JP2019082518A - Optical matrix switch - Google Patents

Abstract

To provide an optical matrix switch which is improved in loss and crosstalk.SOLUTION: An optical switch element 304 comprises a first polarization separation prism 301, a second polarization separation prism 303 and a liquid crystal cell 302 interposed between the prisms, in a z-axis direction. The first and second polarization prisms 301, 303 are constituted of photonic crystallized polarization separation prisms. The liquid crystal cell 302 shifts between an isotropic state with respect to at least incident light and a birefringence state having a phase difference of 180 degrees. The optical switch element 304 selects to emit the incident light which is clockwise or counterclockwise circular polarization light propagated in an xz-plane and having a progress direction inclined by a prescribed angle θ with respect to a z-axis, as circular polarization light having the same progress direction or reverse one rotation direction, or as circular polarization light having a progress direction inclined by an angle -θ with respect to the z-axis and in the same or reverse other rotation direction, depending on the transition of the liquid crystal cell.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an optical switch element formed of a photonic crystal, an optical switch circuit using the same, a two-dimensional or three-dimensional light beam multistage switch circuit, and a variable optical attenuator.

  An optical switch circuit that switches the paths of a plurality of optical signals in optical communication is an important component. FIG. 1 is a principle diagram showing a typical structure of an optical switch circuit, and shows a four-input four-output thermo-optical effect switch circuit coupled by a waveguide in a planar manner.

  As other important optical switch circuits used for optical communication, a wavelength selective switch (WSS) and a MEMS type switch are known. There is a three-dimensional type as shown in FIG. 2 in the MEMS type optical switch circuit. In the three-dimensional MEMS shown in FIG. 2, in order to increase the freedom of selection of the signal path coupled from the optical fiber group 201 on the input side to the optical fiber group 204 on the output side, the MEMS micromirror array 202 , 203 are required to perform multi-gradation analog control of the two-dimensional inclination of the corresponding mirror, which is accompanied by great difficulty.

K. Suzuki, K. Tanizawa, S. Suda, H. Matsuura, T. Inoue, K. Ikeda, S. Namiki, and H. Kawashima, “Broadband silicon photonics 8 × 8 switch based on double-Mach-Zehnder element switches ", Opt. Express 25, 7538-7546 (2017) Ming C. Wu, Olav Solgaard, and Joseph E. Ford, “Optical MEMS for Lightwave Communication,” Journal of Lightwave Technology, vol. 24, No. 12, December 2006.

Unexamined-Japanese-Patent No. 2006-292872 U.S. Patent No. 7092599 Patent No. 3325825

  As a path switch on a plane, a switch on a PLC (Planar Lightwave Circuit) or a silicon optical circuit switch is known. Those multi-input and multi-output optical path switches have a mesh structure as shown in FIG. 1, and signal interchange is performed between adjacent optical lines.

  In FIG. 1, the 2-input × 2-output unit switch element 101 is in the anti-cross state (upper left → upper right or lower left → lower right) and the cross state (upper left) along with heating on / off of the thermo-optical portion represented by a rectangle. → The lower right or lower left → upper right) is switched. As seen from the input of one switch element, the signal destination is only one of two adjacent lines, and the path change is zero or plus or minus 1, that is, the signal jump is at most one stage. In the circuit, the signal lines cross at substantially right angles to minimize crosstalk, but it is still difficult to avoid crosstalk between the signals. In more complex circuits, such as an 8x8 switch circuit (or 16x16 switch circuit), the signal needs to pass through eight (or 16) switch elements before reaching the output end from the input end. It is even more difficult to maintain signal quality, since each signal loss and crosstalk is accumulated eight times (16 times).

  Thus, in an optical switch circuit having a large number of input / output ports, signal loss and crosstalk between signal lines have been considered as problems. Therefore, an object of the present invention is to effectively suppress signal loss and crosstalk between signal lines even in an optical switch circuit having a large number of input / output ports.

[1. Optical switch element]
A first aspect of the present invention relates to an optical switch element. In the optical switch element of the present invention, in the three-dimensional space xyz, circularly polarized incident light that is clockwise or counterclockwise with a traveling direction inclined at a predetermined angle θ with respect to the z axis propagates in the xz plane . The optical switch element includes a first polarization splitting prism, a second polarization splitting prism, and a liquid crystal cell interposed between the first and second polarization splitting prisms in the z-axis direction. The first and second polarization splitting prisms are respectively photonic crystal polarization splitting prisms. The liquid crystal cell can transition between an isotropic state with respect to at least incident light and a birefringent state with a phase difference of 180 degrees by voltage driving. Due to such transition of the liquid crystal cell, the optical switch element according to the present invention emits incident light as circularly polarized light having the same traveling direction and the same or opposite rotation direction, or z It has a traveling direction inclined at an angle -θ with respect to the axis, and can be selected to be emitted as circularly polarized light in the same or opposite other rotation direction.

[1-1. First Specific Example of Photonic Crystal Polarization Splitting Prism]
The photonic crystal polarization splitting prism that constitutes the optical switch element may be of the "split type". The split-type photonic crystal polarization splitting prism includes a half-wave plate of a photonic crystal formed in the xy plane in the three-dimensional space xyz and stacked in the z-axis direction, and single or repeated in the x-axis direction The region is divided into a plurality of strip-like sub-regions in the x-axis direction. In the region, the groove direction of the photonic crystal changes stepwise in the range of 0 ° to 180 ° with respect to the y-axis direction, and in the sub-region, the angle with the y-axis direction is uniform. is there. Such a photonic crystal polarization splitting prism directs light incident in the z-axis direction to the right-handed circularly polarized light in the direction toward the x-axis by an angle from the z-axis and to the −x axis by the same angle from the z-axis It separates and converts into left-handed circularly polarized light and emits it.

[1-2. Second Specific Example of Photonic Crystal Polarization Splitting Prism]
The photonic crystal polarization splitting prism that constitutes the optical switch element may be of a "curved type". A curved photonic crystal polarization separation prism is provided in the xy plane in the three-dimensional space xyz, and includes a half-wave plate of the photonic crystal stacked in the z-axis direction, and single or repeated in the x-axis direction Have one or more regions. The groove direction of the photonic crystal is a curve represented by y = (D / π) log (| cos (πx / D) |) + constant in the approximate range of the average period of the groove, where D is a constant. In such a photonic crystal polarization splitting prism, right-handed circularly polarized light in the direction toward the x-axis by an angle from the z-axis and left-handed circularly polarized light in the direction toward the -x-axis by the same angle from the z-axis Separate, convert and emit.

