WO2016181895A1

WO2016181895A1 - Optical element

Info

Publication number: WO2016181895A1
Application number: PCT/JP2016/063635
Authority: WO
Inventors: 川上　彰二郎; 川嶋　貴之; 俊和居城
Original assignee: 有限会社オートクローニング・テクノロジー; 株式会社フォトニックラティス
Priority date: 2015-05-08
Filing date: 2016-05-06
Publication date: 2016-11-17
Also published as: JPWO2016181895A1

Abstract

[Problem] In a conventional flat plate lens, condensing functionality is weak, and size reduction (decreasing the focal distance and reducing the size of the lens) is significantly limited. [Solution] The present invention provides an optical element for condensing/diverging/refracting linearly polarized light using a plurality of wavelength plate regions of a photonic crystal. Using the features whereby a thin wavelength plate has an extremely large effective refractive index difference, and a phase difference can be continuously varied on a single substrate, it is possible to provide an optical element for condensing/diverging/refracting linearly polarized light. Because a lens can be formed by numerous thin wavelength plates in the present invention, the thickness of the lens as a whole can be made extremely small.

Description

Optical element

The present invention relates to a thin flat optical element that performs condensing, divergence, refraction, polarization separation and synthesis.

Non-Patent Document 1 and Patent Documents 1 to 6 disclose conventional optical elements that can be used as a condensing lens in, for example, an image sensor for a digital camera.

“Three-dimensional periodic structure, method for producing the same, and method for producing the film”, Japanese Patent No. 3325825 “Birefringent Periodic Structure, Phase Plate, Diffraction Grating Polarizing Beam Splitter, and Manufacturing Method Thereof” Japanese Patent Application Laid-Open No. 2001-51122 "Multi-valued wave plate" Japanese Patent Application Laid-Open No. 2010-156896 “Polarized diffraction element”, JP 2010-156895 A "Polarization converter" Japanese Patent Application Laid-Open No. 2012-181385 “Photonic crystal having polarization conversion function”, JP 2013-257371 A

In an optical system, a condenser lens is the most basic universal optical element that is always used. To make it smaller and thinner, there are practical examples widely used in, for example, microlenses used in image sensors for digital cameras. Furthermore, it is highly desirable to make it flat, thin and short focal length, but conventional optical elements are limited to, for example, a focal length of several millimeters and a thickness of several millimeters. There are many restrictions on the use of these devices for device coupling. Therefore, the present invention makes the thickness of the optical element and the focal length drastically improved by setting the thickness of the optical element to several μm to several tens μm and the focal length to several μm to several tens μm or several hundred μm. With the goal.

The first aspect of the present invention relates to an optical element. In the optical element according to the present invention, the plane on which light is incident is divided into a plurality of regions, each region is composed of a wave plate, each wave plate has its own phase difference, and the direction of the slow axis or fast axis between the regions. Each region is adjacent to each other with no gap, and has a function of condensing light incident with one linearly polarized light. That is, the optical element according to the present invention can function as a lens and has a light diverging function and a refractive function in addition to the light condensing function.

In the optical element of the present invention, the plurality of regions are preferably a plurality of circular or annular regions sharing the center, a plurality of elliptical or elliptical regions sharing the center, or a plurality of rectangular regions. . The basic concept of an optical element comprising a wave plate according to the present invention will be described with reference to FIGS. 4 (a) and 4 (b). As shown in FIGS. 4A and 4B, the structure of this optical element is largely divided into three wave plate regions. That is, a circular (or elliptical) region located at the center, an annular (or elliptical) region located around the periphery, and a rectangular region located further around the region. Each region is composed of a number of wave plate unit cells each having a stripe shape or a square shape. In other words, the central circular region is composed of strip-shaped wave plate unit cells extending in the X-axis direction, the surrounding annular region is composed of many rectangular wave plate unit cells, and the surrounding rectangular region is in the Y-axis direction. It consists of a streak-shaped wave plate unit cell extending in a straight line.

