Abstract:
A first multimode interferometer has a first input port to which an optical signal is applied, a first output port, and a second output port. A first optical waveguide is connected to the first output port of the first multimode interferometer. The first optical waveguide has a refractive index changed in response to a trigger signal externally applied. A second optical waveguide is connected to the second output port. A triggering unit supplies, to the first optical waveguide, the trigger signal for changing the refractive index of the first optical waveguide. An optical switch is provided which can increase the processing speed, can reduce the device size, and is free from dependency on the polarization state of an optical signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This invention is based on and claims priority of Japanese patent application 2001-316546, filed on Oct. 15, 2001, the whole contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an optical switch and an optical demultiplexer, and more particularly to an optical switch and an optical demultiplexer which are simplified in structure and control.  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, a wavelength division multiplexing (WDM) optical communication system has been developed as a broadband optical communication system. Other optical communication systems, such as optical time division multiplexing (OTDM) and time wavelength division multiplexing (TWDM), have also been proposed and studied aiming at broader band optical communication.  
           [0006]    The WDM optical communication system is a system for increasing signal density through wavelength multiplexing of an optical signal. The time division systems, such as OTDM and TWDM optical communication systems, are intended to increase signal density by time-dividing an optical signal of the same wavelength and assigning divided optical signals to a number of channels.  
           [0007]    Response speed of an electrical signal is limited by a moving time of carriers in a semiconductor device and hence lower than the response speed of an optical signal. At present, the speed limit of an electrical signal is thought to be about 40 Gbits/s. To process an OTDM signal having speed higher than that limit, an optical signal must be divided through high-speed optical signal processing and demultiplexed to a bit rate, at which electrical processing is feasible.  
           [0008]    In view of the above-mentioned background, an optical device (optical demultiplexer) has recently been studied which is able to demultiplex an optical signal, as it is, without converting the optical signal into an electrical signal. Hitherto, optical demultiplexers of, e.g., non-linear optical loop mirror (NOLM) type, Mach-Zehnder type and polarization separating type, have been proposed.  
           [0009]    [0009]FIG. 15A is a schematic view of a NOLM type optical demultiplexer. An optical signal sig 1  reaches a branch point  102  of an optical fiber loop  101  via an input side optical fiber  100 . At the branch point  102 , the optical signal sig 1  is branched into an optical signal sig 2  propagating in the loop  101  counterclockwise and an optical signal sig 3  propagating in the loop  101  clockwise. The optical signal sig 1  is a signal having four time-division multiplexed channels, i.e., channels #1 to #4.  
           [0010]    A non-linear waveguide  103  is inserted in the optical loop  101  at a position asymmetrical to the branch point  102 . The optical signal sig 2  propagating counterclockwise reaches the non-linear waveguide  103  at timing earlier than the optical signal sig 3  propagating clockwise. A control light pulse con is inputted to the non-linear waveguide  103  immediately after the channel #2 of the optical signal sig 2  has passed the non-linear waveguide  103 . The refractive index of the nonlinear waveguide  103  is changed upon the inputting of the control light pulse con, whereby the phase of a pulse light in each channel #3 and #4 of the optical signal sig 2  is shifted π. In FIG. 15A, a pulse having phase shifted π is represented by hatching.  
           [0011]    Because the optical signal sig 3  reaches the non-linear waveguide  103  at timing delayed from the optical signal sig 2 , only the channel #1 of the optical signal sig 3  has passed the non-linear waveguide  103  at the time when the control light pulse con is inputted to the non-linear waveguide  103 . Therefore, the phase of a pulse light in each of the channels #2 to #4 of the optical signal sig 3  is shifted π.  
           [0012]    When the optical signals sig 2  and sig 3  return to the branch point  102 , the pulses in those ones #1, #3 and #4 of the channels of both the signals, which are in phase, propagate in the input side optical fiber  100 , and the pulse in the out-of-phase channel #2 propagates in an output side optical fiber  105 . Thus, only the signal of one channel can be separated from the time division multiplexed signal sig 1 .  
           [0013]    In the NOLM type optical demultiplexer, the time required for the optical signal to pass the optical loop  101  limits the signal speed achievable in signal processing. Also, the use of an optical fiber loop raises a difficulty in reducing the device size.  
           [0014]    [0014]FIG. 15B is a schematic view of a Mach-Zehnder type optical demultiplexer. Non-linear waveguides  121  and  122  are inserted respectively in two arms of a Mach-Zehnder interferometer  120 . An optical signal sig 10  is branched into two optical signals sig 11 , and sig 12 , which are introduced to the non-linear waveguides  121  and  122 , respectively. A control light pulse con is inputted to the non-linear waveguides  121  and  122  at different timings from each other.  
           [0015]    The control light pulse con is inputted to the non-linear waveguide  121  immediately after a pulse in a channel #1 has passed the non-linear waveguide  121 , and is inputted to the non-linear waveguide  122  immediately after a pulse in a channel #2 has passed the non-linear waveguide  122 . Therefore, the phase of an optical pulse in each of the channels #2 to #4 of the optical signal sig 11  is shifted π after passing the non-linear waveguide  121 , and the phase of an optical pulse in each channel #3 and #4 of the optical signal sig 12  is shifted π after passing the non-linear waveguide  122 .  
           [0016]    When the optical signals sig 11  and sig 12  are combined with each other, the signals in the channels #1, #3 and #4 are introduced to one output optical fiber  125 , and the signal in the channel #2 is introduced to the other output optical fiber  126 .  