[2. Optical switch circuit]
A second aspect of the present invention relates to an optical switch circuit. In the optical switch circuit of the present invention, a plurality of optical switch elements according to the first aspect described above are two-dimensionally or three-dimensionally arranged. Specific examples of the optical switch circuit are a light beam type multistage switch circuit of a two-dimensional structure and a light beam type multistage switch circuit of a three dimensional structure.

[2-1. 2-dimensional structure of light beam type multistage switch circuit]
The light beam type multistage switch circuit of the two-dimensional structure includes N stages of switch subcircuits in the z-axis direction in the three-dimensional space xyz. In the switch sub-circuit of each stage, a plurality of optical switch elements according to the first side surface are arranged in the x-axis direction. The average traveling direction of the entire light beam is the z-axis direction, and the light beam forms a two-dimensional network in the xz plane. In this circuit, 2 ^N input port arrays of predetermined polarization in the input plane and 2 ^N output port arrays of predetermined polarization in the output plane are arranged in the x-axis direction. Each of the optical switch elements selects the emission direction of the light beam so as to direct the light beam incident thereon to one of the other two optical switch elements different in position in the x-axis direction of the next stage It is In the M-th stage (M <N) of the switch circuit, the m-th port of the ports aligned in the x-direction is coupled to the m-th port of the next (M + 1) -th stage, or M) You can choose to combine with the leaped port. A port surface or an intermediate port surface that forms a boundary between each step and a region leading to or behind it comprises a photonic crystal wave plate or a photonic crystal polarization splitting prism that adjusts polarization and bends light in a desired direction. Specific examples of the photonic crystal polarization separation prism are as described above. As a whole effect, the light beam type multistage switch circuit of the two-dimensional structure emits an optical signal train incident on 2N ports on the incident surface from 2N ports on the outgoing surface, and also light different from the light It can be converted into a signal sequence. The light beam type multistage switch circuit of this two-dimensional structure can arbitrarily replace the order of the first stage, the second stage,..., And the Nth stage.

  The light beam type multistage switch circuit of the two-dimensional structure described above is expressed as follows. The present invention is a light beam type multistage switch circuit in a three dimensional space xyz, wherein the average traveling direction of the whole light beam is the z direction, and the light beam forms a two dimensional network in the xz plane. A plurality of switch elements according to the first n side surface are arranged in the x direction to form one stage, and a plurality of stages are arranged in the z direction to form a switch circuit. The coupling destination of the mth switching element in the A-th stage is either the p-th or q-th (q> p) switching element of the (A + 1) -th stage which is the next stage, and the jump width q The value of -p is uniform regardless of m for one stage A, and stages different from stage A can be given different jumping widths. A port surface or an intermediate port surface that forms a boundary between each step and a region leading to or behind it comprises a photonic crystal wave plate or a photonic crystal polarization splitting prism that adjusts polarization and bends light in a desired direction. Specific examples of the photonic crystal polarization separation prism are as described above.

[2-. 3-dimensional light beam type multistage switch circuit]
The light beam type multistage switch circuit of the three-dimensional structure includes 2N switch subcircuits in the z-axis direction in the three-dimensional space xyz. In the switch sub-circuits of each stage, the optical switch elements according to the first side face 2 ^N rows in the x direction and 2 ^N columns in the y direction, arranged in parallel to the xy plane. The light beam forms a three-dimensional network in xyz space, and the average travel direction of the entire light beam is the z direction. The input port group of the predetermined polarization in the input plane and the output port group of the predetermined polarization in the output plane are aligned in the xy plane, and the light beam incident on each of the optical switch elements is in the x-axis direction on the next intermediate port plane It is possible to select whether to lead to one of the other two intermediate ports different in the position of or to one of the other two ports with different values on the next intermediate port surface. Also, let M, 1, 2,. . , N−1, or on the (2M−1) th intermediate port surface, connect the mth row of light beams to the mth row on the 2Mth intermediate port surface, or Alternatively, it is possible to select whether to connect to the m + 2 (N−M) th row, and on the 2nd Mth intermediate port surface, the nth column of the light beam is made the nth column on the (2M + 1) th intermediate port surface It is possible to select the connection or the connection to the (n + 2) th (NM) column. Furthermore, as an effect of all 2N stages, a row of optical signals incident on 2 ^N × 2 ^N ports on the incident surface, and another optical signal of 2 ^N × 2 ^N predetermined polarization ports on the output surface It can be converted to a column. The light beam type multistage switch circuit of this three-dimensional structure can arbitrarily replace the order of the first stage, the second stage, ..., and the second N stages.

  The light beam type multistage switch circuit of the above-described three-dimensional structure can be expressed as follows. The present invention is a light beam type multistage switch circuit in a three-dimensional space xyz, wherein the average traveling direction of the entire light beam is the z direction, and the light beam forms a three-dimensional network in the xyz plane. The switch elements according to the first aspect are arranged in a plurality of rows in the x direction and a plurality of columns in the y direction to form one surface. Said face is an x-switched face such that the signal points on the face can be combined with two points whose y-values of the subsequent face are equal and x-values differ by the jump width Cx, or said faces are The signal point on the surface is a y-shaped switch surface such that the x-values of the subsequent surfaces are equal and can be combined with two points whose y-values differ by a jump width Cy. The jump width Cx for each of the plurality of x-type switch faces is not constant, and the jump width Cy for each of the plurality of y-type switch faces is not constant, and is configured from the x-type switch face and the y-type switch face. A port surface or an intermediate port surface that forms a boundary between each step and a region leading to or behind it comprises a photonic crystal wave plate or a photonic crystal polarization splitting prism that adjusts polarization and bends light in a desired direction. Specific examples of the photonic crystal polarization separation prism are as described above.

[3. Variable Optical Attenuator]
A third aspect of the present invention relates to a Variable Optical Attenuator (VOA). The variable optical attenuator according to the present invention comprises a first polarization splitting prism pair, a second polarization splitting prism pair, and a pair of the first and second polarization splitting prisms in the z-axis direction in a three-dimensional space xyz. And a liquid crystal cell interposed therebetween. Each of the first and second polarization splitting prism pairs is a pair in which the photonic crystal polarization splitting prisms overlap in the z-axis direction. The first pair of polarization splitting prisms is a pair of left-handed and right-handed circularly polarized light traveling along the z-axis and two parallel lights of left-handed and right-handed circularly polarized light. Separating into light. The liquid crystal cell converts each of the two parallel lights into a birefringent state of phase difference θ while keeping the principal axis direction constant by voltage driving. The second polarizing prism pair, two serving unnecessary wave with an incident beam which has passed through the liquid crystal cell, and the same polarization state as the incident beam with the power ratio (sin [theta) ² or (cos [theta]) ² light, the power of the residual Split into parallel beams.

