Abstract:
An optical digital regenerator for digitally regenerating an input signal in an intact optical state. A first operating unit has a first probe light generator for generating a first probe light and a first optical operator for converting a waveform of the first probe light output from a first probe light generator according to an optical intensity waveform of the input signal light. A clock extractor extracts a clock component of the input signal light from a photocurrent generated by the first optical operator. A second optical operating unit has a second probe light generator for generating a second probe light pulsed in accordance with the clock output from the clock extractor and a second optical operator for sampling the second probe pulse light output from the second probe light generator.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical digital regenerator, and more specifically, to an apparatus for regenerating an optical signal in an intact optical state. 
     BACKGROUND OF THE INVENTION 
     As an optical cross-connect node on a large capacity wavelength-division multiplexing optical network in the future, due to the increase of a bit rate per wavelength as well as the increase of the number of multiplexed wavelengths, an optical digital regenerating system for regenerating information as intact optical signals has been widely noticed instead of a system in which optical signals are electrically terminated per wavelength and regeneratively repeated. 
     In a WDM optical network in which an usable wavelength is assigned to each line in advance, it is necessary that an input wavelength and an output wavelength of an optical digital regenerator are the same. However, in a WDM optical network that positively adopts the wavelength conversion, reusing the wavelengths can reduce the number of wavelengths. In this case, an input wavelength and an output wavelength of an optical digital regenerator are not necessarily the same. Accordingly, conventional optical digital regenerators have employed a structure that comprises two-step wavelength converting parts so as to be widely applicable to any optical network. 
     A schematic block diagram of a conventional optical digital regenerator is shown in FIG. 4. A signal light from a trunk line system enters an input terminal  10 . The signal light (wavelength λi) having input the input terminal  10  is divided by an optical coupler  12  and enters a high-speed photodiode  14 . The photodiode  14  converts the signal light into an electric signal and applies it to a clock extracting circuit  16 . The clock extracting circuit  16  extracts a clock component of the signal light from the output of the photodiode  14 . 
     The input signal light (wavelength λi) of the input terminal  10  also enters a wavelength converter  18 . The wavelength converter  18  converts the signal light input from the input terminal  10  into another wavelength λj. The signal light whose wavelength have been converted into λj by the wavelength converter  18  inputs a second wavelength converter  20 . The wavelength converter  20  comprises a clock input terminal besides an optical input terminal to which the output light of the wavelength converter  18  enters. The clock extracted by the clock extracting circuit  16  inputs the clock input terminal after being amplified by an amplifier  20  and phase-shifted (adjusted) by a phase shifter  24 . The wavelength converter  20  superimposes the signal light of the wavelength λj from the wavelength converter  18  on a waveform of an RZ probe pulsed light of wavelength λk formed from the clock input through the clock input terminal. By this operation, the wavelength of the signal light is converted from the wavelength λj into the wavelength λk and at the same time the signal light is retimed and waveform-reshaped. 
     There are conventional structures in which retiming and waveform-reshaping is performed at a first wavelength converter using extracted clocks (for example, see B. Lavigne et al. ‘Experimental analysis of SOA-based 2R and 3R optical regeneration for future WDM networks’, OFC &#39;98, Technical Digest, pp. 324-325, which was published at The Optical Fiber Conference held in February in 1998.). FIG. 5 shows a schematic block diagram of the conventional embodiment. 
     An input signal light (NRZ optical pulse) of wavelength λ 0  from a trunk line system enters an input terminal  110 . The signal light (wavelength λ 0 ) entered the input terminal  110  is divided by an optical coupler  112  and then inputs a clock regenerating circuit  114 . The clock regenerating circuit  114 , which comprises a high-speed photodiode, converts the optical pulse from the optical coupler  112  into an electric signal and electrically extracts a clock component contained in the signal light. An LD driving circuit  116  pulse-drives DFB lasers  118  and  120  in accordance with the clock regenerated by the clock regenerating circuit  114 . The DFB lasers  118  and  120  respectively laser-oscillate at mutually different wavelengths λ 1  and λ 2  and output pulse lights (probe pulse lights) locked with the regenerated clock from the clock regenerating circuit  114 . The probe pulse lights from the DFB lasers  118  and  120  are combined by a multiplexer  122  and then input one facet of a semiconductor optical amplifier (SOA)  124 . 
     The signal light (wavelength λ 0 ) having input the input terminal  110  also enters a port A of an optical circulator  126  and outputs from its port B. The input signal light from the port B enters the other facet of the SOA  124 . While the signal light of the wavelength λ 0  and the probe pulse lights of the wavelengths λ 1  and λ 2  propagate mutually in the opposite directions in the SOA  124 , a pulse waveform or bit information of the signal light is copied to the probe pulsed light due to the cross gain modulation effect. That is, the probe pulse light being output from the SOA  124  toward the port B of the optical circulator  126  becomes an RZ pulse conveying the same bit information with the input signal light (wavelength λ 0 ) of the input terminal  110 . 
     The probe pulse light (wavelength λ 1 +λ 2 ) having output from the SOA  124  enters the port B of the optical circulator  126  and outputs from its port C to be divided into a wavelength λ 1  component and a wavelength λ 2  component by a wavelength demultiplexing element  128 . The respective components of the wavelengths λ 1  and λ 2  demultiplexed by the wavelength demultiplexing element  128  propagate on different optical paths  130   a  and  130   b  and then multiplexed by a multiplexer  132 . The optical paths  130   a  and  130   b  are for example predetermined so that the propagation time of the wavelength λ 2  component is delayed by one half bit period in comparison with that of the wavelength λ 1  component. Therefore, the pulse light becomes an NRZ optical pulse after being multiplexed by the multiplexer  132 . 
     The output light of the multiplexer  132  inputs a first port of a first facet of a Mach-Zehnder interferometer (MZI) type wavelength converter  134  and led to one optical path in the MZI wavelength converter  134 . The CW laser light from a DFB laser  136  inputs a second facet of the MZI wavelength converter  134 . The oscillating wavelength λi of the DFB laser  136  is different from both of the oscillating wavelengths λ 1  and λ 2  of the DFB lasers  118  and  120 . The output light of the DFB laser  136  is divided in the MZI wavelength converter  134 . The divided lights propagate on the two optical paths and combined together again in the MZI wavelength converter  134 . The combined light then outputs from a second port of the first facet of the MZI wavelength converter  134 . On the one optical path in the MZI wavelength converter  134 , the output light of the multiplexer  132  propagates in the opposite direction. The MZI wavelength converter  134  converts the pulse signal of wavelength λ 1 +λ 2  into the wavelength λi. The waveforms are respectively reversed in the SOA  124  and the MZI wavelength converter  134  and nonreversing regenerated optical signal waveforms are obtained as final outputs. The purpose of the wavelength conversion in the MZI wavelength converter  134  is mainly to improve the extinction ratio. 
     An optical band pass filter  138  extracts the wavelength λi components alone from the output light of the second port of the first facet of the MZI wavelength converter  134 . It is for preventing the mixing of lights of the wavelengths λ 0 , λ 1  and λ 2  due to the reflection at each part. 
     In the conventional structure shown in FIG. 4, the optical coupler  12 , the high-speed photodiode  14 , and the clock extracting circuit  16  are indispensable for extracting the clock components. When optical digital regeneration is performed for each wavelength, the required number of the optical couplers  12  and the photodiodes  14  to be disposed on the node are equal to the number of the wavelengths resulting the increase in costs and the decrease in reliability. 
     In the conventional structure shown in FIG. 5, two probe light sources are necessary at the first wavelength conversion and it also reduces the reliability and increases the costs. Also, a wavelength demultiplexing element  128  is needed for dividing two wavelengths λ 1  and λ 2  causing the increase of the costs. Since the oscillating wavelength of DFB lasers  118  and  120  has to coincide with the wavelength dividing characteristics of the wavelength demultiplexing element  128  over a long term, it requires a highly advanced wavelength stabilization technique. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above-mentioned problems and to provide an optical digital regenerator for more reliably regenerating an optical pulse signal in an intact optical state. 
     Another object of the present invention is to provide an optical digital regenerator that requires no high-speed photodiode contributing to the high reliability and the low costs. 
