JP2019113629A - Optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element and an optical circuit which can be easily integrated. An optical element includes a 1/2 wave plate formed on an xy plane in a three-dimensional space x, y, z and laminated in the z-axis direction. The groove direction of the wave plate is a curved line, and the angle with respect to the line changes continuously in the range of 0 degrees to 180 degrees on a line extending radially from the center. The optical element can convert the phase plane of the incident circularly polarized light to be focused or diverged. Further, a polarization separation prism composed of a multi-regional 1/2 wave plate, a plurality of the above optical elements and a plurality of elements composed of a multi-region 1/4 wave plate are arranged at appropriate positions, and the input signal light is polarized and separated. Moreover, by converting the mode diameter by the lens action and converting the polarization state into linear polarization, it is possible to realize an optical circuit having functions of polarization separation, conversion and mode diameter conversion. [Selection diagram] Fig. 2

Description

  The present invention relates to an optical element having a light condensing action, a diverging action, or an action of converting it into parallel light.

  A lens is very widely used as an optical element which condenses light or converts it into divergent or parallel light. Many of them have a three-dimensional shape like a convex lens and a concave lens, and are manufactured as one by one functions, so integration and miniaturization are often accompanied by difficulties. In recent years, a technology that finely processes on the surface of a transparent substrate and changes the phase of each light beam transmitted vertically and changes the position phase of the light beam and tilts the wave front to manipulate the propagation after transmission (graded metasurface: ) Is progressing.

  It is not uncommon for the amount of deformation of the wavefront required at that time to be several times or several tens of times the wavelength. On the other hand, since what can practically be used as a phase change amount of light passing through the surface is a fraction to a few times of 2π radian, an operation to return the phase change amount to zero every 2π radian is necessary. .

The above-described operation of returning the amount of phase change to zero in a sawtooth-like manner every 2π radians can not avoid the scattering of light near the discontinuities and the accompanying errors in amplitude and phase. The following means are known as a method of reducing it. That is,
(A) A minute half-wave plate having various orientations for each region is disposed on the substrate surface without gaps.
(B) It utilizes the property that the phase shift that circularly polarized light receives when passing through the region is equal to twice the angle θ made by the main axis with respect to a certain reference direction.
Specifically, the electric field of light incident in FIG. 1 is, for example, E _x = E ₀ cos (ωt), E _y = E ₀ sin (ωt)
When circularly polarized light is given by ξ, if the half-wave plate having the ξ eta axis as in Fig. 1 and the principal axis in the 挿入 図 axis direction is inserted, the transmitted light becomes circularly polarized in the reverse direction and the relative phase is 2θ It is only known to change.

  When it is necessary to continuously change the phase transition over 2π radians, for example, θ may be defined as in the upper part of FIG. 1, and θ may be continuously changed over π radians, and θ may be continuous. And if monotonically changing by several times π, the phase angle can be changed by any number of 2π radians without discontinuity. If .theta. Increases or decreases approximately linearly with x, the wavefront of the transmitted circularly polarized light undergoes a linear transformation with respect to x. A polarizing prism combining this concept with a photonic crystal has been realized (Non-Patent Document 1).

  On the other hand, coherent methods are widely used in modern optical communications. Here, communication is performed by placing different signals on orthogonal polarizations. Therefore, in the transmission part, it is necessary to combine orthogonal polarized light into one optical path, while in the receiving part it is necessary to separate the received light into orthogonal polarized light and to enter the receiving part.

  In particular, in recent years, an optical circuit having a high refractive index and a strong confinement function represented by silicon photonics has been widely spread. In such an optical circuit, coupling with an optical fiber becomes an issue. That is, the beam diameter of the optical fiber already laid in the world is about 10 microns. On the other hand, in silicon photonics, the beam diameter is several hundred nm in the circuit, and even if the beam diameter conversion portion is provided at the circuit end, the beam diameter is at most 2-3 microns. Therefore, due to the beam diameter mismatch there, power loss of several dB can not be avoided.

  Furthermore, optical circuits with high refractive index often have polarization dependence in their characteristics, and the polarization is separated at the stage of the circuit entrance, one is rotated 90 degrees, and conversion to polarization in which the circuit operates is performed. By doing this, it is sufficient to limit the design of the circuit to only one polarization, because the design is efficient and the yield is improved.

