Abstract:
The present invention provides an optical switch having a photonic crystal structure. An optical switch of the invention has a slab optical waveguide structure whose core ( 35 ) has a two-dimensional photonic crystal structure where two or more media ( 33, 34 ) with different refractive indices are alternately and regularly arranged in a two-dimensional manner. The photonic crystal structure comprises: a line-defect waveguide; and means for altering the refractive index of the line-defect waveguide.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a division of co-pending application Ser. No. 10/105,384, filed on Mar. 26, 2002, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an optical switch. More particularly, the present invention relates to an optical switch having a photonic crystal structure.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical communication requires a mechanism for switching optical signals on and off in accordance with binary signals (i.e., 0 or 1). One of the simplest mechanisms is to directly control on and off of output of a semiconductor laser used as a light source. However, directly controlling on and off of the output of the semiconductor laser deteriorates oscillation stability of the semiconductor laser and above all high-speed switching as high as 10 Gbps (giga-bits/second) or more is difficult to achieve.  
         [0004]     Another technique for controlling on and off of an optical signal is to use an optical switch. According to this technique, constant optical outputs from a semiconductor laser are modulated to be on and off by a device provided at a subsequent stage. This device is referred to as an optical switch. By using an optical switch, light intensity of the semiconductor laser light source can be kept constant. Therefore, a problem of unstable oscillation can be solved. The performance of the optical communication is not influenced by the modulation rate of the semiconductor laser itself.  
         [0005]     Conventional optical switches include a compound semiconductor. Examples of known operation principles include:  
         [0006]     one that utilizes quantum confined Stark effect that leads to a red shift of effective band gap by applying an electric field to a multi quantum well structure;  
         [0007]     one that utilizes change in a refractive index of an active layer due to current injection or a reverse bias voltage application; and  
         [0008]     one that utilizes optical Kerr effect which is a third-order nonlinear optical effect.  
         [0009]     The principle that utilizes quantum confined Stark effect allows some degree of design freedom according to a combination of well layers and barrier layers. However, the number of materials that can be selected by their intrinsic properties such as band-gap energy is limited, which results in limited operating wavelength range.  
         [0010]     According to the principle that utilizes change in the refractive index, obtainable change in the refractive index is small. Therefore, in order to obtain sufficiently large difference between the signal light intensities of ON and OFF states, the length of the optical path needs to be as long as about several-hundreds of microns, which interferes with high-density integration.  
         [0011]     An optical switch that utilizes optical Kerr effect is most likely to achieve high speed. However, this switch requires an additional high-power control light source for obtaining the optical Kerr effect, thereby rendering itself impractical.  
         [0012]     As an optical switch that may overcome the above-mentioned problems, an optical switch that uses photonic crystal has recently been proposed. As one of the publications relating to the optical switch incorporating the photonic crystal, for example, Japanese Patent Laid-Open Application No. 10-90638 discloses a structure of an optical switch. This optical switch includes: a photonic band structure that forms a alternately structure on a two-dimensional plane with two types of optical media having different complex refractive indices where at least one of the optical media is a semiconductor; controlled light parallel to the two-dimensional plane; and means for radiating circularly-polarized light as control light to the alternately structure on the two-dimensional plane along an optical path that does not cross with an optical path of the controlled light, wherein the optical switch changes light transmission of the controlled light according to the control light.  
         [0013]     A photonic crystal has at least two media with different refractive indices alternately and regularly arranged at a three- or two-dimensional on the order of the wavelength of light, namely, at a cycle of sub-micron meter.  
         [0014]     A photonic crystal has high design freedom and thus is receiving great attention as an artificial optical crystal having specific optical property.  
         [0015]      FIG. 1A  is a schematic view showing a two-dimensional photonic crystal as an exemplary structure of the photonic crystal. Here, cylinders made of a second medium  12  are imbedded in a first medium  11  in a two-dimensional triangular lattice arrangement. Such photonic crystal is known to form an energy band structure in response to the light wave present in the photonic crystal.  
         [0016]      FIG. 1B  is a schematic view showing a first Brillouin zone in a reciprocal lattice space corresponding to the triangular lattice shown in  FIG. 1A . Point J is a vertex of the regular hexagon, point X is the midpoint of each side of the hexagon, and r is the center of the hexagon.  