     An optical digital regenerator according to the present invention is an optical digital regenerator for regenerating an input signal light in a intact optical state, comprising a first optical operating unit having a first probe light generator for generating a first probe light and a first optical operator for converting the waveform of the first probe light output from the first probe light generator in accordance with an optical intensity waveform of the input signal light; a clock extractor for extracting a clock component of the input signal light from a photocurrent generated from the first optical operator; and a second optical operating unit having a second probe light generator for generating a second probe light being pulsed in accordance with the clock output from the clock extractor and a second optical operator for sampling the second probe pulse light output from the second probe light generator according to the first probe light from the first optical operating unit. 
     Since the clock of the input signal light is extracted from the first optical operator of the first optical operating unit, high-speed photodiodes for extracting clocks and optical couplers for dividing a signal light become unnecessary. Accordingly, the optical elements can be reduced and, as a result, the reliability improves and the costs reduce. In the second optical operating unit, the signal light can get retiming and reshaping by operating the waveform of the probe pulse light formed from the extracted clock with the waveform of the output light of the first optical operating unit. 
     Assuming that the probe light generated from the first probe light generator is a CW light, the clock extraction becomes easier. The first optical operator comprises for example an EA modulator applied by a predetermined DC bias. 
     The clock extractor comprises a phase adjuster for adjusting the phase of the extracted clock. Therefore, the retiming and reshaping can be performed in appropriate timing at the optical operation, namely the wavelength conversion in the second optical operating unit. 
     On the assumption that a first extractor is disposed in the first optical operating unit, which first extractor extracts the probe light operated by the first optical operator and a second extractor is disposed in the second optical operating unit, which second extractor extracts the probe light operated by the second optical operator, even if the signal light and the probe light enter for example from the opposite directions in the respective first and second optical operators, the waveform-operated probe light can be efficiently extracted separately from the signal light. That is, the disposition of respective elements becomes simpler. 
     When the wavelength λj of the probe light generated from the first probe light generator is different from the wavelength λi of the input signal light, the problem of interference and crosstalk can be reduced or solved. 
     When the wavelength λk of the probe light generated from the second probe light generator is different from the wavelength λj of the probe light generated from the first probe light generator, here again, the problem of interference and crosstalk can be reduced or solved. 
     By providing an optical pulse stretcher for stretching the pulse width of the optical pulse output from the first optical operating unit, it becomes resistant to the jitter of the input signal light and, thus, the signal light pulse of accurate timing can be obtained in the second optical operating unit. The optical pulse stretcher comprises for example a means for dividing an input light into two and combining them after the divided lights have propagated on different optical paths. The optical pulse stretcher comprises a high dispersion medium. The optical pulse stretcher can also comprises a chirped grating fiber and an optical circulator for supplying the input light to the chirped grating fiber and outputting the reflected light from the chirped grating fiber to the outside. The employment of these passive elements assures the stable operation over a long time and, therefore, a highly reliable 3R regenerator can be realized. In the first optical operating unit, probe light generators for two wavelengths are unnecessary. This also contributes to the improvement of reliability. 
     The size of the optical pulse stretcher using the high dispersion medium or the chirped fiber grating can be reduced by providing a phase modulator in the first optical operating unit for modulating the phase of the first probe light output from the first probe light generator in accordance with the clock extracted by the clock extractor. The high dispersion medium comprises for example a high dispersion fiber and its length can be reduced extremely, that is, by half. 
     When the above optical pulse stretcher is employed, it is possible to prevent a bad influence of interference and to improve the stretching effect of the optical pulse by employing at least one of an incoherent light generator and a multi-wavelength light generator as the first probe light generator. 
     The optical pulse stretcher also can comprise media having different propagation characteristics in mutually orthogonal polarization directions. Although the optical path between the first probe light generator and the optical pulse stretcher need to be a polarization preserving type, the optical pulse can be stretched with a very simple structure. 
     Furthermore, by providing an optical gate apparatus, between the output of the first optical operating unit and the input of the second optical operating unit, for optically gating the output light of the first optical operating unit in accordance with the clock output from the clock extractor, the extinction ratio can be improved. Using the optical gate apparatus and the above-described optical pulse stretcher together, the probability of failure at sampling in the second optical operating unit is reduced. 
     When the optical gate apparatus comprises a phase controller for automatically adjusting the phase of the clock output from the clock extractor, the optical gate function of the optical gate apparatus can be followed by the jitter of the input signal light. This operation increases the resistance to the jitter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic block diagram of a first embodiment of this invention; 
     FIG. 2 is a schematic block diagram of an optical crossconnect equipment employing the embodiment; 
     FIG. 3 is a schematic block diagram of a wavelength tunable probe light source suitable for a wavelength division multiplexing transmission system; 
     FIG. 4 is a schematic block diagram of a conventional all-optical signal generator; 
     FIG. 5 is a schematic block diagram of a conventional all-optical signal generator; 
     FIG. 6 is a schematic block diagram of a second embodiment of this invention; 
     FIG. 7 is a schematic block diagram of a third embodiment of this invention; 
     FIG.  8 (A) is a diagram showing timing of an RZ pulse of wavelength λi being input an EA modulator  146 ; 
     FIG.  8 (B) is a diagram showing a phase change at a phase modulator  148 ; 
     FIG. 9 is a schematic block diagram showing another configuration of an optical pulse stretcher; 
     FIG. 10 is a schematic block diagram of a fourth embodiment of this invention; 
     FIG. 11 is a schematic block diagram of a fifth embodiment of this invention; 
     FIG. 12 is a diagram showing a propagation state of optical pulses in fast and slow axes of a polarization dispersion medium  264 , and also showing an optical pulse waveform after the combination; 
     FIG. 13 is a schematic block diagram of a sixth embodiment of this invention; 
     FIG. 14 is a diagram showing the transmission characteristics of an MZ optical intensity modulator  274  against a modulation signal voltage; 
     FIG.  15 (A) is a diagram showing waveform of an input optical signal of an optical gate apparatus  270 ; 
     FIG.  15 (B) is a diagram showing the optical gate characteristics by the MZ optical intensity modulator  274 ; 
     FIG.  15 (C) is a diagram showing an output waveform of the optical gate apparatus  270 ; 
     FIG. 16 shows the absorption loss characteristics of an EA optical modulator against a bias voltage (a reverse bias voltage); 
     FIG. 17 is a block diagram of a phase controller  278 ; 
     FIG. 18 is a diagram showing the relation between an optical gate and an input optical pulse at the MZ optical intensity modulator  274 ; 
     FIG. 19 shows a diagram of the phase discrimination characteristics in the configuration shown in FIG. 17; 
     FIG. 20 shows a schematic block diagram of a modified embodiment in which a signal light and a probe light propagate in the same direction in a wavelength converting element; and 
     FIG. 21 shows an illustration of the configuration of a WDM coupler  320 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention are explained below in detail with reference to the drawings. 
     FIG. 1 shows a schematic block diagram of an embodiment of this invention. Numeral  30  denotes an optical input terminal in which a signal light (wavelength λi) from a trunk line system enters. The signal light (wavelength λi) from the optical input terminal  30  inputs a wavelength converter  32 . The wavelength converter  32  comprises an optical circulator  34  having three ports of A, B and C in which the input light of the port A is sent to the port B and the input light of the port B is sent to the port C, an electroabsorption (EA) type optical modulator  36  biased by a constant DC voltage and a probe light source  38  for generating a CW probe light of the wavelength λj different from the wavelength λi. 
     The signal light (wavelength λi) from the optical input terminal  30  inputs the port A of the optical circulator  34  and the port B of the optical circulator  34  optically couples with one facet of the EA modulator  36  while the probe light (wavelength λj) from the probe light source  38  inputs the other facet of the EA modulator  36 . The output light of the port C of the optical circulator  34  becomes the wavelength-converted output of the wavelength converter  32 . The detailed explanation of such wavelength converter  32  is described in the specification and drawings (especially in FIG. 1) of Heisei 8nen (1996) Japanese Patent Application No. 233796. 
     At an electrode of the EA modulator  36 , a photocurrent according to the intensity change of the input light (the signal light of the wavelength λi from the port B of the optical circulator) is generated. The generated photocurrent is applied to a clock extracting circuit  40  and the clock extracting circuit  40  extracts clock components from the input photocurrent and outputs electrical clock signals. 
     The clock extracted by the clock extracting circuit  40  is applied to a second wavelength converter  46  after being amplified by an amplifier  42  and phase-shifted (phase-adjusted) by a phase shifter  44 . 
