Abstract:
An optical gate according to the invention comprises a polarization divider to divide an optical signal into two orthogonal polarization components and to output them as a first polarization component which precedes in the time base and a second polarization component which follows the first one in the time base; a semiconductor optical amplifier to modulate the phase of the second polarization component output from the polarization divider according to a control light; an assist light supplier to supply to the semiconductor optical amplifier an assist light to help the recovery of the refractive index variation of the semiconductor optical amplifier caused by the control light; a polarization combiner to combine the first and second polarization components of the optical signal transmitting on the semiconductor optical amplifier so as to adjust them in the same time location; and a polarization extractor to extract a predetermined polarization direction component from the output from the polarization combiner.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to an optical gate and an optical phase modulator, and more specifically relates to an optical gate applicable to optical transmission systems, optical network systems and optical switching systems and to an optical phase modulator useful for realizing such an optical gate.  
         BACKGROUND OF THE INVENTION  
         [0002]    To realize an ultra large capacity and ultra high speed optical communication system, it is important to obtain an optical gate element capable of switching ON/OFF of an optical signal using the light. Especially, when a signal transmission rate per wavelength becomes as fast as 10 Gb/s or more, it is difficult to process a signal electrically in terms of the operation speed and the energy consumption. Accordingly, an optical gate or an optical switch to directly turn ON/OFF or to switch the optical signal using another optical signal has enthusiastically developed.  
           [0003]    There are several types of conventional optical switches, for example, one using cross gain modulation (XGM) in a medium whose gain nonlinearly varies according to the intensity of input light such as a semiconductor optical amplifier (SOA), one using cross phase modulation (XPM) in a medium whose refractive index nonlinearly varies according to the intensity of input light such as a SOA, and the one using four wave mixing (FWM).  
           [0004]    The response speed of the optical switch using the FWM is extremely high. However, it has a demerit to need high power for the ON/OFF of the light because its conversion efficiency is small and its wavelength dependency is large.  
           [0005]    The optical switch using the XGM or XPM has a large switching efficiency because both XGM and XPM utilize the phenomena based on the process causing the pumping of actual carriers. It is reported that in the XPM, by switching ON/OFF of the optical signal by a control light pulse in an interference system, the operation speed of 10 Gb/s or more can be realized. However, since the standard XPM utilizes interferometers having two light paths such as Mach-Zehnder interferometers or Michelson interferometers, the circuit tends to be complicated. Furthermore, it is difficult to adjust the operation conditions of the two SOAs in a necessary perfect balance.  
           [0006]    As means to solve the above problems in the XPM, having proposed is an optical switch to have a polarization division interferometer circuit configuration which physically has a single light path by dividing an optical signal into two orthogonal polarization components using a birefringent medium, and combining the two orthogonal polarization components again after passing them through the SOA of the nonlinear medium (for example, see N. S. Patel et al. Optics Letters, vol. 21, pp. 1466-1468, 1996). This optical switch is called as Ultrafast Nonlinear Interferometer (UNI).  
           [0007]    [0007]FIG. 7 shows a schematic diagram of the optical switch disclosed in the above-mentioned paper. An optical signal  212  of wavelength 1550 nm enters an optical signal input port  210 , and a control light  216  of wavelength 1540 nm enters a control light input port  214 . The optical signal  212  contains, for example, a 40 Gbit/s optical clock signal of linear polarization, and the control light  216  contains a 40 Gbit/s optical RZ pulse train synchronized with the optical signal  212 .  
           [0008]    The optical signal output  212  from the optical signal input port  210  enters a 7.5 m long polarization preserving fiber  218  at an angle of 45° of the polarization plane relative to the birefringent axis of the fiber. The polarization preserving fiber  218  functions as a birefringent medium to divide the input optical signal into two polarization components and to output them after separating them in the time base by the amount (12.5 ps) of the polarization mode dispersion of the polarization preserving fiber  218 . A WDM optical coupler  220  combines the output from the polarization preservation fiber  218  with the control light  216  from the control light input port  214 . The timing between the optical signal  212  and the control light  216  is adjusted so that at the output stage of the WDM optical coupler  220 , a control light pulse  226  is located between a preceding optical signal pulse  222  and a following optical signal pulse  224  output from the polarization preserving fiber  218 . The preceding optical signal pulse  222 , the control light pulse  226 , and the following optical signal pulse  224  enter a semiconductor optical amplifier (SOA)  228  in this order.  