           [0017]    Thus, in the Mach-Zehnder type optical demultiplexer, two arms, in which non-linear waveguides are respectively inserted, must be arranged parallel to each other. The device size is therefore increased.  
           [0018]    [0018]FIG. 15C is a schematic view of a polarization separating type optical demultiplexer. An optical signal sig 20  enters a birefringence crystal  130 . The birefringence crystal  130  delays a light in the TM mode by one pulse relative to a light in the TE mode. An optical signal sig 21  having passed the birefringence crystal  130  and a control light pulse con are both inputted to a non-linear waveguide  131 . The control light pulse con is inputted to the non-linear waveguide  131  immediately after a TE-mode pulse in the channel #2 has passed the non-linear waveguide  131 .  
           [0019]    In an optical signal sig 22  having passed the non-linear waveguide  131 , therefore, the phase of the TE-mode optical pulse in each channel #3 and #4 is shifted π, and the phase of the TM-mode optical pulse in each of the channels #2 to #4 is shifted π. The optical signal sig 22  having passed the non-linear waveguide  131  is inputted to another birefringence crystal  132 . The birefringence crystal  132  delays a light in the TE mode by one pulse relative to a light in the TM mode. Accordingly, in an optical signal sig 23  having passed the birefringence crystal  132 , positions of the TM-mode pulses match respectively with positions of the TE-mode pulses in the corresponding channels.  
           [0020]    In the optical signal sig 23 , therefore, the TM-mode pulses and the TE-mode pulses are in phase in the channels #1, #3 and #4, but they have a phase difference therebetween in the channel #2. By introducing the optical signal sig 23  to enter a polarizer  133 , only the pulse of the chancel #2 can be separated.  
           [0021]    Thus, the polarization separating type optical demultiplexer is designed on condition that an inputted optical signal has intensities substantially equal to each other between the TM and TE modes. In general, however, the polarization state of an optical signal having propagated through an optical fiber is not constant. For that reason, the polarization separating type optical demultiplexer is not suitable for practical use.  
           [0022]    As described above, the various types of conventional optical demultiplexers have problems such as a limitation in processing speed, an increased device size, and dependency on the polarization state of an optical signal.  
         SUMMARY OF THE INVENTION  
         [0023]    It is an object of the present invention to provide an optical switch, which can increase the processing speed, can reduce the device size, and is free from dependency on the polarization state of an optical signal.  
           [0024]    Another object of the present invention is to provide an optical demultiplexer using the optical switch.  
           [0025]    According to one aspect of the present invention, there is provided an optical switch comprising: a first multimode interferometer having a first input port to which an optical signal is applied, and at least two output ports; a first optical waveguide connected to one or each of plural first output ports, which is or are selected from the output ports, and allowing a light exiting from the one or plural first output ports to propagate therethrough, the first optical waveguide having a refractive index changed in response to a trigger signal externally applied; a second optical waveguide connected to one or each of plural second output ports, which is or are selected from the output ports, and allowing a light exiting from the one or plural second output ports to propagate therethrough; and trigger for supplying, to the first optical waveguide, the trigger signal for changing the refractive index of the first optical waveguide.  
           [0026]    According to another aspect of the present invention, there is provided an optical switch further comprising a second multimode interferometer having at least two input ports and a first output port, the input ports being connected respectively to an output end of said first optical waveguide and an output end of said second optical waveguide.  
           [0027]    According to still another aspect of the present invention, there is provided an optical demultiplexer comprising: a plurality of drop devices, each of the drop devices having a control light input port to which a control light is applied, an optical signal input port to which an optical signal is applied, and a drop signal output port from which the optical signal is delivered in synchronous with inputting of the control light; a signal waveguide for branching a time-division multiplexed optical signal and introducing a plurality of branched optical signals respectively to the optical signal input ports of the drop devices; and a control waveguide for branching one control light and introducing a plurality of branched control lights to reach the corresponding drop devices at delays gradually shifted in units of a certain time.  
           [0028]    According to still another aspect of the present invention, there is provided an optical demultiplexer comprising: a number N (N is two or larger integer) of drop devices, each of the drop devices having a control light input port to which a control light is applied, an optical signal input port to which an optical signal is applied, and a drop signal output port from which the optical signal is delivered in synchronous with inputting of the control light; a signal waveguide for introducing an optical signal, which is time-division multiplexed at multiplicity of N and has a number N of channels, to the optical signal input port of each of the drop devices; and a control waveguide for branching one control light into a number N of control lights and introducing an i-th (i is an integer not smaller than 1 but not larger than N) one of the branched control lights to the control light input port of an i-th drop device, the signal waveguide and the control waveguide delaying one of the control light and the optical signal relative to the other such that the control light applied to the i-th drop device is in synchronous with an i-th channel of the optical signal applied to the i-th drop device.  
           [0029]    According to still another aspect of the present invention, there is provided an optical demultiplexer comprising: a number N (N is two or larger integer) of drop devices arranged from a first stage to an N-th stage, each of the drop devices having a control light input port to which a control light is applied, an optical signal input port to which an optical signal is applied, a drop signal output port from which the optical signal is delivered in synchronous with inputting of the control light, and a through signal output port from which the optical signal is delivered at least during a period in which the optical signal is not delivered from the drop signal output port; a first signal waveguide for introducing a time-division multiplexed optical signal to the optical signal input port of the first-stage drop device; a second signal waveguide for connecting the through signal output port of each drop device to the optical signal input port of the drop device in a next stage; and a control waveguide for branching one control light and introducing a plurality of branched control lights to reach the corresponding drop devices at delays gradually shifted in units of a certain time toward a most downstream stage.  
