JP2020144165A - Foldable integrated optical circuit - Google Patents

Abstract

To provide an optical circuit which can realize processing of multistage optical signals by fewer optical elements.SOLUTION: The present invention relates to an optical circuit 100 for processing an optical signal, including: a main substrate 110 where a plurality of functional regions 111, 112, 113, and 114 are formed on one flat surface by patterning; and a plurality of reflection members 115, 121, 131, 141, and 116 facing the flat surface of the main substrate, the reflection members reflecting light passing through the functional regions. Light which has entered the optical circuit is reflected by the reflection members and thus passes through each of the functional regions.SELECTED DRAWING: Figure 8

Description

The present invention relates to an optical circuit including a substrate in which a plurality of functional regions are collectively formed on one plane. Specifically, the position of each functional area is formed by collectively forming a plurality of functional areas formed by a plurality of transparent substrates on one plane geometrically and geometrically in the optical circuit of the prior art. It is related to the technology that realized the simplification of the combination, the cost reduction, and the miniaturization of the circuit.

Lenses, prisms, half mirrors, and the like are extremely widely used as optical elements that realize bending, separation, branching, merging, and redistribution of light. Many of them have a three-dimensional shape like a convex lens and a concave lens, and are manufactured as individual functions, so that it is often difficult to integrate and miniaturize them. In recent years, a technique (called gradient metasurface) in which the surface of a transparent substrate is finely processed, the phase of the light beam transmitted perpendicular to it is changed for each location, and the wave surface is tilted to manipulate the propagation after transmission (gradient metasurface). ) Is progressing.

It is not uncommon for the amount of deformation of the wave surface required at that time to be several times or tens of times the wavelength. On the other hand, the amount of phase change of light passing through the surface is practically a fraction to several times that of 2π radians, so it is necessary to perform an operation to return the amount of phase change to zero in a sawtooth wave for each 2π radians. ..

In the above-mentioned operation of returning the amount of phase change to zero in a sawtooth wave every 2π radians, light scattering near the discontinuity and the accompanying amplitude and phase errors are inevitable. The following means are known as a method for alleviating this (Patent Document 1). That is,
(A) Micro 1/2 wave plates having various orientations for each region are arranged on the substrate surface without gaps.
(B) Utilizing the property that the phase transition received when circularly polarized light passes through the region is equal to twice the angle θ formed by the main axis with respect to a certain reference direction.
As far detail, the electric field of the light entering in FIG. 1, for example, _{E x = E 0 cos (ωt} ), E y = E 0 sin (ωt)
In the case of circularly polarized light given in, if the ξη axis is taken as shown in FIG. 1 and a 1/2 wave plate having a main axis in the ξη axis direction is inserted, the transmitted light becomes circularly polarized light in the opposite direction and the relative phase is 2θ. It is known that only changes (Non-Patent Document 1).

On the other hand, the coherent method is widely used in modern optical communication.
In small optical subsystems such as optical transceivers, the means for transmitting light from transmission points T ₁ , T ₂ , ... T _n to reception points R ₁ , R ₂ , ... R _m with branching or merging is
(1) Planar Lightwave Circuit (PLC) (Patent Document 2)
(2) Spatial Optical Circuit (MOC) in which individual parts such as lenses and prisms are spatially arranged in different places (Patent Document 3).
Is the main one. In the present invention, there are a plurality of planes xy ₁ , xy ₂ , xy ₃ , ... perpendicular to the direction of light transmission (z direction), and a signal distribution function, a transfer function, and a signal distribution function between signal points on the planes. And, it has a polarization separation function, and an operation of sending light with branching and merging is performed by a group of optical elements having a periodic structure axis having a plurality of directions.

For example, a PLC circuit that realizes a 90-degree hybrid of an ICR (Intradyne Coherent Receiver) for coherent communication as shown in Patent Document 2 has the following restrictions.
(1) In a PLC circuit, crossovers often occur on the circuit, but interference between signals is difficult to avoid due to the two-dimensional structure.
(2) For each orthogonally polarized light, it is necessary to keep the phase difference between the locally oscillating light of the I phase and the locally oscillating light of the Q phase at π / 2 radians with reference to the instantaneous phase of the signal light. This imposes strict requirements on line length error and line width error between lines, which means low yield and high price of products.
Such a restriction is achieved by arranging polarized gratings (201, 202, 203, 204) as shown in FIG. 2 to make the wiring three-dimensional.
-The crossing of optical paths can be realized without signal interference.-It can be solved by the feature that the symmetry of the structure is high and it is easy to match the line lengths with high accuracy (Patent Document 4).

It has also been proposed to apply an optical circuit similar to coherent communication to object detection by LIDAR (Light Detection and Ranging) (Non-Patent Document 3). The light emitted from one light source is separated into two, one is used as the light returned by hitting the object, and the other is used as it is, and the two lights interfere with each other to increase the distance to the object and the speed of the object. Can be measured. According to the present invention, this interference circuit can be realized compactly and at low cost.

On the other hand, it is possible to realize an optical switch used in optical communication by combining a polarizing grating and a liquid crystal.
An optical switch circuit that switches the paths of a plurality of optical signals is an important component in optical communication. FIG. 2 is a principle diagram showing a typical structure of an optical switch circuit, and shows a 4-input 4-output thermo-optical effect switch circuit connected by a waveguide in a plane.