           [0009]    The SOA  228  is forward biased by a direct power source  230 . For example, the SOA  228  consists of a buried waveguide using the InGaAsP/InP system as an active layer material, and both ends are applied with antireflection coating. When the control light pulse  226  enters, the gain in the SOA  228  instantly decreases due to the stimulated emission, gain saturation occurs, and the carrier density in the SOA  228  decreases. Since the refractive index of the semiconductor depends on the carrier density of the inside (band filing effect), the refractive index variation (which results in XPM) occurs at this point. That is, the refractive index of the SOA  228  varies before and after the entry of the control light pulse  226 . Therefore, the following optical signal pulse  224  receives a phase shift different from that of the preceding optical signal pulse  222  while transmitting in the SOA  228 . Since the amount of the phase shift varies according to the optical intensity and wavelength of the control light pulse  226  and injected electric current of the SOA  228 , the optical intensity and wavelength of the control light pulse  226  and the injected current of the SOA  228  are set so that the amount of the phase variation of the following optical signal pulse  224  caused by the existence and the nonexistence of the control light pulse  226  becomes π. With this configuration, the phase of the following optical signal pulse  224  output from the SOA  228  differs by π according to whether or not the control light pulse  226  exists.  
           [0010]    The optical signal pulses  222  and  224  passed through the SOA  228  enter a 7.5 m long polarization preserving fiber  232  in the direction that the polarization plane of the preceding pulse  222  coincides with the slow axis of the polarization preserving fiber  232  and the polarization plane of the following pulse  224  coincides with the fast axis of the fiber  232 . With this configuration, the time difference between the optical signal pulses  222  and  224  is almost disappeared after they passed through the polarization preserving fiber  232 . To cancel the individual difference of the polarization mode dispersion amount between the polarization preserving fibers  218  and  232 , a polarization phase adjuster  234  is disposed at the output of the polarization preserving fiber  232 . Reference numerals  224   a  and  226   a  denote the preceding optical signal pulse and the following optical signal pulse output from the polarization phase adjuster  234  respectively.  
           [0011]    A polarizer  236  is disposed on the output side of the polarization phase adjuster  234  so that the polarizer  236  passes the light having the same polarization direction with that of the composite polarization of the preceding optical signal pulse  222   a  and the following optical signal pulse  224   a  when the optical signal pulse  226  exists. When the optical signal pulse  226  does not exist, the composite polarization direction of the preceding optical signal pulse  222   a  and the following optical signal pulse  224   a  becomes orthogonal to that of the polarizer  236 . Accordingly, the polarizer  236  passes only the optical signal in the condition that the control light pulse  226  exists out of the optical output from the polarization phase adjuster  234 . An optical bandpass filter  238  exclusively extracts the component having the wavelength equal to that of the optical input signal  212  from the output of the polarizer  236 . With this operation, the remaining components of the control light  216  are removed.  
           [0012]    In the conventional example shown in FIG. 7, the operation of the optical switch is realized using the XPM of the SOA  228 . Since the signals passed through the sole SOA  228  are interfered each other, it is unnecessary to completely balance the operating conditions of two SOAs unlike an interferometer having two optical paths, and accordingly the operation becomes stable. Moreover, its speed is so high that an optical switch as fast as 100 Gbit/s or more has been reported (see K. L. Hall et al., Optics Letters, vol. 23, pp. 1271-1273, 1998).  
           [0013]    However, the recovery time of the XPM in the SOA  228  is as long as approximately 100 ps. Accordingly, although the optical switching speed itself is very high, the pattern effect becomes evident. For example, the characteristics to switch the optical signal  212  vary per pulse when applied to the control light  216  of the random pulse train.  
