JPS63314502A - Double refractive diffraction grating type polarizing plate - Google Patents

Abstract

PURPOSE:To obtain an extremely thin grating type optical polarizing plate by forming an optical diffraction grating of an ion exchange regions having periods on the main plane of a lithium niobate crystal plate and providing means for offsetting the phase change which the ordinary light component of the light waves transmitted through the diffraction grating receives in the ion exchange regions. CONSTITUTION:The lithium niobate crystal plate 1 has the proton ion exchange regions 2 which are periodically formed to provide the optical diffraction grating. Only the surface of, for example, the regions which are not subjected to the ion exchange are etched by as much as a desired depth in order to offset the phase change which the ordinary light component among the light waves to be transmitted through the diffraction grating receives in the ion exchange regions. The polarizing component oscillating in a y-axis direction, i.e., the ordinary light component passes the crystal plate 1 rectilinarly in the form of zero order light 4 when incident light 3 enters this polarizing plate. On the other hand, the extraordinary light component oscillating in the z-axis direction with the polarized incident light 3 oscillating in the y-axis direction emerges from the crystal plate 1 in the form of diffracted light 5 and 6. The thin and small-sized polarizing element is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a birefringent polarizing plate used in various optical devices using semiconductor lasers, and particularly to a grating wall polarizing plate whose diffraction efficiency differs depending on the polarization direction.

(Prior art) A polarizing element, particularly a polarizing beam splitter, is an element that changes the propagation direction of light between orthogonal polarized lights. A dielectric multilayer film is provided on the reflective surface of a total reflection prism that uses the difference in transmission or total reflection to separate optical paths, or is made of an isotropic optical medium such as glass, and the polarization of this dielectric multilayer film is used. Many materials are used that completely reflect or transmit light by taking advantage of the difference in refractive index.

These are used as components for optical isolators and optical circulators such as light source modules for optical fiber communications and optical heads for optical disks. For example, in light source modules for optical communications, polarized light is used as an optical isolator to prevent reflected light from optical fiber connectors from re-entering the semiconductor laser that is the light source. Used in combination with a rotator (Faraday rotator). Further, in an optical head for an optical disk, it is used in combination with a quarter-wave plate as an optical circulator element that efficiently guides information signals from an optical disk substrate to a light receiving optical system without returning them to the light source.

(Problems to be Solved by the Invention) These conventional polarization splitting elements have the disadvantage of being large. Whether it is a polarizing element using an optically anisotropic crystal or a dielectric thin film type polarizing element, it has a reflective boundary surface that is inclined at 45 degrees or more to the optical axis, so at least the diameter of the transmitted beam is It becomes a cube four times as large as . Since the transmitted beam is large when used in an optical disk head, particularly an optical head of a recordable type rather than a read-only type, this conventional polarizing element has a cubic shape with a side of 8 to 10 mm. This is one of the reasons why the size of the optical head for an optical disk is increased.

An object of the present invention is to provide an extremely thin grating-type optical polarizing plate that eliminates the drawbacks of the conventional polarizing elements.

(Means for Solving the Problems) The structure of the diffraction grating type light polarizing plate of the present invention is such that an optical diffraction grating of periodic ion exchange regions is formed on the main surface of a lithium niobate crystal plate. , characterized in that means is provided for canceling the phase change that the ordinary light component of the light wave transmitted through the diffraction grating undergoes in the ion exchange region.

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

(Example 1) FIG. 1 is a perspective view of a diffraction grating type light polarizing plate according to the first example of the present invention, in which 1 is a lithium niobate crystal plate, 2 is a proton ion exchange region, and The optical diffraction grating is provided by periodically forming regions. Furthermore, in order to offset the phase change that the ordinary light component of the light wave transmitted through the first diffraction grating undergoes in the ion exchange region, in this first embodiment, as shown in the cross-sectional view of FIG. Then, only the surface of the area where ion exchange will not be performed is etched to a desired depth.

In this example, a lattice consisting of an ion exchange region and a non-exchange region having a period in the Z-axis direction is formed in the lithium niobate crystal of the X plate. This period may be formed using a commonly used photolithography technique. Proton ions (H
+) When the exchange is performed, the refractive index n6 for extraordinary rays increases by about 0.13, and the refractive index n0 for ordinary rays decreases by about 0.04, compared to the region where no exchange is performed. In this first embodiment, ion exchange is used as a means for eliminating the cardiac phase change that occurs between the ordinary ray and the non-exchanged region by decreasing the refractive index of the ion exchange region. Only the surface of the unetched area is etched to the desired depth.

When incident light 3 enters a phase grating having the cross section shown in FIG. 2 and the structure shown in the perspective view of FIG. Even if a grating with a lower refractive index than the ion exchange region is formed, since the surface of the non-ion exchange region is etched, the phase change received in the plane is uniform and there is no effect of an optical diffraction grating. Therefore, the 0th order light 4 becomes the crystal plate 1.
Go straight through. On the other hand, for the polarized light component vibrating in the Z-axis direction with respect to the incident light 3, that is, the extraordinary light component, the proton ion exchange region 2 has a refractive index of −+ 0.13, and the refractive index of the non-exchange region n6 is periodically different. This means that the light is incident on an optical phase diffraction grating with a step between the two, and the diffracted light beams 5 and 6 are output from the crystal plate 1.