  According to the present invention, signal loss and crosstalk between signal lines can be effectively suppressed even in an optical switch circuit having a large number of input / output ports.

It is a principle view of the optical switch using the plane optical circuit which is prior art. It is the schematic of the optical switch using MEMS technology which is a prior art. It is a block diagram at the time of arranging the optical switch of this invention, and it two-dimensionally. It is a block diagram at the time of arranging the optical switch of this invention in three dimensions. It is a figure which shows an example of the polarization splitter prism (split type) which concerns on 1st Embodiment, and the polarization splitter prism (curved type) which concerns on 2nd Embodiment. It is a figure explaining the operation | movement at the time of the vertical incidence of the polarization separation prism which concerns on 1st Embodiment. It is a figure explaining the operation | movement at the time of oblique incidence of the polarization splitter prism which concerns on 1st Embodiment. It is a figure explaining the structure and operation | movement of the optical switch which concern on 1st Embodiment. It is a figure explaining the input port face of the optical switch which has a port of 8x8 concerning a 4th embodiment. It is a figure explaining layering the optical switch which has a port of 8x8 which concerns on 4th Embodiment into four 4x4 blocks. It is a figure explaining that the optical switch which has a port of 8x8 concerning a 4th embodiment is hierarchized into four 4x4 blocks, and a block is replaced in each section. It is a figure explaining the variable optical attenuator using the liquid crystal which concerns on 5th Embodiment.

[1. Basic principle of optical switch element and optical switch circuit]
FIG. 3 is a principle view of a two-dimensional switch circuit of the present invention. It is the optical switch element 304 shown as an oval that plays a central role. The optical switch element 304 is interposed between the first polarization separation prism 301, the second polarization separation prism 303, and the first and second polarization separation prisms in the z-axis direction in the three-dimensional space xyz. And a liquid crystal cell 302. The first and second polarization splitting prisms 301 and 303 are each formed of a photonic crystal polarization splitting prism. Incident light is incident on the optical switch element 304 having such a structure in the z-axis direction. The incident light is circularly polarized rightward or leftward with a traveling direction inclined at a predetermined angle θ with respect to the z axis, and propagates in the xz plane of the optical switch element 304. The liquid crystal cell 302 can transition between at least an isotropic state (first state) to incident light and a birefringence state (second state) having a phase difference of 180 degrees by voltage driving. The optical switch element 304 emits incident light as circularly polarized light having the same traveling direction and the same rotation direction due to such transition of the liquid crystal cell, or the incident light with respect to the z axis It is possible to select whether the light is emitted as circularly polarized light having a traveling direction inclined at an angle -θ and having a reverse rotational direction. In addition, the optical switch element 304 emits the incident light as circularly polarized light having the same traveling direction and the opposite rotation direction due to the transition of the liquid crystal cell, or the incident light is angled with respect to the z axis It is also possible to select whether the light is emitted as circularly polarized light having the same direction of rotation and a traveling direction inclined by -θ.

  As described above, the light switch element 304 switches the polarization state of the incident light beam by the voltage applied to the liquid crystal cell 302. The photonic crystal prism pairs 301 and 303 sandwiching the liquid crystal cell 302 perform one or both of the function of adjusting the direction of the incident beam and the function of bending the outgoing light beam in a desired direction.

  A plurality of light beams whose connection can be switched enter the optical switch circuit from the input surface (input port surface) P, and output surfaces (a plurality of intermediate port surfaces R and T, and a plurality of optical switch elements Q and U). Output port surface) V is reached. In the optical switch circuit shown in FIG. 3, the light beam has an input port surface P, a first optical switch element Q, a first intermediate port surface R, a second optical switch element S, and a second intermediate port surface T. , And the third optical switch element U and the output port surface V in this order. Each stage is a switch sub-circuit divided by two port surfaces and an optical switch element located therebetween, and is referred to as a first stage, a second stage,... Sequentially from the light beam input side. A plurality of switch elements are arranged side by side in a plane direction (y direction or xy direction) at the center of each stage with respect to the traveling direction of light, and form a switch element surface or a switch element array surface. A light beam passing through the point 305 in FIG. 3 is led to the point 306 or 307 depending on the state of the optical switch 304 (whether it is the first state or the second state). The amount of jump from point 305 to point 307 is four. That is, in the input port plane P, the light beam incident on the uppermost point 305 in the y direction is guided to the fourth uppermost point 307 in the y direction on the first intermediate port plane R. . Such jumping of the signal path from the input port surface to the next port surface is referred to as "jumping" in the present specification. The magnitude of this jump is common in the first stage. Similarly, the amount of jumping between the front and back stages is doubled (or halved).

  The input port plane P, output port plane V, and intermediate port planes R and T have photonic crystal wave plates or photonic crystal polarization splitting prisms, and adjust the polarization state of the light beam to be handled to a desired state, and the desired traveling direction It has a function to make it bend. FIG. 4 is a diagram schematically showing the principle of the three-dimensional switch circuit of the present invention.

  By selecting the distance between the input port surface P and the first intermediate port surface R or the distance between the first intermediate port surface R and the second intermediate port surface T, the jump amount of the above signal path is set to 4. Since it can be set to 2 or 1, the required number of switch stages is small. Also, since the light beam propagates in free space or uniform medium, crosstalk does not occur between signal paths. In addition, since the photonic crystal formed on the glass plate and the liquid crystal cell sandwiched between the glass plates are actually laminated and bonded, the mounting is easy.

[2. Photonic crystal polarization splitting prism]
An optical switch circuit using a photonic crystal polarization separation prism will be described.
Photonic crystals are known, but may be formed, for example, by an auto cloning method (see Patent Document 3). A photonic crystal is a structure whose refractive index changes periodically at a cycle shorter than the operating wavelength of light to be guided. In particular, the wave plate is preferably a photonic crystal formed by an autocloning action. A photonic crystal is a minute periodic structure that functions as an optical element (for example, a polarization separation prism or a wavelength plate). As a specific method of manufacturing a photonic crystal, as disclosed in Patent Document 3, two or more types of different refractive indexes are different on a substrate having a one-dimensionally or two-dimensionally periodic unevenness. There is a method of manufacturing an optical element by sequentially and sequentially laminating a substance (transparent body) and using sputter etching alone or simultaneously with film formation on at least a part of the lamination. This method is also called a self cloning method. And the photonic crystal formed by this auto cloning method is called a self cloning type photonic crystal. In addition, the technique which comprises a wavelength plate using a self-cloning type | mold photonic crystal is well-known. For example, as another method of producing a photonic crystal, there is a method of producing periodic voids by irradiating glass with a femtosecond laser.

