JPH0827406B2 - Birefringent diffraction grating polarizer and method for manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source module for optical fiber communication and a polarizer used in an optical head for optical disks.

[0002]

2. Description of the Related Art A polarizer, especially a polarization beam splitter, is an element that obtains a specific polarized light by changing the propagation directions of light between orthogonal polarized lights. Conventionally, as the polarization beam splitter, a difference in transmission or total reflection due to polarized light on a bonding surface of a crystal with large birefringence such as Glan-Thompson prism or Lotion prism is used.
A dielectric multi-layer film is provided on the reflecting surface of a prism-bonded beam splitter that separates the optical path or isotropic optical medium such as glass. , Those that reflect or transmit light are often used. However, these elements have drawbacks such as large size, low productivity, and high price.

On the other hand, in recent years, a birefringent diffraction grating type polarizer described in Japanese Patent Application Laid-Open No. 63-314502 (Japanese Patent Application No. 62-130144) as a polarizer characterized by being small in size and having high productivity. It has been known. FIG. 4 is a sectional view of the birefringent diffraction grating type polarizer. When the X-plate or Y-plate of the lithium niobate substrate 1 which is a birefringent crystal is subjected to proton exchange with benzoic acid, as an example, it is crystallized for light having a wavelength of 0.78 μm which is generally used in optical disk devices. The refractive index for extraordinary light, which is polarized light parallel to the axis, increases by about 0.11, and the refractive index for ordinary light, which is polarized light perpendicular to the optical axis, decreases by about 0.04.
Therefore, a grating in which an exchange region 2 that has undergone proton exchange and a non-exchange region that has not undergone proton exchange are periodically arranged functions as a diffraction grating. If a phase compensation film 3 having an appropriate thickness is formed on the exchange region in order to cancel the phase difference between the ordinary light passing through the exchange region 2 and the ordinary light passing through the non-exchange region in this lattice, this lattice will be formed against the ordinary light. Does not work as a diffraction grating and can transmit ordinary light without diffraction. So this lattice looks like a mere transparent substrate.

FIG. 5A shows the phase distribution of ordinary light. Λ is the length of one period of the diffraction grating. Reference numeral 7 indicates a phase difference due to the phase compensation film, and reference numeral 8 indicates a phase difference due to the proton exchange region. By changing the depth of the exchange region 2 while satisfying the above-mentioned phase difference cancellation condition for ordinary light, when the phase difference for the extraordinary light is π and the widths of the exchange region and the non-exchange region are both equal to the half cycle 4, the abnormality occurs. The light is completely diffracted. FIG. 5B is a diagram showing the phase distribution of extraordinary light. Here, reference numeral 9 indicates a phase difference due to the proton exchange region.

This polarizer manufacturing method requires an exposure process using two photomasks for forming a pattern of a proton exchange region and a pattern of a phase compensation film. Therefore, each pattern is likely to be displaced, and ordinary light is slightly diffracted, while extraordinary light is slightly transmitted and the extinction ratio is deteriorated. In order to solve this, there is a manufacturing method described in Japanese Patent Application No. 2-266495 as a usage example of our company. FIG. 6 shows the manufacturing method.
As shown in FIGS. 6A and 6B, a proton exchange protection mask 15 having a lattice pattern is formed on the lithium niobate substrate 1, and proton exchange is performed. Next, FIG. 6 (c)
As shown in, the photoresist 16 is applied, and the back surface of the substrate 1 is exposed and developed to leave the photoresist only on the protective mask 15. Then, as shown in FIGS. 6D and 6E, a dielectric material for a phase compensation film is deposited, and finally the photoresist 16 and the protective mask 15 are removed to fabricate a birefringent diffraction grating type polarizer. To do.

In this method, the phase compensation film 3 is formed by using the proton exchange protection mask 15 used to form the proton exchange region 2, so that the manufacturing process can be simplified, and the manufacturing process can be simplified. Positional deviation is unlikely to occur and reproducibility is good.

[0007]

When manufactured by the above self-alignment method, the proton exchange region and the phase compensation film are not displaced, but the proton exchange is performed not only in the depth direction but also under the proton exchange protection mask. Since proton ions, that is, hydrogen ions, are also laterally diffused to the side,
There is a gap between the proton exchange region and the phase compensation film. As a result, the extinction ratio deteriorates.

It is an object of the present invention to provide a birefringent diffraction grating type polarizer having a structure in which deterioration of an extinction ratio is small while being manufactured by a self-alignment method, and a manufacturing method thereof.

[0009]

[Means for Solving the Problems]

(1) The birefringent diffraction grating type polarizer of the present invention forms an optical diffraction grating composed of ion exchange regions periodically provided on the main surface of a crystal plate having optical anisotropy, and In a birefringent diffraction grating type polarizer in which a dielectric film narrower than the width of the ion exchange region is provided substantially in the center of the exchange region, the width of the ion exchange region is wider than half the period of the diffraction grating, and the dielectric film Is narrower than half the period of the diffraction grating. (2) In the method for producing a birefringent diffraction grating-type polarizer of the present invention, an ion-exchange mask for periodically forming ion-exchange regions is formed on the main surface of a crystal plate having optical anisotropy. 1 step, a second step of forming an ion exchange region on the main surface, and a dielectric film is deposited on the ion exchange mask and the ion exchange region, and then the ion exchange mask is removed. In the method for manufacturing a birefringent diffraction grating type polarizer including the step of forming a dielectric film only on the ion exchange region, the width of a portion of the mask for ion exchange that is ion-exchanged is narrower than half the period, Further, the width of the ion exchange region after the ion exchange is wider than half of the cycle.