         [0017]     The solid lines in  FIG. 2  represent the results obtained by calculating an energy band structure of the photonic crystal shown in  FIG. 1A  in Γ-X direction with respect to TM-polarized wave. The vertical axis of the diagram represents normalized Ω=ωa/2πc, where a is a lattice pitch (or a lattice constant), c is light velocity in a vacuum state, ω is angular frequency of the light wave, and k represents the magnitude of the wave vector.  
         [0018]     In the energy band structure shown in  FIG. 2 , light wave mode for light traveling in the Γ-X direction does not exist in an energy range of Ω1-Ω2, where it is referred to as a “photonic band gap (PBG).” 
         [0019]     The above-described optical switch disclosed in the Japanese Patent Laid-Open Application No. 10-90638 controls on and off of light propagating in the photonic crystal by controlling the PBG according to the change in the refractive index of the material. In this conventional example, since light with energy Ω3 is within the PGB, light cannot be propagated in the OFF state. The refractive index of the photonic crystal material is changed by externally radiating control light or by injecting current to the photonic crystal. As a result, the energy band structure defined by the solid lines in  FIG. 2  alters to an energy band structure defined by the dashed lines where a propagation mode becomes effective for the light of Ω3 in the ON state. On the contrary, change in the refractive index alters the state of the light wave with energy of Ω4 from ON state to OFF state.  
         [0020]     The above-described optical switch using a photonic crystal disclosed in Japanese Patent Laid-Open Application No. 10-90638 utilizes a small change in the refractive indices of the media. By setting the wavelength of the controlled light to the edge of the energy band, ON/OFF effect of the light (i.e., function of the optical switch) can be obtained.  
         [0021]     The above-described optical switch using a conventional photonic crystal simply utilizes characteristic of the photonic crystal itself as a bulk structure, but have no regard to the guided wave of light necessary for actually forming an optical circuit, that is, optical confinement in a direction perpendicular to the light propagating direction. Therefore, a recovery efficiency of light that has propagated through the photonic crystal is considered to be extremely low.  
         [0022]     In order to guide the wave of light efficiently, the photonic crystal may be shaped into a narrow waveguide with a width of 1 to 2 μm with which a basic propagation mode can be achieved. In this case, however, the energy band of the bulk structure no longer retains its shape shown in  FIG. 2 , and thus the photonic band gap necessary for ON/OFF control of the propagated light is no longer formed.  
         [0023]     All of the above-described conventional optical switches control ON and OFF of a single channel with a single switch. Therefore, when a multi-channel configuration is considered, a plurality of optical switches need to be prepared for the number of channels.  
         [0024]     Furthermore, the conventional optical switches control entire ON/OFF of light according to control signals such as current, voltage and light. Therefore, when the waveform of the control signal is deteriorated, the waveform of the light is deteriorated as well.  
         [0025]     Thus, the present invention has an objective of providing an optical switch having a photonic crystal structure, which can propagate light efficiently and which can operate at high speed.  
         [0026]     As can be appreciated from the following description, the present invention also has an objective of providing an optical switch, where a multi-channel optical switch that can switch among a plurality of channels with a single switch is realized to make a compact multi-channel switch, or where a waveform shaping function is realized that makes the light intensity to be predetermined range, when the signals are ON.  
       SUMMARY OF THE INVENTION  
       [0027]     The present invention which provides means for solving the above-described problems is an optical switch having a slab optical waveguide whose core has a photonic crystal structure where two or more substances with different refractive indices are alternately and regularly arranged. The photonic crystal structure comprises: a line-defect waveguide; and means for altering the refractive index of the line-defect waveguide. Here, the slab optical waveguide. refers to a waveguide which includes a single- or multi-layered flat media and which allows light to propagate in a direction along the plane. According to the present invention, the refractive index may be altered by current injection or radiation of control light other than the incident light.  
         [0028]     In another aspect of the invention, the photonic crystal structure comprises Mach-Zehnder interferometry using the line-defect waveguide, and means for altering the refractive index of the waveguide at one or both of two branched optical paths.  
