Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     Applicants hereby claim foreign priority benefits under U.S.C. §119 from Japanese Patent Application No. 2010-015929 filed on Jan. 27, 2010, the contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the wavelength domain optical switch which makes it possible to use a cheap lens, makes it possible to correct aberration of the demultiplexed wavelengths produced in a plurality of waveguide type demultiplexing circuits, and is no depending to polarization of incident light. 
     2. Description of the Related Art 
     As shown in  FIG. 6 , a wavelength domain optical switch  600  described in the specification of U.S. Patent Application Publication No. 2006/67611 is constituted by input/output optical fibers  601  to  606 , a collimator lens array  610 , a Wollaston prism  615  constituted by two triangular prisms  616 ,  617  for making a characteristic between horizontal polarized light (y polarization) and vertical polarized light (x polarization) independent, a birefringent plate  620  for setting a phase difference between the horizontal polarized light and the vertical polarized light at zero, a ½-wavelength plate unit  625  constituted by a ½-wavelength plate part  626  and a part  627  that does not affect the polarization, a concave mirror  630 , a cylindrical lens  635 , a grating  642  having an edge prism  641 , a prism  646  for bending light vertically, and an LCOS SLM (Liquid Crystal On Si Spatial Light Modulator)  645 . 
     Further, as shown in  FIGS. 7A and 7B , a waveguide type wavelength selective switch described in the specification of U.S. Pat. No. 7,088,882 employs a MEMS (Micro Electro Mechanical System) micromirror  701 . Here, five waveguide type demultiplexers  703  are disposed on a single substrate  702 , and five further substrates  702  are stacked. 
     There is the following problem in the related art of  FIG. 6 . 
     (1) Since a bulk type grating  642  is used, and therefore a plurality of input beams is dispersed by the single grating  642 . However, the dimensions of the bulk type grating  642  are large, making it difficult to achieve a reduction in size. 
     (2) The collimator lens array  610  is used for the respective input/output optical fibers  601  to  606 , and since the collimator lens array  610  must be aligned with the input/output optical fibers  601  to  606  extremely strictly, a large amount of time is required for assembly. Furthermore, in order to suppress aberration, the collimator lens array  610  must be formed in an aspherical shape, leading to a large increase in cost. 
     (3) Since a complicated optical system is used, the price and assembly cost of the respective optical component increase. Hence, it is difficult to achieve a reduction in cost. 
     There is the following problem in the related art of  FIG. 7 . 
     (1) The plurality of waveguide type demultiplexers  703  is disposed in planar form on the single substrate  702 . When the MEMS micromirror  701  is used, a large reflection angle is permitted, and therefore this structure is possible. However, when this structure is applied to a wavelength domain optical switch such as that of the present invention, the reflection angle of the LCOS SLM is small, and therefore the performance deteriorates dramatically. Further, to increase in the number of switchable ports in the related art, the plurality of substrates  702  (five in this example) on which the respective waveguide type demultiplexers  703  are disposed in planar form are laminated in a thickness direction, and a lens array  704  on which light converges in a vertical direction is provided for each substrate  702 . However, the lens arrays  704  must be aligned with the respective waveguide demultiplexers  703  extremely strictly, leading to an increase in the amount of time required for assembly. Further, in order to suppress aberration, the lens array  704  must be formed in an aspherical shape, leading to a large increase in cost. These difficulties become gradually more insurmountable as the size of the lens array  704  is reduced, and it is therefore extremely difficult to achieve a size reduction. 
     (2) Spectral characteristics (demultiplexed wavelengths or center wavelengths) of the waveguide type demultiplexers  703  provided on the laminated substrates  702  must be strictly aligned such that deviation therebetween is no more than 1% of a demultiplexing interval, for example no more than 0.01 nm in the case of a 1 nm demultiplexing interval, and at current levels of microprocessing precision, it is extremely difficult to achieve this control. Accordingly, yield is extremely poor. 
     A common problem of the related art of  FIGS. 6 and 7  is that the aberration required of the lens arrays is extremely exact, and therefore the lens arrays must be formed in an aspherical shape, leading to an increase in cost. Further, the lens arrays must be aligned with the optical fibers (or the waveguide type demultiplexers) extremely strictly, making mass production extremely difficult. 
     SUMMARY OF THE INVENTION 
     Therefore, the purpose of the present invention is to solve these subjects and to provide the wavelength domain optical switch which makes it possible to use a cheap lens, makes it possible to correct aberration of the demultiplexed wavelengths produced in a plurality of waveguide type demultiplexing circuits, and is no depending to polarization of incident light. 
