JP7048962B2 - Optical element - Google Patents

Description

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

Lenses are extremely widely used as optical elements that collect, diverge, or convert light into parallel light. Many of them have a three-dimensional shape like a convex lens and a concave lens, and are manufactured one by one as a function, so that it is often difficult to integrate and miniaturize them. In recent years, a technology (called gradient metasurface) that manipulates the propagation after transmission by finely processing the surface of a transparent substrate, changing the phase of the light beam that transmits perpendicularly to it at each location, and tilting the wavefront. ) Is progressing.

It is not uncommon for the amount of wavefront deformation 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 possible to be about a fraction to several times that of 2π radians, so it is necessary to return the phase change amount to zero in a sawtooth wave for each 2π radians. ..

In the above-mentioned operation of returning the phase change amount 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 it. That is,
(A) Micro 1/2 wave plates having various orientations for each region are arranged on the surface of the substrate 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.
Specifically, the electric field of the incident light in FIG. 1 is, for example, _Ex = 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. Is known to change only.

When it is necessary to continuously change the phase transition beyond 2π radians, for example, θ may be defined as shown in the upper part of Fig. 1, θ may be continuously changed beyond π radians, and θ may be continuous. Moreover, if the phase angle is monotonically changed by several times of π, the phase angle can be changed by many times of 2π radians without discontinuity. If θ approximately increases or decreases linearly with x, the transmitted circularly polarized wavefront undergoes a linear transformation with respect to x. A polarizing prism that combines this concept with a photonic crystal has been realized (Non-Patent Document 1).

On the other hand, the coherent method is widely used in modern optical communication. There, different signals are put on the orthogonal polarizations for communication. Therefore, in the transmitting portion, it is necessary to combine orthogonal polarizations into one optical path, while in the receiving portion, it is necessary to separate the received light into orthogonal polarizations and incident on the receiving portion.

In particular, in recent years, optical circuits having a high refractive index and a strong confinement action, such as silicon photonics, have become widespread. 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 a beam diameter conversion unit is provided at the end of the circuit, the beam diameter is at most 2 to 3 microns. Therefore, due to the mismatch of the beam diameter there, a power loss of several dB is inevitable.

In most optical circuits with a higher refractive index, the characteristics of the optical circuit depend on the polarization, and the polarization is separated at the entrance stage of the circuit, and one of them is rotated by 90 degrees to convert it into the polarization in which the circuit operates. By doing so, the circuit design need only be limited to one polarization, and the design becomes more efficient and the yield is improved.

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

Proceedings of the 64th JSAP Spring Meeting, Lecture No. 16-a-F202-7

Japanese Patent No. 3325825

The above-mentioned beam phase plane conversion function can be realized by the above-mentioned surface processing, but there is a difficulty as described in the following (1) to (4), for example.

(1) The distance between grooves, or the period of the periodic groove, is at least 1/3 wavelength or more. To control the light beam, we want to control the phase finely for each place, but it is limited by the groove spacing of the wave plate. In fact, before that, in order for the groove to function as a wave plate and have a principal axis direction different from that of the adjacent region, the length of the groove must be at least equal to or more than the distance between the grooves, preferably more than twice. , The size of the minute area cannot be small enough. A specific explanation will be given below. Of each region D in FIG. 1, the region in which the length of the groove in the region is minimized is represented by the symbol d. Similarly, in FIG. 4, the reference numeral d is also defined. Further, the period of the groove (also referred to as “inter-groove unit period”) that is periodically repeated is represented by the reference numeral p. In order to operate as a wave plate, it is necessary that d / p is large to some extent. When d / p is finite, the phase difference due to birefringence in that region is smaller than π and is estimated to be about π (1-p / 2d). In order for the phase difference that should be π to be, for example, 0.95π or more, 0.9π or more, 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.