In the optical element of the present invention, the plane on which light is incident is divided into a plurality of band-like areas, each area is composed of a wave plate, each wave plate has a phase difference, and a slow axis or a speed between the areas. The respective regions may be adjacent to each other with no gap while keeping the axis direction in common, and may have a refraction function at an angle for each polarization with respect to two orthogonal linearly polarized light.

Specifically, the shape of each wave plate unit cell is preferably a rectangle (including a square and parallel irregularities). When the optical element is regarded as a two-dimensional plane having an X axis and a Y axis, the respective wave plate unit cells are formed adjacent to each other with no gap in the X axis direction and the Y axis direction. Here, when the optical element is caused to function as a lens having a condensing, divergence, or refraction function, the optical element is divided into a plurality of regions (preferably three or more or five or more regions) from the central portion toward the outer edge. It is divided. At this time, a plurality of wave plate unit cells having the longest slow axis and the shortest fast axis are arranged in the most central area, and are arranged in each area from the most central area toward the outer edge. The slow axis of each wave plate unit cell is shortened or the fast axis is lengthened. As described above, an optical element in which a plurality of wave plate regions having different phase differences is arranged functions as a lens having a function of condensing, diverging, or refraction with respect to light incident as one linearly polarized light. On the other hand, when the optical element is caused to function as a prism having a polarization separation or synthesis function, the optical element is divided into a plurality of regions (preferably three or more or five or more regions) from one end side to the other end side. . In the region closest to one end, a plurality of wave plate unit cells having the longest slow axis and the shortest fast axis are arranged, and are arranged in each region from the region closest to the one end toward the other end side. The slow axis of each wave plate unit cell is shortened or the fast axis is lengthened. Thus, the optical element in which the wavelength plate unit cell is arranged functions as a prism having a function of polarization separation or synthesis with respect to two orthogonal linearly polarized lights.

It is preferable that the wave plate is a photonic crystal formed by self-cloning action. The photonic crystal has a periodic groove-like structure in a plane and has a structure in which the periodic groove-like structure is laminated in the thickness direction. A photonic crystal is a micro periodic structure that functions as an optical element. As a specific photonic crystal manufacturing method, two or more types of substances (transparent materials) are periodically and sequentially formed on a substrate having two-dimensional periodic irregularities disclosed in JP-A-10-335758. There is a method of manufacturing an optical element by stacking and using sputter etching alone or simultaneously with film formation on at least a part of the stack. This method is also called an autocloning method. A technique for constructing a wave plate using self-cloning crystals is known (for example, JP-A-2008-197399).

One of the plurality of types of transparent bodies forming the photonic crystal is preferably amorphous silicon, niobium pentoxide, or tantalum pentoxide. For example, the photonic crystal can be a combination of niobium pentoxide and tantalum pentoxide, a combination of amorphous silicon and niobium pentoxide, or a combination of amorphous silicon and tantalum pentoxide. Specifically, the self-cloning photonic crystal has a structure in which high refractive index materials and low refractive index materials are alternately stacked in the z direction. The high refractive index material is preferably one of 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 one of fluorides including silicon dioxide, aluminum oxide, and magnesium fluoride, or a combination of two or more of these materials.

In the periodic groove-like (unevenness) structure formed by the self-cloning action, the basic period of the groove is preferably 1/3 or less or 1/5 or less of the light wavelength of incident light. Moreover, the fundamental period of a groove | channel can also be made into 1/6 or less of the light wavelength of incident light, or 1/8 or less.

The second aspect of the present invention relates to an optical component having the optical element according to the first aspect on both surfaces of a single substrate.

In the present invention, after forming the optical element according to the first aspect, after depositing one kind of dielectric film to a certain thickness and flattening the surface, the first optical element is the number of wave plates, The optical component which formed the optical element which concerns on the 1st side surface from which direction or a period differs may be sufficient.

The third aspect of the present invention relates to an optical coupling component that couples two or more optical communication components using the optical element according to the first aspect or the optical component according to the second aspect.

