JPH0576001B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a birefringence polarizing plate using a crystal with optical anisotropy, particularly a diffraction grating type polarizing plate whose diffraction efficiency differs depending on the polarization direction. Relating to a manufacturing 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.

Conventional polarizing beam splitters include 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 a rotation prism, or those made of glass, etc. Totally reflective prisms made of a tropic optical medium A dielectric multilayer film is provided on the reflective surface, and the difference in reflectance due to polarization of this dielectric multilayer film is used to completely reflect or transmit light. There is. 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 areas where no polarization is applied, and optical paths are separated by utilizing differences in diffraction efficiency due to polarization. This diffraction grating type polarizing plate has advantages over conventional polarizing elements, such as being smaller, having higher productivity, and being cheaper. For example, if proton exchange is performed periodically on the main surface of an X plate or a Y plate of lithium niobate, the refractive index for extraordinary rays with a wavelength of 1.3 μm will be approximately 0.09 in the proton exchanged region.
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, the width W of the ion-exchanged region is 55 times the period L of the diffraction grating.
%, the upper limit of the extinction ratio is 20 dB, and when it becomes 60%, the upper limit of the extinction ratio is 14 dB. For this reason,
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. 2a. However, as shown in Figure 2b, due to the phenomenon that ions diffuse from the mask space part to below the line part, the width W of the ion-exchanged area becomes wider than the width S of the mask space. Width W of the area where ions have been exchanged and width L-W of the area where ions have not been exchanged
The difference in extinction ratio causes deterioration of the extinction ratio.

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

(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 below the line parts on both sides is made narrower than the length of a half period of the diffraction 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. 1a. The feature of the present invention is that the width S of the space of this ion exchange mask is
is made narrower than half the period L of the diffraction grating by 2d to the distance at which ions wrap around from the space part of the mask to below the line parts on both sides during ion exchange. As a result, when ion exchange is performed thereafter, the width W of the ion-exchanged region 3 and the width L-W of the non-ion-exchanged region become equal, as shown in FIG. 1b. Finally, by removing the ion exchange mask 2 and attaching a dielectric film 4 such as quartz on the ion exchanged region 3, as shown in FIG.
A diffraction grating type polarizing plate as shown in is obtained. For example, when manufacturing a diffraction grating type polarizing plate for light with a wavelength of 1.3 μm by performing periodic proton exchange on the main surface of a lithium niobate X plate and attaching a quartz film on the proton exchange region, the proton exchange depth is The T is approx.
5 μm, and the thickness t of the quartz film is required to be approximately 300 nm.
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 about 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 about 7.5 μm narrower than the half period length L/2 of the diffraction grating, the width W of the proton exchanged region and the width W of the proton exchanged region can be adjusted. It is possible to make the width L-W of the non-containing region equal, and it becomes possible to manufacture a diffraction grating type polarizing plate with a high extinction ratio.

(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] 1. An optical diffraction grating of periodic ion-exchange regions is formed on the main surface of a crystal plate with optical anisotropy, and the ion-exchanged regions on the main surface are thick and ion-exchanged. In the production of 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 adjusted so that ions move from the mask space to both sides. A method for manufacturing a diffraction grating type polarizing plate, characterized in that the distance that wraps around below the line portion is narrower than the length of a half period of the diffraction grating.