     The signal light of wavelength λj from (the port C of the optical circulator  34  of) the wavelength converter  32  inputs the second wavelength converter  46 . The wavelength converter  46  comprises an optical circulator  48 , similarly to the optical circulator  34 , having three ports of A, B and C, a semiconductor laser amplifier  50  and a probe light source  52  for generating probe pulse light (wavelength λk) whose phase and frequency are adjusted for wavelength conversion at the semiconductor laser amplifier  50 . The probe light source  52  comprises for example a wavelength tunable light source  54  for discretely changing an oscillating wavelength and an optical modulator  56  for intensity-modulating an output laser light from the wavelength tunable light source  54 . A signal, which is obtained by amplifying a clock output from the clock extracting circuit  40  using the amplifier  40  and then phase-shifting (phase-adjusting) with the phase shifter  44 , is applied to the optical modulator  56  as a modulating signal. 
     The semiconductor laser amplifier  50  operates as a wavelength converting element similarly to the EA modulator  36  of the wavelength converter  32 . It is obvious from the previously mentioned Heisei 8 nen (1996) Japanese Patent Application No. 233796 that an EA modulator can be used instead of the semiconductor laser amplifier  50 . Since there is no need for extracting the clock components anymore in the wavelength modulator  46 , the semiconductor laser amplifier  50  is employed. A wavelength converting element can be used as a substitute for the semiconductor laser amplifier  50 , which uses the cross phase modulation effect described in Journal of Lightwave Technology,vol. 14, No. 6, pp. 942-954, 1996. 
     The signal light (wavelength λj) from the wavelength converter  32  inputs the port A of the optical circulator  48  and enters one facet of the semiconductor laser amplifier  50  after being output from the port B of the optical circulator  48 . At the probe light source  52 , the wavelength tunable light source  54  generates a CW light of wavelength λk. The wavelength λk can be discretely changed and also λk can be equal to λi. As previously explained, since the clock output from the clock extracting circuit  40  is applied to the electrode of the EA modulator  56  through the amplifier  42  and the phase shifter  44 , the EA modulator  56  intensity-modulates the output light of the wavelength tunable light source  54  into an RZ waveform according to the clock output from the clock extracting circuit  40 . The output light (wavelength λk) of the probe light source  52  inputs the other facet of the semiconductor laser amplifier  50 , then enters the port B of the optical circulator  48  after being superimposed by the signal light waveform of wavelength λj at the semiconductor laser amplifier  50  and finally outputs from the port C of the optical circulator  48 . The output light of the port C of the optical circulator  48  becomes the wavelength conversion output of the wavelength converter  46  and outputs from an optical output terminal  58  toward the outside (the trunk line system). 
     A method is also applicable in which the RZ probe light is generated using the gain switching effect in accordance with a clock as described in ECOC (European Conference of Optical Communication) &#39;97, 22-25, September. &#39;97 Conference Publication No. 448, Post dead line papers pp. 89-92 by employing a semiconductor laser diode capable of high-speed modulation in place of the EA modulator  56  and wavelength tunable light source  54 . 
     The operation of the embodiment is explained below. The signal light (wavelength λi) inputs the wavelength converter  32  from the optical input terminal  30 . The signal light (wavelength λi) entered the wavelength converter  32  is propagated from the port A to the port B of the optical circulator  34  and enters one facet of the EA modulator  36 . A CW probe light (wavelength λj) output from the probe light source  38  inputs the other facet of the EA modulator  36 . While the signal light of wavelength λi and the CW probe light of wavelength λj transmit in mutually opposite directions in the EA modulator  36 , the waveform of the signal light (wavelength λi) is copied onto the probe light (wavelength λj) as described in Heisei 8 nen (1996) Japanese Patent Application No. 233796. As a result, the probe light (wavelength λj) inputs the port B of the optical circulator  34 , which probe light has substantially the same information with the signal light (wavelength λi) input to the wavelength converter  32 . 
     The signal light wavelength-converted to the wavelength λj by the EA modulator  36 , namely the probe light having transmitted the EA modulator  36 , enters the port B of the optical circulator  34  and outputs to the second wavelength converter  46  from the port C. Consequently, the wavelength of the signal light is converted from λi to λj by the wavelength converter  32 . 
     At an electrode of the EA modulator  36 , a photocurrent is generated, which changes in accordance with the intensity fluctuation of the input light. Since the intensity fluctuating input light is the signal light (wavelength λi) from the port B of the optical circulator  34 , the generated photocurrent has the clock component of the signal light (wavelength λi) entering the optical input terminal  30 . The clock extracting circuit  40  extracts the clock component from the photocurrent. The clock extracted by the clock extracting circuit  40  is amplified by the amplifier  42 , phase-shifted (phase-adjusted) by the phase shifter  44  and then applied to an electrode of an EA modulator  56  of a probe light source  52  in the wavelength converter  46  for retiming and waveform-reshaping by the wavelength converter  46 . The photocurrent is used for extracting the clock, and, therefore, the optical coupler  12  and the high-speed photodiode  14  used in the conventional apparatus become unnecessary. 
     The wavelength converter  46  converts the wavelength of the signal light from λj to λk using basically the same operation with the wavelength converter  32 . The wavelength λk can be either equal or not equal to the wavelength λi of the signal light input to the optical input terminal  30 . 
     The signal light (wavelength λj) input the wavelength converter  46  is transmitted from the port A to the port B of the optical circulator  48  and enters one facet of a semiconductor laser amplifier  50 . In the probe light source  52 , the wavelength tunable light source  54  generates a CW light of wavelength λk and applies it to the EA modulator  56 . The signal light in which the output clock of the clock extracting circuit  40  is amplified by the amplifier  42  and phase-adjusted by the phase shifter  44  is applied as modulation signal to the EA modulator  56 . The EA modulator  56  generates a probe pulse light by intensity-modulating the CW probe light from the wavelength tunable light source  54  according to the modulation signal. The generated probe pulse light (wavelength λk) inputs to the other facet of the semiconductor laser amplifier  50 . 
     In the semiconductor laser amplifier  50 , while the signal light of wavelength λj from the optical circulator  48  and the probe pulse light of wavelength λk from the probe light source  52  propagate in mutually opposite directions, a waveform of the signal light (wavelength λj) is copied or overlapped on the probe light (wavelength λk) by the same operation with the EA modulator  36  of the wavelength converter  32 . In consequence, the probe light (wavelength λk) enters the port B of the optical circulator  48  and outputs from the port C of the optical output terminal  58 , which probe light has basically the same information with the signal light (wavelength λj) that inputs the wavelength converter  46 . That is, the signal light of the wavelength λk is output from the optical input terminal  58  to the outside (the trunk line system). 
     In the wavelength converter  46 , the probe light generated from the probe light source  52  is pulsed at the same repetitive frequency with that of the clock of the signal light (wavelength λi) being input to the optical input terminal  30 . Therefore, the semiconductor laser amplifier  50  performs retiming and waveform-reshaping simultaneously with wavelength conversion. 
     By employing this embodiment for example as an optical crossconnect equipment, a choice of courses becomes more flexible. FIG. 2 shows a schematic block diagram of the optical crossconnect node. 
     Numerals  60   a ,  60   b ,  60   c  and  60   d  denote optical input terminals in which wavelength division multiplexed lights enter from respectively different optical transmission lines. On the assumption that the input lights of the optical input terminals  60   a ˜ 60   d  are wavelength division multiplexed lights comprising M wavelengths (M is, for example, 16 or 32). Numerals  62   a ,  62   b ,  62   c  and  62   d  denote optical amplifiers for amplifying the wavelength multiplexed lights from the optical input terminals  60   a ˜ 60   d . Numerals  64   a ,  64   b ,  64   c  and  64   d  denote wavelength demultiplexing elements (in the concrete, 1×M arrayed waveguide gratings) for demultiplexing the respective output lights of the optical amplifiers  62   a ˜ 62   d  into individual wavelengths. The outputs of the individual wavelengths of the wavelength demultiplexing element  64   a  connect to the respective inputs of an M×4 optical switch  68   a  through all-light signal generators  66   a - 1 ˜ 66   a -M, which have a configuration shown in FIG.  1 . The outputs of the respective wavelengths of the wavelength demultiplexing elements  64   b ,  64   c  and  64   d  also connect the respective inputs of the M×4 optical switches  68   b ,  68   c  and  68   d  through all-light signal generators  66   b - 1 ˜ 66   b -M,  66   c - 1 ˜ 66   b -M and  66   d - 1 ˜ 66   d -M having the configuration shown in FIG.  1 . 