         [0029]     In yet another aspect of the invention, the photonic crystal structure comprises: means for refracting light by making incident light to be oblique with respect to the intrinsic axis of the photonic crystal; and means for altering the refractive index of the photonic crystal. The refractive angle of the photonic crystal is altered according to the change in the refractive index. According to the present invention, a multi-channel optical switch that can switch among a plurality of channels with a single optical switch is realized, thereby achieving a compact multi-channel switch.  
         [0030]     In yet still another aspect of the invention, the refractive index of the photonic crystal alters according to the change in the intensity of light incident to the photonic crystal, whereby switching operation takes place. According to the invention, a waveform shaping function is realized for making the light intensity to be predetermined range, when the signals are ON.  
         [0031]     In yet another aspect of the invention, in an optical switch using a line-defect waveguide of a slab photonic crystal: a core layer of the slab photonic crystal is an i-type, n − -type or p − -type semiconductor; the core layer has an n-type region and a p-type region sandwiching the line-defect waveguide; and the n-type region and the p-type region have an n-type ohmic electrode and a p-type ohmic electrode, respectively.  
         [0032]     According to the present invention, the conductive type of a base material of the core layer is n − -type or p − -type, and an electrode of the same conductive type as the conductive type of the base material of the core layer and an ion-injected region are provided separately from the line-defect optical waveguide. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1A  is a plan view showing an exemplary structure of a two-dimensional photonic crystal;  
         [0034]      FIG. 1B  is a schematic diagram showing vectors of a reciprocal lattice with respect to the two-dimensional photonic crystal;  
         [0035]      FIG. 2  is a diagram showing results of an exemplary calculation for energy bands of the structure shown in  FIG. 1A ;  
         [0036]      FIG. 3A  is a schematic view showing a structure of a first embodiment of the invention;  
         [0037]      FIG. 3B  is a schematic view showing a structure of a photonic crystal layer in the first embodiment shown in  FIG. 3A ;  
         [0038]      FIG. 4  is a diagram showing results of an exemplary calculation for energy bands of the structure shown in  FIG. 3 ;  
         [0039]      FIG. 5  is timing charts for illustrating operations of the first embodiment of the invention;  
         [0040]      FIG. 6  is a schematic view showing a structure of a third embodiment of the present invention;  
         [0041]      FIG. 7  is a schematic view showing a structure of a fourth embodiment of the present invention;  
         [0042]      FIG. 8  is a schematic view showing a structure of a fifth embodiment of the present invention;  
         [0043]      FIG. 9  is a schematic view showing a structure of a sixth embodiment of the present invention;  
         [0044]      FIGS. 10A and 10B  are diagrams for illustrating operations of the sixth embodiment of the present invention;  
         [0045]      FIG. 11  is a schematic view showing a structure of a second embodiment of the present invention; and  
         [0046]      FIG. 12  is a schematic view showing a structure of the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]     Hereinafter, embodiments of the present invention will be described with reference to the drawings.  FIG. 3A  is a schematic view showing a structure of a first embodiment of the invention, and  FIG. 3B  is a plan view showing a photonic crystal layer  38 .  
         [0048]     Referring to  FIG. 3A , an n-type semiconductor clad layer  32 , and the photonic crystal layer  38  including i-type semiconductors  33  (base medium) and  34  having different refractive indices are formed on an n-type substrate  31  in this order.  
         [0049]     A p-type semiconductor clad layer  35  is further provided on the photonic crystal layer  38 , together with an electrode  36  and a lead wire  37  for current injection. The n-type substrate  31  is held at ground potential.  
         [0050]     The i-type semiconductor (base medium)  33  is imbedded with cylinders made of the i-type semiconductor  34  in a dimensional triangular lattice (to have a alternately structure). There is a linear region where a row of the i-type semiconductor  34  is replaced with the base medium  33  in a direction along an input end to an output end of incident light  21 . Hereinafter, this region  39  is referred to as a “line-defect waveguide.” The i-type semiconductor cylinders  34  may be replaced with air, vacuum or an insulator.  
         [0051]     Specifically, the optical switch having the photonic crystal structure shown in  FIGS. 3A and 3B  may be produced, for example, as follows.  