           [0030]    With the features set forth above, an optical switch and an optical demultiplexer, each having a reduced size, can be realized by combining a multimode interferometer and a non-linear waveguide with each other. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIGS. 1A and 1B are schematic plan views of an optical switch according to a first embodiment of the present invention;  
         [0032]    [0032]FIGS. 2A to  2 C show results obtained by simulating propagation of an optical signal through the optical switch according to the first embodiment;  
         [0033]    [0033]FIG. 3 is a schematic plan view of an optical switch according to a second embodiment of the present invention;  
         [0034]    [0034]FIGS. 4A and 4B show results obtained by simulating propagation of an optical signal through the optical switch according to the second embodiment;  
         [0035]    [0035]FIG. 5 is a schematic plan view of an optical switch according to a third embodiment of the present invention;  
         [0036]    [0036]FIGS. 6A and 6B show results obtained by simulating propagation of an optical signal through the optical switch according to the third embodiment;  
         [0037]    [0037]FIG. 7 is a perspective view of a non-linear waveguide (semiconductor optical amplifier) used in the optical switch according to any of the embodiments;  
         [0038]    [0038]FIG. 8 is a schematic view showing an optical system for introducing excitation light to the non-linear waveguide, which is used in the optical switch according to the embodiment;  
         [0039]    [0039]FIGS. 9A to  9 C are schematic plan views of an optical switch according to a fourth embodiment of the present invention;  
         [0040]    [0040]FIG. 10 is a graph showing time-dependent variations in refractive index of each of two non-linear waveguides, which are used in the optical switch according to the fourth embodiments;  
         [0041]    [0041]FIG. 11A is a schematic plan view of an optical switch according to a fifth embodiment of the present invention, and FIG. 1B is a block diagram of the optical switch;  
         [0042]    [0042]FIG. 12 is a schematic plan view of an optical demultiplexer according to a sixth embodiment of the present invention;  
         [0043]    [0043]FIG. 13 is a schematic plan view of an optical demultiplexer according to a seventh embodiment of the present invention;  
         [0044]    [0044]FIG. 14A is a schematic plan view of an optical switch according to an eighth embodiment of the present invention, and FIGS. 14B and 14C show results obtained by simulating propagation of an optical signal through the optical switch; and  
         [0045]    [0045]FIGS. 15A to  15 C are schematic views of conventional optical demultiplexers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    The construction and the operating principle of an optical switch according to a first embodiment of the present invention will be described with reference to FIGS. 1A to  2 C.  
         [0047]    [0047]FIG. 1A is a schematic plan view of the optical switch according to the first embodiment. The optical switch according to the first embodiment comprises a first-stage multimode interferometer (MMI)  1 , a second-stage multimode interferometer (MMI)  2 , and non-linear waveguides  3  and  4 . Each of the MMIs  1  and  2  has a multilayered guide structure made up of a core layer having relative permitivity of 3.25 and clad layers having relative permitivity of 3.18 and sandwiching the core layer from above and below. The core layer has a width (length of one vertical side in FIG. 1A) W1 of 15 μm in a direction perpendicular to the light incident direction, and a length (length of one horizontal side in FIG. 1A) L1 of 320 μm in a direction parallel to the light incident direction. Note that, in FIG. 1A, the length in the light incident direction is scaled down. For example, the core layer is made of InGaAs, and the clad layer is made of InP. The core layer and the clad layer can be formed on a substrate using those materials by metal organic chemical vapor deposition (MOCVD). The waveguides and the multimode interferometers are formed by a combination of lithography and regrowth commonly used in semiconductor processes.  
         [0048]    The non-linear optical waveguides  3  and  4  are each constituted by a semiconductor optical amplifier (SOA). The SOA has a width (length of one vertical side in FIG. 1A) W2 of 2.5 μm and a length (length of one horizontal side in FIG. 1A) L2 of 140 μm. Even when the SOA length is increased over 140 μm for sufficient phase modulation of light passing it, the result of simulation, described below, remains the same. The refractive index of each of the non-linear waveguides  3  and  4  is changed upon optical or electrical excitation. The MMI  1 ,  2  and the non-linear waveguides  3 ,  4  are formed on a single semiconductor substrate.  
         [0049]    The first-stage MMI  1  has one input port  1 A, a first output port  1 B, and a second output port  1 C. The second-stage MMI  2  has a first input port  2 A, a second input port  2 B, a first output port  2 C, and a second output port  2 D. The non-linear waveguide  3  connects the first output port  1 B of the first-stage MMI  1  to the first input port  2 A of the second-stage MMI  2 , and the non-linear waveguide  4  connects the second output port  1 C of the first-stage MMI to the second input port  2 B of the second-stage MMI  2 .  
         [0050]    The first-stage MMI  1  and the second-stage MMI  2  have a line-symmetrical shape with respect to a first imaginary straight line C 1  connecting the centers of the non-linear waveguides  3  and  4 . Also, the guide structure constituted by the first-stage MMI  1 , the second-stage MMI  2 , and the non-linear waveguides  3  and  4  has a line-symmetrical shape with respect to a second imaginary straight line C 2  extending parallel to the light incident direction. The input port  1 A of the first-stage MMI  1  and the first output port  2 C of the second-stage MMI  2  are arranged in point-symmetrical positions with respect to an intersection between the first imaginary straight line C 1  and the second imaginary straight line C 2 . The first output port  2 C and the second output port  2 D of the second-stage MMI  2  are arranged in linear-symmetrical positions with respect to the second imaginary straight line C 2 .  