As other important optical switch circuits used for optical communication, a wavelength selection switch (WSS: Wavelength Selective Switch), a MEMS type switch, and the like are known. There is a three-dimensional type optical switch circuit of the MEMS type. In the three-dimensional type MEMS, in order to increase the degree of freedom in selecting the signal path coupled from the optical fiber group on the input side to the optical fiber group on the output side, the two dimensions of the corresponding mirror of the MEMS micromirror array. It is necessary to control the tilt in multi-gradation analog, which is accompanied by great difficulty.

D. Lin, P. Fan, E. Hasman and M. Brongersma, "Dielectric gradient metasurface optical elements, Science, Applied Optics, 18 July 2014, pp. 298-302. N. Yu and F. Capasso, “Flat optics with designer metasurfaces”, Nature materials, 23 January 2014, pp.139-149. Cristopher V. Poulton, Ami Yaacobi, David B. Cole, Matthew J. Byrd, Manan Raval, Diedrik Vermeulen, and Michael R. Watts, “Coherent solid-state LIDAR with silicon photonic optical phased arrays,” Optics Letters, vol. 42 , No. 20, 15 October 2017, pp.4091-4094. 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.

Japanese Patent No. 3325825 Japanese Unexamined Patent Publication No. 2011-18002 U.S. Pat. No. 7,573,641 International Publication WO2019 / 013320 Pamphlet U.S. Pat. No. 7092599 Japanese Patent No. 3325825

By the way, in order to realize a multifunctional optical circuit, a smaller size, lower cost, and higher precision are required. However, in a planar optical circuit, the area is large because the optical circuit is surrounded only in the two-dimensional plane, and crosstalk always occurs when the optical paths intersect, so that the performance deteriorates.

On the other hand, by arranging optical elements having functions such as polarization grating in series in the traveling direction (x direction) of light, space can be used more efficiently than in a planar optical circuit, and intersection and crosstalk can be suppressed. However, since a plurality of optical elements are required for the processing of a plurality of stages, the entire circuit becomes long and the cost becomes high. For example, when trying to realize an optical circuit that has both polarization separation and 90-degree hybrid used for coherent communication, it is necessary to widen the interval of output light to a certain level or more, but at that time, the separation angle of the polarization grating is increased to some extent or more. If it is set to, a loss will occur. Therefore, there is a problem that the optical path length is inevitably long in order to widen the interval of the output light while suppressing the loss.

Therefore, the first object of the present invention is to realize multi-stage optical signal processing with a small number of optical elements.

Further, as a path switch on a plane, a switch on a PLC (Planar Lightwave Circuit) and a silicon optical circuit switch are known. These multi-input, multi-output optical path switches have a network structure as shown in FIG. 3, and signals are exchanged between adjacent optical lines. In FIG. 3, the 2-input × 2-output unit switch element 301 is in an anti-cross state (upper left → upper right or lower left → lower right) and a cross state (upper left) as the heating on / off of the thermooptical portion represented by the rectangle is performed. → Lower right or lower left → upper right) can be switched. Seen from the input of one switch element, the destination of the signal is only one of the two adjacent lines, and the path change is zero or plus or minus 1, that is, the jump of the signal is at most one stage. In the circuit, the signal lines intersect at almost right angles to minimize crosstalk, but it is still difficult to avoid crosstalk between signals. In more complex circuits, such as 8x8 switch circuits (or 16x16 switch circuits), the signal must pass through 8 (or 16) switch elements before reaching the output end. It is even more difficult to maintain signal quality because each signal loss or crosstalk is accumulated 8 times (16 times). In this way, in an optical switch circuit having a large number of input / output ports, signal loss and crosstalk between signal lines have been considered to be problems.

Therefore, a second 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, and to reduce the size of the circuit configuration. The present invention is for achieving at least one of the first object and the second object.

As a result of diligent studies on means for achieving the above object, the inventors of the present invention collectively form regions having a plurality of functions on one transparent substrate, and arrange reflection mirrors in front of and behind the regions to obtain an optical path. It was found that multi-stage optical signal processing can be realized with a small number of optical elements by sequentially injecting light into different regions of the transparent substrate by bending the light. Then, the present inventors have come up with the idea that the problems of the prior art can be solved based on the above findings, and have completed the present invention. Specifically, the present invention has the following configuration.

The present invention relates to an optical circuit for processing an optical signal. The optical circuit according to the present invention includes a main substrate and a plurality of reflective members. The main substrate is a transparent substrate in which a plurality of functional regions are patterned on one plane. An example of a functional region is a polarizing grating (diffraction grating) region, which provides optical effects such as polarization separation, beam division, beam refraction, and beam superposition on incident light of light. The main substrate is preferably composed of photonic crystals in which each functional region is patterned, but it may be a meta surface as shown in FIG. 1, or a liquid crystal molecule is used. You may. The reflecting members are arranged to face the plane of the main substrate, and are arranged on both sides of the main substrate so as to reciprocally reflect the light transmitted through the functional region. The reflecting member is preferably arranged on both the front surface and the back surface of the main substrate, whereby the reflected light can be repeatedly incident on the main substrate. The optical circuit of the present invention is configured so that the incident light is reciprocally reflected by the reflecting member to pass through each of the plurality of functional regions of the main substrate. In this way, by integrating a plurality of functional regions on one main substrate (collective pattern formation), for example, the number of substrates is smaller than that of the conventional technique in which a plurality of substrates having one functional region are arranged in series. Multi-stage optical signal processing can be realized with an optical element. Further, even in an optical switch circuit having a large number of input / output ports, it is possible to effectively suppress signal loss and crosstalk between signal lines, and to reduce the size of the circuit configuration.

