JPH06289347A - Optical waveguide device and manufacturing method thereof - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to an optical waveguide device such as an optical modulator used for optical communication, and particularly to a structure of an optical waveguide device having a small coupling loss with an optical fiber and a propagation loss and a manufacturing method thereof. The purpose is to provide. [Structure] A plurality of dielectric transparent substrates 1 and 2 having the same refractive index
Is directly bonded via the layer 3 having a lower refractive index than the dielectric transparent substrates 1 and 2, and at least one of the substrates has optical waveguides 5 and 6 in which light is confined. Become.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure for improving the performance of various optical waveguide elements and a method for manufacturing the same, which performs optical intensity modulation using an optical waveguide, optical switching, polarization plane control, propagation mode control and the like. Is.

[0002]

2. Description of the Related Art Conventionally, an optical waveguide device such as an optical modulator, an optical switch, an optical polarization plane control device, an optical propagation mode control device, etc. has been manufactured by using lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ). The optical waveguide of single mode propagation is formed on the dielectric single crystal having the electro-optical effect of, and the shape of the optical waveguide is devised, and the electrode is provided in an appropriate shape, and the light passing through the optical waveguide is controlled by the electro-optical effect. I am going. For example, al. R. Alfernes
s), "Wave Guide Electro-Optic Modulators" I-Y-I-Transactions on Microwave and Techniques, Volume MI T-30, Number 8, 1121 to 1137 (1982)
("Waveguide Electrooptic Modulators" IEEE Transac
tions on Microwave and Techniques, Vol. MTT-30, N
o.8, 1121-1137 (1982)) describes the configuration of such various optical waveguide devices. For the method of manufacturing the optical waveguide, see, for example, Eye. Mr. I. Kamino
w), "Optical Wave Guide Modulators" I-Y-I-Transactions on Microwave and Technology-
Z, Volume MI IT-23, Number
1, pp. 57-70 (1975) ("Optical Wav
eguide Modulators "IEEE Transactions on Microwave
and Techniques, Vol. MTT-23, No.1, 57-70 (1975))
Are described in various. For example, heat treatment of lithium niobate or lithium tantalate at high temperature to remove Li-
By changing the refractive index by out-diffuse, or vapor-depositing a metal such as titanium and thermally diffusing at high temperature, by making the refractive index of the diffusion part a little higher than other parts, I try to trap the light. An example of a Mach-Zehnder type optical modulator using titanium diffusion is described in, for example, Japanese Patent Laid-Open Publication No. 63-261219. In addition, a metal mask is applied to a predetermined part,
A method of forming an optical waveguide by performing proton ion exchange in phosphoric acid at 200 to 300 ° C. to partially change the refractive index is also known. However, out-diffusion (ou
In all manufacturing methods such as t-diffusion), thermal diffusion, and ion exchange from the surface, the optical waveguide is formed by diffusion treatment from the surface, so the cross-sectional shape of the optical waveguide is Therefore, there are various inconveniences.

[0003]

[Problems to be Solved by the Invention]
There is a coupling loss between the optical waveguide and the optical fiber. While the cross-sectional shape of an optical fiber is circular, the shape of a conventional optical waveguide is similar to an inverted triangle because it is diffused from the surface, and the portion with the strongest guided light intensity is near the surface. Therefore, the optical coupling with the optical fiber did not work very well, resulting in a large loss. In optical waveguide devices, reduction of light coupling loss has become an extremely important issue.

There is also a problem that the light propagation loss is increased by performing the diffusion process as compared with that before the diffusion. In the case of a titanium diffusion optical waveguide, a propagation loss of about several dB / cm usually occurs. Reducing propagation loss is also a major issue for optical waveguide devices.

There is also a problem that the light damage is increased by the diffusion process. This is because when light of high intensity or light of short wavelength is introduced into the diffusion type optical waveguide, the propagation loss increases with time. It is considered that this is due to an increase in electron traps in the optical waveguide due to diffusion of ions into the optical waveguide.

