JPH01177505A - Manufacture of diffraction grating type polarizing plate - Google Patents

Abstract

PURPOSE:To obtain a diffraction grating type polarizing plate having a high extinction ratio by equalizing the width of an area which has been brought to ion exchange and the width of an area which has not been brought to ion exchange. CONSTITUTION:Width S of a space of a mask 2 for ion exchange on a substrate 1 of the surface being parallel to an optical axis of a crystal having an optical anisotropy is made narrower than half of a period L of a diffraction grating by a distance 2d in which an ion turns in under a part of lines of both sides from a part of a space of the mask at the time of ion exchange. That is, width W of an area 3 which has been brought to ion exchange and width L-W of an area which has not been brought to ion exchange become equal. In the end, the mask 2 for ion exchange is eliminated and a dielectric film 4 of quartz, etc., is stuck onto the area 3 which has been brought to ion exchange. In such a way, a diffraction grating type polarizing plate having a high extinction ratio is obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to the production of birefringent polarizing plates using crystals having optical anisotropy, especially diffraction grating type polarizing plates whose diffraction efficiency differs depending on the polarization direction. Regarding the method.

(Prior Art) A polarizing element, particularly a polarizing beam splitter, is an element that obtains a specific polarized light by changing the propagation direction of light between orthogonal polarized lights. Such elements are used as components of optical isolators and optical circulators in light source modules for optical fiber communications, optical heads for optical disks, and the like.

Conventionally, polarizing beam splitters are those that separate optical paths by utilizing the difference in transmission or total reflection due to polarization on the light reflecting surface of a crystal with large birefringence, such as a Glan-Thompson prism or Rochon prism, or isotropic beam splitters such as glass. A dielectric multilayer film is provided on the reflective surface of a total reflection prism made of a polarized optical medium, and the difference in reflectance due to the polarization of this dielectric multilayer film is used to completely reflect or transmit light. . However, these devices have drawbacks such as large size, low productivity, and high cost.

A diffraction grating type polarizing plate has periodic ion exchange regions on the main surface parallel to the optical axis of an optically anisotropic crystal, and thick ion exchange regions on the main surface. A thin dielectric film is formed in the untreated area, and the optical path is separated by utilizing the difference in diffraction efficiency due to polarization. This diffraction grating type polarizing plate has advantages over conventional polarizing elements in that it is compact, has high productivity, and is inexpensive. For example, if the main surface of a χ plate or a Y plate of lithium niobate is subjected to periodic proton exchange, the wavelength in the region subjected to proton exchange is 1.3 μn1.
The refractive index for extraordinary rays increases by about 0.09, and the refractive index for ordinary rays decreases by about 0.04. Therefore, the dielectric film thickness in the proton-exchanged region is made thicker than the dielectric film thickness in the non-proton-exchanged region to offset the decrease in the refractive index for ordinary rays in the proton-exchanged region. By doing so, both the first-order or higher-order diffraction efficiency of ordinary rays and the zero-order diffraction efficiency of extraordinary rays can be made zero, and the polarizer becomes a polarizer.

In this diffraction grating type polarizing plate, the extinction ratio deteriorates when the difference between the width of the ion-exchanged region and the width of the non-ion-exchanged region becomes large. For example, when the width W of the ion-exchanged region becomes 55% of the period of the diffraction grating, the upper limit of the extinction ratio becomes 20 dB, and when it becomes 60%, the upper limit of the extinction ratio becomes 14 dB.
d Become B. Therefore, in order to manufacture a diffraction grating type polarizing plate with a high extinction ratio, it is important to make the width W of the ion-exchanged region and the width L-W of the non-ion-exchanged region as equal as possible. Normally, when forming periodic ion exchange regions as described above, ion exchange is performed after covering with a line-and-space metal film mask as shown in FIG. 2(a). However, as shown in FIG. 2(b), due to the phenomenon that ions diffuse from the mask space part to below the line part, the width W of the ion-exchanged region becomes wider than the width S of the mask space. This causes deterioration of the extinction ratio due to the difference between the width W of the ion-exchanged region and the width L-W of the non-ion-exchanged region.