     To achieve this object, the present invention is a wavelength domain optical switch including: an integrated element formed by laminating in a thickness direction three or more waveguide type demultiplexing circuits, each of which includes one or more input/output waveguides for inputting or outputting light, a slab waveguide connected to the input/output waveguide, and an array waveguide constituted by a plurality of waveguides connected to the slab waveguide such that entrance/exit ends of a plurality of waveguides are arranged in the thickness direction in relation to each array waveguide; a first lens having a focal length of F1, which is disposed at a distance F1 from the entrance/exit ends of the integrated element and collects light emitted from the entrance/exit ends; a polarization separation element which is disposed at the distance F1 from the first lens and separates the light emitted from the entrance/exit ends of the integrated element and passed through the first lens into mutually orthogonal X polarization and Y polarization, and emits the X polarization and the Y polarization; a second lens having a focal length F2, which is disposed at a distance F2 from the polarization separation element so as to face the polarization separation element and collects the X polarization and the Y polarization; a ½-wavelength plate which rotates only one of the X polarization and the Y polarization emitted from the second lens spatially by 90 degrees such that the X polarization and the Y polarization have identical polarization directions; a first reflective optical phase modulator which is disposed at the distance F2 from the second lens and reflects the polarization emitted from the second lens and passed through the ½-wavelength plate and the other polarization emitted from the second lens on the second lens; and a second reflective optical phase modulator which is disposed at the distance F2 from the second lens on an identical side to the polarization separation element so as to face the second lens, and inputs light reflected by the first reflective optical phase modulator into one of the waveguide type demultiplexing circuits. 
     An interval between the laminated waveguide type demultiplexing circuits of the integrated element may be within a range of 5 μm to 100 μm. 
     The first reflective optical phase modulator may be controlled to a refractive index distribution for correcting misalignment among demultiplexed wavelengths of the respective waveguide type demultiplexing circuits. 
     The first reflective optical phase modulator is controlled to a refractive index distribution obtained by superimposing a saw-shaped refractive index distribution for polarizing an input light beam in a desired direction, on a refractive index distribution for correcting aberration in the first lens and the second lens. 
     The present invention exhibits the following favorable effects. 
     (1) An inexpensive lens can be used. 
     (2) Deviation among the demultiplexed wavelengths of a plurality of waveguide type demultiplexing circuits can be corrected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an overall perspective image view illustrating a structure of a wavelength domain optical switch according to an embodiment of the present invention; 
         FIG. 1B  is an enlarged view showing entrance/exit ends of an integrated element in order to illustrate the structure of the wavelength domain optical switch according to this embodiment of the present invention; 
         FIG. 1C  is a view showing a phase distribution of a first reflective optical phase modulator in order to illustrate the structure of the wavelength domain optical switch according to this embodiment of the present invention; 
         FIG. 1D  is a view showing an optical path of input light through a second reflective optical phase modulator in order to illustrate the structure of the wavelength domain optical switch according to this embodiment of the present invention; 
         FIG. 1E  is an optical path diagram illustrating reflection angle control in the second reflective optical phase modulator in order to illustrate the structure of the wavelength domain optical switch according to this embodiment of the present invention; 
         FIG. 2  is a view illustrating the structure of the integrated element used in the present invention; 
         FIG. 2A  is a plan view illustrating the structure of the integrated element used in the present invention; 
         FIG. 2B  is an enlarged view showing a multiplexing side entrance/exit end surface in order to illustrate the structure of the integrated element used in the present invention; 
         FIG. 2C  is a perspective view illustrating the structure of the integrated element used in the present invention; 
         FIG. 2D  is an enlarged view showing a demultiplexing side entrance/exit end surface in order to illustrate the structure of the integrated element used in the present invention; 
         FIGS. 3A to 3L  are schematic sectional views illustrating a manufacturing process sequence for manufacturing the integrated element used in the present invention; 
         FIG. 4A  is a sectional view illustrating the structure of and control of the reflective optical phase modulator used in the present invention; 
         FIG. 4B  is a front view illustrating the structure of and control of the reflective optical phase modulator used in the present invention; 
         FIG. 4C  is a graph showing phase variation distributions of optical phase modulation cells in an X axis direction in order to illustrate the structure of and control of the reflective optical phase modulator used in the present invention; 
         FIG. 4D  is a graph showing the phase variation distributions of the optical phase modulation cells in order to illustrate the structure of and control of the reflective optical phase modulator used in the present invention; 
         FIG. 4E  is an optical path diagram showing an equiphase surface in order to illustrate the structure of and control of the reflective optical phase modulator used in the present invention; 
         FIG. 5  is a constitutional diagram showing a communication system employing the wavelength domain optical switch according to the present invention 
         FIG. 6  is a perspective image view of a conventional wavelength domain optical switch; 
         FIG. 7A  is a plan view of a conventional waveguide type wavelength selective switch; 
         FIG. 7B  is a side view of the conventional waveguide type wavelength selective switch; and 
         FIG. 8  is a side view showing positional relationships among respective members constituting the wavelength domain optical switch according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described in detail below on the basis of the attached drawings. 
     As shown in  FIGS. 1A to 1E , a wavelength domain optical switch  100  according to the present invention includes an integrated element  110 , a first lens  130 , a polarization separation element  140 , a second lens  150 , a first reflective optical phase modulator  160 , a ½-wavelength plate  170 , and a second reflective optical phase modulator  180 . 
