JPH05196901A - Optical waveguide and optical waveguide type optical device - Google Patents

Abstract

PURPOSE:To obtain the optical waveguide having an excellent electrooptical effect and optical damage resistance and the optical device having excellent modulation efficiency and stable amplitude modulation. CONSTITUTION:This optical waveguide is constituted by forming a thin-film waveguide 2 consisting of an LiNbO3 single crystal on the surface of an LiTaO3. single crystal substrate 1 and providing a branch part in a part of this thin-film waveguide 2. This optical device is constituted by providing the optical waveguide where the lattice constants of the thin-film waveguide 2 consisting of the LiNbO3 single crystal and the above-mentioned LiTaO3 single crystal substrate 1 are matched and electrodes 7-1, 7-2 along the optical axis of the waveguide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide and a waveguide type optical device, and more particularly to an optical integrated circuit device used for an optical communication system, an optical information processing system, an optical sensor system, etc., particularly an optical switch and an optical device. This is a proposal regarding what is advantageously used as a modulator.

In recent years, the progress of single-mode fibers and single-wavelength lasers has enabled high-speed Gb / s optical transmission. In particular, waveguide-type optical devices, which are excellent in compatibility with single-mode fibers, hold the key to future optical communication technology. In the field of such optical communication, the use of optical devices such as an optical matrix switch and an optical modulator is indispensable for constructing an optical fiber network. The present invention proposes this optical device, that is, a waveguide type switch / modulator, particularly a Mach-Zehnder type optical device and an optical waveguide as a component thereof.

[0003]

2. Description of the Related Art Regarding optical integrated circuit devices such as optical matrix switches and optical modulators, "" Optical Integrated Circuits ", Japan Society of Applied Physics, Optical Communication, P.158- (Asakura Shoten 198
8) ”, etc., Ti-diffused LiN
Generally, a bO ₃ channel waveguide is used. This is because LiNbO ₃ has a relatively large electro-optic constant among inorganic optical crystals and is effective for devices that utilize the electro-optic effect.

Conventionally, there is known an example in which a Mach-Zehnder interferometer is constructed by using an optical waveguide, and an optical switch and an optical modulator are produced by this, and TRRANGANATH et al.
"IEEE J. Quantum. Electron.", QE-13 (4) 290 (1977)
Is reported in.

[0005]

[Problems to be solved by the invention] However, TRRANGANAT
Regarding the Mach-Zehnder type optical modulator proposed by H et al., Koichi Kumagai et al. In "Technical Report" OQE85 (116-132) 1
As reported in 9 (1985), the LiNbO ₃ optical waveguide prepared by the Ti diffusion method has a large optical damage due to the inclusion of Ti, and cannot guide a short wavelength. Li produced by the proton exchange method
The NbO ₃ optical waveguide has the original crystallinity of LiNbO ₃ after the waveguide is formed.
The electro-optic constant is smaller than that of the bulk. Therefore, when this optical waveguide is used as an optical device such as an optical directional coupler or an optical modulator, a large switching voltage and a long working length are required, so that power saving and downsizing cannot be achieved. There was a problem.

[0006]

The inventors of the present invention have diligently studied the above-mentioned problems of the known Mach-Zehnder type modulator, and have a low electro-optical effect.
We have found that the cause is that the lattice constants of the substrate and the crystal material forming the waveguide do not match. Therefore, it has been found that the above-mentioned problems can be solved by matching the lattice constants of the crystal of the substrate and the waveguide to obtain an optical waveguide having an excellent electro-optical effect.

That is, unlike the Ti diffusion method and the like, the optical waveguide of the present invention is manufactured without adding impurities such as transition metals that cause optical damage to the optical waveguide layer. Wave can be guided without loss even at the wavelength.

Moreover, the optical waveguide of the present invention is not limited to simply not diffusing impurities, but if the respective crystals of the substrate and the waveguide are lattice-matched, they are excellent in heat resistance and reach a temperature near the Curie point of LiNbO _3. It exhibits a characteristic that the waveguide characteristics do not change even after the temperature is raised. That is, in the conventional Ti-diffused LiNbO ₃ waveguide and proton-exchanged LiNbO ₃ waveguide, when the temperature is raised to the Curie point of LiNbO ₃ , Ti and protons are further diffused, and the waveguide mode profile changes and the waveguide mode exists. However, the present invention does not have such an adverse effect.

The optical waveguide of the present invention constructed on the basis of such findings forms a LiNbO ₃ single crystal thin film waveguide on the surface of a LiTaO ₃ single crystal substrate, and forms a branching part in a part of this thin film waveguide. And is characterized in that the lattice constants of the LiNbO ₃ single crystal thin film waveguide and the LiTaO ₃ single crystal substrate are matched, and the present invention is a LiTaO ₃ single crystal substrate surface. , While forming a LiNbO ₃ single crystal thin film waveguide,
A branch is provided in a part of the thin film waveguide, and
The lattice constants of the LiNbO ₃ single crystal thin film waveguide and the LiTaO ₃ single crystal substrate are matched, and an electrode for changing the refractive index of this waveguide is attached to at least one of the waveguides of the branched portion. A waveguide type optical device provided is proposed.

[0010]

The function of the optical waveguide according to the present invention is that the refractive index (propagation constant) of the waveguide is formed in the Y-shaped branch portion of the LiNbO ₃ single crystal thin film waveguide formed in or on the LiTaO ₃ single crystal substrate. An electrode for changing the temperature of the LiTaO ₃
The point is that the crystal lattice of the single crystal and the crystal lattice of the LiNbO ₃ single crystal thin film waveguide are matched. In addition, on the (0001) plane of the LiTaO ₃ single crystal substrate 1, (00) of the LiNbO ₃ single crystal waveguide was formed.
01) surface must be formed so that it is laminated.