On the contrary, we want to keep d small for high definition. In the optical element of FIG. 1, the upper limit of d is determined by the request to the element, and the smaller the upper limit is, the higher the performance of the element (because the quantization error is small). On the other hand, since p is required to be one to half an order smaller than that, the benefit of being able to make p smaller is great.

Further, when the groove is curved as shown in FIG. 4, the pitch becomes narrower as the pattern approaches the tangential direction of the circle, and it is necessary to maintain the pitch by reducing (thinning out) the number of grooves. Even in such a case, the pitch interval cannot be made exactly constant, and the pitch interval fluctuates from place to place, and the phase difference deviates from the 1/2 wave plate.

(2) It is necessary to form an antireflection layer on the surface in order to avoid unnecessary reflection of light on the surface of the element, but it is difficult to form a film on the surface-processed wave plate.

(3) When a 1/2 wave plate is realized by microfabrication on the surface of the device, its height is about 100 nm. For example, if the error is 1%, it is necessary to suppress the surface processing accuracy to about 100 × 0.01 = 1 nm or less, which requires a very advanced processing technique. On the other hand, in a photonic crystal, the thickness is on the order of microns even at 1/2 wavelength, so it is sufficient if control of, for example, about 10 μm × 0.01 = 100 nm is possible, which is a value that can be sufficiently handled by a normal thin film process. ..

(4) Since the operation is realized at the boundary with the air on the surface of the element, 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 a problem, and an object thereof is to provide an optical element which is easy to integrate and utilizes a volume effect, instead of a meta surface.

To summarize the effects of the present invention in advance, the present invention exhibits 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 the element that is incident from one side with the film forming surface as a boundary and is emitted to the opposite side.
Second, even if the line-to-line pitch becomes non-uniform or non-uniform due to the curved shape or thinning, the uniformity of the phase difference between the polarizations is maintained (FIG. 5).
Thirdly, as in Examples 3 and 4 described later, by combining photonic crystals, an optical circuit that integrates the functions of polarization separation and beam phase plane conversion is realized.

The first aspect of the present invention relates to an optical element. 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 φ of the wave plate is an odd multiple of π radians. The wave plate is composed of a pattern in which the axial direction θ is approximated to a function of Δθ (Δθ = A × r ² ) from the direction at the center according to the distance r from the center. θ repeats between 0 and 180 degrees. That is, in the wave plate, the axial direction θ of the wave plate at a place advanced by a distance r in the radial direction toward a certain direction from the center is θ = A × r ² with respect to the axial direction at the center using the coefficient A. It consists of a pattern consisting of curves or fold lines that increase in. ^In the optical element of the present invention, when circularly polarized light is incident, the phase difference at a distance r from the center can be expressed as 2A × r2 with respect to the phase of the center. This is the same in every direction from the center. The coefficient A here is expressed as A = ± π / 2fλ (λ is a wavelength), where f is the focal length when the optical element functions as a lens.

As described above, when considered on a straight line traveling in the radial direction from the center, the distance from the center is r, and the change Δθ from the center of the axial direction θ at that point is θ so that Δθ = A × r ² . To set. The coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is the focal length of the lens. Since the phase of the circularly polarized light transmitted through the 1/2 wave plate is proportional to the axial direction θ, it is possible to provide a phase plane that can be expressed by a square function with respect to the distance from the center. In other words, an ideal lens can be realized.

Further, this concept is applied to a pattern of two rotational symmetries, and a pattern as shown in FIG. 2 is considered. Considering that circularly polarized light is incident on such a pattern, it is possible to realize a phase plane that can be represented by the above squared function in any direction from the center, so the phase plane of the circular beam is converted to center symmetry. A lens can be realized. If the coefficient A is positive, it functions as a convex lens, and if it is negative, it functions as a concave lens. This means that the positive and negative directions are reversed when the incident circular polarization is reversed.

