JP2574594B2 - Optical waveguide device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various optical waveguide devices for performing light intensity modulation, optical switching, polarization plane control, propagation mode control, and the like using an optical waveguide, and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for an optical waveguide device used in optical communication equipment as a medium for transmitting a broadband signal.

Conventionally, an optical waveguide element such as an optical modulator,
For optical switches, optical polarization control elements, and optical propagation mode control elements, a single-mode-propagating optical waveguide is formed in a dielectric single crystal having an electro-optical effect such as lithium niobate or lithium tantalate, and the shape is formed. In addition, the electrodes are provided in an appropriate form, and the light passing through the optical waveguide is controlled by the electro-optic effect. In the fabrication of an optical waveguide, light is confined by evaporating a metal, for example, titanium, and thermally diffusing it at a high temperature so that the refractive index of the diffused portion is slightly higher than that of the other portions. Alternatively, an optical waveguide is formed by performing a proton ion exchange in phosphoric acid at 200 to 300 ° C. and changing a part of the refractive index by using a metal mask on a predetermined portion. However, in any of the methods, since the optical waveguide is formed by the diffusion treatment from the surface, the cross-sectional shape of the optical waveguide becomes a shape according to the diffusion, and thus there are various inconveniences.

[0004] One of the major problems is the coupling loss between the optical waveguide and the optical fiber. While the cross-sectional shape of the optical fiber is circular, the shape of the optical waveguide is similar to an inverted triangle due to diffusion from the surface, and the strongest part of the intensity of the guided light is near the surface, The optical coupling with the optical fiber was not sufficient, causing a large loss. In an optical waveguide device, reduction of light coupling loss is an extremely important issue.

[0005] In addition, there is another problem that the light propagation loss is increased by performing the diffusion process as compared with before the diffusion. In the case of a titanium diffused optical waveguide, a propagation loss of about 1 dB / cm usually occurs. Therefore, reduction of the propagation loss is also a major problem of the optical waveguide device.

Another problem is that light damage is increased by the diffusion treatment. This is because, when high-intensity light or short-wavelength light enters the diffusion-type optical waveguide, the propagation loss increases with time. This is thought to be due to an increase in trapping of electrons in the optical waveguide due to diffusion of ions into the optical waveguide.

As a method for forming an optical waveguide that is not of the ion diffusion type, a method using a single crystal epitaxial growth film is known. For example, an optical waveguide in which a mixed crystal film of lithium niobate and lithium tantalate is formed on a lithium tantalate substrate is known. However, this method has some limitations. First, it is difficult to practically obtain a film having a thickness of 5 μm or more due to the problem of the growth rate and strain in the crystal generated during the growth of the epitaxially grown film. The characteristics deteriorate.

Further, conditions for epitaxial growth are limited. It is difficult to form pure lithium niobate on a lithium tantalate substrate because the epitaxial growth is difficult unless the crystal lattice spacing is substantially the same. Therefore, the mixed crystal film is only grown. In the case of lithium niobate, pure lithium niobate is superior to the mixed crystal film in overall optical waveguide characteristics.

Although the same type of epitaxial growth is possible, there is a problem that the substrate has a uniform refractive index because the crystal orientation is the same, and an optical waveguide cannot be formed.

[0010]

However, in the above-mentioned conventional configuration, since the optical waveguide or the optical waveguide element using the epitaxially grown film is formed on the single substrate only by the diffusion method from above, the optical waveguide and the optical fiber are not connected to each other. There are problems such as a large coupling loss, a large propagation loss, and a large optical damage.

An object of the present invention is to solve the above-mentioned conventional problems, and an object of the present invention is to provide an optical waveguide device with less coupling loss to an optical fiber, less propagation loss, and less optical damage, and a method of manufacturing the same.

