JP2003140214A - Method of manufacturing thin film substrate for wavelength conversion element and method of manufacturing wavelength conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a thin film substrate (for example, a lithium niobate thin film substrate having a uniform composition and film thickness over the entire area of a 3-inch wafer) suitable for producing a wavelength conversion element. Further, an optical waveguide having a domain-inverted structure is manufactured using the thin film substrate, thereby providing a high-performance wavelength conversion element. A first step of bonding a first substrate having a second-order nonlinear effect and having a structure in which a nonlinear constant is periodically modulated, a first step of bonding the second substrate, and a thickness of the first substrate. A second step of reducing the thickness to a predetermined thickness to form an optical waveguide, the method for manufacturing a thin film substrate for a wavelength conversion element, wherein the first substrate and the second substrate in the first step The substrate is directly bonded by diffusion bonding by heat treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical drive type optical circuit device in an optical communication system using wavelength division multiplexing or time division multiplexing, and more specifically, to signal light using a difference frequency generation effect generated in a nonlinear optical medium. The present invention relates to a wavelength conversion element and a thin film substrate for a wavelength conversion element for converting the wavelength of the above into another wavelength.

[0002]

2. Description of the Related Art In recent years, in order to increase the communication capacity of an optical communication system, a wavelength division multiplexing (WDM) communication system for multiplexing and transmitting a plurality of lights having different wavelengths has been actively introduced. In such a WDM communication system, in order to effectively use the limited number of wavelengths, it is required to put a wavelength conversion device that converts a signal wavelength into an arbitrary signal wavelength into practical use.

Conventionally, as a wavelength conversion element for converting the wavelength of light, one using a semiconductor optical amplifier, one using four-wave mixing, and the like are known. However, these wavelength conversion elements have not been able to satisfy the conditions required for optical communication systems, such as high efficiency, high speed, wide band, low noise, and polarization independence.

On the other hand, there is known a wavelength conversion element utilizing difference frequency generation by quasi phase matching which is a kind of second-order nonlinear effect. FIG. 8 is a schematic diagram showing the configuration of a quasi phase matching type wavelength conversion element of this type. As shown in the figure, the signal light A having a relatively small light intensity and the control light B having a relatively large light intensity are combined by the combiner 1 to form the nonlinear optical waveguide 2 having the polarization inversion structure. Is incident on. The signal light A is converted into the difference frequency light C having another wavelength in the optical waveguide 2, and is emitted from the waveguide 2 together with the control light B. The emitted difference frequency light C and control light B are separated by the demultiplexer 3. For example, when the control light B has a wavelength λ1 = 0.77 μm, the signal light A having a wavelength λ2 = 1.55 μm is converted into a wavelength λ3.
= 1.53 μm of the difference frequency light C can be converted.

In the conventional method of producing a wavelength conversion element utilizing such quasi phase matching, a proton-exchanged waveguide is produced after producing a periodically poled structure on a nonlinear optical crystal substrate such as lithium niobate. Thus, the wavelength conversion element was manufactured.

On the other hand, in order to improve the optical confinement in the waveguide and to realize highly efficient wavelength conversion utilizing the bulk or near-nonlinear effect, a wavelength conversion element having a ridge type optical waveguide structure. Is proposed.

A conventional method for producing a wavelength conversion element having a ridge-type optical waveguide is that a single crystal film of lithium niobate or the like grown by a liquid phase epitaxial method is provided with an etching mask by ordinary photolithography. The ridge-type optical waveguide was manufactured by removing the single crystal film except the mask portion in the subsequent dry etching process.

On the other hand, as another method for producing a ridge type optical waveguide, a periodically poled structure is formed on a Mg-added lithium niobate substrate and then bonded to a separately prepared lithium niobate substrate using an adhesive. , A Mg-added lithium niobate substrate is thinned by surface grinding to form a ridge-type waveguide by ultra-precision grinding using a dicing saw (Laser Research: Volume 28). No. 9, p601-603).

[0009]

However, since the proton exchange waveguide has a diffusion type refractive index distribution and the waveguide mode is asymmetric, and the surface of the substrate is altered by the proton exchange treatment, the waveguide portion is affected. There was a problem that the non-linear optical effect of was deteriorated.

Further, it is difficult to form a single crystal film by the liquid phase epitaxial method, and it is difficult to form a single crystal film having a uniform composition or film thickness over the entire area of a 3-inch wafer, for example.

Further, in the method of bonding the single crystal film and the substrate using an adhesive, the single crystal film may be cracked when the temperature changes because the adhesive and the single crystal film have different thermal expansion coefficients. In addition to the problem, the SHG light generated in the waveguide deteriorates the adhesive, which increases the waveguide loss during operation and deteriorates the wavelength conversion efficiency. Further, the non-uniformity of the adhesive layer makes the film thickness of the single crystal film non-uniform, which causes a problem that the phase matching wavelength of the wavelength conversion element shifts.