It is given by the diffraction efficiency J02 (Φ) of the 0th order diffracted light by the optically thin diffraction grating. Here, Φ is the phase change that the extraordinary light undergoes due to the diffraction grating. J02 (
Φ)=0, that is, Φ~2.4, and the optical wavelength is 0.
For 8 yen 1m, T or the thickness of the exchange area is 2.3
11m, and for a light wavelength of 1.3pm, 3.
It is sufficient to set it to about 8 yen 1m. Furthermore, the level difference is about 400A for a light wavelength of 0.811m, and the height difference is 11-3p.
The light may be about 700A.

(Example 2) FIG. 3 is a sectional view of a diffraction grating type light polarizing plate according to a second example of the present invention, in which 1 is a lithium niobate crystal plate, 2 is a proton ion exchange region, and this exchange An optical diffraction grating is provided by periodically forming regions. A dielectric film 7 having a desired thickness is provided only on the surface of the ion-exchanged region.

When light enters a phase grating having the cross section shown in FIG. 3 in the same manner as shown in FIG. 1, the phase change due to the decrease in the refractive index of the ion-exchanged region affected by the polarized light component vibrating in the y-axis direction, that is, the ordinary light component, is Even if a grating due to ion exchange is formed by being canceled by the dielectric film provided on the region, there is no effect of an optical diffraction grating, and the light passes straight through the crystal plate 1.

On the other hand, for the extraordinary light component, the refractive index is periodically different from n6+0.13 in the proton ion exchange region to n6 in the non-exchange region, and furthermore, the refractive index is different due to the fact that it is incident on the optical diffraction grating to which a dielectric film is added. The diffracted light is emitted from the crystal plate 1 as diffracted light.

The conditions for the depth of ion exchange and the thickness of the dielectric film so that all the extraordinary light is diffracted and the ordinary light is not diffracted can be determined in the same manner as in Example 1 above, and the light wavelength is 0.8p.
The thickness of the exchange region may be set to about 2.3 pm for a light wavelength of 1.3 μm, and about 3.8 pm for a light wavelength of 1.3 μm. Furthermore, when 5i02 with a refractive index of 1.45 is used as the dielectric film, the thickness of the dielectric film provided on the area subjected to ion exchange is 4
For light of about 00 A and 1.3 pm, it is sufficient to set it to about 700.

(Example 3) FIG. 4 is a cross-sectional view of a diffraction grating type light polarizing plate according to a third example of the present invention, in which 1 is a lithium niobate crystal plate, 2 is a proton ion exchange region, and The optical diffraction grating is provided by periodically forming regions. Furthermore, in order to cancel the phase change that the ordinary light component of the light wave transmitted through the diffraction grating undergoes in the ion exchange region, in this third embodiment, as a means for canceling the phase change that the ordinary light component of the light wave transmitted through the diffraction grating is A dielectric film 7 is provided which has a different thickness on the surface of the area where ion exchange is not performed.

When light enters a phase grating having the cross section shown in Figure 4, the polarized light component vibrating in the y-axis direction, that is, the ordinary light component, undergoes a uniform phase change in the plane and is not affected by the effect of the optical diffraction grating. , passes straight through the crystal plate. On the other hand, the polarized light component vibrating in the biaxial directions, that is, the extraordinary light component, is affected by the optical phase diffraction grating and is emitted from the crystal plate 1 as diffracted light.

Since all the extraordinary light is diffracted and the ordinary light is transmitted without undergoing diffraction, the conditions for the depth of ion exchange and the step difference in the dielectric film may be the same as in Example 2 above.

The method of performing proton exchange on lithium niobate crystals is a well-known method for forming optical waveguides. For example, when a lithium niobate crystal is immersed in benzoic acid heated to about 217°C for about 6 hours, 2. Proton ion exchange at a depth of approximately 311 meters will be realized. Furthermore, by increasing the temperature and time, the exchange depth can be increased to about Qm.

By using the grating-type polarizing element used in the above manufacturing method in the same manner as in the past, that is, in combination with a quarter-wave plate or a Faraday rotator, the same effect as in the past, that is, the optical isolation effect can be obtained. Since this lattice type polarizing element can be formed using a thin lithium niobate crystal plate, a small and thin polarizing element can be obtained.

Note that the accuracy of the above-mentioned ion exchange time does not require very high accuracy. This is because the ion exchange time setting was insufficient. Even if there is a slight deviation from 2(Φ) = 0, the transmission can be reduced to 0 by forming a plurality of diffraction gratings, for example on a lithium niobate crystal screen, or by using multiple plates vertically. The intensity of the secondary light can be made extremely small.

Furthermore, in the above example, the case of proton ion exchange was described, but the same effect can be obtained when immersed in silver nitrate, thallium nitrate, etc., silver ion (Ag+) exchange and thallium ion (TI+) exchange occur, respectively, causing abnormalities. Optical refractive index of 0.13
A moderate increase in refractive index has been confirmed.

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain a thin and compact polarizing element, and furthermore, since it is manufactured using a lithium niobate crystal wafer as a material, it is possible to produce a polarizing element in large quantities and at low cost through batch processing. element can be obtained.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a diffraction grating type light polarizing plate according to a first embodiment of the present invention, FIG. 2 is a sectional view thereof, and FIGS. 3 and 4 are views of different embodiments. FIG. 1... Lithium niobate crystal plate, 2... Ion exchange area 1 Figure and a half 3 Figures

Claims (1)

[Claims] An optical diffraction grating of periodic ion-exchange regions is formed on the main surface of the lithium niobate crystal plate, and the ordinary light component transmitted through the diffraction grating is not ion-exchanged with the ion-exchanged region. 1. A birefringent diffraction grating type optical polarizing plate, characterized in that it is provided with means for canceling phase changes experienced between regions.