JPS5917510A - Optical waveguide - Google Patents

Abstract

PURPOSE:To increase the converging power of light in an optical waveguide part and to reduce the loss of light propagation by incorporating a prescribed structure and a constituting material to an optical waveguide used for an optical IC or the like. CONSTITUTION:A light propagating medium 22 is provided in a substrate 21 and further a covering layer 22 is provided on a part of the surface thereof, whereby, an optical waveguide 24 is constituted. The refractive index of light of the medium 22 which is so formed as to allow the passage of only the waveguide 24 is larger than the refractive indices of the surface of the substrate 21, a layer 23 and the layer 22 which is not covered by the layer 23. Therefore, a thin PbTiO3 film is useful for the medium, a sapphire (alpha-Al2O3) single crystal plate for the substrate and quartz glass for the layer 22. Since the refractive index of this waveguide 24 is larger in the medium 22 than in the other part, the spreading of light in the substrate plane is small and the converging power of the guided light is large. Therefore, the optical waveguide is useful for the higher density and higher integration of optical devices.

Description

[Detailed description of the invention] The present invention relates to an optical waveguide.

As an optical waveguide having a coating layer used in a small optical device or an optical IC, a diffusion type waveguide as shown in FIG. 1A and a thin film type waveguide as shown in FIG. 1A have been used. In this case, in the diffused waveguide shown in FIG. 1a, for example, L i
An optical propagation medium consisting of a Ti diffusion layer 13 was provided on the surface of the NbOa substrate 11, and a coating layer 12 was formed on top of the optical propagation medium to form an optical waveguide.

Furthermore, in the thin film waveguide shown in FIG. 1, for example, LiT
An optical propagation medium made of, for example, a LiNb○3 thin film 15 was provided on the aO3 substrate 14, and a coating layer 16 was formed on top of the medium to form an optical waveguide.

This type of optical waveguide is used not only for light transmission, but also for various optical circuits.
For example, it is used to miniaturize or integrate optical devices and to form optical functional devices or optical ICs. However, in these optical waveguides, the convergence of the guided light, that is, the expansion of the guided light in the in-plane direction of the substrate, is insufficient, and the boundaries of the optical waveguide are unclear, for example, two-dimensional When a plurality of waveguides are integrated, there is a drawback that there is a limit to the degree of integration.

Another disadvantage is that the provision of the coating layer increases optical propagation loss.

The present invention improves the structure of these optical waveguides and their constituent materials, and eliminates the drawbacks of conventional optical waveguides. That is, an object of the present invention is to provide a structure of an optical waveguide and its constituent materials for use in a small six-stage optical device or an optical IC, and further to improve the light convergence ability of the optical waveguide portion and reduce optical propagation loss. reduce

FIG. 2 shows the basic structure of an optical waveguide based on the present invention. That is, a light propagation medium 22 is provided on the surface of the substrate 21, and a coating layer 23 is further provided on a part of the surface of the light propagation medium 22.
An optical waveguide 24 is constructed. In this case, the optical propagation medium 22 is configured so that the light passes only through the optical waveguide 24.
The refractive index of light is larger than that of the surface of the substrate 21 and the covering layer 23 .

In addition, in the light propagation medium 22 , the refractive index of the light of the light propagation medium covered with the coating layer 23 , that is, the optical waveguide 24 is different from that of the light propagation medium 22 not covered with the coating layer 23 .
It is characterized by having a refractive index greater than the refractive index of light of No. 2.

As is clear from FIG. 2, in the optical waveguide according to the present invention, the refractive index of the optical waveguide 24 in the optical propagation medium 22 is larger than that of other parts, so that the optical waveguide is different from the substrate seen in the conventional coated waveguide. There is little light spreading within the plane, and the convergence ability of the guided light is high. Therefore, the above optical waveguide based on the present invention is useful for increasing the density of optical devices and implementing ICs.

Furthermore, the present inventors have discovered that it is important to accurately align the positions of the optical waveguide 24, which is a portion of the optical propagation medium 22 with a high refractive index, and the coating layer 23.

We also discovered that the following materials are optimal for forming this type of optical waveguide, and based on this we succeeded in constructing a high-performance optical waveguide.

That is, the surface part of the crystalline substrate is MqO, a-A12
o3 (S7'fire), spinel, or 5rTi○3, and the light propagation medium is BaTi0P.
If it is composed of either bTiO3 or PLzT-based compound I and the coating layer is composed of quartz glass, silica glass, borosilicate glass, or soda glass, it is easy to form and easy to integrate with various functional devices. It is also easy to convert.

In other words, the optical refractive index of the optical propagation medium constituting the optical waveguide is larger than that of the coating layer or substrate7.
,・ In addition to the basic conditions, important conditions are that the propagation loss of light through the optical waveguide is small and that the substrate can still form an optical waveguide. Furthermore, in order for this type of waveguide to be applied to miniaturized or integrated optical devices, the optical propagation medium constituting the optical waveguide must exhibit, for example, a large electro-optic effect. In addition, by making the coating layer coincide with a portion of the light propagation medium having a large refractive index,
Ease of formation is also important in the selection of these materials.