An object of the present invention is to provide a method of manufacturing a thin film substrate suitable for manufacturing a wavelength conversion device which solves the above problems, and has a uniform composition and film thickness over the entire area of a 3-inch wafer, for example. It is to provide such a thin film substrate of lithium niobate. Another object of the present invention is to provide an optical waveguide having a domain-inverted structure using the above-mentioned thin film substrate, and to provide a high-performance wavelength conversion element.

[0013]

According to a first aspect of the invention for solving the above-mentioned problems, a first substrate having a structure having a quadratic nonlinear effect and a nonlinear constant being periodically modulated, is provided. , A second step of laminating a second substrate, and a second step of reducing the thickness of the first substrate to a predetermined thickness for forming an optical waveguide A method of manufacturing a thin film substrate for a device, characterized in that in the first step, the first substrate and the second substrate are directly bonded by diffusion bonding by heat treatment.

The invention described in claim 2 is the same as claim 1
In the invention described in (3), the heat treatment in the first step is performed at a temperature equal to or lower than the Curie temperature of the first substrate.

The invention according to claim 3 is the same as claim 1
Alternatively, in the invention described in 2, the thickness of the first substrate is set to 20 μm or less in the second step.

The invention according to claim 4 is the same as claim 1
In the invention according to any one of claims 1 to 3, the first group
The plate is LiNbO₃, KNbO₃, LiTaO₃, Li
Nb _(x)Ta_(1-x)O₃(0 ≦ x ≦ 1) or KTiOP
O_FourOr, if they are made of Mg, Zn, Sc, In
Containing at least one selected from the group
It is characterized by

The invention described in claim 5 is the same as claim 1.
The invention according to any one of claims 1 to 4, characterized in that the refractive index of the second substrate is smaller than the refractive index of the first substrate.

The invention described in claim 6 is the same as claim 5
In the invention described in (3), the second substrate is a quartz substrate or a glass substrate.

The invention described in claim 7 is the same as claim 1.
The invention according to any one of claims 1 to 4, wherein the surface layer of the second substrate joined to the first substrate has a refractive index smaller than that of the first substrate. .

The invention according to claim 8 is the same as claim 7
In the invention described in (3), the surface layer of the second substrate is a low melting point glass film or a low melting point glass substrate.

The invention described in claim 9 is the same as claim 1.
In the invention according to any one of claims 1 to 4, the refractive index of the surface layer of the first substrate joined to the second substrate is greater than the refractive index of the main body layer of the first substrate forming the optical waveguide. Is also small.

The invention according to claim 10 is characterized in that, in the invention according to claim 9, the surface layer of the first substrate is a low melting point glass film or a low melting point glass substrate.

The invention according to claim 11 is the invention according to any one of claims 1 to 4, wherein the surface layer bonded to the second substrate of the first substrate has a refractive index of The refractive index of the surface layer of the second substrate, which is smaller than the refractive index of the main body of the first substrate forming the optical waveguide, and which is joined to the first substrate, is also the main body of the first substrate. It is characterized by being smaller than the refractive index of the layer.

The invention according to claim 12 is the invention according to claim 11, wherein the surface layer of the first substrate is a low-melting glass film or a low-melting glass substrate, The surface layer is also a low melting point glass film or a low melting point glass substrate.

The invention according to claim 13 is the invention according to any one of claims 1 to 12, wherein the coefficient of thermal expansion of the second substrate is the coefficient of thermal expansion of the first substrate. It is characterized by almost matching.

A method for manufacturing a wavelength conversion element according to a fourteenth aspect of the present invention is the method for manufacturing a thin film substrate for a wavelength conversion element according to any one of the first to thirteenth aspects. A thin film substrate is produced, and an optical waveguide is produced on the first substrate of the wavelength conversion element thin film substrate in a third step subsequent thereto.

[Operation] In order to improve the efficiency of the wavelength conversion element utilizing the quasi phase matching, the conversion efficiency is in principle proportional to the square of the interaction length (the length of the waveguide having the periodically poled structure). Therefore, increasing the length of the device, that is, increasing the area of the nonlinear optical crystal substrate used for manufacturing the device, and further improving the overlap of the optical fields of the signal light and the control light in the optical waveguide. This is very important. At this time, it is desirable that the excitation light and the signal light incident on the optical waveguide excite the fundamental mode of the optical waveguide, and in order to obtain a high power density in the optical waveguide, It is desirable that the thickness of the nonlinear optical crystal film is approximately 20 μm or less. When the thickness of the optical waveguide layer is 20 μm or more, it is difficult to improve the overlap of the optical electric fields because the signal light and the excitation light are in a multimode state.