         [0052]     First, an n-type InGaAsP clad layer  32  and an i-type InGaAsP semiconductor  33  are deposited on an n-type InP substrate  31 , for example, by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). The i-type semiconductor  33  has a thickness of about 0.3 to 0.5 μm.  
         [0053]     Next, a mask pattern for a two-dimensional triangular lattice is formed on the i-type semiconductor  33  to provide cylindrical holes for achieving photonic crystal by etching.  
         [0054]     After the etching, the mask is removed and i-type InP is grown with which the cylindrical holes are filled in preferentially, thereby forming an i-type semiconductor  34 .  
         [0055]     Then, a p-type InP clad layer  35  is further deposited, and an electrode  36  and a lead wire  37  are provided.  
         [0056]     Alternatively, after etching and removing the mask away, a substrate that has been deposited with a p-type InP clad layer  35  may be placed on the i-type InGaAsP semiconductor  33  so as to leave the p-type clad layer  35  through etching. In this case, the i-type semiconductor  34  is replaced by air. The electrode and the lead wire are attached as described above.  
         [0057]     Hereinafter, operation principle of the first embodiment of the invention will be described.  FIG. 4  is a diagram showing energy bands for TM-polarized wave. Herein, only TM-polarized wave is argued.  
         [0058]     When no change in the refractive index is caused upon current injection or the like in a photonic crystal region other than the line-defect waveguide, an energy band structure will be as defined by the solid lines in  FIG. 4 . There is no propagation mode on the two-dimensional plane for light wave of Ω5 to Ω6.  
         [0059]     Accordingly, when the energy of the incident light  21  is Ω5 to Ω6, light cannot propagate on the two-dimensional plane outside the waveguide, namely, the photonic crystal region. Therefore, the light is highly confined in the above-described line-defect waveguide. For example, light with energy of Ω7 applies to this case. Thus, a minute waveguide as small as about sub-μm which is identical to the size of the lattice pitch can be realized.  
         [0060]     With respect to a direction perpendicular to the two-dimensional plane, light can be confined efficiently in the photonic crystal layer, or a core structure, by appropriately selecting the refractive index and the thickness of the upper and lower clad layers  32  and  35 . The optical switch of the present example is able to confine light in a three-dimensional manner, by which light can propagate and be emitted as light  22  with less loss.  
         [0061]     Hereinafter, a case where current injection is carried out will be described. When the number of carriers in the photonic crystal layer changes (due to plasma effect), a refractive index of at least one of the i-type semiconductors  33  and  34  alters, by which the energy band changes as defined by the dashed lines shown in  FIG. 4 . Since light with energy Ω7 is outside the photonic band gap (PBG) in this case, light leaks to a complete photonic crystal region outside the line-defect waveguide, and as the light propagates through the line-defect waveguide, intensity of the light is attenuated and thus the intensity of the transmitted emitted light  22  becomes extremely weak.  
         [0062]     Based on the same principle, on the contrary to light with energy Ω7, when the energy of the incident light  21  is Ω8, the intensity of the emitted light changes from OFF state to ON state due to current injection.  
         [0063]      FIG. 5  shows timing charts representing intensities of emitted light upon ON/OFF of current (i.e., upon presence or absence of current injection). When current is injected via the electrode  36 , the intensity of the emitted light with energy Ω7 (intensity of emitted light Ω7 in  FIG. 5 ) is zero, i.e., in an OFF state. When the current is not injected, the light is in an ON state. The timing of the ON/OFF states of the intensity of emitted light with energy Ω8 (intensity of emitted light Ω8 in  FIG. 5 ) is opposite to that of the ON/OFF. states of the emitted light with energy Ω7.  
         [0064]     Although two-dimensional triangular lattice is used as an example of the photonic crystal of the present example, the photonic crystal may take other two-dimensional photonic crystal structure such as a square lattice or a regular hexagonal lattice, or a three-dimensional photonic crystal structure such as a diamond structure.  
         [0065]     Although InGaAsP is used as a material for the photonic crystal, other material can also be used. Although current injection by forward bias application is employed to change the refractive index in the present example, equal effect can be obtained when the refractive index is altered by reverse bias application (Franz-Keldish effect).  
