Abstract:
An optical signal processor is disclosed comprising a first optical path having a first electroabsorption optical modulator to be applied with a constant voltage and to absorb light of a signal wavelength, a second optical path having a fixed phase relation with the first optical path relative to a probe wavelength, a probe light introducer for dividing probe light of the probe wavelength into two portions and feeding them respectively into the first and second optical paths, an original signal light introducer for introducing original signal light of the signal wavelength into the first electroabsorption optical modulator, and a combiner for combining both light of the probe wavelength passed through the first and second paths.

Description:
FIELD OF THE INVENTION 
     This invention relates to an optical signal processor, and more specifically, to an optical signal processor for processing a signal in an optical state in an optical network system, an optical switching system and so on. 
     BACKGROUND OF THE INVENTION 
     As an infrastructure for supporting the future information scene, a large capacity optical communication network using a wavelength division multiplexing system has been eagerly researched. In such present situation, a practical optical reproduction technology such as signal wavelength conversion and waveform shaping has been positively studied and examined, since the capacity of an optical network greatly improves if it is possible to reproduce a signal in an optical state at each node of the optical network. 
     In order to obtain such optical reproducer, the following configurations have been basically proposed; a configuration (cf. FIGS. 11 and 12 of the U.S. Pat. No. 5,959,764) employing the cross gain modulation and cross phase modulation of a semiconductor optical amplifier (SOA), a configuration (cf. e.g. FIG. 13 of the above-mentioned patent) utilizing four wave mixing of a semiconductor optical amplifier or an optical fiber, and a configuration (cf. e.g. FIG. 1 of the above-mentioned patent) using cross gain modulation of an electroabsorption (EA) optical modulator. Also, a configuration in which a semiconductor optical amplifier is disposed at each of two optical paths or arms of an interferometer configuration is disclosed in the U.S. Pat. Nos. 6,005,708 and 6,035,078 and EP 0717482. 
     In the foregoing conventional art, the one using the cross gain modulation of the semiconductor optical amplifier (SOA) has a simple configuration. However, this type is not suitable for multistage wavelength conversion since it is unable to produce converted light having a sufficiently high extinction rate. Furthermore, the gain recovery time takes several hundreds picoseconds and so it is not applicable to a high-speed signal of several Gb/s or more because the pattern effect becomes prominent. 
     The configuration, in which the two semiconductor optical amplifiers are disposed to compose a Mach-Zehnder interferometer structure, becomes complicated since original signal light must be input differentially when the high-speed is pursued. Also, this type has another problem that converted light is sensitively affected by only a slight power fluctuation of the original signal light since phase modulation of 180° can be performed with relatively low optical power. 
     Although the one utilizing the four wave mixing of a semiconductor optical amplifier operates at high-speed, its conversion wavelength band is narrow and S/N ratio is deteriorated due to spontaneous emission. When the four wave mixing is used, it is necessary, in principle, to equalize a polarization plane of original signal light with that of pumping light. The configuration of the surrounding optical elements, thus, becomes complicated causing the high production costs. 
     Although the wavelength converter employing the cross gain modulation of an electroabsorption optical modulator has the simplest configuration, this type demands relatively large original signal input light. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the aforesaid problems in the conventional art and provide an optical signal processor for efficiently reproducing (wavelength converting and/or waveform shaping) a signal with a simple configuration. 
     Another object of the present invention is to provide an optical signal processor capable of obtaining a larger extinction rate. 
     Further object of the present invention is to provide an optical signal processor capable of shaping a signal waveform in an optical state. 
     An optical signal processor according to the invention is composed of a first optical path having an electroabsorption optical modulator which is applied with a constant voltage and absorbs light of a signal wavelength, a second optical path having a fixed phase relation with the first optical path relative to a probe wavelength, a probe light introducer for dividing probe light of the prove wavelength into two portions and feeding them respectively into the first and second optical paths, an original signal light introducer for introducing original signal light of the signal wavelength into the first electroabsorption optical modulator, and a combiner for combining both light of the probe wavelength passed through the first and second optical paths. 
