JPH06331843A - Optical waveguide and its production - Google Patents

Abstract

PURPOSE:To provide the optical waveguide consisting of an epitaxial film having good crystallinity. CONSTITUTION:A substrate 7 and the optical waveguide layer 10 consist of the same material and a buffer layer 9 of a different kind lower in refractive index than the optical waveguide layer 10 is formed between this substrate 7 and the optical waveguide layer 10. Inexpensive materials having good crystallinity are usable for the substrate 7. Consequently, the epitiaxial film 10 formed of the same material as the material of the substrate 7 has decreased lattice strains and forms highly safe crystals. There is thus an effect of making it possible to obtain the optical waveguide having excellent optical characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide used in fields such as optical communication and optical information recording, and a manufacturing method thereof.

[0002]

2. Description of the Related Art An optical waveguide used as an optical control element such as an optical modulator or an optical switch using an electro-optical material is surrounded by a medium called a clad having a low refractive index in a region called a core. Is an optical transmission line through which light propagates while repeating total reflection at the boundary surface. In order to realize such a structure, various forming methods are used depending on the waveguide material. For example, in the case of LiNbO ₃ waveguide, LiNbO ₃ is diffused Ti a single crystal substrate surface, a method of forming a waveguide by increasing the refractive index of that portion is taken generally. That is, it is a method of depositing Ti of about 700 Å on the surface of a LiNbO ₃ single crystal substrate, heating the substrate to 1020 to 1050 ° C. in an oxygen atmosphere, and thermally diffusing Ti. In the case of this method, since the substrate is heated to 1000 ° C. or higher, Li and O are evaporated from the LiNbO ₃ substrate, and since Ti is diffused into the waveguide layer, the crystal quality of the waveguide layer is deteriorated, Problems of light propagation loss, deterioration of electro-optical constant, and DC drift occur.

Therefore, in addition to the Ti diffusion method, as shown in FIG. 3, an attempt has been made to form an epitaxial film 2 of LiNbO ₃ on a substrate 1 having a low refractive index and use this as a waveguide layer. LiTaO ₃ is the most promising material for the substrate because it has a lower refractive index than LiNbO ₃ and has a close lattice constant and thermal expansion coefficient. This production method is described in, for example, the proceedings of the Japan Society of Applied Physics, Proceedings, Autumn 1992, 17-p-ZV-2,
It can be manufactured by a binary ion beam sputtering method. This will be described with reference to FIG. 1. In the binary ion beam sputtering apparatus, two ion sources 3 and L
Two Li ₃ NbO ₄ targets 4 composed of i, Nb, and O
And Nb ₂ O ₅ target 5. And
A 1000 kV Ar ion beam is generated from the ion sources 3 and 3, and the ion beam currents IA (mA) and I
The Li ₃ NbO ₄ target 4 and the Nb ₂ O ₅ target 5 are irradiated with B (mA). At this time, Li, Nb generated in the Li ₃ NbO ₄ target 4 and the Nb ₂ O ₅ target 5
Sputtered particles composed of O adhere to the LiTaO ₃ single crystal substrate 7 heated to a substrate temperature of 500 ° C. or higher and grow epitaxially. By this method, the LiNbO ₃ epitaxial film 2 is formed on the LiTaO ₃ substrate 1 having a low refractive index, and an optical waveguide can be obtained.

[0004]

However, the LiT
The optical waveguide obtained by the above method of forming a LiNbO ₃ epitaxial film on an aO ₃ substrate has the following problems. First, since epitaxial growth is a method in which the crystal of the film grows by being controlled by the crystal lattice of the substrate, when the substrate and the film material are different substances and have different lattice constants, the film material grows differently from the original lattice constant. I will end up. for that reason,
For example, the refractive index of the film material may be different from the original value. Also, the LiTaO ₃ substrate and the Li on this substrate
In some cases, such as a combination with an NbO ₃ film, the substrate material is more expensive than the film material and it is difficult to obtain a good quality crystal. The reason for this is thought to be that LiTaO ₃ and LiNbO ₃ are produced by a pulling method in which crystal growth is performed from a melt using seed crystals, but LiTaO ₃ has a higher melting point than LiNbO ₃ . As described above, the film formed on the substrate having high manufacturing cost and poor crystallinity is inevitably expensive,
The crystallinity of the film also deteriorates. It is an object of the present invention to provide an optical waveguide and a method for manufacturing the same, which has a basic object to solve the above problems, and which can prevent deterioration of a film due to a substrate material, deterioration of crystallinity, and cost increase. To aim.

[0005]

In order to solve the above-mentioned problems, an optical waveguide of the present invention is an optical waveguide in which an optical waveguide layer is formed by epitaxial growth on a crystal substrate made of an electro-optical material or a dielectric. The optical waveguide layer is made of the same material, and between the substrate and the optical waveguide layer, a buffer layer made of a different electro-optical material or dielectric material having a lower refractive index than the optical waveguide layer is formed. To do. In the optical waveguide of the second invention, the substrate and the optical waveguide layer are composed of LiNbO ₃ , and the buffer layer is LiTa.
It is characterized by being composed of O ₃ .

