JPH09105813A - Grating polarizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarizer which can extract linear polarized light from natural light without decreasing the light intensity. SOLUTION: This grating polarizer 1 is constituted by laminating a polarization grating 2 which functions as a diffraction grating for unidirectional polarized light and functions not as a diffraction grating, but as a transparent medium for polarized light crossing the light at right angles, and a birefringent sheet type material 3. In this case, the thickness of the birefringent sheet type material 3 is so set that when the natural light is made incident from the side of the polarization grating 2, polarized light (s) diffracted by the polarization grating 2 and polarized light (p) which is transmitted without being diffracted by the polarization grating are transmitted through the birefringent sheet type material 3 to become polarized light beams in the same direction.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a polarizer useful for extracting linearly polarized light from natural light.

[0002]

2. Description of the Related Art Polarizers are used in optical devices such as liquid crystal display devices, polarization microscopes, and polarization interferometers that take out linearly polarized light from natural light having random polarization and utilize the linearly polarized light.

In this case, as the polarizer, conventionally, a polarizing film formed by forming an alignment film of iodine or the like on a thin resin film or the like (that is, a film of thin resin film or the like on which the iodine or the like is oriented and adsorbed) is used. Is widely used.

[0004]

However, when linearly polarized light is extracted from natural light composed of randomly polarized light using a polarizing film using an alignment film of iodine or the like, the polarizing film is oriented to the orientation of the polarizing film among natural light. Since light other than polarized light according to the direction is absorbed, more than half of the light intensity of natural light incident on the polarizing film is lost. Therefore, in an optical device that uses linearly polarized light, it is necessary to increase the light intensity of the light source in order to obtain linearly polarized light with a desired light intensity, or to provide a high-sensitivity sensor. There has been a problem that the cost is high and the size cannot be reduced. In addition, since the light energy absorbed by the polarizing film is changed to thermal energy, the polarizing film is heated, so that the accuracy of the polarizing film becomes unstable and the durability is deteriorated.

The present invention is intended to solve the above problems of the prior art, and an object thereof is to provide a polarizer capable of extracting linearly polarized light from natural light without loss of light intensity.

[0006]

In order to achieve the above object, the present inventor functions as a diffraction grating for polarized light in one direction and as a diffraction grating for polarized light in a direction orthogonal thereto. A polarizing polarizer in which a polarizing grating that does not function but functions as a transparent medium and a birefringent sheet-like material are laminated, and the polarized light is diffracted by the polarizing grating when natural light enters from the polarizing grating side. And a polarized light which is transmitted without being diffracted by the polarizing grating and is made into polarized light in the same direction by being transmitted through the birefringent sheet material.

In a particularly preferred embodiment, there is provided a grating polarizer in which the birefringent sheet-like material is composed of a uniaxial crystal, and its optical axis is perpendicular to the sheet surface of the birefringent sheet-like material.

Further, in the above-mentioned grating polarizer of the present invention, a polarizing film is laminated on the exit side of the birefringent sheet material, and the polarization of outgoing light transmitted through the birefringent sheet material is obtained. Provided is one in which the direction and the polarization direction of the polarizing film match.

In the polarizer of the present invention, when natural light is made incident from the polarization grating side, the light transmitted through the polarization grating is transmitted through the polarization grating diffracted by the polarization grating and the polarization grating without being diffracted by the polarization grating. Then, it is separated into polarized light diffracted by the polarizing grating and polarized light in a direction orthogonal to the polarized light. These polarized lights have a polarization direction when they enter the birefringent sheet material, an incident angle to the birefringent sheet material, a refractive index of the birefringent material forming the polarization grating, a birefringent sheet. The polarization direction changes depending on the refractive index of the sheet material, the thickness of the birefringent sheet material, and the like, and when the birefringent sheet material is emitted, both polarization directions match.
Therefore, according to the present invention, it is possible to obtain polarized light in one direction without losing the intensity of incident natural light.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail with reference to the drawings. In each of the drawings, the same reference numerals represent the same or equivalent components.

FIG. 1 shows a grating polarizer 1 of the present invention.
It is a schematic diagram of the lattice cross-section structure of 1 aspect. As shown in the figure, this grating polarizer 1 is composed of a polarizing grating 2
And a birefringent sheet material 3 are laminated.

