JP3429502B2 - Method for manufacturing domain-inverted region - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a method of manufacturing a domain-inverted region using an electric field and a domain-inverted region manufactured by the method, and is used in the fields of optical information processing and optical application measurement control. The present invention relates to an optical wavelength conversion element using a coherent light source.

[0002]

2. Description of the Related Art By utilizing the polarization inversion phenomenon in which the polarization of a ferroelectric substance is forcibly inverted, it is possible to form a periodic polarization inversion region (polarization inversion structure) inside the ferroelectric substance.
The polarization inversion region thus formed is used for an optical frequency modulator using surface acoustic waves, an optical wavelength conversion element using polarization inversion of nonlinear polarization, and the like. In particular, if the nonlinear polarization of the nonlinear optical material can be periodically inverted, an optical wavelength conversion element with extremely high conversion efficiency can be manufactured. By using this to convert light from a semiconductor laser or the like, it is possible to realize a compact short-wavelength light source that can be applied to the fields of printing, optical information processing, optical application measurement control, and the like.

As a conventional method for forming a periodic domain-inverted region, a method by thermal diffusion of Ti, a method of heat treatment after loading SiO2, a method of performing proton exchange treatment and heat treatment, etc. have been reported. On the other hand, a method of forming a periodic domain-inverted region by utilizing the fact that the spontaneous polarization of a ferroelectric substance is inverted by an electric field has been reported. As a method of utilizing this electric field, for example, a method of irradiating an electron beam on the −C surface of the substrate cut out along the C axis or + C
There is a method of irradiating the surface with positive ions. In any case, due to the electric field formed by the irradiated charged particles,
A deep domain-inverted region of several 100 μm is formed.

As another conventional method for manufacturing a domain-inverted region, a method has been reported in which a comb-shaped electrode is formed on a LiNbO3 substrate or a LiTaO3 substrate and a pulsed electric field is applied to the comb-shaped electrode. No. 121428, Japanese Patent Laid-Open No. 4-19719).

A conventional method of manufacturing a light wavelength conversion element will be described with reference to FIG.

A LiNbO3 substrate 55 as shown in FIG.
In order to manufacture the conventional optical wavelength conversion element 50 using
First, the periodic comb-shaped electrode 51 is formed on the + c surface 55a of the LiNbO3 substrate 55, and the planar electrode 52 is formed on the -C surface 55b. Next, the + C surface 55a is grounded, and the -C surface 55b
The pulse width is typically 100 due to the pulse power supply 56.
A pulse voltage of μs is applied to apply a pulse electric field to the substrate 55. The electric field required to reverse the polarization is about 2
It is 0 kV / mm or more. If the substrate 55 is thick when an electric field of such a value is applied, the crystal of the substrate 55 may be destroyed by the application of the electric field. However, by setting the thickness of the substrate 55 to about 200 μm, it becomes possible to avoid crystal destruction due to application of an electric field, and it is possible to form a domain-inverted region at room temperature.

Further, in order to realize the high efficiency of the light wavelength conversion element 50, a polarization inversion structure having a short period of 3 to 4 μm is required. When the domain-inverted region is formed by applying an electric field, after the polarization of the portion directly below the electrode is inverted,
The domain-inverted region spreads in the direction parallel to the surface of the substrate 55.
Therefore, it becomes difficult to shorten the period of the domain-inverted structure. In order to solve this problem, in the conventional method, the pulse width is 1
By applying a short-time pulse voltage of about 00 μs to the electrodes, the voltage application time is shortened and a domain-inverted structure with a short period is formed.

As described above, in the conventional method, by thinning the substrate 55, it becomes possible to form a domain-inverted region by applying an electric field at room temperature, and by shortening the time for applying an electric field, the period of the domain-inverted structure can be shortened. Has been realized.

Further, a method of manufacturing an optical wavelength conversion element using a conventional method of forming a domain-inverted region is described in, for example, M. Yama.
da, N.Nada, M.Saitoh, and K.Watanabe: "First-order
quasi-phase matched LiNbO3 waveguide periodicall
y poled by applying an external field for efficien
t blue second-harmonic generation ", Appl. Phys.Let
t., 62, pp435-436 (Feb. 1993). According to the disclosed method, after forming a periodically domain-inverted region, an optical waveguide is formed so as to be orthogonal to the domain-inverted region to manufacture an optical wavelength conversion element. In the manufactured optical wavelength conversion element, the power of the incident basic light with an interaction length of 3 mm is 19
When the power is 6 mW, the second harmonic of 20.7 mW is obtained as the output.

Further, a method of manufacturing a domain-inverted region in which a proton exchange treatment and an electric field application are combined is disclosed in, for example, Japanese Unexamined Patent Publication No. 4-264534. According to this method, the entire surface of the substrate is subjected to a proton exchange treatment to form a proton exchange layer, and then a comb-shaped electrode is formed on the surface of the proton exchange layer and a flat electrode is formed on the back surface of the substrate. A polarization inversion region is formed by applying a voltage between these electrodes. By performing the proton exchange treatment, the domain-inverted regions can be easily formed, and a highly uniform periodic domain-inverted structure can be formed.

[0011]

In the conventional method of manufacturing the domain inversion region as described above, it is necessary to apply a high voltage (several kV) and a pulse voltage having a short pulse width (100 μs or less). It is difficult to form such a high-voltage short pulse voltage, and it is difficult to ensure sufficient reproducibility, reliability, and uniformity during application.

When a high voltage and a short pulse voltage is applied to the substrate, the electric field distribution becomes nonuniform in the plane of the substrate.
As a result, there is a problem that the in-plane uniformity of the domain-inverted structure formed is deteriorated. Moreover, since it is difficult to form a uniform domain-inverted structure over a wide range, there is a problem that the domain-inverted structure cannot be mass-produced using a large-sized substrate.

Further, if the applied electric field is non-uniform, the substrate may be cracked, and the yield of device fabrication is lowered. Further, as described above, only a thin film substrate can be used in order to prevent crystal destruction of the substrate even when a high voltage pulse is applied. Therefore, it is difficult to handle the substrate and the workability is not high.

Further, in order to realize a highly efficient optical wavelength conversion element, a domain-inverted region having a short period is required, but in the conventional method for producing a domain-inverted region using an electric field application, a comb-shaped electrode is formed. Since the domain-inverted regions spread around the striped electrode branch and adjacent domain-inverted regions are connected to each other, it becomes difficult to form a domain-inverted region having a short period.

The present invention has been made to solve the above problems, and an object thereof is to form a domain-inverted region with a small pulse electric field intensity when the domain-inverted region is formed by applying an electric field. It is to provide a method of manufacturing an inversion region and an optical wavelength conversion element using the same.

[0016]

According to an aspect of the present invention.
For example, the manufacturing method of the domain-inverted region is a plane perpendicular to the C axis of the crystal.
First of the ferroelectric crystal substrate which is the C plate substrate cut out by
The step of forming a comb-shaped electrode on the surface and the ferroelectric crystal substrate
A step of adhering an optical substrate to the first surface, and
After bonding the substrate, grind the second surface of the ferroelectric crystal substrate.
Polishing process and flat electrode on the polished surface of the ferroelectric crystal substrate
And a step of forming a space between the comb-shaped electrode and the planar electrode.
Applying a constant voltage to the inside of the ferroelectric crystal substrate
Inverting the polarization of a certain region,
Thereby, the above object is achieved. Other stations of the invention
According to the surface, the manufacturing method of the domain-inverted region is strong on the substrate surface.
The step of growing a dielectric crystal in the C-axis direction, and the substrate surface
After growing the ferroelectric crystal in the C-axis direction on the substrate,
A comb-shaped electrode on the front surface of the substrate and a flat electrode on the back surface of the substrate.
Forming the comb-shaped electrode and the planar electrode
Formed so that they are separated from each other in the polarization direction of the ferroelectric crystal
And a predetermined voltage between the comb-shaped electrode and the planar electrode.
Is applied to polarize a predetermined region inside the ferroelectric crystal.
And the step of inverting
The above object is achieved. In one embodiment, the forcing
A nucleus for polarization reversal is generated near the surface of the electric crystal.
It is a surface to grow.

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[0113]

The operation will be described below.

In the present invention, a DC voltage is applied to the substrate in advance in order to form the domain inversion region. If the applied voltage value is set so that the electric field applied by the DC voltage does not exceed the reversal electric field for reversing the spontaneous polarization of the substrate crystal, the polarization reversal due to the electric field does not occur. In this state, when the short pulse voltage is superimposed on the DC voltage, the polarization is inverted by the total electric field of the short pulse electric field accompanying the short pulse voltage and the DC electric field accompanying the DC voltage. Since the superimposed pulse voltage may have a small value, it is possible to apply the pulse with high reproducibility. Moreover, the small pulse voltage increases the uniformity of the electric field applied to the substrate.

According to the present invention, it is possible to uniformly form a periodic domain-inverted region over a large area by the method described above. The reason will be described below.

2 separated from each other in the polarization direction of the ferroelectric crystal
When two electrodes are formed and a voltage is applied between both electrodes to form polarization inversion, the uniformity of the domain inversion region formed is
It depends on the uniformity of the applied electric field distribution. Therefore, according to the present invention, a DC voltage equal to or lower than the reversal electric field for reversing the polarization of the substrate is applied in advance between the electrodes, and under this condition, a short pulse voltage is superposed on the applied voltage. , Manufacture domain inversion regions. Since the voltage applied between the electrodes is a DC voltage, the uniformity of the electric field between the electrodes is high,
A uniform voltage can be applied over a large area. Further, since the intensity of the superimposed pulse voltage can be reduced, the distribution of the electric field applied between the electrodes by the pulse voltage is uniform and the voltage control is easy. Therefore, it becomes possible to form a domain-inverted region having high reproducibility and excellent in-plane uniformity over a large area.

Next, a method for facilitating the formation of a short-period domain-inverted region will be described.

In the domain-inverted region formed by applying an electric field, the polarization is inverted from the + C plane of the substrate. Based on this phenomenon, as a result of various studies on a method for suppressing the lateral expansion of the domain-inverted region, as a result of degrading the crystallinity (ferroelectricity) near the surface of the + C plane of the substrate, the domain-inverted It has been discovered that the occurrence of can be suppressed. Furthermore, when a comb-shaped electrode was formed on the substrate surface (+ C surface) to deteriorate the ferroelectricity in the vicinity of the crystal surface other than the stripe-shaped electrode branches forming the comb-shaped electrode, polarization reversal formed under the electrode branch Lateral expansion of the region is suppressed, and it becomes possible to form a domain-inverted region with a short period. The reason will be described below.

When a comb-shaped electrode composed of striped electrode branches is formed on the + C surface of the ferroelectric crystal substrate and a flat electrode is formed on the -C surface, and a voltage is applied between both electrodes, polarization inversion occurs from the + C surface. , An acicular reversal region is formed under the electrode.
However, when the voltage application is further continued, the inversion region spreads in the width direction with time, making it difficult to form a domain-inversion region with a short period. Then, when the formation process of polarization inversion was observed, it was found that the occurrence of polarization inversion can be suppressed by deteriorating the crystallinity (here, ferroelectricity) near the + C plane where polarization inversion occurs. For example, +
By subjecting the C plane to a proton exchange treatment and changing the crystal on the substrate surface to a proton exchange region, the reversal voltage required for the polarization reversal can be increased by several kV / mm or more, and the occurrence of the polarization reversal can be suppressed. Therefore, by suppressing the ferroelectricity in the vicinity of the surface of the + C plane of the crystal between the striped electrodes that form the comb-shaped electrodes, the widthwise extension of the polarization inversion to a portion other than directly below the electrode branch is suppressed. can do. That is, by providing a region in which the ferroelectricity is deteriorated between the stripe-shaped electrode branches, it is possible to suppress the widthwise extension of the inversion region and form a domain inversion region with a short period.

On the other hand, according to the present invention, in order to make the shape of the domain inversion regions formed on the substrate uniform, a voltage is applied between the electrodes after covering the electrodes formed on the substrate with an insulator. By covering the electrodes with an insulating film, the movement of free charges to the periphery of the electrodes can be suppressed. Therefore, when forming the polarization inversion, it is possible to suppress the lateral expansion of the polarization inversion portion spreading in the peripheral portion of the electrode, and a uniform polarization inversion structure can be obtained.

2 separated from each other in the polarization direction of the ferroelectric crystal
In the formation process in which two electrodes are formed and an electric field is applied to cause polarization inversion, a polarization inversion nucleus is first formed by the application of an electric field, and then a polarization inversion portion is formed in the polarization direction from the polarization inversion nucleus. It grows (forward growth), and the polarization inversion part spreads further to the periphery of the inversion nucleus (lateral growth). However, since the domain-inverted portion expands around the electrode pattern, it becomes difficult to form a domain-inverted pattern having a fine structure. In particular, when forming a periodic domain-inverted layer, it becomes difficult to form a short-period domain-inverted layer. Further, since the lateral growth on the electrode peripheral portion is poor in uniformity, it becomes difficult to fabricate the structure of the domain inversion portion according to the electrode pattern.

As a result of various studies on the expansion phenomenon of the domain-inverted portion, we found that the crystal surface state in which the domain-inverted nuclei occur affects the formation of domain-inversion. In other words, the movement of the surface free charges existing on the ferroelectric surface causes non-uniformity of the electric field distribution applied to the ferroelectric substance and the generation of an electric field around the electrodes, which promotes lateral growth of the domain inversion part. I found out what I was doing. That is, the surface free charge existing on the crystal surface moves to the electrode peripheral portion due to the voltage application, and the electric field is induced not only directly under the electrode but also in the peripheral portion thereof, so that the polarization reversal also spreads to the electrode peripheral portion. Is the mechanism.

Therefore, in order to suppress the lateral growth of the polarization inversion portion, it has been proposed to cover the electrode with an insulator to suppress the movement of the surface free charge to the peripheral portion of the electrode. That is, by covering the electrode formed in the polarization direction of the ferroelectric substance with the insulating film, it is possible to suppress the expansion of the polarization inversion portion around the electrode and form a uniform polarization inversion layer.

Similar means can be used to form the periodic domain-inverted layer necessary for the light wavelength conversion element. That is, by applying a voltage between the comb-shaped electrode formed on the ferroelectric surface and the plane electrode formed on the back surface, when forming the periodic domain-inverted layer, the comb-shaped electrode, which is a fine pattern, is covered with the insulator film. By doing so, it is possible to suppress the expansion of the domain-inverted portion to the peripheral portion of the stripe-shaped electrode that forms the comb-shaped electrode, and form a uniform periodic domain-inverted layer in a short period.