           [0014]    FIGS.  8 ( a )-( d ) show waveform examples showing the pattern effect, and the time variation of the carrier density in the SOA  228 . FIG. 8( a ) shows the waveform of the input optical signal  212 , FIG. 8( b ) shows the waveform of the control light  216 , FIG. 8( c ) shows the waveform of the optical signal output from the optical bandpass filter  238 , and FIG. 8( d ) shows the time variation of the carrier density. As shown in FIGS.  8 ( a ), ( b ), each pulse of the input optical signal  212  has a uniform pulse height, and each pulse of the control light  216  also has a uniform pulse height. When the control light pulses continue, the phase shift amount of the SOA  228  reduces because the XPM is repeated without achieving the complete recovery. Accordingly, when the control light pulses continue, the peak value of the optical output signal from the optical bandpass filter  238  reduces as shown in FIG. 8( c ). This phenomenon is called the pattern effect. As shown in FIG. 8( d ), the amount of the carrier variation caused by the input of the control light pulse, namely the amount of the refractive index variation, differs per control light pulse. This causes the pattern effect.  
           [0015]    Also, in the conventional example, since the interferometer is composed of polarization dividing/combining using the birefringent of the two several meter long polarization preserving fibers  218  and  232 , there is a problem in which the operation tends to be unstable because it is easily affected by the ambient conditions, such as temperature and vibration. In addition, it is not suitable for the mass production due to the structure.  
           [0016]    Furthermore, in the high speed optical transmission systems, it has been expected to realize an optical TDM demultiplexer which not only extract a pulse component of specific time slot from an optical pulse train but also output the remaining time slot pulse component.  
         SUMMARY OF THE INVENTION  
         [0017]    It is therefore an object of the present invention to provide an optical gate to shorten the recovery time of XPM in a SOA and to operate with stability and less pattern effect.  
           [0018]    Another object of the present invention is to provide an optical gate which is compact, outstanding at the stability against long term environmental variation, and suitable for the mass production.  
           [0019]    A further object of the present invention is to provide an optical gate capable of demultiplexing an optical input pulse train into two portions using optical control in the time domain.  
           [0020]    Still a further object of the present invention is to provide an optical phase modulator of high-speed operation to modulate a phase of an optical signal using another light.  
           [0021]    An even further object of the present is to provide an optical phase modulator to stably operate at high speed.  
           [0022]    An optical gate according to the invention consists of a polarization divider to divide an optical signal into two orthogonal polarization components and to output them as a first polarization component which precedes in the time base and a second polarization component which follows the first one in the time base, a semiconductor optical amplifier to modulate a phase of the second polarization component output from the polarization divider according to a control light, an assist light supplier to supply to the semiconductor optical amplifier an assist light to help the recovery of the refractive index variation of the semiconductor optical amplifier caused by the control light, a polarization combiner to combine the first and second polarization components of the optical signal transmitting the semiconductor optical amplifier so as to adjust them in the same time location, and a polarization extractor to extract a predetermined polarization direction component from the output from the polarization divider.  
           [0023]    An optical phase modulator according to the invention consists of a semiconductor optical amplifier to which a control light and optical signal input and which is forward biased and varies its refractive index relative to the optical signal according to the intensity variation of the control light, and an assist light supplier to supply to the semiconductor optical amplifier an assist light to help the recovery of the refractive index variation of the semiconductor optical amplifier caused by the control light. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0024]    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:  
         [0025]    [0025]FIG. 1 shows a schematic diagram of a first embodiment according to the invention;  
         [0026]    [0026]FIG. 2 shows the relation of the gain spectrum of a semiconductor optical amplifier  28  with an optical signal wavelength, control light wavelength, an assist light wavelength;  
         [0027]    FIGS.  3 ( a )-( d ) show waveform examples of the embodiment and the time variation of the carrier density in the semiconductor optical amplifier  28 ;  
         [0028]    [0028]FIG. 4 shows the relation of the gain spectrum of the semiconductor optical amplifier  28  with the optical signal wavelength, control light wavelength, and assist light wavelength when a 1480 nm assist light is used;  
         [0029]    [0029]FIG. 5 shows a schematic diagram of a second embodiment according to the invention;  
         [0030]    [0030]FIG. 6 shows a schematic diagram of a third embodiment according to the invention;  
         [0031]    [0031]FIG. 7 shows a schematic diagram of prior art;  
         [0032]    FIGS.  8 ( a )-( d ) show waveform examples of the prior art and the time variation of the carrier density of the semiconductor optical amplifier  228 ; and  
         [0033]    [0033]FIG. 9 shows a schematic block diagram of a fourth embodiment according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    Embodiments of the invention are explained below in detail with reference to the drawings.  