     With this configuration, cross phase modulation occurs between the original signal light and the probe light in the first electroabsorption optical modulator, and the probe light is phase-modulated. Consequently, by combining the probe light propagated on the first and second optical paths, light (converted signal light) of the probe wavelength can be obtained which waveform varies at high-speed by following a waveform variation of the original signal light. Since the constant voltage is applied to the electroabsorption optical modulator, a carrier generated by the original signal light can be discharged toward the outside at high-speed. Hence, a high-speed response as fast as 10 Gb/s and more can be realized. 
     In the cross phase modulation of the electroabsorption optical modulator, since the intensity variation greater than that of the original signal light can be given to the combined probe light, the extinction rate is improved. Also, it is possible to give compression characteristics to the low intensity part and high intensity part of the original signal light, and therefore the noise can be suppressed and so the waveform is improved to have a steeper shape. 
     Preferably, the original signal light introducer introduces the original signal light into the first optical path so as to propagate in the opposite direction to the probe light in the first electroabsorption optical modulator. This configuration prevents that the remainder of the original signal light passed through the first electroabsorption optical modulator is mixed with the output light of the optical signal processor. 
     Preferably, one of the first and second optical paths has a phase shifter for adjusting the phase difference between the first and second optical paths relative to the probe wavelength into a predetermined value. With this configuration, desired characteristics can be obtained easily, and also an inverter operation can be selected. 
     Preferably, the second optical path has a second electroabsorption optical modulator having the same characteristics with those of the first electroabsorption optical modulator. With this configuration, it becomes easy to adjust or set the phase relation between the first and second optical paths. 
     An optical signal processor according to the invention is also composed of a first optical path having a first electroabsorption optical modulator, which is applied by a constant voltage and absorbs light of a signal wavelength, and a first reflector for reflecting a probe wavelength on one end of the first optical path, a second optical path having a fixed phase relation with the first optical path relative to the probe wavelength and having a second reflector for reflecting the probe wavelength on one end of the second optical path, an original signal light introducer for introducing original signal light of the signal wavelength into the first electroabsorption optical modulator, a combiner/divider for dividing probe light of the probe wavelength into two portions and feeding them respectively into the first and second optical paths as well as combining the two portions of light from the first and second optical paths, and a probe wavelength extracting filter for extracting the optical component of the probe wavelength from the combined output light of the combiner/divider. 
     With this configuration, it is also possible to obtain the same operation effect as a Michelson interferometer optical circuit. 
     Preferably, the original signal light introducer introduces the original signal light into the first optical path so as to enter the first electroabsorption optical modulator in the same direction with the probe light. With this configuration, the original signal light is absorbed while making roundtrips in the first electroabsorption optical modulator. Consequently, since the light intensity of the remained original signal light becomes considerably weak, harmful effects such as interference decrease. 
     Preferably, one of the first and second optical paths has a phase shifter for adjusting the phase difference between the first and second optical paths relative to the probe wavelength into a predetermined value. This configuration makes it possible to easily obtain desired characteristics. Also, an inverter operation can be selected. 
     Preferably, the first reflector is formed on one end face of the first electroabsorption optical modulator. This configuration makes it easier to produce and adjust the optical signal processor. 
     Preferably, the second optical path has a second electroabsorption optical modulator having the same characteristics with those of the first electroabsorption optical modulator. This configuration makes it easier to adjust or set the phase relation between the first and second optical paths. 
     Preferably, the second reflector is formed on one end face of the second electroabsorption optical modulator. This configuration makes it easier to produce and adjust the optical signal processor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a schematic diagram of a first embodiment according to the invention; 
     FIG. 2 shows characteristics of the embodiment in FIG. 1; 
     FIG. 3 shows an example of the waveform shaping effect according to the embodiment in FIG. 1; 
     FIG.  4 ( a ) shows a waveform of an original signal light  12  when a waveform of probe light  16  is an RZ pulse waveform; 
     FIG.  4 ( b ) shows the RZ pulse waveform of the probe light  16 ; 
     FIG.  4 ( c ) shows a waveform of converted signal light  20  when the waveform of the probe light  16  is the RZ pulse waveform; and 
     FIG. 5 shows a schematic diagram of a second embodiment according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the invention are explained below in detail with reference to the drawings. 
