JPH1172809A - Optical wavelength conversion element and its production, optical generator using this element and optical pickup, diffraction element as well as production of plural polarization inversion part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for producing polarization inversion parts improved in characteristics, such as depth, uniformity and effective areas, which are of problem in application to optical wavelength conversion elements, etc., and to provide various kinds of the elements and optical devices using these polarization inversion parts. SOLUTION: The polarization inversion parts 4 are grown from the comb- shaped electrodes 2a of the +C face of a ferroelectric liquid crystal substrate 1 toward the -C face and are subjected to an annealing treatment to decrease the internal electric fields and thereafter the parts made nonuniform in the process of growth are removed by reinverting the polarization from the comb- shaped electrodes 2a of the -C face, by which the uniformity of the polarization inversion parts is improved. The uniform periodic structure of the polarization inversion parts is utilized for the optical wavelength conversion elements, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element to which a coherent light source is applied and which is used in the fields of optical information processing and optical measurement, and the polarization included in such an optical wavelength conversion element. The present invention relates to a method for manufacturing a reversing part. Further, the present invention relates to a light generation device and an optical pickup using the light wavelength conversion element. Further, the present invention relates to a diffraction element used in the above technical field and utilizing a domain-inverted portion.

[0002]

2. Description of the Related Art Polarization reversal, which partially reverses the polarization of a single-polarized ferroelectric crystal, enables control of light waves such as nonlinear optical effects, electro-optical effects, and acousto-optical effects. It is applied in a wide range of fields such as measurement. Above all, application of polarization reversal to an optical wavelength conversion element utilizing a nonlinear optical effect has been actively studied since a small short-wavelength light source can be realized by optical wavelength conversion of a semiconductor laser.

[0003] A conventional method of manufacturing a domain-inverted portion (domain-inverted region) is that a comb-shaped electrode and a bar-shaped electrode (striped electrode) are formed on a substrate surface of an X-plate or Y-plate ferroelectric crystal whose substrate surface is parallel to the direction of polarization. Are formed, and a domain-inverted portion is formed by applying a voltage between the electrodes (Japanese Patent Laid-Open No. 4-335620).
An example of a conventional method for manufacturing a domain-inverted portion will be described with reference to FIG. According to this method, a ferroelectric crystal (eg, LiNb
The first electrode 202 and the second electrode 203 are formed in the C direction (Z direction), which is the polarization direction of the X-plate substrate 201 as O ₃ ), and a voltage is applied between the electrodes by a power supply 205. Then, the polarization of the crystal between the electrodes is inverted, and a periodic polarization inversion portion 204 is formed.

Further, as shown in FIG. 47, one electrode 203 is formed on the side surface of the substrate, and a voltage is applied between the electrode 203 and the other electrode 202 formed on the substrate surface to form a domain-inverted portion 204. A method is also disclosed.

As another conventional method for manufacturing a domain-inverted portion, there has been proposed a method using a substrate in which the crystal polarization direction is inclined from the substrate surface (hereinafter sometimes referred to as an “oblique substrate”). This method differs from the structure shown in FIG. 46 in that the direction of spontaneous polarization of the substrate is inclined with respect to the crystal surface. In this case, when a voltage is applied between the electrodes, the domain-inverted portions are formed so as to dive into the substrate in parallel with the spontaneous polarization of the crystal, that is, along the inclined crystal axis. For this reason, the domain-inverted portions are formed to dig deeper into the substrate.

FIG. 48 shows another example of a conventional method for manufacturing a domain-inverted portion. According to the method as shown in FIG. 48 using a pair of electrodes formed on opposing surfaces of the substrate, the domain-inverted portions are formed in the thickness direction of the substrate from the comb-shaped electrode 202 to the other electrode 203. .

The amount of charge required to form the domain-inverted portion is generally given by (spontaneous polarization Ps) × (electrode area) × 2. On the other hand, the extension of the domain-inverted portion is determined by the value of the ratio W / Λ between the electrode period Λ and the electrode width W, and is considered to extend by a value independent of the thickness of the substrate. For example, the domain-inverted portions formed by the method shown in FIG.
The area of the region where the domain-inverted portions were formed was about 1 μm ² .

FIG. 49 shows an example of the structure of a conventional optical wavelength conversion element.
And FIG. 50. According to these optical wavelength conversion devices, the fundamental wave 208 condensed in the device is focused by achieving phase matching by the polarization inversion unit 207 periodically formed on the substrate 206 of a ferroelectric crystal such as LiTaO ₃ . It is converted to a second harmonic (hereinafter sometimes referred to as “SHG”) 209. In the embodiment shown in FIG. 50, the fundamental wave 208 is SHG in the optical waveguide 210 formed on the surface of the substrate 206.
209.