As a method of forming an optical waveguide without using a process such as diffusion, for example, the above-mentioned Kaminow (Kaminow)
In the reference, a lithium niobate crystal is grown on lithium tantalate, or a thin film of lithium niobate is formed on lithium niobate or lithium tantalate by sputtering, and an optical waveguide is formed on the thin film. The method of forming the layer is described. In Japanese Patent Laid-Open Publication No. 52-23355, again, an optical waveguide is formed by epitaxially growing lithium niobate or the like on a substrate such as lithium tantalate by various methods such as liquid phase, vapor phase and melting. How to do is described. However, there are some problems in the optical waveguide forming method using various thin film crystal growth techniques. First, it is difficult to obtain a film thickness of 5 μm or more practically and the productivity is extremely poor in the epitaxial growth film due to the problem of the growth rate and strain in the crystal generated during the growth. On the other hand, in the case of a thin film having a thickness of 5 μm or less, the coupling characteristics with an optical fiber having a core diameter of about 10 μm, which is a light confined portion, deteriorates.

In addition, there is a severe condition that a good single crystal thin film cannot be obtained unless the intervals of crystal lattices are substantially the same in order to be able to grow crystals. Therefore, it is extremely difficult to form a good lithium niobate crystal film on a lithium tantalate substrate, and for this reason, most of the growth is carried out by growing a niobium-tantalum mixed crystal film. In the case of lithium niobate, pure lithium niobate is more
Excellent overall optical waveguide characteristics.

Although it is possible to epitaxially grow the same kind of material, it is difficult to obtain an effective refractive index difference between the substrate and the grown thin film because the crystal orientation is the same.
However, there is a problem that the substrate becomes uniform and the optical waveguide cannot be formed.

When such a thin film forming technique is used, if the film quality is not good, even if it is thick, the light propagation loss and the optical damage will be large, which is not preferable.

[0010]

In order to solve the above problems, at least two translucent substrates having the same refractive index are directly bonded through a layer having a lower refractive index than the translucent substrate,
At least one of the substrates has an optical waveguide in which light is confined.

[0011]

With the above structure, an optical waveguide device having a small coupling loss with an optical fiber and a small propagation loss and optical damage can be obtained.

[0012]

The following is a description of the configuration and manufacturing method thereof when applied to an optical waveguide device of an embodiment of the present invention, particularly an optical modulator.
A description will be given with reference to the drawings.

(Embodiment 1) A first example of the structure of this embodiment is shown in FIGS. 1 and 2. FIG. 1 shows a case where it is applied to an optical modulator. 1 is a transparent substrate, 2 is a thin transparent substrate having the same refractive index as the substrate 1, and 3 is a substrate 1 and 2. The low refractive index layer formed on the surface of at least one of the substrates directly joins the substrates 1 and 2. 4 is substrate 2
The input / output optical waveguide portion 5 formed at 1 is one of the branched optical waveguides branched from the input portion into two, 6 is the other branched optical waveguide, and 7 and 8 are formed on both sides of the branched optical waveguide 6. It is an electrode. FIG. 2 is a sectional view of the central portion thereof, and in the figure, the names of the respective constituent elements 1, 2, 3, 5, 6, 7, 8 are the same as those in FIG. The branch optical waveguides 5 and 6 have a trapezoidal cross-section head portion, and have a so-called ridge-type optical waveguide structure. The cross-sectional shape of the input / output optical waveguide 4 is also the same. Reference numeral 9 shows a guided light propagation section. The configuration itself of the optical modulator is so-called Mach-Zehnder type, and the light incident from the input part is split into two, and an electric field is applied to one of the branched optical waveguides, and the optical waveguide part is created by the electro-optic effect. The light intensity of the output portion is modulated by changing the refractive index of the light source to change the propagation speed of the guided light so that the phase of the light at the recombination portion is different.