(Object of the Invention) The object of the present invention is to produce a diffraction grating type polarizing plate with a high extinction ratio by making the width of the ion-exchanged region equal to the width of the non-ion exchanged region in the production of a diffraction grating type polarizing plate. The purpose of the present invention is to provide manufacturing technology that enables the production of polarizing plates.

(Structure of the Invention) The present invention provides an optical diffraction grating having periodic ion exchange regions formed on the main surface of a crystal plate having optical anisotropy, and a region on which ion exchange is performed on the main surface. In manufacturing a diffraction grating type polarizing plate, which has a thick dielectric film and a thin dielectric film in the area where ion exchange is not performed, the width of the space of the space-and-line mask for performing the periodic ion exchange is This method of manufacturing a diffraction grating type polarizing plate is characterized in that the distance extending from the space part of the mask to the line parts on both sides is made narrower than the length of a half period of the meandering grating.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram of the manufacturing process of an ion exchange region according to the present invention. That is, first, a metal film such as titanium is uniformly deposited on the substrate 1 on a plane parallel to the optical axis of a crystal having optical anisotropy such as lithium niobate using sputtering, and then Space-and-line patterning is performed on the metal film using the photolithography technique described above to produce an ion exchange mask 2 as shown in FIG. 1(a). The feature of the present invention is that the width S of the space of this ion exchange mask is set by half the period of the diffraction grating by the distance 2d that ions go around from the space part of the mask to below the line parts on both sides during ion exchange. The goal is to make it narrower. As a result, when ion exchange is performed thereafter, the width -W of the ion-exchanged region 3 becomes equal to the width L-W of the non-ion-exchanged region, as shown in FIG. 1(b). Finally, by removing the ion exchange mask 2 and attaching a dielectric film 4 made of quartz or the like on the ion exchanged region 3, a diffraction grating type polarizing plate as shown in FIG. 1(c) is obtained. For example, when manufacturing a diffraction grating type polarizing plate for light with a wavelength of 1.3 μm by periodically performing proton exchange on the main surface of a lithium diophate X plate and attaching a quartz film on the proton exchange region, the proton exchange The depth T is approximately 5 μm, and the thickness t of the quartz film is approximately 300 nm.
is necessary. Through experiments, we have found that the distance d that protons wrap around from the space of the proton exchange mask to the lines on both sides is approximately 150% of the proton exchange depth T when both sides are combined. Therefore, in this case, by making the width S of the space of the proton exchange mask approximately 7.5 μm narrower than the half-period length L/2 of the diffraction grating, the width W of the proton exchanged region The widths L-W of the regions can be made equal, and a diffraction grating type polarizing plate with a high extinction ratio can be manufactured.

(Effects of the Invention) As described above, according to the present invention, a diffraction grating type polarizing plate with a high extinction ratio can be manufactured.

[Brief explanation of the drawing]

FIG. 1 is a process explanatory diagram of a method of manufacturing a diffraction grating type polarizing plate according to the present invention, and FIG. 2 is a process explanatory diagram of a conventional manufacturing method of a diffraction grating type polarizing plate. DESCRIPTION OF SYMBOLS 1... Crystal substrate with optical anisotropy, 2... Ion exchange mask, 3... Ion exchange region, 4... Dielectric film

Claims (1)

[Claims] An optical diffraction grating of periodic ion-exchange regions is formed on the main surface of a crystal plate with optical anisotropy, and the regions on which ion exchange has been performed on the main surface are thick and no ion exchange has been performed. In manufacturing a diffraction grating type polarizing plate with a thin dielectric film, the width of the space of the space-and-line mask for performing the above-mentioned periodic ion exchange is determined so that the ions flow from the space of the mask to the lines on both sides. A method for manufacturing a diffraction grating type polarizing plate, characterized in that the distance that wraps around below the portion is narrower than the length of a half period of the diffraction grating.