 dB/km which is a decreasing ratio of the fiber loss value. When the attenuation of the light power along with the transmission distance caused by the fiber loss and the attenuation of the dispersion are in balanced, the transmission path can be treated as equivalent to a non loss path (K. Tajima “Compensation of soliton broadening in nonlinear optical fibers with loss,” Opt. Lett. vol. 12, pp. 54-56, 1987.), and consequently it can be treated equivalently as the basic soliton travels 1.6 km effective distance without loss. The noise amplification in  FIG. 67  is a result of parametric process by a discrete spectrum of the optical pulse with a very high repetitive characteristic. When a duty ratio (=pulse half width/pulse duration) is large (more than 0.2), the peak gain of the amplified noise is determined by only the duty ratio and transmission distance because the optical soliton train travels without any loss. 
       FIG. 64  shows the peak gain of the amplified noise vs the bit rate and the transmission distance by a numerical calculation. It is assumed that the soliton pulse train with 1 ps pulse width travels in the fiber (dispersion value 3 [ps/nm/km], nonlinear coefficient 20 [l/km/w].) along with the noise. In this case, the dispersion distance is calculated as 0.1 km, and distance L in the drawing is converted to a ratio for the dispersion distance Z, and the noise amplified gain is determined by only the duty ratio and Z. For example, when the repetitive frequency is 160 GHz (duty ratio 0.16) and if the noise amplified gain needs to be less than 15 dB, the maximum distance that the soliton can travel is indicated as 0.5 km from  FIG. 64 , or up to 5 times of the dispersion distance. The fact that the noise amplified gain is determined by the duty ratio and the dispersion distance is truly applicable to the case where the initial soliton power is thought to be relatively very small compared to the arbitrary physical scale and the initial noise power. 
     On the other hand, when the duty ratio is large, the condition of the noise amplification relates to the phase difference between the neighboring pulses in the soliton train.  FIG. 65  shows a calculated spectrum of the output pulse train in the condition wherein the fiber and the pulse are the same parameters as those in the previous case, and the repetitive frequency is 320 GHz and the transmission distance is 1 km. Evidently, it demonstrates that the CS-RZ pulse train wherein the phase difference between neighboring pulses is π has been suppressed more in a noise amplification than the same phase pulse train. 
     Consequently, when the soliton train with high repetitive characteristic travels, it is suggested that the noise amplification is much suppressed in case of adopting a CS-RZ train. The conclusion of the calculation in  FIG. 65  is applicable to the case where the repetitive frequency is 160 GHz, pulse width is 2 ps and the transmission distance is 4 km. 
     The detail of the regenerator has been explained in the above. Those are the devices or combination thereof wherein the nonlinearity of the optical fiber is primarily utilized. In general, since the nonlinearity effect depends on the input light polarization, the improvements of the performance and the stability can be made by realizing a polarization maintaining characteristic of the fiber used in those devices. One of the embodiments is shown in  FIG. 66 . It comprises a polarizer  702  and a polarization maintaining fiber (PMF)  704 . 
     The input light is allowed to pass a single polarized component through the polarizer  702 , then enters the PMF  704 . The polarized wave after passing through the polarizer  702  is optimized to fit the PMF  704 . In order to reduce the loss by the polarizer  702 , it is preferable to place the polarizing controller  706  before the polarizer  702  so that the input light polarizing condition is optimized by the polarizing controller  706 . The optimization of the PM for the nonlinear devices as described above is effective not only to the performance improvements but also to the downsizing of devices. Because the PMF  704  is durable under influence of the loss or the birefringence caused by a fiber bending, the diameter of a bobbin for winding a fiber can be made smaller. As a result of smaller size of the bobbin, the device can be made in a smaller dimension. 
     Consequently, according to the wavelength division multiplexing optical regeneration system and the wavelength division multiplexing optical regeneration method of the present invention, the transmission speed of the wavelength division multiplexing optical light in the wavelength division multiplexing optical regeneration system can be increased to 40 GBit/s of bit rate or higher and also the downsizing and the power saving can be realized. 
     Furthermore, when it passes a plurality of relaying stations which includes the wavelength division multiplexing optical regeneration system of the present invention, the wavelength division multiplexing optical light cab be correctly regenerated at each station, and hence the quality such as a signal intensity, a waveform, a timing or the like of the wavelength division multiplexing light can be maintained. As a result, the optical communication system for long distance can be realized according to the present invention. 
     Therefore, the industry applicability of the wavelength division multiplexing optical regeneration system and the wavelength division multiplexing optical regeneration method according to the present invention is significantly important in the usage value in this communication industry.