     The integrated element  110  includes five input/output ports # 1  to # 5  such that an input/output optical fiber  101  can be connected to each input/output port # 1  to # 5 . Any of the input/output ports # 1  to # 5  may be used as an input port or an output port. In this embodiment, the wavelength domain optical switch  100  is a 1×4 optical switch having one input port and four output ports. 
     The structure of the integrated element  110  will now be described in further detail using  FIGS. 2A to 2D . 
     The integrated element  110  is formed by laminating five waveguide type demultiplexing circuits  114 , in which a core  112  having a high refractive index is buried in cladding  113  having a lower refractive index, onto a substrate  111 . The waveguide type demultiplexing circuit  114  includes five input/output waveguides  115 , a slab waveguide  116  that is connected to the input/output waveguides  115  and structured such that light is held therein in only a thickness direction, and an array waveguide  118  that is connected to the slab waveguide  116  and constituted by a plurality of waveguides  117  that differ in length sequentially by a fixed length. 
     The five waveguide type demultiplexing circuits  114  are laminated in close proximity in a thickness direction of the substrate  111  at intervals of 25 μm, for example. Here, the interval is a distance between an upper end of the core  112  in the waveguide type demultiplexing circuit  114  positioned on a lower side and a lower end of the core  112  in the waveguide type demultiplexing circuit  114  positioned on an upper side in  FIG. 3L . The cladding  113  is interposed between overlapping cores  112 . As a result, a demultiplexing side entrance/exit end surface  120  in which entrance/exit ends  119  of the respective waveguides  117  in the array waveguide  118  are arranged in the thickness direction of the substrate  111  (a lamination direction) is formed on one side of the integrated element  110 , and a multiplexing side entrance/exit end surface  122  in which entrance/exit ends  121  of the plurality of input/output waveguides  115  on each layer are arranged in the thickness direction of the substrate  111  is formed on another side of the integrated element  110 . 
     Hence, the integrated element  110  is formed by laminating the waveguide type demultiplexing circuits  114  in an integrated fashion on the single planar substrate  111 . As shown in  FIG. 2B , the entrance/exit ends  121  of five input/output waveguides  115  are arranged on each of the five layers of waveguide type demultiplexing circuits  114  on the multiplexing side entrance/exit end surface  122 . The five optical fibers  101  are connected to the entrance/exit ends  121  to be used as the input/output ports # 1  to # 5 . An optical fiber array  123  is formed by fitting the five optical fibers  101  into a plurality of parallel V grooves, and the optical fiber array  123  is attached to the multiplexing side entrance/exit end surface  122  at an incline relative to an upper surface of the integrated element  110 . 
     It is known that when the input/output ports are varied in this type of waveguide type demultiplexing circuit, demultiplexed wavelengths also typically vary. In the present invention, however, the first reflective optical phase modulator  160  is used, and therefore the varied demultiplexed wavelengths are corrected. Note that alignment marks  124  are used in a manufacturing method to be described below to realize mask alignment when the respective waveguide type demultiplexing circuits  114  are laminated with a high degree of precision. 
     A method of manufacturing the integrated element  110  will now be described using  FIGS. 3A to 3L . Note that in  FIGS. 3A to 3L , the number of cores  112  in the waveguide type demultiplexing circuit  114  has been reduced to two. 
     As shown in  FIG. 3A , to form the waveguide type demultiplexing circuit  114  of a first layer, first, a core glass  301  constituted by a core film is formed on the substrate  111 , which is constituted by silica glass, using a method such as CVD (Chemical Vapor Deposition). A refractive index of the core glass  301  is set to be approximately 0.2 to 3% higher than that of the silica glass forming the substrate  111 . 
     As shown in  FIG. 3B , a metallic film  302  is formed on the core glass  301  using a sputtering method or the like in order to etch the core glass  301  into a shape having a rectangular cross-section. 
     As shown in  FIG. 3C , a resist film  303  is then applied, whereupon photoresist is formed by exposing the resist film  303  through a photomask  305  having an alignment mark pattern  304 . A resist pattern is then formed by developing the photoresist, whereupon the metallic film  302  is etched. By means of the alignment mark pattern  304 , the waveguide type demultiplexing circuit  114  of the first layer and the alignment marks  124  (see  FIG. 2A ) can be formed simultaneously, and therefore the optical circuits of the subsequently formed second layer, third layer, and so on can be laminated with a high degree of precision. 
     As shown in  FIG. 3D , the core glass  301  is then etched using the metallic film  302  as a mask such that the cores  112  of the waveguide type demultiplexing circuit  114  of the first layer and the alignment marks (not shown) remain on the substrate  111 . 
     As shown in  FIG. 3E , a cladding film  306  covering the cores  112  is formed by coating. The cladding film  306  is constituted by silica glass, and therefore the periphery of the cores  112  having a high refractive index is surrounded by silica glass having a low refractive index. An upper surface of the cladding film  306  is textured to correspond to the cores  112  having a rectangular cross-section. 