     FIG. 1 shows a schematic block diagram of a first embodiment according to the invention. Reference numeral  10  denotes a signal input port to which original signal light  12  of a wavelength λs inputs, reference numeral  14  denotes a probe light input port to which probe light  16  of a wavelength λp inputs, and reference numeral  18  denotes a signal output port for outputting converted signal light  20  of the wavelength λp carrying the same data signal with the one that the original signal light  12  carries. The wavelength λp of the probe light  16  is generally different from the wavelength λs; however it is also possible to be the same with the wavelength λs. In this embodiment, λs is 1.51 μm and λp is 1.55 μm. 
     The probe light input port  14  is connected to a Y-branch 3-dB optical coupler (directional coupler)  22  which functions as an optical splitter, and the signal output port  18  is connected to a Y-branch optical coupler (directional coupler)  24  which functions as a combiner. Two optical paths (or arms)  26  and  28  are disposed between the two optical couplers  22  and  24 . With this configuration, a Mach-Zehnder interferometer is formed. 
     An electroabsorption (EA) optical modulator  30 , a phase shifter  32  and a Y-branch optical coupler (directional coupler)  34  are disposed in that order, from the optical coupler  22  side to the optical coupler  24 . side, on the optical path  26 . The optical coupler  34  is disposed so as to introduce the original signal light  12  from the signal input port  10  to the EA optical modulator  30  on the optical path  26  as well as feed the probe light, passed through the EA optical modulator  30  and phase shifter  32 , into the optical coupler  24 . The phase shifter  32  is composed of, for example, a thin film heater and capable of adjusting a phase of the light propagating on the optical path  26  with the heat. 
     On the other hand, an EA optical modulator  36  and a Y-branch optical coupler (directional coupler)  38  are disposed in that order, from the optical coupler  22  side toward the optical coupler  24  side, on the optical path  28 . The EA optical modulator  36  and the optical coupler  38  (especially the optical coupler  38 ) are not essential since they are disposed only for the purpose of balancing the loss and phase between the probe light propagating on the optical path  28  and the probe light propagating on the optical path  26 . 
     The EA optical modulators  30  and  36  are respectively composed of a waveguide transmission InGaAsP electroabsorption semiconductor optical modulator utilizing the Franz-Keldysh effect, and preset to be transparent to the wavelength λp (here, λp is 1.55 μm) of the probe light  16  and to absorb the wavelength λs (here, λs is 1.51 μm) of the original signal light  12 . To put it concretely, each of the EA optical modulators  30  and  36  has a layer structure in which a 0.3 μm thick InGaAsP electroabsorption layer (λg=1.40 μm), a 0.05 μm thick InGaAsP hole accumulation prevention layer (λg=1.22 μm), a 0.05 μm thick InP cladding layer, a 2 μm thick p-InP cladding layer and a 0.1 μm thick p-InGaAsP contact layer (λg=1.22 μm) are layered on a (100) n-InP substrate in this order, and also an iron doped InP is embedded in the structure. 
     In this embodiment, the optical couplers  22  and  24 , the optical paths  26  and  28 , the EA optical modulators  30  and  36 , the phase shifter  32  and the optical couplers  34  and  38  are hybrid-integrated on a silica substrate  40 . 
     The operation of the embodiment is described below. The EA optical modulators  30  and  36  are fed with a constant voltage of 3.0 volt. Also, the phase shifter  32  is preset so that the phase difference of the probe wavelength λp between the optical paths  26  and  28  becomes π in the condition that the original signal light  12  does not yet enter the signal input port  10 . At this stage, to simplify the explanation, the probe light  16  is assumed as CW light. 