         [0066]     Hereinafter, a second embodiment of the present invention will be described.  FIG. 11  is a schematic view showing a structure of the second embodiment of the invention. Referring to  FIG. 11 , a core layer  72  having a alternately structure of photonic crystal sandwiched by upper and lower clad layers  71  and  73  is formed on a substrate  70 .  
         [0067]     This structure differs from that of the first embodiment in that the substrate  70  can be made of any material and that the clad layers  71  and  73  are dielectric layers. Electrodes  74  and  75  are provided on the surface of the clad layer  73 .  
         [0068]     The core layer having a alternately structure of photonic crystal sandwiched by the clad layers, in order words, a combination of the core layer  72  and the clad layers  71  and  73  shown in  FIG. 11  is sometimes referred to as a “slab photonic crystal.” The clad layers  71  and  73  may be air.  
         [0069]     The core layer  72  is made of a semiconductor, and is of i-type, n − -type (weak n-type) or p − -type (weak p-type) with low conductivity and small conductivity loss.  
         [0070]     The clad layer  73  has holes through which the electrodes  74  and  75  make contact with electrodes  80  and  81  on the core layer  72 .  
         [0071]      FIG. 12  is a top perspective view showing the structure of the second embodiment of the invention shown in  FIG. 11 . The i-type, n − -type or p − -type semiconductor core layer  76  has a photonic crystal structure and is provided with a line-defect optical waveguide  77 .  
         [0072]     On both sides of the line-defect optical waveguide  77 , an n-type region  78  and a p-type region  79  are provided, each having an n-type ohmic electrode  80  and a p-type ohmic electrode  81 , respectively.  
         [0073]     The operation principle of the second embodiment of the invention is the same as that of the first embodiment. Specifically, energy of the optical signal to be switched is within the photonic band gap of the slab photonic crystal. The line-defect optical waveguide formed in the photonic crystal has a propagation mode in the photonic band gap prior to current injection or voltage application to the electrodes.  
         [0074]     When current is injected or a reverse bias voltage is applied between the electrodes  80  and  81 , the photonic band structure of the photonic crystal around the line-defect optical waveguide changes, by which the propagation mode of the line-defect waveguide becomes to be outside the photonic band gap, whereby light can no longer propagate through the line-defect waveguide.  
         [0075]     Alternatively, the propagation mode of the line-defect waveguide that exists outside the photonic band gap in the first place may become to be within the photonic band gap due to current or voltage application, allowing light to propagate therethrough.  
         [0076]     One of the advantages of the second embodiment of the invention is that the clad layers can be made of a non-conductive dielectric. Accordingly, there is no loss of light propagating through the line-defect waveguide  77  due to the clad layers  71  and  73 .  
         [0077]     Hereinafter, a specific example of the second embodiment of the present invention will be described. The triangular lattice photonic crystal structure and the line-defect waveguide as shown in  FIG. 12  are formed in a non-doped Si-layer of an SOI wafer having a structure of non-doped Si (thickness 300 nm)/SiO 2  (thickness 2 micro meter)/non-doped Si-substrate by Si dry etching fabrication using electron beam lithography and chlorine gas.  
         [0078]     After forming a mask by photolithography, arsenic or boron ion injection and subsequent annealing for activation are conducted to form regions corresponding to the n-type region  78  and the p-type region  79 . Then, an ohmic metal is deposited on each of the ion-injected region to form electrodes by sintering. The ion injection for forming the n-type region  78  and the p-type region  79  may be performed prior to the fabrication of the photonic crystal structure. When the conductive type of the base material of the core layer is n − -type or p − -type semiconductor, the electrode (as well as the ion-injected region) having the same conductive type as the conductive type of the base material of the core layer does not need to be in the vicinity of the line-defect optical waveguide but away from it.  
         [0079]     When a plurality of optical switches of the present embodiment are to be arranged, only an electrode having a opposite conductive type to that of the core layer may be provided in the vicinity of each line-defect waveguide while at least one electrode having the same conductive type as that of the core layer may be provided as a common electrode separated from the line-defect waveguides.  