     As shown in  FIG. 3F , the upper surface of the cladding film  306  is flattened using a polishing method such as CMP (Chemical Mechanical Polishing). As a result, the cladding  113  is formed. 
     As shown in  FIGS. 3G to 3L , the waveguide type demultiplexing circuit  114  of the second layer is formed on the cladding  113  in a similar manner to the first layer. At this time, the alignment mark pattern  304  of the photomask  305  is positioned in alignment with the alignment marks of the first layer. The integrated element  110  in which the waveguide type demultiplexing circuits  114  are laminated integrally on a plurality of layers is then manufactured by repeating a similar process. 
     In contrast to a method of forming an integrated element by forming the waveguide type demultiplexing circuit  114  of the first layer on the substrate  111  and then laminating together similar components using an optical adhesive, the manufacturing method for the integrated element  110  described above does not require an optical adhesive, and therefore a corresponding increase in compactness can be achieved in the lamination direction, as shown in  FIG. 3F . 
     Description will now return to the wavelength domain optical switch  100  shown in  FIG. 1 . 
     As shown in  FIG. 1A  and  FIG. 8 , a center of the first lens  130  is disposed at a distance F1 from the entrance/exit end  120  of the integrated element  110 . The first lens  130  collects light emitted from the entrance/exit ends  119  of the waveguide type demultiplexing circuits  114  in the integrated element  110  and therefore functions to collimate light in the lamination direction and a width direction of the waveguide type demultiplexing circuits  114  in the integrated element  110 . A spherical lens, a cylindrical lens, and so on may be used as the first lens  130 . A focal length of the first lens  130  is F1 on both sides. 
     One end surface of the polarization separation element  140  positioned on the first lens  130  side is disposed at the distance F1 from the center of the first lens  130 . The polarization separation element  140  is constituted by a Wollaston prism and is used to separate mutually orthogonal X polarization and Y polarization in the lamination direction of the integrated element  110 . 
     A center of the second lens  150  is disposed at a distance F2 from a center of the polarization separation element  140  such that an upper half of the second lens  150  faces the polarization separation element  140 . The second lens  150  collects both X polarization and Y polarization and therefore functions to collimate light in a parallel direction to the substrate  111  of the integrated element  110 . A spherical lens, a cylindrical lens, and so on may be used as the second lens  150 . A focal length of the second lens  150  is F2 on both sides. 
     In a normal LCOS SLM, a refractive index can only be varied in a uniaxial direction, and therefore only the phase of polarization in a uniaxial direction can be varied. For example, when only the refractive index in a Y axis direction can be varied, only the phase of Y polarization can be varied. However, light typically includes both X polarization and Y polarization components, and moreover, a ratio thereof varies over time. It is therefore necessary to subject the X polarization and the Y polarization to similar phase control. Accordingly, the ½-wavelength plate  170  is disposed between the second lens  150  and the first reflective optical phase modulator  160  on either an optical path of the X polarization or an optical path of the Y polarization so as to cover only one of the polarizations, and makes the polarization directions of the X polarization and the Y polarization the same by rotating one of the X polarization and the Y polarization emitted from the second lens  150  spatially by 90 degrees. 
     A reflective film  404  of the first reflective optical phase modulator  160  positioned on the second lens  150  side is disposed at the distance F2 from the center of the second lens  150 . Note, however, that since the distance F2 is in the order of centimeters whereas respective films  403  to  408  constituting the first reflective optical phase modulator  160  are in the order of several microns, an end surface of the first reflective optical phase modulator  160  positioned on the second lens  150  side can be disposed at the distance F2 from the center of the second lens  150  with substantially no problems. The first reflective optical phase modulator  160  reflects the Y polarization collected by the second lens  150  and Y polarization obtained when the X polarization collected by the second lens  150  is converted by the ½-wavelength plate  170  at an arbitrary angle in each cell. The first reflective optical phase modulator  160  is constituted by a plurality of cells and has a variable refractive index in each cell. By controlling a refractive index distribution of the first reflective optical phase modulator  160  using a control circuit, not shown in the drawings, phase variation can be applied to reflection light in each cell. 
     The second reflective optical phase modulator  180 , similarly to the first reflective optical phase modulator  160 , is constituted by a plurality of cells and has a variable refractive index in each cell. By controlling a refractive index distribution of the second reflective optical phase modulator  180  using a control circuit, not shown in the drawings, phase variation can be applied to reflection light in each cell. The second reflective optical phase modulator  180  reflects light reflected by the first reflective optical phase modulator  160  and collimated by the second lens  150  at an arbitrary angle in each cell such that the reflected light enters one of the plurality of waveguide type demultiplexing circuits  114 . The reflective film  404  of the second reflective optical phase modulator  180  positioned on the second lens  150  side is disposed at the distance F2 from the center of the second lens  150  on the same side as the polarization separation element  140  so as to face a lower half of the second lens  150 . Note, however, that for the same reasons as the first reflective optical phase modulator  160 , substantially no problems arise when an interval between an end surface of the second reflective optical phase modulator  180  positioned on the second lens  150  side and the center of the second lens  150  is set at the distance F2. Hence, the second reflective optical phase modulator  180  is disposed substantially parallel to the polarization separation element  140 . 
