JP2003279746A - Polarization separation element and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a polarization separation element utilizing a photonic crystal polarizer. <P>SOLUTION: Undulate transparent thin films 1 having different refractive indexes 11 are alternately laminated on a slope in a glass board 1 that is formed in a wedge shape to form a polarizer 13. When a wedge angle is set to be θand light is applied from a side where no polarizers are formed, an incident angle to the polarizer becomes θ. TM polarization is transmitted through the polarizer, but is refracted and exits in the direction of ϕ<SB>1</SB>. When the refractive index of air is set to be n<SB>0</SB>and that of the board is set to n, ϕ<SB>1</SB>=sin<SP>-1</SP>(nsinθ/ n<SB>0</SB>)-θ results. Conversely, TE polarization is reflected by the polarizer, reaches the rear surface of the board 10, and is refracted for exiting. In this case, the TE polarization enters the rear surface at an incident angle of 2θ, an emission angle ϕ<SB>2</SB>becomes sin<SP>-1</SP>(nsin2θ/n<SB>0</SB>). As a result, an angle formed by two emission light can be controlled by the values of the refractive index (n) of the board and the wedge angle θ. For example, two beams orthogonally cross each other by setting to n=1.5 and θ=20°. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization separation element that spatially separates two orthogonal polarization components.

[0002]

2. Description of the Related Art Polarizers that spatially separate polarization components are
By using this, polarization-independent devices can be constructed by subjecting separated lights to respective processing operations and then synthesizing them again, which is used in the field of optical communication. Here, as for the separation direction, there are cases where the propagation directions are the same and an offset is provided, and cases where the propagation directions are shifted by 90 ° and separation is performed.

Conventionally, such polarized light separation has been realized by precisely polishing an expensive anisotropic crystal such as rutile or calcite into a wedge shape or a prism shape, but it is extremely expensive as an element. Also, it is difficult to miniaturize.

Further, as a device for separating polarized light, there is also known a polarizing beam splitter in which a glass prism or a flat substrate surface is alternately laminated with multilayer films of dielectric materials having different refractive indexes. Here, the surface is flat. In this structure, by setting the structure of the film and the incident angle so that the Brewster angle is obtained at each interface of the film, the P polarized wave (polarized wave parallel to the incident surface) goes straight and the S polarized wave. (Polarized light perpendicular to the plane of incidence) is reflected in the direction perpendicular to the polarized light, and the polarized light can be separated. However, compared to a prismatic polarizer that uses the birefringence of crystals, it has the drawback of low separation performance (tens of dB).

[0005]

The present invention has two orthogonal
An object of the present invention is to provide an element that spatially separates two polarization components and that is inexpensive and has good separation performance.

[0006]

A polarization separation element of the present invention comprises a wedge-shaped plate (prism) made of an inexpensive material (such as glass) and a photonic crystal polarizer (Japanese Patent Application Laid-Open No. 20-200200).
No. 01-83321).

The concept of the photonic crystal polarizer will be explained. As shown in FIG. 2, a transparent medium having a high refractive index and a low refractive index medium are provided on a glass substrate 3 having a periodic groove array 4 as shown in FIG. The medium having a refractive index is alternately laminated while preserving the shape of the interface. Each of the layers 1 and 2 has a periodicity in the x direction and the z direction, but the y direction may be uniform, or has a periodic or aperiodic structure having a length larger than the x axis direction. Good. When non-polarized light or elliptically polarized light is incident on the periodic structure obtained in this way from the z direction, TE is generated with respect to a polarized wave parallel to the groove array, that is, a y polarized wave, and an x polarized wave orthogonal thereto.
Mode light or TM mode light is induced inside the periodic structure. However, if the frequency of the light is in the TE or TM mode bandgap, the mode cannot propagate in the periodic structure and the incident light is reflected or diffracted. On the other hand, if the frequency of the light is within the energy band, the light passes through the periodic structure while preserving the wave vector. Therefore, this periodic structure operates as a planar polarizer.

The photonic crystal polarizer has a groove array period L.
By adjusting x and the cycle Lz in the stacking direction, the operating wavelength range as a polarizer can be set. As the low refractive index medium, a material containing SiO ₂ as a main component is the most common, has a wide transparent wavelength region, is chemically, thermally, and mechanically stable, and can be easily formed into a film. Si as a high refractive index material
Semiconductors such as and oxides such as TiO ₂ can be used, have a wide transparent wavelength range, and can be used even in the visible light region. On the other hand, semiconductors are limited to the near infrared region, but have the advantage of having a large refractive index.