In the present invention, the polarization grating 2 is a diffraction grating that functions as a diffraction grating for polarized light in one direction, but does not function as a diffraction grating for polarized light in a direction orthogonal thereto and functions as a transparent medium. It is a grid. As such a polarization grating 2, for example, as shown in the same drawing, a lithium niobate layer 4 which is a birefringent material.
It is possible to use the one composed of the ion exchange section 5 and the phase compensation dielectric 6 formed on the ion exchange section 5.

The illustrated polarization grating 2 is generally known as a polarization holographic element having a diffraction effect, and is manufactured as follows (Tech. Digest of MICROOPTICS Conference, 91, 242-245).
(1991)). First, a metal thin film pattern is formed on the surface of the lithium niobate layer 4 which is a birefringent crystal by a photolithography method, and then the surface of the lithium niobate layer 4 is immersed by immersing it in benzoic acid which is heated and melted. The layers are ion-exchanged to form the ion-exchange section 5 in a lattice shape. The ion exchange section 5 acts as a phase grating to increase the refractive index of extraordinary light by about 0.1 and decrease the refractive index of ordinary light by about 0.03. Next, the phase compensating dielectric 6 is formed on the ion exchange section 5 as a phase compensating grating. Here, by appropriately setting the depth of the ion exchange part 5 and the film thickness of the phase compensating dielectric 6, the polarization holographic element is set to have a phase difference of 0 (the same as that of ordinary light) as shown in FIG. (Figure (a))
For extraordinary light, a grating with a phase difference of π ((b) in the figure) can be used (in FIG. 2, the part filled with dots represents the refractive index difference due to the ion exchange part 5, and the shaded part is The refractive index difference due to the phase compensating dielectric 6 is shown.). Therefore, when set in this way, the polarization holographic element becomes a transparent medium for ordinary light and transmits ordinary light, while it becomes a diffraction grating for extraordinary light.

In the present invention, the polarization grating 2
Is not limited to the above-mentioned polarization holographic element. For example, a surface-relief type grating is laminated on the surface of a birefringent sheet material, and the groove portion of the grating has a refractive index for a certain polarization direction and a refractive index of the birefringent sheet material for that polarization direction. It is possible to use a material filled with the same material as the above. In this case, the surface relief type grating can be formed by a generally known method of mechanical engraving, a method of utilizing laser light interference for holography, a method of photolithography and the like.

On the other hand, as the birefringent sheet material 3,
There is no particular limitation as long as it is made of a birefringent material. For example, lithium niobate crystal, lithium oxide, sapphire, KD
Sheet-like materials made of uniaxial crystals such as P and ADP can be preferably used. In addition to these, it is also possible to use those made of biaxial crystals such as glass nitrate. However,
From the viewpoint of preventing the interfacial reflection loss between the polarization grating 2 and the birefringent sheet-like material 3, the birefringent sheet-like material 3 is the birefringent material (that is, the polarized light of FIG. It is preferably formed from a material having the same refractive index as the lithium niobate layer 4) of the grating 2. In this case, both may be integrally formed from the same material. Thereby, the manufacturing process of the grating polarizer 1 can be simplified.

The thickness of the birefringent sheet material 3 is
Two orthogonal polarized lights that are diffracted or transmitted through the polarization grating 2 are respectively reflected by the birefringent sheet material 3
To the birefringent sheet material, the angle of incidence on the birefringent sheet material, the refractive index of the birefringent material forming the polarization grating 2, the birefringent sheet material 3 and the like. The polarization directions of the two polarized lights that were originally orthogonal to each other should be the same when the birefringent sheet material 3 is emitted.

The direction of the optical axis of the birefringent sheet material 3 is not particularly limited, but from the viewpoint of easy design,
It is preferable to be perpendicular to the sheet surface of the birefringent sheet material.

Next, in the grating polarizer shown in FIG. 1, a sheet material made of lithium niobate crystal is used as the birefringent sheet material 3, and its optical axis is perpendicular to the sheet surface of the sheet material. The operation and design method of this grating polarizer will be described with reference to FIG. Here, for the sake of convenience, the polarized light in the paper surface direction is P polarized light, and the polarized light in the direction perpendicular to the paper surface is S polarized light. The polarization grating 2 functions as a diffraction grating only for S-polarized light. Further, in FIG. 3, for convenience of explanation, the polarization grating 2 and the birefringent sheet-like material (lithium niobate crystal) 3 are illustrated separately.