In digital coherent optical communication (coherent optical communication), a light beam to be handled by one optical element is often limited to a predetermined linearly polarized light. In a conventional flat lens, light is collected by a difference in isotropic refractive index, but in a linearly polarizing optical element according to the present invention, a wave plate can be used. Placing the slow axis direction of the wave plate parallel to the electric field of incident light is equivalent to a large phase lag and a high refractive index. The opposite is true when the fast axes are placed in parallel.

Furthermore, in the self-cloning photonic crystal, wave plate regions having various delay phase amounts can be arranged adjacent to each other without any gap on one substrate, so that the light collecting function, the diverging function equivalent to the lens, or A refraction function can be provided. In this way, an extremely thin self-cloning photonic crystal can function as a lens. Therefore, according to the present invention, the thickness of the optical element functioning as a lens can be set to about 1 to several tens of μm, and the focal length can be set to about 1 μm to several tens or several hundreds of μm.

Furthermore, with a photonic crystal wave plate, a condensing function that far exceeds the common sense that a phase difference π equivalent to one wavelength can be realized with a thickness equivalent to eight times the free space wavelength, depending on the choice of materials, between orthogonal polarizations, etc. Therefore, the thickness can be dramatically reduced.

Note that terms are added. The optical element of the present invention can be used both for optical communication and for a visible region with a shorter wavelength. In communication technology, the term polarization is used. In optics centered on visible light, the term polarization is used. In the present application, polarization is used for communication applications, and polarization is used for general optics, but they are equivalent.

For example, components such as a planar optical circuit (PLC) and an optical fiber for coherent optical communication, or a small and elliptical light of an InP optical modulator that can be mounted on a circular optical fiber with a large diameter by flat plate bonding. Benefits such as reductions in manpower, man-hours savings, and cost reductions are significant.

FIG. 1 (a) shows an example of a concavo-convex structure of a film to be given to a periodic structure photonic crystal formed by self-cloning in order to give a predetermined phase difference to an arbitrary region of an integrated wave plate. . FIG. 1B shows an example of an uneven structure of a film to be given to a periodic structure photonic crystal formed by self-cloning in order to give a predetermined phase difference to an arbitrary region of the integrated wave plate. . FIG. 1C shows an example of an uneven structure of a film to be given to a periodic structure photonic crystal formed by self-cloning in order to give a predetermined phase difference to an arbitrary region of the integrated wave plate. . FIG. 2 is a diagram showing actual measurement results for showing that the longitudinal and lateral dimensional ratios determine the respective phase differences in the in-plane unit cell of the photonic crystal shown in FIG. The horizontal axis η represents (1-a / b). In the square unit cell, η is zero, and gradually increases as the vertical and horizontal opening increases, and η is 1 when b is almost infinitely larger than a. FIG. 3 shows a configuration method of an elliptic lens as an example of a condensing lens. In the figure, it is difficult in the prior art to independently realize the horizontal light condensing action and the vertical light converging action for the light traveling perpendicular to the paper surface, but even the function can be realized in the present invention. So how to do that. FIG. 4A shows a perspective view of an elliptic lens as an example of a condensing lens. FIG. 4B is an electron micrograph of the surface of the elliptic lens. FIG. 4C shows a condensing spot of the elliptic lens. Here, a self-cloning photonic crystal condensing lens composed of niobium pentoxide and silicon dioxide and its effect are shown. FIG. 4D shows the focal shape of the elliptic lens. Here, a self-cloning photonic crystal condensing lens composed of niobium pentoxide and silicon dioxide and its effect are shown. FIG. 5 shows an example of an optical circuit that efficiently couples an elongated elliptical beam having a size of about the light wavelength to an optical fiber. FIG. 5A is a cut surface in which the minor axis can be seen by longitudinally cutting the light beam. The vertical axis represents the beam cross section, and the horizontal axis represents the propagation length. The unit is um (micrometer). The left end is the incident surface, the next x is the conversion point (6 um) by the first lens, and the right x is the incident surface of the optical fiber. Since the equiphase surface has a quadratic curve, the second lens is placed here to correct the equiphase surface and make it flat (the optical fiber mode equiphase surface is flat). The arc represents the equiphase surface of the beam. FIG.5 (b) is a figure corresponding to a long diameter similarly. In the first lens, an almost flat equiphase surface is converted to divergence. That is, it has a concave lens action. FIG. 6 shows a concept of a method for realizing the prism function. The prism does not have any particular light condensing action and changes only the traveling direction. When vertically polarized light enters the element shown in FIG. 6, the light is refracted to the left. FIG. 7 shows a more specific method for realizing the prism function. The prism does not have any particular light condensing action and changes only the traveling direction. When vertically polarized light enters the element shown in FIG. 6, the light is refracted to the left. Light deflected in the left-right direction is refracted to the right.