In the above structure, the lattice matching between the superposed single crystals means that the lattice constant of the LiNbO ₃ single crystal thin film is substantially the same as the lattice constant of the LiTaO ₃ single crystal substrate.
That is, 99.81-100.07%, more preferably 99.92-
It means that the range is adjusted to 100.03%.

By thus lattice-matching both single crystals, it is possible to prevent the occurrence of strain during liquid phase epitaxial growth of the LiNbO ₃ single crystal on the LiTaO ₃ single crystal substrate, and as a result, it is possible to obtain a LiNbO ₃ single crystal bulk. Have equivalent electro-optic constants
LiNbO ₃ single crystal thin film waveguides can be formed.

This LiNbO ₃ single crystal thin film is used as a substrate for LiTa
This is because, when an O ₃ single crystal is used, if it is lattice-matched with the lattice constant of this substrate single crystal, it exhibits extremely excellent optical characteristics, and can be formed with a thick film thickness that cannot be obtained by conventional techniques.

[0014] That is, the reason for LiNbO ₃ single crystal thin film exhibits excellent optical properties, and integrated them by LiNbO ₃ single crystal thin film and the LiTaO ₃ single crystal is lattice matched,
This is because a lattice distortion and crystal defects are extremely small, crystallinity is excellent, and a high-quality film without microcracks is obtained.

In the above structure, the method proposed by the present inventors in International Application No. PCT / JP / 90/01207 is desirable as a method for lattice-matching both superposed single crystals. That is, a method of incorporating sodium or magnesium in a LiNbO ₃ single crystal, a method of changing the Li / Nb ratio between 41/59 and 56/44, T
A method of reducing the lattice constant of the LiTaO ₃ single crystal substrate by adding i or the like is well suited. These methods will be described below.

It is generally known that the lattice constant of a LiTaO ₃ single crystal substrate is larger than that of a LiNbO ₃ single crystal. Therefore, this LiNb is added to the LiNbO ₃ single crystal to be superimposed.
When sodium or magnesium, which has the effect of increasing the lattice constant of the O ₃ single crystal, is substituted or doped and contained, the lattice constants of both single crystals can be brought close to each other without causing optical damage.

When Na and Mg are contained in the LiNbO ₃ single crystal thin film, the content of N and Mg is N based on LiNbO ₃ single crystal.
It is desirable to contain a: 0.1 to 14.3 mol% and Mg: 0.8 to 10.8 mol%. The reason for this is that when the content of Na is less than 0.1 mol%, the lattice constant does not become large enough to be lattice-matched with the LiTaO ₃ single crystal substrate, regardless of the amount of addition of Mg. On the other hand, if it exceeds%, the lattice constant becomes too large.
This is because the lattice matching between the LiTaO ₃ single crystal and the LiNbO ₃ single crystal becomes difficult. In addition, regarding the content of Mg, 0.8 mol%
If the amount is less than the above range, the effect of preventing optical damage becomes insufficient, and the amount exceeding 10.8 mol% cannot be contained because magnesium niobate-based crystals are precipitated.

Next, in the present invention, LiNbO₃Li in
By changing the molar ratio of / Nb, LiNbO₃Single crystal thin film
And LiTaO₃As a method of lattice-matching a single crystal substrate,
Using the phase-epitaxial growth method,
At least K₂O, V₂O_Five, Li₂O, Nb₂O_FiveA composition consisting of
It is advantageous to use. The reason for this is K₂O, V ₂O
_FiveActs as a melting agent (flux). As a melting agent
K₂O, V₂O_FiveOf the Li from the melt by using
Since it can prevent the supply of Li,₂O, Nb₂O_FiveComposition of
LiNbO precipitates by changing the ratio₃Li / Nb
The molar ratio can be changed. After all, this Li / Nb model
The lattice constant of the a-axis also changes when the ratio is changed. Sanawa
Li in raw materials₂O, Nb₂O_FiveTo control the composition ratio of
From LiNbO₃Controlling the a-axis lattice constant of a single crystal thin film
And hence LiNbO₃Single crystal thin film and LiTaO₃Single crystal base
The plates can be lattice matched.

Next, in the present invention, as another lattice matching means, lattice matching may be performed by reducing the a-axis lattice constant of the LiTaO ₃ single crystal substrate layer. The method is Li
It is desirable to include Ti in the TaO ₃ single crystal. The reason for this is that Ti atoms or ions have the effect of reducing the a-axis lattice constant of the LiTaO ₃ single crystal substrate layer.

When this Ti atom or ion is contained, its content is preferably 0.2 to 30 mol% with respect to LiTaO ₃ . The reason is that when the Ti content is less than 0.2 mol%, the lattice constant does not become small enough to be lattice-matched with LiNbO _3, and when it exceeds 30 mol%,
Conversely, the lattice constant becomes too small, and in any case LiTaO ₃
This is because the lattice matching between the single crystal substrate and the LiNbO ₃ single crystal thin film cannot be obtained.

The method of manufacturing the waveguide according to the present invention will be described below. Embedded optical waveguide; as shown in Fig. 2 (a), LiTa
After forming the groove 4 in the waveguide forming portion of the O ₃ single crystal plate 1, the thin film of the LiNbO ₃ single crystal film 5 is grown on this substrate while being lattice-matched, and then the unnecessary thin film 5 is removed to form the groove. LiNbO ₃ single crystal is left only in 4 to form the waveguide 2. Ridge type waveguide; LiNbO ₃ on the LiTaO ₃ single crystal substrate 1
A method is used in which the single crystal thin film 5 is grown while being lattice-matched, and then the waveguide forming portion is masked with Ti or the like and dry-etched to remove unnecessary portions to form the waveguide 2. Others: Another method for forming the thin film 5 of LiNbO ₃ single crystal on the LiTaO ₃ single crystal substrate while lattice-matching is lithium oxide-vanadium pentoxide-niobium pentoxide-
Li in a melt composed of sodium oxide-magnesium oxide
There is also a method in which the TaO ₃ single crystal substrate 1 is contacted.