  Therefore, it is clear that it would be useful if an optical element capable of separating orthogonal polarized light and having a function of beam diameter conversion could be realized.

Proceedings of the 64th Annual Conference of the Institute of Applied Physics Spring meeting, Lecture number 16-a-F202-7

Patent No. 3325825

  Although the conversion function of the beam phase plane described above can be realized by the above-described surface processing, for example, there are difficulties as described in the following (1) to (4).

(1) The distance between the grooves and the grooves or the period of the periodic grooves is at least 1/3 wavelength or more. In order to control the light beam, it is desirable to control the phase finely at each place, but it is limited by the groove spacing of the wave plate. Actually, in order for the groove to function as a wave plate and to have a principal axis direction different from that of the adjacent region before that, the length of the groove must be at least equal to, preferably at least twice the distance between the grooves. The size of the micro area can not be sufficiently reduced. A concrete explanation is given below. Of the regions D in FIG. 1, the one with the smallest groove length in the region is represented by a symbol d. Similarly, the symbol d is similarly defined in FIG. Also, a periodically repeated groove cycle (also referred to as “inter-groove unit cycle”) is represented by a symbol p. In order to operate as a wave plate, d / p needs to be large to some extent. When d / p is finite, the phase difference due to birefringence in the region is smaller than π and estimated to be about π (1-p / 2d). In order for the phase difference to be originally π to be, for example, 0.95π or more, or 0.9π or more, or 0.75π or more, or 0.5π or more, d is 10p or more, 5p or more, 2p or more, respectively It is necessary to be p or more.

  Conversely, for high definition, we want to keep d small. In the optical element of FIG. 1, the upper limit of d is determined by the requirements of the element, and the performance of the element is improved as it can be reduced (because the quantization error is smaller). On the other hand, since p is required to be further smaller by one to a half digit, the benefit of being able to reduce p is great.

  When the grooves are curved as shown in FIG. 4, the pitch becomes narrower as the pattern approaches the tangential direction of the circle, and the pitch needs to be maintained by reducing the number of grooves (thinning). Even in such a case, the pitch interval can not be strictly fixed, and the pitch interval varies from place to place, and the phase difference deviates from the half-wave plate.

(2) It is necessary to form an anti-reflection layer on the surface to avoid unnecessary reflection of light on the surface of the element, but it is difficult to form a wave plate by surface processing.

(3) In the case of realizing a half-wave plate by microfabrication on the element surface, the height is about 100 nm. For example, in the case of an error of 1%, it is necessary to suppress the surface processing accuracy to about 100 × 0.01 = 1 nm or less, which requires extremely advanced processing technology. On the other hand, in the case of a photonic crystal, the thickness is on the order of microns even with a half wavelength, so control of, for example, about 10 μm × 0.01 = 100 nm is sufficient. .

(4) Since the operation is realized at the boundary with the air on the element surface, the effect is drastically reduced when the fine structure is filled with the adhesive.

  Therefore, the present invention has been made in view of such problems, and an object thereof is to provide an optical element which is easy to integrate and which uses a volume effect, not a meta surface.

To summarize the effects of the present invention in advance, the present invention exhibits any one or more or all of the following first to third effects.
First, an element is realized in which the phase plane of the beam is converted in an element that is incident from one side with respect to the film formation surface and emitted to the opposite side.
Second, even if the line pitch is nonuniform or nonuniform due to curvilinear shape or thinning out, the uniformity of the phase difference between the polarizations is maintained (FIG. 5).
Thirdly, by combining photonic crystals as in the third and fourth embodiments described later, an optical circuit in which the functions of polarization separation and beam phase plane conversion are integrated is realized.

A first aspect of the present invention relates to an optical element. The optical element includes a wave plate formed in the xy plane in the three-dimensional space x, y, z. The preferred form of the wave plate is a photonic crystal stacked in the z-axis direction. The phase difference φ of the wave plate is an odd multiple of π radian. The wave plate is formed of a pattern in which the axial orientation θ is approximated from the orientation at the center according to the distance r from the center to a function of Δθ (Δθ = A × r ² ). θ repeats between 0 and 180 degrees. That is, in the wave plate, the axial orientation θ of the wave plate at a position where the distance r advances in the radial direction from the center in the radial direction, θ = A × r ^{2 with} respect to the axial orientation at the center using the coefficient A It consists of the pattern which consists of a curve or a line which increases with. In the optical element of the present invention, when circularly polarized light is incident, the phase difference at a distance r from the center with respect to the phase of the central portion can be expressed as 2A × r ² . This is the same for any direction from the center. Here, the coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is a focal length when the optical element functions as a lens.