As the transparent substrate, lithium niobate or lithium tantalate having a large electro-optical effect was used. The respective refractive indices are 2.29 and 2.18 for ordinary light. These refractive indices slightly vary depending on the crystal orientation. In this embodiment, a combination of materials having completely the same material and the same crystal orientation was used. Further, silicon oxide or silicon nitride was used as the low refractive index layer. The indices of refraction are about 1.5 and 2.0, respectively. Since silicon oxide and silicon nitride both have sufficiently small refractive indexes as compared with lithium niobate and lithium tantalate used for the substrate,
By increasing the thickness of the low refractive index layer to a certain extent or more, light can be confined in one of the substrates, and an optical waveguide can be formed. Specifically, the thickness of the thinner substrate 2 on the side where the optical waveguide is formed is 7 μm, and the thickness of the substrate 1 is 4 μm.
The thickness of the low refractive index layer 3 was 00 μm and 2 μm. Further, by forming the optical waveguide forming portion of the substrate 2 into a so-called ridge structure, the portion under the ridge has a larger effective refractive index than the other portions, so that the light is confined under the ridge, whereby the ridge is formed. The structure is such that the lower part functions as an optical waveguide. The height of the ridge head is 2μ
m, the optical waveguide width is 7 μm, and the length of the branch optical waveguide portion is 2 c
m, and the total length of the optical waveguide portion was 3 cm. Aluminum was used for the electrodes.

In this case, the waveguide has a trapezoidal or rectangular head portion and has a uniform refractive index, so that the center of the guided light is near the center of the optical waveguide and has a shape close to a circle. . The cross sections of the input and output optical waveguides have the same shape.
The coupling efficiency with the circular optical waveguide structure of m) becomes extremely good. In fact, the coupling loss with the optical fiber was 0.5 dB or less on one side by adhesively fixing it with an adhesive material having a matching refractive index. When a conventional diffusion-type optical waveguide using lithium niobate or lithium tantalate was used, the bonding loss was 1.0 dB or more by the same adhesive fixing method, which was a great improvement.

Further, since the light transmissive substrate having the optical characteristics as a pure single crystal which is not subjected to the ion diffusion treatment is used as the optical waveguide, the propagation loss of light can be made extremely small. Specifically, in any combination, an optical waveguide propagation loss of 0.5 dB / cm or less was easily obtained, and the characteristics were improved also in this aspect.

Further, the intensity of incident light is changed from 0 dBm to 20 dB.
Although the state of light damage was observed after changing to m, almost no light damage was observed. This is considered to be an effect due to the use of a good single crystal substrate with very few electron traps, since an optical waveguide can be formed without performing diffusion processing. As a result, the effect of improvement was also seen in terms of light damage. The performance as an optical modulator was almost the same as that of a conventional diffusion type optical waveguide using lithium niobate or lithium tantalate. The measurement is 1.3
It was performed at a wavelength of μm.

Here, the direct joining will be described. Direct bonding is a method of bonding inorganic materials to each other by an intermolecular force on the surface or an electrostatic force on the interface without using an organic adhesive. Therefore, if the substrates to be joined have different coefficients of thermal expansion,
Distortion is generated in the joint portion with respect to temperature change, the joint strength is not so large, and reliability with respect to temperature change is poor. Therefore, in order to perform direct bonding with high productivity and high reliability, it is preferable that the substrates to be bonded have the same coefficient of thermal expansion. However, the coefficient of thermal expansion of a single crystal dielectric or a single crystal semiconductor that is generally used for an optical waveguide element greatly differs between materials, and the coefficient of thermal expansion also slightly changes depending on the crystal orientation. Therefore, as in this embodiment, by combining substrates made of the same material and having the same crystal orientation, the coefficients of thermal expansion can be made to be almost completely the same, so that direct bonding with high productivity and reliability can be obtained. Is.