The "small wave plate having various orientations for each region" is realized by periodically arranging deep grooves on the substrate. Periodically formed infinite length grooves on a solid surface cause a greater phase lag for polarization where the electric field is parallel to the groove and for polarization where the electric field is perpendicular to the groove. In a half-wave plate, it is necessary to match the phase difference to π radians, and for design and processing reasons, the distance between grooves is often about 1/3 to 1/2 wavelength, which is 1/4. It never becomes a wavelength.

As shown in FIG. 2, when the convex patterns appear to be gathered 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, and the light is condensed. Become. On the other hand, for counterclockwise circular polarization, the phase advance / delay is reversed and the light becomes divergent. Further, the transmitted light is converted into circularly polarized light in the opposite direction. That is, it acts as a convex lens / concave lens by discriminating the rotation direction of the incident circular polarization. In the pattern of FIG. 3, which has a mirror image pattern with respect to FIG. 2, the relationship of light-collecting and diverging circular polarization is reversed. The effect of converting the incident circular polarization into the reverse circular polarization is the same.

If the incident light is a plane wave, it has a function of condensing or diverging, but if light that is not already a plane wave is incident, a phase plane conversion occurs with respect to the light, and the phase conversion just given by the optical element. When a beam having the opposite distribution to the above is incident, it is converted (colimated) 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 polygonal line. However, if the pattern of FIG. 2 is formed by a polygonal line, a deviation from the ideal line occurs, and this deviation may cause an error in the phase distribution and affect the conversion performance of the phase plane.

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

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 the ratio. It is preferable that they are arranged geometrically (see FIG. 4 and the like).

In the optical element of the present invention, it is preferable that the wave plate is composed of photonic crystals 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 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 waveguided 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 different on a substrate having one-dimensional or two-dimensional periodic irregularities. Examples thereof include a method of manufacturing an optical element (wave plate) by periodically and sequentially laminating substances (transparent bodies) and using spatter etching alone or simultaneously with film formation on at least a part of the laminated material. This method is also called the self-cloning method. The photonic crystal formed by this self-cloning method is called a self-cloning type 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 a periodic void by irradiating the glass with a femtosecond laser.

The multiple types of transparent substances that form self-cloning photonic crystals are fluorides such as amorphous silicon, niobium pentoxide, tantalum pentaoxide, 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.

More specifically, the optical element of the present invention is a wave plate (curved line type) in which the spindle direction is continuously changed, or a wave plate (folded line type) in which the main axis direction is approximated by a broken line, and each region thereof. The wave plate of No. 1 has a periodic structure in the plane and is composed of a photonic crystal in which the periodic structure is laminated in the thickness direction. The photonic crystal may be formed by a self-cloning method (see Patent Document 1).

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. It is preferable that the inter-groove unit period of the in-plane periodic structure is 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 minimum in-plane wave plate groove length is equal to or longer than the inter-groove unit period. The upper limit of the in-plane minimum value of the wave plate groove length is preferably 50 times or less of the inter-groove unit period p.

Further, when the pitch p of the convex portion of the wave plate (the pitch when the pattern is a straight line) is p ₀ , the convex portion or the concave portion is within 0.7 · p ₀ ≦ p ≦ 1.4 · p ₀ . It is preferable that they are arranged geometrically so as to branch and merge. As shown in FIG. 5, the self-cloning photonic crystal has a small change in phase difference with respect to pitch fluctuation. 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 an incident predetermined circular polarization. This optical element forms a wave plate with a phase difference φ in which each region is an odd multiple of π radians, converts the phase plane of the incident circularly polarized light into either a convex surface, a concave surface, or a flat surface, and is a reverse circle. It is polarized and emitted.

Since the optical element of the present invention based on the self-cloning photonic crystal wave plate is a volume type, which is fundamentally different from the inclined meta surface, antireflection treatment is applied to the surface and its lower part, and an adhesive is used. It can be easily connected to other optical elements. 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.