As described above, when considered on a straight line advancing in the radial direction from the center, the distance from the center is r, and the change Δθ from the center of the axial orientation θ at that point is Δθ = A × r ² θ Set The coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is a focal length as a lens. The phase of the circularly polarized light passing through the half-wave plate is proportional to the axial orientation θ, so it can provide a phase surface that can be represented by a square function with respect to the distance from the center. That is, an ideal lens can be realized.

  Furthermore, this concept is applied to two rotation symmetric patterns, and a pattern as shown in FIG. 2 is considered. Considering that circularly polarized light is incident on such a pattern, the phase plane of the circular beam can be converted into central symmetry because a phase plane that can be represented by the above square function can be realized in any direction from the center. A lens can be realized. When the coefficient A is positive, it functions as a convex lens, and when it is negative, it functions as a concave lens. This is reversed when the incident circularly polarized light is reversed.

  The “microwave plate having various orientations in each area” is realized by periodically arranging deep grooves in the substrate. An infinite array of grooves formed periodically on the solid surface results in a greater phase lag for polarization where the electric field is parallel to the grooves and for polarization where the electric field is perpendicular to the grooves. In a half-wave plate, it is necessary to make the phase difference equal to π radian, and for design and processing reasons, the distance between grooves is often about 1/3 wavelength to 1/2 wavelength, 1/4 It will not be the wavelength.

  As shown in FIG. 2, when the convex patterns appear to gather clockwise from the periphery toward the center, when a plane wave of clockwise circular polarization is incident, the phase advances from the center to the periphery, Become. On the other hand, with respect to left-handed circularly polarized light, the phase lead / lag is reversed to become divergent light. Also, the transmitted light is converted into reverse circularly polarized light. That is, the rotation direction of the incident circularly polarized light is determined to act as a convex lens or a concave lens. In the pattern of FIG. 3 having a mirror image pattern with respect to FIG. 2, the relationship of the circularly polarized light collected and diverged is reversed. The effect of the incident circular polarization being converted into counter-rotating circular polarization is identical.

  If the incident light is a plane wave, it functions as focusing or divergence, but when light that is not a plane wave has already entered, conversion of the phase plane occurs to that light, and the phase conversion given by the optical element When a beam having an inverse distribution to is incident, it is converted (collimated) into a plane wave.

  Although the patterns in FIGS. 2 and 3 are drawn as a set of curves, they may be a set of line segments, that is, a broken line. However, when the pattern of FIG. 2 is formed by a broken line, a deviation from an ideal line occurs, and this deviation may give an error to the phase distribution and may affect the conversion performance of the phase plane.

  In each embodiment of the present invention, the wave plate is particularly preferably a half wave plate. In this case, the optical element of the present invention converts the phase plane of circularly polarized light entering from −z direction to + z into either convex, concave, or flat surface, and emits it as reverse circularly polarized light.

  In the optical element having the above-described curved groove, the other splits and merges so that the ratio of the maximum value to the minimum value in the area of one of the distance between the adjacent protrusion and recess is within two times It is preferable to be geometrically arranged (see FIG. 4 etc.).

  In the optical element of the present invention, the wave plate is preferably made of a photonic crystal laminated in the z-axis direction. In this case, the inter-groove unit period of the photonic crystal is 40 nm or more and 1/4 or less of the wavelength of incident light, and the period in the thickness direction of the photonic crystal is 1/4 of the wavelength of incident light It is preferable that it is the following.

  Photonic crystals are known, but may be formed, for example, by an auto cloning method (see Patent Document 1). A photonic crystal is a structure whose refractive index changes periodically at a cycle shorter than the operating wavelength of light to be guided. In particular, the wave plate is preferably a photonic crystal formed by an autocloning action. A photonic crystal is a minute periodic structure that functions as an optical element. As a specific method of manufacturing a photonic crystal, as disclosed in Patent Document 1, two or more kinds of different refractive indexes are different on a substrate having periodical irregularities in one or two dimensions. There is a method of manufacturing an optical element (wave plate) by sequentially and sequentially laminating a substance (transparent body) and using sputter etching alone or simultaneously with film formation on at least a part of the lamination. This method is also called a self cloning method. And the photonic crystal formed by this auto cloning method is called a self cloning type photonic crystal. In addition, the technique which comprises a wavelength plate using a self-cloning type | mold photonic crystal is well-known. For example, as another method of producing a photonic crystal, there is a method of producing periodic voids by irradiating glass with a femtosecond laser.