As the translucent substrate, other than lithium niobate or lithium tantalate, other translucent dielectric substrate having an electro-optical effect, barium titanate (refractive index: 2.4), Potassium niobate (refractive index: 2.
Even if 2) or potassium titanophosphate (refractive index: 1.7) is used, substantially the same effect can be obtained when an optical waveguide is formed by direct bonding via a low refractive index layer.
In addition to silicon oxide or silicon nitride, zinc oxide (refractive index: 2.0), aluminum oxide (refractive index: 1.6), or indium oxide (refractive index:
2.0) various metal oxides, various soda glass (refractive index: about 1.5) and optical glass (refractive index: 1.5-
Even if 1.8) is used, the same effect as the similar configuration can be obtained.

(Embodiment 2) A first example of a method of manufacturing an optical waveguide device of this embodiment will be described.

First, the surfaces of two mirror-polished dielectric transparent substrates having the same refractive index and the same thermal expansion coefficient are made extremely clean, and at least one of the substrates has a predetermined thickness of silicon oxide or silicon nitride. Layer chemical vapor deposition (CVD)
It was formed by sputtering or sputtering. After that, the surface of each substrate was hydrophilized. Specifically, it was performed by immersing in a solution of hydrogen peroxide-ammonia-water. After that, by immersing the surface in pure water, the water constituents are made to adhere to the surface, and then immediately and evenly overlaid, and the water, hydroxyl groups, and hydrogen adsorbed on the substrate surface easily and directly A bond was obtained. In this state, heat treatment at a temperature of 100 ° C. or higher further strengthened the bond. Next, the substrate on the side where the optical waveguide is to be formed was thinned by mechanical polishing and etching. After thinning the thickness from 10 to 7 μm, an etching mask is formed on the thinned substrate by the photolithography technique in the pattern of the optical waveguide structure shown in Example 1, and the portions other than the optical waveguide portion are removed by etching.
It was removed by etching by μm. Chrome-gold was used as the mask. A hydrofluoric acid-based etching solution was used as the etching solution. After that, the mask was removed, and the electrodes were formed by usual photolithography and etching techniques. Thus, the structure of the optical waveguide device shown in Example 1 was obtained.

The coupling characteristic of this element with the optical fiber, the propagation loss, and the optical damage characteristic are as described in the first embodiment.

(Embodiment 3) A second example of the method of manufacturing the optical waveguide device of the present embodiment will be described.

In the same manner as in Example 2, a dielectric transparent substrate having a low refractive index layer on the surface of at least one substrate was prepared. After that, stack both substrates, and
A direct junction was obtained when a DC voltage of 0 V was applied. At that time, when the substrates were heated, direct bonding became possible in a shorter time. In any case, the direct bonding strength could be strengthened at a lower temperature than the method shown in Example 2. afterwards,
By performing the same treatment as that in Example 2, an optical waveguide device having the structure shown in Example 1 was obtained. The characteristics of these devices are as described in the first embodiment.

(Embodiment 4) A third example of the method of manufacturing the optical waveguide device of the present embodiment will be described.

In the same manner as in Example 2, a dielectric translucent substrate having a low refractive index layer on at least one substrate surface was prepared, and the surface thereof was subjected to hydrophilic treatment and water treatment, and superposed. , The substrates were directly bonded to each other. Then, when a direct current voltage of 100 to 2000 V was applied to the bonded portions of both substrates in the same manner as in Example 3, the direct bonding was strengthened. In this case, a stronger bond strength than that shown in Example 3 was obtained at a lower temperature. Then, the same treatment as in Example 2 was performed to obtain the optical waveguide device having the structure in Example 1. The characteristics of these devices are as described in the first embodiment.

(Embodiment 5) A fourth example of the method of manufacturing the optical waveguide device of the present embodiment will be described.

On at least one surface of the translucent substrate, a layer made of a low melting point glass material having a lower refractive index than that was formed by sputtering or the like. Next, the low-melting glass layers were superposed on each other and heated to near the melting point of the low-melting glass to directly bond the substrates. Then, the same treatment as in Example 2 was performed to obtain the optical waveguide device having the structure in Example 1. The characteristics of these devices are as described in the first embodiment.

