JPH0830767B2 - Hologram element - 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 hologram element having a function of a polarizer used in an optical head for an optical disk.

[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, polarization beam splitters, such as Glan-Thompson prisms and Lotion prisms, which use the difference in transmission or reflection due to polarized light on the bonding surface of crystals with large birefringence, are used to separate the optical path, or isotropic materials such as glass. A multi-layer prism that is made of a transparent optical medium is provided with a dielectric multilayer film on the reflecting surface of a beam splitter, and the one that reflects or transmits light is often used by utilizing the difference in interference due to the polarization of the dielectric multilayer film. ing.

However, these devices have drawbacks such as large size, low productivity, and high price. Further, the above bulk-type polarizer has only a polarizer function, and when used in an optical head for an optical disc, it is difficult to combine a focus error detection function and a track error detection function, and it is difficult to downsize the optical head.

On the other hand, in recent years, a hologram element having a small size and high productivity and a polarizer function has been disclosed in Japanese Patent Laid-Open No. 63-314.
A birefringent diffraction grating type polarizer described in Japanese Patent No. 502 is known.

FIG. 4 is a sectional view of this birefringent diffraction grating type polarizer. Lithium niobate substrate which is a birefringent crystal 1
When the X-plate or the Y-plate is subjected to proton exchange with benzoic acid, for example, with respect to light having a wavelength of 0.78 μm which is generally used in optical disk devices, it is polarized light parallel to the crystal optical axis of the substrate. The refractive index for certain extraordinary light is about 0.
11 increases, 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 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, the lattice for the ordinary light is formed. 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 is a diagram showing the phase distribution of ordinary light. Reference numeral 7 is a phase difference due to the phase compensation film,
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 a half cycle, an abnormality occurs. The light is completely diffracted. Figure 5
(B) is a figure showing a phase distribution of extraordinary light. Here, reference numeral 9 indicates a phase difference due to the proton exchange region.

Further, as an example of utilizing this birefringent diffraction grating type polarizer for an optical head device, Japanese Patent Laid-Open No. 3-291
There are hologram elements described in Japanese Patent No. 37 and Japanese Patent Application Laid-Open No. 3-29129. Since these hologram elements have the cross-sectional structure shown in FIG. 4 because they have a polarizer function, the grating pattern has different diffraction directions as shown in FIG. 6 to detect the focus error signal and the track error signal of the optical head. Is formed from a plurality of lattice areas. As a similar grid pattern, there is a pattern in which a grid area is divided as shown in FIG.

As a method of manufacturing this hologram element, there is a manufacturing method described in JP-A-4-143788. FIG. 8 shows the manufacturing method. FIG. 8 (a),
After forming a photoresist film 18 having a lattice pattern on the lithium niobate substrate 1 as shown in (b) and (c), the lattice pattern is transferred to the lower metal film 14 by etching to transfer protons. The protection mask 15 is formed. Then, proton exchange is performed as shown in FIG. Next, a photoresist is applied, and the back surface of the substrate 1 is exposed and developed to form a protective mask 1 as shown in FIG.
Photoresist 16 is left only on 5. And FIG.
As shown in (f) and (g), a dielectric material for the phase compensation film is deposited, and finally the photoresist 16 and the protective mask 15 are removed to produce a birefringent grating polarizer. In this method, the self-alignment method is used in which the phase compensation film 3 is formed 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 misalignment can be prevented. Reproducibility is also good.

[0009]

In the above-mentioned conventional self-alignment method of manufacturing, it is necessary to form a proton exchange protection mask in order to form the proton exchange region and the phase compensation film, and the manufacturing method is complicated. Has become.
An object of the present invention is to provide a hologram element having a structure in which the manufacturing method can be further simplified while manufacturing by a self-alignment method, and a manufacturing method thereof.

[Means for Solving the Problems]

In the hologram element of the present invention, an optical diffraction grating composed of ion exchange regions periodically provided on the main surface of a crystal plate having optical anisotropy, and other than the ion exchange region are provided.
An index having an index of refraction almost equal to that of the crystal plate on the region
It is characterized in that a diffraction grating is formed by providing an electric body.

Further , at least one of the hologram elements
It is characterized in that an antireflection film is formed on the surface.

[0012]

The operation and principle of the present invention are as follows. In order to simplify the manufacturing method of the hologram element of the present invention, FIG.
The structure is as shown in. The dielectric phase compensation film is formed not on the conventional proton exchange region but on the region not proton exchanged. Diffraction of this grating uses the principle of utilizing the difference in refractive index change between ordinary light and extraordinary light in the proton exchange region as in the conventional technique. That is, with respect to ordinary light, the phase difference between the light passing through the proton exchange region and the light passing through the dielectric phase compensation film is 0 or an integer multiple of 2π which is substantially equivalent to no phase. Transmits without being diffracted. On the other hand, the extraordinary light can be completely diffracted by setting the phase difference to be an odd multiple of π and making the widths of the proton exchange region and the phase compensation film region equal. The phase relation of these is given by the following equation.