[0009]

However, the conventional method of manufacturing the domain-inverted portion has the following problems. For example, although it is remarkably observed particularly in the mode as shown in FIG. 48, in the vicinity of the + C plane where the domain-inverted nuclei occur, even if a uniform domain-inverted structure almost equal to the electrode shape is formed,- The shape of the domain-inverted portion was disturbed as approaching the C-plane, and the uniformity of the domain-inverted portion became insufficient near the -C-plane. For this reason, there is a problem that it is difficult to use the domain-inverted portion deep from the substrate, and it is difficult to produce, for example, an optical wavelength conversion element having an optical waveguide formed on the −C plane.

In addition, the conventional method of manufacturing a domain-inverted portion using an X-plate or Y-plate substrate has a problem that it is difficult to form a deep domain-inverted portion. For this reason, the depth of the domain-inverted portion on a substrate whose spontaneous polarization direction is parallel to the substrate surface is 1
It was limited to about μm or less. Furthermore, since the uniformity of the domain-inverted portions to be formed is not good, it is difficult to form the domain-inverted portions over a wide area, and there is a problem that the interaction length is limited to about 10 mm.

On the other hand, in a conventional method of manufacturing a domain-inverted portion using a substrate whose substrate surface and the polarization direction are inclined, a domain-inverted portion having a relatively large depth is formed, but 1) an effective area that can be used for an optical wavelength conversion element. 2) The depth of domain inversion is limited to about 2 μm, and it is difficult to form a deep domain inversion part.

The domain-inverted portion formed on such an oblique substrate is formed so as to sneak into the substrate from the substrate surface. Therefore, the existence of the deep domain-inverted portion overlapping the optical waveguide near the substrate surface is limited to a part of the formed domain-inverted portion. For this reason, there is also a problem that the domain-inverted portion that can be used in the optical waveguide type optical wavelength conversion element is limited to a small area. For example, in the conventional optical wavelength conversion element as shown in FIG. 50, since an oblique substrate is used, a deep polarization inversion portion is formed and the overlap with the optical waveguide is improved, but the polarization inversion portion is used. The available effective area is small, and the number of optical waveguides overlapping the domain-inverted portions is limited to about one in the substrate.

As described above, the conventionally manufactured domain-inverted portions are not necessarily sufficient in terms of depth, uniformity, effective area, and the like in consideration of application to an optical wavelength conversion element. Was.

The present invention has been made in view of the above circumstances, and has been completed to solve the above-mentioned conventional problems. An object of the present invention is to improve a domain-inverted portion, and particularly to an optical wavelength suitable for generating short-wavelength light. Providing a conversion element and a method for manufacturing the same,
Another object of the present invention is to provide a method for manufacturing a domain-inverted portion, which is required to provide such an optical wavelength conversion element. In addition, another object of the present invention is to provide a light generation device and an optical pickup including the above-described light wavelength conversion element, and a diffraction element using the above-described polarization inversion portion.

[0015]

In order to achieve the above object, the present invention provides the following method for manufacturing a domain-inverted portion. The first method for manufacturing a domain-inverted portion includes a step of forming a domain-inverted portion by applying a voltage in a polarization direction of a substrate of a ferroelectric crystal, and reducing an internal electric field of the substrate caused by the application of the voltage. The method includes a step of performing a process, and a step of re-inverting at least a part of the polarization of the domain-inverted portion by applying a voltage in a direction opposite to the voltage in the polarization direction. According to such a manufacturing method, in particular, the uniformity of the domain-inverted portions can be improved, and a domain-inverted portion that is formed deep and can be used effectively can be provided.

A second method for manufacturing a domain-inverted portion includes a step of forming a domain-inverted portion by applying a voltage in the direction of polarization of a substrate of a ferroelectric crystal, and a process of forming a domain-inverted portion near the surface of the substrate. Forming a reversal stabilizing portion. According to such a manufacturing method, the domain-inverted portion is stabilized, and a domain-inverted portion suitable for an optical wavelength conversion element or the like can be provided.

In a third method of manufacturing a domain-inverted portion, a ferroelectric crystal substrate having a first surface and a second surface opposed to each other has a first pattern having a stripe pattern portion on the first surface. Forming an electrode, covering the first electrode with a first insulating film, forming a second insulating film on the second surface, and projecting the first electrode in a polarization direction. Forming a second electrode on the second insulating film so as to cover a region on the second surface defined by the second insulating film via the second insulating film; Applying a voltage between the first electrode and the second electrode to grow a domain-inverted portion from the striped pattern portion of the first electrode to the second electrode. According to such a manufacturing method, it is possible to provide a domain-inverted portion excellent in uniformity and having a short period and a large area.