As another method for forming a highly efficient light wavelength conversion element, the movement of surface free charges can be suppressed by shaving the crystal surface between the stripe electrodes forming the comb-shaped electrodes. As a result, it is possible to suppress the expansion of the domain-inverted portion to the peripheral portion of the comb-shaped electrode and form a uniform periodic domain-inverted layer in a short period.

Further, by using the formed periodic domain-inverted layer having a short period and uniform, it becomes possible to manufacture a highly efficient optical wavelength conversion element.

[0128]

BEST MODE FOR CARRYING OUT THE INVENTION Before explaining the embodiments of the present invention, first, polarization reversal of a ferroelectric substance will be described.

The ferroelectric substance has a bias of electric charge in the crystal due to spontaneous polarization. By applying an electric field opposite to the spontaneous polarization, the direction of the spontaneous polarization in the ferroelectric substance can be changed.

The direction of spontaneous polarization depends on the type of crystal (material). Crystals such as LiTaO3 and LiNbO3 are C
Since these crystals have spontaneous polarization only in the axial direction, in these crystals, the polarization is − in the + direction along the C axis or vice versa.
There are only two directions. By applying an electric field,
The polarization of these crystals is rotated by 180 degrees and is oriented in the opposite direction. This phenomenon is called polarization reversal. The electric field necessary to cause the reversal of polarization is called an inversion electric field, and in crystals such as LiNbO3 and LiTaO3,
It takes a value of about 20 to 30 kV / mm at room temperature.

Making a crystal having a single polarization direction in a ferroelectric is called "single domainization of polarization". In order to achieve this polarization in a single domain, a method of applying an electric field at high temperature after crystal growth is generally performed.

(First Embodiment) FIG. 1A is a schematic perspective view for explaining a method of forming a domain inversion region according to the first embodiment of the present invention.

In order to manufacture the optical wavelength conversion element 100 using the LiTaO3 substrate 1 as shown in FIG. 1A, first, the periodic comb-shaped electrode 2 is formed on the + C plane 1a of the LiTaO3 substrate 1, and − The planar electrode 3 is formed on the c-plane 1b. Next, the DC power source 4 is connected to the comb-shaped electrode 2 on the + C surface 1a and -C.
The pulse power supply 5 is connected to the flat electrode 3 on the surface 1b. With such a configuration, a DC voltage having a predetermined voltage level on which a pulse voltage is superposed as necessary is applied to LiTaO
3 can be applied to the substrate 1.

In order to prevent the occurrence of discharge when a voltage is applied,
The substrate 1 is placed in an insulating liquid or vacuum (10 −6 Torr or less), and a DC voltage is applied. When the polarization reversal occurs, a current (referred to as “reversal current”) that is proportional to the size of the ferroelectric spontaneous electrode and the electrode area flows between the comb-shaped electrode 2 and the planar electrode 3.

First, the relationship between the applied DC electric field Ecw and pulse electric field Epp and the polarization inversion phenomenon will be described below.

First, an example in which only the pulse power supply 5 is used and only the pulse voltage is applied to form the domain inversion region will be described. In this case, in the configuration of FIG. 1A, a pulse voltage is applied to the LiTaO 3 substrate 1 via the planar electrode 3 by the pulse power supply 5. Without using the DC power supply 4,
The comb electrode 2 is grounded.

A pulse width of 10 is applied to the substrate 1 having a thickness of 0.2 mm.
When a pulse voltage of about 0 μs is applied, a pulsed electric field is applied to the substrate 1. At this time, about 20 kV / mm (1
It means that a voltage of 20 kV per mm is applied). Further, when the same operation was performed on the substrate 1 having a thickness of 0.3 mm, polarization reversal similarly occurred when an electric field of 20 kV / mm or more was applied, but the substrate 1 was easily cracked and the yield was low.

Next, the thickness of the substrate 1 is kept constant at 0.2 mm,
The area of the plane electrode 3 is set to 10 mm × 10 mm, and the cycle 3
An attempt was made to form a periodic domain inversion region of μm. However, when the area of the planar electrode 3 is increased in this way, the thickness becomes 0.
Even the 2 mm substrate 1 was frequently cracked. Further, there is a problem that the domain-inverted regions are only partially formed, or the formed domain-inverted regions are not uniform in shape.
It is considered that these are because the thickness and shape of the planar electrode 3 to which the pulse voltage is applied are actually non-uniform, so that the electric field applied to the substrate 1 is partially concentrated. Further, the uniformity of the periodic structure of the formed domain-inverted regions also deteriorated.

As described above, in the case of forming the domain inversion region by applying only the pulsed electric field by the application of the pulse voltage,
In order to make the shape of the periodic domain-inverted regions uniform, the size of the entire region where the domain-inverted structure is formed must be about 3 mm × 3 mm or less. Further, with a substrate having a thickness of 0.3 mm or more, it is difficult to form a domain-inverted region having a sufficient size, or the formed domain-inverted region has poor in-plane uniformity.

Next, an example in which only the DC power source 4 is used and only the DC voltage is applied to form the domain inversion region will be described. In this case, in the configuration of FIG.
Then, a DC voltage is applied to the LiTaO 3 substrate 1 via the comb-shaped electrode 2. The plane electrode 3 is grounded without using the pulse power supply 5.

When a DC voltage is applied to the substrate 1 having a thickness of 0.2 mm, a DC electric field is applied to the substrate 1. About 20k
When a voltage corresponding to an electric field strength of V / mm or more was applied, a reversal current flowed and the polarization was reversed. Further, when the polarization reversal characteristics with respect to the thickness of the substrate 1 were measured, it was possible to form the polarization reversal region when the thickness of the substrate 1 was 0.5 mm or less, but when the thickness was 0.5 mm or more, the substrate 1 A crack was generated in the film, making it difficult to form the domain inversion region. This is because when the thickness of the substrate 1 increases and the voltage level to be applied to obtain the electric field strength required for forming the domain-inverted regions increases, the domain-inverted regions cannot be formed on the thick substrate. It is considered that the substrate 1 was cracked by applying a voltage exceeding the breakdown voltage of the crystal.

However, when a DC voltage is applied, since the in-plane uniformity of the electric field applied to the substrate 1 is high, it is possible to apply an electric field having a large strength.

Regarding the formation of the periodic domain-inverted regions by the application of the DC voltage, the formation of the periodic domain-inverted regions was tried by applying a DC electric field of about 20 kV / mm under various conditions as follows. It was That is, the period of the comb electrodes 2 is 2 μm to 10 μm, and the thickness of the substrate 1 is 0.2 mm to
When the voltage application time was set to 0.5 mm and the voltage application time was set to 0.5 seconds to 10 seconds, a periodic domain-inverted structure having a period of 5 μm or less was not formed in any case. This is a comb-shaped electrode 2
It is shown that it is difficult to form a periodic structure with a short period by applying only a DC voltage, because the domain-inverted region formed immediately below the electrode spreads laterally at a high speed.

As described above, when only a DC voltage (electric field) or only a pulse voltage (electric field) is applied, it is difficult to uniformly form a domain-inverted structure with a short period over a large area. On the other hand, according to the present invention, a DC voltage (electric field)
By superimposing a pulse voltage (electric field) on the substrate and applying the pulse voltage to the substrate, the above object can be achieved.

FIG. 1B shows a change with time of the electric field strength applied to the substrate 1 by the voltage application according to this embodiment. In the following description, the DC electric field applied by the DC voltage from the DC power supply 4 is Ecw, and the pulse electric field applied by the pulse voltage (single pulse here) from the pulse power supply 5 is Epp. In the present invention, as shown in FIG.
A pulse electric field Epp is superimposed on the DC electric field Ecw and applied to the substrate 1.

The pulse electric field Epp typically has a pulse width of 100 μs or less, and is 0.5 ms in the following description. Further, with the pulse voltage actually applied to the substrate, the voltage change of a predetermined amplitude at the time of its rise and fall does not occur instantaneously, but requires a certain amount of time. In FIG. 1B, in order to simplify the description, these points are omitted and the pulse electric field Epp is drawn as an impulse waveform.

The polarization inversion characteristics were measured using the values of Ecw and Epp as parameters. In addition, the voltage value at which polarization reversal (reversal voltage value) was measured through the measurement of the reversal current. As a result, it has been clarified that the polarization is inverted when the total value of Ecw + Epp is about 20 kV / mm or more, regardless of the individual values of Epp or Ecw.

By the way, in forming the domain-inverted region by applying an electric field, it is necessary to pay attention to the re-inversion phenomenon of polarization. On the other hand, according to the method of manufacturing the domain-inverted region in the present embodiment, it is possible to solve the conventional problems caused by the re-inversion phenomenon of polarization. This point is shown in FIG.
This will be described below with reference to (a) to (e) and FIG.

In forming the domain inversion region by applying a conventional pulse voltage, the pulse width of the applied pulse voltage,
Even if the rising speed and the falling speed are adjusted, the domain inversion region may not be finally formed inside the substrate.
In order to investigate the cause of this phenomenon, the present inventors measured the reversal current flowing with the occurrence of polarization reversal as well as the application of the pulse voltage. The results are shown in FIGS. 2 (a) to 2 (d).

FIGS. 2A and 2B show the waveform of the voltage applied to the substrate in the conventional method and the waveform of the reversal current flowing with it. When a pulse voltage (depicted here as a negative pulse voltage) is applied to the substrate as shown in FIG. 2 (a) and a voltage exceeding the inversion voltage is applied to the substrate, it is shown in FIG. 2 (b). Thus, the reversal current flows and polarization reversal occurs. However, in the process of ending the application of the pulse voltage and decreasing the applied voltage toward zero, a current in the opposite direction to the reversal current flows. This is the re-inversion current that flows when the polarization that has been inverted once is re-inverted and returns to the initial state. As described above, in the conventional method of applying only the pulse voltage (electric field), the domain-inverted region may not be eventually formed due to the influence of the re-inversion phenomenon of the polarization. This re-inversion phenomenon indicates that the domain-inverted region immediately after formation is unstable.

The voltage value at which the polarization re-inverts and the re-inversion current flows depends on the rate of change of the voltage when the applied pulse voltage is returned to the zero level. This relationship is shown in FIG. This graph is a graph when a pulsed voltage of −5 kV is applied to the substrate using a LiTaO 3 substrate having a thickness of 0.2 mm as the substrate. Actually, this relationship changes depending on the value of the load resistance of the power source, but the graph of FIG. 3 shows the relationship when the load resistance value is 1 MΩ. According to FIG.
As the rate of change of the pulse voltage increases, re-inversion of polarization occurs and the voltage value at which re-inversion current starts to flow increases, and gradually approaches the inversion voltage value.

On the other hand, according to the method of this embodiment,
As shown in FIG. 2C, a voltage obtained by superposing a pulse voltage on a DC voltage is applied to the substrate. At this time, the magnitude of the DC voltage is set to a voltage value at which polarization reversal does not occur and re-reversal current does not flow. As a result, when the applied voltage is decreased from the pulse voltage value toward the zero level, the reversal current does not flow as shown in FIG. 2D, and the formed polarization inversion layer can be maintained. The DC voltage applied after this polarization reversal may be applied at the above-mentioned predetermined voltage level for at least a predetermined time, for example, several seconds.

For example, when the LiTaO3 substrate having the thickness of 0.2 mm is used as described above, the DC voltage is set to -2 k.
V and pulse voltage may be −3 kV, and a maximum voltage of −5 kV may be applied to the substrate. Further, the rate of change of the applied voltage to the zero level after forming the domain-inverted region may be 100 V / sec. Furthermore, the pulse width of the pulse voltage can be optimized by calculating the total amount of charge flowing out to the substrate from the waveform of the reversal current. By performing the process of this embodiment based on these settings, the domain-inverted regions can be formed uniformly over the entire substrate.

Although the LiTaO3 substrate is used in the above description, similar results can be obtained with the LiTaO3 substrate.

The reverse current flows only when a voltage exceeding the reverse voltage is applied to the substrate. In order to accurately control the shape (area) of the domain-inverted region to be formed, it is necessary to control the total amount of inversion current. Such control can be performed by accurately controlling the magnitude of the inversion current and the time during which the applied voltage exceeds the inversion voltage value. For that purpose, for example, as shown in FIG. 2E, when the applied voltage is changed from the pulse voltage value to the zero level, first, the voltage is changed stepwise below the inversion voltage level, and then gradually changed. Good.

In the method of manufacturing the domain inversion region of the present embodiment as described above, the total value of Ecw + Epp is 21.
When the relationship between the value of Ecw and the thickness of the substrate 1 on which the domain-inverted region can be formed is determined with constant kV / mm, FIG.
The results shown in (a) are obtained. That is, by increasing the value of Ecw, the domain inversion region can be formed even in a thick substrate without cracking the substrate. For example, when Ecw is 5 kV / mm or more, the thickness is 0.3 m.
When the Ecw is 10 kV / mm or more on the m substrate, the domain inversion region can be formed on the substrate having a thickness of 0.4 mm or more.

Increasing the value of Ecw improves the in-plane uniformity in the domain inversion region to be formed. The domain-inverted region having the best uniformity is formed when Ecw is set to a value slightly smaller than the electric field where the domain-inversion actually occurs (that is, the inversion field). Although the specific numerical value varies slightly depending on the substrate, when the reversal electric field is about 20 kV / mm, the value of Ecw may be set to 19.9 kV / mm, for example. By setting Ecw to the above value, the domain inversion region can be formed in a large area of 20 mm × 20 mm or more.

Next, in the case of forming a domain-inverted region by applying a DC voltage (electric field) on which a pulse voltage (electric field) is superposed, consideration will be given to shortening the period of the domain-inverted region formed.

The domain-inverted region formed by applying the electric field is generated just below the electrode and then expands in the lateral direction. Therefore, even if an attempt is made to form a short-period domain-inverted region, the domain-inverted regions formed adjacent to each other will contact each other and the periodic structure will not be formed. Therefore, the influence of the applied voltage shape on the lateral expansion of the domain-inverted region will be examined below.

In the structure of FIG. 1A, the + C surface 1a of the substrate 1
A comb-shaped electrode 2 having a width of each stripe-shaped electrode branch is 10 μm, and a flat electrode 3 is formed on the -C plane 1b, and a DC voltage in which a pulse voltage is superimposed is applied between the electrodes 2 and 3. . The pulse width is 5 μs, and the total value of the electric field strength Ecw + Epp applied by voltage application is 21 kV / m.
When the value of Ecw is changed as a parameter while keeping m as a constant value, the width between the domain-inverted regions and Ecw is shown in FIG.
The relationship shown in (b) is obtained. Specifically, the larger the applied DC electric field Ecw, the smaller the lateral expansion of the domain inversion region. The width of the domain-inverted region formed when Ecw is 5 kV / mm or less is substantially equal to the width of the domain-inverted region formed when the pulse voltage is applied alone.