[0012]

In order to achieve the above object, an optical waveguide device and a method of manufacturing the same according to the present invention provide a single crystal dielectric substrate having an electro-optic effect and having a different refractive index depending on the crystal orientation. By changing at least two substrates bonded by a silicon film, a silicon oxide film or a silicon nitride film, an optical waveguide in which light is confined in one single crystal dielectric substrate due to a difference in refractive index due to a difference in crystal orientation. And controlling the light passing through the optical waveguide by an electro-optic effect, or at least two single-crystal dielectric substrates having an electro-optic effect and having different refractive indices by a silicon film or a silicon oxide film or a silicon nitride film. The bonded substrate has an optical waveguide in which light is confined in one of the single-crystal dielectric substrates due to the difference in the refractive index, and the light passing through the optical waveguide is controlled by an electro-optic effect. To have.

[0013]

According to the present invention, an optical waveguide device having a small coupling loss and a propagation loss with an optical fiber and a small optical damage can be obtained.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical waveguide device according to an embodiment of the present invention, in particular, a configuration when applied to an optical modulator and a method of manufacturing the same will be described.
This will be described with reference to the drawings.

(Embodiment 1) As shown in FIGS. 1 and 2, when applied to an optical modulator, the optical waveguide device of this embodiment was directly bonded to a lithium niobate substrate 1 and a lithium niobate substrate 1. Lithium niobate thin plate 2 having a different crystal orientation from lithium niobate substrate 1, input / output optical waveguide 3 formed on lithium niobate thin plate 2, one of two branches from the input part The optical waveguide 4 and the other second
, The electrodes 6 and 7 formed on both sides of the second branch optical waveguide 5, and the silicon oxide film 8 joining the lithium niobate thin plate 2 to the lithium niobate substrate 1. As shown in FIG. 2, the names of the components in the cross section of the central portion are the same as those in FIG. The first and second branch optical waveguides 4 and 5 have trapezoidal cross-section heads, and have a so-called ridge-type optical waveguide structure. The cross-sectional shape of the input / output optical waveguide 3 is also the same, and has a guided light propagation section 9. The configuration itself of the optical modulator is what is called a Mach-Zehnder type, in which light incident from the input portion of the optical waveguide 3 is branched into two, and an electric field is applied to one of the branched second branched optical waveguides 5. The intensity of light at the output of the optical waveguide 3 is modulated by changing the refractive index of the optical waveguide by the electro-optic effect to change the propagation speed of the guided light so that the phase of the light at the recombination is different. It is like that.

Lithium niobate has a large difference in dielectric constant between the crystal optical axis direction and a direction perpendicular to the crystal optical axis, and the refractive index also changes accordingly. The refractive index for ordinary rays is 2.
29, whereas for extraordinary rays, 2.20
Of the refractive index. If there is a difference of about 0.01 or more in the refractive index, light can be confined to the one having a larger refractive index, and an optical waveguide can be formed. In the case of this embodiment, the crystal axes are selected and joined so that the refractive index of the lithium niobate thin plate 2 is larger than the refractive index of the lithium niobate substrate 1 in the light propagation mode. The junction silicon oxide film 8 in the meantime has a refractive index of about 1.46 and is lower than that of lithium niobate, but its thickness is set to 0.5 μm, which is much smaller than the optical waveguide blocking thickness. Light incident on the thin plate 2 is confined in the thin plate. Further, by providing the ridge structure on the lithium niobate thin plate 2, the effective dielectric constant is higher in the lower portion of the ridge than in the other portions, so that the light is confined in the lower portion of the ridge, and thus the guided light propagation in the lower portion of the ridge. The part 9 acts as an optical waveguide.

In this case, the waveguide has a uniform trapezoidal or rectangular head and a uniform refractive index, so that the center of the guided light is close to a circle at the center of the optical waveguide. The cross section of the input / output optical waveguide 3 has the same shape, and therefore, the coupling efficiency with the optical fiber circular optical waveguide structure having a diameter of about 10 μm is extremely good.

A typical practical value of each dimension is that the thickness of the lithium niobate substrate 1 is 600 microns, the thickness of the lithium niobate thin plate 2 is 7 microns, the height of the ridge head is 3 microns, and the optical waveguide 3 4 and 5 are 7 microns wide,
The thickness of the silicon oxide film 8 is 0.5 μm, the first and second
The length of the branched optical waveguides 4 and 5 is 2 cm, and the length of the entire optical waveguide is 3 cm. Electrodes 6 and 7 used aluminum. With the above-described configuration, the coupling loss with the optical fiber is reduced to 0.3 dB or less on one side by bonding and fixing using an adhesive whose refractive index is matched. When a conventional titanium diffused optical waveguide is used, the coupling loss is about 0.5 to 1.0 dB by the same adhesive fixing method.
Was greatly improved. The performance as an optical modulator was almost the same as that of a conventional titanium diffused optical waveguide.