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

  More specifically, the optical element of the present invention is a wave plate (curved type) in which the main axis direction changes continuously or a wave plate (broken line type) in which the main axis direction is approximated by a broken line. The wave plate of is composed of a photonic crystal having a periodic structure in the plane and the periodic structure being stacked in the thickness direction. The photonic crystal may be formed by an auto cloning method (see Patent Document 1).

  The inter-groove unit period of the periodic structure in the surface forming each wave plate and the unit period in the thickness direction of the wave plate are both equal to or less than a quarter of the wavelength of light incident on the optical element. The inter-groove unit period of the in-plane periodic structure is preferably 40 nm or more. In addition, it is assumed that the wavelength of the light which injects into an optical element is normally chosen from 400 nm-1800 nm.

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

In addition, assuming that the pitch p of the convex portions of the wave plate is p ₀ (the pitch when the pattern is a straight line), the convex portions or the concave portions are arranged such that 0.7 · p ₀ ≦ p ≦ 1.4 · p ₀ It is preferable to geometrically arrange so as to branch and merge. In the self-cloning type photonic crystal, as shown in FIG. Therefore, it is possible to reduce the phase shift from the half-wave plate when the pitch changes.

  A preferred embodiment of the optical element according to the present invention is an optical element operating on predetermined incident circularly polarized light. This optical element is a wave plate of phase difference φ in which each region is an odd multiple of π radian, and the phase plane of the incident circularly polarized light is converted to a convex surface, a concave surface, or a plane, and a reverse circle It emits polarized light.

  The optical element of the present invention based on a self-cloning type photonic crystal wave plate is fundamentally different from the inclined meta surface and is of a volume shape, so that an antireflection treatment is performed on the surface and the lower portion, or an adhesive is used. It can be easily connected to other optical elements. It is a volume type, and the characteristics can be kept almost constant even if the number of laminations is large (for example, 2 times), the lamination period and the in-plane period are small (for example, 1⁄2) while maintaining the total thickness of the laminations. Therefore, high definition of the structure is possible.

When it is desired to input or output linearly polarized light instead of circularly polarized light, an optical element that can be input / output as linearly polarized light is realized by the presence of quarter wavelength plates in front and back.
For example, as in the compound optical element 601 of FIG. 6, by inserting the quarter wavelength plate 604 on the incident side, the incident linearly polarized light can be converted to circularly polarized light by 604 and incident on 605 to realize the condensing action. .
Alternatively, as in the complex optical element 602, the quarter wavelength plate 607 may be disposed on the output side, and the function of converting the circularly polarized light collected in 606 into linearly polarized light and emitting the linearly polarized light can be realized.
Alternatively, quarter wavelength plates 608 and 610 may be disposed on the incident side and the output side, respectively, as in the composite optical element 603, and the linearly polarized incident light may be collected and emitted as linearly polarized light.

A second aspect of the present invention relates to an optical circuit using the optical element of the first aspect.
When a half-wave plate having the structure of Non-Patent Document 1 shown in FIG. 1 is used as the first optical element 701 having a polarization separation function as shown in FIG. 7, the incident light is rotated clockwise and counterclockwise. It can be separated into circularly polarized light.

  The optical element 701 having such a polarization separation function has, for example, the following structure. That is, the optical element 701 is a wave plate having a phase difference formed in the xy plane in the three-dimensional space x, y, z and having an odd multiple of π radian, and has a band-like width D parallel to the y-axis direction. Are repeated one or more times in the x-axis direction. The groove formed in the wave plate is a curve which is coincident with the curve y = (D / π) log (│cos (πx / D) │) + constant in the range of the discretization error. The optical element 701 separates linearly polarized light incident from the −z direction to the + z direction into two circularly polarized lights orthogonal to each other at a power ratio of 1: 1. In the separated circularly polarized light, the rotation direction and the bending angle have an inverse relationship of positive and negative.