         [0080]     Hereinafter, a third embodiment of the invention will be described.  FIG. 6  is a schematic view showing a structure of the third embodiment of the invention. Referring to  FIG. 6 , the structure of the present embodiment differs from that of the first embodiment shown in  FIG. 3  in that, control light  41  which is different from the incident light  21  is used for altering the refractive index of the semiconductor as the photonic crystal.  
         [0081]     The operation principle of the third embodiment of the invention will be described. Referring to the energy band shown in  FIG. 4 , the solid lines define an energy band of the case where the control light  41  is not radiated, while the energy band alters, for example, as defined by the dashed lines in  FIG. 4  when the control light  41  is radiated and carriers are excited to alter the refractive index of the i-type semiconductor  33  or  34  as the photonic crystal. Accordingly, in the same manner as the first embodiment of the invention, when the energy of the incident light  21  is Ω7, the output light  22  can be switched on or off according to the presence or absence (ON or OFF) of the control light  41 .  
         [0082]     Change in the number of carriers in the photonic crystal (plasma effect) is one factor that causes alteration of the refractive index by light radiation. In addition, optical Kerr effect as the third-order nonlinear optical characteristic of the medium may be another factor. Optical Kerr effect is a phenomenon represented by the following equation (1). 
 
 n=n   0 + n   2 * I    (1) 
 
 where n is a refractive index of the medium under radiation of the control light  41 , n 0  is a refractive index under no radiation, n 2  is a value proportional to a third-order nonlinear optical constant, and I is light intensity. 
 
         [0083]     Hereinafter, a fourth embodiment of the present invention will be described.  FIG. 7  is a plan view of a photonic crystal structure according to the fourth embodiment of the invention. Referring to  FIG. 7 , the present embodiment uses a line-defect waveguide to establish Mach-Zehnder interferometry. Two electrodes  36  and  36 ′ are attached such that carriers can be injected efficiently into the two branched channels.  
         [0084]     Next, operation principle of the fourth embodiment of the invention will be described. The incident light  21  is split at a Y-shaped junction of the line-defect waveguide to be transmitted through two optical paths  40  and  40 ′ as upper and lower branches. The beams that propagate through the two optical paths are recombined at the other Y-shaped junction and output as emitted light  22 . Since the lengths of the optical paths of the upper and lower branches are equal when no current is injected, the phase difference between the two branched beams is zero (in-phase), and thus the intensity of the output light  22  (which is the coherent light of the recombined beams) is the maximum. Even when the lengths of the two optical paths are different, the phase difference is equivalent to zero as long as the difference between the lengths of the optical paths is a multiple of the wavelength.  
         [0085]     When a current is injected through at least one of the electrodes of the upper and lower branches to alter the refractive index, the effective optical path length of that part changes. When a phase difference n is caused between the optical paths  40  and  40 ′ of the upper and lower branches, the recombined beams as the coherent light weakens each other and the intensity of the output light  22  becomes the minimum.  
         [0086]     Accordingly, the emitted light  22  can be switched on/off according to on/off of the current injection. In order to alter the refractive index, instead of current injection, application of a reverse bias, or radiation of control light other than the incident light may also be employed.  
         [0087]     Hereinafter, a fifth embodiment of the invention will, be described.  FIG. 8  is a schematic view showing the fifth embodiment of the invention. Referring to  FIG. 8 , a photonic crystal  50  is a perfect crystal with no line-defect waveguide. This is an optical switch utilizing a super prism phenomenon. As described in a document (Applied Physics Letters vol. 74, pp. 1370-1372 (1999)), the super prism phenomenon has an anomalous dispersion characteristic where its refractive angle changes by 50 degrees or more with a wavelength difference of only 1%.  
         [0088]     In other words, when the refractive index is slightly altered while the wavelength of the incident light is fixed, the energy band changes and due to which the refractive angle changes greatly.  
         [0089]     Next, operation principle of the fifth embodiment of the invention will be described with reference to  FIG. 8 . Light that is incident, prior to current injection, to the photonic crystal  50  via the waveguide  51  in an oblique manner with respect to the intrinsic axis (Γ-X or Γ-J) of the photonic crystal  50  propagates through the photonic crystal as indicated by the solid arrow in  FIG. 8  due to scattering characteristic of the photonic crystal. In this case, the light is output to the waveguide  52  of channel  1  (ch 1 ).  