         [0051]    In a state in which the refractive indexes of both the non-linear waveguides  3  and  4  are not changed as shown in FIG. 1A, an optical signal introduced through the input port  1 A of the first-stage MMI  1  passes the non-linear waveguides  3  and  4 , and then exits from the first output port  2 C of the second-stage MMI  2 .  
         [0052]    [0052]FIG. 1B shows a state in which the refractive index of the non-linear waveguide  3  is changed upon optical or electrical excitation. The non-linear waveguide  3  subjected to a change in the refractive index is indicated by hatching. In that state, the symmetry of an optical circuit is lost and the optical signal is delivered from the second output port  2 D as well as the first output port  2 C. The state in which the optical signal does not exit from the second output port  2 D of the second-stage MMI  2  corresponds to an off-state, and the state in which the signal light exits from the second output port  2 D corresponds to an on-state  
         [0053]    By changing the refractive index intermittently, the optical switch can be shifted from the off-state to the on-state.  
         [0054]    [0054]FIGS. 2A to  2 C show results obtained by simulating optical paths of the optical switch shown in FIGS. 1A and 1B based on the beam propagation method. FIGS. 2A, 2B and  2 C show respectively a state in which the refractive indexes of both the non-linear waveguides  3  and  4  are not changed, a state in which the refractive index of the non-linear waveguide  4  is changed, and a state in which the refractive index of the non-linear waveguide  3  is changed. Note that, in FIGS. 2A to  2 C, white areas represent portions in which the light intensity is high.  
         [0055]    As shown in FIG. 2A, it is confirmed that in the state in which the refractive indexes of both the non-linear waveguides  3  and  4  are not changed, the incident light passes the two non-linear waveguides  3  and  4  and then exits from the first output port  2 C of the second-stage MMI  2 , but does not exit from the second output port  2 D of the second-stage MMI  2 . As shown in FIGS. 2B and 2C, it is confirmed that in the state in which the refractive index of one of the non-linear waveguides  3  and  4  is changed, the optical signal exits from both the first output port  2 C and the second output port  2 D of the second-stage MMI  2 .  
         [0056]    Thus, since the optical signal does not essentially exit from the second output port  2 D of the second-stage MMI  2  in the state in which the refractive indexes of both the non-linear waveguides  3  and  4  are not changed, an RZ (Return to Zero) switch is realized. Consequently, an optical switch having superior characteristics from practical point of view is obtained.  
         [0057]    An optical switch according to a second embodiment of the present invention will be described below with reference to FIGS. 3, 4A and  4 B.  
         [0058]    [0058]FIG. 3 is a schematic plan view of the optical switch according to the second embodiment. The optical switch according to the second embodiment comprises, as with the optical switch according to the first embodiment, a first-stage MMI  11 , a second-stage MMI  12 , and non-linear waveguides  13  and  14 . Those components are connected in the same relation as those of the optical switch according to the first embodiment, but are different in shape and size from them.  
         [0059]    A width W3 and length L3 of a core layer of the first-stage MMI  11  are respectively 15 μm and 130 μm. A width W4 and length L4 of a core layer of the second-stage MMI  12  are respectively 15 μm and 80 μm. Symmetry is lost with respect to a third imaginary straight line C 3  passing middle points of the non-linear waveguides  13  and  14  in the longitudinal direction. Symmetry is maintained with respect to a fourth imaginary straight line C 4  extending parallel to the light incident direction. An input port  11 A of the first-stage MMI  11  is located on the fourth imaginary straight line C 4 .  
         [0060]    [0060]FIGS. 4A and 4B show results obtained by simulating optical paths of the optical switch shown in FIG. 3 based on the beam propagation method. FIG. 4A shows a state in which the refractive indexes of both the non-linear waveguides  13  and  14  are not changed, and FIG. 4B shows a state in which the refractive index of the non-linear waveguide  13  is changed. Note that, in FIGS. 4A and 4B, white areas represent portions in which the light intensity is high.  
         [0061]    As shown in FIG. 4A, it is confirmed that in the state in which the refractive indexes of both the non-linear waveguides  13  and  14  are not changed, the incident light passes the two non-linear waveguides  13  and  14  and then exits from the two output ports  12 C and  12 D of the second-stage MMI  12  with intensities almost equal to each other. This is because the optical switch is line-symmetrical with respect to the fourth imaginary straight line C 4 .  
         [0062]    As shown in FIG. 4B, it is confirmed that in the state in which the refractive index of the non-linear waveguide  13  is changed, the intensity of the light exiting from the first output port  12 C of the second-stage MMI  12  is weakened, while the intensity of the light exiting from the second output port  12 D of the second-stage MMI  12  is intensified. Since the intensity of the output light is thus changed, switching operation can be achieved. Unlike the first embodiment, however, an RZ switch is not realized.  
         [0063]    The construction and the operation of an optical switch according to a third embodiment of the present invention will be described below with reference to FIGS. 5, 6A and  6 B.  