The present invention also provides the above-mentioned optical waveguide in a form in which the waveguide is embedded in a LiTaO ₃ single crystal substrate 1 as shown in FIG. 1 (a), or as shown in FIG. 1 (b). As described above, a Mach-Zehnder type optical interferometer is constructed by a channel type optical waveguide formed by bulging on the substrate 1 as a ridge type, and an optical device for an optical modulator or an optical switch is obtained. suggest. An optical modulator that applies an electric field along the optical axis of the LiNbO ₃ single crystal thin film waveguide that is lattice-matched with the LiTaO ₃ single crystal substrate has a very high modulation efficiency and is stable. It is a long-lived optical device that provides improved amplitude modulation. In the optical device of the present invention, that is, the optical modulator, the channel type optical waveguide 2 is provided on the substrate 1 as shown in FIG. , 2-2, which is a 1-input-1-output type.

The operating principle of such an optical modulator will be described below with reference to FIG. In this optical modulator, a branched LiNbO ₃ single crystal waveguide 2 is formed in the vicinity of the surface (0001) of a LiTaO ₃ single crystal substrate 1, and at least a part of the waveguide is divided into two regions. Have

Such a waveguide is obtained by using the LiTaO ₃ single crystal substrate 1
Of the branched LiNbO ₃ single crystal waveguide on the (0001) plane of
The 001) faces must be formed so that they are laminated. Also, the a-axis lattice constant of LiTaO ₃ single crystal substrate and LiNbO ₃
The lattice constants of the a-axis of the single crystal thin film waveguide are matched.

Then, the LiNbO ₃ single crystal waveguides 2-1 and 2-
Electrodes 3-1 and 3-2 having an appropriate structure are provided on 2 to induce a change in the refractive index through the electro-optic effect of the LiNbO ₃ single crystal waveguides 2-1 and 2-2 to guide the guided light. Is electrically controlled to perform intensity modulation of output light.

As the electrodes 3-1, 3-2,
Planar electrodes are preferred. It suffices if this electrode is formed on at least one of the branched waveguides, and it is desirable to form an electrode on each branched waveguide.

In this structure, the guided light is split into two at the input side branch portion and guided to the optical waveguides 2-1 and 2-2. At least one of the divided optical waveguides 2-1 and 2-2 is provided with an electrode having an appropriate structure, and when a voltage is applied, a phase difference (electric field concentration under the electrode causes Therefore, the propagation constant of the waveguide changes so as to increase or decrease, thereby causing a phase difference Δφ caused between the two waveguides. 2 with a phase difference
When two guided lights are combined / interfered at the branch on the output side, the intensity of the output light changes according to the phase difference Δφ. When an electrode is formed in each of the branched waveguides, the guided light undergoes a phase change of Δφ and −Δφ in each of the branched waveguides, so that the phase difference between the waveguides is 2Δφ, and Compared with the case where the electrode exists only on the waveguide, the phase difference can be doubled and the efficiency is high. When Vπ is a half-wave voltage (voltage at which the phase difference is π / 2), the phase difference generated between the two waveguides by applying a voltage is

[0028]

[Equation 1]

Is given by Here, 1 is the electrode length, d is the electrode spacing, Γ is the applied electric field reduction coefficient, λ is the wavelength of the guided light,
r is the electro-optic constant of the LiNbO ₃ single crystal thin film, and V is the applied voltage. Further, the output light Po when the light with the power of Pi is input is

[0030]

[Equation 2]

Is given by Here, rp is the power distribution ratio of each waveguide. In this way, when the phase difference Δφ is given by changing the voltage applied to the electrodes, the intensity of the output light can be modulated.

When Δφ is π / 2, the two guided lights interfere with each other and disappear. As a result, the guided light can be turned on / off (switched).

Next, a method of manufacturing such an optical modulator will be described. As a manufacturing method of the LiNbO ₃ single crystal waveguide, a groove is formed in the waveguide formation portion of the LiTaO ₃ single crystal substrate 1, the LiNbO ₃ single crystal thin film is formed while being lattice-matched, and then an unnecessary portion is removed to form the groove 4. leaving the LiNbO ₃ single crystal 5 only during either the waveguide 2, or, LiNbO ₃ on LiTaO ₃ single crystal substrate 1
A method is used in which a single crystal film is formed while being lattice-matched, then the waveguide formation portion is masked with Ti or the like, and unnecessary portions are removed by dry etching to form the waveguide 2.

As a method for forming a LiNbO ₃ single crystal thin film while lattice-matching, a LiTaO ₃ single crystal substrate 1 is brought into contact with a melt composed of lithium oxide-vanadium pentoxide-iobium pentoxide-sodium oxide-magnesium oxide. .

As a means for changing the refractive index on the LiNbO ₃ single crystal waveguide 2 thus manufactured, the electrode 7-
1 is set. The electrode 7-2 is preferably formed by depositing a metal film such as aluminum by vapor deposition or sputtering. A buffer layer 6 such as alumina may be provided between the waveguides 2-1 and 2-2 and the electrodes 7-1 and 7-2.

The waveguide of the present invention becomes a single mode waveguide under the following conditions. That is, Na in the LiNbO ₃ waveguide
When contained in the range of 0.1 to 14.3 mol% and Mg 0.8 to 10.8 mol%, the waveguide thickness T (μm) and wavelength λ
(μm), TM mode: 1.9 <(T + 0.7) / λ <5.7 TE mode: 0.29 <(T + 0.04) / λ <1.19 is desirable. In addition, the condition of the single mode of the ridge type waveguide is that the waveguide width W (μm) and the step difference ΔT (μ
m), TM mode: W ≦ (4λ−0.5) (λ ² 10T + 2.0) TE mode: W ≦ (0.04λ ³ + 0.1λ ² ) / ΔT +
It is 2.5 λ. The above range is a condition specific to a LiNbO ₃ waveguide lattice-matched with a LiTaO ₃ substrate using Na and Mg.