Further, since a lithium niobate thin plate having optical properties as a pure single crystal not subjected to ion diffusion processing is used as the optical waveguide, light propagation loss can be extremely reduced. Specifically, 0.1 dB / c
An optical waveguide propagation loss of m or less was easily obtained. In the case of the titanium diffused optical waveguide, the characteristic was significantly improved since the value was 0.5 to 1.0 dB / cm.

Further, the intensity of the incident light is changed from 0 dBm to 30 dB.
m, the state of light damage was observed, but almost no light damage was observed. This is considered to be the effect of using a pure single crystal lithium niobate thin plate with very few electron traps as the optical waveguide. The measurement light was 1.
Performed at a wavelength of 3 microns.

(Embodiment 2) A second embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 3, when the structure of the optical waveguide device of this embodiment is applied to an optical modulator, a lithium tantalate substrate 10 and a niobium bonded to the lithium tantalate substrate 10 by a silicon nitride film 8a are used. The names and functions of the components from the lithium oxide thin plate 11, the optical waveguide 3 to the electrode 7 are the same as in the first embodiment.

Lithium tantalate and lithium niobate have different refractive indices. For ordinary light, lithium tantalate is 2.175 and lithium niobate is 2.29. Also in this case, there is an appropriate difference of 0.115 in the refractive index,
Light can be effectively confined to the lithium niobate thin plate having a large refractive index, and an optical waveguide can be formed. As a result, the light incident on the lithium niobate thin plate 11 is confined in the thin plate, and the effective refractive index of the lower portion of the ridge becomes larger than that of the other portions by providing the ridge structure. Trapped in the lower part, the lower part of the ridge acts as an optical waveguide. In bonding the silicon nitride film 8a has a refractive index of about 1.9, but less than the lithium niobate, the thickness single crystal dielectric base
Due to the difference in the refractive index between the plates, the bonded composite substrate
Light is closed in the single-crystal dielectric substrate with the higher refractive index.
Since the thickness is set to 0.5 μm, which is significantly thinner than the thickness at which the indentation is possible, the light incident on the lithium niobate thin plate 11 is confined in the thin plate.

In this case, the shape of the waveguide is exactly the same as that of the first embodiment. Therefore, the coupling efficiency of the optical fiber with the circular optical waveguide structure is extremely good.

A typical practical value of each dimension is that of the silicon nitride film 8.
a thickness 0.5 a, lithium tantalate substrate 1
0 has a thickness of 600 microns, and the others are the same as in the first embodiment. With the above-described configuration, the coupling loss with the optical fiber was 0.3 dB or less on one side, as in the first embodiment, and was significantly improved.

The propagation loss is 0.1 d as in the first embodiment.
B / cm was easily obtained. The same effect as in Example 1 was obtained for optical damage.

(Embodiment 3) Hereinafter, a third embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 4, when the structure of the optical waveguide device of this embodiment is applied to an optical modulator, a lithium tantalate substrate 10 and a silicon film 8 are formed on the lithium tantalate substrate 10.
The names and functions of the lithium niobate thin plate 11 joined by b and other components are the same as those in the second embodiment.

In this case, as in the case of Example 2, the difference in the refractive index between lithium tantalate and lithium niobate causes
Light incident on the lithium niobate thin plate 11 is confined in the thin plate, and furthermore, by providing a ridge structure, the effective dielectric constant of the lower part of the ridge becomes larger than that of the other parts. Therefore, the lower part of the ridge acts as an optical waveguide. The junction silicon film 8b has a lower refractive index than lithium niobate.
Although light having a wavelength of 3 μm is absorbed, since the thickness is set to 0.5 μm, which is much smaller than the optical waveguide cutoff thickness, light incident on the lithium niobate thin plate 11 is confined in the thin plate.