[0010]

This type of polarizer separates ordinary light and extraordinary light into 0th-order diffracted light and 1st-order or higher-order diffracted light, respectively, as described above. The extinction ratio of the 0th-order diffracted light is mainly determined by the small amount of extraordinary light that is transmitted without being diffracted. As a factor for transmitting the extraordinary light, the phase difference may not be a predetermined value, and the ratio of the line to the space of the grating may not be 1: 1. In the case of the manufacturing method by the self-alignment method described in the related art, since the width of the proton exchange region 2 and the phase compensation film 3 do not match in the structure of the polarizer as shown in FIG. 1, the phase distribution is shown in FIG. As shown in, the latter factor is essential. However, as shown in FIG. 2B, the transmission of extraordinary light can be suppressed by averaging the line-and-space ratio of the lattice between the phase compensation film and the proton exchange region to 1: 1. as a result,
A high extinction ratio of 0th order light can be achieved.

On the other hand, the extinction ratio of the first-order light is mainly determined by the slight diffracted light amount of the ordinary light. When manufactured by the self-alignment method described above, the phase distribution is as shown in FIG. At this time, the ordinary ray is slightly diffracted, but the period of this grating is half of the original period, so that the diffraction angle is twice the original diffraction angle. As a result, ordinary light does not wrap around to the primary light, and a high extinction ratio becomes possible.

[0012]

1 is a sectional view of a first embodiment of a birefringent diffraction grating type polarizer of the present invention. As the phase compensation film 3, Nb ₂ O having a refractive index of 2.2 which is almost the same as that of the lithium niobate substrate 1
_{5 When a} dielectric film is used, in order to satisfy the phase condition described in the prior art, the thickness of the phase compensation film 3 is about 95 nm for the light of wavelength 0.78 μm, and the proton exchange region The depth of 2 needs to be about 2.4 μm. When an X-cut lithium niobate substrate is used, the lateral diffusion distance of the proton exchange region is about 2 μm due to the difference in the diffusion coefficient in the crystal axis direction, and the width of the phase compensation film 3 is the same as that of the proton exchange region. It is about 4 μm narrower than the width. As for the relation between the width of the lattice and the extinction ratio of the 0th-order light, in the case of a lattice with a period of Λ = about 50 μm, when the width of the proton exchange region is equal to the half period, the extinction ratio of about 20 dB is When the width is 2.8 μm wider than the half cycle, an extinction ratio of about 30 dB is obtained. Further, when the width of the phase compensation film 3 becomes equal to or wider than the half cycle, the extinction ratio deteriorates. on the other hand,
The extinction ratio of the primary light is about 30 dB regardless of the above grating width.

FIG. 3 is a sectional view showing a second embodiment.
Antireflection films 5 and 6 are applied. A SiO ₂ film is most suitable for the air layer as the antireflection film. The method of manufacturing the above-mentioned polarizer is the same as the procedure described in the prior art as a process, but the width of the transparent portion of the protective mask 15 for proton exchange is narrower than the half period of the lattice, and The proton exchange region 2 is formed to have a width wider than a half cycle. As an example, in the above case, the width of the transparent portion of the proton exchange protection mask 15 is about 1.2 μm from the half cycle.
It should be narrowed to some extent.

[0014]

EFFECT OF THE INVENTION In the birefringence diffraction grating type polarizer manufactured by the manufacturing method by the self-alignment method with high manufacturing accuracy and few manufacturing steps, a high extinction ratio can be realized by adopting the structure of the present invention.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of a birefringent diffraction grating type polarizer of the present invention.

FIG. 2 is a diagram showing a phase distribution of the birefringent diffraction grating type polarizer of the present invention.

FIG. 3 is a cross-sectional view showing a second embodiment of the birefringent diffraction grating type polarizer of the present invention.

FIG. 4 is a cross-sectional view of a conventional birefringent diffraction grating type polarizer.

FIG. 5 is a diagram showing a phase distribution of a conventional birefringent diffraction grating type polarizer.

FIG. 6 is a diagram showing a method for manufacturing a birefringent diffraction grating type polarizer.

[Explanation of symbols]

 1 Lithium niobate substrate 2 Proton exchange region 3 Phase compensation film 4 Half cycle 5, 6 Antireflection film 7 Phase difference amount of phase compensation film 8, 9 Phase difference amount of proton exchange region 15 Proton exchange protection mask 16 Photoresist

Claims (2)

[Claims] 1. An optical diffraction grating composed of periodically provided ion-exchange regions is formed on the main surface of a crystal plate having optical anisotropy, and the ion-exchange region is formed substantially at the center of the ion-exchange region. In a birefringent diffraction grating type polarizer provided with a dielectric film narrower than the width of the diffraction grating, the width of the ion exchange region is wider than half of the period of the diffraction grating, and the width of the dielectric film is half of the period of the diffraction grating. A birefringent diffraction grating type polarizer characterized by being narrower. 2. A first step of forming an exchange mask for periodically forming ion exchange regions on the main surface of a crystal plate having optical anisotropy, and ion exchange regions on the main surface. Second step of forming, and after depositing a dielectric film on the ion exchange mask and the ion exchange region, the ion exchange mask is removed to form a dielectric film only on the ion exchange region. In the method for manufacturing a birefringent diffraction grating type polarizer having at least a step, a width of a portion of the ion exchange mask which is ion-converted is narrower than half of a cycle,
The method for producing a birefringent diffraction grating type polarizer is characterized in that the width of the ion exchange region after the ion exchange is wider than half of the period.