         [0035]    [0035]FIG. 1 shows a schematic diagram of a first embodiment according to the invention. An optical signal  12  of wavelength 1550 nm enters an optical signal input port  10 , and a control light  16  of wavelength 1540 nm enters a control light input port  14 . For example, the optical signal  12  consists of a 40 Gbit/s optical clock signal of linear polarization, and the control light  16  consists of a 40 Gbit/s RZ optical pulse train synchronized with the optical signal  12 .  
         [0036]    The optical signal  12  from the optical signal input port  10  enters a 7.5 m long polarization preserving fiber  18  at an angle of 45° of the polarization plane relative to the birefringent axis of the fiber. The polarization preserving fiber  18  functions as a birefringent medium to divide the input optical signal into two polarization components and to output them after separating them in the time base by 12.5 ps, which is the amount of the polarization mode dispersion of the polarization preserving fiber  18 . A WDM optical coupler  20  combines the optical output from the polarization preserving fiber  18  with the control light  16  from the control light input port  14 . The timing between the optical signal  12  and the control light  16  is adjusted so that at the output stage of the WDM optical coupler  20 , a control light pulse  26  is located between a preceding optical signal pulse  22  and a following optical signal pulse  24  output from the polarization preserving fiber  18 . The preceding optical signal pulse  22 , control light pulse  26 , and following optical signal pulse  24  continuously enter a semiconductor optical amplifier (SOA)  28  in this order.  
         [0037]    The SOA  28  is forward biased by a direct power source  30 . Similarly to the SOA  228 , the SOA  28  consists of a buried waveguide using the InGaAsP/InP system as an active layer material, for example, and its both ends are applied with antireflection coating. The optical intensity and wavelength of the control light pulse  26  are set so that the amount of the phase variation of the following optical signal pulse  24  caused by the existence and the nonexistence of the control light pulse  26  becomes π. With this configuration, the phase of the following optical signal pulse 24  output from the SOA  28  differs by π according to whether or not the control light pulse  26  exists.  
         [0038]    The optical signal pulses  22  and  24  passed through the SOA  28  enter a 7.5 m long polarization preserving fiber  32  through ports B and C of an optical circulator  40 . The polarization preserving fiber  32  is disposed so that the polarization plane of the preceding pulse  22  coincides with the slow axis of the polarization preserving fiber  32  and the polarization plane of the following pulse  24  coincides with the fast axis of the fiber  32 . With this configuration, the time difference between the optical signal pulses  22  and  24  is almost disappeared after they passed through the polarization preserving fiber  32 . To cancel the individual difference of the polarization mode dispersion amount between the polarization preserving fibers  18  and  32 , a polarization phase adjuster  34  is disposed at the output of the polarization preserving fiber  32 . Reference numerals  24   a  and  26   a  denote the preceding optical signal pulse and the following optical signal pulse output from the polarization phase adjuster  34  respectively.  