It is assumed that the incident light travels in the z direction in the three-dimensional space composed of the xyz orthogonal coordinate system, and the polarized light grating region is formed as the above functional region on the xy plane. In this case, the period of the polarization grating region (that is, the diffraction grating period) is P, and when the polarization grating has a phase difference of 1/2 wavelength of the period P, the incident light having a wavelength λ is included in this polarization grating region. When is incident, it is preferable that the circular polarization incident from the direction tilted by θ = sin-1 (λ / 2P) in the + x direction from the z direction becomes the reverse circular polarization tilted by θ in the + x direction. .. According to such a configuration, it is possible to theoretically convert the incident light which is circularly polarized light into circularly polarized light in the opposite direction and bend it in the opposite direction without loss.

In the optical circuit according to the present invention, it is preferable that at least one of the plurality of functional regions or all of the plurality of functional regions is a polarization grating region having a phase difference of 1/4 wavelength. In this case, since the reflecting member is provided immediately after the polarized light plating region, the incident light reciprocates twice in the same polarized light plating region, and the polarized light having a phase of 1/4 wavelength is substantially relative to the incident light. It is preferable to function as a polarization grating region having a phase difference of 1/2 wavelength. By doing so, for example, even when the main substrate is entirely formed of a 1/4 wave plate, it can be partially functioned as a 1/2 wave plate, and a plurality of different functional regions are the same. It becomes easy to form all at once on the substrate.

In the optical circuit according to the present invention, it is preferable that the main substrate has a plurality of polarized grating regions made of photonic crystals formed in a pattern on one plane. Further, the plurality of polarized light grating regions include those having functions of polarization separation, beam division, beam refraction, and beam superposition (superposition). As a result, the optical circuit according to the present invention is preferably configured to function as a composite circuit of a polarizing separator and a 90-degree hybrid circuit.

In the optical circuit according to the present invention, the functional region may include one or more polarizing gratings and one liquid crystal variable wavelength plate. In this case, the optical circuit of the present invention may selectively couple the input light to one of the plurality of output ports.

In the optical circuit according to the present invention, the functional region may include one or more polarizing gratings and one liquid crystal variable wavelength plate. In this case, the optical circuit of the present invention may distribute the input light over a plurality of output ports at an arbitrary ratio.

According to the optical circuit of the present invention, multi-stage optical signal processing can be realized with a small number of optical elements. Further, according to the optical circuit of the present invention, even in an optical switch circuit having a large number of input / output ports, signal loss and crosstalk between signal lines can be effectively suppressed, and the circuit configuration can be miniaturized.

FIG. 1 shows an example of a polarization grating realized by using an inclined meta surface. FIG. 2 shows an example of a combined circuit of a polarization separation circuit and a 90-degree hybrid circuit. FIG. 3 shows an example of a multi-input, multi-output optical path switch having a network structure. FIG. 4 shows the basic principle of the optical circuit according to the present invention. FIG. 5 is an application example of the optical circuit shown in FIG. FIG. 6 shows an example of a curved polarization grating made of a photonic crystal. FIG. 7 shows a method of collectively forming a portion that functions as a 1/4 wave plate and a portion that functions as a 1/2 wavelength plate on a single substrate using a mirror. FIG. 8 shows an example of the cross-sectional structure of the yz plane of the optical circuit according to the present invention. FIG. 9 is a schematic development of the optical path of the optical circuit according to the present invention, and shows the optical path seen from the xz plane. FIG. 10 shows a layout example (xy plane) of the main board which is the core of the optical circuit. FIG. 11 is a schematic development of an optical circuit using a liquid crystal according to the present invention, and shows an optical path seen from the xz plane. FIG. 12 shows an example of electrical wiring for driving the optical circuit of FIG.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The present invention is not limited to the forms described below, and includes those appropriately modified from the following forms to the extent apparent to those skilled in the art.

[1. Basic configuration]
The basis of the present invention is to accurately align the functional regions by collectively forming a plurality of functional regions having different optical signal transmission paths and processing on the plane of one main substrate geometrically and geometrically. The purpose is to reduce the manufacturing cost of the optical circuit and to reduce the size of the optical circuit.

Examples of the functional area include polarized light grating disclosed in International Publication WO2019 / 01332020 (Patent Document 4). Polarization grating enables polarization separation, beam refraction, beam division, or beam superposition of incident light, and by combining them, a polarization separation circuit and a 90-degree hybrid circuit that can be used in coherent communication. The function can be realized. In this case, Patent Document 4 describes a configuration in which a plurality of optical elements (functional regions) are arranged in series in the traveling direction of light, but in the present invention, the idea is changed and all the functional regions are combined into one. A pattern is collectively formed on the plane of the main substrate. Then, reflective members (mirrors, etc.) are arranged in front of and behind the main substrate so that the incident light on the main substrate reflects the reflective members and is introduced into a plurality of functional regions formed on the main substrate. To do.