         [0090]     Upon current injection, the refractive index of the medium forming the photonic crystal changes, by which the refractive angle also changes. As a result, the path of the propagating light changes as represented by the dashed arrow shown in  FIG. 8  and the light is output to the waveguide  52 ′ of channel  2  (ch  2 ).  
         [0091]     Although the case where two waveguides are used on the output side is described in the above-described example, the number of the waveguides used may be one, or three or more. Of course, instead of current injection, the refractive index may be altered by other means such as application of a reverse bias or radiation of control light other than the incident light.  
         [0092]     Hereinafter, a sixth embodiment of the present invention will be described.  FIG. 9  is a schematic view showing a structure of the sixth embodiment of the invention The photonic crystal serves as a waveguide  60  with a width of about 1 to 2 micro meter in the direction perpendicular to the light propagating direction in a two-dimensional plane. The waveguide  60  is sandwiched by clad layers  61  and  61 ′. The sixth embodiment of the invention performs switching according to the intensity of the incident light  21  itself.  
         [0093]     Next, operation principle of the sixth embodiment of the invention will be described with reference to  FIGS. 4, 10A  and  10 B.  
         [0094]     First, a case where the energy of the incident light  21  is Ω8 (see  FIG. 4 ) will be described. When the intensity of the incident light is weak, the energy band will be as represented by the solid lines in  FIG. 4 , where the energy of the light is outside the PBG and thus the light can propagate through the complete crystal waveguide. However, when the intensity of the incident light becomes stronger to some extent, the refractive index of the photonic crystal alters due to the intensity of the incident light itself as can be appreciated from Equation (1), according to which the energy band alters as represented by the dashed lines shown in  FIG. 4  where the energy of the light falls within the photonic band gap (PBG), whereby the light can no longer propagate.  
         [0095]      FIG. 10A  is a graph in which intensities of the emitted lights are plotted with respect to the intensity of the incident light with energy Ω8 based on the above-described example. When the intensity of the incident light becomes stronger to some extent, the emitted light reaches the limit and subsequently declines gradually. This may be assumed as a limiter function. The limiter function may be utilized as a shaping function for making the intensity of the light to be predetermined range, when the signals are ON.  
         [0096]     On the contrary to the case of incident light with an energy of Ω8, for incident light with an energy of Ω7 (see  FIG. 4 ), the intensity of the transmitted light is decreased when the intensity of the incident light is weak while the intensity of the transmitted light greatly increases when the intensity of the incident light is strong as shown in  FIG. 10B . This may be assumed as a function that increases the contrast of the signal light intensities between on and off states. Of course, the limiter function and the contrast increasing function can be combined.  
         [0097]     Similar to the embodiments of the first and second embodiments described with reference to  FIGS. 1A, 1B  and  11 , the core layer having the photonic crystal structure in the above-described third to sixth embodiments can also be sandwiched by upper and lower clad layers.  
         [0098]     According to each embodiment, the operating wavelength for switching is determined not by the characteristic of the structural material but by the structure of the photonic crystal, which gives more design freedom. By using a photonic crystal, a compact functional element can be realized. Since a flat micro-optical circuit, a semiconductor laser and the like used in optical communication have waveguide structures, the photonic crystal preferably has a waveguide structure as well.  
         [0099]     According to the present invention, light is confined in a three-dimensional manner to efficiently guide the light, and thus the present invention has an advantage of enabling high-speed operations. Furthermore, since the operating wavelength for switching is determined not by the characteristic of the structural material but by the structure of the photonic crystal, the design freedom is increased.  
         [0100]     According to the present invention, a multi-channel optical switch which can switch among a plurality of channels with a single optical switch is realized, thereby realizing a compact multi-channel switch.  
         [0101]     According to the present invention, a waveform shaping function is realized which can make the light intensity to be predetermined range when the signal is ON.  
         [0102]     The invention may be embodied in other specific forms without departing from the spirit or essential characteristic thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and rage of equivalency of the claims are therefore intended to be embraced therein.  
         [0103]     The entire disclosure of Japanese Patent Application No. 2001-106318 (Filed on Apr. 4, 2001) including specification, claims, drawings and summary are incorporated herein by reference in its entirety.