     The respective four outputs of the M×4 optical switch  68   a  connect first inputs of 4×1 optical couplers  70   a ,  70   b ,  70   c  and  70   d , the respective four outputs of the M×4 optical switch  68   b  connect second inputs of the 4×1 optical couplers  70   a ˜ 70   d , the respective four outputs of the M×4 optical switch  68   c  connect third inputs of the 4×1 optical couplers  70   a ˜ 70   d  and the respective four outputs of the M×4 optical switch  68   d  connect fourth inputs of the 4×1 optical couplers  70   a ˜ 70   d.    
     The outputs of the 4×1 optical couplers  70   a ˜ 70   d  respectively connect optical output terminals  74   a ˜ 74   d  through optical amplifiers  72   a ˜ 72   d . The respective optical output terminals  74   a ˜ 74   d  connect mutually different optical transmission lines. 
     The operation of the crossconnect equipment depicted in FIG. 2 will be explained below. The input wavelength multiplexed lights of the input terminals  60   a ˜ 60   d  are respectively amplified by the optical amplifiers  62   a ˜ 62   d , input the wavelength demultiplexing elements  64   a ˜ 64   d  and demultiplexed into individual signal wavelengths. The signal lights of the individual wavelengths output from the wavelength demultiplexing elements  64   a ˜ 64   d  connect to the respective inputs of the M×4 optical switches  68   a ˜ 68   d  after getting retiming, waveform-reshaping and, if necessary, wavelength-converting by the all-light signal regenerators  66   a - 1 ˜ 66   a -M,  66   b - 1 ˜ 66   b -M,  66   c - 1 ˜ 66   c -M and  66   d - 1 ˜ 66   d -M having the configuration shown in FIG.  1 . 
     The M×4 optical switches  68   a ˜ 68   d  can distribute the M input lights at a desired combination to four output ports. The output lights of the first outputs of the M×4 optical switches  68   a ˜ 68   d  are combined (wavelength division multiplexed) by a 4×1 optical coupler  70   a , the output lights of the second output ports of the M×4 optical switches  68   a ˜ 68  are combined (wavelength division multiplexed) by a 4×1 optical coupler  70   b , the output lights of the third output ports of the M×4 optical switches  68   a ˜ 68  are combined (wavelength division multiplexed) by a 4×1 optical coupler  70   c  and the output lights of the fourth output ports of the M×4 optical switches  68   a ˜ 68  are combined (wavelength division multiplexed) by a 4×1 optical coupler  70   d . The respective combined output lights of the optical couplers  70   a ˜ 70   d  are optically amplified by the optical amplifiers  72   a ˜ 72   d  and output to the outside from output terminals  74   a ˜ 74   d.    
     The individual wavelengths after the separation of the wavelengths are, if necessary, wavelength-converted by the all-light signal regenerators  66   a ˜l-M,  66   b - 1 ˜M,  66   c - 1 ˜M and  66   d - 1 ˜M, and selected their output destinations by the M×4 optical switches  68   a ˜ 68   d . Therefore, at joints of a plurality of ring networks, the transmission of light signals beyond the rings can be realized and, furthermore, the selection of courses in accordance with events such as a breakage also becomes possible. 
     In the embodiment shown in FIG. 1, the output wavelength λk should be equivalent to a wavelength used in an optical network as close as possible. As an element for wavelength-demultiplexing the number of wavelength division multiplexed wavelengths, for example 32 wavelengths, an arrayed waveguide grating has been considered as a promising one. FIG. 3 shows a schematic block diagram of wavelength tunable laser having wavelength selective characteristics by using the arrayed waveguide grating. In FIG. 3, a wavelength tunable probe light source comprises a fiber ring laser in which an arrayed waveguide grating  80  for demultiplexing wavelengths, an optical switch  82  for selecting an oscillating wavelength, an optical amplifier  84 , an EA modulator  86  for pulsating a laser light with an extracted clock (the output of a phase shifter  44 ), a semiconductor laser amplifier  88  for coinciding a longitudinal mode-locked frequency with an extracted clock frequency and optical couplers  90  and  92  are connected as forming a ring. 
     The arrayed waveguide grating  80  demultiplexes light from one output of the optical coupler into 32 wavelengths of λ 1 ˜λ 2  and the optical switch  82  selects one wavelength (namely, an oscillating wavelength) from the outputs of respective wavelengths of the arrayed waveguide grating  80 . The light having the wavelength selected by the optical switch  82  is amplified by the optical amplifier  84  and enters the EA modulator  86 . Since a clock signal from the phase shifter  44  is applied to an electrode of the EA modulator  86 , the incident light is intensity-modulated by the clock and becomes a mode-locked short pulse light (RZ pulse light). 
     The output light of the EA modulator  86  entered the semiconductor laser amplifier  88  is amplified and phase-adjusted. The output light of the semiconductor laser amplifier  88  is divided into two portions by the optical coupler  90 ; one portion enters the optical coupler  92  and the other portion enters the high-speed waveform monitor  94 . The high-speed waveform monitor  94  converts the incident light to an electrical signal and monitors its optical pulse waveform. To obtain a desired optical pulse waveform, the high-speed waveform monitor  94  controls the bias to be applied to the semiconductor laser amplifier  88  using the bias controlling circuit  96 . As a result, the mode-locked frequency matches up with the extracted clock frequency. 
     The light entered the optical coupler  92  is divided in two; one enters the arrayed waveguide grating  80  and the other outputs to the outside as a probe pulse light. By this operation, individual wavelengths can be selected that coincide with a plurality of wavelengths specified on optical networks. 
     By providing an optical pulse stretcher between the wavelength converters  32  and  46  for stretching an optical pulse width, the margin to the jitter can be improved. FIG. 6 shows a schematic block diagram of the modified embodiment. Identical elements are labeled with reference numerals common to those in FIG.  1 . Numeral  98  shows an optical pulse stretcher for widening a pulse width of the optical pulse output from (the port C of the optical circulator  34  of) the wavelength converter  32 . 
     In the structure such as the conventional structure shown in FIG. 5 that employs two probe lights having mutually different wavelengths and combines them after their propagation on different optical path lengths, it is necessary to highly stabilize the two wavelengths because not only the two probe light sources are needed but also the two wavelengths must be demultiplexed with extraordinary precision. Therefore, it is difficult to maintain the high reliability over a long term and also the costs increase. 
     However, in an embodiment shown in FIG. 6, the optical pulse stretcher  98  comprises a Mach-Zehnder interferometer of a silica glass waveguide and, thus, the stable transmission characteristics over a ling term can be maintained as well as the reliability of the whole apparatus improves. In the optical pulse stretcher  98 , the optical pulse (wavelength λj) input to the optical pulse stretcher  98  from the wavelength converter  32  is divided into two by a divider  98   a  and propagated on two optical paths  98   b  and  98   c  having different optical path lengths. The lights propagated on the two optical paths  98   b  and  98   c  are combined by a combiner  98   d  and enter the port A of the optical circulator  48  of the second wavelength converter  46 . The difference of the optical path lengths of the two optical paths  98   b  and  98   c  are adjusted so that the pulse width is widened approximately twice due to the combining by the combiner  98   d . When the combining is performed by the combiner  98   d , interference occurs at the point where the optical pulses propagated on the two optical paths  98   b  and  98   c  are superimposed each other in a time domain and it causes fluctuation of the optical pulse waveform after the combining. To solve the problem, a heater  98   e  should be disposed on one optical path  98   c . By adjusting the temperature of the heater  98   e , the difference of the optical phases between the optical paths  98   b  and  98   c  is controlled so that the waveform after the combining becomes the optimum shape (a waveform having a flat peak). 
     As the alternative to the heater  98   e , the divider  98   a  and the combiner  98   d  are replaced with polarization beam splitters in which a TE wave (or a TM wave) and a TM wave (or a TE wave) respectively propagate on the optical paths  98   b  and  98   c . In this case, the polarization should be preserved on the line path between the probe light source  38  and the optical pulse stretcher  98 . 