  The transparent bodies forming the self-cloning type photonic crystal are fluorides such as amorphous silicon, niobium pentoxide, tantalum pentoxide, titanium oxide, hafnium oxide, silicon dioxide, aluminum oxide, magnesium fluoride and the like. It is preferable that it is either. From among these, two or more species having different refractive indices can be selected and used as a photonic crystal. For example, combinations of amorphous silicon and silicon dioxide, niobium pentoxide and silicon dioxide, and tantalum pentoxide and silicon dioxide are desirable, but other combinations are also possible. Specifically, the self-cloning type photonic crystal has a structure in which high refractive index materials and low refractive index materials are alternately stacked. The high refractive index material is preferably tantalum pentoxide, niobium pentoxide, amorphous silicon, titanium oxide, hafnium oxide or a combination of two or more of these materials. The low refractive index material is preferably silicon dioxide, aluminum oxide, a fluoride containing magnesium fluoride, or a combination of two or more of these materials.

  More specifically, the photonic crystal polarization splitting prism (optical element) used in the present invention is a wave plate (split type) in which the principal axis orientation is different for each region, or a wave plate in which the principal axis orientation changes continuously. (Waveform type), and the wave plate of each area | region is comprised with the photonic crystal which has a periodic structure in surface, and the said periodic structure was laminated | stacked in the thickness direction. The photonic crystal may be formed by an autocloning method.

  The split-type photonic crystal polarization separation prism forms a photonic crystal on a substrate having a pattern as shown in 501 of FIG. 5 using a self-cloning method. In the xy plane, a plurality of regions D are periodically and repeatedly formed in at least the x-axis direction. The lengths in the x-axis direction of the plurality of regions D are preferably equal. Each region D is further divided into a plurality of subregions in the x direction. The number of divisions of each area D can be set to 3 to 21 and is preferably an odd number such as 5, 7, 9, 11, 13, 15, 17, 19 or the like. The subregions included in each region D preferably have substantially the same width in the x direction. "Substantially equal width" means to allow an error of ± 2% based on the width d of the subregion located at the center in the x direction.

  Further, a plurality of grooves are periodically formed in each sub region. The widths of the grooves are substantially all equal. Also, the grooves are formed from end to end in the x direction in each sub region. In the region D, in the subregion located at the center in the x direction, grooves extending parallel to the x axis direction are periodically and repeatedly formed in the y direction. On the other hand, in the region D, in the sub-regions located at both left and right ends in the x direction, grooves extending in parallel to the y direction are formed. Therefore, the angle θ between the grooves formed in the left and right subregions is 90 degrees with respect to the groove formed in the central subregion. In such a subregion, the length of the groove is the largest and corresponds to the effective dimension in the y direction of the entire element.

  In addition, a plurality of sub-regions are located on the left and right, respectively, between the central sub-region and the sub-regions at both left and right ends. Then, a plurality of grooves are periodically and repeatedly formed in the y direction also in each sub region located between them. Also, the angles of the grooves formed in certain sub-regions are all equal. However, the angle θ of the groove of each sub-region located between the two sub-regions is set so as to gradually approach 90 degrees as it approaches the sub-regions at both left and right ends from the central sub-region. For example, four subregions are provided between the central subregion and the left and right subregions, respectively. The angle of the groove in the central subregion is 0 degrees, and the angle of the grooves in the left and right subregions is 90. In this case, the inclination angle θ becomes steeper by 22.5 degrees in order from the area closer to the central sub-area. Thus, each region D is divided into a plurality of sub-regions having equal widths in the x-direction, grooves having equal angles are periodically formed in each sub-region, and sub-regions located at the center in the x-direction The angle of the groove monotonously increases toward the sub-regions located at both ends.

  Under such premise, in each sub-region, the inter-groove unit period p (see FIG. 5) of the periodic structure is a quarter or less of the wavelength of the incident light (for example, selected from 400 nm to 1800 nm) It becomes. The lower limit value of the inter-groove unit period p is 40 nm. Moreover, in the thickness direction (z direction), the unit period of the two types of transparent media having different refractive indexes is also 1⁄4 or less of the wavelength of light. The lower limit value of the unit cycle in the thickness direction is 40 nm. Then, the in-plane minimum value d (see FIG. 5) of the groove length among the plurality of regions D is 1 or more times the inter-groove unit period p described above. The upper limit of the in-plane minimum value d of the groove length is considered to be 50 times the inter-groove unit period p described above. Here, as shown in FIG. 5, since the widths in the x direction of a plurality of sub-regions formed in a certain region D are all equal, the in-plane minimum value d of the groove length in the region D is basically The length of the groove formed in the sub region located at the center of this region D is obtained. The length of the groove tends to be longer as the groove in the region on the left and right sides in the x direction.

In the three-dimensional space xyz, the traveling direction of light is taken as the z axis. An optical element (polarization separation prism) having a period D is disposed in the xy plane, the inclination from the x axis of its slow axis direction is θ, and light emitted upon entering clockwise polarized light has the following formula Is represented by
When the phase difference φ of the optical element (polarization separation prism) is π, the above equation can be arranged as follows:
It becomes. Thus, the emitted light is converted to counterclockwise circularly polarized light. Further, since θ depends only on x, a phase difference depending on x is generated. When θ changes in proportion to x in proportion to 0 during one period D, the slope of the phase of the first term and the second term of the output wave at x = D, x = 0, It changes by 2π. Therefore, the light is bent by ψ = sin ⁻¹ (λ / D) with respect to the z axis and the light is emitted. Similarly, when left-handed circularly polarized light is incident, light is converted to right-handed circularly polarized light, bent by −ψ = −sin ⁻¹ (λ / D) with respect to the z-axis, and emitted.