Hereinafter, the present invention will be described in detail based on Examples 1 to 6.

A method of creating a wave plate or wave plate region having a plurality of types of phase differences on a single substrate by the same process will be described. FIG. 1A shows an upper surface of a wave plate having a rectangular (rectangular) shape having a short side a and a long side b as a unit cell. Wave plates made of a photonic crystal produced by a self-cloning method in which the long side b is almost infinitely larger than a are known (Patent Documents 1 to 6). On the other hand, FIG. 1 shows an example of the structure of the wave plate according to the present invention. A structure in which a ridge is provided every length b is shown in FIG. 1 (a), and a structure in which a valley is provided every other length b is shown in FIG. 1 (b). Further, FIG. 1A may be modified as shown in FIG. 1C, and anisotropy can be reduced by forming infinitely long peaks and grooves in a finite length. By arranging a plurality of such wave plate regions with controlled phase differences without leaving a gap and keeping the main axis direction in common, the phase difference can be selected independently for each location. In the present application, the slow axis and the fast axis are collectively referred to as the main axis in the wave plate.

Next, a method for adjusting the phase difference of the wave plate created by the self-cloning method for each region is shown. In a photonic crystal wave plate made of niobium pentoxide and silicon dioxide, the long side b of the unit cell rectangle is made two times, three times, and four times the short side a so as to have a common number of layers and a common thickness. Each film was evaluated for the phase difference. The result is shown in FIG. The relative phase difference is approximately proportional to (1-a / b).

In the inventive idea of the present application, the optical element has a structure in which a plurality of wave plate regions are adjacent to each other without a gap while keeping the slow axis and / or fast axis directions in common. Further, each wave plate unit cell has a basic structure of a rectangle having a short side a and a long side b (a square wave plate having the short side a and the long side b equal in length exists). Also good). That is, when considering a two-dimensional coordinate system of orthogonal X-axis and Y-axis, each rectangular wave plate is formed adjacent to the X-axis direction and the Y-axis direction. In the present specification, for convenience, a rectangle that is long in the Y-axis direction (the vertical direction in the figure) is referred to as a “longitudinal rectangle”, and a rectangle that is long in the X-axis direction (the horizontal direction in the figure) is referred to as a “horizontal rectangle”. Yes. As shown in FIG. 1A and the like, each wave plate region has a convex portion or a concave portion, and ridges or valleys are formed between the wave plate regions. The boundaries of each wave plate region are clearly delimited by valleys. For example, in the example of FIG. 1A, each wave plate has a recess, and a ridge is formed at the boundary between adjacent wave plate regions. In the example of FIG. 1A, the ridges between the wave plate regions are continuous in an infinite length along the X axis and the Y axis. In the example of FIG. 1B, each wave plate region has a convex portion, and a valley is formed at the boundary between adjacent wave plate regions. In the example of FIG. 1B, the valleys between the wave plate regions are connected to the infinite length along the X axis and the Y axis. In the example of FIG. 1C, each wave plate region has a recess, and a ridge is formed at the boundary between adjacent wave plate regions. However, in the example of FIG. 1C, the ridges are connected to the infinite length along the Y axis, but have a finite length along the X axis. That is, the ridges extending in the X-axis direction are formed so as to be offset in the Y-axis direction.