For example, as shown in FIG. 4A, conventionally, different polarization grating regions 411, 421, 431 were formed on a plurality of substrates on 410, 420, and 430, respectively. On the other hand, in the present invention, as shown in FIG. 4B, polarized grating regions 441, 442, 443 composed of photonic crystals having different functions in the three regions are collectively patterned on one main substrate 440. It is formed. Specifically, in the optical circuit of FIG. 4B, light incident from the left side of the drawing passes through the first polarized light grating region 441, is then reflected by the first mirror 450, and then is second polarized light. It is configured to pass through the grating region 442, then be reflected by the second mirror 460, and finally pass through the polarization grating region 443 and be emitted. In this case, it is possible to collectively form a photonic crystal containing polarized grating regions 441, 442, 443 having different optical functions on one main substrate 440. Therefore, the optical circuit shown in FIG. 4B can be manufactured at a lower cost than that shown in FIG. 4A. Further, in the optical circuit shown in FIG. 4B, since the patterns in each of the three regions are created by a lithography technique such as electron beam drawing, the positional relationship between them can be controlled with high accuracy. In the past, it was necessary to align three sheets, but that is no longer necessary. In the example shown in FIG. 4, the polarization grating region is three locations, but it can be further increased.

Further, as shown in FIG. 5, a half mirror 530 is adopted as a reflecting member arranged in front of and behind the main board 510 to output a part of the light introduced into the optical circuit to the outside or transmit only a certain region. It is also possible to control the propagation length by making it reflect by another mirror. If the component is oblique to the direction perpendicular to the paper surface of FIG. 5, the position in the direction perpendicular to the paper surface changes depending on the propagation length, so that the position can be controlled in three dimensions. Further, as shown in FIG. 5, it is also possible to output light in the same direction as the incident direction. That is, the incident light to the optical circuit passes through the first polarization grating region 511, is then reflected by the first mirror 520, then passes through the second polarization grating region 512, and then the half mirror 530. Part of it is reflected and the rest is transmitted. The transmitted light of the half mirror 530 is output in the same direction as the light incident on the optical circuit. Further, only the reflected light of the half mirror 530 passes through the third polarization grating region 513 and is reflected by the second mirror 540. The reflected light of the second mirror 540 is also output in the same direction as the light incident on the optical circuit.

In a multi-stage optical circuit, it is advantageous that the light loss of each reflection mirror is small (for example, the reflection efficiency is 99% or more) and the diffraction efficiency of the polarizing grating to the desired order / desired polarization is high (for example, 98% or more). It is a condition. The former is widely used as a dielectric multilayer reflective film. The latter has been demonstrated by self-cloning photonic crystals with Nb ₂ O ₅ / SiO ₂ . In this way, it is possible to realize an optical circuit having a high degree of freedom by combining an optical element in which regions having a plurality of optical functions are collectively formed on one substrate and a reflection mirror. Of course, the number of main substrates on which the functional region is formed does not have to be one, and may be a plurality.

[2. Functional area]
A case where the functional region formed on the main substrate is achieved by polarization grating having a fine structure pattern as shown in FIG. 6 will be described. The polarizing grating used in the present invention is a wave plate in which a fine uneven structure is patterned, and the ratio of 0th-order light and 1st-order light can be changed depending on the phase difference of the wave plate. Specifically, when the incident light is circularly polarized light and the phase difference is 1/4, half the power passes as it is as 0th order light. The power of the other half becomes circularly polarized light in the opposite direction, and is diffracted at an angle of θsin-1 (λ / P) when the period P of the polarizing grating is set as the primary diffraction light and the wavelength is λ. If the phase difference is 1/2, the process proceeds to the first-order diffracted light. In this way, the function given to each functional region can be changed by the phase difference of the polarization grating. A method for realizing polarization separation, branching, refraction, and superposition of light by a polarization grating of such a pattern is known, and is described in detail in, for example, International Publication WO2019 / 013320 (Patent Document 4). ..

The optical element includes a wave plate formed on the xy plane in the three-dimensional spaces x, y, and z. A preferred form of the wave plate is a photonic crystal laminated in the z-axis direction. The phase difference θ of the wave plate is not an integral multiple of π radians. The optical element has one or more regions that are single or repeated in the x-axis direction. That is, a band-shaped region having a width D parallel to the y-axis direction is repeated once or a plurality in the x-axis direction. The axial direction of this wave plate is curved, and the angle with respect to the y-axis direction continuously changes in the range of 0 degrees to 180 degrees. Specifically, the groove formed in the wave plate is a curve that matches the curve y = (D / π) log (| cos (πx / D) |) + constant within the range of the discretization error. For example, the wave plate is formed with a groove along its axial direction.
The optical element applies circular polarization incident from the −z direction to the + z direction with a power ratio of sin ² (θ / 2): cos ² (θ / 2), and when the incident circular polarization is clockwise, it is counterclockwise. It is separated and converted into a component that is circularly polarized and bends in the + x direction on the xz plane and a component that is clockwise and goes straight. On the other hand, when the incident circularly polarized light is counterclockwise, the component that bends in the −x direction on the xz plane due to the clockwise circularly polarized light and the component that travels straight through the counterclockwise circularly polarized light are separated and converted and emitted.

It is particularly preferable that the wave plate is a quarter wave plate. In this case, the optical element of the present invention separates and converts the circularly polarized light incident on the + z direction from the −z direction with a power (light amount) equal to the bending component and the straight-moving component, and emits the polarized light. ..

In the above-mentioned optical element having a curved groove, the other branches and merges so that the ratio of the maximum value to the minimum value inside the region of one of the adjacent convex portions and concave portions is within twice. It is preferable that they are arranged geometrically.

