JP2541548B2 - Diffraction grating type optical polarizer - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringent polarizing plate for use in various optical devices using semiconductor lasers, and more particularly to a lattice type polarizing plate having different diffraction efficiency depending on the polarization direction.

(Prior Art) A polarization element, especially a polarization beam splitter, is an element that makes the propagation directions of light different between orthogonal polarizations, and is a polarization on a light reflection surface of a crystal with large birefringence such as a Glan-Thompson prism or a Lotion prism. Is used to separate the optical path by utilizing the difference in transmission or total reflection due to, and a dielectric multilayer film is provided on the reflecting surface of a total reflection prism made of an isotropic optical medium such as glass. It is often used that totally reflects or transmits light by utilizing the difference in refractive index due to polarized light.

These are used as parts constituting an optical isolator and an optical circulator in a light source module for optical fiber communication, an optical head for optical disks, and the like. For example, in a light source module for optical communication, as a light isolator that prevents reflected light from an optical fiber connector or the like from re-entering a semiconductor laser that is a light source, a polarization that rotates the polarization by 45 ° by using the Faraday effect of a magneto-optical material is used. It is used in combination with a rotor (Faraday rotator). Further, in an optical head for an optical disk, it is used in combination with a 1/4 wavelength plate as an optical circulator element that efficiently guides an information signal from an optical disk substrate to a light receiving optical system without returning it to a light source.

(Problems to be Solved by the Invention) These conventional polarization separation elements have a drawback that they are large in size. Whether it is a polarizing element using an optically anisotropic crystal or a polarizing element of a dielectric thin film type, it has a reflective boundary surface that is arranged at an angle of 45 ° or more with respect to the optical axis, so at least the transmitted beam diameter It becomes a cube of √2 times. Since the transmitted beam is large when the optical disk head is used for a recordable type optical head, not for reproduction only, this conventional polarizing element is a cube whose one side is 8 to 10 mm.
This is one of the reasons for increasing the size of the optical head for optical disks.

An object of the present invention is to provide an extremely thin grating type optical polarizing plate which eliminates the above-mentioned drawbacks of the conventional polarizing element.

(Means for Solving Problems) The structure of the diffraction grating type optical polarizing plate of the present invention is such that an uneven optical diffraction grating is formed on the main surface of an optically isotropic substrate, and the surface of the diffraction grating is The main refractive index is covered with a liquid crystal having a refractive index equal to that of the isotropic substrate.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment) FIG. 1 is a perspective view of a diffraction grating type optical polarizing plate according to an embodiment of the present invention, in which 1 is a quartz substrate, 2 is a ferroelectric liquid crystal, and 3 is a quartz substrate sandwiching the liquid crystal. It is a cover glass for supporting. As shown in the cross-sectional view of FIG. 2, the quartz substrate 1 is provided with a periodic diffraction grating having irregularities on the surface in contact with the liquid crystal 2. This diffraction grating can be formed by a dry etching process such as an ion milling method using CF ₄ or C ₃ F ₈ as a reaction gas.

A quartz substrate 1 having a refractive index of 1.45 is formed with a concave-convex grid, and between this and the cover glass 3, a ferroelectric having a refractive index of 1.45 for ordinary rays and a refractive index of 1.73 for extraordinary rays is formed. Liquid crystal 2 is filled. The filled liquid crystal molecules are subjected to an alignment force by the stripe-shaped irregularities of the lattice formed on the quartz substrate 1 and molecularly aligned in one direction, and the liquid crystal layer in the cell has optical uniaxiality. Of the polarization components of the light wave incident on the cell configured as described above, the polarization component vibrating in the periodic direction (z-axis direction) of the grating has a refractive index n ₀ = 1.73 of the liquid crystal layer. Since the refractive index of the substrate is 1.45, it acts as an optical diffraction grating for this polarized component. On the other hand, for the polarization component along the direction of the line of the grating (y-axis direction), the refractive index of the liquid crystal layer is n ₀ = 1.45, which is equal to the refractive index of the quartz substrate. The role of the diffraction grating does not play.

When the incident light 4 enters the phase grating having the cross section shown in FIG. 2 and having the structure shown in the perspective view of FIG. 1 (the cover glass is deviated for the sake of clarity in FIG. 1), y The polarization component that oscillates in the time direction undergoes a uniform phase change in the plane and has no effect of the optical diffraction grating, so that it becomes 0th-order light 5 and goes straight through the cell. On the other hand, the polarized component of the incident light 4 vibrating in the z-axis direction is incident on the optical diffraction phase grating, and becomes diffracted lights 6 and 7 and exits from the cell.

The design condition for realizing the optical element is such that all the polarized components of the incident light 4 that vibrate in the z-axis direction are diffracted. The diffraction efficiency of the _0th- order diffracted light by the optically thin diffraction grating is given by J ₀ ² (Φ). Here, J ₀ is the 0th-order Bessel function, and Φ is the phase change that extraordinary light undergoes by the diffraction grating. The condition that all extraordinary rays are diffracted and do not appear in the 0th order light component 5 is J ₀ ² (Φ) = 0, that is, Φ ~
2.4, and in the case of the combination of the ferroelectric liquid crystal 2 and the quartz substrate 1 described above, the groove depth of the grating provided on the quartz substrate is about 1.1 μm for a light wavelength of 0.8 μm, and about 1.8 μm for a light wavelength of 1.3 μm. And set it.

Furthermore, if the grating is formed in a sawtooth shape, the diffracted light will be +
Only the 1st-order diffracted light becomes.

The substrate material used is not limited to quartz, and other optical glass or resin substrates can be used by selecting a liquid crystal material.

The same effect as the conventional one, that is, the optical isolation effect can be obtained by using the grating type polarizing element manufactured by the above-described manufacturing method in the same manner as the conventional one, that is, by combining the quarter-wave plate and the Faraday rotator. Since this lattice type polarizing element can be formed using a thin plate, a small and thin polarizing element can be obtained.

(Effects of the Invention) As described above, according to the present invention, a thin and small polarizing element can be obtained, and furthermore, since ordinary optical glass or resin can be used as a substrate, a large amount by batch processing can be obtained. An inexpensive polarizing element can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view of a diffraction grating type optical polarizing plate according to an embodiment of the present invention, and FIG. 2 is a sectional view thereof. 1 ... Quartz substrate, 2 ... Ferroelectric liquid crystal, 3 ... Cover glass, 4 ... Incident light, 5-7 ... Diffractive outgoing light.

Claims (1)

(57) [Claims] 1. An optically isotropic substrate is provided with a concave-convex periodic grating on the main surface thereof, and the surface of the periodic grating has a refractive index in which one of the main refractive indices is equal to the refractive index of the isotropic substrate. A diffraction grating type optical polarizing plate characterized by being covered with a liquid crystal.