When the incident light is linearly polarized, when the slow axis of the wave plate coincides with the polarization of the linearly polarized light, that portion delays the wavefront of the light, in other words, behaves in the same way as a medium having a large refractive index. Similarly, when the fast axis of the wave plate coincides with the polarization of linearly polarized light, that portion relatively advances the wavefront of light, in other words, behaves equivalently to a medium having a small refractive index. FIG. 3 shows a configuration of an elliptic lens as an example of a condensing lens (optical element) according to the present invention. As shown in FIG. 3, a plurality of wave plate regions (rectangles) having different phase differences are arranged, and the phase lag of the wave plate is maximized in the central region with respect to predetermined linearly polarized light, and the phase lag decreases toward the periphery. If each wave plate region is disposed in the optical element, an optical element composed of a plurality of wave plate regions can have a lens action. That is, the condensing lens configured in this manner exhibits a condensing, divergence, or refraction function for one linearly polarized light according to the polarization state of incident light. Since the phase difference changes stepwise, generally the phase lag is not a smooth function of the space and has a minute quantization error, but its influence is small in the condensing system.

More specifically, as shown in FIG. 3, the condensing lens (elliptical lens) having a lens action is formed with a plurality of vertically long wave plate regions having relatively long sides at the center, and the center A plurality of vertically long wave plate regions with relatively long sides are formed around the substrate, and a plurality of substantially square wave plate regions (with short sides and long sides being equal) are formed around it. A plurality of horizontally long waveplate regions having short sides are formed, and a plurality of horizontally long waveplate regions having relatively long sides are formed around the periphery. As described above, the condensing lens includes the first area corresponding to the central portion, the second area positioned around the first area, the third area positioned around the second area, and the periphery of the third area. 4 and the fifth region located around the fourth region, and the length and direction of the long side of the wavelength plate region are different in each region. The phase difference is different. In the example shown in FIG. 3, the condensing lens is configured with a five-stage phase distribution from the first to fifth regions. As described above, in the elliptical lens shown in FIG. 3, the phase lag of the wave plate is maximized in the first region located in the center with respect to the predetermined linearly polarized light, and the phase lag decreases toward the periphery. Each wave plate region is arranged.

When the phase distribution of a lens (optical element) having a condensing / dispersing / refractive function is stepped as in the present invention, the degree of approximation to the ideal phase change amount to be realized as the number of steps increases. Will increase. If the phase change amount to be realized is 180 degrees from end to end, that is, equivalent to a half wavelength, and the number of steps is n, one stage represents (180 / n) degrees, and the maximum error is (90 / n) degrees. The loss due to phase mismatch is 0.4 dB when n = 3, 0.22 dB when n = 4, and 0.14 dB when n = 5. Thus, the loss due to the phase mismatch can be reduced as the number of stages (n) is increased. Accordingly, the number of stages of the phase distribution can be selected according to the purpose. Here, the number of stages n of the phase distribution is preferably an integer of 3 or more or 5 or more.

In order to increase the number of steps, it is desirable to reduce the size of the rectangle constituting each phase plate region. In the self-cloning method, the size of the short side can be set to about 1/3 of the wavelength used for niobium pentoxide and tantalum pentoxide, and is usually used for amorphous silicon. It can be about 1/5 of the wavelength. It is also possible to make the value smaller than these values while maintaining the ratio between the in-plane period and the thickness direction period. For example, niobium pentoxide and tantalum pentoxide normally have a wavelength of 1/5 or less, and amorphous silicon normally has a wavelength of 1/8 or less. You can also choose.

When creating a wave plate condensing device by the self-cloning method, the approximate relationship between the required maximum phase difference and the film thickness to be formed is as follows.

For example, at the optical communication wavelength of 1550 nm, the same thickness as the half-wave plate may be used to make the phase difference between the minimum and maximum refractive index in the plane to be π radians. 12 μm. When a larger value, for example, 2π radians, is required, a film thickness of about 12 μm is further laminated in proportion to the film thickness.