If it is desired to enter or emit linearly polarized light instead of circularly polarized light, an optical element capable of input and output as linearly polarized light is realized by having 1/4 wave plates in front and behind.
For example, by inserting a 1/4 wave plate 604 on the incident side as in the composite optical element 601 of FIG. 6, the incident linear polarization is converted into circular polarization by 604 and incident on 605 to realize a light collecting action. ..
Alternatively, as in the composite optical element 602, a 1/4 wave plate 607 is arranged on the emission side, and the action of converting the circularly polarized light focused by the 606 into linear polarization and emitting the light can be realized.
Alternatively, as in the composite optical element 603, 1/4 wave plates 608 and 610 are arranged on the incident side and the emitted side, respectively, and the incident linear polarization is condensed and emitted as linear polarization.

The second aspect of the present invention relates to an optical circuit using the optical element of the first aspect.
As shown in FIG. 7, when a 1/2 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, 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 on the xy plane having an odd multiple of π radian in the three-dimensional spaces x, y, and z, and is a band-shaped region having a width D parallel to the y-axis direction. Is repeated singly or multiple times in the x-axis direction. 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 discretization error. The optical element 701 separates the linearly polarized light incident from the −z direction to the + z direction into two circularly polarized light orthogonal to each other with a power ratio of 1: 1. In the separated circular polarization, the positive and negative relations of the rotation direction and the bending angle are opposite to each other.

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

Next, as the second optical element, the optical element 702 having the pattern of FIG. 2 or FIG. 3 is arranged on one sheet as shown in FIG. 7, for example. Each beam separated into clockwise and counterclockwise circularly polarized light by the first optical element is incident on the second optical element and becomes, for example, focused light. In this case, the light is obliquely incident on the second optical element, but the photonic crystal wave plate does not have the characteristic of being so sensitive to the incident angle as shown in FIG. 8, so that the same function is realized. can do.

Alternatively, as shown in FIG. 9, a second optical element 902 having the same structure is arranged behind the first optical element 901 having a polarization separation function (for example, the structure shown in FIG. 1). Then, the light is separated by the first optical element 901, and the light propagating in a slanting manner is bent in parallel by the second optical element 902. In this case, the polarization state is converted into circular polarization in the opposite direction before and after the second optical element. If a third optical element 903 having the structure shown in FIG. 2 or FIG. 3 according to the present invention is arranged after this, the light collecting function can be similarly realized.

Further, after the first optical element 1001 (eg, the structure shown in FIG. 1), the second optical element 1002 (the structure shown in FIG. 1), and the third optical element (eg, the structure shown in FIG. 2 or 3). For example, by arranging the 1/4 wave plate 1004 as shown in FIG. 10, a circuit having a linearly polarized light output can be realized. If the 1/4 wave plate 1004 is a 1/4 wave plate in which two beams are orthogonal to each other as in the first pattern 1005, linear polarization in the same direction can be output. Alternatively, if the 1/4 wave plate 1004 is made into a 1/4 wave plate in which two beams are oriented in the same axis as in the second pattern 1006, linear polarized light that is orthogonal to each other can be output. The "orthogonal polarization state" is orthogonal to both clockwise and counterclockwise circular polarization, and is also orthogonal to vertical linear polarization and horizontal linear polarization. They are still orthogonal and both are the same in the sense that they multiplex orthogonal polarization in communication.

Since the optical elements used in the optical circuit on this side surface are each formed on one plate, multi-parallelization is easy. Therefore, it is easy to make multiple parallel circuits with the above functions at the same time.

As described above, using the polarization separation prism as shown in FIG. 1, the lens of the present invention, and the multi-region 1/4 wave plate, the incident light is separated into orthogonal polarizing portions, and each is converted into focused light. , It is possible to realize a function of converting a polarization state into orthogonal or parallel linear polarization.

According to the present invention, it is possible to provide an optical element that can be easily integrated. Further, the present invention can suppress the generation of light scattering and unnecessary light components due to discontinuity by increasing the definition and curving of the structure. Further, according to the present invention, it is possible to reduce the volume as a part, the footprint, and the manufacturing cost by excellent workability such as surface treatment, cleaning, and adhesive treatment.