         [0064]    The optical switches according to the first and second embodiments are each of a two-stage construction of MMIs. The optical switch according to the third embodiment comprises one MMI  21  and two non-linear waveguides  22  and  23 . A width W5 and length L5 of a core layer of the MMI  21  are respectively 15 μm and 320 μm. The MMI  21  has one input port  21 A, a first output port  21 B, and a second output port  21 C.  
         [0065]    The non-linear waveguides  22  and  23  are connected respectively to the first output port  21 B and the second output port  21 C.  
         [0066]    [0066]FIGS. 6A and 6B show results obtained by simulating optical paths of the optical switch shown in FIG. 5 based on the beam propagation method. FIG. 6A shows a state in which the refractive indexes of both the non-linear waveguides  22  and  23  are not changed, and FIG. 6B shows a state in which the refractive index of the non-linear waveguide  22  is changed. Note that, in FIGS. 6A and 6B, white areas represent portions in which the light intensity is high.  
         [0067]    As shown in FIG. 6A, it is confirmed that in the state in which the refractive indexes of both the non-linear waveguides  22  and  23  are not changed, the incident light propagates through the two non-linear waveguides  22  and  23  with intensities almost equal to each other.  
         [0068]    As shown in FIG. 6B, it is confirmed that in the state in which the refractive index of the non-linear waveguide  22  is changed, the intensity of the light exiting from the second output port  21 C is intensified, while the intensity of the light exiting from the first output port  21 B is weakened. Since the intensity of the output light is thus changed, switching operation can be achieved. As with the second embodiment, however, an RZ switch is not realized.  
         [0069]    A description is now made of a practical method for changing the refractive index of each of the non-linear waveguides used in the first to third embodiments.  
         [0070]    [0070]FIG. 7 is a schematic perspective view of a semiconductor optical amplifier (SOA) constituting the non-linear waveguide. The SOA has a structure in which an active layer  200  having a gain for amplification of light is sandwiched by a p-type semiconductor layer  201  and an n-type semiconductor layer  202 . The active layer  200  is formed as a quantum well layer or a semiconductor layer made of a semiconductor material having a smaller band gap than those of the semiconductor layers  201  and  202  on both sides. For example, the active layer  200  is made of InGaAsP, and the semiconductor layers  201  and  202  on both sides are made of InP.  
         [0071]    Upon a forward bias being applied to the active layer  200 , a carrier distribution in the active layer  200  is brought into an inverted population state and the refractive index of the active layer  200  is changed. When an optical signal  203  enters the active layer  200  through one end surface thereof in such a state, the optical signal is subjected to phase modulation depending on the refractive index of the active layer  200  and then exits from the other end surface on the opposite side.  
         [0072]    Thus, the refractive index of the non-linear waveguide can be changed by applying an electrical signal to the non-linear waveguide constituted by the SOA.  
         [0073]    The method for electrically changing the refractive index of the non-linear waveguide has been described above with reference to FIG. 7. In that method, however, the response speed of the optical switch is limited by the processing speed of an electrical signal. To achieve higher-speed switching, it is preferably that the refractive index of the non-linear waveguide be changed using an optical signal. A method for changing the refractive index with an optical signal will be described below.  
         [0074]    [0074]FIG. 8 is a schematic sectional view of an optical system for changing the refractive index of the non-linear waveguide in the optical switch according to the first embodiment shown in FIGS. 1A and 1B. A pair of reflecting mirrors  31  and  32  arranged so as to sandwich the non-linear waveguide  3  therebetween with their reflecting surfaces positioned to face each other. A control light waveguide  33  is arranged above the first-stage MMI  1  parallel to the substrate surface. A reflecting mirror  30  is arranged in an obliquely opposite relation to an exit end of the control light waveguide  33  for reflecting a control light con having exited from the control light waveguide  33 .  
         [0075]    The pair of reflecting mirrors  31  and  32  can be each formed of, e.g., a dielectric or a multilayered film of semiconductors. The obliquely located reflecting mirror  30  can be formed by obliquely etching an end surface of the control light waveguide  33 .  
         [0076]    The control light con having exited from the control light waveguide  33  is reflected by the obliquely located reflecting mirror  30  toward the substrate (nonlinear waveguide). The control light con reflected by the reflecting mirror  30  is then repeatedly reflected by the pair of reflecting mirrors  31  and  32 . While repeating the reflection, the control light con excites the non-linear waveguide  3  and changes the refractive index thereof.  
         [0077]    The construction and the operation of an optical switch according to a fourth embodiment of the present invention will be described below with reference to FIGS. 9A to  9 C.  
         [0078]    [0078]FIG. 9A is a schematic plan view of the optical switch according to the fourth embodiment. In addition to the first-stage MMI  1 , the second-stage MMI  2 , and the non-linear waveguides  3  and  4  according to the first embodiment shown in FIGS. 1A and 1B, the optical switch according to the fourth embodiment further comprises a control light introducing MMI  40  and two waveguides  41  and  42 . While the first-stage MMI  1  has only one input port  1 A in the first embodiment, another input port  1 D is provided in the fourth embodiment at a position symmetrical to the (first) input port  1 A with respect to the second imaginary straight line C 2 .  
         [0079]    The control light introducing MMI  40  has a first input port  40 A, a second input port  40 B, a first output port  40 C, and a second input port  40 D. The waveguide  41  connects the first output port  40 C of the control light introducing MMI  40  to the first input port  1 A of the first-stage MMI  1 . Also, an optical signal sig is combined with a control light propagating through the waveguide  41  and then introduced to the first input port  1 A of the first-stage MMI  1 . The optical signal sig has wavelength of, e.g., 1.55 μm, and the control light has wavelength, e.g., 1.3 μm or 1.48 μm, shorter than that of the optical signal.  