When natural light L enters the polarizing grating 2 of the grating polarizer 1 as shown in FIG.
The polarization grating 2 acts as a homogeneous transparent sheet having a refractive index n _p for the p-polarized light p. Therefore, this light p is transmitted without being diffracted. On the other hand, with respect to the S-polarized light s, the polarization grating 2 acts as a phase grating to diffract the light s. Therefore, the p-polarized light p
Is incident on the birefringent sheet material 3 as it is when it is incident on the polarization grating 2, and the S-polarized light s is
The light enters the birefringent sheet material 3 after being bent at a diffraction angle according to the spatial frequency of the polarization grating 2. In this case, the spatial frequency μ and the diffraction angle θ are n _s sin (θ) = μ λ according to a known grating formula (where, n _s is a birefringent material (lithium niobate layer 4) is the refractive index for S-polarized light, and λ is the wavelength of light.

On the other hand, the birefringent sheet material 3 made of lithium niobate crystal has a uniaxial crystal structure, and the optical axis is perpendicular to the sheet surface of this sheet material.
This means that the traveling direction of light that is incident perpendicularly to the sheet surface and the direction of the optical axis are the same. Therefore, the p-polarized light p transmitted through the polarization grating 2 is transmitted while maintaining the p-polarized light without changing the polarization. On the other hand, S-polarized light s diffracted by the polarization grating 2
Has an incident angle θ (= diffraction angle), and thus is polarized due to the birefringence of the birefringent sheet material 3. This state,
This will be described in more detail with reference to FIG.

As shown in FIG. 4, the ξ-axis and the η-axis are considered by sandwiching the polarization direction B of the S-polarized light in a plane A perpendicular to the traveling direction of the S-polarized light s. The refractive indices are n _ξ and n _η , respectively. In this case, the ξ component when the S-polarized light s passes through the birefringent sheet-like material 3 is n _ξ d
/ Cos (θ) and the η component is n _η d / co
It will pass through the optical path length of s (θ). Here, d is the thickness of the birefringent sheet material 3.

The thickness of the birefringent sheet material 3 is such that the optical path difference Δ between the ξ component and the η component is Δ = n _ξ / cos (θ) −n _η d / cos (θ) = λ / 2. When d is determined, the birefringent sheet material 3 becomes a half-wave plate for the S-polarized light s, and the polarization direction thereof is rotated by 90 °. Therefore, when the birefringent sheet material 3 is incident, S
The polarized light s becomes P-polarized light after passing through the sheet-shaped material 3.

Therefore, this grating polarizer 1
, The p-polarized light p at the time of incidence does not diffract and the polarization direction does not change and passes through the grating polarizer 1 as it is, but the s-polarized light s at the time of incidence is transmitted by the polarization grating 2. The light is diffracted, and the birefringent sheet material 3 further rotates the polarization direction by 90 ° to become P-polarized light. Therefore, only the P-polarized light is emitted from the birefringent sheet material 3. In this case, the grating polarizer 1 does not absorb light in one polarization direction unlike the conventional polarizing film. Therefore, this grating polarizer 1 becomes a polarizer with a very small loss of light intensity.

The above description of the operation of the grating polarizer 1 is completely the same when the relationship between the P-polarized light and the S-polarized light is reversed.

In the grating polarizer 1 of the present invention, a known polarizing film may be provided on the exit side of the birefringent sheet material 3 if necessary. In this case, the polarization direction of the polarizing film is made to coincide with the polarization direction of the light transmitted through the birefringent sheet material 3. Thereby, the birefringent sheet material 3
Even if the polarization direction of the light emitted from is not completely single, the grating polarizer 1 can emit light of a single polarization direction.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments.

A grating polarizer having the structure shown in FIG. 1 was produced as follows.

First, Tech. Digest of Second Optoelectr
Polarizing grating 2 was produced according to the method described in onics Conference, 166-169 (1988). That is, a metal thin film pattern of the grating pattern is formed on the surface of the lithium niobate crystal by a photolithography method,
This was immersed in benzoic acid melted by heating to carry out ion exchange, and a phase grating composed of an ion exchange part was formed on the surface of the lithium niobate crystal. Next, a dielectric grating for phase compensation was formed on the ion exchange section. In this way, a polarization grating (spatial frequency μ = 400 lines / mm) having a phase difference π for extraordinary rays and a phase difference 0 for ordinary rays was obtained.

For the sake of explanation unified with the description of FIGS. 3 and 4, it is assumed that for the obtained polarization grating 2, the ordinary ray is P-polarized and the extraordinary ray is S-polarized.
Further, it is known that the refractive index of ordinary rays of lithium niobate at room temperature is 2.29 and the refractive index of extraordinary rays is about 2.2.