     Identically constituted reflective optical phase modulators do not have to be used as the first reflective optical phase modulator  160  and the second reflective optical phase modulator  180 , but it is assumed here for ease of description that identically constituted reflective optical phase modulators are used. This reflective optical phase modulator will now be described in detail using  FIGS. 4A to 4E . 
     As shown in  FIG. 4A , a reflective optical phase modulator  401  is formed by laminating an electrode (an ITO, for example)  403 , the reflective film  404 , an SiO 2  film  405 , an alignment film  406 , a liquid crystal layer  407 , the SiO 2  film  405 , the electrode  403 , and a thin film-form glass substrate  408  in sequence on an Si substrate  402  formed with an electronic circuit. 
     As shown in  FIG. 4B , the reflective optical phase modulator  401  includes a plurality of cells  409  arranged horizontally and vertically such that the refractive index of each cell  409  can be controlled independently. More specifically, by applying a voltage to each cell  409 , an alignment direction (birefringence) of the liquid crystal layer  407  is controlled, and as a result, the phase of a light beam that enters and is reflected by an upper surface of the reflective optical phase modulator  401  can be modulated in each cell  409 . 
     A phase variation required to reflect light beams entering the respective cells  409  of the reflective optical phase modulator  401  is at most approximately 2π. Therefore, in the cells arranged in the X axis direction, as shown in  FIG. 4C , the phase applied to the light beam does not exceed 2π, and a phase distribution is set at a saw tooth-shaped phase distribution  411  that is equivalent to a linear phase distribution indicated in the drawing by a broken line  410 . 
     Further, in locations removed from a central portion of a lens, it is typically impossible to collect light in an ideal manner, and as a result, aberration occurs. Therefore, a parabola-shaped phase distribution  412 , as shown in  FIG. 4D , is applied to the cells arranged in the Y axis direction by varying the voltages applied to the respective cells  409  of the reflective optical phase modulator  401 , and as a result, a collection deviation caused by aberration between the first lens  130  and the second lens  150  can be corrected. By superimposing a similar saw tooth-shaped phase distribution to the X axis phase distribution  411  on the parabola-shaped phase distribution  412 , phase distributions  413   a ,  413   b ,  413   c  are obtained. 
       FIG. 4D  shows only the three phase distributions  413   a ,  413   b ,  413   c  in the X axis direction, but since these three distributions are located respectively on an A-A line, a B-B line and a C-C line of  FIG. 4B , in actuality, phase variation in the entire reflective optical phase modulator  401  forms a two-dimensional distribution. The X axis direction positions are varied in accordance with the lamination positions of the waveguide type demultiplexing circuits  114  such that the distribution on the A-A line corresponds to the input/output port # 2 , for example, and the distribution on the B-B line corresponds to the input/output port # 3 , for example. The phase distributions  413   a ,  413   b ,  413   c  are formed by superimposing a saw tooth-shaped phase distribution on the parabola-shaped phase distribution  412 , and the resulting phase distribution corrects a demultiplexing characteristic of the corresponding waveguide type demultiplexing circuit  114 . 
     By applying a refractive index distribution that brings about the phase distributions shown in  FIG. 4D  to the reflective optical phase modulator  401 , the Y axis direction phases of light beams of each wavelength are varied such that the light beams exit the reflective optical phase modulator  401  at different angles. In other words, as shown in  FIG. 4E , equiphase surfaces L 1 , L 2 , L 3  of the light beams reflected by the reflective optical phase modulator  401  during spatial propagation are different at each wavelength. As a result, a light beam of a predetermined wavelength, from among the light beams emitted by the waveguide type demultiplexing circuit  114  having the input port, enters a desired waveguide type demultiplexing circuit  114  having an output port. 
     Note that the reflective optical phase modulator  401  used as the first reflective optical phase modulator  160  and the second reflective optical phase modulator  180  is preferably subjected to temperature control using a heater or a Peltier element so that a temperature thereof remains constant. 
     Next, functions and an optical signal transmission method of the wavelength domain optical switch  100  according to the present invention will be described. 
     When a beam (a wavelength multiplexed optical signal) of various wavelengths input from the optical fiber  101  enters the waveguide type demultiplexing circuit  114  directly in the middle of the integrated element  110  in the lamination direction, beams are emitted from the entrance/exit ends  119  of the respective waveguides  117  arranged on the multiplexing side entrance/exit end surface  120  in different directions for each wavelength. When these beams pass through the first lens  130 , the beams of the respective wavelengths enter the polarization separation element  140  as mutually offset parallel beams. The beams that pass through the polarization separation element  140  are separated into two groups, namely an X polarization group and a Y polarization group, whereupon the respective groups enter the upper half of the second lens  150 . Having passed through the second lens  150 , the two polarization groups (the X polarization group and the Y polarization group) respectively form parallel beams that enter the first reflective optical phase modulator  160 . At this time, the X polarization group forming one of the two separated polarization groups passes through the ½-wavelength plate  170  before entering the first reflective optical phase modulator  160 . In the ½-wavelength plate  170 , the polarization direction of the X polarization group is rotated 90° spatially to become Y polarization, whereupon the Y polarization enters the first reflective optical phase modulator  160 . 