In the case of glass bonding, the heat treatment temperature is
If the melting point of the low-melting glass used is higher than the melting point, the melt-bonding will be easy. However, even if the temperature is not raised to that temperature, the glass will be bonded even if it is held near the softening point of the glass used. In that case, the film thickness at the time of forming the glass layer was almost maintained. When the heat treatment was carried out at the melting point or higher, the film thickness generally became thinner than the initial film thickness, but there was no particular adverse effect. As the glass, glass having a melting point of 300 ° C. to 800 ° C. was used, and good characteristics were obtained by heat treatment at an appropriate temperature according to the melting point, that is, a temperature equal to or higher than the softening point.

In each of Examples 1 to 5 described above, since the substrates have exactly the same coefficient of thermal expansion, the heat treatment temperature for improving the adhesive strength can be increased more easily. In that case, even if the processing for thinning is performed by strong polishing or the like, the effect of no peeling or stable operation at higher temperatures as an optical waveguide element was obtained.

The direct bonding by hydrophilization and water treatment shown in Examples 2 and 4 was carried out by adding water in water to the surface of each substrate.
It is considered that hydroxyl groups, hydrogen, etc. were adsorbed on the surface and bonded by the binding force of the ions. When heat treatment is performed in this state, water gradually escapes from the bonding interface, hydrogen in the hydroxyl group and hydrogen adsorbed directly escapes, and the remaining oxygen reacts with the element on the surface of the dielectric translucent dielectric substrate, It is believed that the bond is strengthened.

In the direct bonding method of applying a DC voltage to the bonding interface shown in Examples 3 and 4, it is considered that ions move at the bonding interface and the bonding is performed by the electrostatic force.

In each of the embodiments, the representative dimensions are described, but the dimensions are not particularly limited as long as a good optical waveguide is formed.

Further, in the embodiment, the example of the optical modulator having the Mach-Zehnder structure has been described, but the optical modulator is not limited to this structure, and the same applies to any structure using an optical waveguide. It is clear in principle. It is also clear that the structure of the optical modulator can be used as an optical switch if it is modulated in a switching manner. Further, it is obvious that the present invention can be applied to an optical waveguide element that can be realized by controlling the optical waveguide guided light by various means, for example, an optical polarization plane control element, an optical propagation mode control element, an optical phase matching control and the like.

In the present embodiment, the optical waveguide element is formed on only one of the two dielectric transparent substrates, but it is also possible to form the optical waveguide element on both substrates. It is also possible to directly bond three or more dielectric translucent substrates and form an optical waveguide element on each.

[0036]

The present invention has the above-described structure and manufacturing method, and therefore exhibits the following effects.

Since a uniform layered structure is obtained as the optical waveguide, the cross-sectional shape of the optical waveguide has good symmetry, and the center of light propagation can be set at the center of the thin plate, and its thickness can be freely set. As a result, the coupling loss with the optical fiber can be significantly reduced.

Further, since the optical waveguide has a large degree of freedom in selecting the material composition, for example, a good single crystal dielectric substrate not subjected to diffusion treatment can be used, so that the light propagation loss is small and the optical damage is small. An optical waveguide device can be obtained.

Further, since they are made of the same material, they have exactly the same coefficient of thermal expansion, so that the productivity and reliability of direct joining can be improved, and the characteristics are stable even at high temperatures.

In this embodiment, an example of the structure of the optical modulator is shown. However, since the feature of this embodiment lies in the structure of the optical waveguide itself, it is basically applied to various optical waveguide devices using the optical waveguide. It can be widely and generally applied, and is not limited to an optical modulator, but can be applied to an optical waveguide device such as an optical switch, a polarization plane control, a propagation mode control, and an optical phase matching control.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a sectional view of the first embodiment of the present invention.