When natural light L is incident on the obtained polarization grating 2 as shown in FIG. 3, the phase difference with respect to the P-polarized light p is 0. Therefore, the polarization grating 2 refracts against the P-polarized light p. It becomes a homogeneous transparent medium with a ratio of 2.29, and the light p is transmitted as it is without being diffracted. on the other hand,
Since the phase difference π is given to the S polarized light s, it becomes a phase grating and diffracts the light s. In this case, the diffraction angle θ of the light s
Is the spatial frequency of the polarization grating 2 = 400 /
Since the distance is set to mm, for example, the diffraction angle θ = 5.51290 ° for light having a wavelength λ = 550 nm.

A lithium niobate sheet as a birefringent sheet material 3 was attached to the polarizing grating 2 having such characteristics. In this case, the optical axis of the lithium niobate sheet was set to be perpendicular to the sheet surface. The thickness d of the lithium niobate sheet 3 is
For the light s entering the lithium niobate sheet 3, d = 311 μm so as to form a half-wave plate.

That is, the lithium niobate sheet 3 is a homogeneous substance having a refractive index of 2.29 in the optical axis direction, and has a refractive index of 2.28912 for light incident at an incident angle of 5.51290 °. It has two refractive indices of 2.29. Therefore, the P-polarized light p that has been transmitted through the polarization grating 2 as it is becomes a homogeneous medium having a refractive index of 2.29, and the light p is transmitted while maintaining the P-polarized light without changing its polarization. On the other hand, since the S-polarized light s enters at an incident angle of 5.51290 °, the refractive index n _ξ =
2.29 and the refractive index n _η = 2.28912 can be set. Therefore, in this embodiment, the optical path difference Δ between the ξ component and the η component is Δ = n _ξ / cos (θ) −n _η d / cos (θ) = λ / 2 at the wavelength λ = 550. The thickness d of the birefringent sheet material 3 is 311
μm.

The thus-obtained grating polarizer 1 of the embodiment converted the natural light incident from the polarization grating 2 side into P-polarized light with high selectivity and emitted it. Further, in this case, the loss ratio of the light intensity was extremely small.

[0034]

According to the present invention, linearly polarized light can be extracted from natural light without loss of light intensity. Therefore, in an optical device that uses linearly polarized light, it is not necessary to unnecessarily increase the intensity of the light source or increase the sensitivity of the sensor, and it is possible to reduce the manufacturing cost of the device and downsize the device. .

[Brief description of the drawings]

FIG. 1 is a sectional view of a grating polarizer of the present invention.

FIG. 2 is an explanatory diagram of a phase distribution in a polarization grating.

FIG. 3 is an explanatory view of the action of the grating polarizer of the present invention.

FIG. 4 is an explanatory view of the action of a birefringent sheet material.

[Explanation of symbols]

 1 Grating Polarizer 2 Polarizing Grating 3 Birefringent Sheet-like Material 4 Lithium Niobate Layer 5 Ion Exchange Section 6 Phase Compensation Dielectric

Claims (6)

[Claims] 1. A polarization grating that functions as a diffraction grating for polarized light in one direction, and does not function as a diffraction grating for polarized light in a direction orthogonal thereto, and functions as a transparent medium, and a birefringent sheet-like shape. When a natural light enters from the polarizing grating side, the polarized light diffracted by the polarizing grating and the polarized light transmitted without being diffracted by the polarizing grating are birefringent. A grating polarizer, which is configured to be polarized in the same direction by transmitting a material sheet material. 2. The grating polarizer according to claim 1, wherein the optical axis of the birefringent sheet material is perpendicular to the sheet surface of the birefringent sheet material. 3. The grating polarizer according to claim 1, wherein the birefringent sheet-shaped material is a uniaxial crystal. 4. The grating polarizer according to claim 1, wherein the birefringent material and the birefringent sheet material forming the polarization grating are made of the same refractive index material. 5. The grating polarizer according to claim 4, wherein the birefringent material and the birefringent sheet-shaped material forming the polarization grating are integrally formed of the same material. 6. The birefringent sheet material is laminated on the exit side with a polarizing film so that the polarization direction of the outgoing light of the birefringent sheet material and the polarization direction of the polarizing film coincide with each other. ~
5. The grating polarizer according to any one of 5 above.