As described above, by superimposing the pulse voltage on the DC voltage and applying it to the substrate 1, it is possible to obtain a voltage of 20 kV /
The formation of the domain inversion region, which requires the application of a pulse electric field as large as about mm, can be formed by the application of a pulse electric field of several kV / mm. Along with this, it becomes easy to make the domain-inverted regions to be formed uniform and to shorten the period. Especially 5
Applying a DC electric field of kV / mm or more and reducing the DC electric field applied after the application of the pulsed electric field are effective for forming a uniform and short-period domain-inverted region.

In this embodiment, an electric field is applied between the electrodes 2 and 3, but even if a magnetic field is applied instead of the electric field,
Similar domain-inverted regions can be formed. For example,
By applying a strong magnetic field of 10 kHz or more in the + Z direction, it becomes possible to form the domain inversion region as in the case of applying an electric field. Further, by shortening the magnetic field application time to form a pulse shape, it is possible to form a domain-inverted region with a short period.

Further, in the present embodiment, a single pulse is superposed as the pulse electric field, but the same effect can be obtained by superposing a plurality of pulses instead. When a single pulse is superposed, the shape of the domain inversion region formed can be controlled by the amplitude and pulse width of the applied pulse voltage. On the other hand, when a plurality of pulses are superimposed, it is possible to control the shape of the domain-inverted region formed by the number of applied pulse electric fields, which is effective in improving the in-plane uniformity of the domain-inverted region. .

The comb-shaped electrode 2 is formed on the + C plane 1a of the substrate 1 because the formation of polarization inversion nuclei occurs on the + C plane 1a. Even if the comb-shaped electrode 2 is formed on the -C face 1b to form a periodic domain-inverted structure, the domain-inverted region rapidly expands in the lateral direction (direction parallel to the surface of the substrate 1). It is difficult to form a short-period domain-inverted structure.

(Second Embodiment) In the present embodiment, the first
A method of manufacturing an optical wavelength conversion element having a stripe-shaped optical waveguide by utilizing a domain-inverted region manufactured according to the embodiment of the present invention will be described. 5 (a) to (c),
FIG. 6 is a perspective view showing a method for manufacturing the light wavelength conversion element 200 in the present embodiment.

As described in the first embodiment, a DC electrode on which a pulse voltage Epp is superimposed by forming a comb-shaped electrode on the + C surface and a flat electrode on the -C surface of a LiTaO3 substrate 1 having a thickness of 0.3 mm. The periodic domain-inverted regions 9 are formed by applying the voltage Ecw. The applied voltage is, for example, the magnitude of the DC electric field Ecw = 19.5 kV / mm, and the magnitude of the pulse electric field Epp =
The pulse width of the pulse voltage is set to 0.5 ms to obtain 1.5 kV / mm. In addition, a current detection function is added to the DC power supply to detect a reversal current, and polarization inversion is formed by superposition of the pulse voltage, and at the same time, a DC electric field of
kV / mm or less. As a result of the above, FIG.
As shown in (a), a domain-inverted region 9 having a period of 3.5 μm is formed over the entire region having a size of 10 mm × 10 mm.

By forming a striped optical waveguide so as to be orthogonal to the formed periodic domain inversion region 9, an optical waveguide type optical wavelength conversion element is manufactured. However, in general, the refractive index of the region where the polarization is inverted is higher than that of the substrate 1. Therefore, in the periodic domain-inverted region 9 produced by applying the electric field as described above, the refractive index changes periodically, and the propagation loss of the optical waveguide formed is significantly increased.

In order to solve the above problems, the substrate 1 is annealed in an oxygen atmosphere before forming the optical waveguide to reduce the change in the refractive index of the domain inversion region 9 to make the refractive index distribution uniform. Turn into

When the annealing temperature was changed between 300 ° C. and 600 ° C., it became clear that the annealing process at a temperature of 400 ° C. or higher significantly reduces the change in the refractive index. However, if the annealing temperature is further raised and the treatment is performed at 580 ° C. or higher, the domain-inverted region once formed is reduced. In particular, when the annealing treatment at such a high temperature is continuously performed for 60 seconds or more, the domain inversion region disappears almost completely.

From the above results, the annealing temperature is 580 ° C.
The following is preferable. Further, when the temperature is lowered after the annealing treatment, by setting the temperature lowering rate to 5 ° C./sec or less, it is possible to further reduce the change in the refractive index and obtain a uniform refractive index distribution.

After the periodic domain inversion regions are formed according to the first embodiment, annealing is performed under the above conditions. Further, after that, in the steps shown in FIGS. 5A to 5C, the optical wavelength conversion element 2 having a stripe-shaped optical waveguide is formed.
00 is manufactured.

When the optical waveguide is formed after the domain-inverted regions 9 are formed, it is desirable to carry out the process at a low temperature so as not to affect the already formed periodic domain-inverted structure. For example, the Curie temperature of LiTaO3 is about 600 ° C, so it is desirable to carry out the process at a temperature below that. Therefore, in the present invention,
Proton exchange treatment that can create an optical waveguide at low temperature is performed.

In the proton exchange treatment, a substrate immersed in an acid is heat-treated to exchange metal ions in the substrate with protons in the acid to form a layer having a high refractive index. For example, in the case of LiTaO3 substrate,
Li and protons are exchanged. Since the non-linearity of the region subjected to the proton exchange treatment is reduced to about half the original value of the substrate, it is necessary to perform the annealing treatment after the proton exchange treatment to recover the non-linearity.

Specifically, as shown in FIG. 5A, first, a Ta mask layer 10 is deposited by a sputtering method on the surface of the substrate 1 in which the periodic domain inversion regions 9 are formed. The thickness of the Ta mask layer 10 is typically 10
nm to 500 nm, preferably 20 nm to 100 nm,
For example, it is 40 nm. Subsequently, as shown in FIG. 5B, the Ta mask layer 10 is subjected to patterning by photolithography and subsequent dry etching to form a stripe-shaped opening 10a corresponding to the stripe-shaped optical waveguide. To do. Then, the Ta mask layer 1 is heat-treated in pyrophosphoric acid at 260 ° C. for 16 minutes.
Proton exchange processing is performed through the 0 opening 10a to form a proton exchange waveguide 11 as shown in FIG. After that, the Ta mask layer 10 is removed, and further heat treatment is performed at 420 ° C. for 60 seconds for the purpose of reducing the loss of the waveguide and recovering the nonlinearity of the waveguide, whereby the striped optical waveguide 11 is completed. .

A perspective view of the manufactured light wavelength conversion element 200 is shown in FIG. In the light wavelength conversion element 200 of FIG. 6, both the element length and the working length are 10 mm, and the period of the domain inversion region 9 is 3.5 μm. Also, the optical waveguide 1
1 has a width of 4 μm and a depth of 3 μm. Further, as shown by arrows 9a and 9b in FIG. 6, the polarization directions in the domain-inverted region 9 and other portions are opposite to each other.

According to this embodiment, the domain-inverted regions can be formed uniformly over a large area, and the operating characteristics of the manufactured optical wavelength conversion device 200 are improved. In particular, the optical waveguide 11
The light propagating through the laser can be sufficiently overlapped with the domain-inverted region 9 formed to a deep position, and a highly efficient optical wavelength conversion element can be manufactured.

In the above description of this embodiment, the LiTaO3 substrate is used as the substrate 1. Or KTP
(KTiOPO4) substrate, KNbO3 substrate, LiNbO
3 substrate or L doped with MgO, Nb, Nd, etc.
An iTaO3 substrate or a LiNbO3 substrate can be used. Alternatively, a LiNb (1-x) TaXO3 substrate (0 ≦ x ≦, which is a mixed crystal of LiTaO3 and LiNbO3,
The same light wavelength conversion element can also be manufactured by 1). In particular,
Of the above, the LiNbO3 substrate has a high non-linear optical constant, and is therefore effective for manufacturing a highly efficient optical wavelength conversion element.

In the above description of this embodiment, the optical waveguide 11 is formed on the + C surface 1a of the substrate 1. However, since the domain-inverted region 9 is formed until reaching the back surface of the substrate 1, even if the optical waveguide 11 is formed on the −C surface 1b of the substrate 1, it is possible to manufacture an optical wavelength conversion element having similar performance. You can When the optical waveguide is formed on the −C plane as described above, the flat electrode is only formed on the −C plane and the pattern of the comb-shaped electrodes is not formed, so that the surface is less rough. Therefore, a waveguide with less waveguide loss can be manufactured, and a highly efficient optical wavelength conversion element can be manufactured.

As the optical waveguide, other optical waveguides such as a Ti diffusion waveguide, an Nb diffusion waveguide, an ion implantation waveguide can be used instead of the waveguide formed by the above-mentioned proton exchange treatment.

In the proton exchange treatment, orthophosphoric acid, benzoic acid, sulfuric acid, etc. can be used in addition to the above-mentioned treatment using pyrophosphoric acid.

Further, the mask for the proton exchange treatment is not limited to the Ta mask, but Ta2O5, P
A mask made of a material having acid resistance such as t or Au may be used. (Third Embodiment) In order to form a domain-inverted region having a short period, the width of the domain-inverted region needs to be suppressed to a desired period or less. The polarization reversal formed by applying an electric field through the comb-shaped electrode formed on the + C surface of the substrate occurs from the + C surface directly below the stripe-shaped electrode branches that form the comb-shaped electrode, and in the -C axis direction. grow up. However, at the same time, the stripe-shaped electrode branches also spread in the width direction (that is, in the direction parallel to the surface of the substrate). As a result, the width of the domain-inverted region widens, making it difficult to form a domain-inverted structure with a short period.

On the other hand, the inventor of the present application considered a method of suppressing the expansion of the domain-inverted region in the width direction. As a result, it was considered that by suppressing the occurrence of polarization in the gap between the stripe-shaped electrode branches on the + C surface of the substrate, the spread of the domain-inverted region in the width direction can be suppressed. Therefore, as a result of investigating a method of suppressing the occurrence of polarization inversion, it was found that + of LiTaO3 crystal or LiNbO3 crystal
It was found that by deteriorating the ferroelectricity in the vicinity of the C plane, the occurrence of polarization reversal in that portion can be suppressed. For example,
By performing the proton exchange treatment on the + C plane surface of LiTaO3, the value of the reversal electric field for reversing the polarization is several kV / mm.
It was revealed that the increase of the polarization reversal was suppressed.

Using the results of the study by the inventors, an attempt was made to shorten the domain inversion domain. Fig.7 (a)-
This will be described with reference to (c).

FIG. 7 (a) is a sectional view of the LiTaO3 substrate 1 in the measurement system previously described as FIG. 1 (a), and the same constituent elements as those in FIG. 2 (a) are designated by the same reference numerals. ing.

In the comb-shaped electrode 2 provided on the + C surface 1a of the LiTaO3 substrate 1 in FIG. 7A, stripe-shaped electrode branches having a width of 10 μm are formed with a gap of 10 μm. On the other hand, the planar electrode 3 is formed on the -C surface 1b of the substrate 1. At this point, the polarization inside the substrate 1 is pointing upward in the drawing, as indicated by the arrow 9b in FIG. 7 (a).

When a voltage is applied between the comb-shaped electrode 2 and the flat electrode 3 to apply an electric field to the substrate 1, when the electric field strength inside the substrate 1 reaches about 20 kV / mm or more, the comb-shaped electrode 2 A domain inversion region 9 is formed immediately below the electrode branch. The direction of polarization in this polarization inversion region 9 is shown in FIG.
As shown by an arrow 9a in FIG. 9B, it is downward in the drawing and is reversed from before the electric field is applied.

However, the domain-inverted regions 6 thus formed further extend in the width direction of the electrode branches (that is, in the direction parallel to the surface of the substrate 1). When such growth in the width direction progresses, the adjacent domain-inverted regions 9 may eventually come into contact with each other.

On the other hand, as shown in FIG.
Proton exchange treatment is applied to the vicinity of the surface of the + C plane 1a between the electrode branches to form a proton exchange region 7. In such a proton exchange region 7, its ferroelectricity is deteriorated, and the lateral expansion of the domain inversion region 6 is suppressed. As a result, as shown in FIG. 7C, the domain-inverted region 9 having the same width as the electrode branch is formed just below each electrode branch.

Further, the effect of the presence or absence of the proton exchange treatment on the lateral expansion of the domain inversion region 9 is shown in FIG.
And it demonstrates with reference to FIG.8 (b).

In FIG. 8B, the vertical axis represents the width W of the formed domain inversion region 9. This domain-inverted region 9 is shown in FIG.
As shown in (a), it is formed using a comb-shaped electrode 2 having a width of 10 μm. In other words, FIG.
The width W of the domain-inverted region shown on the vertical axis in (b) represents the degree of expansion of the domain-inverted region 9.

On the other hand, the horizontal axis of FIG. 8B shows the voltage applied between the comb-shaped electrode 2 and the plane electrode 3. Specifically, a voltage in which the pulse voltage Epp is superimposed on the DC voltage Ecw is applied, and the DC component Ecw of the applied voltage is kept constant at 3 kV. The vertical axis of FIG. 8B shows the value of the sum E = Ecw + Epp of the DC voltage Ecw and the pulse voltage Epp. The pulse width of the pulse voltage is 3 ms.

In FIG. 8B, the + C plane in the gap between the striped electrode branches of the comb-shaped electrode 2 is subjected to the proton exchange treatment (2
The relationship between the width W of the domain-inverted region 9 and the magnitude E of the applied voltage in the sample subjected to 60 ° C. for 20 minutes) and the sample not subjected to such a proton exchange treatment is shown.

As shown in FIG. 8B, the width W of the domain-inverted region 9, that is, the widthwise extension of the domain-inverted region 9 increases with an increase in the applied voltage E regardless of the presence or absence of the proton exchange treatment. However, when the proton exchange treatment is not performed, uniform polarization reversal is not formed when the applied voltage is 5.5 kV or less. Therefore, the width W of the domain-inverted region at this time is 2.7 μm.
m is the minimum value of W that can be obtained. That is, in the sample not subjected to the proton exchange treatment, the width of the stripe-shaped electrode branch of the comb-shaped electrode 2 used for forming the domain-inverted region 9 is 2
Despite being μm, the width W of the domain-inverted region 9 actually formed is at least 2.7 μm.