     The pulse width of the optical pulse is widened by the optical pulse stretcher  98  and then a probe pulse light is sampled by the optical pulse of the stretched pulse width at the wavelength converter  46 . As a result, an optical pulse output can be obtained excellent both in timing and waveform. Since the optical pulse stretcher  98  comprises a passive element that can maintain the stable transmission characteristics over a long time, this embodiment is capable of preserving the stable optical regenerating function for a long period of time. Needless to say, trouble seldom occurs. 
     The optical pulse stretcher  98  can comprise a high dispersion medium for example such as a high dispersion fiber used for compensating accumulative wavelength dispersion. However, when an existing high dispersion fiber is used, a long fiber between several km and several tens km is required. If a high dispersion fiber of a higher wavelength dispersion value is obtained, it is obvious that the fiber length can become shorter accordingly. To cope with the problem, using phase modulation together can shorten the length of the high dispersion medium. FIG. 7 shows a schematic block diagram of the embodiment. In the embodiment shown in FIG. 7, a phase modulator is disposed in the first wavelength converter, which phase modulator phase-modulates a probe light in synchronization with an input light pulse. It uses that a pulse width of an optical pulse widens in a time domain in a high dispersion medium due to the chirping by the phase modulation. 
     The structure and operation of the embodiment shown in FIG.  7  will be explained. An RZ optical pulse signal of wavelength λi from a trunk line system inputs an input terminal  140 . The RZ optical pulse input the optical input terminal  140  enters the port A of an optical circulator  144  of a wavelength converter  142 . The optical circulator  144  is an optical element for outputting the input light of the port A from the port B and outputting the input light of the port B from the port C. The port B of the optical circulator  144  optically couples with one facet of an EA modulator  146  and a CW probe light of wavelength λj (≠λi) output from a probe light source inputs the other facet of the EA modulator  146  through a phase modulator  148 . Similarly to the EA modulator  36 , the EA modulator  146  is applied by a constant DC voltage. 
     In the same way as the EA modulator  36 , a photocurrent in accordance with the intensity change of the input light is generated in a electrode of the EA modulator  146 . The generated photocurrent is applied to a clock extractor  152 . The clock extractor  152  extracts and regenerates a clock component from the input photocurrent. The clock extracted by the clock extractor  152  is amplified by an amplifier  154 , phase-shifted (phase-adjusted) by a phase shifter  156 , and applied to a phase modulator  148 . From this operation, the phase modulator  148  modulates the phase of the probe light output from the probe light source in synchronization with the RZ optical pulse of wavelength λi input the EA modulator  146  from the port B of the optical circulator  144 . FIGS.  8 (A) and  8 (B) are diagrams showing the relation between the phase modulation at the phase modulator  148  and the RZ optical pulse of wavelength λi input the EA modulator  146  from the port B of the optical circulator  144 . FIG.  8 (A) shows timing of the RZ optical pulse and FIG.  8 (B) shows the phase change at the phase modulator  148 . In FIG.  8 (A), the vertical axis and the horizontal axis respectively show optical intensity and time. In FIG.  8 (B), the vertical axis and the horizontal axis respectively show phase and time. 
     In the EA modulator  146 , The RZ optical pulse of wavelength λi and the cw probe light of wavelength λj propagate in mutually opposite directions. By this operation, the optical intensity of the probe light of wavelength λj reflects the optical intensity change of the RZ optical pulse of wavelength λi and consequently a wavelength of an optical carrier conveying the signal is converted from the wavelength λi to the wavelength λj. 
     The probe light of wavelength λj output from the EA modulator  146  inputs the port B of the optical circulator  144  and outputs from its port C toward the outside of the wavelength  142 . The output light from the port C of the optical circulator  144  enters a second wavelength converter  162  via an optical band pass filter  158  for transmitting only the wavelength λj and an optical pulse stretcher  160  for stretching the optical pulse width in a time domain. The optical band pass filter  158  is disposed for preventing the component of wavelength λi to be provided to the rear stage. The optical pulse stretcher  160  for example comprises a high dispersion medium having a large wavelength dispersion at the wavelength λj such as a high dispersion fiber. This operation uses the characteristics that the pulse width of the optical pulse stretches before and after of time when an optical pulse whose phase fluctuates in the relation shown in FIG. 8 transmits in a medium of large wavelength dispersion. The phase modulation at the phase modulator  148 , concretely the quantity of the phase adjusting by the phase shifter  156  is determined so as to cause the above pulse width stretch. For example, in case of 10 Gbit/s transmission rate, the optical pulse stretcher  160  stretches the pulse width of the optical pulse of wavelength λj output from the optical filter  158  from 20˜30 ps to 40˜60 ps, namely approximately twice in width. 
     The output light of the optical pulse stretcher  160  enters the port A of the optical circulator  164  of the second wavelength converter  162  and outputs from the port B. The output light of the port B of the optical circulator  164  enters one facet of the EA modulator  166 . The wavelength converter  162  comprises a second probe light source  168  for generating a CW probe light of wavelength λk different from the wavelength λj. λk also can be equal to λi. The probe light having output from the probe light source  168  enters an EA modulator  170 . A phase shifter  172  adjusts the phase of output of the amplifier  154 , namely the phase of the clock extracted and regenerated by the clock extractor  152 , and applies it to the electrode of the EA modulator  170 . The EA modulator  170  modulates the intensity of the output light from the probe light source  168  in accordance with the output of the phase shifter  172 . The output of the EA modulator  170  has an RZ optical pulse train of the same frequency with that of the clock extracted and regenerated by the clock extracting circuit  152  and inputs the other facet of the EA modulator  166 . 
     In the EA modulator  166 , the output light (wavelength λj) from the port B of the optical circulator  164  and the RZ pulsed probe light (wavelength λk) from the EA modulator  170  transmit in mutually opposite directions. Similarly to the EA modulator  146 , the RZ probe light is gated or sampled by the intensity of the output light (wavelength λk) from the port B of the optical circulator  164 . As a result, the optical carrier for conveying the signal is converted from the light of wavelength λj to the light of wavelength λk. The alternative to the EA modulator  166 , a semiconductor laser amplifier can be employed. In terms of the gain, the semiconductor laser amplifier is more preferable. However, considering that non-inversion output waveform can be obtained and in respect of the signal to noise ratio, the EA modulator is more preferable. 
     The probe light of wavelength λk transmitted through the EA modulator  166  is what the pulse signal light being input the optical input terminal  140  is the so-called 3R (Retiming, Reshaping and Regeneration) regenerated. The probe light of wavelength λk transmitted the EA modulator  166  inputs the port B of the optical circulator  164  and outputs from its port C. An optical band pass filter  174  for transmitting through only the wavelength λk is connected to the port C of the optical circulator  164  and thus the component of the wavelength λk alone is extracted from the output light of the port C of the optical circulator  164 . The output light of the optical band pass filter  174  outputs from an optical output terminal  176  to the outside. The optical band pass filter  174  prevents that for example the light of wavelength λj reflected by the facet of the EA modulator  166  is output toward the outside. 
     In the embodiment shown in FIG. 7, since the phase modulation by the phase modulator  148  is used together with the optical pulse stretcher  160  using the high dispersion medium, the length of the high dispersion medium used in the optical pulse stretcher  160  can be greatly shortened and, therefore, the optical pulse stretcher itself, ultimately the whole embodiment shown in the FIG. 7 can become smaller. In the conventional structure shown in FIG. 5, the two probe light sources  118  and  120 , the wavelength demultiplexing element  128  such as an AWG, the two optical paths  130   a  and  130   b  having the predetermined difference between the optical paths, and the multiplexer  132  are indispensable for stretching the optical pulse width in a time domain. However, in this embodiment, the single probe light source  150  is sufficient and the multiplexing/demultiplexing becomes unnecessary. Therefore, the costs can be reduced and the reliability can be improved. Furthermore, since the elements requiring adjustment are reduced, the production and the maintenance become easier. 
     As mentioned above, the optical pule stretcher  160  can comprise a high dispersion optical fiber used for compensating accumulative wavelength dispersion, which is formed only with the so-called dispersion equalizing fiber. However, as shown in FIG. 9, it is also possible to use an apparatus in which a chirped fiber grating and an optical circulator are combined. The chirped fiber grating can be regarded the same with the high dispersion fiber in terms of its function. In the structure shown in FIG. 9, the output light of the optical band pass filter  158  enters a port A of an optical circulator  180 . A chirped fiber grating  182  connects to a port B of the optical circulator. The chirped fiber grating  182  is a grating element in which a reflection wavelength is shifted in its longitudinal direction. In this embodiment, the long wavelength reflection side of the grating connects to the port B of the optical circulator  180 . The reflection wavelength at the middle point of the chirped fiber grating  182  is predetermined as the wavelength λj. The port C of the optical circulator  180  connects to the port A of the optical circulator  164 . 