The present inventors diligently studied a method of manufacturing a thin film substrate made of a non-linear optical crystal capable of producing such a long wavelength conversion element and having a film thickness of 20 μm or less. As a result, the coefficient of thermal expansion is approximately the same as that of a substrate made of an optical crystal having a nonlinear effect,
After bonding non-linear optical crystals of the same type, optical crystals of different types or glass substrates, etc. by direct bonding by diffusion,
The inventors have found a method for producing a nonlinear single crystal thin film substrate suitable for producing an optical waveguide by adjusting the nonlinear optical crystal substrate so as to have a film thickness of 20 μm or less by a method such as grinding, polishing or etching.

In the present invention, the non-linear optical crystal substrate (first substrate) on which the periodically poled structure is formed is such that microparticles are not present as much as possible on the second substrate prepared separately in the first step. After being directly laminated in a clean atmosphere, they are heat-treated in an electric furnace to be diffusion-bonded. Here, the temperature for heat-treating the substrate is preferably equal to or lower than the Curie temperature of the first substrate.
It is not preferable to perform the heat treatment at a temperature equal to or higher than the Curie temperature because the periodically poled structure may be disturbed. In particular, when LiTaO ₃ is used as the nonlinear optical crystal substrate, the Curie temperature is 650 ° C., so heat treatment at a temperature higher than 650 ° C. is not preferable because it causes disorder of the domain-inverted structure.

The bonded substrates can be used as a thin film substrate for a wavelength conversion element by polishing, grinding or etching until the thickness of the nonlinear optical crystal substrate becomes 20 μm or less in the second step.

The present invention differs from the conventional method in that an adhesive is not used for joining the substrates and the substrates are directly joined.

When a wavelength conversion element is subsequently manufactured using the thin film substrate of the present invention, in the subsequent third step, a ridge type optical waveguide is manufactured by ultraprecision grinding using a dicing saw. Alternatively, a ridge type optical waveguide can be produced by dry etching or wet etching.

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) FIG. 1 is a flow chart showing a process for producing a thin film substrate for a wavelength conversion element according to a first embodiment of the present invention.

As shown in FIG. 1, in the present Example 1, Z-cut Zn was added as a first substrate 11 in which a periodically poled structure was prepared in advance so that the phase matching condition was satisfied in the 1.5 μm band. Using LiNbO ₃ substrate,
A Z-cut Mg-added LiNbO ₃ substrate was used as the second substrate 12 to prepare a thin film substrate for wavelength conversion element. Both the first substrate 11 and the second substrate 12 are made by adding an additive to LiNbO ₃ and have substantially the same thermal expansion coefficient. In addition, the refractive index of the second substrate 12 is smaller than that of the first substrate 11 by changing the kind of the additive. The first and second substrates 11,
Reference numeral 12 is a 3-inch wafer whose both surfaces are optically polished, and has a substrate thickness of 300 μm.

In the first step, the surfaces of the prepared first and second substrates 11 and 12 are made hydrophilic by ordinary acid cleaning or alkali cleaning, and then these two substrates 11 and 12 are made hydrophilic.
12 was superposed in a clean atmosphere in which microparticles were not present as much as possible. Then, the superposed first and second substrates 11 and 12 are put into an electric furnace and heated at 400 ° C. for 3 hours.
Diffusion bonding was performed by heat treatment for a time. The bonded substrates were void-free, with no microparticles or the like sandwiched between the bonded surfaces, and no cracks or the like were generated even when returned to room temperature.

Next, in the second step, a polishing apparatus whose flatness of the polishing platen is controlled is used to perform polishing until the thickness of the first substrate 11 of the bonded substrates reaches 20 μm. did. After the polishing process, a polishing process could be performed to obtain a mirror-polished surface. When the parallelism of the substrate (difference between the maximum height and the minimum height) was measured using an optical parallelism measuring machine, submicron parallelism was obtained over almost the entire area except for the circumference of the 3-inch wafer, and The thin film substrate 13 suitable for producing the conversion element could be produced. Since this thin film substrate 13 was produced by directly bonding the first substrate 11 and the second substrate 12 by diffusion bonding by heat treatment without using an adhesive, a uniform composition over the entire area of the 3-inch wafer,
It had a film thickness.

As the first substrate 11, X-cut Zn
Using the added LiNbO ₃ substrate, X is used as the second substrate 12.
Even when the cut Mg-added LiNbO ₃ substrate was used, the thin film substrate 13 for a wavelength conversion element could be manufactured by the same method as in Example 1.

Next, in the third step, the manufactured thin film substrate 13 was used, and a wavelength conversion element was manufactured using a dry etching process as a means for manufacturing an optical waveguide.
That is, after a waveguide pattern is formed on the surface of the thin film substrate 13 (first substrate 11) by a normal photolithography process, the substrate is set in a dry etching device and CF ₄ gas is used as an etching gas to form the thin film substrate 13.
A ridge type optical waveguide was produced by etching the surface of the (first substrate 11). FIG. 2 is a view showing a cross section of the substrate after etching. As shown in Fig. 2, the height is 8μ
m, a ridge type optical waveguide 14 having a waveguide width of about 8 μm,
The thin film substrate 13 (first substrate 11) could be produced. As shown in the drawing, the optical waveguide 14 has a mesa shape because the etching selectivity between the mask and the film is not large in the dry etching process.