In the above-mentioned optical element having a curved groove, the curve is represented by y = (D / π) log (| cos (πx / D) |) + constant, where D is the width of the region. Is preferable.

In the optical element of the present invention, the wave plate is preferably composed of photonic crystals laminated in the z-axis direction. In this case, the unit period between the grooves of the photonic crystal is 40 nm or more and 1/4 or less of the wavelength of the incident light, and the period in the thickness direction of the photonic crystal is 1/4 of the wavelength of the incident light. The following is preferable.

Although the photonic crystal is known, it may be formed by, for example, a self-cloning method (see Patent Document 1). A photonic crystal is a structure in which the refractive index changes periodically in a period shorter than the operating wavelength of the guided light. In particular, the wave plate is preferably a photonic crystal formed by a self-cloning action. A photonic crystal is a microperiodic structure that functions as an optical element. As a specific method for producing a photonic crystal, as disclosed in Patent Document 1, two or more kinds of different refractive indexes are provided on a substrate having one-dimensional or two-dimensional periodic irregularities. An example is a method of manufacturing an optical element (wave plate) by periodically and sequentially laminating substances (transparent bodies) and using sputter etching alone or at the same time as film formation on at least a part of the laminate. This method is also called the self-cloning method. The photonic crystal formed by this self-cloning method is called a self-cloning photonic crystal. A technique for constructing a wave plate using a self-cloning photonic crystal is known. For example, as another method for producing a photonic crystal, there is a method for producing periodic voids by irradiating glass with a femtosecond laser.
Similarly, as a technique for constructing a wave plate, a method using a liquid crystal can be mentioned.

The plurality of types of transparent substances forming the self-cloning photonic crystal are fluorides such as amorphous silicon, niobium pentoxide, tantalum pentoxide, titanium oxide, hafnium oxide, silicon dioxide, aluminum oxide, and magnesium fluoride. It is preferably either. Two or more kinds having different refractive indexes can be selected from these and used for photonic crystals. For example, a combination of amorphous silicon and silicon dioxide, niobium pentoxide and silicon dioxide, and tantalum pentoxide and silicon dioxide is desirable, but other combinations are also possible. Specifically, the self-cloning photonic crystal has a structure in which a high refractive index material and a low refractive index material are alternately laminated in the z direction. 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 a fluoride containing silicon dioxide, aluminum oxide, magnesium fluoride, or a combination of two or more of these materials.

Both the inter-groove unit period of the in-plane periodic structure forming each wave plate and the unit period in the thickness direction of the wave plate are one-fourth or less of the wavelength of the light incident on the optical element. The unit period between grooves of the in-plane periodic structure is preferably 40 nm or more. It is assumed that the wavelength of the light incident on the optical element is usually selected from the range of 400 nm to 1800 nm.

Further, among the wave plates in a plurality of regions, the in-plane minimum value of the wave plate groove length is equal to or longer than the inter-groove unit period. The upper limit of the in-plane maximum value of the wave plate groove length is preferably 50 times or less of the inter-groove unit period p.

In the case of a wave plate (curve type) in which the principal axis orientation changes continuously, if the pitch p of the convex portion (pitch when the pattern is a straight line) is p ₀ , 0.7 · p ₀ ≦ p ≦ 1. It is preferable that the convex or concave portions are geometrically arranged so as to branch or merge so as to be within 4.p ₀ . In the self-cloning photonic crystal, the change in phase difference is small with respect to the fluctuation in pitch. Therefore, the phase shift from the half-wave plate when the pitch changes can be reduced.

A preferred embodiment of the optical element according to the present invention is an optical element that operates with respect to incident predetermined circularly polarized light. In this optical element, each region forms a wave plate with a phase difference θ that is not an integral multiple of π radians, and the incident circular polarization is the same as the reverse circular polarization and the intensity ratio is sin ² (θ / 2): Branched at cos ² (θ / 2).

Since the optical element of the present invention based on the self-cloning photonic crystal wave plate has a volume shape, which is fundamentally different from the inclined metasurface (for example, Non-Patent Documents 1 and 2: gradient metasurface), it is reflected on the surface and the lower part thereof. It is possible to easily perform preventive treatment and connect to other optical elements using an adhesive. It is a volume type, and the characteristics are kept almost constant even if the number of layers is increased (for example, twice) and the layer period and in-plane period are decreased (for example, 1/2) while maintaining the total thickness of the layers. Therefore, it is possible to increase the definition of the structure.

By the way, when a polarizing grating is formed using a photonic crystal wave plate produced by the self-cloning method of Patent Document 1, the phase difference is controlled by the number of layers of transparent materials. That is, the number of layers of the 1/4 wave plate needs to be approximately half the number of layers of the 1/2 wave plate. Therefore, in the case of a photonic crystal wave plate, it is difficult to simultaneously form a region that functions as a 1/4 wavelength plate and a region that functions as a 1/2 wavelength plate on the plane of the same single substrate. There are challenges. The present invention proposes a solution to the problem.