The operation in the case where the light beam is vertically incident will be described with reference to FIG. A straight line PQR having straight lines L ₁ and L ₂ equal in height in the y direction parallel to the x axis, passing from a point P on L ₁ through a point Q on the optical element 601 to a point R on L ₂ Define. In this case PQ is perpendicular to the element 601. For the convenience of description, it is assumed that the refractive indices of the media on both sides of the optical element are equal. The light ray QR ₊ has a wave number 2π / D in the x direction, and QR ₋ has the same −2π / D, with R ₋ and R ₊ taken on the straight line L ₂ across R. When clockwise circularly polarized light is incident along PQ, it becomes left-handed circularly polarized light and is bent to QR ₊ (see FIG. 6A). On the other hand, the case where the incident polarization is left-handed circular polarization will be described using FIG. 6 (b). When left-handed circularly polarized light is incident perpendicularly to the element 602, it becomes right-handed circularly polarized light and is bent into QR ₋ (see FIG. 6B).

  In the above description, the wave numbers in the y direction are all 0 for the sake of convenience, but the same applies to the case where they have a common wave number. Further, also in the case where the refractive index of the medium that fills the space on both sides of the optical elements 601 and 602 is not common (for example, air and glass), the light rays are associated not by the spatial shape but by the wave number. When a plurality of optical elements are used, the direction defining the cycle may be selected in a different direction for each element, and divided into a plurality of regions in one element, each having a different cycle direction and cycle. It may be

Further, the operation in the case where a ray of light is obliquely incident will be described with reference to FIG. A straight line PQR having straight lines L ₁ and L ₂ equal in height in the y direction parallel to the x axis, passing from a point P on L ₁ through a point Q on the optical element 701 to a point R on L ₂ Define. For the convenience of description, it is assumed that the refractive indices of the media on both sides of the optical element are equal. For light of wavelength λ in the propagation medium, the light rays PQ, QR have equal wave numbers in the x direction. _P across the P on the straight line _{L 1} -, take _{P +,} light _{P +} Q has a wave number + 2 [pi / D in the x direction, beams P _- Q is also made to have -2.pi. / D. When the light ray P + Q is left-handed circularly polarized light, the emitted light becomes right-handed circularly polarized light, is bent and travels in the QR direction. On the other hand, when the light ray P-Q is right-handed circularly polarized light, the emitted light becomes left-handed circularly polarized light, is bent, and also travels in the QR direction. In this case, the QR direction is perpendicular to the elements 701 and 702.

  In the above description, the wave numbers in the y direction are all 0 for the sake of convenience, but the same applies to the case where they have a common wave number. Further, also in the case where the refractive index of the medium filling the space on both sides of the optical elements 701 and 702 is not common (for example, air and glass), the light rays are associated not by the spatial shape but by the wave number. When a plurality of optical elements are used, the direction defining the cycle may be selected in a different direction for each element, and divided into a plurality of regions in one element, each having a different cycle direction and cycle. You may

  Here, although the split type photonic crystal polarization separation prism has been described, the pattern 501 in FIG. 5 can be replaced with the pattern 502 in FIG. The pattern 502 in FIG. 5 shows a so-called curved photonic crystal polarization splitting prism. That is, the basic configuration of the curved photonic crystal polarization splitting prism (optical element) is a wavelength plate formed of an xy plane in the three-dimensional space x, y, z, and made of a photonic crystal stacked in the z-axis direction (More specifically, a half wave plate). In the wave plate, a band-like width D area parallel to the y-axis direction is repeated in the x-axis direction. The groove direction of the photonic crystal is a curve which coincides with the curve 503 y = (D / π) log (| cos (πx / D) |) + constant in the range of the discretization error.

In addition, as shown in 502 of FIG. 5, by making the pattern (convex part or concave part) of the photonic crystal into a curved shape, the pattern becomes sparse in the central part within one cycle, and denser as it gets closer to the end This makes it difficult to make patterns. Therefore, the pitch between patterns in the central portion is taken as a reference as in 504, and it is set as p ₀ . The two patterns are merged at a position where p ₀ falls below a certain threshold pitch. Although the pitch immediately after merging is 2p _0, it becomes denser as it gets closer to the end, so it merges again when it becomes smaller than the threshold length. By repeating the above operation, an ideal optical axis distribution can be realized while changing within a certain range of the pitch. Assuming that the threshold pitch is 0.5 p ₀ , the change range of the pitch is between 0.5 p _{0 and} 2.0 p ₀ . That is, they are geometrically arranged such that the ratio of the maximum value to the minimum value of one of the intervals between adjacent projections and depressions is within 4 times, preferably within 2 times, and the other is branched and merged. . In the example shown in 504, the white part is a recess and the black part is a protrusion. That is, in the case of a wavelength plate (curved type) in which the main axis orientation changes continuously, assuming that the pitch p of the convex portion (pitch when the pattern is a straight line) is p ₀ , 0.5 · p ₀ ≦ p ≦ 2 The projections or recesses are geometrically arranged so as to branch and merge so as to be within p ₀ .
By continuously changing the axial orientation in this manner, higher efficiency can be realized as compared with the divided ones.
Photonic crystal polarization separation prisms have, for example, free space wavelength λ ₀ 1.55 μm
・ High refractive index material a-Si
・ Low refractive index material SiO ₂
· Prism period D 10 μm
・ Slow axis refractive index n _s 2.713
・ Fast axis refractive index n _f 2.486
・ Thickness of the entire stack λ ₀ / (ns _s −n _f ) × φ / (2π)
If the medium is quartz, the separation angle Φ = 6.1 degrees.

  Further, as in the optical switch element 804 shown in FIG. 8, two curved photonic crystal separating prisms 801 and 803 are disposed so that the directions of the curves are opposite to each other, and a glass substrate is interposed therebetween. It is also possible to arrange a liquid crystal cell 802 across nematic liquid crystal molecules. A transparent electrode is formed on the glass substrate, and an electric field is applied perpendicularly to the glass. For example, when a birefringence control type liquid crystal is used, it becomes isotropic when an electric field is applied, and when an electric field is not applied, birefringence depending on the orientation can be given.

  As a result, the liquid crystal cell 802 can turn on / off the phase difference, and if the appropriate voltage is selected, the phase difference becomes (2n + 1) π (n = 0, 1, 2,...) When the voltage is off. Functions as a variable birefringence plate with zero retardation. When the phase difference is zero, when clockwise circularly polarized light is incident, it is clockwise, and when left circularly polarized light is incident, it is emitted counterclockwise. Therefore, as in the optical switch element 804 shown in the dotted-lined ellipse in FIG. 8, when the phase difference is zero, the light coming from above travels upward and the light coming from below travels downward. On the other hand, when the phase difference of the liquid crystal cell is π, clockwise circularly polarized light is emitted as left-handed circularly polarized light, and left-handed circularly polarized light is emitted as right-handed circularly polarized light. As a result, the light coming from the top goes down and the light coming from the bottom goes up. As described above, the light path can be switched by turning on and off the voltage applied to the liquid crystal cell 803.