         [0039]    A polarizer  36  is disposed on the output side of the polarization phase adjuster  34  so that the polarizer  36  passes the light having the same polarization direction with that of the composite polarization of the preceding optical signal pulse  22   a  and the following optical signal pulse  24   a  when the optical signal pulse  26  exists. When the optical signal pulse  26  does not exist, the composite polarization direction of the preceding optical signal pulse  22   a  and the following optical signal pulse  24   a  becomes orthogonal to that of the polarizer  36 . Accordingly, the polarizer  36  passes only the optical signal in the condition that the control light pulse  26  exists out of the optical outputs from the polarization phase adjuster  34 . An optical bandpass filter  38  exclusively extracts the component having the wavelength equal to that of the optical input signal  12  from the output of the polarizer  36 . With this operation, the remaining components of the control light  16  are removed.  
         [0040]    In this embodiment, a CW light source  42  of wavelength 1590 nm is disposed to supply a CW light into a port A of an optical circulator  40 . The optical circular  40  supplies the CW light from the CW light source  42  to the SOA  28  in the opposite direction with the optical signal. It is also applicable to use a 3 dB coupler instead of the optical circulator  40 .  
         [0041]    As explained referring to FIGS.  8 ( a )-( d ), the pattern effect is a phenomenon as follows. Owing to the entry of the control light, the gain is saturated and the carrier density is reduced causing the refractive index variation. The recovery time (the time in which the carrier density returns to the normal condition) of the refractive index variation is approximately several 100 ps. Since this time is about one figure longer than the repetition time of the optical signal, the refractive index variation by the control light differs per control light pulse according to whether or not the preceding control light pulse exists. Namely, when the control light pulses continuously transmit, the carrier density in the SOA is gradually lowered and the variation amount is also reduced. Conversely, when the control light pulse transmits immediately after the condition in which the control light did not exist for a while, the variation amount returns to the beginning state.  
         [0042]    In this embodiment, by introducing the CW light (assist light) of wavelength 1590 nm into the SOA  28 , the recovery time of the carrier density and refractive index can be shortened to the extent approximately equal to the repetition time of the optical signal. The reason for the shortening is explained below.  
         [0043]    [0043]FIG. 2 shows the relation of the gain spectrum in the SOA  28  to the wavelengths of the optical signal  12 , control light  16 , and assist light. Reference numeral  50  denotes the gain spectrum before the entry of the control light  16 . At this stage, the gain spectrum shows gain for the optical signal  16  of wavelength 1550 nm and is almost transparent for the assist light of wavelength 1590 nm. Accordingly, the assist light does not affect the gain spectrum of the SOA  28  in the condition that the control light  16  does not exist. Reference numeral  52  shows the gain spectrum immediately after the control light  16  enters the SOA  28 . At this stage, the gain is reduced due to the gain saturation caused by the entry of the control light  16  of wavelength 1540 nm, and the assist light of wavelength 1590 nm is absorbed by the SOA  28 . This absorbed carrier helps the gain spectrum to recover rapidly and to return to the beginning characteristic  50 . The recovery time shortens as the intensity of the assist light increases. For instance, the recovery time is shortened approximately 30 ps when the assist light is 50 mW.  
         [0044]    FIGS.  3 ( a )-( d ) show waveform examples of the optical signal  12 , control light  16 , and optical output from the optical bandpass filter  38 , and the time variation of the carrier density in the SOA  28 . FIGS.  3 ( a )-( c ) show the waveform of the optical signal  12 , the waveform of the control light  16 , and the waveform of the optical signal output from the optical bandpass filter  38  respectively, and  3 ( d ) shows the time variation of the carrier density. As easily understandable by comparing to FIGS.  8 ( a )-( d ), in this embodiment, the optical signal output from the optical bandpass filter  38  synchronizes with the control light  16  and has the uniformed peak value. As shown in FIG. 3( d ), since the recovery time of the carrier density in the SOA  28  is sufficiently short, a constant variation of the carrier density relative to the control light pulse occurs without depending on the pattern.  
         [0045]    The operation of the SOA  28  when the control light pulse enters is identical to that of the prior art. Accordingly, it is possible to realize the switching rate approximately equal to that of the prior art.  