[0037]

【Example】

Example 1 (1) Precipitation from the melt composition was possible for a mixture of Na ₂ CO ₃ 22 mol%, Li ₂ CO ₃ 28 mol%, V ₂ O ₅ 40 mol%, and Nb ₂ O ₅ 10 mol%. A mixed raw material obtained by adding 2 mol% of MgO to the theoretical amount of LiNbO ₃ is put in a platinum crucible, and the contents of the platinum crucible are heated to 1100 ° C. in an air atmosphere in an epitaxial growth and growth apparatus. Melted. Then, the obtained melt in the crucible was stirred with a propeller at a rotation speed of 100 rpm for 12 hours. (2) On the other hand, in order to prepare a substrate, the (001) plane of a LiTaO ₃ single crystal having a thickness of 2 mm was optically polished and then chemically etched. Next, the portion where the waveguide 2 shown in FIG. 1 was formed was patterned by photolithography, and a Ti mask was formed by the lift-off method. Further, a groove having a width of 10 μm and a depth of 3.5 μm was formed by Ar plasma etching, and the Ti mask was peeled off. (3) Thereafter, the melt is gradually cooled to 915 ° C. at a cooling rate of 60 ° C. per hour, while the substrate 1 is preheated at 915 ° C. for 30 minutes and then 30 rpm in the slowly cooled melt. It was immersed for 4 minutes while rotating at the speed of. The growth rate of LiNbO ₃ was 1 μm / min. (4) The substrate 1 is pulled up from the solution body, and the rotation speed is 1000 rp.
Rotate at m for 30 seconds to shake off the melt, then 1 ℃ /
The film was gradually cooled to room temperature at a rate of a minute, and a LiNbO ₃ single crystal thin film containing sodium and magnesium having a thickness of about 4 μm was formed on the substrate material. (5) The amounts of sodium and magnesium contained in the obtained LiNbO ₃ single crystal thin film were 3 mol% and 2 respectively.
It was mol%. The lattice constant (a-axis) of the thin film is 5.
The refractive index measured at 156 A and incident light wavelength of 1.15 μm is 2.235.
It was ± 0.001. (6) A wavelength of 1.55μ is added to the optical waveguide manufactured in this way.
When the m and TM mode laser light was guided, the guided mode was single and the propagation loss measured by the prism movement method was 1 dB / cm or less. Moreover, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (7) Further, this optical waveguide was kept at 1000 ° C. for 12 hours in a water vapor atmosphere, and when the TM mode laser light with a wavelength of 1.55 μm was guided again, the waveguide mode was single and the propagation loss was Was less than 1 dB / cm. Moreover, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (8) Next, a silica buffer layer was formed by sputtering directly on the optical waveguide 2 obtained as described above, and subsequently an aluminum electrode was vapor-deposited. When the electro-optic constant of this optical waveguide was measured, it was found to be almost the same as that of the bulk LiNbO ₃ single crystal.

Example 2 (1) Li ₂ CO ₃ 44.3 mol%, V ₂ O ₅ 46.4 mol%, Nb ₂ O ₅
9.3 mol% and Na ₂ CO ₃ to Li ₂ CO ₃ 27.2 mol%
LiNb that can be precipitated from the melt composition for a mixture of
A mixed raw material obtained by adding 5 mol% of MgO to the theoretical amount of O ₃ was placed in a platinum crucible, and the mixture in the platinum crucible was heated to 1050 ° C. in an air atmosphere in an epitaxial growth and growth apparatus.
It melted by heating to. Then, the obtained melt in the crucible was stirred with a propeller at a rotation speed of 100 rpm for 20 hours. (2) On the other hand, a 1 mm thick LiTaO ₃ single crystal (001) surface was optically polished and then chemically etched to prepare a substrate. Next, patterning was performed on the portion where the waveguide 2 shown in FIG. 1 was formed by photolithography, and a Ti mask was formed by the lift-off method. Further, by Ar plasma etching, a groove with a width of 1.5 μm and a depth of 0.8 μm is formed,
The Ti mask was peeled off. (3) Thereafter, the melt is gradually cooled to 938 ° C. at a cooling rate of 60 ° C. per hour, while the substrate 1 is cooled to 938 ° C. at 30 ° C.
It was preheated for 1 minute and then immersed in the slow cooling melt for 2.5 minutes while rotating at a speed of 35 rpm. The growth rate of LiNbO ₃ was 1.6 μm / min. (4) The substrate 1 is pulled up from the solution body, and the rotation speed is 1000 rp.
Rotate at m for 30 seconds to shake off excess melt, then 1
Approximately 4 μm on the substrate material after slowly cooling to room temperature at a rate of ° C / min.
Sodium- and magnesium-containing LiNbO ₃ single-crystal thin films with a thickness of 1 μm were formed. (5) A wavelength of 0.5 is added to the optical waveguide manufactured as described above.
When laser light of 15 μm and TM mode was guided, the guided mode was single and the propagation loss was 1 dB / cm or less. Moreover, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (6) Furthermore, this optical waveguide is exposed to water vapor in an atmosphere of 12
When the TM mode laser beam having a wavelength of 0.515 μm was guided again by keeping the temperature at 1000 ° C. for a time, the guided mode was single and the propagation loss was 1 dB / cm or less. Moreover, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (7) Next, a silica buffer layer was formed by sputtering directly on the optical waveguide 2, and subsequently an aluminum electrode was vapor-deposited. When the electro-optic constant of this optical waveguide was measured, it was found to be almost the same as that of the bulk LiNbO ₃ single crystal.