  The optical element 701 having a polarization separation function may be a broken line type. That is, a band-like width D area parallel to the y-axis direction is repeated singly or in plurality in the x-axis direction, and the area of the width D is further divided into a plurality of band-like subareas parallel to the y-axis. The slow axis β is uniform within the same subregion. The slow wave axis β is represented by β = (180 × x1 / D) degrees + constant clockwise with respect to the x coordinate x1 of the center line of the sub region. The broken-line-type optical element 701 separates linearly polarized light incident from the −z direction to the + z direction into two circularly polarized lights orthogonal to each other at a power ratio of 1: 1. In the separated circularly polarized light, the rotation direction and the bending angle have an inverse relationship of positive and negative.

  Next, as a second optical element, for example as shown in FIG. 7, the optical element 702 of the pattern of FIG. 2 or FIG. 3 is disposed on one sheet. The respective beams separated into clockwise and counterclockwise circularly polarized light by the first optical element are incident on the second optical element and become, for example, focused light. In this case, although light is obliquely incident on the second optical element, the same function is realized because the photonic crystal wave plate does not have a characteristic sensitive to the incident angle as shown in FIG. can do.

  Alternatively, as shown in FIG. 9, the second optical element 902 having the same structure is disposed behind the first optical element 901 having a polarization separation function (for example, the structure shown in FIG. 1). Then, the lights separated by the first optical element 901 and propagated to each other in a transparent manner are bent in parallel by the second optical element 902. In this case, the polarization state is converted to reverse circular polarization before and after the second optical element. After that, if the third optical element 903 having the structure shown in FIG. 2 or 3 according to the present invention, for example, is disposed, the light collecting function can be realized similarly.

  Furthermore, after the first optical element 1001 (e.g. the structure shown in FIG. 1), the second optical element 1002 (the structure shown in FIG. 1) and the third optical element (e.g. the structure shown in FIG. 2 or 3) For example, by arranging a quarter wavelength plate 1004 as shown in FIG. 10, a circuit having a linearly polarized light output can be realized. If the quarter wavelength plate 1004 is, for example, a quarter wavelength plate in which two beams are orthogonal to each other as in the first pattern 1005, linearly polarized light in the same direction can be output. Alternatively, if the quarter-wave plate 1004 is, for example, a quarter-wave plate in which the two beams face the same axis as in the second pattern 1006, linearly polarized light orthogonal to each other can be output. The “orthogonal polarization state” is orthogonal to both right-handed circular polarization and left-handed circular polarization, and vertical linear polarization and horizontal linear polarization are also orthogonal. There is no change to being orthogonal, and both are the same in the sense of multiplexing orthogonal polarization in communication.

  The optical elements used in the optical circuit of the present aspect are formed on a single plate, so that multiple parallelization is easy. Therefore, it is also easy to simultaneously make a circuit having the above functions in parallel.

  As described above, the incident light is separated into orthogonal polarization parts using the polarization separation prism as shown in FIG. 1, the lens of the present invention, and the multi-region quarter-wave plate, and each is converted into focused light The function of converting the polarization state into orthogonal or parallel linear polarization can be realized.

  According to the present invention, an optical element that can be easily integrated can be provided. Further, according to the present invention, it is possible to suppress the generation of the light scattering and the unnecessary light component originating from the discontinuity by the high definition of the structure and the curving. Further, according to the present invention, the processability such as surface treatment, cleaning, adhesion treatment is excellent, and the volume as a part, the footprint, and the manufacturing cost can be reduced.

It is a polarization grating realized using a gradient metasurface which is a prior art. It is a figure which shows the pattern of the optical element of this invention. It is a figure which shows the pattern of the optical element of this invention. It is a figure which shows an example of the optical element (curved type) which concerns on 1st Embodiment, and the optical element (curved type) which concerns on 2nd Embodiment. It is a figure which shows the sensitivity of the phase difference with respect to the pitch change in the case of a photonic crystal. It is a figure which shows the function of the optical element which concerns on 2nd Embodiment. It is a figure which shows the function of the optical element which concerns on 3rd Embodiment, and has shown the compound optical element which consists of one polarization splitter prism and the optical element of this invention. It is a figure which shows the incident angle dependence of the phase difference of a photonic crystal wave plate. It is a figure which shows the function of the optical element which concerns on 3rd Embodiment, and has shown the composite optical element which consists of two polarization splitting prisms and this invention. It is a figure which shows the function of the optical element which concerns on 3rd Embodiment, and has shown the compound optical element which consists of two polarization splitting prisms, the optical element of this invention, and a quarter wavelength plate. It is a figure which shows the function of the optical element which concerns on 1st Embodiment. It is a figure which shows the simulation result of the condensing action of the optical element which concerns on 1st Embodiment. It is a figure which shows the example of a design of the compound optical element which has the function of polarization separation and mode diameter conversion for connecting between different optical circuits based on 3rd, 4th embodiment.