     The CW probe light  16  from the probe light input port  14  is divided into two portions by the optical coupler  22 ; one portion enters the EA optical modulator  30  on the optical path  26  and the other enters the EA optical modulator  36  on the optical path  28 . As already mentioned, since the EA optical modulators  30  and  36  are transparent to the probe wavelength λp, the probe light transmits the EA optical modulators  30  and  36  with almost no loss. The phase shifter  32  adjusts the phase of the probe light passed through the EA optical modulator  30  and the optical coupler  34  divides the output light from the phase shifter  32  into two portions. The probe light attenuated by 3 dB at the optical coupler  34  enters the optical coupler  24 . On the other hand, the probe light passed through the EA optical modulator  36  is divided into two portions by the optical coupler  38 . The probe light attenuated by 3 dB at the optical coupler  38  enters the optical coupler  24 . The optical coupler  24  combines the probe light from the optical coupler  34  and the probe light from the optical coupler  38 . The combined output light of the optical coupler  24  is output as the converted signal light  20  from the signal light output port  18  toward the outside. 
     Since the phase difference of the probe light between the optical paths  26  and  28  are adjusted to become π by the phase shifter  32 , the phase difference of the probe light entered the optical coupler  24  from the optical couplers  34  and  38  becomes π in the condition that the original signal light  12  does not yet enter the signal input port  10 . So, the intensity of the light combined by the optical coupler  24 , namely the intensity of the light (the converted signal light  20 ) of the probe wavelength λp output from the signal output port  14  becomes zero in principle (nearly zero in practice). on the other hand, in the condition that the original signal light  12  enters the signal input port  10 , the original signal light  12  is introduced to the optical path  26  by the optical coupler  34 , then transmits the phase shifter  32  and enters the EA optical modulator  30  in the opposite direction to the probe light. The EA optical modulator  30  absorbs the input original signal light. Owing to the absorption, a carrier is generated in the EA optical modulator  30 , and so the effective refractivity at the waveguide part of the EA optical modulator  30  varies according to the intensity variation of the original signal light  12 . The variation of the effective refractivity varies the phase of the probe light propagating in the EA optical modulator  30 . That is, the cross phase modulation between the original signal light and the probe light occurs in the EA optical modulation  30 . The carrier generated due to the absorption in the EA optical modulator  30  is sent to the outside circuit by the electric field, and thus this embodiment can respond at an ultrahigh speed. Even if the data rate of the original signal light  12  is 10 Gbit/s, this embodiment can sufficiently follow it. 
     FIG. 2 shows a characteristic diagram of the embodiment when the CW probe light  16  of a predetermined optical intensity enters the probe light input port  14 . In FIG. 2, the horizontal axis represents the optical intensity of the original signal light  12  and the vertical axis represents the intensity of the light (the converted signal light  20 ) of the probe wavelength λp output from the signal output port  18 . Needless to say, the EA optical modulator  30  is set so as to give the phase variation of π to the probe light according to the estimated peak optical intensity of the original signal light  12 . 
     Obviously from FIG. 2, the embodiment shown in FIG. 1 shows strong nonlinearity toward the optical intensity of the original signal light  12 , more specifically, shows binary characteristics for compressing the parts with weak optical intensity and strong optical intensity of the original signal light  12  as well as stretching the parts with medium optical intensity. By utilizing those characteristics, it is possible to shape a waveform and compress a noise. Moreover, when the optical intensity of the original signal light  12  becomes stronger, the embodiment shown in FIG. 1 shows optical limiter characteristics wherein the optical intensity of the converted signal light is fixed even if the optical intensity of the original signal light  12  increases, and thereafter shows, as the optical intensity of the original signal light  12  becomes even stronger, negative characteristics wherein the optical intensity of the converted signal light  20  inversely becomes weaker. 
     FIG. 3 shows an example of the waveform shaping effect according to the embodiment in FIG.  1 . Reference numeral  42  represents the input/output characteristics of the embodiment shown in FIG. 2, numeral  44  represents the waveform of the original signal light  12  entered the signal input port  10 , and numeral  46  represents the waveform of the converted signal light  20  of the wavelength λp output from the signal output port  18 . Obviously the waveform  46  of the converted signal light  20  is obtained by improving the waveform  44  of the original signal light  12  so as to change more drastically. 
     The embodiment shown in FIG. 1 works at the inverter operation when the phase difference between the optical paths  26  and  28  relative to the probe wavelength λp is adjusted to become zero by the phase shifter  32  in the condition that the original signal light  12  does not yet enter. That is, the waveform of the converted signal light  20  is basically the inverted one of the signal waveform of the original signal light  12 . 
