JPH06130224A - Polarizing beam splitter - Google Patents

Abstract

PURPOSE:To attain the light weight and miniaturization of the whole optical system by arranging optical multilayer film protruded on a transparent substrate and a material having, specific refractive index between the optical multilayer films, and comprising the optical multilayer film by applying specific film repeatedly. CONSTITUTION:Assuming an equivalent refractive index for first polarized light as M1 and that for second polarized light as N2 when an incident direction is decided, a diffraction grating in which the optical multilayer film 2 is stacked partially and the material 3 with refractive index N2 is arranged in an area where no optical multilayer film 2 is stacked is generated. When light is made incident on such diffraction grating, it functions as the diffraction grating of refractive index change type of refractive index N1 and refractive index N2 for the first polarized light, and light is emitted as diffracted light 5. As for the second polarized light, a function as the diffraction grating is not provided since the refractive index of the optical multilayer film 2 coincides with that of the arranged material 3, and light is transmitted as it is as non-diffracted light 6. Therefore, it is possible to comprise a polarizing beam splitter of plate shape by forming such diffraction grating on a thin substrate 1.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a polarizing beam splitter. More specifically, the present invention relates to a polarization beam splitter used in an optical information reading device such as a magneto-optical disk and an optical information writing device.

[0002]

As a conventional polarizer, (i) a birefringent polarizer utilizing crystal birefringence like a Nicol prism, and (ii) a polymer like a polaroid. Dichroic polarizer utilizing dichroism of light, (iii) Layered with reflective polarizer that uses reflected light of S-polarized light (polarized light component having a vibration plane perpendicular to the incident plane) by utilizing the property of polarization angle There is a transmission type polarizer called a type polarizer. Thin film polarizers that use thin films are classified into prism type and flat plate type. Those that use the polarization angle as described in (iii) above, S-polarized light and P-polarized light in the case of oblique incidence (vibration plane parallel to the incident plane There is one that utilizes the difference in the opaque band of the polarized component. Such a polarizer is cheaper than other polarizers, can obtain an excellent polarization degree in both reflected light and transmitted light, and is mainly used as a beam splitter.

FIG. 5 shows an example of a conventional polarization beam splitter. This polarization beam splitter is of a type called McNile type, and has a structure in which a prism 15 sandwiches a thin film layer 14 in which two layers having different refractive indexes are laminated. In order to separate the polarized light using this structure, it is necessary to satisfy the following formula (1). sin ² θ _G = n _H ² n _L ² / n _G ² (n _H ² + n _L ² ) (1) (where θ _G is the angle of incidence, n _H is the higher refractive index of the two layers, n _L is the lower index of refraction of the two layers and n _G is the index of refraction of the prism material.) In such a McNile polarization beam splitter,
When θ _G = 45 °, n _H = 2.1, and n _L = 1.38, n _G = 1.63 is obtained from the above formula (1), and it is necessary to form a prism with a material having this refractive index. There is.

In the polarization beam splitter as described above, the refractive index of the material used is limited, and it is very difficult to reduce the cost even by using a prism. Further, when the size and weight of the optical system are reduced, the size and weight of the prism impose restrictions.

[0005]

Thus, according to the present invention, an optical multilayer film projecting on a transparent substrate and a material having a refractive index n are disposed between the optical multilayer films. A polarization beam splitter is characterized in that the optical multilayer film is formed by repeating symmetrical films, and the refractive index n is made of a material having an equivalent refractive index for the first polarized light or an equivalent refractive index for the second polarized light. Provided.

FIG. 1 shows an example of the polarization beam splitter of the present invention. The optical multilayer film 2 configured by repeating such symmetrical films can be equivalently replaced with a layer having one refractive index when the wavelength of light, the incident angle, and the polarization direction are fixed. In other words, this means that when the incident light 4 is obliquely incident, the refractive index differs depending on the polarization direction.

When the incident direction is determined, the equivalent refractive index for the first polarized light is N1 and the equivalent refractive index for the second polarized light is N2. An optical multilayer film 2 is partially laminated, and a diffraction grating in which a material 3 having a refractive index N2 is arranged in a region where the optical multilayer film 2 is not laminated is produced. When light is incident on this diffraction grating, it acts as a diffraction grating of the refractive index change type having refractive indexes N1 and N2 for the first polarized light, and is emitted as diffracted light 5.
Regarding the second polarized light, since the refractive index of the optical multilayer film 2 and the refractive index of the disposing material 3 are the same, the second polarized light does not function as a diffraction grating and is transmitted as the non-diffracted light 6 as it is. Therefore,
By forming this diffraction grating on a thin substrate, it becomes possible to form a flat plate type polarization beam splitter as shown in FIG.

As the transparent substrate that can be used in the present invention, a commonly used substrate such as glass or plastic can be used. As the configuration of the optical multilayer film, it is preferable to configure the optical multilayer film so that the diffraction efficiency is maximized. If the equivalent refractive index for S-polarized light is Ns and the equivalent refractive index for P-polarized light is Np, the film thickness is It is preferable to set the film thickness d so that d = λcosθ / | Ns−Np |.

Further, it is preferable that the optical multilayer film has a basic structure of a three-layer film in which the first layer and the third layer have the same layer thickness and the same refractive index, and the basic structure is further laminated. As a material for forming such a layer, a refractive index of 1.4 to
2.5 materials are preferred, eg SiO ₂ , TiO ₂ ,
Examples include ZnS and MgF ₂ . As a method of laminating the optical multilayer film, a method of forming a fine diffraction grating pattern by a dry etching method such as reactive ion etching or ion milling of a film laminated by an electron beam evaporation method, a sputtering method or the like can be used.