  The following Example 1, Example 2, Example 3, and Example 4 of the present invention will be described.

  The present embodiment relates to the optical element according to the first aspect described above. First, an optical element will be described which converts a plane wave of right-handed circularly polarized light, which is vertically incident, into counterclockwise circularly polarized light and condenses it.

The optical arrangement of the optical elements is shown in FIG. The basic configuration of the optical element is a wave plate made of a photonic crystal that is formed in the xy plane in the three-dimensional space x, y, z, and is stacked in the z-axis direction. As shown in FIG. 2, the wave plate is a pattern in which the axial orientation θ can be represented by θ = A × r ² (A is an appropriate coefficient) according to the distance r from the center. Here, the coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), assuming that the focal length f when the optical element functions as a lens.

In addition, as shown in FIG. 4, by making the pattern (convex part or concave part) of the photonic crystal curved, the pattern becomes sparse in the central part within one cycle and becomes denser as it gets closer to the end Breaks down. Therefore, the pitch between patterns in the central portion is taken as a reference, and it is set as p ₀ . The two patterns are merged at a position where p ₀ falls below a certain threshold pitch. Although the pitch immediately after merging is 2p _0, it becomes denser as it gets closer to the end, so it merges again when it becomes smaller than the threshold length. By repeating the above operation, an ideal optical axis distribution can be realized while changing within a certain range of the pitch. Assuming that the threshold pitch is 0.5 p ₀ , the change range of the pitch is between 0.5 p _{0 and} 2.0 p ₀ . That is, they are geometrically arranged such that the ratio of the maximum value to the minimum value of one of the intervals between adjacent projections and depressions is within 4 times, preferably within 2 times, and the other is branched and merged. . In the example shown in FIG. 4, the white part is a concave part, and the black part is a convex part. That is, in the case of a wavelength plate (curved type) in which the main axis orientation changes continuously, assuming that the pitch p of the convex portions (pitch when the pattern is a straight line) is p ₀ , 0.5 · p ₀ ≦ p ≦ 2 ··· The projections or recesses are geometrically arranged so as to branch and merge so as to be within p ₀ .

On the other hand, the optical element of the present invention is not limited to a curve as described above, and can be expressed by a so-called broken line approximation. That is, it is a wavelength plate which is formed in the xy plane in the three-dimensional space x, y, z, and is made of a photonic crystal stacked in the z-axis direction. The wave plate is a pattern represented by a broken line which approximates to a curve whose axis orientation θ can be represented by θ = A × r ^{2 as the} distance r from the center as shown in FIG. Here, A is represented by A = ± π / 2fλ (λ is a wavelength), where A is a focal length f when functioning as a lens.

The result of simulation when right-handed circularly polarized light passes through the above-mentioned structure is shown in FIG. The cross section of the intensity distribution in the xz plane of light traveling in the z direction is shown. It can be clearly seen how the incident beam is focused by the structure. Further, since the phase plane is continuously converted, it is understood that a beam with a mode diameter of 10 microns is efficiently converted into a mode diameter of 4 microns without unnecessary stray light.
The conditions of analysis are as follows.
· Wavelength λ 1.55 μm
・ High refractive index material a-Si
・ Low refractive index material SiO ₂
・ Slow axis refractive index n _s 2.713
・ Fast axis refractive index n _f 2.486
· The total thickness of the laminated _{λ / (n s -n f)} × 0.5
・ Coefficient A = 0.5π / 5 ²

  This is the case of normal incidence, but since the incident angle dependency of the photonic crystal wave plate is less dependent on the incident angle as shown in FIG. 8, it works also for light entering obliquely. In that case, light has a wave number in the x and y directions, but the wave number possessed by the incident light is preserved even for the outgoing light.