As shown in FIG. 3, a plurality of optical waveguides 14 were produced in parallel with the thin film substrate 13 which was a 3-inch wafer.
Then, the substrate 13 was cut into strips for each of the optical waveguides 14, and both end surfaces 14a of the optical waveguides 14 were optically polished to fabricate a wavelength conversion element 15 having a length of 60 mm. Wavelength of 0.77 μm in the manufactured wavelength conversion element 15
When the control light and the signal light with a wavelength of 1.55 μm were incident, wavelength-converted light with a wavelength of 1.53 μm was obtained, and wavelength conversion could be realized with high efficiency.

(Embodiment 2) FIG. 4 is a flow chart showing the steps for producing a thin film substrate for a wavelength conversion element according to Embodiment 2 of the present invention.

As shown in FIG. 4, in the second embodiment, a LiNbO ₃ substrate having a periodically poled structure is prepared as the first substrate 21, and a quartz substrate is used as the second substrate 22. Thus, a thin film substrate for wavelength conversion element was produced. Thermal expansion coefficient of the plane perpendicular to the Z axis of the crystal is 13.6 × 10 ^-6 / K, close to the thermal expansion coefficient of the LiNbO _3, also the refractive index of the LiNbO ₃ is 2.1 On the other hand, since the refractive index of quartz is as small as 1.53, it is suitable as an example of the embodiment of the present invention.

In the second embodiment as well, by the same method as in the first embodiment, the thin film substrate for the wavelength conversion element similar to that in the first embodiment can be manufactured.
That is, in the first step, the prepared first and second substrates 2
The surfaces of the substrates 1 and 22 were made hydrophilic by ordinary acid cleaning or alkali cleaning, and then these two substrates 21 and 22 were superposed in a clean atmosphere in which microparticles were not present as much as possible. Then, the stacked first and second substrates 11 and 12 were placed in an electric furnace and heat-treated at 400 ° C. for 3 hours to perform diffusion bonding. The bonded substrates were void-free, with no microparticles or the like sandwiched between the bonded surfaces, and no cracks or the like were generated even when returned to room temperature.

Next, in the second step, a polishing apparatus in which the flatness of the polishing platen is controlled is used until the first substrate 21 of the bonded substrates has a thickness of 20 μm. did. After the polishing process, a polishing process could be performed to obtain a mirror-polished surface. When the parallelism of the substrate was measured by using an optical parallelism measuring instrument, a parallelism of submicron was obtained almost all over the periphery of the 3-inch wafer, and the thin film substrate 23 suitable for manufacturing the wavelength conversion element was obtained. Could be made. This thin film substrate 23
Is a first substrate 21 and a second substrate 22 without using an adhesive.
And 3 were directly bonded by diffusion bonding by heat treatment, and thus had a uniform composition and film thickness over the entire area of the 3-inch wafer.

In addition to the Zn-added LiNbO ₃ as the first substrate 21, Mg-added LiNbO ₃ and Sc-added L
iNbO ₃ , In-added LiNbO ₃ , LiTaO ₃ , L
iNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KNb
Even when O ₃ , KTiOPO _4, or the like was used, a similar thin film substrate 23 for wavelength conversion element could be produced.

Also, when a glass substrate whose coefficient of thermal expansion is close to that of the first substrate and whose refractive index is smaller than that of the first substrate is used as the second substrate, the same wavelength is obtained. A thin film substrate for a conversion element could be produced. Here, as the material of the glass substrate, glass materials such as multi-component quartz glass, phosphate glass, fluoride glass, and tellurite glass can be used. It is possible for a person skilled in the art of manufacturing glass materials to appropriately prepare a glass substrate having a desired coefficient of thermal expansion or refractive index by appropriately adjusting the composition of the glass material.

Next, a wavelength conversion element having a ridge type optical waveguide was manufactured by the same method as in Example 1 using the manufactured thin film substrate for wavelength conversion element. When the control light having a wavelength of 0.77 μm and the signal light having a wavelength of 1.55 μm were incident on the manufactured wavelength conversion element, wavelength converted light having a wavelength of 1.53 μm was obtained, which was higher than that of the wavelength conversion element manufactured in Example 1. We were able to achieve wavelength conversion with high efficiency. This is because in Example 2, by using a quartz substrate or a glass substrate as the first substrate, it is possible to increase the relative refractive index difference between the first substrate and the second substrate. This is because the confinement of the optical electric field in the light can be made stronger than that in the wavelength conversion element manufactured in the first embodiment.