         [0080]    As shown in FIG. 9B, when a control light pulse con is applied to the second input port  40 B of the control light introducing MMI  40 , the control light pulse con passes both the waveguides  41  and  42  and then enters the non-linear waveguide  3 . Accordingly, the non-linear waveguide  3  is excited and its refractive index is changed. A thus-resulting state is the same as that shown in FIG. 2C. Hence, the optical signal sig exits from both the first output port  2 C and the second output port  2 D of the second-stage MMI  2  with intensities almost equal to each other.  
         [0081]    As shown in FIG. 9C, when the control light pulse con is applied to the first input port  40 A of the control light introducing MMI  40 , the control light pulse con reaches the non-linear waveguide  4 . Accordingly, the non-linear waveguide  4  is excited and its refractive index is changed.  
         [0082]    [0082]FIG. 10 shows time-dependent variations in refractive index of each of the non-linear waveguides  3  and  4 . Curves n 3  and n 4  represent the refractive indexes of the non-linear waveguides  3  and  4 , respectively. At time t 1 , as shown in FIG. 9B, the control light pulse con is applied and the refractive index of the non-linear waveguide  3  is changed. The refractive index having changed is restored to its original value at a predetermined time constant. At time t 2 , as shown in FIG. 9C, the control light pulse con is applied and the refractive index of the non-linear waveguide  4  is changed. The non-linear waveguides  3  and  4  are designed such that the refractive index n 4  of the non-linear waveguide  4  is substantially equal to the refractive index n 3  of the non-linear waveguide  3  at that time t 2 .  
         [0083]    When the control light pulse con is applied as shown in FIG. 9C and the refractive indexes of the non-linear waveguides  3  and  4  are both changed similarly, symmetry of the optical circuit is restored and maintained. As with the state of FIG. 9A, therefore, the optical signal sig exits only from the first output port  2 C of the second-stage MMI  2 , and the optical signal sig does not exit from the second output port  2 D of the second-stage MMI  2 . Consequently, the optical signal sig is delivered from the second output port  2 D of the second-stage MMI  2  during a period between the time t 1  and t 2 , but is not delivered from the second output port  2 D after the time t 2 .  
         [0084]    Then, as shown in FIG. 10, the control light pulse con is applied through the second input port  40 B of the control light introducing MMI  40  at time t 3 , and is applied through the first input port  40 A thereof at time t 4 . As a result, the optical signal sig can be delivered from the second output port  2 D of the second-stage MMI  2  during a period between the time t 3  and time t 4 .  
         [0085]    By repeating the above-described operation periodically, the optical signal sig can be delivered from the second output port  2 D only during a desired period. The control made at the time t 1  and t 3  in FIG. 10 is called push control, and the control made at the time t 2  and t 4  is called pull control. Thus, the optical switch according to the fourth embodiment is able to perform the push-pull control.  
         [0086]    The light delivered from the output port is subjected to filtering through a wavelength filter, whereby only the optical signal can be taken out while cutting the control light. This results in an improved S/N ratio of the optical signal.  
         [0087]    The construction and the operation of an optical switch according to a fifth embodiment of the present invention will be described below with reference to FIGS. 11A and 11B.  
         [0088]    [0088]FIG. 11A is a schematic plan view of the optical switch according to the fifth embodiment. A description is made of the difference between the optical switch of the fifth embodiment and the optical switch of the fourth embodiment shown in FIG. 9A.  
         [0089]    While the optical signal sig and the control light pulse con are combined with each other in the waveguide  41  in the fourth embodiment, a combining MMI  50  combines the optical signal sig and the control light pulse con with each other in the fifth embodiment.  
         [0090]    The optical signal sig is introduced to a first input port  50 A of the combining MMI  50 . The control light pulse con having exited from the first output port  40 C of the control light introducing MMI  40  is applied to a second input port  50 B of the combining MMI  50 . The optical signal sig and the control light pulse con having exited from an output port of the combining MMI  50  are introduced to the first input port  1 A of the first-stage MMI  1 .  
         [0091]    A control light branching MMI  60  is arranged upstream of the control light introducing MMI  40 . The control light branching MMI  60  has an input port  60 A, a first output port  60 C and a second output port  60 D. The first output port  60 C is connected to the first input port  40 A of the control light introducing MMI  40  through a waveguide  62 , and the second output port  60 D is connected to the second input port  40 B of the control light introducing MMI  40  through a waveguide  61 . The waveguide  62  is longer than the waveguide  61 . In other words, the waveguide  62  constitutes a delay circuit.  
         [0092]    The control light pulse con is applied through the input port  60 A of the control light branching MMI  60 . The control light pulse con is substantially equally divided and exits from the first output port  60 C and the second output port  60 D. A control light pulse con 2  passing the waveguide  62  reaches the control light introducing MMI  40  at timing delayed from a control light pulse con 2  passing the waveguide  61 . This delay time corresponds to the period from the time t 1  to t 2  shown in FIG. 10. Therefore, the push-pull control can be performed by applying only one the control light pulse con.  