On the other hand, when the proton exchange treatment is carried out, the lateral expansion of the domain inversion region 9 is suppressed. As a result, it is possible to form the domain inversion region 9 having the width W substantially equal to the width of the stripe-shaped electrode branch of the comb-shaped electrode 2. As described above, it has been clarified that the lateral expansion of the domain inversion region 9 is suppressed by performing the proton exchange treatment.

Next, an example in which the above results are applied to the formation of periodic domain-inverted regions will be described with reference to FIGS. 9 (a) to 9 (c). 9 (a) to 9 (c), the same components as those shown in the drawings described with reference to the above have the same reference numerals.

First, as shown in FIG. 9A, the L of the C plate is
The comb-shaped electrode 2 is formed on the + C surface 1a of the iTaO3 substrate 1 and the planar electrode 3 is formed on the -C surface 1b. Of these, the planar electrode 3 can be, for example, a Ta film having a thickness of about 500 nm. Next, using the comb-shaped electrode 2 as a mask, the + C plane 1a in the gap between the striped electrode branches is subjected to a proton exchange treatment. This proton exchange treatment is typically 26
Perform for 30 minutes in pyrophosphoric acid at 0 ° C. As a result, FIG.
As shown in (b), a proton exchange region 7 having a depth of about 0.4 μm is formed. Next, as shown in FIG.
The substrate 1 that has been subjected to the above treatment is placed in an insulating liquid or in a vacuum, and a comb-shaped electrode 2 is formed by using a DC power supply 4 and a pulse power supply 5.
A voltage is applied between the flat electrode 3 and the flat electrode 3 to apply an electric field to the substrate 1. As a concrete applied electric field, for example, DC electric field Ecw = 18 kV / mm, pulse electric field Epp = 3 kV /
mm is superimposed.

According to the above steps, the period is 2 μm.
It is possible to form the domain inversion region 9 having a thickness of 10 μm. If the gap between the striped electrode branches of the comb-shaped electrode 2 is not subjected to the proton exchange treatment, it is difficult to uniformly form the domain-inverted regions having a period of 3 μm or less. On the other hand, when proton exchange is performed between the stripe electrode branches according to the present embodiment, it is possible to uniformly form the short-period domain-inverted regions 9 having a period of 2.5 μm or less.

Next, the influence of the time of the proton exchange treatment on the formation of the domain inversion region 9 will be described.

When the proton exchange treatment is carried out using pyrophosphoric acid at 260 ° C., the effect of suppressing the expansion of the domain inversion region 9 is exhibited by setting the proton exchange time to 5 minutes or longer. Further, by performing the proton exchange treatment for 10 minutes or more, the domain-inverted region 9 having good in-plane uniformity can be obtained. However, if the proton exchange treatment is performed for a very long time, the adjacent proton exchange regions 7 come into contact with each other, and the periodic domain-inverted structure 9 cannot be formed. From this, it is effective to carry out the proton exchange treatment for 10 minutes or more, and for a time within the limit that the adjacent proton exchange regions 7 do not contact each other.

The deterioration of the ferroelectricity of the substrate 1 caused by the proton exchange treatment means the deterioration of the crystalline state in which the spontaneous polarization can be inverted. Specifically, it shows a state in which the spontaneous polarization is small, a state in which the reversal electric field of the spontaneous polarization is increased, or a state in which the crystal no longer exhibits ferroelectricity. For example, when the proton exchange treatment is applied, the strain of the crystal structure becomes small, so that the spontaneous polarization becomes extremely small.

Alternatively, a similar phenomenon can be obtained by a treatment other than proton exchange. For example, when the surface of the substrate 1 is ion-implanted, the crystal structure collapses and becomes closer to a random structure, and spontaneous polarization is not exhibited.

As described above, by changing the crystal structure on the crystal surface where the polarization inversion occurs, the expansion of the polarization inversion region 9 can be suppressed. LiTaO described above
In the case of 3 substrate or LiNbO 3 substrate, the domain inversion region 9
Occurs from the + C plane 1a. Alternatively, depending on the type of crystal, polarization reversal may occur not from the + C plane but from the −C plane or another plane. However, in that case, by similarly deteriorating the ferroelectricity of the surface where the polarization inversion occurs, the expansion of the polarization inversion region can be similarly suppressed.

In the above description of this embodiment, the proton exchange treatment is used as a means for deteriorating the ferroelectricity of the substrate surface. Alternatively, the same effect can be obtained by ion exchange of Zn, Cd, etc., implantation of proton ions, He ions, Si ions, Au ions, etc., metal diffusion such as Ti diffusion, or MgO diffusion.

In the above description of this embodiment, the proton exchange treatment is performed using pyrophosphoric acid. Alternatively, the same effect as described above can be obtained by performing the proton exchange treatment using orthophosphoric acid, benzoic acid, sulfuric acid or the like.
Further, although the Ta film is used as the metal film forming the electrode in the above description, another film may be used as long as it has appropriate heat resistance. Specifically, a film made of a material such as Ti, Pt, or Au can be used.

In the above description of this embodiment, the LiTaO3 substrate is used as the substrate 1. Alternatively, LiNb
An O3 substrate, a LiTaO3 substrate or a LiNbO3 substrate doped with MgO, Nb, Nd or the like can be used. Alternatively, a LiNb (1-x) TaXO3 substrate (0 ≦ x ≦, which is a mixed crystal of LiTaO3 and LiNbO3,
The same light wavelength conversion element can also be manufactured by 1). These crystals can easily form the ferroelectric deterioration layer by the proton exchange treatment, as in the above description. Therefore, the periodic domain inversion region can be easily manufactured by applying an electric field. In particular, LiNbO3 is effective for producing a highly efficient optical wavelength conversion element because it has a high nonlinear optical constant.

On the other hand, as the substrate 1, KTP (KTiO
It is also possible to use a PO4) substrate or a KNbO3 substrate. Because these substrates have high nonlinear optical constants,
It is effective for manufacturing a highly efficient optical wavelength conversion element. Among these, in the KTP substrate, when the crystal surface in the gap between the electrode branches of the comb-shaped electrode is deteriorated by ion exchange, the crystallinity of the surface can be changed by the treatment with Rb ions. By applying an electric field thereafter, a deep domain-inverted region is formed.

(Fourth Embodiment) In the present embodiment, the third embodiment
A method for producing an optical wavelength conversion element by utilizing the periodically domain-inverted region produced by performing the proton exchange treatment according to the embodiment of the present invention will be described.

If an optical waveguide is formed on the formed domain-inverted region as described in the second embodiment, the structure shown in FIG.
An optical wavelength conversion element having the shape shown in (c) can be manufactured. However, in the LiTaO3 substrate 1 in which the periodic domain-inverted regions 9 are formed according to the third embodiment,
The proton exchange treatment is periodically performed on the C-plane 1a. Therefore, there is a periodical refractive index distribution (refractive index difference between the substrate 1 and the domain inversion region 9). Therefore, in order to form a low-loss optical waveguide, it is necessary to make the refractive index distribution uniform by annealing.

10 (a) to 10 (d), the temperature of the annealing treatment carried out for the above-mentioned purpose is the same as that of the domain inversion region 9 formed.
FIG. 6 is a cross-sectional view that schematically shows the influence on the shape of FIG. Figure 1
0 (a) shows a state where the annealing treatment is not performed, and FIGS. 10B to 10D show states after the annealing treatment is performed at 450 ° C., 500 ° C., and 550 ° C., respectively.

Before performing the annealing treatment, as shown in FIG. 10A, the needle-shaped microdomains 8 remain near the surface of the + C surface 1a of the substrate 1. Such microdomains 8 deteriorate the periodicity of the periodic domain-inverted structure including the domain-inverted regions 9. On the other hand, FIG.
As shown in (b) to (d), such microdomains 8 disappear by performing the annealing treatment.

However, when the annealing temperature exceeds 500 ° C., as shown in FIGS. 10C and 10D, the formed periodic domain-inverted regions 9 disappear from the vicinity of the + C plane 1a of the substrate 1. To do. When the annealing temperature or the annealing time is increased, the domain-inverted region 9 reaching the deeper portion disappears.

If the periodic domain inversion structure disappears from the vicinity of the surface of the substrate 1, the desired optical wavelength inversion function cannot be obtained. Therefore, in order to improve the uniformity of the formed periodic domain inversion structure and form a low-loss optical waveguide,
It is desirable to set the annealing temperature to 500 ° C. or lower.

According to the above method, since the period of the domain inversion structure can be shortened to 2 μm, the phase matching wavelength can be shortened to 740 nm. Therefore,
It is possible to generate ultraviolet light having a wavelength of 370 nm.

As described above, according to this embodiment, a uniform domain-inverted region can be obtained, so that the characteristics of the optical wavelength conversion element can be improved. In addition, by forming the domain-inverted region deep in the substrate, a highly efficient optical wavelength conversion element can be manufactured. Further, by shortening the period of the domain inversion region, it becomes possible to generate the second harmonic having a short wavelength.

In the above description, the change in the refractive index formed by the proton exchange treatment before the electric field is applied is removed by the annealing treatment, but it can be removed by another method.

The thickness of the proton exchange layer formed on the surface of the substrate by the proton exchange treatment is only about 1 μm. Therefore, the proton exchange layer can be easily removed by optically polishing the substrate surface. Specifically, L
Using a polishing diamond solution that is generally used for optical polishing of iNbO3 substrate and LiTaO3 substrate,
The proton exchange layer on the substrate surface is removed by polishing the substrate surface on a polishing cloth. After that, if a proton exchange optical waveguide is formed by the method described above, a highly efficient light wavelength conversion element can be formed similarly.

Alternatively, the proton exchange layer on the surface of the substrate can be removed by wet etching or dry etching.

In the case of wet etching using an etching solution, for example, HF and HNO 3 are used as the etching solution in a ratio of 2:
The temperature of the etching liquid is adjusted to 60 by using the liquid mixed in 1.
Etching is performed at about ℃. The etching rate for wet etching is usually different between the + C plane and the −C plane, but since the values are almost the same after the proton treatment, etching is performed without unevenness appearing on the polarization-inverted surface. . This makes it possible to form an optical waveguide with low loss.

In the case of dry etching, for example, CF4
Alternatively, etching can be performed using a gas such as CHF3.
For example, if the RF power is set to about 100 W using a reactive ion etching device, an etching rate of about several tens nm / min can be obtained. The etching rate on the surface of the substrate after the proton exchange treatment is higher than that of the substrate before the treatment, and a more efficient process can be performed.

(Fifth Embodiment) In this embodiment, another method for forming a highly efficient optical wavelength conversion element will be described.

The efficiency of the optical wavelength conversion element depends on the power density of the light propagating through the optical waveguide. Therefore, if the optical waveguide having a strong confinement is formed, a more efficient optical wavelength conversion element can be manufactured. In the present embodiment, a ridge type optical waveguide is used as the optical waveguide in order to manufacture an optical wavelength conversion element having an optical waveguide with a strong confinement.

11 (a) to 11 (d) illustrate a method of forming the ridge type optical waveguide 17a on the domain inversion region 9 according to the present embodiment. The domain-inverted regions 9 may be formed according to the method of the first embodiment or the third embodiment described above.

First, the LiTaO3 substrate 1 on which the periodic domain-inverted regions 9 are formed is subjected to a proton exchange treatment, and the result shown in FIG.
As shown in FIG. 1 (a), the proton exchange layer 1 is formed on the surface of the substrate 1.
Form 7. The proton exchange treatment is performed, for example, at a temperature of 26
It can be carried out by immersing the substrate 1 in pyrophosphoric acid at 0 ° C. for 20 minutes. Next, as shown in FIG. 11B, a resist pattern 12 for forming an optical waveguide is formed on the proton exchange layer 17 by photolithography. Then, using the resist pattern 12 as a mask, CHF3
Dry etching is performed in a gas atmosphere. Thereby, the proton exchange layer 17 is etched by about 300 nm. Further, by removing the resist pattern 12, as shown in FIG. 11C, a ridge 17a is partially formed.
The proton exchange layer 17 having the is formed. Further, an annealing treatment is performed, for example, at a temperature of 420 ° C. for 60 seconds,
An optical wavelength conversion element 500 including the ridge type optical waveguide 17a as shown in FIG. 11D is obtained.

In the optical wavelength conversion element 500 manufactured as described above, the thickness of the optical waveguide 17a is changed from the conventional 2 μm to 1.5 μm, and the width is changed from the conventional 4 μm to 3 μm.
Each can be reduced. With such miniaturization of the optical waveguide, the power density of light propagating through the optical waveguide can be increased to 1.5 times that of the conventional one. As a result, the conversion efficiency of the light wavelength conversion element is increased to about twice that of the conventional one.

(Sixth Embodiment) An optical wavelength conversion device 600 according to a sixth embodiment of the present invention will be described with reference to FIG.

In the light wavelength conversion element 600, LiTaO3 is used.
Periodic domain-inverted regions 9 are formed inside the substrate 1. A proton exchange region 7 is provided in the gap between the domain inversion regions 9 near the surface of the substrate 1. Further, the reflection films 14 are deposited on both ends of the substrate 1 on which the periodic domain inversion regions 9 are formed after polishing. The reflective film 14 is
Typically, 90% or more of the fundamental wave having a wavelength of 800 nm is reflected.

When the light (fundamental wave) 23 of the semiconductor laser 21 is incident on such an optical wavelength conversion element 600 via the condensing optical system 22, the incident fundamental wave 23 is applied to the reflection film 14 on both end surfaces of the substrate 1. Is multiple-reflected and resonates inside the substrate 1. That is, the optical wavelength conversion element 600 functions as a resonator, and the increased fundamental power thereof causes the incident fundamental wave 23 to be efficiently converted into the second harmonic wave 24 and emitted.

In the light wavelength conversion element 600, the periodic polarization inversion region 9 formed inside the substrate 1 is used as it is in a bulk shape to convert the wavelength of incident light. Fundamental wave 23
Since a sufficient overlap between the polarization inversion region 9 and the polarization inversion region 9 can be obtained, highly efficient light wavelength conversion can be performed.

Further, the proton exchange region 7 formed between the domain-inverted regions 9 near the surface of the substrate 1 has a function of preventing the domain-inverted region 9 from being deteriorated. When polarization inversion is caused by applying an electric field, a deep polarization inversion region 9 can be formed, but a large strain is applied to the crystal of the substrate 1. Such strain causes the formed domain-inverted region 9 to change with time. For example, in several weeks to several months, the shape of the domain inversion region 9 gradually changes, and the optical wavelength conversion element 600
Operating characteristics may change. On the other hand, when the proton exchange region 7 is formed between the domain-inverted regions 9, such a change in shape of the domain-inverted region 9 is prevented, and a stable optical wavelength conversion element in which the operating characteristics do not change with time. 6
00 can be configured.

