JP2000284121A - Polarized light separating film and polarized light separating prism - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a high extinction ratio with a small number of layers and to adopt wavelength in a wide band by using gallium oxide as the material of low refractive index layers used as layers of a polarized light separating film. SOLUTION: A polarized light separating film is formed on the junction face of a junction type prism to obtain the objective polarized light separating prism. An ordinary glass material having a refractive index of 1.52 is used as the prism material on the incident side. The polarized light separating film is obtained by depositing many combinations of a high refractive index layer and a low refractive index layer and further forming a high refractive index layer. Gallium oxide (GaO2) having a refractive index of about 1.2-1.3 is used as the material of the low refractive index layers. Tantalum oxide (Ta2O5) having a refractive index of 2.1 is used as the material of the high refractive index layers combined with the low refractive index layers in accordance with the McNeil conditions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization separation film for separating incident light into s-polarized light and p-polarized light, and a polarization separation prism using the polarization separation film.

A polarization splitting film used for optics is widely used as a detection optical system for an optical disk and an illumination optical system for a projector. Particularly for projector applications, there is a growing demand for a wider wavelength band that can be separated by one polarization separation film.

As described above, the polarization splitting film can realize a higher extinction ratio (ratio of p-polarized light transmittance and s-polarized light transmittance) with a smaller number of layers, and can further broaden the wavelength band. is needed.

[0004]

2. Description of the Related Art As a polarizing beam splitter for separating incident light into s-polarized light and p-polarized light, two layers having different refractive indices (a high refractive index layer and a low refractive index layer) are formed on a bonding surface of a bonding prism made of glass or the like. There is known a polarization splitting prism in which a polarization splitting film is formed by alternately depositing a plurality of refractive index layers. As a combination of the refractive indices used for the layers of the polarization separation film, a MacMeille type polarization separation film is known. In this McNail-type polarization separation film, the transmittance of p-polarized light has a high value in a wide wavelength range, but the reflectance of s-polarized light and the usable wavelength range are limited by the combination of the refractive indices of the film materials used. .

FIG. 3 explains the reflectance of p-polarized light at the interface of the optical thin film, and explains the Brewster angle (polarization angle). FIG. 3 shows that light is incident from the high refractive index n _H layer side at an incident angle θ _H to the interface where the material layer having the high refractive index n _{H and} the material layer having the low refractive index n _L are joined. The s-polarized component is reflected, the p-polarized component is transmitted,
The state where the transmitted light is refracted at the refraction angle θ _L and exits into the low refractive index n _L layer is shown. The reflection coefficient r _p for p-polarization component in this case is expressed by the following equation.

R _p = (n _H / cos θ _H −n _L / cos
θ _L ) / (n _H / cos θ _H + n _L / cos θ _L ) Here, from the condition that all p-polarized light is transmitted, that is, the condition that r _p = 0, n _H / cos θ _H = n _L / cos θ _L θ _H = tan ⁻¹ (n _H / n _L ) is derived as the Brewster angle (polarization angle). The high refractive index n _H and the low refractive index n _L satisfying this condition, the incident angle θ _H ,
The combination of the refraction angles θ _L is known as a MacMeille condition. That is, when the McNail condition is satisfied, the p-polarized light component is transmitted without being reflected at the interface.

On the other hand, the s-polarized light component has a reflectance and a usable wavelength range based on the refractive index n _H of the high refractive index layer and the refractive index n _L of the low refractive index layer used for the polarization separation film. Will be decided.

A case where the above-described McNail condition is applied to a polarization splitting film of a junction type polarization splitting prism as illustrated in FIG. 4 will be considered. Now, assuming that the refractive index n ₀ of the material of the incident side prism (here, ordinary glass) is 1.52 and the apex angle θ _{0 of the} prism is 45 degrees, an example of the combination of the refractive indices of the layer materials satisfying the McNail condition is as follows. By calculation, for example, when the refractive index of the low refractive index layer is 1.38, the refractive index of the high refractive index layer is 1.71.

Substances that can be used as each layer of the polarization separation film
Is commonly used as a low refractive index material
And MgF_Two, SiO_TwoA for high refractive index materials
l_TwoO _Three, Y_TwoO_Three, SiO, TiO_Two, Ta_TwoO_FiveWhat
What is it? The combination of refractive indexes that can be used for the polarization separation film is
In reality, it depends on whether there is a practical low refractive index material.
Is determined. An example is the low refractive index commonly used today.
The material has a refractive index (ie, n_L) Is 1.38 Mg
F_Two(Magnesium fluoride), combined with this
The refractive index (ie, n_H) Is 1.7
Y of 0_TwoO_Three(Yttrium oxide).