  The substrate of the liquid crystal cell 802 may be shared with the substrate of the photonic crystal or may be independent. The function of the liquid crystal cell is to switch the phase difference, and the type of liquid crystal and the method of applying an electric field may be appropriately known. Of course, the directions of the patterns of the polarizing prisms on the input side and the output side may be the same. It only reverses the light path when the voltage is on and off. In addition, the liquid crystal cell may have a phase difference (2n + 1) π (n = 0, 1, 2,...) When a voltage is applied, and may have no phase difference when the liquid crystal cell is off. It only changes the output direction when on or off.

[3.2-dimensional optical switch circuit]
The configuration for reducing the interference and crosstalk between adjacent lines of signal light by making the optical switch circuit into a two-dimensional type (planar arrangement type) is shown below.

FIG. 3 shows an 8 input × 8 output optical switch circuit. Light having a predetermined free space wavelength and a predetermined polarization (for example, right-handed circularly polarized light) is incident on the input port surface P at the left end. The input port P is configured by a polarization separation prism row, and the first to fourth ports 308 are arranged so that the light to be output is left-handed circularly polarized light and emitted obliquely downward; The eight ports 309 are arranged so that the light to be output becomes right-handed circularly polarized light and is emitted obliquely upward. In the optical switching element column Q, whether voltage is applied to the liquid crystal cell 302 and the liquid crystal cell becomes isotropic, or whether the voltage applied to the liquid crystal cell 302 becomes zero and the liquid crystal cell becomes a wave plate with a phase difference of 180 °. select. Thereby, the light from P column m port is
When m = <4, it is input to the m-th port or the (m + 4) -th port of R column,
When m> = 5, it is input to the m-th port or the (m-4) -th port of R column,
Combined. In contrast to coupling between adjacent lines only as shown in FIG. 1, in the optical switch circuit of the present invention, destinations selected by on / off of voltage are separated by four ports. That is, the jump width of the switch is four.

  As shown in FIG. 3, in the optical switch array S after the intermediate port plane R, switching between lines separated by 2 is selected. Further, in the optical switch element array U after the intermediate port surface T, switching between adjacent ports (between lines separated by only one) is performed.

As apparent from the above description, in the optical switch element of N stages, path selection (switching) is possible such that the difference between ports is at most 2 ^{N -1} , and adjacent lines as shown in FIG. Compared to switches only between them, the number of switches can be significantly reduced, and insertion loss and crosstalk can be significantly reduced.

  Specific examples of the structure and characteristics of the optical switch circuit are shown. N = 3 (8 x 8) switch with channel spacing of 0.5 mm, prism pattern period of 5 um, propagation space of silica glass, light wavelength of 1550 nm, in-medium bending angle of 12.3 degrees, optical switch circuit If the radius of the input beam is chosen to minimize the overlap of the tails of the light beam after propagation, the propagation distance is 18.4 mm and the inter-signal crosstalk is -87.2 dB. Losses and crosstalk can be made very small.

[4.3-dimensional optical switch circuit]
The example which comprises the optical switch circuit of a three-dimensional type (space wiring type | mold) using the optical switch element which concerns on this invention is shown.

FIG. 4 shows an optical switch circuit of 16 inputs × 16 outputs (4 rows and 4 columns each). Light having a predetermined free space wavelength and a predetermined polarization (for example, right-handed circularly polarized light) is incident on the left end input port surface P portion. Optical switch element arrays Q, S, U, and W are disposed between the polarization separation prism arrays disposed on the port surfaces P, R, T, V, and X, respectively. In the example shown in FIG. 4, a ray incident on the input port plane P proceeds parallel to the xz plane, and path selection between ports in the x direction is performed by the first optical switch element array Q, so that The jump width 2 is coupled to the first intermediate port surface R of the next stage. The vertical point row (first optical switch element row Q) between the input port surface P and the first intermediate port surface R is a layer lower than the optical switch element in the y direction in the y direction. Also indicate that they are similarly provided. The same applies to the following second to fourth optical switch element arrays S, U, and W. Subsequently, in the section between the first intermediate port surface R and the second intermediate port surface T, the light beam travels parallel to the yz plane, and the path between the ports in the y direction by the second optical switch element array S A selection is made and coupled with the second intermediate port face T of the next stage with a jump width 2 in the y-direction. Similarly, in the section between the second intermediate port surface T and the third intermediate port surface V, the light beam travels parallel to the xz plane, and switching is performed between the ports adjacent in the x direction. In the section between the intermediate port plane V and the output port plane X, the light beam travels parallel to the yz plane, and switching is performed between adjacent ports in the y direction.
Specific examples of the structure and characteristics of the optical switch circuit are shown. In the 16 input × 16 output switch of FIG. 4, the channel spacing is 0.25 mm, the prism pattern period is 10 μm, the propagation space is silica glass, the light wavelength is 1550 nm, the in-medium bending angle is 6.1 degrees, and the optical switch circuit If the radius of the input beam is chosen so as to minimize the overlap of the tails of the light beam after propagation of S., the propagation distance is 14.1 mm and the inter-signal crosstalk is -56.8 dB. Losses and crosstalk can be made very small.

  In the embodiment shown in FIG. 4, the number of input ports and output ports is 4 × 4 = 16. The number of input / output ports can be increased to 8 × 8 = 64, respectively, in the same way. The penalty, that is, the increase in the number of stages required, is only two.

[5. Multiple block method]
There is a multiple block method as an embodiment for variably connecting the signal port on the input side and the signal port on the output side with a high degree of freedom. In the multiple block method, free compatibility can be provided between blocks of the same size. In order to make the effects of the invention easy to understand, the principle of the structure of FIG. 4 will be described as it is by enlarging the size from 4 × 4 to 8 × 8. FIG. 9 is an input / output surface of a switch circuit having 8 × 8 ports on both the input surface and the output surface. Divide the input surface into four 4x4 blocks. Furthermore, hierarchy can be realized such as dividing each 4 × 4 block into four 2 × 2 blocks. Returning to the original, as shown in FIG. 10, the 4 × 4 block divided into four may be freely rearranged according to the following configuration.