[Explanation of symbols]

 1, 2 Transparent substrate 3 Low refractive index layer 4 Input / output optical waveguide section 5 First branched optical waveguide 6 Second branched optical waveguide 7, 8 Electrode 9 Guided light propagation section

Claims (18)

[Claims] 1. A plurality of dielectric transparent substrates having the same refractive index and the same thermal expansion coefficient are directly bonded to each other through a layer having a lower refractive index than the dielectric transparent substrate, and at least one of the substrates is bonded. An optical waveguide device having an optical waveguide in which light is confined. 2. The optical waveguide device according to claim 1, wherein the dielectric transparent substrate is made of a substance having an electro-optical effect. 3. The optical waveguide device according to claim 1, wherein the dielectric transparent substrate is made of lithium niobate or lithium tantalate. 4. The optical waveguide device according to claim 1, wherein the low refractive index layer at the joint portion is a metal oxide. 5. The optical waveguide element according to claim 1, wherein the low refractive index layer at the junction is made of silicon oxide. 6. The optical waveguide device according to claim 1, wherein the low refractive index layer at the junction is made of silicon nitride. 7. The optical waveguide device according to claim 1, wherein the low refractive index layer at the joint portion is made of glass. 8. The optical waveguide element according to claim 1, wherein the low refractive index layer at the joint portion has a thickness of at least a thickness sufficient for confining light in the transparent substrate. . 9. A plurality of dielectric translucent substrates having the same refractive index and the same thermal expansion coefficient are made to have a smooth and clean surface, and at least one of the substrates is made of a material having a lower refractive index than the substrate. After forming a layer and hydrophilizing and water-treating both substrate surfaces, they are directly bonded by superposing them through the low refractive index layer and heated at 100 ° C. or higher to improve the direct bonding strength. A method for manufacturing an optical waveguide element, characterized in that an optical waveguide is formed on at least one of the substrates. 10. A plurality of dielectric translucent substrates having the same refractive index and the same thermal expansion coefficient are made to have smooth and clean surfaces, and at least one of the substrates is made of a material having a lower refractive index than the substrates. An optical waveguide element characterized in that a layer is formed, both substrates are superposed via a low refractive index layer and directly bonded by applying a voltage to the substrate bonding portion, and then an optical waveguide is formed on at least one substrate. Manufacturing method. 11. A plurality of dielectric translucent substrates having the same refractive index and the same thermal expansion coefficient are made to have a smooth and clean surface, and at least one of the substrates is made of a material having a lower refractive index than that of the substrate. After forming a layer, and hydrophilizing treatment and water treatment on both substrate surfaces, by superposing them directly through the low refractive index layer and directly bonding, by applying a voltage to the substrate bonding portion,
A method for manufacturing an optical waveguide device, characterized in that an optical waveguide is formed on at least one of the substrates after improving the direct bonding strength. 12. A light-transmissive substrate having a plurality of refractive indexes and the same thermal expansion coefficient, the surface of which is made smooth and clean, and at least one surface of the substrate has a low melting point made of a material having a refractive index lower than that of the substrate. An optical waveguide characterized in that a glass layer is formed, superposed through the low-melting glass layer, bonded by heat treatment by an adhesive force of the low-melting glass layer, and then an optical waveguide is formed on at least one substrate. Device manufacturing method. 13. The method of manufacturing an optical waveguide device according to claim 10, wherein the transparent substrate is made of a substance having an electro-optical effect. 14. The transparent substrate is made of lithium niobate or lithium tantalate.
14. The method for manufacturing an optical waveguide device according to any one of 1 to 13. 15. The method of manufacturing an optical waveguide device according to claim 10, wherein the low refractive index layer at the joint portion is a metal oxide. 16. The method of manufacturing an optical waveguide device according to claim 10, wherein the low refractive index layer at the junction is made of silicon oxide. 17. The method of manufacturing an optical waveguide device according to claim 10, wherein the low refractive index layer at the junction is made of silicon nitride. 18. The method for manufacturing an optical waveguide device according to claim 10, wherein the low refractive index layer at the joint portion is made of glass.