Furthermore, since the LiTaO3 substrate 1 has a strong pyroelectric effect, when the temperature of the substrate 1 changes, pyroelectric charges are accumulated on the crystal surface in the substrate 1 and an electric field is generated.
When an electric field is generated, the refractive index changes due to the electro-optic effect. Therefore, the phase matching characteristics of the light wavelength conversion element 600 are affected, and the output becomes unstable. On the contrary,
Since the proton exchange region 7 has an electric resistance that is lower than that of the LiTaO3 substrate 1 by about one digit, the proton exchange region 7 is formed on the surface of the substrate 1 to increase the moving speed of electric charges generated by the pyroelectric effect and generate an electric field. Can be prevented. As a result, the optical wavelength conversion device 600 that can maintain stable output characteristics even when the external temperature changes can be configured.

As described above, the optical wavelength conversion element 600 functions as a resonator. For that purpose, to reach a position deeper than the beam diameter of the fundamental wave 23 that is multiply reflected, typically up to a depth of several tens of μm or more. The domain-inverted region 9 must be formed uniformly. By applying an electric field, a periodic domain-inverted region 9 that is uniform up to a depth of several 100 μm is obtained.
Is formed, the high-efficiency resonator-type optical wavelength conversion element 6
00 can be produced.

In the above description of this embodiment, the substrate 1
The bulk-type optical wavelength conversion element 600 that performs wavelength conversion by utilizing the domain-inverted region 9 formed inside is described.
On the other hand, as described in the second, fourth or fifth embodiment, an optical waveguide can be formed on the surface of the substrate 1 to form an optical wavelength conversion element of the optical waveguide type. In that case, as described in connection with the second embodiment, the annealing process is required to reduce the periodical refractive index change existing between the substrate 1 and the domain inversion region 9. On the other hand, in this embodiment, since the proton exchange region 7 having a higher refractive index than that of the substrate 1 is formed in the gap between the polarization inversion regions 9 near the surface of the substrate 1, the polarization inversion region 9 as described above is formed. The refractive index difference between the substrate and the substrate 1 is reduced. Therefore, it is possible to manufacture a low-loss optical waveguide, and a high-efficiency optical wavelength conversion element can be configured.

(Seventh Embodiment) A method for forming a domain inversion region according to a seventh embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to (c).

First, as shown in FIG. 13A, each stripe is formed on the + C plane 1a of a LiTaO3 substrate 1 (a substrate cut out in a plane perpendicular to the C axis of the crystal) of a C plate having a thickness of 200 μm, for example. A comb-shaped electrode 2 is formed in which the width of each electrode branch is 1.2 μm. On the other hand, the planar electrode 3 is formed on the −C surface 1b of the substrate 1. These electrodes 2 and 3 are
For example, a Ta film having a thickness of about 60 nm can be used. Next, as shown in FIG. 13B, a 200 nm-thick insulating film 34 made of SiO 2 is deposited by a sputtering method so as to cover the comb-shaped electrode 2 on the surface of the + C surface 1a. After that, the comb-shaped electrode 2 is grounded, and then a negative pulse voltage (typically, pulse width 3 ms) is applied to the planar electrode 3.
In order to prevent the occurrence of discharge, the entire substrate 1 is placed in an insulating liquid or in a vacuum of 10 −6 Torr or less when a voltage is applied.

The lateral expansion of the domain-inverted region 9 formed as described above will be described with reference to FIGS. 14 (a) to 14 (c).

In FIG. 14A, the vertical axis represents the width W of the formed domain-inverted region 9, and the horizontal axis represents the comb-shaped electrode 2 and the planar electrode 3.
Indicates the voltage (absolute value) applied between and. On the other hand, FIG.
4B and FIG. 14C are cross-sectional views showing the shape of the domain-inverted region 9 to be formed.

In FIG. 14A, for comparison, SiO 2 is used.
Data is also shown when the insulating film 34 is not deposited on the comb-shaped electrode 2. According to this, when the SiO 2 insulating film 34 is not formed, the width W of the domain inversion region 9 formed does not become smaller than 1.7 μm. In addition, FIG.
As schematically shown in (b), the domain-inverted region 9 obtained when the SiO 2 insulating film 34 is not formed has a non-uniform shape, and its width W greatly varies within a range of ± 30% or more. . Further, when the applied voltage is less than 5.5 kV, there is a portion where the polarization is not inverted inside the substrate 1,
It is observed that only a small area has polarization inversion. Therefore, when the SiO 2 insulating film 34 is not deposited on the comb-shaped electrode 2, it is necessary to apply a voltage of 5.5 kV or more in order to stably form the domain inversion region 9 over a wide area of the substrate. .

On the other hand, when the SiO 2 insulating film 34 having a thickness of 200 nm is deposited on the comb-shaped electrode 2 according to the present embodiment, the width W of the domain-inverted region 9 to be formed is set to the stripe-shaped electrode branch of the comb-shaped electrode. The width can be reduced to 1.5 μm. Further, as shown in FIG. 15C, the domain-inverted regions 9 having a uniform shape are formed, and the variation in width thereof is suppressed to ± 5% or less. Furthermore, 4.9
By applying a voltage of kV or more, the domain inversion region 9 can be formed over a wide range corresponding to the entire electrode.

As described above, by covering the comb-shaped electrode 2 with the insulating film 34, the widthwise extension of the domain-inverted region 9 is suppressed over a wide range of the applied voltage, and the domain-inverted region 9 having a uniform shape is suppressed. Can be formed.

Next, the insulating film 3 formed on the comb-shaped electrode 2
The characteristics required for No. 4 will be described.

First, the effect of the resistivity of the insulating film 34 will be examined based on the results of measuring the widths of the domain-inverted regions formed above by depositing insulating films having different resistivities. As a result, when an insulating film having a resistivity of 1015 Ω · cm or more is formed, the domain inversion region 9 formed has a spread of 1 or less.
It is suppressed to about μm, and the width variation is reduced to about ± 10%. If the resistivity of the insulating film is further increased,
For an insulating film of 1016 Ω · cm or more, the spread of the width of the domain inversion region 9 to be formed is further suppressed, and the variation is also suppressed within ± 5%, and the more uniform domain inversion region 9 is formed.
Is formed.

Considering the above results, the domain-inverted region 9 having a period of about 5 μm can be formed by using an insulating film having a resistivity of about 1015 Ω · cm, but the period is 4 μm.
When the domain-inverted region 9 having a short period of m or less is formed, it is desirable to use an insulating film having a resistivity of 1016 Ω · cm or more.

Next, the thickness of the insulating film 34 and the domain-inverted region 9 to be formed are taken as an example in which a SiO 2 film (having a resistivity of about 1017 Ω · cm) is deposited on the comb-shaped electrode 2 with different thicknesses. The relationship with the width of is explained.

When the thickness of the SiO 2 film is 20 nm or more, the effect of suppressing the expansion of the domain inversion region 9 in the width direction can be obtained. Further, when the thickness of the SiO 2 film is 100 nm or more, the variation in the width of the domain inversion region 9 is reduced to about ± 10%. Furthermore, the thickness of the SiO2 film is 200n.
When it is m or more, the variation of the width of the domain inversion region 9 is reduced to ± 5% or less, and the spread thereof is 0.2
It can be suppressed to less than μm. However, if the thickness of the SiO2 film is 2
Even if the thickness exceeds 00 nm, no further improvement is observed.

On the other hand, when the SiO 2 film is thin, the insulating liquid (resistivity: 1015 Ω · cm) is the ambient atmosphere when the electric field is applied.
Under the influence of, the sufficient effect cannot be obtained for suppressing the expansion of the domain-inverted region 9 in the width direction.

In the above description, the insulating film 34 is made of SiO 2.
Although two films are used, an insulating film made of another material can be used. For example, when a domain-inverted region is formed in the same manner as above on a substrate in which a Ta2O5 film is deposited to a thickness of 200 nm by the sputtering method, the same characteristics as when using a SiO2 film are obtained. However, when the organic polymer film is used as the insulating film, the suppression effect is about half that of the SiO2 film or the Ta2O5 film.

In addition, in the above description, the insulating film 34 is deposited by sputtering. The sputtering method has a large kinetic energy when the material of the film to be deposited is sputtered from the target and adheres to the substrate. Therefore, it has a great influence on the free electric charge on the surface of the substrate 1, and is excellent in the effect of suppressing polarization inversion. However, other film forming methods such as EB vapor deposition method, CVD method,
The insulating film 34 may be deposited by an ion beam sputtering method, a sol-gel method, or the like.

In the above description, the comb-shaped electrode 2 is formed on the + C surface 1a of the substrate 1. LiNbO3 substrate and LiTa
The formation of the domain-inverted region in O3 starts with the formation of the domain-inverted nucleus on the + C plane 1a. Therefore, by providing the comb-shaped electrode 2 on the + C surface 1a as described above, the pattern of the comb-shaped electrode 2 can be accurately transferred to the pattern of the domain inversion region. On the other hand, even if a comb-shaped electrode is formed on the -C surface 1b of the substrate 1, a highly uniform periodic domain-inverted structure is not formed.

In the above description of this embodiment, the LiTaO3 substrate is used as the substrate 1. Or KTP
(KTiOPO4) substrate, KNbO3 substrate, LiNbO
3 substrate or L doped with MgO, Nb, Nd, etc.
An iTaO3 substrate or a LiNbO3 substrate can be used. Alternatively, a LiNb (1-x) TaXO3 substrate (0 ≦ x ≦, which is a mixed crystal of LiTaO3 and LiNbO3,
The same light wavelength conversion element can also be manufactured by 1). In particular,
Of the above, the LiNbO3 substrate has a high non-linear optical constant, and is therefore effective for manufacturing a highly efficient optical wavelength conversion element.

(Eighth Embodiment) A method for forming a domain inversion region according to an eighth embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to (d).

First, as shown in FIG. 15 (a), the comb-shaped electrode 2 is formed on the + C surface 1a of a LiTaO3 substrate 1 (a substrate cut out in a plane perpendicular to the C axis of the crystal) of a C plate having a thickness of 200 μm, for example. , The planar electrodes 3 are formed on the -C surface 1b. These electrodes 2 and 3 can be, for example, a Ta film having a thickness of about 60 nm. In the comb-shaped electrode 2, for example, the stripe-shaped electrode branches have a period of 3.8 μm and the width of each electrode branch is 1.9 μm.

Next, a resist having a thickness of 1 μm is deposited on the comb-shaped electrode 2 and the substrate 1 not covered with the comb-shaped electrode 2 is formed by reactive ion etching in a CHF 3 gas atmosphere.
Etching the surface of. After that, by removing the resist, as shown in FIG.
+ C plane 1 of the substrate 1 between the striped electrode branches of
The groove 18 is formed in a. The removal depth by etching is, for example, 0.1 μm.

Subsequently, including the comb-shaped electrode 2 and the groove 18,
The S of 200 nm thickness is formed so as to cover the + C surface 1a of the substrate 1.
The insulating film 34 made of iO2 is deposited by the sputtering method. Then, the comb-shaped electrode 2 is grounded, and a negative pulse voltage (typically, a pulse width of 3 ms is applied to the planar electrode 3).
Then, an amplitude of 5.2 kV) is applied. In order to prevent the occurrence of discharge, the entire substrate 1 is placed in an insulating liquid or in a vacuum of 10 −6 Torr or less when a voltage is applied.

Through the above steps, the domain-inverted regions 9 are formed only under the respective electrode branches of the comb-shaped electrode 2, and the pattern of the electrode 2 is completely transferred to the domain-inverted structure. In particular, by removing the surface of the substrate 1 around the stripe-shaped electrode branches of the comb-shaped electrode 2 by etching to form the groove 18, the movement of charges on the surface of the substrate 1 is reduced. As a result, the widthwise expansion of the domain inversion region 9 is suppressed,
Uniform periodic domain inversion regions 9 are formed.

(Ninth Embodiment) As described in each of the above embodiments, by applying a voltage in the polarization direction of the ferroelectric crystal using the comb-shaped electrode, the periodic polarization inversion region Can be formed. However, depending on the crystal, there are cases where polarization inversion cannot be easily formed, or where it is difficult to form a short-period polarization inversion region even if polarization is inverted. In the present embodiment described below, a method for easily forming a domain-inverted region will be described for a crystal in which the domain-inverted region is difficult to form by such a conventional technique.

In order to facilitate the polarization reversal in the ferroelectric substrate, it is considered that a thin substrate is used and a large electric field is applied with a small inversion voltage. However, since a thin substrate does not have sufficient strength, it becomes very difficult to carry out an electrode forming process and the like. Therefore, in this embodiment, in order to easily handle a thin substrate,
A domain-inverted region is formed in the substrate by the steps shown in FIGS.

First, as shown in FIG. 16A, the comb-shaped electrode 2 is formed on the + C surface 31a of the MgO-doped LiNbO3 substrate 31 having a thickness of 0.5 mm. Next, in FIG.
As shown in (b), MgO-doped LiNbO is formed on the LiNbO3 substrate 32 having the extraction electrode 20 formed on the surface thereof.
3 Bond the substrate 31. At this time, the LiNbO3 substrate 3
2 extraction electrode 20 and MgO-doped LiNbO3 substrate 3
The comb-shaped electrode 2 of 1 is electrically contacted. Subsequently, the MgO-doped L in such a state of being adhered
The iNbO3 substrate 31 is optically polished to reduce its thickness to 50 μm as shown in FIG. Then, Figure 1
As shown in FIG. 6D, the flat electrode 3 is formed on the polished surface of the MgO-doped LiNbO 3 substrate 31. Then, as shown in FIG. 16E, the pulse power supply 5 is connected between the extraction electrode 20 of the LiNbO 3 substrate 32 and the planar electrode 3 of the MgO-doped LiNbO 3 substrate 31 to apply a pulse voltage. As a result, the domain-inverted regions 9 are formed in the MgO-doped LiNbO3 substrate 31 with a short period.

The MgO-doped LiNbO3 substrate is a promising material for the wavelength conversion element because it has a high nonlinear optical constant and excellent light damage resistance. However, it has been difficult to form a periodic domain-inverted structure with the conventional technique.
On the other hand, according to the method of the present embodiment described above, the domain-inverted regions 9 can be formed on the MgO-doped LiNbO3 substrate 31 with a period of 3 μm. Thus, the MgO-doped L in which the periodic domain-inverted regions 9 are formed
By using the iNbO3 substrate 31, it is possible to manufacture a highly efficient optical wavelength conversion element, and it is possible to generate high-output SHG light.