     The reason for having the X polarization group pass through the ½-wavelength plate  170  but having the Y polarization group enter the first reflective optical phase modulator  160  without passing through the ½-wavelength plate  170  is to ensure that the reflective optical phase modulator  401  acts on (controls the reflection light direction of) only one type of polarization (here, the Y polarization). In a case where the reflective optical phase modulator  401  is to be applied only to the X polarization, the Y polarization should be passed through the ½-wavelength plate  170 . 
     A case in which light enters all of the input/output ports # 1  to # 5  of the integrated element  110  will now be considered. Beams are emitted from the respective entrance/exit ends  119  of the multiplexing side entrance/exit end surface  120  in different directions for each wavelength, and therefore images # 1 BU to # 5 BU generated by five beams of the X polarization group that is separated by the polarization separation element  140  and spatially rotated 90° by the ½-wavelength plate  170  are projected onto the upper half of the first reflective optical phase modulator  160 . Meanwhile, images # 1 BL to # 5 BL generated by five beams of the Y polarization group separated by the polarization separation element  140  are projected onto the lower half of the first reflective optical phase modulator  160 . 
     The images # 1 BU to # 5 BU, # 1 BL to # 5 BL generated by the total of ten beams are analogous to the distribution of the beams emitted from the respective waveguide type demultiplexing circuits  114  of the integrated element  110  but disposed upside-down. The reason why the images formed by the beams are analogous to the beam distribution is that the images formed by the beams are subjected to Fourier transform twice by the first lens  130  and the second lens  150 . A magnification ratio B of the images formed by the beams and the beam distribution is given by B=F2 /F1. Hence, the phase distribution of the beam distribution projected onto the first reflective optical phase modulator  160  is identical to that of the respective waveguide type demultiplexing circuits  114 . Therefore, since the phase distribution of the liquid crystal cells in the parts, onto which the respective beams are projected, is changed to a complementary distribution, the center wavelength, which varied previously among the respective beams, is corrected. This will be explained below using  FIG. 1C . 
     The beam image # 1 BU and the beam image # 1 BL, for example, are converted images obtained by converting the beam distribution emitted from the waveguide type demultiplexing circuit  114  corresponding to the port # 1 , and therefore collecting positions thereof are different whereas amplitudes and phases thereof are identical. As shown in the drawing, the phase distribution of the waveguide type demultiplexing circuit  114  deviates from a desired phase distribution (broken line) due to manufacturing process variation. By applying a phase distribution that has been inverted relative to this phase distribution to the first reflective optical phase modulator  160 , the resulting phase distribution can be substantially aligned with the desired phase distribution, and therefore the varying phase distributions of the respective waveguide type demultiplexing circuits  114  can be corrected to substantially perfectly aligned phase distributions. In other words, the demultiplexed wavelengths (determined by the incline of the phase distribution) of the five waveguide type demultiplexing circuits  114  can be corrected so as to match each other. 
     A beam reflected by the first reflective optical phase modulator  160  passes through the lower half of the second lens  150  and is projected onto the second reflective optical phase modulator  180 . Here, an image formed by the projected beam is a Fourier-transformed image of the beam distribution from the first reflective optical phase modulator  160  (which is analogous to the beam distributions from the respective waveguide type demultiplexing circuits  114  of the integrated element  110 ), and therefore, if the image formed by the beam projected onto the first reflective optical phase modulator  160  has an elliptical Gauss distribution in which the X axis is the long axis, the image formed by the beam projected onto the second reflective optical phase modulator  180  has an elliptical distribution rotated 90° such that the Y axis is the long axis. As a result, beams of respective wavelengths are projected onto locations corresponding to respective wavelengths on the Y axis. The first reflective optical phase modulator  160  performs phase correction such that the demultiplexed wavelengths of the respective waveguide type demultiplexing circuits  114  all match, and therefore the total of ten beam distributions (including polarization) relating to the respective wavelengths form a single beam distribution that is projected onto the second reflective optical phase modulator  180 . 