The method of manufacturing the optical waveguide of the present invention is
In a method of manufacturing an optical waveguide in which an optical waveguide layer is formed by epitaxial growth on a crystal substrate made of an electro-optical material or a dielectric, a heterogeneous material having a lattice constant and a thermal expansion coefficient close to that of the substrate material is epitaxially grown on the substrate. A buffer layer is formed, an optical waveguide layer is epitaxially grown on the buffer layer with the same material as the substrate, and a buffer layer having a lower refractive index than the optical waveguide layer is interposed between the substrate and the optical waveguide layer. Is characterized by. As the electro-optical material or the dielectric of the substrate, LiNbO ₃ which is inexpensive and has excellent crystallinity can be exemplified. The lattice constant of this buffer layer is a method in which the epitaxial growth method is a method in which crystal growth is controlled by the crystal lattice of the substrate, and therefore it is desirable to be relatively close to the substrate. By epitaxially growing a film made of the same material as the substrate on this buffer layer, a film having a lattice constant substantially matching that of the substrate can be obtained, and an optical waveguide layer having no distortion and good crystallinity can be obtained.

[0007]

According to the method of the present invention, an inexpensive and excellent crystallinity material is used, which is used for the substrate and the optical waveguide layer,
Moreover, by producing an optical waveguide in which a buffer layer made of a different material having a lower refractive index than both is provided between the two, the deterioration of the film due to the defect of the substrate caused by the substrate defect generated when the substrate with poor crystallinity, the crystal It is possible to improve the deterioration of sex.
That is, a film epitaxially grown on a substrate having excellent crystallinity and further epitaxially grown on a buffer layer of a different type from the substrate inherits the crystallinity of the substrate as it is and has good crystallinity.

[0008]

EXAMPLE Next, an example of the present invention will be described with reference to FIG. First, Z-cut LiNb whose surface is optically polished
The O ₃ single crystal is set as the substrate 7 in the sputtering device 8. In this state, the temperature of the substrate 7 is kept at room temperature. The ordinary refractive index of the substrate 7 was 2.28 as measured by the Abeles method before being installed in the sputtering device 8. Then, an Ar ion beam 6 with an acceleration voltage of 1000 V is generated from the two Kauffman ion sources 3 and 3, and a Li ₂ CO ₃ target 4 and Ta ₂ O ₅ are generated.
The targets 5 were respectively irradiated with an ion beam current of 30 mA to generate sputtered particles of Li, Ta, and O from the two targets, and the sputtered particles were deposited on the LiNbO ₃ single crystal substrate 7. Next, this sample was taken out from the spattering apparatus 8 and heat-treated in oxygen at 600 ° C. for 3 hours to perform crystallization and decarbonization. This film 9
The ordinary refractive index was measured by the prism coupler method and found to be 2.175.

In this way, L was formed on the LiNbO ₃ single crystal.
The sample in which the iTaO ₃ film 9 was formed as a buffer layer was placed again as a substrate in the sputtering device 8 and the substrate temperature was raised to 480 ° C. The targets 4, 5
Are changed to Li ₃ NbO ₄ and Nb ₂ O ₅ , respectively. Next, the accelerating voltage 1 from the Kaufman type ion sources 3 and 3.
The Ar ion beam 6 of 000 V is generated and the ion beam current is set to 21 mA for the Li ₃ NbO ₄ target 4 and Nb.
_{The 2} O ₅ target 5 was set to 38 mA, and the LiNbO ₃ film 10 was formed on the substrate by the binary simultaneous ion beam sputtering method.
Was vapor-phase epitaxially grown. The ordinary light refractive index of the LiNbO ₃ film 10 thus formed as the optical waveguide layer was 2.28. When the film 10 obtained by this method was observed by reflection high-energy electron diffraction and X-ray diffraction,
It was confirmed that the single crystal thin film was epitaxially grown in the C-axis direction and had good crystallinity.
When the -Ne laser light was introduced and the light propagation loss was measured, it was 1.0 dB / cm or less, and the efficiency was very high.

[0010]

As described above, according to the optical waveguide of the present invention, an inexpensive material having good crystallinity is used for the substrate (Li
NbO ₃ etc.) and a different material (L
By epitaxially growing (iTaO ₃ etc.), an epitaxial film (LiNbO ₃ etc.) formed on the same material as the substrate has a small lattice distortion, becomes a crystal with high integrity, and has low cost and excellent optical characteristics. There is an effect that an optical waveguide (LINbO ₃ etc.) can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus used for manufacturing the present invention.

FIG. 2 is a sectional view of an optical waveguide obtained by the present invention.

FIG. 3 is an enlarged cross-sectional view showing an example of a conventional waveguide.

[Explanation of symbols]

3 Ion Source 7 Substrate 8 Ion Sputtering Device 9 LiTaO ₃ Film 10 LiNbO ₃ Film

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuya Sano 1-3, Takanodai, Yotsukaido, Chiba Stock Company Japan Steel Works (72) Inventor Ryuji Makoto 1-3, Takanodai, Yotsukaido, Chiba Stock Company Inside Japan Steel Works

Claims (3)

[Claims] 1. In an optical waveguide in which an optical waveguide layer is formed by epitaxial growth on a crystal substrate made of an electro-optical material or a dielectric, the substrate and the optical waveguide layer are made of the same material, and between the substrate and the optical waveguide layer. And an optical waveguide having a buffer layer formed of a different electro-optical material or dielectric material having a refractive index lower than that of the optical waveguide layer. 2. The optical waveguide according to claim 1, wherein the substrate and the optical waveguide layer are made of LiNbO ₃ , and the buffer layer is made of LiTaO _3. 3. A method of manufacturing an optical waveguide in which an optical waveguide layer is formed by epitaxial growth on a crystal substrate made of an electro-optical material or a dielectric, wherein the substrate material has a different lattice constant and thermal expansion coefficient from each other. A buffer layer is formed by epitaxially growing a material, an optical waveguide layer is epitaxially grown on the buffer layer with the same material as the substrate, and a buffer layer having a lower refractive index than the optical waveguide layer is provided between the substrate and the optical waveguide layer. A method for manufacturing an optical waveguide, characterized in that