If it is desired to achieve the same function with a thinner film thickness, a self-cloning photonic crystal of a combination of amorphous silicon and silicon dioxide may be used, and the required thickness is about 1/3 of the previous case. Amorphous silicon is a transparent material in the optical communication wavelength band but is opaque in the visible range, so it is selected according to the purpose.

An example of such a condensing device pattern is shown in FIG. An example is shown in which the light converging power in the left-right direction and the light converging power in the vertical direction are separately determined for light incident perpendicularly to the paper surface. In this example, incident light needs to be linearly polarized waves in the vertical direction.

As described above, the elliptic lens shown in FIG. 3 is divided in the order of the first region, the second region, the third region, the fourth region, and the fifth region from the center toward the outer edge. A plurality of vertically long wave plate regions having relatively long sides are formed in the first region, a plurality of vertically long wave plate regions having relatively long sides are formed in the second region, and a substantially square shape is formed in the third region. A plurality of wave plate regions (which are equal in short side and long side) are formed, a plurality of horizontally long wave plate regions having relatively long sides are formed in the fourth region, and a horizontally long plate having relatively long sides in the fifth region. A plurality of the wavelength plate regions are formed.

In FIG. 3, a vertically long rectangle represents a wave plate whose slow axis is in the vertical direction, and a horizontally long rectangle represents a wave plate whose slow axis is in the left and right direction. Squares are isotropic, ie, the phase difference is zero. As described with reference to FIG. 2, the phase difference of the wavelength plate increases as the ratio of the vertical and horizontal dimensions of the rectangle greatly differs from 1.

This structure makes it possible to form an elliptic lens that collects light independently in the X and Y directions.

In addition, as shown in FIG. 3, a wave plate with a slow axis in the vertical direction and a large phase difference is arranged in the first region at the center of the elliptic lens, and the slow axis is in the vertical direction in the surrounding second region. A wave plate having a small phase difference is arranged, and an isotropic wave plate having a phase difference of zero is arranged in the third region around the wave plate. Further, in the fourth region around the third region, a wave plate having a small phase difference with the slow axis in the left-right direction, that is, the fast axis in the vertical direction, is arranged, and in the fifth region around the slow region, the slow axis is A wave plate having a large phase difference is arranged in the left-right direction, that is, the fast axis is the up-down direction. Here, when the incident light is linearly polarized light, when the slow axis of the wave plate coincides with the polarization of the linearly polarized light, the portion delays the wavefront of the light, in other words, behaves in the same manner as a medium having a large refractive index. Similarly, when the fast axis of the wave plate coincides with the polarization of linearly polarized light, that portion relatively advances the wavefront of light, in other words, behaves equivalently to a medium having a small refractive index. Therefore, various wave plates (rectangular shapes) having different phase differences are arranged as shown in FIG. 3, and the phase delay of the wave plate is maximized at the central portion with respect to predetermined linearly polarized light, and the phase delay decreases toward the periphery. If each wave plate is arranged in this manner, an optical element composed of a plurality of wave plates can have a lens action. That is, the condensing lens configured in this manner exhibits a condensing, divergence, or refraction function for one linearly polarized light according to the polarization state of incident light.

4 (a) shows the configuration of the condensing lens according to the present invention, FIG. 4 (b) shows an electron micrograph of the surface, FIG. 4 (c) shows the condensing spot, and FIG. (D) shows the focal shape. FIG. 4B shows the surface of the stacked photonic crystal that serves as a light incident surface. The photonic crystal is a self-cloning multilayer film having a total thickness of 12 μm, and consists of an inner cylinder having a diameter of 5.8 μm, an outer cylinder having an outer shape of 8.2 μm, an inner diameter of 5.8 μm, and an outer region. The slow axis of the inner cylindrical part is represented by the X axis. Let Y be the slow axis of the outer region orthogonal to it. The middle ring is a stack of west lattices and is isotropic with respect to X and Y. In this structure, an incident light beam having a wavelength of 1.55 μm polarized in the X direction and having a diameter of about 10 μm propagates in the air after passing through the lens by 6.3 μm, and then has a diameter as shown in FIGS. 4 (c) and 4 (d). Light was condensed to about 3 μm.