Further, as a material having a refractive index n, which is disposed between the above-mentioned protruding optical multilayer films, an epoxy resin or the like can be cited. This material may be provided only between the protruding optical multilayer films, or may be formed so as to cover the entire optical multilayer film as shown in FIG. When it is formed so as to cover the optical multilayer film, its film thickness is preferably 100 μm or more.

Here, the principle of polarization in the case of using an optical multilayer film having a symmetrical three-layer film as a basic structure will be described. Consider a case where light of wavelength λ is incident on a three-layer film including a layer having a refractive index n _{R and} a layer having a refractive index n _Q as shown in FIG. 3 at an angle θ (the first and third layers have a symmetric structure). The refractive indices of the eyes are equal.)
The optical multilayer film having such a symmetrical structure has a wavelength of light,
By fixing the incident angle and the polarization direction, it is possible to equivalently replace the layer with one refractive index. The equivalent refractive index can be calculated from the equations (2) and (3) shown below.

[0012]

[Equation 1] Therefore, when the incident direction is determined, the equivalent refractive index N1 for one polarized light and the equivalent refractive index N2 for the other polarized light have different values. An optical multilayer film is partially projected, and a relief type diffraction grating in which a material having a refractive index N2 is arranged in a region where the optical multilayer film is not projected is manufactured. When light is incident on such a diffraction grating, as described above, it acts as a refractive index change type diffraction grating with a refractive index of N1 and N2 for one polarized light, but for the other polarized light,
Since the refractive index of the optical multilayer film and the refractive index of the filling material are the same, they do not function as a diffraction grating and light is transmitted as it is.
Therefore, even when the three-layer type optical multilayer film is used, a flat plate type beam splitter can be configured.

[0013]

EXAMPLES Specific examples of the polarization beam splitter of the present invention are shown below, but the invention is not limited thereto. As shown in FIG. 4, a first layer of SiO ₂ 7 (having a refractive index of 1.4) was formed on the glass substrate 1 by electron beam evaporation.
6) TiO ₂ 8 having a film thickness of λ / 2 (refractive index 2.
3) and the third layer has a thickness of λ / 4 of SiO ₂ 9 (refractive index 1.
46) were laminated, and the optical multilayer film 2 was formed by the reactive ion etching method so that the distance between the individual optical multilayer films was 10 μm. The equivalent refractive index for S-polarized light of this optical multilayer film is Ns, and the equivalent refractive index for P-polarized light is N
p is Ns = 1.5 when the incident angle is 30 °.
2, Np = 1.74.

Next, a material 3 having a refractive index n is formed between the optical multilayer films. It is necessary to select a material whose refractive index n of this material 3 is equal to either Ns or Np. In the case of this embodiment, an epoxy resin having a refractive index n = 1.52 is arranged between the optical multilayer films 2. . Further, in the present embodiment, the maximum diffraction efficiency occurs when the film thickness d becomes d = λcosθ / | Ns−Np |, and therefore, when calculated using the above numerical values, it becomes approximately 4λ. Therefore, a structure in which four three-layer structures of the above optical multilayer film are laminated is most preferable because the diffraction efficiency is increased. Further, even if the three-layer structure is laminated, the equivalent refractive index is the same, so the condition of the three-layer structure is directly applied.

[0015]

According to the method of the present invention, it acts as a refractive index changing type diffraction grating for one polarized light, but for the other polarized light, the refractive index of the optical multilayer film and the refractive index of the filling material. Since the rates are the same, they do not function as a diffraction grating and light is transmitted as it is. Therefore, since it has a function as a flat plate type polarization beam splitter and can be formed on a flat plate without using a prism, it can be made thin and lightweight, and the entire optical system can be made lightweight and compact.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a polarizing beam splitter of the present invention.

FIG. 2 is a schematic sectional view of a polarization beam splitter of the present invention.

FIG. 3 is an enlarged cross-sectional view of an optical multilayer film having a three-layer structure.

FIG. 4 is a schematic sectional view of a polarizing beam splitter of the present invention shown in an embodiment.

FIG. 5 is a schematic sectional view of a conventional polarization beam splitter.

[Explanation of symbols]

1 Transparent Substrate 2 Optical Multilayer Film 3 Material with Refractive Index n 4 Incident Light 5 Diffracted Light 6 Nondiffracted Light 7 SiO ₂ Film 8 TiO ₂ Film 9 SiO ₂ Film 14 Thin Film Layer 15 Prism

Claims (3)

[Claims] 1. An optical multilayer film projectingly provided on a transparent substrate, and a material having a refractive index n is disposed between the optical multilayer films,
The polarization beam splitter, wherein the optical multilayer film is formed by repeating symmetrical films, and the refractive index n is made of a material having an equivalent refractive index for the first polarized light or an equivalent refractive index for the second polarized light. . 2. The polarizing beam splitter according to claim 1, wherein the optical multilayer film is composed of at least three laminated layers, and the refractive index and the layer thickness of the first layer and the third layer of the laminated layers are equal to each other. 3. The polarization beam splitter according to claim 1, wherein the entire optical multilayer film is coated with a material having a refractive index n.