JPS6271907A - Grating optical device - Google Patents

Abstract

PURPOSE:To obtain extremely uniform grating structure by setting an antireflection film provided on the rear surface of a substrate in a manner as to satisfy antireflection conditions for both of light irradiation in the stage of forming the grating pattern and light irradiation in the stage of use. CONSTITUTION:The antireflection film 30 is formed on the rear surface of the substrate 5. The substrate 5 consists of a y-cut LiNbO3 crystal and the antireflection film 30 consists of an SiO2 film (1.48 refractive index). The antireflection conditions in both the manufacturing stage and using stage hold if the coincident point of the min. values of the curves A and B is found. The min. values of the two curves near film thickness d=1.2mum (shown by an arrow) coincide with each other and such value is the value to be determined. Namely, the film thickness (d) of the antireflection film 30 is made 1.12mum. The reflected luminous flux by the antireflection film 30 is thus substantially obviated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a grating optical device, and particularly to a grating optical device in which a grating pattern is formed by interference of a plurality of light beams during manufacture. Grating optical devices are used, for example, as optical coupling parts in optical equipment using optical waveguides.

[Conventional technology and its top points]

Conventionally, grating optical devices have generally been fabricated by two-beam interferometry.

FIG. 8 is a schematic cross-sectional view showing the outline of fabrication of a grating optical device by two-beam interferometry.

In the figure, 5 is an optical waveguide substrate, for example, L s
Consists of NbOs. Reference numeral 4 denotes an optical waveguide layer formed on the surface of the substrate 5, and the layer 4 is formed by performing TI diffusion or proton exchange from the surface of the material of the substrate 5. 3 is a photoresist layer provided on the surface of the optical waveguide layer 4. Two coherent light beams 1.2 are incident on the photoresist layer 3 in different directions from above the layer 3.
forming an interference fringe pattern with a desired pitch,
The resist layer 3 is formed into stripes by a predetermined development and fixing process, and etching is performed using this as a mask to form a grating in the optical waveguide layer 4.

Therefore, when the two light beams 1 and 2 are irradiated, a part of the light beam that has progressed into the substrate 5 is reflected by the back surface 8 of the substrate 5 to generate reflected light beams 6 and 7, and the reflected light beams are applied to the photoresist. 3, the desired interference fringe pattern formed by the incident beams 1, 2 will be disturbed. That is, when a typical LINbO is used as the substrate 5, the reflectance at the back surface 8 is approximately 24, assuming that the incident angle of the incident light beams 1 and 2 is 45'.
The noise caused by the reflected light beams 6 and 7 from the surface is quite large.

As a means of removing such noise caused by reflection on the back surface of the substrate, a method of immersing the back surface of the substrate in a liquid such as matching oil having a refractive index almost equal to that of the substrate may be considered, but this method requires a complicated device configuration and There are problems such as the lack of suitable matting oil in some cases.

Further, as a means for removing noise caused by the back surface of the substrate, a method of applying an antireflection film to the back surface can be considered. FIG. 9 is a schematic cross-sectional view showing the outline of the production of a grating optical device by this method, and is a diagram similar to FIG. 8. Here, the only difference from the case shown in FIG. 8 is that an antireflection film 9 is formed on the back surface of the substrate 5. Thus, after the light beams 1 and 2 travel into the substrate 5, most of the light beams 10 and 2 become light beams 10 and 11 that pass through the antireflection film 9.

By the way, as the various antireflection films mentioned above, hyperelectric films with good optical properties are generally used, and since such films are formed by vapor deposition or sputtering, it is difficult to peel off once formed. Therefore, the antireflection coating 9 remains intact even when the grating optical device is used.

FIGS. 10 and 11 are schematic cross-sectional views of a grating optical device fabricated by the two-beam interference method with the antireflection film 9 provided thereon. FIG. 10 shows the case where it is used as an optical coupling part for outputting from an optical waveguide, and FIG. 11 shows the case where it is used as an optical coupling part for inputting to an optical waveguide.

In FIG. 10, guided light 21 traveling through the optical waveguide layer 4
is diffracted by the grating 20 and becomes a diffracted light beam 22 toward the air side and a diffracted light beam 23 toward the substrate side. A portion of the diffracted light beam 23 is reflected by the anti-reflection film 9 to produce a reflected light beam 24, and the remainder passes through the anti-reflection film 9 and becomes an output light beam 25. Here, since the antireflection film 9 generally does not sufficiently satisfy antireflection conditions for the diffracted light beam 23, the intensity of the reflected light 240 does not become zero.