Example 3 (1) Li ₂ CO ₃ 40.5 mol%, V ₂ O ₅ 50.5 mol%, Nb ₂ O ₅
9.0 mol% and Na ₂ CO ₃ to Li ₂ CO ₃ 23.4 mol%
LiNb that can be precipitated from the melt composition for a mixture of
A mixed raw material obtained by adding 5 mol% of MgO to the theoretical amount of O ₃ was put into a platinum crucible, and the mixture in the platinum crucible was
1050 in an air atmosphere in an epitaxial growth and growth system.
The contents of the crucible were dissolved by heating to ℃. Next, the obtained melt was stirred with a propeller at a rotation speed of 100 rpm for 19 hours. (2) On the other hand, the substrate is made of (00 mm of LiTaO ₃ single crystal with a thickness of 2 mm.
1) The surface was prepared by optical polishing. (3) Thereafter, the melt is gradually cooled to 915 ° C. at a cooling rate of 60 ° C. per hour, while the substrate 1 is preheated at 938 ° C. for 30 minutes, and then at 20 rpm in the slowly cooled melt. It was immersed for 2.5 minutes while rotating. The growth rate of LiNbO ₃ is 1.
It was 6 μm / min. (4) The substrate 1 is pulled up from the solution body, and the rotation speed is 1000 rp.
Rotate at m for 30 seconds to shake off excess melt, then 1
Approximately 4 μm on the substrate material after slowly cooling to room temperature at a rate of
Sodium- and magnesium-containing LiNbO ₃ single-crystal thin films with a thickness of 1 were formed. (5) The amounts of sodium and magnesium contained in the obtained LiNbO ₃ single crystal thin film were 3 mol% and 2 respectively.
It was mol%. The lattice constant (a-axis) of the thin film is 5.
The refractive index measured at 156 A and incident light wavelength of 1.15 μm is 2.235.
It was ± 0.001. (6) When the propagation loss of the slab-type waveguide thus obtained was measured by the prism moving method, the wavelength was 0.83.
It was less than 1 dB / cm in μm and TM mode. (7) The waveguide was patterned to form a Ti mask, and Ar plasma etching was performed to produce a ridge-type channel waveguide. The waveguide shape was 10 μm in width and 1 μm in etching depth. (8) A wavelength of 1.55 is added to the optical waveguide manufactured in this way.
When the laser light of μm and TM mode was guided, the guided mode was single and the propagation loss was the same as that of the slab waveguide. Also, when the change with time of the emitted light was measured,
There was no change for at least 24 hours. (9) Furthermore, when this waveguide was kept at 1000 ° C for 12 hours in a water vapor atmosphere and a TM mode laser beam with a wavelength of 1.55 µm was guided again, the waveguide mode was single and the propagation loss was Was less than 1 dB / cm. Further, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (10) Next, a silica buffer layer was formed directly on the optical waveguide 2 by sputtering, and then an aluminum electrode was deposited. When the electro-optic constant of this optical waveguide was measured, it was found to be almost the same as the value of the bulk LiNbO ₃ single crystal.

Example 4 (1) Li ₂ CO ₃ 39.7 mol%, V ₂ O ₅ 46.0 mol%, Nb ₂ O ₅
14.3 mol% and Na ₂ CO ₃ to Li ₂ CO ₃ 14.5 mol%
LiNb that can be precipitated from the melt composition for a mixture of
A mixed raw material obtained by adding 5 mol% of MgO to the theoretical amount of O ₃ was placed in a platinum crucible, and the mixture in the platinum crucible was heated to 1050 ° C. in an air atmosphere in an epitaxial growth and growth apparatus.
It melted by heating to. Then, the obtained melt in the crucible was stirred with a propeller at a rotation speed of 100 rpm for 21 hours. (2) On the other hand, the substrate is made of (00 mm) LiTaO ₃ single crystal with a thickness of 1 mm.
1) The surface was prepared by optical polishing. (3) Thereafter, the melt is gradually cooled to 940 ° C. at a cooling rate of 60 ° C. per hour, while the substrate 1 is cooled to 940 ° C. at 30 ° C.
After preheating for a minute, it was immersed in the slow cooling melt for 1 minute while rotating at 25 rpm. The growth rate of LiNbO ₃ is 1 μm /
It was a minute. (4) The substrate 1 is pulled up from the solution body, and the rotation speed is 1000 rp.
Rotate at 30 m for 30 seconds, shake off the melt on the excess melt, then slowly cool to room temperature at a rate of 1 ° C / min, and deposit LiNb containing sodium and magnesium with a thickness of about 1 µm on the substrate material.
An O ₃ single crystal thin film was formed. (5) The amounts of sodium and magnesium contained in the obtained LiNbO ₃ single crystal thin film were 3 mol% and 2 respectively.
It was mol%. The lattice constant (a-axis) of the thin film is 5.
The refractive index measured at 156 A and incident light wavelength of 1.15 μm is 2.235.
It was ± 0.001. (6) When the propagation loss of the slab-type waveguide thus obtained was measured by the prism moving method, the wavelength was 0.83.
It was less than 1 dB / cm in μm and TM mode. (7) The waveguide was patterned to form a Ti mask, and Ar plasma etching was performed to produce a ridge-type channel waveguide. The waveguide shape has a width of 1.5 μm and an etching depth of 0.5 μm. (8) A wavelength of 0.515 is added to the optical waveguide manufactured in this way.
When the laser light of μm and TM mode was guided, the guided mode was single and the propagation loss was 1 dB / cm or less, which is the same as the value of the slab waveguide. Further, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (9) Furthermore, when this optical waveguide was held at 1000 ° C for 12 hours in a water vapor atmosphere and the TM mode laser light with a wavelength of 0.515 µm was guided again, the waveguide mode was single and the propagation loss was Was less than 1 dB / cm. Moreover, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (10) Next, a silica buffer layer was formed by sputtering directly on the optical waveguide 2, and then an aluminum electrode was deposited. When the electro-optic constant of this optical waveguide was measured, it was found to be almost the same as that of the bulk LiNbO ₃ single crystal.