         [0046]    In this embodiment, by supplying the CW assist light, to which the SOA  28  is practically transparent in the condition that the control light pulse does not exist and which is absorbed by the SOA  28  after the gain spectrum variation caused by the control light pulse, to the SOA  28 , the recovery time of the carrier density is shortened and consequently the pattern effect can be removed or greatly reduced. Theoretically, although the assist light should be introduced to the SOA  28  immediately after the carrier variation caused by the control light pulse  26  has changed the phase of the following optical signal  24 , to achieve such timing is very troublesome, and thus to use the CW assist light is sufficient in practice.  
         [0047]    In this embodiment, although the wavelength of the assist light is set to the 1590 nm in which the gain of the SOA  28  becomes practically zero, the same operation effect can be obtained using any wavelength for the assist light as long as it is longer than those of the optical signal  12  and control light  16  and shorter than the band edge wavelength of the SOA  23 , namely the wavelength in which the gain in the longer wavelength side becomes zero.  
         [0048]    The embodiment shown in FIG. 1 has the function to switch a 40 Gbit/s optical clock signal with a control light pulse. This function is considered the wavelength conversion from the control light wavelength of 1540 nm to the optical signal wavelength of 1550 nm. Also, when the optical signal consists of a 40 Gbit/s RZ light and the control light consists of a 10 Gbit/s light, the embodiment shown in FIG. 1 functions as a demultiplexer to demultiplex an optical pulse in a desired time slot or an optical gate to extract an optical pulse in a desired time slot. The embodiment shown in FIG. 1 also functions as an optical pulse generator to cut out an optical pulse from a CW light by a control light pulse. As explained above, the embodiment shown in FIG. 1 can perform the several functions according to the conditions of the optical signal and control light and has little pattern effect in all functions.  
         [0049]    It is obviously possible to utilize rutile crystal (TiO 2 ) instead of the polarization preserving fibers  18  and  32 . For instance, the length of the rutile crystal to give 12.5 ps of the PMD amount is approximately 15 mm. Because the rutile crystal is greatly compact and mechanically stable compared to the polarization preserving fiber, the optical switching characteristics are not affected by the external circumstances such as the ambient temperature and vibration etc., and lead to the stable operation.  
         [0050]    Also, the wavelength of the assist light can be set to 1480 nm. FIG. 4 shows the gain spectrum of the SOA  28  in this condition. Reference numeral  60  denotes the gain spectrum before the control light  16  enters. This is identical to the characteristic curve  50  in FIG. 2. Reference numeral  62  denotes the gain spectrum when the control light  16  of wavelength 1540 nm enters. Although the 1480 nm wavelength is transparent when the control light  16  does not exist, it is absorbed when the control light  16  enters. Accordingly, even the assist light having the wavelength 1480 nm is used, the SOA  28  operates in the same way when the assist light of wavelength 1590 nm is used.  
         [0051]    Generally, the wavelength of the assist light is selectable within a range of +/−50 nm to the wavelength 1480 nm in which the gain in the shorter wavelength side becomes zero.  
         [0052]    It is possible to transmit the optical signal and the control light in the opposite directions in the SOA. FIG. 5 shows a schematic block diagram of such modified embodiment.  
         [0053]    An optical signal  112  of wavelength 1550 nm enters an optical signal input port  110 , and a CW assist light of wavelength 1480 nm from an assist light source  116  enters assist light input port  114 . For example, the optical signal  112  consists of a 40 Gbit/s optical clock signal of linear polarization.  
         [0054]    The optical signal  112  from the optical signal input port  110  enters a rutile crystal  118  at an angle of 45° of the polarization plane relative to the birefringent axis of the rutile crystal. The rutile crystal  118  divides the input optical signal into two polarization components and output them after separating them in the time base by 12.5 ps. A WDM optical coupler  120  combines the output light fromthe rutile crystal  118  with the assist light fromthe assist light input port  114 . A pulse  122  illustrated at the output stage of the WDM optical coupler  120  is an optical signal pulse preceded by the birefringent medium  118  in the time base, and a pulse  124  is a following signal pulse which is delayed in the time base. The preceding optical signal pulse  122 , following optical signal pulse  124  and assist light enter a semiconductor optical amplifier (SOA)  128 .  