In this case, the shape of the waveguide is exactly the same as that of the first embodiment, and the coupling efficiency of the optical fiber with the circular optical waveguide structure is extremely good.

A typical practical value of each dimension is the same as that of the second embodiment except that the thickness of the silicon film 8b is 0.5 μm. With the above configuration, the coupling loss with the optical fiber is also 0.3% on one side, as in the first embodiment.
dB or less, which is a significant improvement.

The propagation loss is 0.1 d as in the first embodiment.
B / cm was easily obtained. The same effect as in Example 1 was obtained for optical damage.

(Embodiment 4) Next, an example of a method for manufacturing an optical waveguide device of the present embodiment will be described with reference to FIG.

First, the surfaces of two mirror-polished lithium niobate substrates having different crystal orientations are extremely cleaned, and then a silicon oxide film is formed on one surface of each substrate to a thickness of 0.25 μm by a plasma CVD method. did.

Next, each of the silicon oxide film surface extremely clean by etching, and was subjected to hydrophilic treatment. Specifically, when the surface layer of the silicon oxide film was slightly removed by etching with a hydrofluoric acid-based etchant, the silicon oxide film could be cleaned and the surface could be hydrophilized. Under such conditions, clean the surface with pure water
When a hydroxyl group was attached to the surface of the silicon oxide film and immediately overlapped, a bond was easily obtained by the hydroxyl group adsorbed on the surface of the silicon oxide film. Although a sufficiently strong joint was obtained as it was, a heat treatment at a temperature of 100 to 1100 ° C. in this state further strengthened the joint.

Next, a lithium niobate substrate having a crystal orientation with a higher refractive index was thinned by mechanical polishing and etching. After thinning to 7 microns,
An etching mask was formed on the thinned lithium niobate substrate by the photolithography technique in the pattern of the optical waveguide structure shown in Example 1, and portions other than the optical waveguide portion were etched away by 3 μm by etching. Cr was used as a mask, and etching was performed by reactive ion etching using CF4 gas. After that, the mask was removed, and aluminum electrodes 6 and 7 were formed by ordinary photolithography and etching techniques. Thus, the structure of the optical waveguide device shown in Example 1 was obtained. The coupling characteristics of the device to an optical fiber, the propagation loss, and the optical damage characteristics were all the same as in Example 1.

The heat treatment of the silicon oxide film 8 is performed at 100 to 110
In the range of 0 ° C., the higher the heat treatment temperature, the higher the bonding strength.

(Embodiment 5) Next, another example of the method of manufacturing the optical waveguide device of this embodiment will be described with reference to FIG.

In the same manner as in Example 4, the surfaces of the mirror-polished lithium tantalate and lithium niobate substrates were extremely cleaned, and a silicon nitride film of 0.25 μm was formed on one surface of each substrate by plasma CVD. It was formed to a thickness.

Next, cleaned and hydrophilized substantially similar manner the silicon nitride film surface of Example 4, to further washed with pure water
And attach hydroxyl groups to the surface,
By overlapping with the silicon nitride,
Bonding was easily achieved by the hydroxyl groups adsorbed on the surface of the base film . Further, by subjecting this to a heat treatment at a temperature of 100 to 1100 ° C., a stronger bond was obtained.

Thereafter, aluminum electrodes 6 and 7 were formed in the same manner as in Example 4, and the structure of the optical waveguide device shown in Example 2 was obtained.

The coupling characteristics of the device to an optical fiber, the propagation loss, and the optical damage characteristics were all the same as in Example 2.

The heat treatment of the silicon nitride film 8a is performed at 100 to 110.
In the range of 0 ° C., the higher the heat treatment temperature, the higher the bonding strength.

(Embodiment 6) Next, another example of the method of manufacturing the optical waveguide device of this embodiment will be described with reference to FIG.

In the same manner as in Example 5, the surfaces of the mirror-polished lithium tantalate and lithium niobate substrates were extremely cleaned, and then an amorphous silicon film was formed on one surface of each substrate by plasma CVD. It was formed to a thickness of 25 microns.