Here, as shown in FIG. 7, a glass plate on which a reflection mirror is formed is arranged on a part of the back side of a transparent substrate on which a polarizing grating made of, for example, a 1/4 wave plate is formed. Then, the portion of the transparent substrate through which the incident light is transmitted as it is functions as a 1/4 wave plate, and the portion that reflects the incident light substantially passes (reciprocates) the incident light twice. Functions as a 1/2 wave plate. In this way, by separating the part where the reflection mirror is placed and the part where the reflection mirror is not placed on the back of the transparent substrate, the part that functions as a 1/4 wave plate and the part that functions as a 1/2 wave plate on one transparent substrate The parts to be formed can be collectively formed in a plane. That is, the light reflected by the mirror bounces back. When light is input by combining a 1/4 wave plate and a mirror, the light reciprocates on the 1/4 wave plate, and the reflected light returns by feeling a phase difference corresponding to the 1/2 wave plate. By applying this idea to the above-mentioned polarizing grating and combining a polarizing grating formed of a 1/4 wave plate and a mirror, it is possible to realize a polarizing grating that partially functions as a 1/2 wavelength plate.

[3. Optical circuit: Synthesis of polarization separation circuit and 90-degree hybrid circuit]
Subsequently, with reference to FIGS. 8 to 10, an optical circuit having the functions of a polarization separation circuit and a 90-degree hybrid circuit will be described. The optical circuit 100 has a three-dimensional structure composed of an orthogonal coordinate system of xyz, and incident light travels in the x-axis direction. FIG. 8 is a cross-sectional view of an optical circuit 100xz plane according to an embodiment. Further, FIG. 9 is a trial development in which the optical path of the incident light in the optical circuit 100 is linear. FIG. 10 is a cross-sectional view taken along the line XX of FIG. 8 and shows an xy plane of the main substrate 110 in which a plurality of functional regions are formed.

The basic function of the optical circuit 100 will be described with reference to FIG. First, the signal light and the locally oscillated light are incident on the optical circuit 100 from the left side in the drawing. These signal lights and locally oscillated light enter the first polarized light grating 111 (functional region A: polarized light separation) and are separated into clockwise and counterclockwise circular polarization, respectively. The first polarization grating 111 for polarization separation may have a pattern similar to that of the curved line shown in FIG. 6, for example, and the phase difference may be π radians.

Next, the circularly polarized light output from the first polarized light grating 111 is incident on the second polarized light grating 112 (functional region B: beam division), and is divided into light having half the power. The second polarization grating 112 for beam division may have a pattern similar to that of the curved line shown in FIG. 6, for example, and the phase difference may be π / 2 radians. At this time, the lattice pattern formed on the second polarized light grating 112 can give an arbitrary phase difference to each circularly polarized light by shifting the phase of the pattern for each incident position of the circularly polarized light. For example, the phase of the clockwise signal light separated by the first polarization grating 111 may be advanced by 90 degrees with respect to the other counterclockwise signal light to make the phase difference between the two signal lights 90 degrees. Alternatively, another phase difference can be given.

Next, the light divided into two by the second polarization grating 112 is directed to the fourth polarization grating 114 (functional region D: superposition), but at that time, the light divided into two One is refracted through the third polarization grating 113 (functional region C: refraction) and then incident on the fourth polarization grating 114, and the other of the bisected light is the third. It directly enters the fourth polarized light plating 114 without passing through the polarized light plated 113. Here, when the phase difference of the third polarized light grating 113 is π radian, the input circularly polarized light is bent and output as a reversely rotated circularly polarized light. Since all the beams output from the first polarization grating 111 are circularly polarized light, the direction of the beams can be changed. The bending angle of the beam is determined by the period of polarization grating. The orientation and period of this pattern can be set to various values. Therefore, by arranging such an element, the beam can be guided to an arbitrary position while being bent.

Next, the signal light and the locally transmitted light that have passed through the first polarized grating 111 and the second polarized grating 112 are introduced into the fourth polarized grating 114 (functional region D: superposition). The signal light introduced into the fourth polarizing grating 114 and the locally oscillated light cause superposition interference. As a result, it is possible to obtain output light in which the signal light and the locally oscillated light interfere with each other. That is, according to the configuration of the optical circuit 100, it is possible to output a total of eight beams in which the signal light whose polarization plane is adjusted in an arbitrary direction and the locally oscillated light interfere with each other. Immediately after the fourth polarization grating 114, a fifth polarization grating 151 for parallelization (colimating) is provided. The eight beams emitted from the fourth polarized grating 114 are returned to a parallel state by passing through the fifth polarized grating 151.

FIG. 9 is for explaining the optical function of the optical circuit 100. The optical circuit 100 according to the present invention actually has the three-dimensional structure shown in FIG. 8, whereby the function shown in FIG. 9 is achieved. Conceptually, it is considered to fold the paper surface of FIG. 9 on the reflective surface (115, 121, 131, 141, 116, 141) shown in FIG. Of the reflective surfaces shown in FIG. 9, the paper surface is folded in a mountain fold at the portion (115, 131, 116) indicated by the broken line, and the paper surface is folded in a valley at the portion (121, 141, 141) indicated by the alternate long and short dash line. Then, it can be seen that the first to fourth polarization gratings 111 to 114 all overlap. FIG. 8 shows the configuration as viewed from the side. By constructing the optical circuit 100 three-dimensionally using such a concept, all the functional regions of the first to fourth polarizing gratings 111 to 114 are collectively formed on the plane of one main substrate 110. It becomes possible to do.