As a result of searching for different constituent materials of the optical waveguide shown in FIG. 2, the present inventors found that, for example, Pb
TiO3 thin film on sapphire (α-A1203) substrate
7 confirmed that the single crystal plate is still useful as a coating layer of quartz glass. In other words, with this kind of constituent material, by introducing a thin film formation technique called sputtering method, it is possible to realize an optical waveguide having the structure according to the present invention at a relatively low temperature, and it is possible to realize an optical waveguide having a structure according to the present invention, which can be used for optical integrated devices such as optical ICs. It is suitable for realizing

Next, in order to better understand the present invention, an example of the procedure for forming an optical waveguide according to the present invention and constituent material elements will be specifically explained.

First, a single crystal plate of sapphire (0001), for example, is used as a substrate. A PbTiO3 thin film is formed on this substrate surface by, for example, high frequency sputtering. In this case, the substrate temperature is maintained at about 60°C, and the chemical composition is PbT.
If there is no deviation from the chemical equivalence of iO3, then (
A transparent single-crystal thin film of P b TiOs with a 111) plane is formed to form a light propagation medium.

FIG. 3 shows an example of an X-ray diffraction pattern of a P b T z Os thin film formed by the method described above.

Furthermore, the present inventors instantaneously heat-treated the PbTiO3 thin film formed on the sapphire C-plane.
We found that the refractive index increases slightly by doing this. Further, the P b T 103 thin film was locally heat-treated, and a coating layer made of, for example, quartz glass was formed by, for example, a high-frequency sputtering method 91, by precisely matching the heat-treated portion. . In this case, it was confirmed that it is important to precisely align the coating layer with the portion of the light propagation medium that has a high refractive index. We found that by precisely matching the positions, the in-plane spread of guided light is reduced, and optical propagation loss is significantly reduced.

Currently, it is not fully understood why instantaneous heat treatment slightly increases the refractive index and reduces optical propagation loss, but defects, distortions, etc. in the film are removed by heat treatment. I don't think so.

In the above explanation, a simple crystal plate of sapphire (0001) was used as the substrate, but similar effects can be obtained using MgO, S
It was confirmed that it could be obtained by using the (10o) plane of r Ti O3 single crystal or the (110) plane of spinel (MgO-A1203) single crystal as a substrate. In this case, P b
The TiOs thin film grows on the (1oo) plane.

Furthermore, in addition to P b T 10 a, the optical propagation medium is
For B a TiOs and PLZT thin films, PLZ
T (9/65/35), P L T , P
The inventors have discovered that even a thin film with a monoskite structure can be formed using the same formation process as PbT103 and has a large electro-optic effect, and is therefore effective as a constituent material of the optical waveguide according to the present invention. confirmed. That is, the electro-optic effect exhibits a value several tens of percent or more of that of a normal bulk material.

It has been confirmed that, in addition to quartz glass, for example, silica glass, borosilicate glass, and soda glass can be used for the above-mentioned coating layer.

Furthermore, the present inventors used BGO (Bi12G) as a substrate.
eo2o) single crystal is used, and the light propagation medium is BTO (Bi
12TiO2o) or BS○(B112 S 10
20) It was confirmed that it can be made into a thin film. Note that the substrate is a LiTaO single crystal plate and the optical waveguide is LiNb.
The same results were observed when using an O3 thin film.

However, the same effects as the gist of the present invention can be obtained by changing the chemical composition, crystal orientation, etc. of materials other than those for the substrate or optical waveguide shown in the specific examples. For example, it is only necessary that the basic conditions of the structure of the present invention are satisfied even for ■-■ group compounds, and G a P
The optical propagation medium is composed of G a A, s. This is effective for infrared rays.

It may also be a ■-■ group compound, for example, Zn on the substrate.
It is preferable to use a 5e single crystal and ZnTe as a light propagation medium. In addition, these TI-Vl group compounds, such as Zn
O, ZnS, CdS, Zn5e, ZnTe or a compound thereof is used as a light propagation medium, and a-A120 is used as a substrate.
3 may be used. When using ZnO for an optical waveguide, for example, the a of the (0001) or (0112) plane
A 120s single crystal substrate is used, and ZnO is deposited on it.
When a film is deposited by magnetron sputtering, a good single crystal thin film with a light propagation loss of 2 dB/m or less can be epitaxially grown even when the substrate temperature during sputter deposition is as low as 300 to 4°C. Useful for forming wave paths.

Furthermore, in the configuration of the optical waveguide according to the present invention shown in FIG. 2, the present inventors have determined that the optical absorption coefficients of the coating layer, the optical propagation medium, and the substrate are α. .