In an optical transceiver (optical transceiver), the shape of an optical mode of a transmitting element, for example, an InP optical modulator, is often small and non-circular (the minor axis is about 1 μm, the major axis is several μm, for example, 3 μm or less). . Since a normal optical fiber has a circular shape with a diameter of about 10 μm, it is usually not easy to efficiently couple between them. However, in digital coherent communication, each waveguide of a transmission circuit or a reception circuit handles only light of a specific polarization, so that a light collection system integrated by applying the light collection system according to the present invention is not used. Can be made. The beam of the narrow waveguide is approximated by an elliptical Gaussian wave, the short diameter (diameter) is about 0.92 μm, and the long diameter is 3.0 μm (respectively expressed by the full width of power exp (−2)). An example of an optical circuit that converts it into a circular beam with a diameter of 10 μm, which is a mode of a standard optical fiber for communication, is shown in FIG.

The optical circuit is formed by integrally stacking the following four parts.
(A) A first high refractive index layer, for example, a layer made of amorphous silicon and having a thickness of 6 μm. (B) A first condensing element, for example, a photonic crystal made of amorphous silicon and silicon dioxide.
(C) A high refractive index layer, for example, a layer made of amorphous silicon and having a thickness of 40 μm. (D) A second condensing element, for example, a photonic crystal made of amorphous silicon and silicon dioxide.
The part (A) is in contact with the thin waveguide, and the part (D) is in contact with the optical fiber.

The first light collecting element and the second light collecting element shown in FIG. 5 will be described below.
In a three-dimensional space, the traveling direction of light is the z-axis, the minor axis direction of the narrow waveguide perpendicular to the z-axis is the x axis, and the major axis direction is the y axis. Each of the first and second optical elements is formed so as to approximate the phase difference represented by the following quadric surface with a step function.
First: phase difference = Ax ² + By ² + C
Second: phase difference = Dx ² + Ey ² + F
At that time, the light beam propagates as shown in FIG. 5A when observed on the xz plane and as shown in FIG. 5B when observed on the yz plane.
However, A = 0.91 rad / μm ^ 2
B = 0.03 rad / μm ^ 2
D = −0.07 rad / μm ^ 2
E = 0.034 rad / μm ^ 2
And
C and F are constants for setting the phase difference at the center point x = y = 0. A negative value of D means divergent (concave lens).

In order to realize a microprism with an optical element composed of a large number of wave plates, a photonic crystal wave plate whose concept is shown in FIG. 6 can be used. The relationship between the rectangle in the figure and the principal axis / phase difference of the wave plate region that it represents is the same as that described for FIG. In addition, the short side and long side of the rectangle in the figure have a simple integer ratio both within one rectangle and between different rectangles, but this is only for convenience of explanation and drawing, and there is a large degree of freedom. Similarly, the number of partial wave plates in the whole element and the number of partial wave plates in the entire element can be freely selected depending on the purpose.

More specifically, the entire optical element shown in FIG. 6 is formed in a rectangular thin plate shape. As shown in FIG. 6, in the optical element, a plurality of horizontally long rectangular wave plate regions are formed in the rightmost first region, and a substantially square (short side and long side are formed in the second region located on the left side thereof. Are formed), a plurality of vertically long rectangular wave plate regions are formed in the third region located on the left side, and the fourth region located on the left side has a long longitudinal side. A plurality of rectangular wave plate regions are formed, and a plurality of horizontally long rectangular wave plate regions with longer long sides are formed in the fifth region located on the left side thereof.

In such an optical element, the delay is increased if the electric field component of the polarized light of the incident light is parallel to the long side of the rectangular wave plate region, and the delay is reduced if it is perpendicular to the long side of the rectangular wave plate. This element does not have a condensing or diverging function. For this reason, the parallel beam takes an optical path determined for each polarized light while being kept parallel. Therefore, it operates as a polarization separation element or a polarization composition element. As described above, the optical element shown in FIG. 5 has a function of separating or synthesizing two orthogonal linearly polarized lights, that is, a function as a prism.