Specifically, a main substrate 110 having an xy plane is arranged in the center of the optical circuit 100 (the traveling direction of light is the z direction). The main substrate 110 is made of a transparent substrate such as a photonic crystal, and the above-mentioned first to fourth polarization gratings 111, 112, 113, 114 are collectively formed on the plane thereof. An example of a flat pattern on which the polarization grating of the main substrate 110 is formed is shown in FIG. As shown in FIGS. 8 and 10, on the main substrate 110, in order from the top of the paper, the first polarized light grating 111, the second polarized light grating 112, the third polarized light grating 113, and the polarized light grating 114 are y. It is formed in the direction. A transmission region in which no polarization grating exists is provided between the first polarization grating 111 and the second polarization grating 112.

Further, in the present embodiment, the reflection mirrors 115 and 116 are superposed on the first polarization grating 111 and the third polarization grating 113, respectively. Therefore, as shown in FIG. 8, the light incident on the main substrate 110 is mirror-finished by the reflection mirrors 115 and 116 immediately after passing through the first and third polarizing gratings 111 and 113 via the transparent substrate. It reflects and passes through the first and third polarizing gratings 111 and 113 again. Therefore, as described above, even when the entire main substrate 110 is formed to function as a 1/4 wave plate, the first and third polarizing gratings 111 and 113 are 1 /. It will function as a two-wave plate.

A plurality of sub-boards 120, 130, 140, 150 are laminated on both front and rear sides of the main board 110 in the z direction. Each sub-board may be formed of photonic crystals like the main board 110, or a transparent material such as a glass plate may be used. The first sub-board 120 is arranged to face the front side of the main board 110, and the second sub-board 130 is arranged to face the back side of the main board 110. Further, the third sub-board 140 is sandwiched between the main board 110 and the first sub-board 120, and the fourth sub-board 150 is sandwiched between the main board 110 and the second sub-board 130. As described above, in the optical circuit 100 according to the present embodiment, the main substrate and the sub substrate are laminated in a total of five layers. Reflective mirrors 121, 131, and 141 are partially formed on the first to third sub-boards 120, 130, and 140, respectively, and the rest is a transmission region. Further, the fourth sub-board 150 is partially formed with a fifth polarization grating 151 for parallelization, and the rest is a transmission region.

As shown in FIG. 8, the signal light and the locally oscillated light are incident from the transmission region of the first sub-board 120. At this time, the incident angle of the light is inclined by a predetermined angle θ with respect to the parallel line with respect to the z-axis. The predetermined angle θ may be a value exceeding 0 degrees, for example, 1 degree to 10 degrees. Then, these lights are incident on the first polarizing grating 111 of the main substrate 110 via the transmission region of the third auxiliary substrate 140, and immediately after that, are reflected by the reflection mirror 115. These lights are separated into clockwise and counterclockwise circular polarizations by the first polarization grating 111, respectively. The light (circularly polarized light) polarized by the first polarization grating 111 is reflected again by the reflection mirror 121 of the first sub-board 120 via the transmission region of the third sub-board 140. This reflected light passes through the transmission region of the third sub-board 140, the transmission region between the first polarization grating 111 and the second polarization grating 112 on the main substrate 110, and the transmission region of the fourth sub-board 150. The light is reflected by the reflection mirror 131 of the second sub-board 130. This reflected light again enters the second polarizing grating 112 of the main substrate 110 via the transmission region of the fourth sub-board 150, and is divided into light having half the power. The light divided by the second polarizing grating 112 goes straight as it is and is reflected by the reflection mirror 141 of the third auxiliary substrate 140. This reflected light is incident on the third polarizing grating 113 of the main substrate 110 and immediately after that, is reflected by the reflection mirror 116. The third polarizing grating 113 refracts one optical path of the light divided into two by the second polarizing grating 112. The other side of the light divided into two goes straight without incident on the third polarizing grating 113 (see FIG. 9). After that, each light is reflected again by the reflection mirror 141 of the third auxiliary substrate 140 and is incident on the fourth polarization grating 114. The signal light introduced into the fourth polarizing grating 114 and the locally oscillated light cause superposition interference. As a result, the output light in which the signal light and the locally oscillated light interfere with each other is emitted. After that, the output light is incident on the fifth polarizing grating 151 of the fourth sub-board 150, and the optical path is parallelized. As a result, a total of eight light beams are emitted from the optical circuit 100. As described above, in the optical circuit 100 of the present invention shown in FIG. 8, the optical functional regions (polarization separation, division, refraction, superposition interference) shown in FIG. 9 are integrated on one main substrate 110, and the optical functional regions thereof are integrated. Reflective members (mirrors) are arranged in the front and rear to make the optical path three-dimensional.

Further, FIG. 10 shows a layout example of the main board 110 having a plurality of functional areas A to D. From this figure, it can be seen that the polarization grating regions for realizing the polarization separation function, the division function, the bending function, and the superposition function are integrated on the plane of one main substrate 100. By collectively forming each of the functional regions A to D in this way, it becomes easy to form the respective functional regions in parallel, and as a result, the phase difference error as a 90-degree hybrid device can be minimized. That is, when the functional region is made of individual elements as in the prior art, there is a problem that the phase error becomes large unless the alignment of each element is performed accurately, and high accuracy is required at the time of assembly. .. On the other hand, by collectively forming the functional regions A to D on one element (main substrate 100) as shown in FIG. 10, high parallelism of each functional region can be guaranteed.