  FIG. 11 shows that the PR part of the circuit of FIG. 4 is increased in size from 4 × 4 to 8 × 8 (section A), the same as the RT section (section B), and the PR part is again 4 × 4 to 8 × It shows three sections of what is 8 (section C). In order to display a ray moving horizontally or vertically in a three-dimensional space, section A in FIG. 11 represents horizontal movement, and in this section, the p-axis represents the x-axis. Section C is also the same. Section B represents the movement of the vertical plane of the ray, where the p-axis represents the y-axis. In other words, in the section A between the input port surface and the first intermediate port surface, the value of y of the ray is constant and only the value of x changes. The same applies to section C. In the section B, the value of x of the ray is constant and only the value of y changes. That is, in the sections A and C, (1 + 3) and (2 + 4) in FIG. 10 can be interchanged, and in the section B, (1 + 2) and (2 + 4) in FIG. It is shown in FIG. 10 in the 3-section structure of FIG. The blocks 1, 2, 3, 4 of size 4 × 4 on the input surface of section A can be arbitrarily replaced without changing the internal configuration on the output surface of section C.

  According to the same concept, subblocks of size 2 × 2 that are inside blocks of size 4 × 4 can also be arbitrarily replaced in three sections. The same operation as above is also possible for subblocks of size 2 × 2. Alternatively, if sub-blocks of size 2 × 2 are arranged with 4 signals each including two signals each having two polarization degrees of freedom from two optical fibers, swapping corresponds to swapping between fibers. Only the section is enough.

Summarizing the above, there are 2 ^N × 2 ^N signal ports each on the input and output planes, which are divided into four layers to realize free replacement between blocks of the same size. -1) One switch face will be sufficient, and the advantage is more remarkable as N is larger.

[6. Variable Optical Attenuator]
Subsequently, a variable optical attenuator to which the principle of the optical switch element is applied will be described.
FIG. 12 shows an example of the variable optical attenuator. The variable optical attenuator includes a first polarization splitting prism pair 1201 and 1202, a second polarization splitting prism pair 1204 and 1205, and their first and second polarizations in the z-axis direction in the three-dimensional space xyz. And a liquid crystal cell 1203 interposed between the separation prism pair. The first and second polarization splitting prism pairs 1201 and 1202 and 1204 and 1205 are pairs of photonic crystal polarization splitting prisms overlapping in the z-axis direction. The first polarization splitting prism pair 1204 and 1205 is a pair of left-handed and right-handed circularly polarized light, which travels along the z-axis and is a single light of a mixed state of left-handed and right-handed circularly polarized light. Separates into parallel lights of books. The liquid crystal cell 1203 converts each of the two parallel lights into a birefringent state of phase difference θ while keeping the principal axis direction constant by voltage driving. The second pair of polarizing prisms 1204 and 1205 is not required to have the remaining power and the light of the same polarization state as the incident beam having the power ratio (sin θ) ² or (cos θ) ² for the incident beam having passed through the liquid crystal cell 1203 Split into two parallel beams.

  When the voltage applied to the liquid crystal cell 1203 is continuously changed, the phase difference to be applied can also be continuously changed. The photonic crystal polarization splitting prisms 1201, 1202, 1204, 1205 are all in the same direction. It can be said that mixed polarized light incident on the first polarization splitting prism pair 1201 and 1202 is synthesized by changing the ratio of arbitrary right-handed circularly polarized light and left-handed circularly polarized light. Therefore, the first polarized light separating prism pair 1201 and 1202 separates the light into clockwise and counterclockwise circularly polarized light by the first prism 1201 and the second prism 1202 no matter what polarized light is incident. The direction is corrected in parallel, and the liquid crystal cell 1203 is made incident. In the liquid crystal cell 1203, the polarization state changes according to the given phase difference. For example, when right-handed circularly polarized light is incident, it becomes elliptically polarized light as the phase difference increases, and linearly polarized light when the phase difference is π / 2. When the phase difference further increases and the phase difference becomes π, it becomes counterclockwise circularly polarized light. Thus, in the liquid crystal cell 1203, the polarization state is changed and emitted according to the applied voltage. Therefore, the third polarizing prism 1204 further separates the light into the upper and lower portions depending on the polarization state of the incident light. This separation ratio depends on the phase difference given by the liquid crystal cell. The necessary light is combined by the fourth polarizing prism 1205 and output from the output port 1206. Unwanted light is separated into upper and lower parts by the polarization separation prism 1204, then converted back into parallel light by the polarization prism 1205, and emitted from other than the output port 1206. These unnecessary light may be absorbed by, for example, a black object.

  The amount of light output in this manner can be controlled by the phase difference given by the liquid crystal cell. Therefore, the configuration of FIG. 12 functions as a variable attenuator that can control the amount of transmitted light by the voltage applied to the liquid crystal cell. The polarization state of the emitted light is the same as that of the incident light. The present variable attenuator does not require alignment between elements, and has an industrially excellent feature.

  In optical switches with a large number of input / output ports, those with excellent characteristics due to insertion loss and crosstalk have been difficult to realize, but a large number of input / output ports can be realized by the simple composite structure of the photonic crystal prism and liquid crystal cell. It can be realized and is effective in an optical communication system.

Claims (11)