Next, cleaned and hydrophilized amorphous silicon film surface in much the same way as in Example 4, further washed with pure water
And attach hydroxyl groups to the surface.
In the same manner as in Example 4 ,
Bonding was easily obtained by the hydroxyl groups adsorbed on the surface of the porous silicon film . Further, by subjecting this to a heat treatment at a temperature of 100 to 1100 ° C., a stronger bond was obtained.

Thereafter, aluminum electrodes 6 and 7 were formed in the same manner as in Example 5, and the structure of the optical waveguide device shown in Example 3 was obtained.

The characteristics of this device for coupling to an optical fiber, the propagation loss, and the optical damage characteristics were all the same as in Example 3.

The heat treatment of the amorphous silicon film is performed at 100 to 110
In the range of 0 ° C., the higher the heat treatment temperature, the higher the bonding strength. However, when the heat treatment temperature was higher than the crystallization temperature of the amorphous silicon, the amorphous silicon film was changed to a polycrystalline silicon film, but the bonding state was similarly maintained.

In Examples 4, 5, and 6, the bonding strength generally increased as the temperature increased during the heat treatment at the time of bonding. However, when the temperature exceeds 1100 ° C., lithium escapes from the surface of the lithium niobate or lithium tantalate plate, so that the surface characteristics are greatly deteriorated and the performance as an optical waveguide device is deteriorated. Therefore, the heat treatment temperature is 1
100 ° C. or less was preferred.

In the case of joining lithium niobate shown in Example 4, since the coefficient of thermal expansion was the same, the heat treatment temperature for improving the joining strength could be more easily performed at a higher temperature. In this case, there were obtained effects such as no peeling even if the processing for thinning was performed by strong polishing or the like, and the optical waveguide element could operate stably up to a higher temperature.
Therefore, in the case of bonding substrates of the same type, a substrate having high bonding strength and stable up to high temperatures was obtained.

The thickness of the junction can be set to 0.1 by changing the conditions of the plasma CVD in any of the embodiments.
To 3 microns.

In the embodiments, typical dimensions have been described. However, the dimensions are not particularly limited as long as a good optical waveguide is formed.

In the above embodiments, examples of lithium niobate and lithium tantalate have been described as examples of single crystal dielectrics. However, the same applies to the case of using other single crystal dielectrics having an electro-optical effect. What can be done is clear in principle.

In the embodiment, only an example of the relationship between a specific substrate and a bonding film is shown. However, the combination may be changed in various ways. For example, a silicon nitride film or a silicon nitride film may be used instead of the silicon oxide film in the method of the fourth embodiment. Similar configurations and effects were obtained by using a silicon film or using a silicon oxide film by the method of the fifth embodiment. Further, by directly joining the single crystal dielectric substrates via a silicon film, a silicon oxide film, or a silicon nitride film, a new effect can be obtained in application and manufacturing. In manufacturing, even if there is some unevenness or small dust at the bonding interface, it is taken into the film to some extent and flattened in the process of forming a silicon, silicon oxide, or silicon nitride film.
This has the effect of improving the production yield of direct bonding.
When a silicon film is used, the initial film quality may be amorphous or polycrystalline, and the state after heat treatment depending on the heat treatment temperature may be either amorphous or polycrystalline.

The configuration shown in the first embodiment is particularly effective for a mode splitter because the optical waveguide portion and the substrate portion have different refractive indexes for ordinary and extraordinary rays.

[0057]

As is apparent from the above embodiments, according to the structure and the manufacturing method of the present invention, a uniform layered structure can be obtained as an optical waveguide. The center of light propagation can be substantially at the center of the thin plate, and its thickness can be made freely, thereby greatly reducing the coupling loss with the optical fiber.

Further, since a pure single-crystal dielectric thin plate which has not been subjected to a diffusion treatment is used as the optical waveguide, an optical waveguide element with small light propagation loss and small optical damage can be obtained.

In the case of a bonded substrate made of the same material, since the thermal expansion coefficient is the same, heat treatment for improving the bonding strength can be performed more easily at a higher temperature. Is stable.

In this embodiment, an example of the configuration of the optical modulator is shown. However, since the feature of this embodiment lies in the configuration of the optical waveguide, various types of optical waveguide devices using the optical waveguide are basically used. That can be applied to optical waveguide devices such as optical switches, polarization plane control, and propagation mode control as well as optical modulators. It is.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the configuration of a first embodiment of the present invention.