     By setting the waveform of the probe light  16  to be an RZ pulse waveform synchronized with the pulse waveform of the original signal light  12 , the signal waveform is more improved and thus the S/N ratio is also improved. FIGS.  4 ( a ),  4 ( b ) and  4 ( c ) show the waveforms of the original signal light  12 , the probe light  16  and the converted signal light  20  respectively. In FIGS.  4 ( a ),  4 ( b ) and  4 ( c ), the horizontal axis and the vertical axis express time and optical intensity respectively. It is known from FIG. 4 that, in the original signal light  12 , the overlap of adjacent optical pulses and the pulse-like noise at the part where any optical pulse does not exist are removed, and the pulse wave forms are reshaped. Also, the S/N ratio is improved. 
     Furthermore, by setting the probe light  16  to be pulse light having a specific pulse pattern, it becomes possible to extract or remove a specific time slot part of the original signal light  12 . 
     In the embodiment shown in FIG. 1, to make it easily understandable, the EA optical modulators  30  and  36  are disposed on the optical paths  26  and  28  respectively. However, in principle, it is sufficient that the EA optical modulator  30  is disposed on the optical path  26  alone. Also, when the optical couplers  34  and  38  are composed of wavelength multiplexing optical couplers corresponding to the optical signal wavelength λs and probe wavelength λp, the 3 dB loss owing to each of the demultiplexing of the probe wavelength λp and the multiplexing of the optical signal wavelength λs can be solved. 
     In the foregoing explanation, although the probe wavelength λp is defined as a wavelength that is not absorbed by the EA optical modulators  30  and  36 , there is no difficulty for the operation in principle even if the wavelength is absorbed to a certain extent. In this case, however, the inverter operation is not suitable since the loss becomes larger and the interference circuit becomes asymmetry when the wavelength λp is absorbed. 
     In the embodiment, each component is hybrid-integrated on the InP substrate; however, it is also possible to realize the structure with optical fiber components having almost the same functions. 
     Also, although a waveguide InGaAsP electroabsorption type optical modulator of Franz-Keldysh effect is used as the EA optical modulator in the embodiment, any other absorption type optical modulator is applicable to the invention as far as it has the function for forcibly discharging, faster than the thermal diffusion, electrons and/or holes generated in the optical modulator due to the absorption by using an electric field formed in the optical modulator out of a voltage fed into the optical modulator. For example, waveguide type MQW optical modulators using the quantum Stark effect of multiquantum wells (MQW) of semiconductors can be used. 
     As already explained referring to FIG. 3, since the embodiment has nonlinear characteristics between the optical amount of the original signal light  12  and that of the converted signal light  20 , it is also applicable to optical waveform shaping. That is, the embodiment has optical threshold characteristics in which the optical intensity of the converted signal light  20  becomes drastically stronger relative to that of the original signal light. Also, the embodiment has an optical limiter function in which the optical output level of the converted signal light  20  is limited when the optical intensity of the original signal light  12  is strong. Such nonlinear characteristics are effective to suppress strength of noise at a space level and a mark level of an intensity-modulated optical signal. When it is used for such purpose of noise control, the probe wavelength λp can be identical with the wavelength s of the original signal light  12 . 
     Apparently, a similar operating effect can be obtained when the optical couplers  22 ,  24 ,  34  and  38  are respectively composed of an Y-shaped waveguide or a multimode interferometer (MMI) type optical combiner/splitter circuit. In such case, this embodiment can be formed on an InP substrate by using a semiconductor process technology; namely it can be obtained with a monolithic integrated structure. A semiconductor optical amplifier for amplifying the light of the wavelength λp also can be disposed between the optical coupler  24  and the signal output port  18 . 
     It is also applicable to feed assist light into the signal input port  10  in order to adjust the absorption characteristics of the EA optical modulator  30  for the wavelength λs light. The wavelength of the assist light should be a wavelength, e.g. 1.52 μm, to be absorbed at the EA optical modulator  30 . By adjusting the intensity of the assist light, it becomes possible to control the absorption characteristics of the EA optical modulator  30  for the wavelength λs light from the outside. That is, the cross phase modulation action between the original signal light and the probe light in the EA optical modulator  30  can be controlled from the outside. 