Note that, as described in the eighth embodiment, the comb-shaped electrode 2 is replaced by S in the process of this embodiment.
By covering with an insulating film such as an iO 2 film, the uniformity of the periodic structure of the domain-inverted regions formed can be increased.

When the optical wavelength conversion element formed according to this embodiment is applied to a pyroelectric sensor, if a material having high thermal conductivity is used as the optical substrate, the response speed of the pyroelectric sensor can be increased. it can.

Furthermore, an optical waveguide type optical wavelength conversion element can be manufactured by utilizing the polarization inversion structure formed as described above. In that case, FIG.
Before carrying out the series of steps shown in (e), first, MgO
An optical waveguide is formed on the + C surface 31a of the doped LiNbO3 substrate 31.

The optical waveguide is manufactured, for example, by the following steps. A Ta film is deposited on the + C surface 31a of the substrate 31 by sputtering, and a stripe-shaped optical waveguide pattern is formed by photolithography and dry etching. Then, heat treatment is performed in pyrophosphoric acid at 230 ° C. for 8 minutes to perform proton exchange to form a proton exchange waveguide. Then, heat treatment is further performed at 300 ° C. for 10 minutes to form an optical waveguide. After that, when the steps shown in FIGS. 16A to 16E are performed,
Periodic domain-inverted regions are formed in the optical waveguide. As a result, the striped optical waveguide 3 as shown in FIG.
The optical wavelength conversion element 900 having the number 3 can be formed.

In the optical wavelength conversion element 900, the optical waveguide 33
Fundamental wave 2 incident on the inside of the element 900 from the incident portion 15 of
3 is limitedly propagated in the optical waveguide 33. In the process, the fundamental wave 23 is converted into the second harmonic wave 24 and taken out from the emitting portion 16 to the outside. At this time, the fundamental wave 23
Is propagated only in the optical waveguide 33, so that the element 9
The power density of the fundamental wave 23 inside 00 increases. Further, the interaction length between the propagating fundamental wave 23 and the domain-inverted structure 9 also increases. As a result, the highly efficient wavelength conversion element 900 is realized.

A LiNbO 3 substrate 31 is formed on the optical waveguide 33.
By adhering, it is possible to prevent dust and the like from accumulating on the optical waveguide 33 and causing waveguide loss. Also, the optical waveguide 3
By depositing a material having a refractive index close to the refractive index of the substrate on 3, the refractive index distribution of the optical waveguide 33 can be made symmetrical. As a result, the optical waveguide 3
The electric field distribution of the light propagating in 3 has a symmetrical structure, and the fundamental wave 2
The coupling efficiency of 3 becomes higher. The comb-shaped electrodes of the light wavelength conversion element 900 of FIG. 17 must be formed of transparent electrodes in order to reduce the loss of the optical waveguide 33.

In the above description of this embodiment, the MgO-doped LiNbO3 substrate is used as the ferroelectric substrate 31. Alternatively, in addition, MgO-doped LiTaO3 substrate, Nd-doped LiNbO3 substrate, Nd-doped LiNb
O3 substrate, KTP substrate, KNbO3 substrate, Nd and MgO
LiNbO3 substrate doped with or Nd and Mg
It may be a LiTaO3 substrate doped with O and the like.

Of these, since the substrate made of Nd-doped crystal is capable of laser oscillation, it is possible to simultaneously generate the fundamental wave by laser oscillation and the second harmonic by wavelength conversion. Therefore, a short wavelength light source having highly efficient and stable operation characteristics can be manufactured.

Since the KNbO3 substrate has a high nonlinear optical constant and is excellent in light damage resistance,
A high output light wavelength conversion element can be formed.

On the other hand, in the above description of this embodiment, the LiNbO3 substrate is used as the substrate 32 attached to the ferroelectric substrate 31, but a substrate made of another material may be used as long as it is an optically flat substrate. it can. In particular, the ferroelectric substrate 3
It is preferable to use the substrate 32 made of a material having a thermal expansion coefficient equal to 1 from the viewpoint that the application of thermal strain to the ferroelectric substrate 31 can be reduced.

(Tenth Embodiment) If liquid phase crystal growth, vapor phase crystal growth, laser abrasion or the like is used,
Thin film crystals of ferroelectric materials can be formed. By using such a ferroelectric thin film crystal, the domain-inverted regions can be formed even in a material in which the periodic domain-inverted regions are difficult to form. Furthermore, by using a thin film as an optical waveguide, a highly efficient optical waveguide type optical wavelength conversion element can be constructed.

A method for forming the domain inversion region 9 in the ferroelectric thin film 30 formed by crystal growth will be described below as the tenth embodiment of the present invention.

The MgO-doped LiNbO3 crystal is
Although it is a highly nonlinear material excellent in light damage resistance, it is difficult to form a periodic domain-inverted region therein according to the conventional technique. Therefore, in the present embodiment, first, LiT
MgO-doped LiNbO3 layer 30 on aO3 substrate 31
Crystal growth, and then the grown MgO-doped Li
Periodic domain-inverted regions 9 are formed inside the NbO 3 layer 30.

A method of forming the domain-inverted region 9 in this embodiment will be described with reference to FIGS. 18 (a) to 18 (c).

First, as shown in FIG. 18 (a), the + C plane 32a of the LiTaO3 substrate 32 of C plate (the substrate cut out by the plane perpendicular to the C axis of the crystal) was doped with 5 mol% of MgO to a thickness of 2 μm. LiNbO3 layer 30 of is deposited by liquid phase epitaxy. Next, as shown in FIG. 18B, the comb-shaped electrode 2 was formed on the grown LiNbO 3 layer 30 and LiTa.
The planar electrodes 3 are respectively formed on the -C surface 32b of the O3 substrate 32. These electrodes 2 and 3 can be, for example, a Ta film having a thickness of about 60 nm. The comb-shaped electrode 2 has a stripe-shaped electrode branch period of 3.8 μm, for example.
Then, the width of each electrode branch is set to 1.9 μm. Then, a pulse voltage is applied between the comb-shaped electrode 2 and the planar electrode 3 by the pulse power supply 5. As a result, periodic domain-inverted regions 9 are formed inside the MgO-doped LiNbO3 layer 30.

Since the ferroelectric film formed by the crystal growth method has a low impurity concentration and can change the crystal structure, a highly efficient optical wavelength conversion element can be formed. Furthermore, since a thin film can be easily formed, a highly uniform periodic domain-inverted structure can be easily formed.

Note that, as described in the eighth embodiment, the comb-shaped electrode 2 is replaced by S in the process of this embodiment.
By covering with an insulating film such as iO2, it is possible to increase the uniformity of the periodic structure of the domain inversion regions 9 formed.

Further, by using the periodic domain-inverted regions 9 formed according to this embodiment, the ridge type optical waveguide 3 is formed.
A method of forming the optical wavelength conversion device 1000 having 0a will be described with reference to FIGS.

First, the Mg in which the periodic domain-inverted regions 9 are formed by the steps of FIGS.
A striped Ti film 29 as shown in FIG. 19A is formed on the O-doped LiNbO 3 layer 30. Then T
Using the i film 29 as a mask, the MgO-doped LiNbO3 layer 30 is etched by an ECR etching apparatus. After that, by removing the Ti film 29, as shown in FIG.
As shown in FIG. 9 (b), the MgO-doped LiNbO 3 layer 3
The stripe portion 30a is formed at 0. This Ti film 2
The striped portion 30a of the MgO-doped LiNbO3 layer 30 which is covered with 9 and remains unetched has a size of, for example, 6 μm in width and 0.3 μm in height. In addition, MgO-doped LiNb other than the stripe portion 30a
The thickness of the O3 layer 30 is typically 1 by etching.
It becomes as thin as 0 μm.

Since the refractive index of the MgO-doped LiNbO3 layer 30 is smaller than that of the LiTaO3 substrate 32, the MgO-doped LiN layer formed as described above is used.
The stripe portion 30a of the bO3 layer 30 is the optical waveguide 30.
functions as a. Therefore, by the steps shown in FIGS. 19A and 19B, the optical waveguide 30a having the periodic domain-inverted regions 9 is formed in the steps of FIGS. 7A to 7C.

Further, the optical wavelength conversion element 1000 is formed by optically polishing both end faces of the formed optical waveguide 30a. Since MgO-doped LiNbO3 is a material having a high nonlinear optical constant, the optical wavelength conversion element 1000 formed in this way can perform highly efficient wavelength conversion. Furthermore, since MgO-doped LiNbO3 is a material having excellent light damage resistance, wavelength conversion with high output becomes possible.

In the above description of this embodiment, the ferroelectric thin film 30 formed by liquid layer growth is used. Alternatively, the ferroelectric thin film 30 can be grown by another growth method such as vapor phase growth or laser abrasion.

In particular, when laser abrasion is used to form the ferroelectric thin film 30, a strained superlattice thin film of a ferroelectric crystal can be formed on the substrate 32. Since the superlattice thin film has a larger strain than the crystal forming the thin film, it has a large nonlinear constant. Also in the process of this embodiment, a highly efficient optical wavelength conversion element can be formed by depositing the ferroelectric thin film 30 by laser abrasion and then forming the periodic domain-inverted regions 9.

Further, the ferroelectric thin film 30 may be formed by a method other than crystal growth. For example, a thin film crystal can be formed by bonding a ferroelectric crystal onto an optical crystal substrate, and then polishing and etching the ferroelectric crystal. 18 (a) to 18 (c) for a thin film formed by laminating such crystals.
In the step of forming a periodic domain-inverted region,
An optical waveguide is formed in the steps of (a) and FIG. 19 (b).
By using such crystal bonding, it is possible to form a periodic domain-inverted structure even in a material in which crystal growth is difficult, for example, a nonlinear material such as KNbO3, KTP, BBO.

(Eleventh Embodiment) In this embodiment, a method of manufacturing an optical wavelength conversion device using a domain-inverted region formed by the steps described in the eighth embodiment will be described. The structure of the formed light wavelength conversion element is shown in FIG.

In order to realize a high-performance light wavelength conversion element, it is necessary to form a short period periodic domain inversion region having a uniform structure over a long distance. For example, LiNb
In order to generate blue light in the wavelength range of 400 nm by wavelength conversion using a domain-inverted structure formed on a crystal such as O3, LiTaO3, or KTP, a domain-inverted region having a period of 3 to 4 μm has a length of 10 mm. It is necessary to form it uniformly to some extent. As described above, in order to form a domain-inverted region having a short period, it is necessary to suppress the spread of the domain-inverted region in the electrode width direction as much as possible, and in order to form a uniform periodic structure, Uniformity is required. In consideration of the above points, the domain-inverted region obtained by the manufacturing method shown in the eighth embodiment is very effective for manufacturing a highly efficient optical wavelength conversion element.

The manufacturing method of the light wavelength conversion element is shown in FIG.
This is substantially the same as the method described with reference to (a) to (d).

That is, LiT having a thickness of 0.2 mm
The comb-shaped electrode 2 is formed on the + C1a surface of the aO3 substrate 1 and the -C surface 1b.
The planar electrode 3 is formed on the. The cycle of the comb-shaped electrode 2 is 3.8μ
The width of each stripe electrode branch forming the comb-shaped electrode 2 is 1.9 μm. On the other hand, the size of the flat electrode 3 is 3 mm × 10 mm. After forming the comb-shaped electrode 2,
The surface of the substrate 1 around each stripe electrode branch (+ C plane 1
The groove 18 is formed by etching a) by 100 nm.
Then, a 200 nm-thickness SiO2 layer 34 is deposited on the comb-shaped electrode 2 on the + C surface 1a by a sputtering method.
After that, a pulse voltage is applied between the electrodes 2 and 3.
The pulse voltage to be applied has, for example, a pulse width of about 3 ms.
And the peak value is 5.1 kV.

Through the above steps, the periodic domain-inverted regions 9 having a width of 1.9 μm and a duty ratio of 50% are uniformly formed in the substrate 1 over the length of 10 mm. In particular, the domain-inverted region 9 formed reaches the bottom of the substrate 1, that is, the -C plane 1b.

After that, as shown in FIG. 20, the entrance surface 25 and the exit surface 26 of the substrate 1 are optically polished to a wavelength of 850.
A 145 nm thick SiO2 film 19 that functions as an antireflection film 19 for the fundamental wave 23 of nm is deposited. Thereby, the optical wavelength conversion element 1100 shown in FIG. 20 is configured.

Light of a Ti: Al2O3 laser as a fundamental wave 23 is incident on the manufactured light wavelength conversion element 1100,
The SHG characteristics were measured. Specifically, the light (fundamental wave) 23 emitted from the laser 21 is condensed and incident on the incident surface 25 of the light wavelength conversion element 1100 by the condensing optical system 22.
The incident fundamental wave 23 is wavelength-converted when propagating inside the element 1100, and is emitted from the emission surface 26 as a second harmonic (SHG light) 24 having a half wavelength of the fundamental wave 23. The conversion efficiency from the fundamental wave 23 to the second harmonic wave 24 is
It becomes maximum when the focused spot diameter is φ37 μm.

FIG. 21 shows the relationship between the wavelength of the input fundamental wave 23 (phase matching wavelength) and the power of the output second harmonic wave 24 (SHG output). The focused spot diameter of the fundamental wave 23 is fixed to the above-mentioned φ37 μm. As shown in FIG. 21, the SHG output becomes maximum when the phase matching wavelength is 850 nm, and the full width at half maximum of the wavelength tolerance at this time is 0.12 nm. This value is the theoretical value of 0.1
The value is very close to nm, and the periodically poled structure is
This means that the element is formed uniformly over a length of 10 mm.

Next, the power of the input fundamental wave 23 and S
The relationship with the HG output is shown in FIG. When the fundamental wave input power is 300 mW, an SHG output of 4.2 mW is obtained. The conversion efficiency at this time is 1.4%.
This value is equal to the theoretical value, indicating that the formed domain-inverted structure has an ideal shape.

The groove 18 formed between the domain-inverted regions 9 near the surface of the substrate 1 has a function of preventing deterioration of the domain-inverted regions 9. When polarization inversion is caused by applying an electric field, a deep polarization inversion region 9 can be formed, but a large strain is applied to the crystal of the substrate 1. Such strain causes the formed domain-inverted region 9 to change with time. For example, in several weeks to several months, the shape of the domain-inverted region 9 may gradually change and the operating characteristics of the optical wavelength conversion element 900 may change. On the other hand, the polarization inversion region 9
By forming the groove 18 between the two, it is possible to prevent such a change in shape of the domain-inverted region 9 and to form a stable optical wavelength conversion element with no change in operating characteristics over time.