(Embodiment 3) FIG. 5 is a flow chart showing the steps for producing a thin film substrate for a wavelength conversion element according to Embodiment 3 of the present invention.

As shown in FIG. 5, in Example 3, a Mg-doped LiNbO ₃ substrate having a domain-inverted structure prepared in advance was prepared as the first substrate 31, and the second substrate 32 had a substrate thickness of A thin film substrate for a wavelength conversion element was produced using a composite substrate in which a low melting point glass film 34 (surface layer) having a thickness of 50 μm was formed on a LiNbO ₃ substrate 32 (main body layer) having a thickness of 300 μm.

The low melting point glass film 34 was manufactured by using a glass paste. That is, a transparent low-melting-point glass film 34 was prepared by baking a LiNbO ₃ substrate 33 coated with a glass paste so as to have a uniform film thickness in an electric furnace.

Here, SiO is used as the low melting point glass.₂-R
₂OB₂O₃System, SiO₂-R₂OB₂O-PbO
System, SiO₂-B₂O-PbO system, SiO₂-PbO-
TeO₂System, B₂O₃-PbO type, SiO₂-B₂O₃
-ZnO-PbO system, B₂O ₃-ZnO-PbO system, B
₂O₃-RO system, TeO₂-R₂O type, TeO₂-R ₂
O-La₂O₃System, TeO₂-ZnO system (R is monovalent or
Can be a glass material such as a divalent element)
However, this is not the case. In addition, a set of these glass materials
The desired coefficient of thermal expansion and
It is possible to obtain a low-melting-point glass film having a certain rate,
Glass paste is used to adjust the composition of such glass materials.
It can be appropriately done by the manufacturer concerned.

The low melting point glass used for the second substrate 32 of the third embodiment has a coefficient of thermal expansion substantially equal to that of LiNbO ₃ and has a refractive index smaller than that of LiNbO _3. Since the composition is adjusted, it is suitable as an example of the embodiment of the present invention.

In the third embodiment as well, by the same method as in the first embodiment, the same thin film substrate for wavelength conversion element as in the first embodiment can be manufactured.
That is, in the first step, the prepared first and second substrates 3
After the surfaces of the substrates 1, 32 are made hydrophilic by ordinary acid cleaning or alkaline cleaning, these two substrates 31, 32 are
Were superposed in a clean atmosphere in which microparticles were not present as much as possible so that the surface (lower surface) of the first substrate 31 overlaps the surface (upper surface) of the low melting point glass film 34 of the second substrate 32. Then, the stacked first and second substrates 31 and 32 were placed in an electric furnace and heat-treated at 400 ° C. for 3 hours to perform diffusion bonding. The bonded substrates were void-free, with no microparticles or the like sandwiched between the bonded surfaces, and no cracks or the like were generated even when returned to room temperature.

Next, in the second step, a polishing apparatus in which the flatness of the polishing platen is controlled is used to perform polishing until the thickness of the first substrate 31 of the bonded substrates becomes 20 μm. did. After the polishing process, a polishing process could be performed to obtain a mirror-polished surface. When the parallelism of the substrate was measured using an optical parallelism measuring instrument, a parallelism of submicron was obtained over almost the entire area except for the periphery of the 3-inch wafer, and the thin film substrate 35 suitable for manufacturing the wavelength conversion element was obtained. Could be made. This thin film substrate 35
Is a first substrate 31 and a second substrate 32 without using an adhesive.
And 3 were directly bonded by diffusion bonding by heat treatment, and thus had a uniform composition and film thickness over the entire area of the 3-inch wafer.

Besides, LiNbO _{3 is used} as the first substrate.
In addition, Zn-added LiNbO ₃ , Sc-added LiNbO ₃ ,
In-added LiNbO ₃ , LiTaO ₃ , LiNb _(x) T
a _{(1 -x)} O ₃ (0 ≦ x ≦ 1), KNbO ₃ , KTiOP
Even when O ₄ or the like was used, a similar thin film substrate for wavelength conversion element could be produced.

Further, a low melting point glass formed on the second substrate
When vapor deposition or sputtering is used as the method for forming the film
The same second substrate 32 can be manufactured in
It was We also prepared a substrate made of low-melting glass,
Melting point glass substrate and LiNbO ₃Overlaid with the substrate
Later, by bonding by diffusion bonding by heat treatment
Therefore, the second substrate could be prepared.