     In the structure shown in FIG. 9, the output light from the optical band pass filter  158  inputs the port A of the optical circulator  180 , outputs from the port B, and then enters the chirped fiber grating  182 . The chirped fiber grating  182  practically functions similarly to the high dispersion fiber and the pulse width of the reflected pulse light of the chirped fiber grating  182  stretches wider than that of the input pulse light in a time domain. The pulse light reflected at the chirped fiber grating enters the port B of the optical circulator  180 , outputs from its port C, and then enters the port A of the optical circulator  164 . 
     When the length of the chirped fiber grating  182  is about 1 cm, the optical pulse having several femto-seconds in pulse width is stretched only several ps. However, if the chirped fiber grating  182  of several meters long is realized, the pulse width stretching of the same degree with the high dispersion fiber described above can be obtained. 
     The chirped fiber grating  182  can be replaced with a high dispersion fiber with its termination being totally reflective. In this case, it is obvious that the length of the high dispersion fiber can be reduced to half compared to the case that the high dispersion fiber in a transmissible state is used. 
     When the high dispersion medium is used as a means for stretching the optical pulse width, the pulse width stretching effect by the wavelength dispersion can be increased by using a Fabry-Perot laser or a multi wavelength laser as the light source instead of using a single longitudinal mode oscillation laser. 
     FIG. 10 shows a schematic block diagram of the embodiment. An RZ optical pulse signal of wavelength λi from the trunk line system enters an input terminal  210 . The RZ optical pulse input the optical input terminal  210  is optically amplified by an erbium doped optical amplifying fiber (EDFA)  212  and enters a first wavelength converter  214 . In the first wavelength converter  214 , the output light from the optical amplifying fiber  212  inputs the port A of an optical circulator  216 , outputs from its port B, and then enters one facet of an EA modulator  218 . A probe light source  220  comprises an incoherent light source or a multi wavelength light source and generates a probe light of the wavelength λj different from the wavelength λi of the input signal light. The output light of the probe light source  220  is optically amplified by an EDFA  222  and enters the other facet of the EA modulator  218 . Similarly to the EA modulators  36  and  144 , the EA modulator  218  is applied by a constant DC voltage. 
     In the EA modulator  218 , the RZ optical pulse of the wavelength λi and the CW probe light of the wavelength λj propagate in mutually opposite direction. Consequently, the optical intensity of the probe light of the wavelength λj reflects the optical intensity change of the RZ optical pulse of the wavelength λi and, therefore, the wavelength of the optical carrier conveying the signal is converted from the wavelength λi to the wavelength λj. 
     The probe light of the wavelength λj out put from the EA modulator  218  enters the port B of the optical circulator  216  and outputs from its port C toward the outside of the wavelength converter  214 . The output light of the port C of the optical circulator  216  enters a second wavelength converter  230  through an optical band pass filter  224  for passing through only the wavelength λj and an optical pulse stretcher  226  for stretching the optical pulse width in a time domain. 
     The optical band pass filter  224  is disposed for preventing that the component of the wavelength λi is supplied to the rear stage. The optical pulse stretcher  226  has a structure using an optical pulse stretcher  98  comprising Mach-Zehnder interferometer shown in FIG. 6, a high dispersion medium having a large wavelength dispersion at the wavelength λj such as a high dispersion fiber or a chirped grating fiber shown in FIG.  9 . In an embodiment shown in FIG. 10, since the probe light source  220  generates a wide spectrum light such as an incoherent light source or a multi wavelength light source, for example even if a high dispersion fiber is used, its length becomes shorter and, thus, also it becomes unnecessary that the probe light is phase-modulated in advance at the wavelength converter  226  in the first stage. When the Mach-Zehnder interferometer shown in FIG. 6 is employed as the optical pulse stretcher  226 , the influence of the interference is reduced or defused by using an incoherent light source as the probe light source  220 . 
     The output light of the optical pulse stretcher  226  inputs the second wavelength converter  230 , is amplified by an EDFA  232 , enters the port A of an optical circulator  234  and outputs from the port B. The output light from the port B of the optical circulator  234  enters one facet of an EA modulator  236 . The wavelength converter  230  comprises a second probe light source  238  for generating a CW probe light of the wavelength λk different from the wavelength λj. λk also can be equal to λi. The probe light output from the probe light source  238  enters an EA modulator  240 . The EA modulator  240 , as to be described later, intensity-modulates and RZ-pulsates the probe light output from the probe light source  238  in accordance with the clock generated from the optical pulse that inputs the input terminal  210 . The probe light being RZ pulsed by the EA modulator  240  is optically amplified by an EDFA  242  and enters the other facet of the EA modulator  236 . 
     Similarly to the EA modulators  36  and  144 , at the electrode of the EA modulator  218 , a photocurrent also generates in accordance with the intensity change (concretely the intensity change of the signal light that inputs the input terminal  210 ) of the input light. The generated photocurrent is applied to a clock extracting circuit  246  and the clock extracting circuit  246  extracts and regenerates the clock component from the input photocurrent. The clock extracted by the clock extracting circuit  246  is phase-shifted by a phase shifter  248 , amplified by an amplifier  250 , and applied to an EA modulator  241  as its driving signal. As mentioned above, the EA modulator  240  converts the output light of the probe light source  238  to an RZ optical pulse according to the output voltage from the amplifier  250 . The output of the EA modulator  240  shows an RZ optical pulse train of the same frequency with that of the clock extracted and regenerated by the clock extracting circuit  146 . 
     In the EA modulator  236 , the output light (wavelength λj) from the port B of the optical circulator  234  and the RZ pulsed probe light (wavelength λk) from the EDFA  242  propagate in mutually opposite directions. By the similar function performed in the EA modulator  218 , the RZ pulsed probe light is sampled or gated in accordance with the intensity of the output light (wavelength λj) from the port B of the optical circulator  234 . Consequently, the optical carrier conveying the signal is changed from the light of the wavelength λj to the light of the wavelength λk. 
     The probe light of the wavelength λk transmitted through the EA modulator  236  is the light formed by 3D-regenerating the pulsed signal light that inputs the optical input terminal  210 . The probe light of the wavelength λk transmitted through the EA modulator  236  input the port B of the optical circulator  234  and outputs from its port C. The port C of the optical circulator  234  connects to an optical band pass filter for passing only the wavelength λk through. The optical band pass filter  252  allows to pass through only the wavelength λk component from the output light of the port C of the optical circulator  234 . The output light of the optical band pass filter  252  is output from an optical output terminal  254  toward the outside. Since the optical band pass filter  252  is disposed, it is prevented that the unnecessary light, such as the light of the wavelength λj reflected by the facet of the EA modulator  236 , is output the outside. 
     FIG. 11 shows a schematic block diagram of the embodiment in which an optical pulse width is stretched utilizing the propagation time difference of polarization. Identical elements are labeled with reference numerals common to those in FIG.  10 . In this embodiment, similarly to the embodiment shown in FIG. 1, a laser light source of a single wavelength with a narrow spectrum is employed as a probe light source in a first wavelength converter without using a incoherent light source or a multi wavelength light source. In this embodiment, the optical pulse width is stretched using the difference of the propagation delay times between TE and TM waves of the probe light. 
     A wavelength converter  214   a  comprises a probe light source  260  that laser-oscillates at a single wavelength λj of a narrow spectrum in place of the probe light source  220  in the wavelength converter  214 . The wavelength converter  214   a  converts the signal light of the wavelength λi input from the input terminal  210  into the wavelength λj and outputs it from the port C of the optical circulator  216  toward the outside. Similarly to the embodiment shown in FIG. 10, the output light from the port C of the optical circulator  216  enters the optical band pass filter  224  for passing only the wavelength λj through and is extracted the wavelength λj component alone there. 