  In the simulation shown in FIG. 12, the way of collecting parallel light was calculated, but when light travels in the reverse direction, that is, when divergent light having a certain phase plane is incident, it is converted into parallel light. This is similar to a normal lens. For example, it is apparent that a beam emitted from the focal plane of FIG. 12 with a beam diameter at the focal plane is converted into parallel light by a lens.

  Further, there is no limitation on the refractive index of the incident and outgoing medium, and even if the element is buried in the adhesive even if it is emitted to the air, it operates with appropriate design.

  Also, it is easy to arrange a plurality of optical elements on one substrate, and it is also possible for each to have patterns of different parameters.

  The optical element of the present invention operates on circularly polarized light as described above, and the emitted light is also circularly polarized light. However, it is not always common to use circularly polarized light in an optical circuit, and rather linear polarized light is often easier to use. Now consider the case where the input and output are linearly polarized light.

  For example, as in the composite optical element 601 of FIG. 6, by disposing the quarter-wave plate 604 in front of the optical element 605, it is possible to once convert incident linearly polarized light into circularly polarized light and operate it.

  Similarly, as in the composite optical element 602 of FIG. 6, it is also possible to transmit the light output as circularly polarized light from the optical element 606 through the 1⁄4 wavelength plate 607 and output it as linearly polarized light. Further, as shown by the compound optical element 603 in FIG. 6, the linearly polarized light enters the optical element 608 by sandwiching the front and back of the optical element 608 with the quarter wavelength plates 609 and 610. It is also possible to emit linearly polarized light. As shown in FIG. 8, since the incident angle dependency of the photonic crystal wave plate is not sensitive, the expected operation can be realized even if the light is condensed and obliquely enters the 1⁄4 wavelength plate. If it is necessary to change the direction of linearly polarized light, the axis of each quarter wave plate may be selected as appropriate.

In this case, although it is possible to arrange the optical elements one by one, for example, after forming a multilayer structure having the pattern shown in FIG. 2 by the self cloning method, for example, the surface is planarized using a dielectric of SiO ₂ and further there. The different optical elements can be fused by forming another pattern on the other and forming another photonic crystal. The planarization process may be bias sputtering, etching, CMP, or mechanical polishing. By using this method, it is possible to integrate the above-mentioned 1⁄4 wavelength plate and an optical element having a lens function into one. In this case, either one may be upper or lower, or it may be a three-stage structure of an optical element having a lens function between upper and lower quarter wavelength plates.

  In the structure shown in FIG. 1, it is possible to realize a polarization separation prism that separates incident light into right-handed circularly polarized light and left-handed circularly polarized light. Therefore, this polarization separation prism is disposed as a first optical element, and the optical element of the present invention is disposed as a second optical element behind it. In that case, if the pattern shown in 703 of FIG. 7 is arranged at the position where the separated beam is incident, the circularly polarized light in reverse direction is incident on each area, the phase plane is converted, and the light is collected on both It can have an effect. The distance between the two lenses is uniquely determined by the separation angle of the front prism and the distance between the prism and the lens.

In this case, light is obliquely incident on the lens as shown in FIG. As described above, since the characteristics of the photonic crystal wave plate are insensitive to the incident angle, there is no problem for the incident at an angle of about 10 degrees from the perpendicular, but considering the subsequent optical system, two beams are It is preferred that they be parallel. Therefore, another polarization separation element having the same function is disposed behind the first optical element as a second optical element. Then, light obliquely separated by the first optical element is obliquely incident on the second optical element, but is bent in the same direction as the incident direction on the first optical element by the second optical element. As a result, the two beams become parallel. In this case, it is important that the patterns of the first optical element and the second optical element (specifically, the pattern 904 in FIG. 9) be the same. In this case, it should be taken into consideration that the circularly polarized light is converted into the backward circularly polarized light by the second optical element. By arranging the optical element of the present invention after this, it is possible to obtain light condensed in parallel.