Next, a wavelength conversion element having a ridge type optical waveguide was manufactured by the same method as in Example 1 using the manufactured thin film substrate for wavelength conversion element. When the control light having a wavelength of 0.77 μm and the signal light having a wavelength of 1.55 μm were incident on the produced wavelength conversion element, wavelength converted light having a wavelength of 1.53 μm was obtained, and the wavelength conversion produced in Example 1 was performed. We were able to realize wavelength conversion with higher efficiency than the device. This is the third embodiment.
In this case, since the relative refractive index difference between the first substrate 31 and the second substrate 32 can be increased, that is, by forming the low melting point glass film 34 on the second substrate 32, the low melting point glass film Since the difference between the refractive index of 34 and the refractive index of the first substrate 31 can be increased, the confinement of the optical electric field in the optical waveguide can be made stronger than that of the wavelength conversion element manufactured in Example 1. This is because.

(Embodiment 4) FIG. 6 is a flow chart showing the steps for producing the thin film substrate for a wavelength conversion element of this embodiment.

As shown in FIG. 6, in Example 4, as the first substrate 41, a low-thickness 50 μm film was formed under the Mg-added LiNbO ₃ substrate 43 (main body layer) in which the domain-inverted structure was prepared in advance. A composite substrate having a melting point glass film 44 (surface layer) was prepared, and a LiNbO ₃ substrate having a thickness of 300 μm was used as the second substrate 42 to prepare a thin film substrate for wavelength conversion element.

Fabrication on the first substrate 41 in Example 4
The low melting point glass film 44 thus formed has a periodic polarization inversion structure.
Nonlinear optical crystal substrate (Mg-doped LiNbO₃Board 4
Non-linear light with a refractive index smaller than that of 3)
Scientific crystal substrate (Mg added LiNbO₃Thermal expansion of substrate 43)
It is necessary to have a coefficient of thermal expansion close to the coefficient. Low
The refractive index of the melting point glass film 44 is a nonlinear optical crystal substrate (Mg
Added LiNbO₃When it is larger than the refractive index of the substrate 43)
Is a nonlinear optical crystal substrate (Mg-doped LiNbO ₃Board 4
The signal light and control light incident on 3) satisfy the condition of total reflection.
As a result, it is possible to dissipate in the low melting point glass film 44.
The ridge type optical waveguide cannot be formed. Well
In addition, the coefficient of thermal expansion of the low melting point glass film 44 is a nonlinear optical crystal.
Substrate (Mg added LiNbO₃The coefficient of thermal expansion of the substrate 43)
When the difference is large, warpage occurs on the first substrate 41.
In addition to being a cause, the first substrate 41 is not affected by temperature changes.
It is not preferable because it may cause a crack.

The second substrate 42 prepared in this embodiment is also used.
The coefficient of thermal expansion of is preferably close to the coefficient of thermal expansion of the low melting point glass film 44 formed on the first substrate 41. This is because warpage or cracks do not occur when the first substrate 41 and the second substrate 42 are bonded by heat treatment. On the other hand, the refractive index of the second substrate 42 is 5 when the film thickness of the low melting point glass film 44 formed on the first substrate 41 is 5
When it is less than or equal to μm, it preferably has a value smaller than the refractive index of the first substrate 41. This is a nonlinear optical crystal substrate (Mg-doped LiNbO ₃ having a periodically inverted polarization structure).
Since the skirts of the optical fields of the signal light and the control light incident on the substrate 43) extend beyond the interface between the low melting point glass film 44 and about 5 μm into the low melting point glass film 44, the second substrate 42
This is because if the refractive index of the surface layer is larger than that of the low melting point glass film 44, the photo-electric field will dissipate into the second substrate beyond the low melting point glass film. On the other hand, when the thickness of the low melting point glass film 44 is 5 μm or more, there is no limitation on the refractive index of the second substrate 42.

The low melting point glass film 44 formed on the first substrate 41 was formed by the same method as in Example 3. That is,
A transparent low-melting-point glass film 4 having a thickness of 50 μm was obtained by coating a Mg-added LiNbO ₃ substrate 43 with a glass paste applied so as to have a uniform film thickness, and firing it in an electric furnace.
4 was produced.

The glass composition of the low melting point glass used in Example 4 was adjusted so that its thermal expansion coefficient was approximately equal to that of LiNbO ₃ and that its refractive index was smaller than that of LiNbO _3. There is. The glass composition can be adjusted to have a desired coefficient of thermal expansion and a desired refractive index by a person skilled in the art of manufacturing low melting glass. Further, as the low melting point glass, the glass material as described in Example 3 can be used.

Then, as shown in FIG. 6, in the first step, the surfaces of the prepared first and second substrates 41 and 42 were made hydrophilic by ordinary acid cleaning or alkali cleaning.
Next, the two substrates 41 and 42 were superposed in a clean atmosphere in which microparticles were not present as much as possible so that the low melting point glass film 44 of the first substrate 41 overlaps the surface of the second substrate 42. Then, the first and second substrates 41 and 42 that were overlapped were put in an electric furnace and heat-treated at 400 ° C. for 3 hours to perform diffusion bonding. The bonded substrates were void-free, with no microparticles or the like sandwiched between the bonded surfaces, and no cracks or the like were generated even when returned to room temperature.