     A polarization adjuster  262  adjusts the polarization of the output light from the optical band pass filter  224  so as to be at an angle of 45° to the principal axis (the fast axis or the slow axis) of a polarization dispersion medium  264  and input sit the polarization dispersion medium  264 . It is obvious that if such adjustment of polarization is unnecessary, the polarization adjuster  262  can be omitted. The polarization dispersion medium  264  comprises a polarization preserving fiber or a silica optical waveguide (PLC) having large polarization dispersion as same as the input optical pulse width. Needless to say, in this embodiment, the light path between the probe light source  260  and the polarization dispersion medium  264  comprises a transmission medium for preserving a plane of polarization. 
     The length of the polarization dispersion medium  264  is adjusted so that the optical pulse of the slow axis is delayed about 1 pulse width compared with the optical pulse of the fast axis in the process of the propagation and the optical pulses of the orthogonal polarization directions are combined having a gap of the extent of the optical pulse width in a time domain at the output stage of the polarization dispersion medium  264 . That is, the polarization dispersion medium  264  stretches the optical pulse width using the difference of the propagation time of both polarization directions. FIG. 12 is a diagram showing the propagation condition of an optical pulse in the fast axis and an optical pulse in the slow axis and an optical pulse waveform after the combining. 
     The signal light that inputs the input terminal  210  contains the possibility whose polarization fluctuates at random since it propagates a long transmission line. Therefore, in the embodiment shown in FIG. 11, the length of the polarization preserving fiber between the port B of the optical circulator  216  and the EA modulator  218  needs to be short enough so that the input signal light is not affected by the polarization dispersion while it is propagating the fiber. 
     The pulse light (wavelength λj), its optical pulse width being stretched by the polarization dispersion medium  264 , enters the wavelength converter  230 . The operation of the wavelength converter  230  is not described here since it is the same with the embodiment shown in FIG.  10 . 
     An embodiment of a larger extinction ratio and an improved S/N ratio compared to the embodiment shown in FIG. 11 will be explained. FIG. 13 shows a schematic block diagram of the modified embodiment. Identical elements are labeled with reference numerals common to those in FIG.  11 . 
     In the embodiment shown in FIG. 13, an optical gate  270  is disposed between the optical band pass filter  224  and the polarization adjuster  262 . The optical gate  270  gates the signal pulse portion from the output light of the optical band pass filter  224  in accordance with the clock extracted by the clock extracting circuit  246 . In this embodiment, to increase the capacity for following the fluctuation of the jitter, the timing of gating is automatically adjusted according to the gating result. 
     The modified part of the embodiment shown in FIG. 13 will be explained in detail. In the optical gate  270 , an EDFA  272  amplifies the output light of the optical band pass filter  224  and an MZ optical intensity modulator  274  gates the marked portion of the signal light from the output light of the EDFA  272 . The operation of the MZ optical intensity modulator  274  will be described later. A divider  276  supplies the most of the output light from the MZ optical intensity modulator  274  to the polarization adjuster  262  and supplies the rest to a phase controller  278 . The phase controller  278  comprises a photodiode for converting an incident light into an electric signal and a circuit for generating a phase controlling signal that controls the phase of the clock in accordance with the output of the photodiode. Since the photodiode can be low-speed as long as it can follow the clock frequency of the signal, it can be obtained low-priced. A phase shifter  280  shifts the phase of the clock extracted by the clock extracting circuit  246  by a quantity specified by the phase error signal from the phase controller  278 . The clock phase-shifted by the phase shifter  280  is amplified by an amplifier  282  and applied to the MZ intensity modulator  274  as a modulation signal or an optical gate controlling signal. 
     The MZ optical intensity modulator  274  is feedback-controlled by the divider  276 , the phase controller  278 , the phase shifter  280  and the amplifier  282  so that the average power of the output light from the MZ optical intensity modulator  274  becomes the maximum. 
     The clock waveform output from the clock extractor  246  is generally a sine wave. The transmission characteristics of the MZ optical intensity modulator  274  against the modulation signal voltage usually shows a sine wave state as shown in FIG.  14 . Accordingly, when the voltage amplitude of the sine wave modulation signal (the output of the amplifier  282 ) and the bias voltage are respectively adjusted to around Vp and Vp/2, the MZ optical intensity modulator  274  can perform the rectangular optical gate operation. FIGS.  15 (A)˜ 15 (C) show diagrams of the optical gate operation. FIGS.  15 (A)˜ 15 (C) respectively show a waveform of the input optical signal of the optical gate  270 , the optical gate characteristics of the MZ optical intensity modulator  274 , and an output waveform of the optical gate  270 . By this optical gate operation, the optical gate  270  can improve the signal to noise ratio and the extinction ratio of the input optical pulse signal. Although the noise still remains in the space (the signal ‘0’ part), the noise can be removed if the operation bias voltage of the EA modulator  236  in the second wavelength converter  230  is properly adjusted. The reason is because absorption loss characteristics of the EA modulator against the bias voltage (the reverse bias voltage) compresses the noise in the space part proportional to the depth of the bias voltage, as shown in FIG. 16 (see Edagawa et al, OFC &#39;97, TuO 5, pp. 77-78, 1997). In FIG. 16, the vertical and horizontal axes show respectively transmission rate and the intensity of input light. 
     As the MZ optical intensity modulator  274  used for the optical gate operation can use a narrow band width clock signal for modulation without requiring the wide bandwidth modulation characteristics, it can be obtained easily and inexpensive. The MZ modulator that can be used for this purpose is for example a modulator comprising a LiNbO 3  (lithium niobate) optical waveguide. 
     Because the phase controller  278  is disposed, it becomes possible that the optical gate operation can automatically follow the jitter fluctuation of the input optical pulse. The jitter generated in the all-optical regenerator can be also absorbed, if a phase controller similar to the phase controller  278  is disposed in the second wavelength converter  230  for controlling the amount of the phase shift by the phase shifter  248  so that the average power of the output light of the EA modulator  240  becomes the maximum. 
     FIG. 17 shows a schematic block diagram of the phase controller  278 . The controlling circuit  284  outputs the phase error signal for controlling the amount of the phase shift by the phase shifter  280 . A low frequency oscillator  286  oscillates at an extremely low frequency, for example at a several kHz, compared to a bit rate of the transmission optical pulse. The output of the low frequency oscillator  286  and the error signal output from the controlling circuit  284  are added and applied to the control input of the phase shifter  280 . By this operation, the phase shifter  280  shifts the phase of the clock signal from the clock extracting circuit  246  according to the amount specified by the error signal and modulates (dithers) the clock with the output frequency of the low frequency oscillator  286 . The clock phase-shifted and phase-modulated by the phase shifter  280  is amplified by the amplifier  282  and applied to the MZ optical intensity modulator  274  after being biased by the bias voltage output from the bias power supply  292 . The relation of the optical gate and the input optical pulse in the MZ optical intensity modulator  274  is shown in FIG.  18 . The timing of the optical gate fluctuates according to the oscillation frequency of the low frequency oscillator  286 . 
     As described above, one portion of the output light of the MZ optical intensity modulator  274  is split by the divider  276  and enters a low speed photodetector  288  in the phase controller  278 . The photodetector  288  converts the optical pulse from the divider  276  into an electric pulse and supplies it to a mixer  290 . The mixer  290  is applied by the output of the low frequency oscillator  286 . The mixer  290  multiplies both inputs, synchronously detects (locking-in detects) the modulation signal introduced at the phase shifter  280 , and supplies the detecting result to the controlling circuit  284 . By this locking-in detection, as shown in FIG. 19, the phase discriminating characteristics can be obtained according to the difference between the average phase of the optical gate and the optical pulse phase. The controlling circuit  284  generates the above mentioned phase error signal for changing the phase shift amount of the phase shifter  280  toward the direction so that the input optical pulse enters in the optical gate in the MZ optical intensity modulator  274  in accordance with the output of the mixer  290  and applies it to the phase shifter  280 . 
     When such an automatic phase controlling loop is employed, the phase shift amount of the phase shifter  280  can be automatically adjusted so that the phase error signal is locked to approximately zero. The range of the control bandwidth is sufficient if it covers the bandwidth of the jitter to be occurred on the transmission line. In this structure, even if the optical intensity of the input optical pulse fluctuates, as long as it is a single peak optical pulse, the stable automatic phase control can be obtained. However, if the phase displacement amount by the modulation signal output from the low frequency oscillator  286  is excessive, the intensity fluctuation noise occurs in the output light of the MZ optical intensity modulator  274  and deteriorates the signal quality. Therefore, the amplitude of the modulation signal preferably should be kept to the minimum within a necessary condition. 