         [0055]    The SOA  128  is forward biased by a direct power source  130 . For example, similarly to the SOAs  28  and  228 , the SOA  128  consists of a buried waveguide using the InGaAsP/InP system as an active layer material, and both ends are applied with antireflection coating.  
         [0056]    The optical signal output side of the SOA  128  connects to a port B of the optical circulator  132 , and a control light  134  enters a port A of the optical circulator  132 . The wavelength of the control light  134  can be any wavelength as far as the SOA  128  has gain, and can be equal to that of the optical signal  112 . The optical circulator  132  transmits the control light  134  entered the port A to the port B, and accordingly the control light  134  enters the SOA  128  to propagate in the opposite direction with the optical signal. Similarly to the control light  16 , the control light  134  consists of, for example, a 40 Gbit/s RZ optical pulse train. The timing of the control light  134  relative to the optical signal  112  is adjusted in advance so that the each pulse of the control light  134  enters the SOA  128  after the preceding optical signal pulse  122  is output from the SOA  128  and before the following optical signal pulse  124  enters the SOA  128 . Also, the optical intensity and wavelength of the control light pulse  134  are set so that the amount of the phase variation of the following optical signal pulse  124  caused by the existence and the nonexistence of the control light pulse  134  becomes π. With this configuration, the phase of the following optical signal pulse  124  output from the SOA  128  differs by π according to whether or not the control light pulse  134  exists.  
         [0057]    Since the assist light of wavelength 1480 nm enters the SOA  128  through an optical coupler  120 , the variation of the carrier density caused by the control light  134  recovers rapidly due to the absorption of the assist light of wavelength 1480 nm. This function is similar to that of the embodiment shown in FIG. 1.  
         [0058]    The optical signal pulses  122  and  124  passed through the SOA  128  enter the rutile crystal  136  through the ports B and C of the optical circulator  132 . The rutile crystal  136  is disposed so that the polarization plane of the preceding pulse  122  coincides with the slow axis of the rutile crystal  136  and the polarization plane of the following pulse  124  coincides with the fast axis of the rutile crystal  136 . With this configuration, the time difference between the optical signal pulses  122  and  124  is almost disappeared after they passed through the rutile crystal  136 . To cancel the individual difference of the polarization mode dispersion amount between the rutile crystals  120  and  136 , apolarization phase adjuster  138  is disposed at the output of the rutile crystal  136 . Reference numerals  122   a  and  124   a  denote the preceding optical signal pulse and the following optical signal pulse output from the polarization phase adjuster  138  respectively.  
         [0059]    A polarizer  140  is disposed on the output side of the polarization phase adjuster  138  so that the polarizer  140  passes the light having the same polarization direction with that of the composite polarization of the preceding optical signal  122   a  and the following optical signal pulse  124   a  when the control light pulse  134  exists. When the control light pulse  134  does not exist, the composite polarization direction of the preceding optical signal  122   a  and the following optical signal pulse  124   a  becomes orthogonal to that of the polarizer  140 . Accordingly, the polarizer  140  passes only the optical signal in the condition that the control light pulse  134  exists out of the optical outputs from the polarization phase adjuster  138 . An optical bandpass filter  142  exclusively extracts the components having the wavelength equal to the optical input signal  112  from the output of the polarizer  140 . With this operation, the remaining components of the assist light are removed.  
         [0060]    In the embodiment shown in FIG. 5, since the optical signal and the control light propagate in the opposite directions in the SOA  128 , the wavelength of the optical signal and that of the control light can be identical. This configuration is effective when the wavelength conversion function is unnecessary. The effect for suppressing the pattern effect is identical to that of the first embodiment.  