It is a polarization grating realized by using a gradient metasurface, which is a conventional technique. 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 (curve type) which concerns on 1st Embodiment and the optical element (curve 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 shows the composite optical element which consists of one polarization separation 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 shows the composite optical element which consists of two polarization separation prisms and this invention. It is a figure which shows the function of the optical element which concerns on 3rd Embodiment, and shows the composite optical element which consists of two polarization separation prisms, the optical element of this invention, and a quarter wave 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 design example of the composite optical element which has the function of polarization separation and mode diameter conversion for connecting different optical circuits which concerns on 3rd and 4th Embodiment.

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

In this embodiment, the present invention relates to an optical element according to the first aspect described above. First, an optical element that converts a vertically incident clockwise circularly polarized plane wave into counterclockwise circularly polarized light and condenses it will be described.

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

Further, as shown in FIG. 4, by making the pattern (convex or concave portion) of the photonic crystal curved, the pattern becomes sparse in the central part within one cycle, and becomes denser toward the end. Goes bankrupt. Therefore, the pitch between patterns in the central part is taken as a reference, and it is set as _p0 . Two patterns are merged at a position where _p0 is below a certain threshold pitch. The pitch immediately after merging becomes 2p ₀ , but since it becomes denser toward the end, it is rejoined when it becomes less than the threshold length. By repeating the above operation, the ideal optical axis distribution can be realized while the pitch changes within a certain range. Assuming that the threshold pitch is 0.5p ₀ , the pitch change range is between 0.5p ₀ and 2.0p ₀ . That is, they are geometrically arranged so that the ratio of the maximum value and the minimum value of the distance between one of the adjacent convex portions and the concave portions is within 4 times, preferably within 2 times, and the other branches and merges. .. In the example shown in FIG. 4, the white portion is a concave portion and the black portion is a convex portion. That is, in the case of a wave plate (curved type) in which the main axis direction changes continuously, if the pitch p (pitch when the pattern is a straight line) of the convex portion is p ₀ , then 0.5 · p ₀ ≦ p ≦ 2 ·. The protrusions or recesses are geometrically arranged so as to branch or merge so as to be within _p0 .

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

FIG. 12 shows the results of a simulation in which the clockwise circular polarization passes through the above-mentioned structure. The cross section of the intensity distribution in the xz plane of the light traveling in the z direction is shown. You can clearly see how the incident beam is focused by the structure. Further, since the phase plane is continuously converted, it can be seen that the beam having a mode diameter of 10 microns is efficiently converted to a mode diameter of 4 microns without unnecessary stray light.
The analysis conditions 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
・ Thickness of the entire laminate λ / ( _ns − n _f ) × 0.5
・ Coefficient A = 0.5π / 5 ²

This is the case of vertical incident, but since the incident angle dependence of the photonic crystal wave plate is small as shown in FIG. 8, it operates even for light entering diagonally. In that case, the light has a wave number in the x and y directions, but the wave number of the incident light is also preserved in the emitted light.

In the simulation shown in FIG. 12, the state of condensing parallel light was calculated, but when the light travels in the opposite 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 clear that the beam emitted from the focal plane in FIG. 12 with the beam diameter at the focal plane is converted into parallel light by the lens.

Further, there is no limitation on the refractive index of the incident / emitting medium, and even if the element is emitted into the air or the element is embedded in the adhesive, it will operate if an appropriate design is made.

Further, it is easy to arrange a plurality of optical elements on one substrate, and each of them can have a pattern of different parameters.

The optical element of the present invention operates with respect to 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 optical circuits, and linearly polarized light is often easier to use. Therefore, consider the case where the input and output are linearly polarized.