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

FIG. 3 is a perspective view showing the configuration of a second embodiment of the present invention.

FIG. 4 is a perspective view showing the configuration of a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Lithium niobate substrate 2, 11 Lithium niobate thin plate 3 Optical waveguide for input / output 4 First branch optical waveguide 5 Second branch optical waveguide 6, 7 Electrode 8 Silicon oxide film 8a Silicon nitride film 8b Silicon film 9 Conduction Wave light propagation part 10 Lithium tantalate substrate

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-44025 (JP, A) JP-A-64-18121 (JP, A) JP-A-52-145240 (JP, A) JP-A-49-440 98258 (JP, A) JP-A-58-223106 (JP, A) JP-A-60-51700 (JP, A) JP-A-4-116816 (JP, A) JP-A-63-49732 (JP, A) JP-A-4-123018 (JP, A) APPL. PHYS. LETT. VOL. 56 NO. 24 p. 2419-P. 2421 (June 1990)

Claims (12)

(57) [Claims] [Claim 1 further comprising an electro-optical effect, a single-crystal dielectric substrates at least two of the same type refractive index differs by crystal orientations, by changing the crystal orientation, a predetermined portion of said single crystal dielectric substrates The formed silicon film or silicon oxide film or nitride
Via the silicon oxide film, the silicon film or the silicon oxide film or
Direct bonding by the hydroxyl group or oxygen on the silicon nitride film surface
Out of the composite substrate, the single crystal having the higher refractive index
In the dielectric substrate, the refractive index between the single crystal dielectric substrate
Having an optical waveguide confined by the difference in
An optical waveguide device characterized by the following . 2. A has the electro-optical effect, low refractive index different
Without even a single-crystal dielectric substrates of the two dissimilar, the single crystal
Silicon film or oxidation formed on predetermined part of dielectric substrate
The silicon film or the acid is interposed through a silicon film or a silicon nitride film.
Hydroxyl or oxygen on the surface of the silicon nitride film or silicon nitride film.
Of the directly bonded composite substrates, the high refractive index
The single-crystal dielectric substrate in one of the single-crystal dielectric substrates
Optical waveguide confined by difference in refractive index between
An optical waveguide device comprising: 3. The optical waveguide device according to claim 1, wherein lithium niobate or lithium tantalate is used as the single crystal dielectric. 4. The optical waveguide device according to claim 1, wherein light intensity modulation, optical switching, polarization plane control, or propagation mode control is performed. 5. The silicon film according to claim 1, wherein the silicon film is amorphous.
An optical waveguide device as described in the above. 6. The silicon film according to claim 1, wherein the silicon film is polycrystalline.
An optical waveguide device as described in the above. 7. A light source having an electro-optic effect and having different refractive indices.
A process to clean the surface of at least two single-crystal dielectric substrates
And a silicon film at a predetermined position on the single crystal dielectric substrate.
Forming a silicon oxide film or a silicon nitride film;
After the surface of the film is made hydrophilic, it is superposed and joined.
And an optical waveguide element comprising a heat treatment step.
Child manufacturing method. 8. The difference in the refractive index depends on the crystal orientation.
8. The method for manufacturing an optical waveguide device according to claim 7, wherein
Law. 9. A substrate on the side where an optical waveguide is formed after bonding.
The board is thinned to a thickness that effectively works as an optical waveguide
And forming an optical waveguide on the processed substrate
And a step of further forming an electrode.
9. The method of manufacturing an optical waveguide device according to claim 7, wherein
Law. As claimed in claim 10 single crystal dielectric, characterized by using lithium niobate or lithium tantalate
A method for manufacturing an optical waveguide device according to claim 7 . 11. This is to perform the light intensity modulation or optical switching or polarization control, or propagation mode control
10. The method for manufacturing an optical waveguide device according to claim 7, wherein: 12. A heat treatment temperature of 100-1100
℃ characterized by a temperature range of claims 7 to 9 Symbol
Manufacturing method of the above-mentioned optical waveguide element.