Comparative Example 1 (1) Same as the place where a waveguide was formed in Example 1 on the Z plane of a LiNbO ₃ single crystal plate having a dimension of about 5 × 15 mm, a thickness of 0.5 mm, and a C axis as the thickness direction. After Ti was vapor-deposited at a position of about 100 A, patterning was performed and heat treatment was performed at 1000 ° C. for 10 hours to thermally diffuse Ti to form an optical waveguide. (2) The optical waveguide manufactured in this way has a wavelength of 0.515
When the μm and TM mode laser light was guided, the emitted light became unstable in a few seconds, and the waveguide could not be guided.

Comparative Example 2 (1) The same position as the portion where the waveguide was formed in Example 1 on the Z plane of the LiNbO ₃ single crystal plate having dimensions of 5 × 15 mm and a thickness of 1 mm and having the C axis as the thickness direction. After Ti is vapor-deposited to a thickness of about 100 A, patterning is performed and heat treatment is performed at 1000 ° C. for 10 hours.
Ti was thermally diffused to form a waveguide. (2) Wavelength of 1.55μ in the optical waveguide manufactured in this way
When the m and TM mode laser light was guided, the guided mode was single and the propagation loss was 1 dB / cm or less. (3) This waveguide in a water vapor atmosphere for 12 hours at 1000 ° C
When the laser light of TM mode was guided again with the wavelength held at 1.55 μm, no guided light could be confirmed.

Comparative Example 3 (1) First, the substrate was made of a LiTaO ₃ single crystal (00 mm) having a thickness of 2 mm.
1) Prepare the surface by optical polishing, and after chemical etching,
As shown in FIG. 1, a groove was formed in the portion where the optical waveguide 2 was formed. The dimensions of the groove and the like were the same as those in Example 1. (2) On the other hand, Li ₂ CO ₃ 50 mol%, V ₂ Ol ₆ 40 mol%, Nb ₂ O
_A mixed raw material consisting of ₅ 10 mol% was heated to 1000 ° C. to obtain a melt. (3) Thereafter, the melt was gradually cooled to 915 ° C. at a cooling rate of 60 ° C. per hour, while the substrate 1 was preheated at 915 ° C. for 30 minutes, and then the melt was cooled at 30 rpm in the slowly cooled melt. It was immersed for 4 minutes while rotating at a speed. The growth rate of LiNbO ₃ was 1 μm / min. (4) The substrate 1 is pulled up from the solution body, and the rotation speed is 1000 rp.
The melt was shaken off on the melt at m 2 for 30 seconds and then gradually cooled to room temperature at a rate of 1 ° C./min to obtain a LiNbO ₃ single crystal thin film having a thickness of about 4 μm on the substrate material. (5) When the propagation loss of the obtained slab waveguide was measured by the prism moving method, it was found to be 10 dB / cm in the wavelength mode of 0.83 μm and TM mode. (6) Further, the LiNbO ₃ single crystal thin film was subjected to ion beam etching to remove unnecessary portions to form a LiNbO ₃ single crystal waveguide. The waveguide shape was 10 μm in width and 1 μm in etching depth. (7) A wavelength of 1.55 is added to the optical waveguide manufactured in this way.
When the laser light of μm and TM mode was guided, the waveguide mode was single and the propagation loss was 10 dB / cm, which was the same as the value of the slab waveguide. Further, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (8) Next, a silica buffer layer was formed by sputtering directly on the optical waveguide 2, and then an aluminum electrode was deposited. When the electro-optic constant of this optical waveguide was measured, it was found to be 1/10 of the bulk LiNbO ₃ single crystal.

Comparative Example 4 (1) First, the substrate 1 was made of (2) thick LiTaO ₃ single crystal (0
The (01) surface was prepared by optical polishing, chemically etched, and then, as shown in FIG. 1, a groove was formed in the portion where the waveguide 2 was formed. The dimensions of the groove and the like were the same as those in Example 1. (2) On the other hand, Li ₂ CO ₃ 50 mol%, V ₂ O ₅ 40 mol%, Nb ₂ O
_A mixed raw material consisting of ₅ 10% was heated to 1000 ° C. to obtain a melt. (3) Thereafter, the melt was gradually cooled to 915 ° C. at a cooling rate of 60 ° C. per hour, while the substrate 1 was preheated at 915 ° C. for 30 minutes, and then the melt was cooled at 30 rpm in the slowly cooled melt. It was immersed for 4 minutes while rotating at a speed. The growth rate of LiNbO ₃ was 1 μm / min. (4) The substrate 1 is pulled up from the solution body, and the rotation speed is 1000 rp.
The melt was shaken off on the melt at m 2 for 30 seconds and then gradually cooled to room temperature at a rate of 1 ° C./min to obtain a LiNbO ₃ single crystal thin film having a thickness of about 4 μm on the substrate material. (5) When the propagation loss of the obtained slab waveguide was measured by the prism moving method, it was found to be 10 dB / cm in the wavelength mode of 0.83 μm and TM mode. (6) Further, the LiNbO ₃ single crystal thin film was subjected to ion beam etching to remove unnecessary portions to form a LiNbO ₃ single crystal waveguide. The waveguide shape was 10 μm in width and 1 μm in etching depth. (7) A wavelength of 1.55 is added to the optical waveguide manufactured in this way.
When the laser light of μm and TM mode was guided, the waveguide mode was single and the propagation loss was 10 dB / cm, which was the same as the value of the slab waveguide. Moreover, when the change with time of the emitted light was measured, there was no change for at least 24 hours. (8) Further, when this optical waveguide was kept at 1000 ° C. for 12 hours in a water vapor atmosphere, cracks occurred in the substrate.