α (and a8, (2, ≧ 10af, 10α
It was found that materials with a rating of 8 are effective.

By using the above-mentioned materials, it is possible to easily form an optical waveguide in which the coating layer accurately matches the portion of the optical propagation medium having a high refractive index.

That is, P b T formed on a sapphire substrate
A coating layer made of quartz glass is formed on the 103 thin film by, for example, a high frequency sputtering method at a portion to be used as an optical waveguide. Next is infrared radiation, which is opaque to quartz glass but transparent to zaphire substrates or P b T 103 thin films.

That is, infrared rays with a wavelength of 5 to 6 μm are irradiated.

By infrared irradiation, only the quartz glass layer of the covering layer is efficiently heated. As a result, the optical propagation medium P b T
In the 10a thin film, only the portion in contact with the quartz glass is efficiently heat-treated, and the refractive index of that portion is slightly higher, reducing light propagation loss. In this way, it is possible to easily form an optical waveguide according to the present invention in which the portion of the optical propagation medium having a high refractive index and the coating layer accurately match. At this time, if the difference in light absorption coefficient is about 10 times, it can be formed efficiently.

That is, if the difference in light absorption coefficient is about 10 times, it means that even if the coating layer and the light propagation medium have the same film thickness, the absorbed infrared energy will be different by about 10 times. Therefore, when the difference in light absorption coefficient is larger, localized heat treatment can be achieved even with a thinner coating layer,
This shows that more advanced microfabrication can be achieved.

However, the characteristics imposed on the substrate are not necessarily required for the entire substrate, and only the surface of the substrate needs to be satisfied.

As is clear from the above description, the optical waveguide according to the present invention has excellent optical propagation characteristics and can provide an optical waveguide with extremely clear boundaries.

Using current semiconductor manufacturing technology, the width accuracy of the propagation path is 1 μm or less, in the so-called submicron range.

Furthermore, by configuring the optical waveguide with a material that has a large electro-optic effect, it becomes possible to further downsize and integrate the original device, which is effective in forming integrated functional devices such as optical ICs.

14 pages

[Brief explanation of the drawing]

Figures 1a and b are diagrams showing the structure of a conventional thin-film optical waveguide, respectively. Figure 2 is a diagram showing the basic structure of an optical waveguide based on the present invention. Figure 3 is an example of the characteristics of a thin-film optical waveguide. FIG. 11.14.21...Substrate, 13, 15, 22
...Light propagation medium, 12,16.23 ---Covering layer. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Fig. 12 Fig. 2 4 Fig. 3 Mi 2θ 7''Easy' shoulder 45

Claims (5)

[Claims] (1) A substrate, a light propagation medium provided on the substrate, and a coating layer provided on a part of the light propagation medium, the light propagation medium having a light refractive index of at least The refractive index of light of the light propagation medium covered by the coating layer is greater than the refractive index of light of the surface of the substrate and the coating layer, and the refractive index of the light of the light propagation medium covered by the coating layer is not covered by the coating layer. An optical waveguide characterized by having a refractive index greater than that of a propagation medium. (2) At least the surface part of the substrate is Mq○, α-Aρ2
o3 (sapphire), spinel, and 5rTlo3, the light propagation medium is composed of at least one of BaTiO3, PbTiO2, and PLZT-based compounds, and the coating layer is quartz glass or silica glass. 2. The optical waveguide according to claim 1, wherein the optical waveguide is made of at least one of the following: borosilicate glass, and soda glass. (3) At least the surface portion of the substrate is B 112 G
e O20, and the light propagation medium is B 11
2 T 1020 or B s 12 S 102o
2. The optical waveguide according to claim 1, wherein the coating layer is made of at least one of quartz glass, silica glass, borosilicate glass, and soda glass. (4) At least the surface portion of the substrate is composed of LiTaO3, the optical propagation medium is composed of L I N b Os, and the coating layer is composed of at least one of quartz glass, borosilicate glass, and soda glass. An optical waveguide according to claim 1, characterized in that: (5) At least the surface portion of the substrate is made of GaP, the optical propagation medium is made of GaAs,
Claim 1 3/, characterized in that the coating layer is made of at least one of quartz glass, silica glass, borosilicate glass, and soda glass. −・ The optical waveguide described. (6) At least the surface part of the substrate is composed of α-A1203, and the optical propagation medium is Zn○, ZnS, CclS.
, Zn5e. A patent claim characterized in that the coating layer is made of ZnTe or one of these compounds, and the coating layer is made of at least one of quartz glass, silica glass, borosilicate glass, and soda glass. The optical waveguide according to scope 1. (72 The light absorption coefficient of the coating layer, light propagation medium and substrate is
α each. , (If and α8, α.≧
10. The optical waveguide according to claim 1, wherein the optical waveguide is 10af and 10α8.