Next, in the second step, the thickness of the nonlinear optical crystal substrate in which the polarization inversion structure of the bonded substrate is formed is 10 by using the polishing apparatus in which the flatness of the polishing platen is controlled.
Polishing was performed until it became μm. After the polishing process, a polishing process could be performed to obtain a mirror-polished surface. When the parallelism of the substrate was measured using an optical parallelism measuring machine, except for the periphery of the 3 inch wafer,
Submicron parallelism was obtained over almost the entire area, and the thin film substrate 45 suitable for producing a wavelength conversion element could be produced.

In addition, LiNbO _{3 is used} as the first substrate.
In addition, Zn-added LiNbO ₃ , Sc-added LiNbO ₃ ,
In-added LiNbO ₃ , LiTaO ₃ , LiNb _(x) T
a _{(1 -x)} O ₃ (0 ≦ x ≦ 1), KNbO ₃ , KTiOP
Even when O ₄ or the like was used, a similar thin film substrate for wavelength conversion element could be produced.

Also, when vapor deposition or sputtering was used as the method for producing the low melting point glass film to be produced on the first substrate, the same first substrate could be produced.
Further, a first substrate is also prepared by preparing a substrate made of low-melting glass, superposing this low-melting glass substrate and a nonlinear optical crystal substrate having a domain-inverted structure, and then bonding them by heat treatment. I was able to.

Also, a composite substrate having a low melting point glass film as used in Example 3 formed on the surface is used as the second substrate, and a nonlinear substrate having the domain-inverted structure of Example 4 is used as the first substrate. By using a composite substrate in which a low-melting glass film is formed under the optical crystal substrate, stacking these composite substrates so that the low-melting glass films of the respective substrates overlap each other, and then performing diffusion bonding by heat treatment. A similar thin film substrate for a wavelength conversion element could be produced even when they were bonded together. In this case, even if the low-melting glass films formed on the first substrate and the second substrate are thin, it is possible to increase the thickness of the low-melting glass film as a whole by bonding these low-melting glass films. it can.

Next, a wavelength conversion element having a ridge type optical waveguide was manufactured by the same method as in Example 1 using the manufactured wavelength conversion element thin film substrate. When the control light having a wavelength of 0.77 μm and the signal light having a wavelength of 1.55 μm were incident on the manufactured wavelength conversion element, wavelength converted light having a wavelength of 1.53 μm was obtained, which was higher than that of the wavelength conversion element manufactured in Example 1. We were able to realize highly efficient wavelength conversion. This is because the low melting point glass film was formed on the first substrate in this example,
Since the difference between the refractive index of the main body layer of the first substrate (Mg-doped LiNbO ₃ substrate) and the refractive index of the low melting point glass film of the surface layer can be increased, the optical electric field is confined in the optical waveguide. This is because it can be made stronger than the wavelength conversion element manufactured in Example 1.

(Embodiment 5) FIG. 7 is a sectional view of a ridge type waveguide processed according to the present embodiment 5. In the fifth embodiment, the wavelength conversion element thin film substrate 1 manufactured in the first embodiment is used.
No. 3 was used, and the wavelength conversion element 17 was manufactured using the ultra-precision grinding processing technique using a dicing saw as a manufacturing means of the ridge type optical waveguide 16. Ridge part (optical waveguide 16)
Had a width of 6 μm and a groove depth of 50 μm.

The substrate (3
(Inch wafer) is cut into strips, and the end face of the waveguide is optically polished to obtain a wavelength conversion element 17 having a length of 60 mm.
Was produced. Wavelength 0.77 μm for the manufactured wavelength conversion element
When the control light and the signal light with a wavelength of 1.55 μm were incident, wavelength-converted light with a wavelength of 1.53 μm was obtained, and highly efficient wavelength conversion could be realized. In addition, Examples 2, 3 and 4
A similar wavelength conversion element could be manufactured using the thin film substrate for wavelength conversion element manufactured in.

[0072]

As described above, according to the present invention, it is possible to provide a thin film substrate for a wavelength conversion element having a uniform composition and a film thickness over a large area. Therefore, if the thin film substrate for wavelength conversion element of the present invention is used, a long wavelength conversion element can be manufactured, and it is effective in improving the wavelength conversion efficiency.

[Brief description of drawings]

FIG. 1 is a flow chart showing a process of manufacturing a thin film substrate for a wavelength conversion element according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a cross section of the wavelength conversion element according to the first embodiment of the present invention.

FIG. 3 is a diagram showing how the wavelength conversion element of Example 1 of the present invention is cut out.

FIG. 4 is a flow chart showing a process of manufacturing a thin film substrate for a wavelength conversion element according to a second embodiment of the present invention.

FIG. 5 is a flowchart showing steps of producing a thin film substrate for a wavelength conversion element according to Example 3 of the present invention.