For example, by arranging the 1/4 wave plate 604 in front of the optical element 605 as in the composite optical element 601 of FIG. 6, it is possible to temporarily convert the incident linear polarization into circular polarization and operate it.

Similarly, as in the composite optical element 602 of FIG. 6, it is also possible to transmit the light output from the optical element 606 in circular polarization through the 1/4 wave plate 607 and output it as linear polarization. Further, as in the composite optical element 603 of FIG. 6, by sandwiching the front and rear of the optical element 608 between the 1/4 wave plates 609 and 610, linearly polarized light is incident on the optical element 608 from the 1/4 wave plate 610 in the subsequent stage. It is also possible to emit linearly polarized light. As shown in FIG. 8, since the incident angle dependence of the photonic crystal wave plate is not sensitive, the expected operation can be realized even if the light is focused and diagonally enters the 1/4 wave plate. If it is necessary to change the direction of the linear polarization, the axis of each 1/4 wave plate may be appropriately selected.

In this case, the optical elements can be arranged one by one, but for example, after forming a multilayer structure having the pattern shown in FIG. 2 by the self-cloning method, the surface is flattened using, for example, a dielectric of SiO ₂ , and further there. By forming another pattern and forming another photonic crystal, different optical elements can be fused. The flattening process may be bias sputtering, etching, CMP, or mechanical polishing. By using this method, the above-mentioned 1/4 wave plate and an optical element having a lens function can be integrated into one sheet. In this case, either one may be up or down, and a three-stage structure of an optical element having a lens function between the upper and lower 1/4 wave plates may be used.

With the structure shown in FIG. 1, it is possible to realize a polarization separation prism that separates incident light into clockwise and counterclockwise circular polarization. Therefore, this polarization separation prism is arranged as the first optical element, and the optical element of the present invention is arranged behind it as the second optical element. At that time, if the pattern shown in 703 of FIG. 7 is arranged at the position where the separated beam is incident, the circular polarizations in opposite directions are incident on each region, the phase plane is converted, and the light is focused on both. It can have an action. The distance between these two lenses is uniquely determined by the separation angle of the prism in the previous stage and the distance between the prism and the lens.

In this case, as shown in FIG. 7, light is obliquely incident on the lens. As mentioned above, the characteristics of the photonic crystal wave plate are insensitive to the incident angle, so there is no problem with the incident at an angle of about 10 degrees from the vertical, but considering the optical system after that, two beams are It is preferably parallel. Therefore, another polarization separation element having the same function is arranged behind the first optical element as the second optical element. Then, the 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 light incident on the first optical element by the second optical element. As a result, the two beams are 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) are the same. In this case, it should also be taken into consideration that the circular polarization is converted into the reverse circular polarization by the second optical element. By arranging the optical element of the present invention after this, light condensed in parallel can be obtained.

Further, as described above, the emitted light can be converted into linearly polarized light. Consider a case in which a 1/4 wave plate is further inserted behind the optical element of the present invention as shown in FIG. For example, if the quarter wave plate passes through a region where the axes are orthogonal to each other as shown in 1005 in FIG. 10, linear polarization in the same direction can be obtained. Alternatively, if both beams pass through the region of the same axial direction as in the 1/4 wave plate shown in the pattern 1006 in FIG. 10, orthogonal polarization can be obtained. It is clear that the direction of the obtained linear polarization is determined by the axial orientation of the 1/4 wave plate 1004.

By combining the polarization separation prism, the optical element of the present invention, and the 1/4 wave plate in this way, it is possible to realize an optical circuit having a function of separating incident beams by polarization, condensing them, and emitting them as linearly polarized light. can. In optical communication, these elements separate the light transmitted through the optical fiber into orthogonally polarized light, and perform polarization separation and conversion while converting the mode diameter to another optical circuit such as silicon photonics whose operating polarization is fixed. Can be done and incident.

Furthermore, as described above, this optical circuit can be stacked and formed on a single substrate, and the optical circuit can be realized with the minimum required thickness, which makes it possible to reduce the size of the optical circuit. It will be useful.