FIG. 5 shows a practical design example of the polarization splitting film.
And the material of the low refractive index layer is MgF _Two(N_L= 1.3
8) using Y as a material for the high refractive index layer_TwoO_Three(N_H
= 1.70) to form a polarization separation film.
Excess rate / relative wave number characteristics are shown. In the figure, the horizontal axis is the relative wave number
(Relative Wave Number), the vertical axis is transmittance (Transmittanc
e). Here, the relative wave number is a predetermined center wavelength (for example,
For example, the wavelength is represented by a magnification with respect to 550 nm).
Thus, the relative wave number 1 corresponds to the center wavelength. Also, in the figure
(HL)ⁿH [however, n = 5 in the example of FIG. 5]
The polarization separation film is composed of n layers of a combination of a high refractive index layer and a low refractive index layer.
The high refractive index layer was formed by adding one layer by itself.
And Therefore, in the case of this example, the number of deposited layers is 11
Becomes In FIG. 5, (1) is the characteristic of the p-polarized light component, and (2) is
This is the characteristic of the s-polarized light component. As can be seen from FIG.
The p-polarized light has a transmittance of almost 100%. On the other hand, s-polarized light
Wavelengths in the range of 0.85 to 1.15 are reflected.
However, there are components that pass through, which is sufficient for the extinction ratio
Is not obtained.

FIG. 6 shows a design example of a conventional polarization splitting film for broadening the wavelength of reflected light (s-polarized light component). In this design example, fifteen high-refractive-index layers and low-refractive-index layers are provided, and a single high-refractive-index layer is further added thereto, and the thickness of each layer is changed little by little. This is for widening the bandwidth. As shown in the figure, according to this design example, the wavelength of the reflected light (s-polarized light component) is widened to the range of the relative wave number of 0.7 to 1.3, but the extinction ratio is still sufficient. Things have not been obtained.

As a method of further increasing the reflectance of the s-polarized light component, a prism material to be used (glass) having a higher refractive index, for example, a high refractive index having a refractive index of about 1.7 to 1.8 is used. There is a method using a rate glass. When a high refractive index glass is used, the difference between the combination of the high refractive index and the low refractive index that satisfies the above-mentioned McNail condition becomes large, and the reflectance of the s-polarized light component becomes large.

[0013]

Conventionally, a polarizing beam splitting film which can be practically used has not yet realized a sufficient extinction ratio, and also has an insufficient wavelength broadening band. Although a design method as shown in FIG. 6 is also possible to achieve a wide band, in this case, the number of layers to be deposited increases, and the manufacturing time is increased. There is also a method of using a high-refractive-index glass as a material for the prism. However, such a high-refractive-index glass is expensive and has a problem that processing is difficult.

The present invention has been made in view of the above problems, and has as its object to realize a higher extinction ratio with a smaller number of layers and to further widen the wavelength band.

[0015]

In order to solve the above-mentioned problems, a polarized light separating film according to the present invention comprises a high-refractive-index layer and a low-refractive-index layer alternately deposited on the surface of a transparent object in a multilayer manner. Gallium oxide (GaO ₂ ) is used as the material of the refractive index layer. Further, a polarization splitting prism according to the present invention is obtained by forming the above-mentioned polarization splitting film on the junction surface of the junction type prism.

[0016]