An optical switch element in which incident light that is circularly polarized clockwise or counterclockwise with a traveling direction inclined at a predetermined angle θ with respect to the z axis in the three-dimensional space xyz propagates in the xz plane,
a first polarization separation prism, a second polarization separation prism, and a liquid crystal cell interposed between the first and second polarization separation prisms in the z-axis direction,
Each of the first and second polarization splitting prisms is a photonic crystal polarization splitting prism,
The liquid crystal cell can transition between an isotropic state with respect to at least the incident light and a birefringence state having a phase difference of 180 degrees by voltage driving.
By the transition of the liquid crystal cell, the incident light is emitted as circularly polarized light having the same traveling direction and one rotation direction of the same or the opposite, or a traveling direction inclined at an angle -θ with respect to the z axis An optical switch element that can be selected to be emitted as circularly polarized light in the same or the other opposite rotational direction. In the optical switch element according to claim 1,
The photonic crystal polarization splitting prism
A photonic crystal half-wave plate formed in the xy plane and stacked in the z-axis direction in the three-dimensional space xyz;
have one or more regions that are single or repeated in the x-axis direction,
The area is divided into a plurality of band-like sub-areas in the x-axis direction,
The groove direction of the photonic crystal is
In the region, the angle with respect to the y-axis direction changes stepwise in the range of 0 ° to 180 °, and
In the sub-regions, the angles with respect to the y-axis direction are uniform,
light incident in the z-axis direction,
clockwise circularly polarized light in a direction from the z axis toward the x axis,
left-handed circularly polarized light in the direction from the z axis towards the -x axis by the same angle,
An optical switch element that emits light separated and converted. In the optical switch element according to claim 1,
The photonic crystal polarization splitting prism
A photonic crystal half-wave plate formed in the xy plane and stacked in the z-axis direction in the three-dimensional space xyz;
have one or more regions that are single or repeated in the x-axis direction,
The groove direction of the photonic crystal is a curve represented by y = (D / π) log (| cos (πx / D) |) + constant, where D is a constant, in the approximate range of the average period of the groove, And,
light incident in the z-axis direction,
clockwise circularly polarized light in a direction from the z axis toward the x axis,
left-handed circularly polarized light in the direction from the z axis towards the -x axis by the same angle,
An optical switch element that emits light separated and converted. An optical switch circuit comprising a plurality of the optical switch elements according to claim 1 arranged two-dimensionally or three-dimensionally. A light beam type multistage comprising switch sub-circuits of N stages in the z-axis direction in the three-dimensional space xyz, and a plurality of optical switch elements according to claim 1 arranged in the x-axis direction in the switch sub-circuits of each stage A switch circuit,
The average direction of travel of the entire light beam is the z-axis direction,
The light beam creates a two-dimensional network in the xz plane,
2 ^N output ports sequence of a predetermined polarization in the 2 ^N input ports columns and output surface of the predetermined polarized light at input plane are aligned in the x-axis direction,
Each of the optical switch elements directs the emission direction of the light beam so as to direct the light beam incident thereon to one of the other two optical switch elements having different positions in the x-axis direction of the next stage. You choose
In the M-stage switch circuit (M <N), or the m-th of the ports arranged in the x direction therein binds to the m-th port of the next (M + 1) stages, or 2 to m ^{(N- M)} Do you combine with the port that only jumped,
A port surface or an intermediate port surface that forms a boundary between each step and a region leading to or behind it comprises a photonic crystal wave plate or a photonic crystal polarization separation prism that adjusts polarization and bends light in a desired direction,
An optical beam type multistage switch circuit capable of converting an optical signal train incident on 2 ^N ports on an incident surface to an optical signal train different from an input, which is emitted from 2 ^N ports on an outgoing surface. The light beam type multistage switch circuit according to claim 5, wherein the order of the first stage, the second stage, ..., and the Nth stage is arbitrarily replaced. A light beam type multistage switch circuit in a three-dimensional space xyz,
The average direction of travel of the entire light beam is the z-direction,
The light beam creates a two-dimensional network in the xz plane,
A plurality of switch elements according to claim 1 are arranged in the x direction to form one stage,
A plurality of the stages are arranged in the z direction to form a switch circuit,
The coupling destination of the mth switching element in the A-th stage is either the p-th or q-th (q> p) switching element of the (A + 1) -th stage which is the next stage, and the jump width q The value of -p is uniform regardless of m for one stage A, and different stages from stage A can be given different jump widths,
A port face or an intermediate port face forming a boundary between each step and a region leading to or behind it is provided with a photonic crystal wave plate or a photonic crystal polarization splitting prism for adjusting polarization and bending light in a desired direction. circuit. In three-dimensional space xyz, a switch sub-circuits of 2N stages in the z-axis direction, 2 ^N rows in the x direction aligned parallel to the optical switch element xy plane according to claim 1 in the switch sub-circuits at the respective stages , Light beam type three-dimensional multistage switch circuit forming 2 ^N rows in the y direction,
The light beam creates a three-dimensional network in xyz space,
The average direction of travel of the entire light beam is the z-direction,
The input port group of the predetermined polarization in the input plane and the output port group of the predetermined polarization in the output plane are aligned in the xy plane, and the light beam incident on each of the optical switch elements is in the x-axis direction on the next intermediate port plane It is possible to select whether to lead to one of the other two intermediate ports different in the position of or to one of the other two ports with different values on the next intermediate port surface,
M 1, 2. . , N-1 if any one
On the (2M-1) th intermediate port surface, it is determined whether the mth row of light beams is connected to the mth row on the 2Mth intermediate port surface or the m + 2th ^(NM) row. It is selectable,
On the second M intermediate port surface, it is possible to select whether the nth column of light beams is to be connected to the nth column on the (2M + 1) th intermediate port surface or to be connected to the n + 2 ^(NM) column And
As an effect of all 2N stages, a row of optical signals incident on 2 ^N × 2 ^N ports on the entrance plane is divided into another optical signal series of 2 ^N × 2 ^N predetermined polarization ports on the exit plane. A light beam type multistage switch circuit that can be converted. The light beam type multistage switch circuit according to claim 8, wherein the order of the first stage, the second stage,... And the second N stages is arbitrarily replaced. A light beam type multistage switch circuit in a three-dimensional space xyz,
The average direction of travel of the entire light beam is the z-direction,
The light beam forms a three-dimensional network in the xyz plane,
The switch elements according to claim 1 are arranged in a plurality of rows in the x direction and a plurality of columns in the y direction to form one surface.
Said face is an x-switched face such that the signal points on the face can be combined with two points whose y-values of the subsequent face are equal and x-values differ by the jump width Cx, or The signal point on the surface is a y-shaped switch surface such that the x-values of the subsequent surfaces can be combined with two points that are equal and y-values differ by a jump width Cy,
The jump width Cx for each of the plurality of x-type switch faces is not constant, and the jump width Cy for each of the plurality of y-type switch faces is not constant, and is composed of the x-type switch face and the y-type switch face,
A port face or an intermediate port face forming a boundary between each step and a region leading to or behind it is provided with a photonic crystal wave plate or a photonic crystal polarization splitting prism for adjusting polarization and bending light in a desired direction. circuit. A first polarization splitting prism pair, a second polarization splitting prism pair, and a liquid crystal cell interposed between the first and second polarization splitting prism pairs in the z-axis direction in the three-dimensional space xyz ,
Each of the first and second polarization splitting prism pairs is a pair in which the photonic crystal polarization splitting prisms overlap in the z-axis direction,
The first pair of polarization separation prisms is a pair of left-handed and right-handed circularly polarized light, which travels along the z-axis and is a single light of a mixed state of left-handed and right-handed circularly polarized light. Split into parallel light,
The liquid crystal cell converts each of the two parallel beams into a birefringence state of phase difference θ by voltage driving while keeping the principal axis direction constant.
It said second polarizing prism pairs, the incident beam that has passed through the liquid crystal cell, having a power ratio (sin [theta) ² or (cos [theta]) ^2, unnecessary wave with the light of the same polarization state as the incident beam, the power of the residual Variable optical attenuator that splits into two parallel beams.