In the manufacturing method, first, as shown in FIG. 1, periodic grooves 4 are formed on a quartz glass substrate 3 by electron beam lithography and dry etching. Reference numeral 5 is a non-reflection coating layer. On this substrate, SiO ₂ and Si
Using the target of No. ₂ , the SiO ₂ layer and the Si layer are alternately laminated by the bias sputtering method (FIG. 2). At that time, it is important to form the film while preserving the shape of the periodic unevenness in the x-axis direction of each layer. The reason why the regular layered structure shown in FIG. 2 is formed on the substrate is that the deposition of neutral particles from the target by scattered incidence, the sputter etching by normal incidence of Ar ions, and the re-deposition of deposited particles. It can be explained by the superposition of the three effects of adhesion.

When light is vertically incident on such a normal photonic crystal polarizer, the difference in propagation direction between transmitted light and reflected light is 180 °, as shown in FIG. If the incident angle is inclined to θ, it becomes 180-θ, but θ is at most 2 due to the characteristics of the polarizer.
The upper limit is about 0 °.

The polarization separation element of the present invention comprises a transparent substrate,
It is the same as the conventional photonic crystal polarizer in that it consists of a polarizer in which wavy transparent thin films with different refractive indices are alternately laminated on top of it, but the thickness of the transparent substrate is not uniform and it is wedge-shaped ( Claim 1). By using a wedge-shaped substrate in this way, the light reflected by the polarizer can be totally reflected inside the substrate or refracted at a large angle when it goes out of the substrate to separate the polarized light. A wide range of corners can be controlled. Of course, since expensive anisotropic crystals such as rutile and calcite, which have been associated with conventional polarizing prisms, are not required, the feature is that they can be provided at a low cost and above all can be made compact.

This polarization separation element has two spectral patterns. In the polarization beam splitting element according to claim 2, when light is incident on the substrate from the surface opposite to the polarizer, one polarized wave passes through the polarizer and then is refracted and emitted to the outside. The polarized wave is reflected by the polarizer and is emitted while refracting from the surface of the substrate opposite to the polarizer. With this pattern, as the wedge angle is increased, 2
The angle formed by the two emitted lights can be gradually reduced from 180 °, and ultimately the two emitted lights can be made orthogonal to each other (claim 3).

In the polarization separation element according to the fourth aspect, when light is incident on the substrate from the surface opposite to the polarizer, one polarized wave passes through the polarizer and then is refracted and emitted to the outside. The other polarized wave is reflected by the polarizer, is totally reflected by the surface of the substrate opposite to the polarizer, and is emitted while refracting from the side surface of the substrate. With this pattern, as the wedge angle is increased, the angle formed by the two emitted lights becomes smaller, and ultimately both emitted lights can be made parallel and have an offset ( Claim 5).

The polarization splitting element of the present invention is one in which a photonic crystal polarizer is formed on a wedge-shaped substrate. To make it easier, the photonic crystal polarizer is formed on a substrate of uniform thickness, Even if an additional substrate made of the same material whose thickness changes like a wedge is joined to the substrate, the same effect can be obtained (claim 6). Alternatively, the photonic crystal polarizer may be formed on a substrate having a uniform thickness, and then the substrate may be polished into a wedge shape (claim 7).

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The polarization separation element shown in FIG. 4 is formed in a wedge shape, and a groove array 14 is formed on a slope of a quartz glass substrate 10, on which a wavy transparent thin film 11 having a different refractive index is provided.
The polarizer 13 is formed by alternately stacking two layers.
When the wedge angle is θ and light is incident from the side where the polarizer 13 is not formed, the incident angle with respect to the polarizer is θ. TM
The polarized light passes through the polarizer, but is then refracted and goes out in the direction of φ ₁ . The refractive index of air is n ₀ , and the refractive index of the substrate is n
Then, φ ₁ = sin ⁻¹ (n sin θ / n ₀ ) −θ. On the other hand, the TE polarized light is reflected by the polarizer, reaches the back surface of the substrate 10, is refracted, and goes out. At this time, since the light is incident on the back surface at the incident angle 2θ, the exit angle φ ₂ = sin ⁻¹ (n sin2θ / n ₀ ).

Therefore, the refractive index n of the substrate and the wedge angle θ
The angle between the two emitted lights can be controlled by the value of. For example, if n = 1.5 and θ = 20 °, the two beams are orthogonal.

Although the wave directions of the thin films 11 and 12 in FIG. 4 are parallel to the paper surface, they may be perpendicular to the paper surface as shown in FIG. In this case, the directions of the transmitted and reflected polarized waves are switched.

The polarization separation element shown in FIG. 4 is a wedge-shaped substrate 10.
The groove array 4 was provided on the substrate, and the polarizer was laminated on the groove array. However, as another manufacturing method, the groove array was provided on the substrate of uniform thickness, the polarizer was formed on the groove array, and then the substrate was polished to form a wedge shape. You may Furthermore, as shown in FIG. 7, the substrate 10 having a uniform thickness is used.
A groove array may be provided on a to form the polarizer 13, and then the additional substrate 10b of the same material, the thickness of which changes like a wedge, may be bonded to the substrate 10a with an adhesive. This makes it easier to make in some cases.