FIG. 6 is a flowchart showing a process of manufacturing a thin film substrate for a wavelength conversion element according to Example 4 of the present invention.

FIG. 7 is a diagram showing a cross section of a wavelength conversion element according to a fifth embodiment of the present invention.

FIG. 8 is a diagram showing a configuration of a quasi phase matching type wavelength conversion element.

[Explanation of symbols]

Reference Signs List 11 First substrate (Zn-added LiNbO ₃ substrate) 12 Second substrate (Mg-added LiNbO ₃ substrate) 13 Thin film substrate for wavelength conversion element 14 Ridge type optical waveguide 14a Waveguide end face 15 Wavelength conversion element 16 Ridge type optical waveguide 17 Wavelength conversion element 21 First substrate (LiNbO ₃ substrate) 22 Second substrate (crystal substrate) 23 Thin film substrate for wavelength conversion element 31 First substrate (Mg-added LiNbO ₃ substrate) 32 Second substrate 33 LiNbO ₃ substrate 34 low melting point glass film 35 thin film substrate for wavelength conversion element 41 first substrate 42 second substrate 43 Mg-added LiNbO ₃ substrate 44 low melting point glass film 45 thin film substrate for wavelength conversion element

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masao Yube             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation (72) Osamu Tadaga, inventor             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation (72) Inventor Hiroyuki Suzuki             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation F-term (reference) 2K002 AB12 BA03 CA02 CA03 EA07                       FA27 FA29 GA04 HA21

Claims (14)

[Claims] 1. A first step of bonding a first substrate having a structure having a quadratic nonlinear effect and a nonlinear constant being periodically modulated, and a second substrate, and the first substrate. A thin film substrate for a wavelength conversion element having a second step of reducing the thickness to a predetermined thickness for forming an optical waveguide, wherein the first step includes the first step. A method for manufacturing a thin film substrate for a wavelength conversion element, which comprises directly bonding the substrate and the second substrate by diffusion bonding by heat treatment. 2. The method for manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein the heat treatment in the first step is performed at a temperature equal to or lower than the Curie temperature of the first substrate. Method for manufacturing thin film substrate for device. 3. The method for manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein the thickness of the first substrate is 20 μm or less in the second step. Method for manufacturing thin film substrate for device. 4. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein the first substrate is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , Li.
Nb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1) or KTiOP
A method of manufacturing a thin film substrate for a wavelength conversion element, which contains O ₄ or at least one of them selected from the group consisting of Mg, Zn, Sc and In as an additive. 5. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein the second substrate has a refractive index smaller than that of the first substrate. A method for manufacturing a thin film substrate for a wavelength conversion element, comprising: 6. The method for manufacturing a thin film substrate for a wavelength conversion element according to claim 5, wherein the second substrate is a quartz substrate or a glass substrate. 7. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein a refractive index of a surface layer of the second substrate bonded to the first substrate is: A method for manufacturing a thin film substrate for a wavelength conversion element, which is smaller than the refractive index of the first substrate. 8. The wavelength conversion element according to claim 7, wherein the surface layer of the second substrate is a low melting point glass film or a low melting point glass substrate. Method for manufacturing thin film substrate. 9. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein a refractive index of a surface layer of the first substrate bonded to the second substrate is A method of manufacturing a thin film substrate for a wavelength conversion element, which is smaller than a refractive index of a main body layer of the first substrate forming a waveguide. 10. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 9, wherein the surface layer of the first substrate is a low melting glass film or a low melting glass substrate. Method for manufacturing thin film substrate. 11. The method for manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein a refractive index of a surface layer of the first substrate bonded to the second substrate is The refractive index of the surface layer of the second substrate, which is smaller than the refractive index of the main body of the first substrate forming the waveguide, and which is bonded to the first substrate is also the main body layer of the first substrate. A method of manufacturing a thin film substrate for a wavelength conversion element, which is smaller than the refractive index of. 12. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 11, wherein the surface layer of the first substrate is a low melting glass film or a low melting glass substrate, and the surface of the second substrate. A method for producing a thin film substrate for a wavelength conversion element, wherein the layer is also a low melting point glass film or a low melting point glass substrate. 13. The method of manufacturing a thin film substrate for a wavelength conversion element according to claim 1, wherein the thermal expansion coefficient of the second substrate is the thermal expansion coefficient of the first substrate. A method for manufacturing a thin film substrate for a wavelength conversion element, which is characterized in that they are substantially identical. 14. A thin film substrate for a wavelength conversion element is manufactured by the method for manufacturing a thin film substrate for a wavelength conversion element according to claim 1, and the wavelength conversion is performed in a subsequent third step. A method of manufacturing a wavelength conversion element, characterized in that an optical waveguide is formed on the first substrate of the element thin film substrate.