     In the embodiment, although the probe light  16  is sent from the outside, it is applicable to use an InP substrate instead of the silica substrate and form a semiconductor laser light source for generating the probe light  16  on the substrate. Needless to say, it is possible to use other substrates. 
     FIG. 5 shows a schematic block diagram of a second embodiment according to the invention. The embodiment employs a Michelson interferometer structure. Original signal light  52  of a wavelength λs (e.g. 1.52 μm) enters a signal input port  50 , and CW probe light  56  of a wavelength λp (e.g. 1.55 μm) enters a probe light input port  54 . In the embodiment, the original signal wavelength λs and the probe wavelength λp must be different from each other. 
     The original signal light  52  entered the signal input port  50  propagates a silica waveguide  58  and enters a phase shifter  62  via an Y-shaped branch line  60 . The configuration and function of the phase shifter  62  are identical with those of the phase shifter  32  of the embodiment in FIG.  1 . The original signal light passed through the phase shifter  62  propagates a silica waveguide  64  and enters an electroabsorption (EA) optical modulator  66 . The EA optical modulator  66  is, for example, composed of a waveguide transmission InGaAsP electroabsorption semiconductor element with Franz-Keldysh effect, and the internal configuration is basically identical with that of the EA optical modulator  30  of the embodiment shown in FIG. 1. A constant voltage of 3.0 volt is applied to the EA optical modulator  66 . The EA optical modulator  66  is composed of a medium that is almost transparent so as not to absorb the probe wavelength 
     In the EA optical modulator  66 , on an end face  66   a  side connecting to the silica waveguide  64 , a region (not shown in the diagram) is formed for matching a spot-size of the light propagating on the silica waveguide  64  and that of the light propagating in the EA optical modulator  66 . To put it concretely, the region reduces the stripe width of the electroabsorption layer as approaching to the end face  66   a  connecting to the silica waveguide  64 , the stripe width is 1.5 μm in the EA optical modulator  66 , and makes the stripe width to become 0.5 μm at the end face  66   a.  Also, a nonreflective coating is formed on the end face  66   a  for the original signal wavelength λs and probe wavelength λp. Although the coupling loss of the silica waveguide  64  and the EA optical modulator  66  is 6 dB when any spot size converter is not disposed, it is reduced to 4 dB when the above spot size converter is disposed. 
     An end face  66   b  on the opposite side of the EA optical modulator  66  is formed as a complete reflective surface for the probe wavelength λp. Although the end face  66   b  can completely reflect the original signal wavelength λs, the reflected light becomes an obstacle in that case. 
     The probe light  56  input the probe light input port  54  enters a port A of an optical circulator  68 . The optical circulator  68  is an optical element for outputting the input light of the port A from a port B and outputting input light of the port B from a port C. The probe light output from the port B of the optical circulator  68  transmits an optical filter  70 , which passes the probe wavelength λp through and does not pass the original wavelength λs, and enters an Y-shaped branch line  72  to be divided into two portions. 
     One portion of the probe light divided at the Y-shaped branch line  72  enters the EA optical modulator  66  through the Y-shaped branch line  60 , the phase shifter  62 , and the silica waveguide  64 . On the other hand, the other portion of the probe light divided at the Y-shaped branch line  72  enters an EA optical modulator  78  through an Y-shaped branch line  74  and a silica waveguide  76 . The EA optical modulator  78  has the same configuration with the EA optical modulator  66 . That is, the EA optical modulator  78  has, on an end face  78   a  side connecting to the silica waveguide  76 , a region (not shown in the diagram) for matching a spot-size of the light propagating on the silica waveguide  76  and that of the light propagating in the EA optical modulator  78 . The same nonreflective coating used for the end face  66   a  is formed on the end face  78   a,  and the same reflective surface used for the end face  66   b  is formed on an end face  78   b.    