Comparative Example 5 (1) Ta was deposited to a thickness of about 1 μm on the Z surface of a LiNbO ₃ single crystal substrate having a dimension of about 5 × 15 mm and a thickness of 0.5 mm, and the C axis was the thickness direction, and then patterned. , RIE, width 3μ
m waveguide pattern was formed. This was dipped in H ₄ P ₂ O ₇ and heat-treated at 250 ° C. for 30 minutes to remove Ta, thereby forming a waveguide. (2) A wavelength of 1.55 is added to the optical waveguide manufactured in this way.
When the laser light of μm and TM mode was guided, the guided mode was single and the propagation loss was 3 dB / cm. (3) This waveguide is used for 1000 hours in a water vapor atmosphere for 1000 hours.
When held at ℃ and guided TM mode laser light with a wavelength of 1.55 μm again, guided light could not be confirmed.

Table 1 shows the results of comparative measurement of various characteristics of the waveguides of Examples 1, 2, 3, and 4 and Comparative Examples 1, 2, 3, 4, and 5 described above. As is clear from the results shown in Table 1, the optical waveguide of the present invention can obtain characteristics superior to those of the conventional optical waveguide. That is, in the optical waveguide according to the present invention, since the lattice constants of the LiNbO ₃ single crystal waveguide 2 and the LiTaO ₃ single crystal 1 are matched, there is no distortion in the crystal lattice of the LiNbO ₃ single crystal waveguide 2, and Unlike Ti diffused LiNbO ₃ optical waveguides and diffused optical waveguides such as proton-exchanged optical waveguides, impurities are not diffused, so that it has excellent annealing resistance.

[0047]

[Table 1]

Example 5 (1) Na ₂ CO ₃ 22 mol%, Li ₂ CO ₃ 28 mol%, V ₂ O ₅ 40
Mol%, Nb ₂ O ₅ 10 mol%, MgO 2 mol% to the theoretical amount of LiNbO _{3 which} can be precipitated from the melt composition was added to the platinum crucible, and the mixture was placed in an epitaxial growth and growth apparatus under an air atmosphere. The contents of the crucible were dissolved by heating to 1100 ° C. Further, the melt was stirred with a propeller at a rotation speed of 100 rpm for 12 hours. (2) The (0001) plane of a LiTaO ₃ single crystal having a thickness of 2 mm was optically polished and then chemically etched, and then a groove was formed by ion beam etching in the portion where the waveguide 2 shown in FIG. 1 was formed. The shape of the groove is a rectangle with a width of 7 μm and a depth of 5 μm, and the branch angle is 1.5 °. Melt 60 ℃ per hour
After slowly cooling to 915 ℃ at the cooling rate of
After preheating at 30 ° C. for 30 minutes, it was immersed in the melt for 8 minutes while rotating at 100 rpm. Growth rate of LiNbO ₃ is 1 μm
/ Min. (3) Pulling up the substrate 1 from the melt and rotating at 1000 rpm
The melt was shaken off on the melt for 30 seconds and then gradually cooled to room temperature at a rate of 1 ° C./min to obtain a sodium-magnesium-containing LiNbO ₃ single crystal thin film having a thickness of about 8 μm on the substrate material. (4) The amounts of sodium and magnesium contained in the obtained LiNbO ₃ single crystal thin film were 3 mol% and 2%, respectively.
It was mol%. The lattice constant (a-axis) of the thin film is 5.1
56, the refractive index measured at an incident light wavelength of 1.15 μm is 2.235 ± 0.
It was 001. (5) Further, unnecessary portions were removed by polishing the LiNbO ₃ single crystal thin film to form a buried LiNbO ₃ single crystal waveguide having a depth of 3 μm. (6) After forming the alumina buffer layer 6 directly on the waveguide 2 by sputtering, the aluminum electrode 7 was formed by vapor deposition. The electrodes had a planar electrode structure and the length covering the waveguide was 6 mm, and the electrode interval was 20 μm. (7) When a laser beam with a wavelength of 1550 nm is applied to the branching interference type optical modulator formed in this way at 10 mW, the modulation band is
It was 3.0 GHz and the half-wave voltage was 8V. Also,
The characteristics were not changed even after 5 hours with the laser light being guided.

Example 6 (1) The (0001) plane of a LiTaO ₃ single crystal having a thickness of 2 mm was optically polished and then chemically etched. It was immersed in the melt for 5 minutes in the same manner as in Example 1. The growth rate of LiNbO ₃ is
It was 1 μm / min. (2) Pulling the substrate 1 out of the melt and rotating at 1000 rpm
After the melt was shaken off for 30 seconds on the melt, the melt was gradually cooled to room temperature at a rate of 1 ° C./min to obtain a sodium-magnesium-containing LiNbO ₃ single crystal thin film having a thickness of about 5 μm on the substrate material. (3) Further, the film thickness of the LiNbO ₃ single crystal thin film was adjusted by polishing to form a LiNbO ₃ single crystal thin film having a thickness of 2.8 μm. Then, a ridge type LiNbO ₃ single crystal waveguide having a width of 5 μm and a step of 1.8 μm was formed by the technique of photolithography and ion beam etching. (4) After forming the alumina buffer layer 6 on the waveguide 2 by sputtering, the aluminum electrode 7 was formed by vapor deposition. The electrodes had a planar electrode structure with a length of 5 mm covering the waveguide and an electrode interval of 10 μm. (5) When a laser beam with a wavelength of 633 nm is applied to the branching interference type optical modulator formed in this way at 15 mW, the modulation band is
It was 3.5 GHz and the half-wave voltage was 2V. Also,
No change was observed in the characteristics even after the lapse of 3 hours with the laser light guided.