FIG. 2 is a graph showing various calculations of combinations of the high refractive index and the low refractive index of the layer material of the polarization separation film according to the above-mentioned McNail condition, for the polarization separation film of the polarization separation prism shown in FIG. is there. Here, a low refractive index glass (for example, BK7) having a refractive index n ₀ of 1.52 is used as a material of the incident side prism, and the apex angle θ _{0 of the} prism is 45 degrees. According to the calculation result of FIG. 2, as the value of the low refractive index n _L decreases, the high refractive index n _{L associated} therewith decreases.
It can be seen that the difference between _H and the value increases. Therefore, based on this finding, in the present invention, gallium oxide (GaO ₂ ), which is a substance having a refractive index of about 1.2 to 1.3, is used as the material of the low refractive index layer. As a result, the difference between the low refractive index and the high refractive index of the combination of the layer materials can be made large, thereby realizing a high reflectance and a wide operating wavelength range for s-polarized light.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing transmittance / relative wave number characteristics of a polarization splitting prism as one embodiment of the present invention. This polarization splitting prism uses a normal glass material (specifically, BK7) having a refractive index of 1.52 as a prism material on the incident side. The polarization separation film formed on the prism bonding surface has a layer structure of (HL) ⁵ H, that is,
It has a structure in which five combinations of a high refractive index layer and a low refractive index layer are deposited, and a single high refractive index layer is further added thereto. The thickness of each layer of this layer structure follows the design example of a normal polarization separation film. Here, as the low refractive index layer, gallium oxide (GaO ₂ ) having a refractive index (n _L ) of 1.25 is used. n _H) is tantalum oxide of 2.1 (Ta ₂ O ^5).

The gallium oxide (Ga) used here
O ₂ ) is a substance disclosed in U.S. Pat. No. 5,474,851 for use as an antireflection film, for example, and its production method is also a known one disclosed in the same patent. This gallium oxide has a refractive index of 1.2 to 1.3.

FIG. 1 is a diagram showing the transmittance / relative wave number characteristics of the polarization splitting prism having the above-described configuration. In the figure, the horizontal axis represents the relative wave number, the vertical axis represents the transmittance, (1) is the characteristic of the p-polarized component,
(2) is the characteristic of the s-polarized light component. As can be seen from FIG. 1, the transmittance of the p-polarized light component is almost 100%, and almost all of the s-polarized light component is reflected (transmittance is 0) in the range of the relative wave number of 0.7 to 1.3. . As can be seen by comparing the characteristic of FIG. 1 with the characteristic of the conventional design example shown in FIG. 5, according to the present invention, a high extinction ratio can be obtained in a very wide wavelength range with a small number of layers as a polarization separation film. Things.
For example, as compared with the conventional design example for widening the band shown in FIG. 6 (using MgF ₂ for the low refractive index layer), the conventional design example requires the deposition of 31 layers on the polarization separation film. In the embodiment, a higher extinction ratio can be obtained with 11 layers. Also, as the material of the prism, it is not necessary to use a material having a high refractive index, and ordinary glass (for example, BK7 having a refractive index of 1.52) is sufficient, and it is inexpensive and easy to manufacture.

In the present invention, when it is desired to further increase the wavelength band, the conventional design method shown in FIG. 6 in which the number of layers to be deposited is increased and the thicknesses of the layers are gradually changed can be used. .

In the above description, the polarized light separating film of the present invention is
Although the design in which the refractive index of each layer constituting the multilayer film is set according to the McNail condition has been described as an example, the present invention is not limited to this, and the refractive index is determined by another method. Of course, the present invention can be applied to a typical polarization separation film.

[0022]

As described above, according to the present invention, it is possible to obtain a remarkably high extinction ratio as compared with the related art while forming the polarization splitting film with a small number of layers, and to obtain a sufficient extinction ratio. Can be obtained in a wide wavelength range. Further, since a normal glass material is sufficient as the prism material in order to obtain a high extinction ratio, it is not necessary to use a material having a high refractive index, and the polarization splitting prism can be manufactured easily and at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating transmittance / relative wave number characteristics of a polarization splitting prism as one embodiment of the present invention.

FIG. 2 is a diagram for explaining McNail conditions.

FIG. 3 is a diagram illustrating a Brewster angle (polarization angle).

FIG. 4 is a diagram for explaining a function of a polarization splitting prism.

FIG. 5 is a diagram showing transmittance / relative wave number characteristics of polarization separation bristles according to a conventional typical design example (using MgF ₂ ).

FIG. 6 shows a conventional design example (MgF
₂ is a diagram showing the transmittance / relative wave number characteristics of the polarization separation brhythm by using ( ₂ ).

[Explanation of symbols]

n _H high refractive index n _L low refractive index n ₀ refractive index of incident light side prism material θ _H incident angle θ _L refractive angle θ θ ₀ prism apex angle

Claims (2)

[Claims] 1. A polarized light separating film comprising high refractive index layers and low refractive index layers alternately deposited on the surface of a transparent object, and using gallium oxide as a material for the low refractive index layer. 2. A polarized light separating prism wherein the polarized light separating film according to claim 1 is formed on a bonding surface of a bonded type prism.