In FIG. 11, an input light beam 26 is partially reflected by the anti-reflection film 9 to produce a reflected light beam 28, and the rest passes through the anti-reflection film 9 and becomes an incident light beam on the grating 20. 29 is a coupled waveguide light.

Also here, since the anti-reflection film 9 generally does not sufficiently satisfy the anti-reflection conditions for the input light beam 26, the intensity of the reflected light beam 28 does not become O.

As described above, the anti-reflection film applied to the back surface of the substrate during the fabrication of the grating optical device is formed in such a way as to match the anti-reflection conditions at the time of fabrication. When used, the anti-reflection conditions generally differ, resulting in insufficient anti-reflection, and the problem arises that the light flux reflected by the anti-reflection film causes deterioration in device performance (for example, a decrease in coupling efficiency and a decrease in S/N). .

In order to solve this problem, it is possible to further apply a correction film after forming the grating structure so as to satisfy the anti-reflection conditions during use, but this would complicate the manufacturing process.

[Means for solving problems]

According to the present invention, in order to solve the problems of the prior art as described above, an antireflection film is provided on the back surface of the substrate.
Provided is a grating optical device having an fF characteristic that the antireflection film is set to satisfy antireflection conditions for both light irradiation during grating pattern formation and light irradiation during use. .

〔Example〕

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

FIG. 1 is a schematic cross-sectional view showing the outline of the fabrication of a first embodiment of the grating optical device of the present invention. In this figure, the same members as in FIG. 9 are given the same reference numerals, and explanations thereof will be omitted here. An antireflection film 30 is formed on the back surface of the substrate 5. The substrate 5 is made of y-ka) LiNbO3 crystal,
Further, the antireflection film 30 is made of a 5IO2 film (refractive index: 1.48).

2 and 3 are schematic cross-sectional views of a grating optical device fabricated by the three-beam interferometry with the anti-reflection film 30 attached, and FIG. 2 shows the output light from the optical waveguide. The case is shown in which it is used as a coupling part, and FIG. 3 shows the case in which it is used in an optical coupling part for input to an optical waveguide. In these figures, the same members as in FIGS. 10 and 11 are denoted by the same reference numerals, and description thereof will be omitted here.

As shown in Figure 1, the incident angle of light on the air side during fabrication (
As shown in FIGS. 2 and 3, the incident angle (outgoing angle) of light on the air side during use is assumed to be h.

In Fig. 1, an argon laser (wavelength λF = 0.488 μm) is used as the incident light beams 1 and 2, and the incident angle θy is set to 2.
The relationship between the thickness d of the antireflection film 3o and the reflectance R when the angle is 6° is as shown by curve A in FIG. The position where the minimum value of the curve A is obtained is the antireflection condition at the time of manufacture.

The period A of the grating pattern produced by the three-beam interferometry as described above is determined by the following equation.

Δ=λ2/2s stone θ2 Here, λF= 0.488 ttm, θ2=26°
Then, 71=0.56 μm.

In FIGS. 2 and 3, when a semiconductor laser (wavelength λ, ==Q, 831 μm) is used as the input and output light beam 25°26, the above grating pattern period A=0.56
In μm, θ8=45°. Anti-reflection film 3 at this time
The relationship between the film thickness d at 0 and the reflectance at 1 is as shown by curve B in FIG. The position where the minimum value of the curve B is obtained is the anti-reflection condition during use.

In order to satisfy the anti-reflection conditions both during fabrication and use, it is only necessary to find a point where the minimum values of curves mA and B match, and the two curves should be found near the film thickness d = 1.12 μm (indicated by the arrow). It can be seen that the minimum values of are the same, and this value is the desired value. That is, in this embodiment, the antireflection film 30
The film thickness d of is 1.12 μm, therefore,
As shown in the figure and FIG. 3, substantially no reflected light beam is generated by the anti-reflection M30.

In the graph of FIG. 4, C n = 2.2561 (λ =
0.488μm) / n=2.1729 (λ=0
．． 831μff1)').

FIGS. 5(a) to 5(f) are manufacturing process diagrams of the grating optical device of this example.