Example 7 (1) A branching interference type optical modulator was manufactured in the same manner as in Example 2. The difference from the second embodiment is the size of the waveguide and the size of the electrode. Waveguide size is width 6μm, thickness 2.5μ
It is a ridge type LiNbO ₃ single crystal waveguide with m and a step of 2.0 μm. Regarding the size of the electrodes, the length covering the waveguide was 3 mm, and the electrode interval was 15 μm. (5) 10 mW of laser light having a wavelength of 1300 nm was made incident on the branch interference type optical modulator thus formed. When the applied voltage was adjusted and the phase difference of the light passing through the two waveguides was set to π / 2, an extinction ratio of 15 dB was observed.

Comparative Example 6 (1) Same as the place where the waveguide was formed in Example 1 on the Z plane of a LiNbO ₃ single crystal plate having a dimension of 5 × 15 mm and a thickness of 0.5 mm and having the C axis as the thickness direction. After Ti is vapor-deposited at a position of about 100 A, patterning is performed and heat treatment is performed at 950 ° C. for several hours.
A waveguide was formed by thermally diffusing Ti. (2) Next, alumina was deposited as a buffer layer directly on the waveguide, and further an aluminum electrode was formed by vapor deposition. (3) When 10 mW of laser light with a wavelength of 633 nm was made to enter the branch interference type optical modulator thus formed, it was found that optical damage occurred and there was a problem in stability.

Comparative Example 7 (1) A (0001) plane of a LiTaO ₃ single crystal having a thickness of 2 mm was optically polished and then chemically etched, and then a groove was formed in the waveguide 2 shown in FIG. 1 by ion beam etching. Formed by. The shape of the groove is a rectangle with a width of 7 μm and a depth of 5 μm, and the branch angle is 1.5 °. (2) Then, Li ₂ CO ₃ 50 mol%, V ₂ O ₅ 40 mol%, Nb
_A raw material consisting of ₂ O ₅ 10 mol% is heated to 1000 ° C to form a melt, and the melt is cooled at a cooling rate of 60 ° C per hour.
The substrate 1 was gradually cooled to 915 ° C., and the substrate 1 was preheated at 915 ° C. for 30 minutes and then immersed in the melt for 8 minutes while rotating at 100 rpm. The growth rate of LiNbO ₃ was 1 μm / min. (3) Pulling up the substrate 1 from the melt and rotating at 1000 rpm
The melt was shaken off on the melt for 30 seconds and then slowly cooled to room temperature at a rate of 1 ° C / min.
An NbO ₃ single crystal thin film was obtained. (4) Further, the LiNbO ₃ single crystal thin film is removed by ion beam etching to remove unnecessary portions, and the LiNbO ₃ single crystal waveguide 2
Formed. (5) After forming the alumina buffer layer 6 directly above the waveguide 2 by sputtering, the aluminum electrode 7 was formed by vapor deposition. (6) When 5 mW of laser light having a wavelength of 1550 nm is incident on the branch interference type optical modulator formed in this way, the modulation band is
It was 2.5 GHz and the half-wave voltage was 8V. Also,
The extinction ratio was 12 dB. Further, the applied voltage was periodically changed to repeatedly turn on and off the laser beam, and the extinction ratio after 2 hours had decreased to 8 dB.

As can be seen from the above examples, the optical modulator of the present invention has high modulation efficiency and stable amplitude modulation. That is, since the lattice constants of the LiNbO ₃ single crystal waveguide 2 and the LiTaO ₃ single crystal 1 are matched, there is no distortion in the crystal lattice of the LiNbO ₃ single crystal waveguide 2 and even if the refractive index change due to the electric field is repeated. The refractive index of the LiNbO ₃ single crystal waveguide 2 does not change greatly from the initial value.

[0054]

As described above, since the optical waveguide of the present invention does not decrease the electro-optical constant and has high optical damage resistance, it is an excellent waveguide used as an optical device such as an optical modulator or an optical switch. In addition, an optical waveguide having excellent annealing resistance can be obtained. Further, the optical device according to the present invention is useful as a modulator or an optical switch that can achieve stable phase modulation and optical power control even with high-power light, and since it becomes an optical device with a longer life, it is industrially available. The effect of contributing to is extremely large.

[Brief description of drawings]

FIG. 1 is an optical modulator of the present invention.

FIG. 2 is a method for manufacturing a Mach-Zehnder type optical waveguide of the present invention.

FIG. 3 is a method for manufacturing an optical modulator of the present invention.

[Explanation of symbols]

1 LiTaO ₃ Single Crystal Substrate 2 LiNbO ₃ Single Crystal Waveguide 3 Incident Light 4 Groove 5 LiNbO ₃ Single Crystal Thin Film 6 Buffer Layer 7 Electrode

Claims (2)

[Claims] A LiNbO ₃ single crystal thin film waveguide is formed on a surface of a LiTaO ₃ single crystal substrate, and a branch portion is provided in a part of the thin film waveguide, and the LiNbO ₃ single crystal thin film waveguide is formed. An optical waveguide in which the lattice constants of the LiTaO ₃ single crystal substrate and the LiTaO ₃ single crystal substrate are matched. 2. An LiNbO ₃ single crystal thin film waveguide is formed on the surface of a LiTaO ₃ single crystal substrate, and a branch portion is provided in a part of this thin film waveguide, and the LiNbO ₃ single crystal thin film waveguide is formed. And the LiTaO ₃ single crystal substrate are matched in lattice constant, and a waveguide type optical device is provided in which at least one waveguide of the branched portion is provided with an electrode for changing the refractive index of the waveguide. .