[0013] _{{(n d -n out) ·} T d -Δn o · T
_{p} · 2π / λ = 2nπ} {(n d -n out) · T d -Δn e · T p} · 2π /
λ = (2m + 1) π where n _d and T _d are the refractive index and thickness of the phase compensation film, and T _p
Is the depth of the proton exchange layer, Δn _e , Δn _o are extraordinary rays of the proton exchange layer, the amount of change in the refractive index of ordinary light, n _out is the refractive index outside the hologram substrate, λ is the wavelength of light, m,
n is an integer.

On the contrary, the ordinary light is defined as an integer multiple of 2π which is equivalent to 0 or substantially no phase difference between the light passing through the proton exchange region and the light passing through the dielectric phase compensation film for extraordinary light. In contrast, the polarizer function can be realized also by setting the phase difference to be an odd multiple of π and making the widths of the proton exchange region and the phase compensation film region equal. In this case, the extraordinary ray is transmitted without being diffracted, and the ordinary ray is completely diffracted.
These phase relationships are given by: _{{(N d -n out) ·} T d -Δn e · T p} · 2π /
_{λ = 2m'π {(n d -n} out) · T d -Δn o · T p} · 2π /
λ = (2n ′ + 1) π where m ′ and n ′ are integers.

In this structure, since the phase compensation film of the dielectric is first formed by patterning, it can be used also as a protective mask for proton exchange, so that the number of manufacturing steps can be reduced.

[0016]

1 is a sectional view of a first embodiment of a hologram element of the present invention. When a dielectric film having a refractive index of 2.2 which is almost the same as that of the lithium niobate substrate 10 is used as the phase compensation film 12, the thickness of the phase compensation film 12 is set as an example for light having a wavelength of 0.78 μm. By setting the wavelength of about 240 nm and the depth of the proton exchange region 11 to about 2.6 μm, the polarizer function can be realized by satisfying the conditions of the phase difference of the extraordinary light being 0 and the phase difference of the ordinary light being π described in the action. Nb ₂ O ₅ or the like is used as this dielectric film.

FIG. 2 is a diagram for explaining an embodiment of the manufacturing method of the present invention. As shown in FIG. 2A, a photoresist 17 is applied on the lithium niobate substrate 10 and patterned using a photomask. Next, FIG.
As shown in (b) and (c), the phase compensating film 12 is formed by depositing a dielectric for the phase compensating film and then immersing it in a solvent of the photoresist to remove the photoresist 17. Finally, as shown in FIG. 2B, the substrate is immersed in a proton exchange source to form a proton exchange region 11. Examples of the proton exchange source include benzoic acid and pyrophosphoric acid.

FIG. 3 is a sectional view showing a second embodiment of the hologram element, which is provided with antireflection films 5 and 6.

The hologram element of the present invention can also be used in an optical head by forming a lattice pattern as shown in FIGS.

[0020]

By adopting the structure of the present invention, the method of manufacturing a hologram element can be greatly simplified and the cost of the element can be reduced. Further, in the manufacturing method of the present invention, since the device can be manufactured with high accuracy due to the self-alignment method, a high extinction ratio can be realized.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of a hologram element of the present invention.

FIG. 2 is a diagram illustrating an embodiment of the method for manufacturing a hologram element according to the present invention.

FIG. 3 is a sectional view showing a second embodiment of the hologram element of the present invention.

FIG. 4 is a cross-sectional view of a conventional hologram element.

FIG. 5 is a diagram showing a phase distribution of a conventional hologram element.

FIG. 6 is a diagram showing a lattice pattern of a conventional hologram element.

FIG. 7 is a diagram showing a lattice pattern of a conventional hologram element.

FIG. 8 is a diagram showing a conventional method for manufacturing a hologram element.

[Explanation of symbols]

 1, 10 Lithium niobate substrate 2, 11 Proton exchange region 3, 12 Phase compensation film 5, 6 Antireflection film 7 Phase difference of phase compensation film 8, 9 Phase difference of proton exchange region 14 Metal film 15 Protective mask for proton exchange 16, 17, 18 photoresist

Claims (2)

[Claims] 1. An optical diffraction grating comprising ion exchange regions periodically provided on a main surface of a crystal plate having optical anisotropy, and a bending of the crystal plate on a region other than the ion exchange region.
A hologram element characterized in that a diffraction grating is formed by providing a dielectric material having a refractive index substantially the same as the folding index . 2. At least one of the hologram elements
Claims characterized in that an antireflection film is formed on the surface
Item 1. The hologram element according to item 1.