         [0093]    [0093]FIG. 11B is a block diagram of an optical switch  70 , in which an internal optical circuit of the optical switch shown in FIG. 11A is represented as a black box. The optical switch  70  has a control light input port  70 C to which the control light pulse con is applied, an optical signal input port  70 S to which the optical signal sig is applied, and two output ports  70 T and  70 D. The control light input port  70 C corresponds to the input port  60 A of the control light branching MMI  60  shown in FIG. 11A, and the optical signal input port  70 S corresponds to the input port  50 A of the combining MMI  50  shown in FIG. 11A. Further, the output ports  70 T and  70 D correspond respectively to the output ports  2 C and  2 D of the second-stage MMI  2  shown in FIG. 11A.  
         [0094]    When the control light pulse con is applied through the control light input port  70 C, the optical signal sig is delivered from the output port  70 D for a certain period. The output port  70 D is hence called a drop signal output port. Also, the other output port  70 T is called a through signal output port. In this specification, the optical switch  70  is called a drop device.  
         [0095]    [0095]FIG. 12 is a schematic plan view of an optical demultiplexer according to a sixth embodiment of the present invention. The optical demultiplexer according to the sixth embodiment comprises four drop devices  70 ( 1 ) to  70 ( 4 ), four optoelectronic transducers  75 ( 1 ) to  75 ( 4 ), an optical signal waveguide  72 , and a control light waveguide  71 . Each of the drop devices  70 ( 1 ) to  70 ( 4 ) is the same as the drop device  70  according to the fifth embodiment shown in FIG. 11B.  
         [0096]    An optical signal sig, which is time-division multiplexed at multiplicity of 4 and contains pulses of channels #1 to #4, is branched into four optical signals by the optical signal waveguide  72 . The branched optical signals sig are introduced to respective optical signal input ports of the drop devices  70 ( 1 ) to  70 ( 4 ).  
         [0097]    A control light pulse con is branched into four control light pulses con 1  to con 4  by the control light waveguide  71 . The branched control light pulses con 1  to con 4  are applied to respective control light input ports of the drop devices  70 ( 1 ) to  70 ( 4 ). The four control light pulses con 1  to con 4  reach the corresponding drop devices  70 ( 1 ) to  70 ( 4 ) at delays gradually shifted in units of a certain time. More specifically, at the time when the pulse in the channel #i of the signal sig reaches the drop device  70 ( i ), the control light pulse con i  reaches the drop device  70 ( i ). The pull control is thereby performed. Then, until arrival of the pulse in the channel #(i+1), the push control is completed.  
         [0098]    Thus, only the pulse in the channel #i is delivered from a drop signal output port of the drop device  70 ( i ). It is therefore possible to demultiplex the time-division multiplexed optical signal sig and to obtain individual signals in respective channels. From an optical signal of 160 Gbits/s, for example, four optical signals of 40 Gbits/s can be obtained. The optical signal in the channel #i is inputted to the optoelectronic transducer  75 ( i ) for conversion into an electrical signal.  
         [0099]    [0099]FIG. 13 is a schematic plan view of an optical demultiplexer according to a seventh embodiment of the present invention. While the four drop devices are connected in parallel in the optical demultiplexer of the sixth embodiment, the optical demultiplexer of the seventh embodiment comprises four drop devices  70 ( 1 ) to  70 ( 4 ) connected in series. Stated otherwise, a through signal output port of the drop device  70 ( i ) is connected to an optical signal input port of the drop device  70 (i+1) in the next stage. Optoelectronic transducers  75 ( 1 ) to  75 ( 4 ) are connected to respective drop signal output ports of the drop devices  70 ( 1 ) to  70 ( 4 ).  
         [0100]    An optical signal sig, which is time-division multiplexed at multiplicity of 4, is introduced to the optical signal input port of the first-stage drop device  70 ( 1 ). A control light pulse con is branched into four control light pulses con 1  to con 4 . The branched control light pulses con 1  to con 4  are applied to respective control light input ports of the drop devices  70 ( 1 ) to  70 ( 4 ).  
         [0101]    A control light waveguide  80  delays the control light pulses con 1  to con 4  by respective predetermined periods of time so that at the time when a pulse in the channel #i of the signal sig reaches the drop device  70 ( i ), the control light pulse con i  reaches the drop device  70 ( i ). Upon the control light pulse cone reaching the drop device  70 ( i ), the pull control is performed in the drop device  70 ( i ). Then, until arrival of the pulse in the channel #(i+1), the push control is completed.  
         [0102]    Thus, only the pulse in the channel #i is delivered from a drop signal output port of the drop device  70 ( i ). It is therefore possible to demultiplex the time-division multiplexed optical signal sig and to obtain individual signals in respective channels. The optical signal in the channel #i is inputted to the optoelectronic transducer  75 ( i ) for conversion into an electrical signal.  
         [0103]    The sixth and seventh embodiments have been described in connection with the case of demultiplexing an optical signal multiplexed at multiplicity of 4. Generally, when demultiplexing an optical signal multiplexed at multiplicity of N, a number N of drop devices are connected in parallel or in series.  
         [0104]    Also, in the sixth and seventh embodiments, one control light pulse is branched and a plurality of branched control light pulses are applied so as to reach the corresponding drop devices at delays gradually shifted in units of a certain time. Accordingly, there is no need of generating the control light pulse for each of the time-division multiplexed channels.  
         [0105]    Advantages of the sixth and seventh embodiments will now be described while comparing both the embodiments with each other.  