The polarization beam splitting element shown in FIG. 5 is adapted to utilize total reflection. In the figure, the same members as those in FIG. 4 are designated by the same reference numerals. The angle between the upper surface of the substrate and the right side surface is θ ₂ . When light is incident from the side on which the polarizer is not formed, the incident angle on the polarizer is θ ₁ . TM polarized light passes through the polarizer and is refracted at that time, φ ₁ = sin ^-1 (nsi
Propagation occurs at nθ ₁ / n ₀ ) −θ ₁ . On the other hand, the TE polarized light is reflected by the polarizer and reaches the back surface of the substrate. At this time, the incident angle is 2θ ₁
Next, it causes total reflection. After that, the light reaches the side surface and the emission angle φ ₂ = θ ₂ −sin ⁻¹ (n sin (θ ₂ −2θ ₁ ) / n ₀ ). Therefore, the angle formed by the two beams can be controlled by the values of the refractive index n of the substrate and the wedge angles θ ₁ and θ ₂ . For example, when n = 2.53, θ ₁ = 20 °, and θ ₂ = 40 °, the propagation directions of the two beams match.

In FIG. 5, the wave direction is parallel to the paper surface, but it may be perpendicular to the paper surface. In this case, the directions of the transmitted and reflected polarized waves are switched.

EXAMPLE The polarization separation element shown in FIGS. 4 and 5 was manufactured by the following method. First, a fine groove array was processed on the substrate surface by photolithography and dry etching. The pattern had a period of 0.5 μm, the concave and convex portions were 1: 1 and the depth was about 0.2 μm. Next, using a bias sputtering apparatus, alternating multilayer films of Si and SiO ₂ were laminated by the self-cloning method. The period is 570 nm, 10 periods, and the thickness ratio of the Si layer and the SiO ₂ layer is 3: 7. The deposition conditions are Ar gas pressure 6 mTorr for SiO ₂ deposition, target high-frequency power 600 W, substrate high-frequency power 100 W, and Ar gas pressure 1 mTorr for Si deposition.
The target high frequency power was 600W. In this way, a serrated surface shape was formed.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a substrate forming a photonic crystal polarizer.

FIG. 2 is a conceptual diagram of a photonic crystal polarizer body.

FIG. 3 shows the action when light is applied to the photonic crystal polarizer.

FIG. 4 is a sectional view of a polarization beam splitting element.

FIG. 5 is a cross-sectional view of a polarization separation element that also utilizes total reflection.

6 is a diagram in which the wave directions of the thin film of FIG. 4 are shifted by 90 °.

FIG. 7 shows a structure in which the substrates of the polarization beam splitting element are bonded together.

[Explanation of symbols]

10 substrates 11 thin film 12 thin film 13 Polarizer 14 groove rows

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Nao Sato             1-2-5 Tomizawa Minami, Taihaku Ward, Sendai City, Miyagi Prefecture             Nar Tomizawa 302 (72) Inventor Osamu Ishikawa             2-21-8 Shiomidainan, Shichigahama-cho, Miyagi-gun, Miyagi Prefecture F-term (reference) 2H049 BA05 BA45 BC01                 4G059 AA08 AA11 AB01 AB09 AB11                       AC01 AC09 BB01 GA02 GA04                       GA14

Claims (7)

[Claims] 1. A polarization splitting element comprising a transparent substrate and a polarizer in which wavy transparent thin films having different refractive indices are alternately laminated on the transparent substrate, and the substrate is not uniform in thickness but wedge-shaped. . 2. When light is incident on the substrate from a surface opposite to the polarizer, one of two orthogonal polarization components passes through the polarizer and then exits while refracting. The polarized light separating element according to claim 1, wherein the other polarized light is reflected by the polarizer and is emitted to the outside while refracting from the surface of the substrate opposite to the polarizer. 3. The polarization beam splitting element according to claim 2, wherein an angle formed by the two outgoing lights is 90 °. 4. When light is incident on the substrate from a surface opposite to the polarizer, one polarized light of two orthogonal polarization components passes through the polarizer and then is refracted and emitted outside. And the other polarized wave is reflected by the polarizer, is totally reflected by the surface of the substrate opposite to the polarizer, and is emitted from the side surface of the substrate while being refracted. The polarization separation element according to 1. 5. The polarization separation element according to claim 4, wherein both emitted lights are parallel to each other and have an offset. 6. The polarization beam splitting element according to claim 1, wherein the polarizer is laminated on a substrate having a uniform thickness, and an additional substrate having a wedge-shaped thickness is bonded to the substrate. Manufacturing method. 7. The method according to claim 1, wherein the polarizer is laminated on a substrate having a uniform thickness, and then the substrate is polished into a wedge shape.
4. The method for manufacturing the polarization separation element according to 4 or 5.