     In the EA optical modulator  66 , the probe light is reflected by the end face  66   b  and reenters the silica transmission line  64 . However, in the EA optical modulator  66 , cross phase modulation occurs between the original signal light  52  and the probe light  56  and the optical phase of the probe light  56  is influenced and varies with the optical intensity waveform variation of the original signal light  52 . The probe light which phase is modulated at the EA optical modulator  66  transmits the silica waveguide  60  and the phase shifter  62  and is divided into two portions at the Y-shaped branch line  60 . One portion of the probe light divided at the Y-shaped branch line  60  enters the Y-shaped branch line  72 . 
     On the other hand, in the EA optical modulator  78 , the probe light is reflected by the end face  78   b,  reenters the silica transmission line  76  and is divided into two portions by the Y-shaped branch line  74 . One portion of the probe light divided at the Y-shaped branch line  74  enters the Y-shaped branch line  72 . 
     Consequently, entered the Y-shaped branch line  72  are the probe light which made a roundtrip to the first optical path or arm composed of the Y-shaped branch line  60 , the phase shifter  62 , the silica waveguide  64  and the EA optical modulator  66  and the probe light which made a roundtrip to the second optical path or arm composed of the Y-shaped branch line  74 , the silica waveguide  76  and the EA optical modulator  78 . Similarly to the embodiment in FIG. 1, the phase shifter  62  is preset so that the phase difference of the probe wavelength λp between the first and second optical paths becomes π in the condition that the original signal light  52  does not yet enter the signal input port  50 . Accordingly, the combined output light intensity of the Y-shaped branch line  72  becomes practically zero owing to the interference when the original signal light  52  does not yet enter the signal input port  50 . By setting the optical intensity of the original signal light  52  appropriate for cross phase modulation characteristics of the EA optical modulator  66 , the waveform of the combined output light of the Y-shaped branch line  72  becomes a waveform that varies more steeply, while reducing noise light and reflecting the signal waveform of the original signal light  52 . That is, the light output from the Y-shaped branch line  72  toward the optical filter  70  contains the light of the wavelength λp, namely converted signal light, which carries the same signal with the original signal light  52 . Since the light output from the Y-shaped branch line  72  toward the optical filter  70  contains the original signal wavelength λs, the optical filter  70  is disposed for removing it. 
     The optical filter  70  extracts the converted signal light of the wavelength λp alone from the output light of the Y-shaped branch line  72  and feeds the extracted light into the port B of the optical circulator  68 . The optical circulator  68  outputs the converted signal light from the optical filter  70  toward a signal output port  80  through the port C. Converted signal light  82  of the wavelength λp for carrying the same signal with the original signal light  62  is output from the signal output port  80  toward the outside. 
     In the embodiment in FIG. 5, the silica waveguide  58 , the Y-shaped branch line  60 , the phase shifter  62 , the silica waveguide  64 , the EA optical modulator  66 , the optical filter  70 , the Y-shaped branch lines  72  and  74 , the silica waveguide  76  and the EA optical modulator  78  can be formed on a substrate  84 . 
     Wavelength conversion characteristics of the embodiment shown in FIG. 5, more specifically the characteristics of optical intensity variation of the converted signal light  82  for the optical intensity of the original signal light  52  is basically the same with those (FIG. 2) of the embodiment shown in FIG.  1 . Similarly to the embodiment shown in FIG. 1, by making the probe light  56  to be an optical pulse synchronized with the original signal light  52 , it becomes possible to improve the shaping effect of a pulse waveform as well as effectively suppress the noise in the absence of a pulse. It is obvious that the inverter operation can be realized when the phase difference between the first and second optical paths are adjusted to become zero. 
     In the foregoing description, although the embodiment using a 1.5 μm band electroabsorption optical modulator is explained, the invention is not restricted to the wavelength band and optical element. 
     As readily understandable from the aforementioned, according to the invention, signal light can be reproduced in an optical state with a simple configuration. At the same time, it is possible to convert a wave length of the signal light. In principle, by optimizing an electroabsorption semiconductor element and an interferometer optical circuit, an extinction ratio larger than that of original signal light can be obtained. By adjusting the interferometer optical circuit, a signal waveform can be improved, e.g. elimination of the noise and so on, and also the optical phase distortion of the original signal light can be removed. 
     While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.