       FIG. 1D  shows the manner in which the ten beams enter the second reflective optical phase modulator  180 . One of the two polarization groups separated by the polarization separation element  140  (light beams # 1 DU to # 5 DU corresponding to the beam images # 1 BU to # 5 BU) enters the second reflective optical phase modulator  180  in a downwardly oriented diagonal direction, while the other polarization group (light beams # 1 DL to # 5 DL corresponding to the beam images # 1 BL to # 5 BL) enters the second reflective optical phase modulator  180  in an upwardly oriented diagonal direction. In the respective polarization groups, entrance angle differences between the respective light beams are determined in accordance with the lamination intervals between the respective waveguide type demultiplexing circuits  114  such that if the waveguide type demultiplexing circuits  114  are laminated at equal intervals, the entrance angle differences take an identical angle α. As shown in  FIG. 1D , an angle between the two separated polarization groups is set at θ. 
     The second reflective optical phase modulator  180  is constituted by a plurality of liquid crystal cells, and by varying the refractive indices of the respective cells, a virtual mirror is realized. As shown in  FIG. 1E , when an angle φ of a mirror surface of this virtual mirror is inclined downward by the angle α from a vertical plane (when φ=α), the light beam (# 1 DL) is reflected at an angle of the light beam (# 2 DU), and the light beam (# 1 DU) is reflected as the light beam (# 2 DL). In other words, the light beams (# 1 DL, # 1 DU) indicated by solid lines in  FIG. 1E  are switched to the light beams (# 2 DU, # 2 DL) indicated by broken lines. 
     Description will now return to the method of transmitting an optical signal employed in the wavelength domain optical switch  100 . Here, the wavelength domain optical switch  100  is a 1×4 optical switch in which the central input/output port # 1  is used as the input port and the remaining input/output ports # 2  to # 5  are used as the output ports. The following description will focus on a single wavelength demultiplexed by the waveguide type demultiplexing circuit  114 . 
     A beam input into the central input/output port # 1  serving as the input port from the optical fiber  101  is demultiplexed by the waveguide type demultiplexing circuit  114 . A beam of a single wavelength, from the demultiplexed beam, passes through the first lens  130  and is then separated into two by the polarization separation element  140 . One of the two beams passes through the second lens  150  and the ½-wavelength plate  170  while the other passes through the second lens  150  alone. Thus, the two beams are formed into identical polarization that is projected onto the first reflective optical phase modulator  160  (as the beam images # 1 BU, # 1 BL). The two projected beams are reflected after undergoing phase correction, thereby passing back through the second lens  150  so as to enter the second reflective optical phase modulator  180  at different angles (as the light beams # 1 DU, # 1 DL). The two light beams are projected onto the second reflective optical phase modulator  180  as a single beam distribution. 
     Here, as shown in  FIG. 1E , by applying an appropriately inclined phase distribution to the second reflective optical phase modulator  180 , the light beam # 1 DL, for example, is reflected as the light beam # 2 DU, whereupon the light beam # 2 DU travels back along the optical path shown in  FIG. 1 . More specifically, the beam that is reflected as the light beam # 2 DU forms the beam image # 2 BU on the first reflective optical phase modulator  160  and is then reflected thereby. 
     The beam that is reflected as the beam image # 2 BU is subjected to 90° polarization rotation by the ½-wavelength plate  170  and then passes through the second lens  150  and the polarization separation element  140  so as to enter the waveguide type demultiplexing circuit  114  connected to the input/output port # 2 . The beam is then output from the input/output port # 2 . 
     Meanwhile, in the second reflective optical phase modulator  180 , the light beam # 1 DU is reflected as the light beam # 2 DL, whereupon the light beam # 2 DL travels back along the optical path shown in  FIG. 1 . More specifically, the beam that is reflected as the light beam # 2 DL forms the beam image # 2 BL on the first reflective optical phase modulator  160  and is then reflected thereby. 
     The beam that is reflected as the beam image # 2 BL then passes through the second lens  150  and the polarization separation element  140  so as to enter the waveguide type demultiplexing circuit  114  connected to the input/output port # 2 . The beam is then output from the input/output port # 2 . 
     A switching operation in which both the X polarization and the Y polarization of a beam input into the input/output port # 1  are emitted from the input/output port # 2  is thus completed. Thus, switching can be performed independently of the input polarization. 
     A switching operation from the input/output port # 1  to the input/output port # 2  was described above, but by varying the phase distribution incline applied to the second reflective optical phase modulator  180 , switching can also be performed from the input/output port # 1  to the input/output ports # 3 , # 4 , # 5 . 
     More specifically, when the angle of the virtual mirror surface is φ, the entrance angle difference between the respective light beams is α, and the angle between the polarization groups is θ, the following effects are obtained. 
     When φ=0, the light beam # 1 DU is reflected at an angle θ+4α to become the light beam # 1 DL and the light beam # 1 DL is reflected at an angle θ+4α to become the light beam # 1 DU. Therefore, the beam output from the input/output port # 1  returns to the input/output port # 1 . This corresponds to a case in which switching is not performed. 
     When φ=α/2, the light beam # 1 DU is reflected at an angle θ+5α to become the light beam # 3 DL and the light beam # 1 DL is reflected at an angle θ+3α to become the light beam # 3 DU. Therefore, the beam output from the input/output port # 1  returns to the input/output port # 3 . In other words, switching is performed from the input/output port # 1  to the input/output port # 3 . 