First, both sides of the LiNbO3 crystal substrate 5 (Y-cut) are optically polished [Fig. 5 (,):l], and then the optical waveguide layer 4 is formed by T1 diffusion, proton exchange, etc. [Fig. 5 (b)].
], Next, an antireflection film 30 having a thickness d that satisfies the above antireflection conditions is formed by vapor deposition or sputtering [fifth
Figure (C)]. Note that it is desirable to measure the film thickness in real time in order to set the film thickness of the antireflection film 30 as accurately as possible. Next, on the optical waveguide layer 4, an optical resist 3 such as Microposit 1350 having sensitivity in the visible range is applied using a spinner (3500 rpm, t1i thickness 0.5 μm).
L. Two beam interference is performed using the argon lasers 1 and 2 as described above [FIG. 5(d)], and the shift resist is developed and fixed into a relief-like grating. Subsequently, the surface of the optical waveguide layer 40 is processed using a dry etching technique such as ion beam processing using the grating butter 7 of the photoresist as a mask [FIG. 5(,)).
Further, the photoresist is removed to obtain an optical device having a relief type grating structure 20 [FIG.
f)].

FIG. 6 is a schematic cross-sectional view showing the outline of the fabrication of the second embodiment of the present invention. In this figure, members similar to those in FIG. 1 are designated by the same reference numerals, and description thereof will be omitted here.

In this embodiment, the photosensitive material 40 is applied relatively thickly to the surface of the substrate 50. In order to perform the same three-beam interferometry as in the above embodiment, an incident light beam of 1°2 is made incident. The angle of incidence of the incident light beams 1 and 2 is θF.

FIG. 7 is a schematic cross-sectional view of the grating optical device thus produced. In this figure, members similar to those in FIG. 2 are designated by the same reference numerals, and description thereof will be omitted here.

#) is exposed by three-beam interferometry as shown in Figure 1,
After undergoing a predetermined maintenance and fixing process, the photosensitive material 40 forms a layer 41 having a grating structure as shown in FIG.
becomes. In FIG. 2, 42 is the incident light flux, and 43
is a transmitted light beam, and 44 is a diffracted light beam subjected to Sibrag diffraction by the grating structure. Diffraction beam 44
The angle of emission into the air is θ8.

Even in such a grating optical device that performs Bragg diffraction, the wavelength and incidence angle θ of light when forming the grating are generally different from the wavelength and incidence angle θR of light during use, so the above-mentioned first embodiment is different. The film thickness of the antireflection film 30 is determined using the same method as in , so as to satisfy the antireflection conditions both during manufacture and during use.

〔Effect of the invention〕

According to the present invention as described above, the following effects can be obtained.

(1) During fabrication, reflected noise from the back surface of the substrate can be reduced and an extremely uniform grating structure can be obtained.

(2) During use, it reduces reflection on the back surface of the substrate and enables highly efficient optical coupling.

(3) Since the antireflection film once formed functions as a part of the device until the end, it is effective in simplifying the process and has the effect of protecting the substrate during the process.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing the fabrication of the device of the present invention. 2 and 3 are schematic cross-sectional views of the device of the present invention. FIG. 4 is a graph showing the relationship between the thickness of the antireflection film and the reflectance. FIGS. 5(a) to 5(f) are process diagrams for manufacturing the device of the present invention. FIG. 6 is a schematic cross-sectional view showing the fabrication of the device of the present invention. FIG. 7 is a schematic cross-sectional view of the device of the present invention. FIGS. 8 and 9 are schematic cross-sectional views showing the production of a conventional grating optical device. FIGS. 10 and 11 are schematic cross-sectional views of conventional grating optical devices. 3: Photoresist layer, 4: Optical waveguide layer, 5: Substrate, 2
0: grating, 30: antireflection film. Agent Patent Attorney Minoru Yamashita Child 1 Figure 2 @ Figure 3 Figure 6 Figure 7 Figure 8

Claims (2)

[Claims] (1) In a grating optical device that has a grating structure on the surface of the substrate and is manufactured by including a step of forming a grating pattern by irradiating a photosensitive material with light, an antireflection film is provided on the back surface of the substrate, and the antireflection film is A grating optical device, characterized in that it is set to satisfy anti-reflection conditions for both light irradiation during grating pattern formation and light irradiation during use. (2) The grating optical device according to claim 1, wherein an optical waveguide is formed on the surface of the substrate, and a grating structure is provided in the optical waveguide.