  Furthermore, as mentioned earlier, the emitted light can also be converted into linearly polarized light. As shown in FIG. 10, it is assumed that a quarter-wave plate is further inserted behind the optical element of the present invention. For example, if the 1⁄4 wavelength plate transmits the respective beams orthogonal to each other at 1005 in FIG. 10, linearly polarized light in the same direction can be obtained. Alternatively, as in the quarter-wave plate shown by the pattern 1006 in FIG. 10, orthogonal polarization can be obtained by transmitting both beams in the same axial direction. It is apparent that the orientation of the obtained linear polarization is determined by the axial orientation of the quarter-wave plate 1004.

  Thus, by combining the polarization separation prism, the optical element of the present invention, and the 1⁄4 wavelength plate, it is possible to realize an optical circuit having a function of polarization separation, condensing and emitting as linearly polarized light. it can. Such an element separates the light transmitted through the optical fiber into orthogonal polarized light in optical communication, and performs polarization separation and conversion while converting the mode diameter to another optical circuit such as silicon photonics whose operating polarization is fixed. You can do it and let it go.

  Furthermore, as described above, this optical circuit can be formed by being stacked on a single substrate, and it becomes possible to realize the optical circuit with the minimum necessary thickness, so that the optical circuit can be miniaturized. It will be useful.

  Here, conversion from a large beam diameter to a small beam diameter, such as coupling from an optical fiber to silicon photonics, is assumed. However, because the circuits are reciprocal, it is of course also possible to combine small beams into large ones. For example, it is assumed that a beam having the same polarization direction and having different signals modulated is output from a modulator configured by silicon photonics. If FIG. 10 is traced from right to left, linearly polarized light is converted to circularly polarized light orthogonal to each other by the 1⁄4 wavelength plate 1004, and collimated light is generated by the third optical element 1003, and the first and second optical elements 1001 and 1002 are made. It is obvious that it is also possible to couple to one light path.

  For example, FIG. 13 shows an example of designing an optical circuit for polarization separation and coupling of a 10-micron beam, which is a typical mode diameter of an optical fiber, to a silicon photonic circuit having a mode diameter of 2 microns. Since the mode diameter of the silicon photonics circuit is very small, typically 0.45 microns, it is assumed that the mode diameter conversion function is provided in the silicon photonics circuit. For example, when an optical circuit having a mode diameter of 10 microns and a mode diameter of 2 microns is directly coupled, the loss due to mode diameter mismatch there is 8.3 dB. On the other hand, it is clear from the calculation using the beam propagation method that it is possible to improve to 1.5 dB when the lens of the present invention is used.

Claims (4)

The three-dimensional space x, y, z includes a wave plate of phase difference φ formed in the xy plane, and the phase difference φ is an odd multiple of π radian,
The wave plate consists of a curve in which the axial orientation θ at a location where the distance r advances in the radial direction from the center in the radial direction with respect to the axial orientation at the center by a factor A, θ = Ar ² Composed of patterns,
The coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is a focal distance in free space.
Transform the wavefront of circularly polarized light that is incident from the −z direction to the + z direction,
An optical element that converts it into focused light or divergent light or parallel light. The three-dimensional space x, y, z includes a wave plate of phase difference φ formed in the xy plane, and the phase difference φ is an odd multiple of π radian,
The wave plate consists of a broken line in which the axial orientation θ at a position where the distance r advances in the radial direction from the center in the radial direction by θ = Ar ^{2 with} respect to the axial orientation at the center using a coefficient A Composed of patterns,
The coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is a focal distance in free space.
Transform the wavefront of circularly polarized light that is incident from the −z direction to the + z direction,
An optical element that converts it into focused light or divergent light or parallel light. An optical element according to claim 1 or 2, wherein
The wave plate is composed of a photonic crystal stacked in the z-axis direction,
The inter-groove unit period of the photonic crystal is 40 nm or more and 1/4 or less of the wavelength of the incident light,
An optical element in which the period in the thickness direction of the photonic crystal is 1/4 or less of the wavelength of incident light. As a first optical element group, it has one or two polarizing prisms for separating incident light into right-handed circularly polarized light and left-handed circularly polarized light,
The optical element according to claim 1 or 2 is disposed as a second optical element,
It has a quarter wave plate as the third optical element,
The light incident from the first optical element group is separated into left-handed and right-handed circularly polarized light by the first optical element group,
It passes through the second optical element and becomes focused light,
It passes through the third optical element and is condensed as linearly polarized light,
Compound optical element that has the function of separating incident light into orthogonal polarized light and collecting it.