Here, it is assumed that the beam diameter is converted from a large beam to a small beam diameter, such as a coupling from an optical fiber to silicon photonics. However, because the circuit is reciprocal, it is of course possible to couple from a small beam to a large beam. For example, it is assumed that a modulator composed of silicon photonics outputs a beam having the same polarization direction and modulated by different signals. Tracing FIG. 10 from right to left, the linearly polarized light is converted into circularly polarized light orthogonal to each other by the 1/4 wave plate 1004, parallel light is generated by the third optical element 1003, and the first and second optical elements 1001, 1002. It is clear that it is also possible to combine them into one optical path.

For example, FIG. 13 shows an example of designing an optical circuit in which a beam having a mode diameter of 10 microns, which is a typical mode diameter of an optical fiber, is polarized and separated and coupled to a silicon photonics circuit having a mode diameter of 2 microns. Since the mode diameter of the silicon photonics circuit is usually very small, 0.45 micron, it is assumed that the silicon photonics circuit has a mode diameter conversion function. For example, if an optical circuit having a mode diameter of 10 microns and a mode diameter of 2 microns is directly coupled, the loss due to the mode diameter mismatch there is 8.3 dB. On the other hand, it has been clarified by the calculation using the beam propagation method that when the lens of the present invention is used, it can be improved up to 1.5 dB.

Claims (4)

As the first optical element group, one or two polarizing prisms that separate incident light into clockwise and counterclockwise circular polarization are provided, and a second optical element is arranged after the first optical element group. It is a composite optical element
The second optical element is
In the three-dimensional space x, y, z, a wave plate having a phase difference φ formed on the xy plane is provided, and the phase difference φ is an odd multiple of π radians.
The wave plate consists of a curve in which the axial direction θ at a place advanced by a distance r in the radial direction from the center increases by θ = Ar ² with respect to the axial direction at the center using the coefficient A. Consists of patterns,
The coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is the focal length in free space.
Converts the circularly polarized wavefront incident from the -z direction to the + z direction,
It converts focused light, divergent light, or parallel light .
The light incident on the first optical element group is separated into counterclockwise and clockwise circularly polarized light by the first optical element group.
It passes through the second optical element and becomes focused light.
Composite optical element. As the first optical element group, one or two polarizing prisms that separate incident light into clockwise and counterclockwise circular polarization are provided, and a second optical element is arranged after the first optical element group. It is a composite optical element
The second optical element is
In the three-dimensional space x, y, z, a wave plate having a phase difference φ formed on the xy plane is provided, and the phase difference φ is an odd multiple of π radians.
The wave plate is composed of a broken line in which the axial direction θ at a place advanced by a distance r in the radial direction from the center increases by θ = Ar ² with respect to the axial direction at the center using the coefficient A. Consists of patterns,
The coefficient A is expressed as A = ± π / 2fλ (λ is a wavelength), where f is the focal length in free space.
Converts the circularly polarized wavefront incident from the -z direction to the + z direction,
It converts focused light, divergent light, or parallel light .
The light incident on the first optical element group is separated into counterclockwise and clockwise circularly polarized light by the first optical element group.
It passes through the second optical element and becomes focused light.
Composite optical element. The composite optical element according to claim 1 or 2.
The wave plate is composed of photonic crystals laminated in the z-axis direction.
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.
The period in the thickness direction of the photonic crystal is 1/4 or less of the wavelength of the incident light.
Composite optical element. The composite optical element according to claim 1 or 2.
A 1/4 wave plate is further provided as a third optical element after the second optical element .
The light incident from the first optical element group is separated into counterclockwise and clockwise circularly polarized light by the first optical element group.
It passes through the second optical element and becomes focused light.
It is transmitted through the third optical element, becomes linearly polarized light, and is condensed.
A composite optical element having the function of separating incident light into orthogonally polarized light and condensing it.

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