     When the EA modulator is used instead of the MZ optical intensity modulator  274 , if the phase of the optical gate shifts against the signal light pulse, as explained in the Heisei 9 nen (1997) Japanese patent application No. 189748, (one portion of) the signal light is absorbed and the photocurrent flowing the electrode of the modulator is increased. By utilizing this function, also a similar automatic phase control can be realized. 
     The optical gate  270  can be applied to the respective embodiment shown in FIGS. 1,  6  and  10 . 
     In FIGS. 6 and 7, optical amplifiers (EDFA) are not shown among the elements. However, as a practical matter, optical amplifiers are inserted at the positions corresponding to the optical amplifiers  212 ,  222 ,  232 , and  242  in the embodiment shown in FIG.  10 . It is obvious that at the points where optical level is sufficient, the optical amplifiers become unnecessary. 
     In the above embodiments, the signal light and the probe light are inputted into the wavelength converting elements, namely the EA modulators  36 ,  146 ,  166 ,  218  and  236  so that the signal light and the probe light propagate in the mutually opposite directions in the wavelength converting elements. However, it is also possible to modify that the signal light and the probe light propagate in the same direction. 
     FIG. 20 shows a modified schematic block diagram of the embodiment shown in FIG. 13 in which the signal light and the probe light propagate in the same direction in a wavelength converting element. An RZ optical pulse signal of a wavelength λi from the trunk system enters an optical input terminal  310 . The RZ optical pulse having inputted the optical input terminal  310  is optically amplified by an EDFA  312  and then enters a first wavelength converter  314 . 
     In the first wavelength converter  314 , a probe light source  316  generates a probe light having a narrow spectral single wavelength λj, which is different from the wavelength λi of the input signal light. The output light from the probe light source  316  is optically amplified by an EDFA  318  and enters a WDM coupler  320 . The output light of the EDFA  312  also inputs the WDM coupler  320  and the WDM coupler  320  multiplexes both incident lights and supplies the multiplexed light to an EA modulator  322 . The WDM coupler  320  comprises for example a dielectric multilayer filter as shown in FIG.  21 . It is capable of multiplexing the signal light and the probe light at an insertion loss of between 1 and 2 dB. Though a fiber coupler can be also employed as the WDM coupler  320 , it brings a branching loss of not less than 3 dB. 
     The EA modulator  322 , similarly to the EA modulators  36 ,  146  and  218 , is also applied by a constant DC voltage. In the EA modulator  322 , the RZ optical pulse of the wavelength λi and the CW probe light of the wavelength λj propagate in the same direction. While they propagate in the EA modulator  322 , the optical intensity of the probe light of the wavelength λj reflects the optical intensity change of the RZ optical pulse of the wavelength λi and the wavelength of an optical carrier conveying the signal is converted from λi to λj. 
     An optical band pass filter  324  exclusively extracts a wavelength λj component from the output light (the original signal light of the wavelength λi and the probe light of the wavelength λj) of the EA modulator  322 . The output light of the optical band pass filter  324  becomes the output light of the first wavelength converter  314 . Since the output light of the EA modulator  322  includes both original signal light of the wavelength λi and probe light of the wavelength λj, the wavelength converter  314  necessarily must comprises the optical band pass filter  324  at its output stage. 
     The EA modulator  322 , similarly to the EA modulators  36 ,  146  and  218 , also generates photocurrent in accordance with the intensity change of the incident lights (concretely, the intensity change of the original signal light of the wavelength λi) from its electrode. The generated photocurrent is applied to a clock extracting circuit  326 . The clock extracting circuit  326 , similarly to the clock extracting circuit  246 , extracts and regenerates clock components from the input photocurrent. 
     The output light (of the optical band pass filter  324 ) of the wavelength converter  314  is applied to an optical gate  328 . The optical gate  328  comprises the same structure with the optical gate  270  and operates the same way. That is, the optical gate  328  takes out the signal pulse part from the output light of the optical band pass filter  324  according to the clock extracted by the clock extracting circuit  326 . 
     The output light of the optical gate  328  enters an optical pulse stretcher  330 . The optical pulse stretcher  330 , similarly to the embodiments shown in FIGS. 11 and 13, comprises a polarization adjuster  332  identical with the polarization adjuster  262  and a polarization dispersion medium  334  identical with the polarization dispersion medium  264 . The optical pulse stretcher  330  operates equally to the polarization adjuster  262  and the polarization dispersion medium  264  in the embodiments shown in FIGS. 11,  13  and stretches the optical pulse width of the output optical pulse from the optical gate  328  in the time domain. Since the optical pulse stretcher  330  stretches the optical pulse width using the polarization, the optical path between the probe light source  316  and the polarization dispersion medium  334  inevitably needs to be a transmission medium for maintaining a state of polarization. 
     The output light of the optical pulse stretcher  330  enters a second wavelength converter  336 . The clock extracted and outputted by the clock extracting circuit  326  also inputs to the second wavelength converter  336  after being phase-adjusted by a phase shifter  338  and amplified by an amplifier  340 . 
     In the second wavelength converter  336 , an EDFA  342  optically amplifies the output light from the optical pulse stretcher  330  and applies it to a WDM coupler  344 . A probe light source  346  generates a second probe light of a wavelength λk being different from the wavelength λj of the first probe light. The wavelength λk can be equal to the wavelength λi of the original signal light. The probe light from the probe light source  346  enters an EA modulator  348 . The output signal of the amplifier  340  is applied to an electrode of the EA modulator  348 . That is, the EA modulator  348  is driven according to the clock output from the clock extracting circuit  326  and converts the probe light output from the probe light source  346  into an RZ pulse by intensity-modulating it. The probe light being converted into the RZ pulse light by the EA modulator  348  is optically amplified by an EDFA  350  and applied to the WDM coupler  344 . 
     The WDM coupler  344  multiplexes the output lights of the EDFAs  342 ,  350  and applies the multiplexed light to an EA modulator  352 . The WDM coupler  344 , similarly to the WDM coupler  320 , also comprises a dielectric multilayer filter. The EA modulator  352 , similarly to the EA modulator  236 , is applied by a constant DC voltage. In the EA modulator  352 , the output light (comprising the first probe light of the wavelength λj) from the optical pulse stretcher  330  and the second probe light of the wavelength λk being converted into the RZ pulsed light by the EA modulator  348  propagate in the same direction. While they are propagating, the second probe pulse light of the wavelength λk is sampled by the output light of the optical pulse stretcher  330 . As a result, a wavelength of an optical carrier for conveying the signal is converted from λj to λk and, at the same time, the pulse waveform gets reshaping and the signal pulse gets retiming. 
     The optical band pass filter  354  exclusively extracts wavelength λk components from the output light (comprising the first probe light of the wavelength λj and the second probe light of the wavelength λk) of the EA modulator  352 . The output light from the optical band pass filter  354  is output from an output terminal  356  to the outside as the output light of the second wavelength converter  336 . Since the output light of the EA modulator  352  includes the components of both wavelengths λj and λk, the wavelength converter  336  necessarily must comprise the optical band pass filter  354  at its output stage. 
     Propagating the signal light and probe light in the same direction is applicable to the other embodiments, namely to each embodiment shown in FIGS. 1,  6 ,  7  and  11 . 
     The WDM coupler  344 , similarly to the WDM coupler  320 , also comprises a dielectric multilayer filter as shown in FIG.  21 . 
     In the above embodiments, it is preferable to automatically control their phase adjusting amount of the phase shifter  44 ,  172 ,  248  and  338 . For instance, it is sufficient if phase controlling circuits are disposed to the outside, which can automatically control the phase adjusting amount of the phase shifters  44 ,  172 ,  248  and  338  in accordance with the output lights from the output terminals  58 ,  176 ,  254  and  356 . The structure of the phase controlling circuits can be the same with the phase controlling circuit  278 . 
     As can be readily understood from the foregoing descriptions, according to this invention, a signal light can be regenerated in an intact optical state and get retiming and reshaping using relatively a fewer elements. Also, the costs can be reduced since there is no need to employ costly high-speed photodiodes. The reliability improves because the number of elements can be reduced. Furthermore, according to the invention, it becomes tolerant to the jitter and, therefore, the signal to noise ratio and the extinction ratio are improved and the missing signals are effectively prevented.