         [0061]    [0061]FIG. 6 shows a schematic diagram of a third embodiment according to the invention. In the embodiment shown in FIG. 6, the control light combined with an assist light of wavelength 1480 nm enters a SOA from the back. The identical elements with those in FIG. 5 are labeled with the common reference numeral. A light source  150  outputs the assist light of wavelength 1480 nm. An optical coupler  152  combines the assist light from the light source  150  with the control light  134  and supplies the combined light to a port A of the optical circulator  132 . With this operation, both assist light and control light enter the SOA  128  together from the back. That is, the optical coupler  153  is disposed instead of the optical coupler  120 .  
         [0062]    In the embodiment shown in FIG. 6, since the optical output from the polarizer  140  does not contain the assist light and control light, the optical bandpass filter  142  can be omitted.  
         [0063]    In the embodiments shown in FIGS. 1, 5 and  6 , although only the specific polarization components are extracted by the polarizers  36  and  140 , such a configuration is also possible that the orthogonal two polarization components are split and output by a polarizing beam splitter. With this configuration, a light-controlled optical TDM demultiplexer is obtained to demultiplex an optical pulse train into two portions in the time domain. FIG. 9 shows a configuration example in which the embodiment shown in FIG. 1 is modified as described above. The identical elements with those in FIG. 1 are labeled with the common reference numeral.  
         [0064]    An optical output from the polarization phase adjuster  34  enters a polarizing beam splitter  44 . The polarizing beam splitter  44  is disposed in the same polarization direction with that of the polarizer  36  relative to the optical output from the polarization phase adjuster  34 . The polarization of the signal pulse component on the time slot according to the timing in which the control light  26  does not exist are orthogonal to the polarization of the signal pulse component on the time slot according to the timing in which the control light  26  exists. Therefore, the polarizing beam splitter  44  splits the pulse component on the time slot according to the timing of the existence of the control light  26  and the other pulse components from the signal light  12 , and output the former pulse component to the optical bandpass filter  38  and the latter to the optical bandpass filter  46 . The optical bandpass filter  46  consists of the same filter characteristics with those of the optical bandpass filter  38 .  
         [0065]    As described above, the embodiment shown in FIG. 9 functions as an optical TDM demultiplexer to demultiplex the optical signal  12  into two portions in the time domain under the control of the control light  26 . In this embodiment, it is selectable which time slot is output to which filter by the control light  26 .  
         [0066]    Also, in this embodiment, the polarizing beam splitter is used to divide the optical signal into two portions at the final stage, the optical signal is divided into two portions efficiently, that is, with a little loss.  
         [0067]    In the above embodiments, although the optical signal wavelength is set to 1550 nm and the wavelength of the control light is set to 1540 nm, they are only examples. The optical signal can have any wavelength as far as it is longer than the wavelength in which a SOA has gain, and the control light can have any wavelength as far as it is the wavelength to have gain of the SOA. Accordingly it does not matter which wavelength of the optical signal or control signal is longer. Also, the assist light should not necessarily be a single wavelength laser light; a multi mode laser light is also applicable.  
         [0068]    Although the embodiment of the 1.5 μm wavelength band is explained as the amplification band of the SOA, any amplification band between 0.4 to 2.0 μm which can be realized by the appropriate semiconductor is applicable. Although the InGaAsP/InP system is explained as the active layer material, the other materials such as GaAs/AlGaAs, InAlGaAs/InP, and InGaAlP/GaAs systems, as well as other material such as III-V semiconductor and II-IV semiconductor are applicable.  
         [0069]    As readily understandable from the aforementioned explanation, according to the invention, it is possible to realize a polarization separation interferometer optical gate to shorten the recovery time of the XPM caused by the control light in the semiconductor optical amplifier and to operate stably having smaller pattern effect. This optical gate is compact, outstandingly stable against a long term ambient variation, and suitable for mass production.  
         [0070]    Also, it is possible to realize an optical TDM demultiplexer to be controllable by a control light and to demultiplex an optical signal into two portions in the time domain in the combination of any time slots.  
         [0071]    Furthermore, it is possible to realize a phase modulator to modulate a phase of a light with another light and to operate at high speed with no or little pattern effect.  
         [0072]    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.