When forming the light wavelength conversion element, the width W of the stripe-shaped electrode branch included in the comb-shaped electrode 2 is set to the comb-shaped electrode 2.
It is preferable to have a relationship of W ≦ Λ / 2 with the period Λ of. The reason will be described below.

Under the applied voltage condition in which the polarization inversion is uniformly formed over the entire electrode, the width Wd of the polarization inversion region formed under the striped electrode branch is slightly wider than the width W of the electrode branch. On the other hand, the efficiency of the light wavelength conversion element becomes maximum when the relationship between the period Λ and the width Wd of the domain inversion region is Λ / 2 = Wd. Therefore, in order to set the value of Wd to Λ / 2, the width W of the electrode should be set equal to or smaller than Λ / 2 in consideration of the widthwise extension of the domain-inverted region. Is preferred.

Further, according to this embodiment, a resonator type optical wavelength conversion element can be formed. In that case, in the configuration of FIG. 20, after polishing both end faces of the LiTaO 3 substrate 1 in which the periodic domain-inverted regions 9 are formed, the fundamental wave 2 having a wavelength of 800 nm is used instead of the antireflection film 19.
A reflective film 14 that reflects 99% or more of 3 is deposited. When the fundamental wave 23 is incident on such a light wavelength conversion element, the fundamental wave 23 is multiply reflected by the reflection films 14 formed on both end surfaces of the substrate 1 and resonates inside the substrate 1. That is, the light wavelength conversion element functions as a resonator, and the increased fundamental power converts the incident fundamental wave 23 into the second harmonic wave 24 with high efficiency.

As described above, in order for the optical wavelength conversion element to function as a resonator, the domain-inverted region is reached to a position deeper than the beam diameter of the fundamental wave 23 that is multiply reflected, typically to a depth of several tens of μm or more. 9 should be formed uniformly. By applying the electric field, the uniform periodic domain-inverted regions 9 are formed to a depth of about several hundreds of μm, so that a highly efficient resonator type optical wavelength conversion element can be manufactured.

(Twelfth Embodiment) In this embodiment, a method of manufacturing a highly efficient optical wavelength conversion device having a high power density and a long interaction length will be described. Specifically, an optical waveguide is formed inside the formed uniform and short-period domain-inverted region. Specifically, first, periodic domain-inverted regions are formed on the LiTaO3 substrate by the method described in the above-described embodiments. After that, an optical waveguide is formed by a proton exchange treatment.

As a forming method at this time, for example, the following processes can be considered. A Ta mask layer corresponding to the pattern of the optical waveguide to be formed is formed on the + C surface of the substrate on which the periodic domain-inverted regions are formed, and the temperature is set to 260 ° C.
Heat treatment for 16 minutes in pyrophosphoric acid and 4 in air
A proton exchange waveguide is formed by heat treatment at 20 ° C. for 5 minutes.

However, when both end surfaces of the optical waveguide formed by the above process are optically polished and the output characteristics of the SHG light output by inputting the fundamental wave into the optical waveguide are measured,
Only about half the theoretical conversion efficiency can be obtained. As for the cause of this low conversion efficiency, the inventors have found that a part of the periodic domain-inverted region disappears inside the optical waveguide. That is, it was revealed that the conversion efficiency was lowered because the domain-inverted region that had been formed disappeared in the range from the surface to a depth of about 0.6 μm by the manufacturing process of the optical waveguide. Also, L
In the iNbO3 substrate or the substrate of the mixed crystal of LiNbO3 and LiTaO3, the recession from the surface portion of the domain inversion region is similarly observed.

Therefore, in order to prevent the polarization inversion region from being affected by the manufacturing process of the optical waveguide, in the present embodiment, the optical waveguide 11 is manufactured by the steps shown in FIGS.

As shown in FIG. 23A, the + C plane 1 of the substrate 1 on which periodic domain-inverted regions (not shown) are formed.
A Ta mask layer 10 corresponding to the pattern of the optical waveguide 11 to be formed is formed on a. Next, heat treatment in pyrophosphoric acid at 260 ° C for 20 minutes and 420 ° C in air.
23B, the proton exchange waveguide 11 is formed in the portion of the substrate 1 corresponding to the opening of the Ta mask layer 10 by heat treatment for 5 minutes. After that,
By reactive ion etching in CHF3 gas, T
While the a mask layer 10 is removed, the surface of the substrate 1 is further removed by etching by 0.5 μm. At this time, the surface portion of the proton exchange optical waveguide 11 is similarly removed by etching to remove the deteriorated portion of the periodic domain-inverted region existing near the surface of the optical waveguide 11.

When both end faces of the optical waveguide 11 formed by the above process are optically polished and the output characteristics of the SHG light output by inputting the fundamental wave into the optical waveguide are measured, for example, a fundamental wave input of 100 mW is input. A second harmonic output of 20 mW was obtained. The conversion efficiency at this time is a value equal to the theoretical value, and by the process based on this embodiment,
A highly efficient light wavelength conversion element has been obtained.

In the above description of this embodiment, the optical waveguide 11 is formed on the + C surface 1a of the substrate 1. However, the domain inversion region is the back surface of the substrate 1, that is, −
Since it is formed until it reaches the C surface 1b, the optical waveguide 1
Even if 1 is formed on the -C surface 1b of the substrate 1, it is possible to manufacture an optical wavelength conversion element having similar performance. When the optical waveguide 11 is thus formed on the -C plane 1b, the -C plane 1
Since only the planar electrode is formed on the surface b and the pattern of the comb-shaped electrode is not formed, the surface is less rough. Therefore, a waveguide with less waveguide loss can be manufactured, and a highly efficient optical wavelength conversion element can be manufactured.

As the optical waveguide, other optical waveguides such as Ti diffusion waveguide, Nb diffusion waveguide, and ion implantation waveguide can be used instead of the waveguide formed by the above-mentioned proton exchange treatment.

Of these, the diffusion temperature must be set to 1000 ° C. or higher in order to manufacture the optical waveguide by utilizing the diffusion treatment. However, the Curie temperature of LiTaO3 is 600 ° C. and the Curie temperature of LiNbO3 is 1000 ° C., both of which are equal to or lower than the diffusion temperature. Therefore, when the optical waveguide is formed by the diffusion process according to the conventional technique after forming the domain-inverted regions, all the domain-inverted regions that have been formed disappear. On the other hand, by forming the domain-inverted region after forming the optical waveguide, it is possible to form the periodic domain-inverted region inside the optical waveguide formed by diffusion, which makes it possible to manufacture a highly efficient optical wavelength conversion element. It will be possible.

In the proton exchange treatment, orthophosphoric acid, benzoic acid, sulfuric acid, etc. can be used in addition to the above-mentioned treatment using pyrophosphoric acid.

Also, the mask for the proton exchange treatment is not limited to the Ta mask, but Ta2O5, P
A mask made of a material having acid resistance such as t or Au may be used.

(Thirteenth Embodiment) As a thirteenth embodiment of the present invention, referring to FIGS. 24 (a) and 24 (b), a bulk type optical wavelength conversion device having a modified periodic polarization inversion structure. 1300 will be described.

According to the present invention, in the bulk type optical wavelength conversion device which uses the substrate on which the periodic type polarization inversion region is formed as a bulk, the device is inclined at a certain angle with respect to the optical axis of the incident fundamental wave. It is possible to change the polarization inversion period for the light traveling inside the element. By utilizing this point, it is possible to adjust the fluctuation of the oscillation wavelength of the incident fundamental wave and the fluctuation of the phase matching wavelength due to the change of the environmental temperature.

However, the period range of the domain-inverted structure which can be varied by adjusting the angle of the element is defined by Snell's law which depends on the refractive index of the substrate. Therefore,
The period cannot be changed significantly.

For example, in the case of an optical wavelength conversion element in which a domain-inverted region is formed inside a LiTaO3 substrate in parallel to its end face, the optical wavelength conversion element is 12 with respect to the optical axis of the fundamental wave incident on the substrate.
When tilted, the period of the domain-inverted structure is only 1.02 times as large as when the fundamental wave is perpendicularly incident on the incident surface of the device.

Therefore, the light wavelength conversion element 13 of this embodiment is used.
At 00, as shown in FIG. 24A, inside the substrate 1, the incident surface 15 and the emitting surface 16, or at least the domain-inverted region 9 inclined at an angle θ with respect to the incident surface 15 is formed. For that purpose, in the manufacturing process of the domain-inverted regions 9 in the embodiments described so far,
When forming the comb-shaped electrode 2 on the + C surface 1a of the substrate 1,
It may be formed by inclining by an angle θ with respect to the end surface of the substrate 1. Since other features of the forming process are substantially the same, the description thereof is omitted here.

As described above, the domain-inverted regions 9 are the incident surface 15 and the exit surface 16 of the substrate 1, or at least the incident surface 1.
In the optical wavelength conversion element 1300 which is tilted at an angle θ with respect to 5, the polarization inversion period for light traveling inside the element 1300 by tilting the element 1300 with respect to the optical axis of the incident fundamental wave 23. When adjusting the phase matching range by changing, it becomes possible to adjust over a wider range.

For example, in the case of the optical wavelength conversion element 1300 in which the domain-inverted region 9 is formed inside the LiTaO 3 substrate 1 in the direction inclined by 45 degrees with respect to the incident surface 15, the light is emitted from the laser 21 and condensed. When the substrate 1 is tilted 12 degrees with respect to the optical axis of the fundamental wave 23 that is incident through the optical system 22,
The period of the domain-inverted structure is 1.12 times that of the case where the fundamental wave 23 is incident vertically on the incident surface 15 of the element 1300. Thus, compared to prior art devices,
The range of angle adjustment is increased more than 5 times. As a result, the tolerance of the phase matching wavelength is expanded and the optical wavelength conversion element 130 is expanded.
0 can be used more easily.

Furthermore, by tilting the domain inversion region 9 with respect to the incident beam, it becomes possible to expand the tolerance of the phase matching temperature. This occurs because the focused beam (fundamental wave) obliquely crosses the periodic domain-inverted structure, so that phase matching of components having an angle with respect to the optical axis of the focused beam occurs in a wide range. Is.

In FIG. 24A, the polarization inversion region 9
Are inclined with respect to both the incident surface 15 and the exit surface 16 of the substrate 1, but the exit surface 16 and the polarization inversion region 9 are formed.
You may form so that it may become parallel with.

Next, a bulk type optical wavelength conversion device 1300
A method for separating the fundamental wave 23a and the higher harmonic wave 24, which are emitted from the emission surface 16 in a, will be described with reference to FIG.

In the optical wavelength conversion element, the fundamental wave 23 incident from the laser 21 through the condensing optical system 22 from the incident surface 15 of the element 1300a is converted into a harmonic wave 24 while propagating inside the element 1300a. After that, the converted higher harmonic wave 24 is emitted from the emitting surface 16, but at this time, the unconverted fundamental wave component 23a also emerges from the emitting surface 16 to the outside at the same time. Therefore, it becomes necessary to separate the unconverted fundamental wave component 23a from the converted harmonic wave 24.

At this time, as shown in FIG. 24B, the light of the fundamental wave 23 incident on the emission surface 16 of the bulk type optical wavelength conversion element 1300a in which the periodic domain-inverted regions 9 are formed. When tilted with respect to the axis, the refractive index felt by each is different due to the wavelength dispersion of the fundamental wave and harmonics,
The fundamental wave 23a and the harmonic wave 24 can be emitted at different emission angles (that is, θ1 and θ2) to separate them. Specifically, since the refractive index nf felt by the fundamental wave and the refractive index ns felt by the higher harmonic are different inside the element 1300a, the emission angles for the respective elements are different based on Snell's law.

(Fourteenth Embodiment) In the present embodiment, the results of the study by the present inventors regarding the influence of the process of forming the optical waveguide on the periodic domain-inverted region will be further explained, and more preferable based on the result of the study. A method of manufacturing the optical wavelength conversion element including the process of forming the optical waveguide will be described.

Consider a process of forming an optical waveguide by subjecting the + Ta plane of the LiTaO 3 substrate on which the periodically domain-inverted regions have been formed, to a proton exchange treatment. For example,
For such a proton exchange treatment, the substrate is heated to a temperature of 26
It is immersed in pyrophosphoric acid at 0 ° C. for a heat treatment for 16 minutes, and then an annealing treatment is performed at a temperature of 420 ° C. for 5 minutes in the air. This process is a low temperature process performed at a temperature lower than the Curie temperature (about 600 ° C.) of the LiTaO 3 substrate.

However, when the optical waveguide is formed by the above process, the periodic domain-inverted regions formed are
It may disappear in the depth direction from the + C surface of the substrate. The depth at which the domain-inverted region disappears keeps a constant value after increasing to some extent with the annealing time, but if the annealing condition is constant, it mainly depends on the time of the proton exchange treatment as shown in FIG. To do. Therefore, it is considered that the formed polarization inversion region disappears due to the thermal diffusion of the proton during the proton exchange. Even if the annealing temperature is lowered to 300 ° C., the polarization inversion region may still disappear, which causes a problem that the optical wavelength conversion element that realizes highly efficient wavelength conversion cannot be stably manufactured.

The disappearance of the domain-inverted region described above occurs only on the + C plane of the substrate. On the −C plane, disappearance of the domain-inverted region due to the formation of the optical waveguide is not recognized.

In this embodiment, in order to prevent the influence of the disappearance phenomenon of the domain-inverted region due to the implementation of the manufacturing process of the optical waveguide, the optical waveguide is formed by the proton exchange treatment prior to the formation of the domain-inverted region. A wavelength conversion element is manufactured.

26 (a) to 26 (d) are perspective views schematically showing steps of manufacturing the optical wavelength conversion element 1410 having the stripe-shaped embedded optical waveguide 11 according to the present embodiment. .

First, a Ta mask layer (not shown) having openings corresponding to the pattern of the optical waveguide is deposited on the surface of the substrate 1, and a proton exchange treatment is performed by a heat treatment in pyrophosphoric acid. As a result, the stripe-shaped embedded proton exchange optical waveguide 11 as shown in FIG. 26A is formed. After that, an annealing treatment is performed to reduce the difference in polarization inversion characteristics between the portion where the proton exchange is performed (optical waveguide 11) and the other portion. After that, FIG.
As shown in (b), the comb-shaped electrode 2 and the plane electrode 3 are formed on the + C surface and the -C surface of the substrate 1, respectively. Then, a predetermined electric field is applied to the substrate 1 through the electrodes 2 and 3 to form periodic domain-inverted regions 9 as shown in FIG. Further, if the comb-shaped electrode 2 and the plane electrode 3 are removed, the optical wavelength conversion element 1 having the embedded optical waveguide 11 in the stripe shape shown in FIG.
410 is obtained.