         [0106]    In the sixth embodiment, since the optical signal sig is evenly divided into four rays, the intensity of the optical signal sig inputted to each drop device  70 ( i ) is about ¼ of the intensity of the original optical signal sig. In the seventh embodiment, however, since one ray of original optical signal sig passes the four drop devices  70 ( 1 ) to  70 ( 4 ) successively, the signal intensity is hardly reduced. As a result, in the seventh embodiment, the intensity of the optical signal in each separated channel can be maintained at a high level.  
         [0107]    In the seventh embodiment, each time the optical signal sig passes the drop device  70 ( i ), the signal purity is reduced. For example, the signal waveform is deformed, or noise is mixed, or jitter occurs. On the other hand, in the sixth embodiment, deterioration of the signal purity hardly occurs.  
         [0108]    In the sixth embodiment, branches of the control light waveguide  71  cross branches of the optical signal waveguide  72 . Therefore, due care is required in design of the waveguides.  
         [0109]    The construction and the operation of an optical switch according to an eighth embodiment of the present invention will be described below with reference to FIGS. 14A to  14 C. While the first-stage MMI and the second-stage MMI are connected by two non-linear waveguides in the first to seventh embodiments described above, both the MMIs may be connected using three or more waveguides. In that case, at least one of the three or more waveguides requires to be a non-linear waveguide. In the eighth embodiment, four waveguides are used to connect the first-stage MMI and the second-stage MMI.  
         [0110]    [0110]FIG. 14A is a schematic plan view of the optical switch according to the eighth embodiment. A first-stage MMI  91  and a second-stage MMI  92  are connected to each other by four waveguides  93 ,  94 ,  95  and  96 . The first-stage MMI  91  has a first input port  91 A and a second input port  91 B, and the second-stage MMI  92  has a first output port  92 A and a second output port  92 B.  
         [0111]    The first-stage MMI  91 , the second-stage MMI  92  and the waveguides  93  to  96  are line-symmetrical with respect to an imaginary straight line C 2  extending parallel to the light incident direction. The waveguides  94  and  95  are arranged in symmetrical positions with respect to each other. The waveguides  93  and  96  are arranged outside the waveguides  94  and  95 , respectively. The waveguides  94  and  95  are non-linear waveguides, whereas the waveguides  93  and  96  are normal waveguides.  
         [0112]    Each of the first-stage MMI  91  and the second-stage MMI  92  has a width W6 of 12 μm and a length L6 of 345 μm. A length L7 of each of the waveguides  93  to  96  is 100 μm. The waveguides  93  and  96  have a width W7 of 1.5 μm, and the waveguides  94  and  95  have a width W8 of 1.0 μm. The two input ports  91 A and  91 B are arranged at opposite ends of one side of the first-stage MMI  91  on the input side, and the two output ports  92 A and  92 B are arranged at opposite ends of another side of the second-stage MMI  92  on the output side.  
         [0113]    [0113]FIGS. 14B and 14C show results obtained by simulating optical paths of the optical switch of this embodiment based on the beam propagation method. FIG. 14B shows a state in which the refractive indexes of both the non-linear waveguides  94  and  95  are not changed, and FIG. 14C shows a state in which the refractive indexes of both the non-linear waveguides  94  and  95  are changed. A core portion of each of the first-stage MMI  91 , the second-stage MMI  92 , and the waveguides  93  and  96  has the refractive index of 3.25, and a clad portion around the core portion has the refractive index of 3.18. The refractive index of the core portions of the waveguides  94  and  95  is 3.25 in the state of FIG. 14B, but 3.18 in the state of FIG. 14C. Enclosed curves in FIGS. 14B and 14C represent equi-light intensity lines.  
         [0114]    As shown in FIG. 14B, it is confirmed that in the state in which the refractive indexes of both the non-linear waveguides  94  and  95  are not changed, an optical signal introduced through the second input port  91 B of the first-stage MMI  91  passes the four waveguides  93  to  96  and then exits from the second output port  92 B of the second-stage MMI  92 . The optical signal does not exit from the first output port  92 A of the second-stage MMI  92 .  
         [0115]    As shown in FIG. 14C, it is confirmed that in the state in which the refractive indexes of both the non-linear waveguides  94  and  95  are changed, an optical signal introduced through the second input port  91 B of the first-stage MMI  91  passes the two waveguides  93  and  96  on the opposite outer sides and then exits from the two output ports  92 A and  92 B of the second-stage MMI  92 . The optical signal exiting from the first output port  92 A has higher intensity than that exiting from the second output port  92 B.  
         [0116]    As seen from the simulation results shown in FIGS. 14B and 14C, the optical switch according to the eighth embodiment can be used as a drop device in which the second output port  92 B of the second-stage MMI  92  serves a through signal output port and the first output port  92 A thereof serves as a drop signal output port.  
         [0117]    In any of the optical switches and the optical demultiplexers according to the first to eighth embodiments, a plurality of optical elements can be of a monolithic structure formed on a single semiconductor substrate. The device size can be therefore reduced. However, the optical switch and the optical demultiplexer are not necessarily required to be in a monolithic structure, and an optical fiber or an optical crystal can also be used to form the waveguide.  
         [0118]    Further, since the operations of the optical switches and the optical demultiplexers according to the first to eighth embodiments are not dependent on the polarization state of an optical signal, the optical signal having exited from an optical fiber can be processed in a desired manner.  
         [0119]    The present invention has been described above in connection with the preferred embodiments, but the present invention is not limited to the illustrated embodiments. It is apparent to those skilled in the art that, for example, various modifications, improvements, and combinations thereof can be made on the present invention.