     When φ=α, as described above, the light beam # 1 DU is reflected at an angle θ+6α to become the light beam # 2 DL and the light beam # 1 DL is reflected at an angle θ+2α to become the light beam # 2 DU. Therefore, the beam output from the input/output port # 1  returns to the input/output port # 2 . In other words, switching is performed from the input/output port # 1  to the input/output port # 2 . 
     When φ=−α/2, the light beam # 1 DU is reflected at an angle θ+3α to become the light beam # 4 DL and the light beam # 1 DL is reflected at an angle θ+5α to become the light beam # 4 DU. Therefore, the beam output from the input/output port # 1  returns to the input/output port # 4 . In other words, switching is performed from the input/output port # 1  to the input/output port # 4 . 
     When φ=−α, the light beam # 1 DU is reflected at an angle θ+2α to become the light beam # 5 DL and the light beam # 1 DL is reflected at an angle θ+6α to become the light beam # 5 DU. Therefore, the beam output from the input/output port # 1  returns to the input/output port # 5 . In other words, switching is performed from the input/output port # 1  to the input/output port # 5 . 
     Further, beams of a large number of wavelengths are projected onto the second reflective optical phase modulator  180 , and therefore, by subjecting cell groups in regions of the respective projected wavelengths to phase distribution control independently, the beams of the respective wavelengths can be switched independently. 
     As described above, according to the present invention, an expensive aspherical lens array that was problematic in the related art is not used to collect light from the plurality of laminated waveguide type demultiplexing circuits  114 , and instead, the single inexpensive first lens  130  is used. As a result, a reduction in cost is achieved. This reduction in cost is made possible by employing the integrated element  110  in which the laminated waveguide type demultiplexing circuits  114  are laminated at extremely narrow intervals of 5 μm to 100 μm to form an integrated body. This lamination interval can only be realized through a similar microprocessing technique to that used for a semiconductor LSI, such as photolithography or dry etching. Conventionally, when a plurality of optical fiber arrays are overlapped, the lamination interval is several hundred μm, and therefore, when a plurality of waveguides are adhered to each other, the waveguide interval reaches several thousand μm. In such a case, a lens array must be used to limit the device to practicable dimensions. 
     Further, in the integrated element  110  manufactured using a similar microprocessing technique to that of an LSI, it is important to align the demultiplexed wavelengths (center wavelengths) of the respective laminated waveguide type demultiplexing circuits  114  strictly (to a demultiplexing interval of no more than 1%) because when the demultiplexed wavelengths are not aligned, extremely large loss occurs. With a conventional microprocessing technique, it is extremely difficult to align the demultiplexed wavelengths strictly, and therefore, in order to align the demultiplexed wavelengths strictly, the demultiplexed wavelengths must be corrected using a certain method after the integrated element  110  is manufactured. In the present invention, the first reflective optical phase modulator  160  is used to perform wave surface correction such that beams emitted from the respective laminated waveguide type demultiplexing circuits  114  have identical demultiplexed wavelengths. With this technique it has become possible for the first time to use an integrated element  110  manufactured through a microprocessing technique. 
     According to the present invention, an inexpensive, small, high-performance, flexible wavelength domain optical switch can be realized, enabling great advancements in optical systems and optical networks of the future. 
     Next, other embodiments of the present invention will be described. 
     The light emitted from the waveguide type demultiplexing circuit  114  has an elliptical beam distribution, and therefore, when an ellipticity thereof is large, a semi-cylindrical lens may be used as the first lens  130 . Alternatively, semi-cylindrical lenses may be combined. 
     In the wavelength domain optical switch  100  shown in  FIG. 1A , two three-dimensional lenses (the first lens  130  and the second lens  150 ) are used, but the respective lenses may be realized by combining semi-cylindrical two-dimensional lenses. 
     Furthermore, inexpensive lenses are more likely to include aberration. Therefore, lens aberration is preferably corrected by providing each cell of the second reflective optical phase modulator  180  with an aberration-correcting phase distribution, as in the embodiment described above. 
     In the embodiment described above, the waveguide type demultiplexing circuits  114  are laminated on five layers in the integrated element  110 , but by laminating three layers, an optical switch having one input and two outputs can be formed, and by laminating three or more layers, an optical switch having one input and two or more outputs can be formed. 
     Next, a method of using the wavelength domain optical switch  100  according to the present invention will be described. As shown in  FIG. 5 , the wavelength domain optical switch  100  may be used in each node of a metro core  501 , and may also be applied to a normal optical signal splitting/inserting (an optical Add/Drop) system or an optical cross-connect system. Note that a conventional wavelength domain optical switch is used in a comparatively large-scale system such as a main line system or a metro core, but since a large cost reduction can be achieved with the present invention, the wavelength domain optical switch can be introduced into a wide range of systems such as a metro edge and an access system, thereby enabling groundbreaking developments in the field of optical networks. 
     While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present.

Technology Category: g