27 (a) to 27 (d) are perspective views schematically showing steps of manufacturing the optical wavelength conversion element 1420 having the ridge-shaped optical waveguide 17a according to the present embodiment.

First, the + C plane of the substrate 1 is subjected to a proton exchange treatment and an annealing treatment to form a proton exchange layer (slab type optical waveguide) 17 shown in FIG. 27 (a).
Then, as shown in FIG. 27B, the comb-shaped electrode 2 and the planar electrode 3 are formed on the back surface (-C surface) of the proton exchange layer 17 and the substrate 1, respectively. Then, a predetermined electric field is applied to the substrate 1 through these electrodes 2 and 3, and
Periodic domain-inverted regions 9 as shown in (c) are formed inside the substrate 1. After that, the comb-shaped electrode 2 and the planar electrode 3
And the proton exchange layer 17 is processed into a stripe shape to form a ridge-type optical waveguide 17a. As a result, the optical wavelength conversion element 1420 having the stripe-shaped ridge-type optical waveguide 17a shown in FIG. 27D is obtained.

In the embedded optical waveguide 11, there is a slight difference in the proton distribution between the portion where the proton exchange was performed (optical waveguide 11) and the other portions. On the other hand, in the above process of forming the ridge-type optical waveguide 17a, since the proton exchange layer 17 is formed over the entire surface of the substrate 1, the proton distribution in the electrode surface during the application of the electric field for forming the domain-inverted region. There is no non-uniformity.
As a result, a domain-inverted region having a uniform in-plane distribution can be formed. Further, since the proton exchange layer 17 is also present on the side surface of the optical waveguide 17a, the strength against mechanical breakage is improved and the light damage resistance is also excellent. As a result, the ridge-type optical waveguide 17a has twice or more the optical damage resistance strength as compared with the case of the embedded optical waveguide 11.

28 (a) to 28 (d), a stripe-shaped high refractive index layer 44 is further provided on the surface of the proton exchange layer (slab type optical waveguide) 17 as a loading type optical waveguide 44. It is a perspective view which shows typically the process of manufacturing the wavelength conversion element 1430 according to this embodiment.

First, the + C plane of the substrate 1 is subjected to a proton exchange treatment and an annealing treatment to form a proton exchange layer (slab type optical waveguide) 17 shown in FIG. 28 (a).
After that, as shown in FIG. 28B, the comb-shaped electrode 2 and the planar electrode 3 are formed on the back surface (-C surface) of the proton exchange layer 17 and the substrate 1, respectively. Then, a predetermined electric field is applied to the substrate 1 through these electrodes 2 and 3, and
Periodic domain-inverted regions 9 as shown in (c) are formed inside the substrate 1. After that, the comb-shaped electrode 2 and the planar electrode 3
And the stripe-shaped high refractive index layer 44 is formed on the proton exchange layer 17. As a result, the optical wavelength conversion element 1430 having the stripe-shaped optical waveguide 44 shown in FIG. 28D is obtained.

Since the optical waveguide 44 in the optical wavelength conversion element 1430 has a stronger confinement characteristic than the ridge type optical waveguide 17a, a highly efficient optical wavelength conversion element can be obtained.

As described above, by forming the optical waveguide prior to the formation of the domain-inverted regions, it is possible to prevent the periodic domain-inverted structure from disappearing. As a result, an optical wavelength conversion element having excellent light damage resistance and realizing highly efficient wavelength conversion can be obtained.

(Fifteenth Embodiment) FIG. 29 (a) -FIG.
A method of forming a domain-inverted region according to the present embodiment using front-side inversion of polarization and re-inversion in a partial region will be described with reference to (d).

As shown in FIG. 29 (a), LiTaO3
Planar electrodes 43 and 3 are formed on the + C surface 1a and the −C surface 1b of the substrate 1, respectively. At this time, the polarization inside the substrate 1 is directed upward in the drawing, as indicated by an arrow 41a in FIG.

Next, an electric field is applied to the substrate 1 at room temperature through the electrodes 3 and 43 to invert the polarization in almost the entire area inside the substrate 1. As a result, as shown in FIG. 29B, the domain-inverted region 41 having the downward polarization direction in the drawing indicated by the arrow 41b is formed.

After that, the flat electrode 43 on the + C surface 1a of the substrate 1 is removed, and instead, the comb-shaped electrode 2 is formed as shown in FIG. 29 (c). Then, a voltage is applied to the comb-shaped electrode 2 and the plane electrode 3 to apply a predetermined electric field to the substrate 1 to re-invert the polarization of the region 49 immediately below each stripe-shaped electrode branch of the comb-shaped electrode 2. by this,
As shown in FIG. 29 (d), a region 49 having a polarization direction in the direction indicated by an arrow 41a and a region 41 having a polarization direction in the opposite direction indicated by an arrow 41b are periodically arranged alternately. Form.

According to the above method, the polarization is inverted once in almost the entire area of the inside of the substrate 1 to obtain the -C plane 1b.
A uniform periodic domain-inverted structure is formed even in the vicinity of. In particular, in the -C plane 1b, the polarization inversion region does not disappear when the optical waveguide is formed by using the proton exchange treatment and the annealing treatment. Therefore, a highly efficient optical wavelength conversion element can be realized by forming the periodic domain-inverted structure according to the present embodiment and then forming the optical waveguide on the -C plane.

[0339]

As described above, according to the present invention,
Comb-shaped electrodes and flat electrodes are formed on the + C surface and the -C surface of the substrate, respectively, and a DC electric field on which a pulse electric field is superimposed is applied to the substrate via both electrodes. This makes it possible to apply an electric field having a uniform intensity distribution to the substrate and form a domain-inverted region having a uniform periodic structure. Further, a highly efficient optical wavelength conversion element is manufactured by further forming an optical waveguide on the formed periodic polarization inversion region or using the formed periodic polarization inversion region as it is as a bulk.

In order to form a domain-inverted region having a short period, it is necessary to prevent the domain-inverted region from expanding laterally from immediately below the comb-shaped electrode. On the other hand, the gap between the stripe-shaped electrode branches of the comb-shaped electrode on the surface of the substrate is subjected to a proton exchange treatment to deteriorate the ferroelectricity of that portion, thereby suppressing the lateral inversion of the domain-inverted region. To be done. As a result, the domain inversion region is formed with a short period. As a result, a highly efficient light wavelength conversion element is manufactured.

Alternatively, a comb-shaped electrode and a planar electrode covered with an insulating film are formed on the front surface and the back surface of the ferroelectric crystal, respectively, and a pulsed electric field is applied between both electrodes, whereby the free charge on the surface of the substrate is reduced. Can be suppressed, and an electric field having a uniform intensity distribution can be applied to the crystal. As a result, a domain-inverted region having a uniform periodic structure is formed.

Further, the crystal surface between the striped electrode branches of the comb-shaped electrode is cut to form a groove, and then an insulating film is formed on the crystal surface to apply an electric field, thereby forming a uniform periodic structure. A domain inversion region having the same can be formed.

By forming an optical waveguide in the formed periodic polarization inversion region, it is possible to manufacture a highly efficient optical wavelength conversion device.

In order to form a domain-inverted region having a short period, it is necessary to prevent the domain-inverted region from expanding laterally from immediately below the comb electrodes. At this time, LiTaO3, LiN
Surface free charges of a ferroelectric substrate such as bO3 move on the crystal surface when an electric field is applied, and cause nonuniformity of the electric field distribution in the electrode peripheral portion. It has been discovered by the study of the inventors that such non-uniformity of the electric field distribution promotes lateral expansion of the domain inversion region. Therefore, in order to suppress the movement of such surface free charges, by covering the entire comb-shaped electrode formed on the surface where the polarization inversion nuclei are formed with an insulating film, lateral expansion of the polarization inversion region is suppressed. As a result, the domain inversion region is formed with a short period. This allows
A highly efficient light wavelength conversion element is manufactured.

[Brief description of drawings]

FIG. 1A is a perspective view showing a method of forming a domain-inverted region according to an embodiment of the present invention, and FIG. 1B is a schematic diagram showing a change with time of an electric field strength applied to a substrate for forming a domain-inverted region. Graph

FIG. 2A is a waveform of a voltage applied to a substrate in the conventional method (b) A waveform of a reversal current flowing therewith (c) A waveform of a voltage applied to a substrate of the present invention (d) A waveform of a reversal current flowing therewith ( e) Examples of other applied voltage waveforms in the present invention

FIG. 3 is a graph showing a relationship between a change rate of a pulse applied voltage and a voltage value at which re-reversal current flows.

FIG. 4A is a graph showing the relationship between the intensity of a DC electric field applied to a substrate and the thickness of a substrate on which a domain-inverted region can be formed. FIG. 4B is the intensity of a DC electric field applied to a substrate and the width of a domain-inverted region. Graph showing the relationship with

FIG. 5 is a perspective view showing a process of forming a light wavelength conversion element according to an embodiment of the present invention.

6 is a perspective view of a light wavelength conversion element manufactured in the process of FIG.

FIG. 7 is a sectional view showing a method of forming a domain inversion region according to an embodiment of the present invention.

FIG. 8A is a sectional view showing the positional relationship between the comb-shaped electrode and the domain-inverted region. FIG. 8B is a graph showing the relationship between the applied voltage and the width of the domain-inverted region.

FIG. 9 is a cross-sectional view showing a method of forming a domain inversion region according to an embodiment of the present invention.

FIG. 10 is a sectional view showing the relationship between the annealing temperature and the shape of the domain inversion region.

FIG. 11 is a perspective view showing a process of forming a light wavelength conversion element according to an embodiment of the present invention.

FIG. 12 is a perspective view showing an optical wavelength conversion element according to an embodiment of the present invention.

FIG. 13 is a sectional view showing a method of forming a domain inversion region according to an embodiment of the present invention.

14A is a graph showing the relationship between the applied voltage and the width of the domain inversion region, FIG. 14B is a cross-sectional view schematically showing the shape of the domain inversion region to be formed, and FIG. 14C is the shape of the domain inversion region to be formed. Sectional view schematically showing

FIG. 15 is a sectional view showing a method of forming a domain inversion region according to an embodiment of the present invention.

FIG. 16 is a cross-sectional view showing a method of forming a domain inversion region according to an embodiment of the present invention.

17 is a perspective view of an optical wavelength reversal element manufactured by using the polarization reversal region obtained by the process of FIG.

FIG. 18 is a sectional view showing a method of forming a domain inversion region according to an embodiment of the present invention.

FIG. 19 is a perspective view showing a process of forming a light wavelength conversion element according to an embodiment of the present invention.

FIG. 20 is a perspective view showing an optical wavelength conversion element according to an embodiment of the present invention.

FIG. 21 is a graph showing the relationship between the input fundamental wave and the output second harmonic power in the optical wavelength conversion element of the present invention.

FIG. 22 is a graph showing the relationship between the power of the input fundamental wave and the power of the output second harmonic wave in the optical wavelength conversion element of the present invention.

FIG. 23 is a cross-sectional view showing a method of forming an optical waveguide according to an embodiment of the present invention.

FIG. 24A is a perspective view showing an optical wavelength conversion element according to an embodiment of the present invention, and FIG. 24B is a cross-sectional view illustrating a method of separating a converted higher harmonic wave from an unconverted fundamental wave.

FIG. 25 is a graph showing the relationship between the processing time of proton exchange for forming an optical waveguide and the depth at which polarization inversion disappears.

FIG. 26 is a perspective view showing a process of forming a light wavelength conversion element according to an embodiment of the present invention.

FIG. 27 is a perspective view showing a process of forming a light wavelength conversion element according to an embodiment of the present invention.

FIG. 28 is a perspective view showing a process of forming a light wavelength conversion element according to an embodiment of the present invention.

FIG. 29 is a schematic cross-sectional view showing a method for forming a domain-inverted region according to an embodiment of the present invention.

FIG. 30 is a schematic perspective view showing a method of forming a domain-inverted region in a conventional method.

[Explanation of symbols]

1, 32, 55 substrate 2.51 comb-shaped electrode 3,52 flat electrode 4 DC power supply 5 pulse power supply 7,17 Proton exchange region 8 micro domains 9 Polarization reversal region 10 Ta mask layer 11, 33, 44 Optical waveguide 12 Resist 14 Reflective film 15 incident part 16 Output part 18 grooves 19 Antireflection film 20 Extraction electrode 21 laser 22 Focusing optical system 23 fundamental wave 24 Second harmonic 25 Incident end face 26 Emitting end face 29 Ti film 30 MgO-doped LiNbO3 thin film 31 MgO-doped LiNbO3 substrate 34 Insulating film

Continuation of front page (56) Reference JP-A 64-18121 (JP, A) JP-A 5-289134 (JP, A) JP-A 6-186603 (JP, A) JP-A 6-281982 (JP , A) JP-A-5-173213 (JP, A) JP-A-60-30115 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/37 G02B 6/12 C30B 29/30 C30B 33/00 JISST file (JOIS)

Claims (3)

(57) [Claims] 1. C cut out in a plane perpendicular to the C axis of the crystal
Forming a comb-shaped electrode on the first surface of the ferroelectric crystal substrate that is a plate substrate ; adhering an optical substrate to the first surface of the ferroelectric crystal substrate; and attaching the optical substrate to the first surface. After adhering, a step of polishing the second surface of the ferroelectric crystal substrate, a step of forming a flat electrode on the polished surface of the ferroelectric crystal substrate, and a predetermined step between the comb-shaped electrode and the flat electrode. Applying a voltage to invert the polarization of a predetermined region inside the ferroelectric crystal substrate. 2. A step of growing a ferroelectric crystal on the surface of a substrate in the C-axis direction, and a step of growing the ferroelectric crystal on the surface of the substrate in the C-axis direction, and thereafter, the surface of the substrate. to form a comb-shaped electrode, a step of forming a flat electrode on the back surface of the substrate, comb-type electrodes and said flat
The surface electrodes are separated from each other in the polarization direction of the ferroelectric crystal.
A step of urchin formed, by applying a predetermined voltage between the comb electrodes and said planar electrode, a step of reversing the polarization of a predetermined area of the interior of the ferroelectric crystal, the polarization inversion region encompassing Production method. 3. The method for producing a domain-inverted region according to claim 2, wherein the surface of the ferroelectric crystal is a surface on which nuclei for domain-inversion are generated.