Next, examples of the optical circuit according to the present invention will be described with reference to FIG. In this embodiment, a functional region is formed by sandwiching a liquid crystal element having a variable phase difference between two polarizing gratings, and a switch function or a distribution function of an optical signal is realized. In this example, the case of 1 input and 8 outputs will be described for the sake of simplicity, but the same function can be obtained in the case of any multi-input and multi-output. In addition, the case where the polarization of the incident light is known is illustrated, but it is omitted because it is clear that it can be separated by the polarization grating when two polarization modes are mixed as in an optical fiber.

In FIG. 11, the light is incident from the left in the z direction. Light passes through the functional regions 1151, 1152, 1153, 1154 in order as a function, but as a shape, it reciprocates in the plus or minus z direction in the folding structure, tilts only a small angle from the xz plane, and is monotonous in the y direction. Proceed in the plus y direction. However, 1140 (3 lines) and 1141 (3 lines) in the figure represent valley folds and mountain fold positions, respectively. If this is reversed, it proceeds in the minus y direction.

The functional regions 1151, 1153, 1154 are formed on the same xy plane. In FIG. 12, the regions of 1170, 1171 (2 pieces) and 1172 (4 pieces) are composite portions of the polarizing grating and the liquid crystal, and the light beam of the signal passes through the regions. The functions of 1170, 1171 and 1172 control the light of 1151, 1152 and 1153 stages in FIG. 11 in order, respectively. In the enlarged view of the small circle of 1152, the light incident at an angle θ from the left is directly evolved by the polarization grating 1160, and the polarization state of the light is changed by the liquid crystal variable wave plate 1161. If the phase difference of the liquid crystal portion of the liquid crystal wave plate is changed to binary between 0 and π, the possible optical path 1167 or 1168 is selected. On the other hand, when the phase difference is an angle φ between 0 and π, the power of the incident light is divided into cos ² φ / 2: sin ² φ / 2 between the two rays 1167 and 1168. By continuously changing the phase difference, light can be distributed to any ratio between the optical paths 1167 and 1168. The above switch function and distribution function are provided in all the functional areas of FIG. As a switch, it is possible to connect the input light from the left end to any of the eight output ports on the right end. As a distribution function, the input light from the left end can be divided into all eight output ports on the right end at an arbitrary distribution ratio.

The phase difference of the liquid crystal wave plate is changed by applying a voltage of several volts to each of the pair of transparent electrodes 1163 and 1164 sandwiching the liquid crystal layer 164 via the connector electrode of 1173. As the liquid crystal, for example, a nematic liquid crystal can be used. As described above, in the conventional straight-ahead optical system, as compared with the case where a large number of functional element plates are used, since they can be collectively produced on one plate, the effects of cost reduction and miniaturization are remarkable.

As described above, in the specification of the present application, in order to express the content of the present invention, the embodiments of the present invention have been described with reference to the drawings. However, the present invention is not limited to the above embodiment, and includes modifications and improvements which are obvious to those skilled in the art based on the matters described in the present specification.

100 ... Optical circuit 110 ... Main board 111-114 ... First to fourth polarization grating (functional area)
115, 116 ... Reflective mirror 120 ... 1st sub-board 121 ... Reflective mirror 130 ... 2nd sub-board 131 ... Reflective mirror 140 ... 3rd sub-board 141 ... Reflective mirror 150 ... 4th sub-board 151 ... Fifth Polarization grating

Claims (7)

An optical circuit that processes optical signals
A main board in which multiple functional areas are patterned on one plane,
A plurality of reflective members, which are arranged to face the plane of the main substrate, are on both sides of the main substrate, and reciprocally reflect light transmitted through the functional region, are provided.
An optical circuit in which light incident on the optical circuit is reciprocally reflected by the reflecting member to pass through each of the plurality of functional regions of the main substrate. In a three-dimensional space consisting of an orthogonal coordinate system of xy, when the incident light travels in the z direction and the polarization grating region is formed on the xy plane,
When the incident light having a wavelength of λ is incident on the polarized light having a phase difference of 1/2 wavelength of the period P, a circle incident from a direction inclined by θ = sin-1 (λ / 2P) in the + x direction from the z direction. The optical circuit according to claim 1, wherein the polarized light is emitted as circularly polarized light in the reverse direction tilted by θ in the + x direction. At least one of the plurality of functional regions is a polarization grating region having a phase difference of 1/4 wavelength.
Since the reflecting member is provided immediately after the polarized grating region, the incident light reciprocates in the same polarized grating region, and the polarized grating causes a phase difference of 1/2 wavelength with respect to the incident light. The optical circuit according to claim 1 or 2, which functions as a polarizing grating region having. The main substrate has a plurality of polarized grating regions made of photonic crystals formed in a pattern on one plane.
The plurality of polarization grating regions include those having functions of polarization separation, beam division, beam refraction, and beam superposition.
The optical circuit according to any one of claims 1 to 3, which is configured to function as a composite circuit of a polarizing separator and a 90-degree hybrid circuit. The functional region includes one or more polarizing gratings and one liquid crystal variable wavelength plate.
The optical circuit according to claim 1, wherein the input light to the optical circuit is selectively coupled to one of a plurality of output ports. The functional region includes one or more polarizing gratings and one liquid crystal variable wavelength plate.
The optical circuit according to claim 1, wherein the input light to the optical circuit is distributed over a plurality of output ports at an arbitrary ratio. The optical circuit according to claim 5 or 6, wherein the polarization grating is a self-cloning type photonic crystal.