FIG. 7 shows a surface configuration of a polarization splitting prism for a wavelength of 1.55 μm according to the present invention. The photonic crystal wave plate is composed of Nb ₂ O ₅ / SiO ₂ , and the interval in the X direction (lateral direction) of the groove row extending in the Y direction (vertical direction) is 0.4 μm, which is almost a quarter wavelength, Mark it as a. This polarization splitting prism is mainly composed of 13 regions, and each region is composed of a rectangular unit cell (including one in which one side extends to the end of the element) having a common long side. If the long side of the rectangular unit cell is written as b, the long side of the rectangular unit cell in the mth area is b = 12a / (m−1).
That is, b = ∞, 12a, 6a, 4a, 2.4a,.
It becomes.

For the incident light polarized in the Y direction (longitudinal direction), the effective refractive index is higher toward the left side of the figure, and the light is deflected to the left side. The opposite is true for deflection in the X direction (lateral direction). The photonic crystal is a self-cloning multilayer film having a total thickness of 48 μm, and it is composed of 13 band-like regions arranged in the X direction. The i-th band from the left is further composed of four peak / valley structures with a period d parallel to the Y-axis, and valleys parallel to the X-axis and having an interval equal to b / I. In this structure, an incident light beam having a wavelength of 1.55 μm polarized in the X direction is directed to the right by about 1/12 radians, and an incident light beam having a wavelength of 1.55 μm polarized in the y direction is also directed to the left by about 1/12 radians. To be biased.

In the present specification, the case where the wavelength plate region as a unit is a rectangle (including a square and parallel unevenness) has been exemplified. However, the wave plate as a unit is not limited to a rectangle, and may be a polygon having a slow axis and a fast axis, such as an elongated hexagon, and may be arranged without a gap. Further, in this specification, when the wave plate as a unit is a rectangle, the case where the long side and the short side form a simple integer ratio has been shown in many cases, but it is of course possible to use a more general ratio. Is possible.

The state in which the light to be processed by any optical element is a specific linearly polarized light can be widely used for coherent optical communication in the optical communication region and in the laser optical system even in the visible region. There are a wide range of applications such as short focal length characteristics, small characteristics, and high integration.

Claims

The plane on which the light is incident is divided into a plurality of regions, each region is composed of a wave plate, each wave plate has a phase difference, and each region has a common slow axis or fast axis direction between the regions. An optical element that is adjacent to each other with no gap and has the function of condensing light incident on a single linearly polarized light.
The plurality of regions are:
A plurality of circular or annular regions sharing a center,
The optical element according to claim 1, wherein the optical element is a plurality of elliptical or elliptical annular regions sharing a center or a plurality of rectangular regions.
The plane on which light is incident is divided into a plurality of band-shaped regions, each region is made of a wave plate, each wave plate has its own phase difference, and each region has a common slow axis or fast axis direction. An optical element in which the regions are adjacent to each other with no gap and has a function of refraction at an angle for each polarization with respect to two orthogonal linearly polarized light.
The element according to claim 1, wherein the wave plate is a photonic crystal having a periodic groove-like structure in a plane, and the periodic groove-like structure is stacked in a thickness direction.
The photonic crystal is a laminate in which a plurality of types of transparent bodies having different refractive indexes are laminated in the thickness direction,
The optical element according to claim 4, wherein one of the plurality of types of transparent bodies is amorphous silicon, niobium pentoxide, or tantalum pentoxide.
The optical element according to claim 4, wherein the wavelength plate has a fundamental period of grooves that is not more than one-fifth of an optical wavelength of incident light in the periodic groove-shaped structure.
An optical component having the optical element according to claim 1 or 3 on both surfaces of a single substrate.
After the first optical element according to claim 1 or 3 is formed, a thickness of one type of dielectric film is formed and the surface is flattened. The manufacturing method of the optical component which forms the 2nd optical element of Claim 1 or Claim 3 from which the number, direction, or period of these differs.
An optical coupling component that couples two or more optical communication components using the optical element according to claim 1.