A fourth method for manufacturing a domain-inverted portion includes the steps of: first, second, and third electrodes being arranged in this order in a polarization direction of a ferroelectric crystal substrate whose polarization direction is substantially parallel to the substrate surface. Forming and applying a voltage between the first electrode and the second electrode to form a striped polarization along the polarization direction between the first electrode and the second electrode. Forming a reversal portion, and applying a voltage between the first electrode and the third electrode to move the stripe-shaped polarization reversal portion from the second electrode to the third electrode. And a step of growing. According to such a manufacturing method, not only the effective area is enlarged, but also a domain-inverted portion formed deeper can be provided.

A fifth method for manufacturing a domain-inverted portion is that a first electric field component parallel to the polarization direction of the ferroelectric crystal and a second electric field component perpendicular to the polarization direction are provided on the substrate of the ferroelectric crystal. A step of growing a domain-inverted portion in the polarization direction by applying an electric field having the following characteristics: According to such a manufacturing method, a deep domain-inverted portion having excellent uniformity can be provided.

The sixth method of manufacturing the domain-inverted portion is that the surface between the first electrode and the second electrode formed on the surface of the ferroelectric crystal substrate has a higher resistance value than the ferroelectric crystal. Forming a low-resistance portion having a low resistance, and applying a voltage between the first electrode and the second electrode to grow a domain-inverted portion in a polarization direction of the ferroelectric crystal. It is characterized by including. According to such a manufacturing method,
It is possible to provide a deep and large-area domain-inverted portion having excellent uniformity.

According to the seventh method of manufacturing the domain-inverted portion, a step of forming a concave portion on the surface of the ferroelectric crystal substrate, a step of forming a first electrode in the concave portion, Growing a domain-inverted portion from the first electrode in the polarization direction of the ferroelectric crystal by applying a voltage between the electrode and a second electrode formed on the surface. Features. According to such a manufacturing method, it is possible to provide a deep domain inversion portion. This concave portion can also be used, for example, for alignment between an optical waveguide formed to use the formed domain-inverted portion as an optical wavelength conversion element and a light source.

According to the eighth method for manufacturing a domain-inverted portion, a step of forming an insulating film on the surface of the ferroelectric crystal substrate, and a step of forming a recess penetrating the insulating film on the surface. Forming a conductive film in at least a region including the concave portion on the surface, and using the conductive film in the concave portion as a first electrode between the first electrode and a second electrode formed on the surface; Growing a domain-inverted portion in the direction of polarization of the ferroelectric crystal from the first electrode by applying a voltage. According to such a manufacturing method, a deep domain-inverted portion having excellent uniformity can be provided.

A ninth method for manufacturing a domain-inverted portion includes a step of forming a first electrode on a surface of a ferroelectric crystal substrate, and a step of forming an insulating film on the surface including the first electrode. ,
Forming a second electrode at a position on the surface of the insulating film different from the position at which the first electrode is formed; and applying a voltage between the first electrode and the second electrode. Growing a domain-inverted portion in the polarization direction of the ferroelectric crystal from the first electrode by applying the voltage. According to such a manufacturing method, a deep domain-inverted portion having excellent uniformity can be provided.

According to a tenth method for manufacturing a domain-inverted portion, an electrode pair comprising a comb-shaped first electrode and a second electrode arranged in a direction in which the comb shape extends on a surface of a ferroelectric crystal substrate. Forming two or more such that the combs are oriented in the same direction, and applying a voltage between the first electrode and the second electrode, so that the first Growing a domain-inverted portion on the second electrode side. According to such a manufacturing method, it is possible to provide a deep, large-area domain-inverted portion having excellent uniformity.

The optical wavelength conversion device of the present invention is provided by forming two or more domain-inverted portions on the substrate of a ferroelectric crystal by any one of the above manufacturing methods so as to be layers substantially parallel to each other. Is done. In addition, according to the present invention, there is provided an optical wavelength conversion element having the following characteristics, which is different from a conventional element.

The first optical wavelength conversion element includes a ferroelectric crystal substrate, a domain-inverted portion formed in the substrate so as to form layers substantially parallel to each other, and the polarization inverting portion near the surface of the substrate. A polarization inversion stabilizing portion formed in the domain inversion portion.

[0027] The second optical wavelength conversion element includes a ferroelectric crystal substrate whose polarization direction is substantially parallel to the substrate surface, an optical waveguide formed in the substrate along the surface, and And two or more domain-inverted portions having a depth of 2 μm or more formed in the substrate so as to periodically traverse the optical waveguide.

The third optical wavelength conversion element comprises a substrate of a ferroelectric crystal in which the polarization direction and the substrate surface have an inclination of 1 ° to 5 °, and a substrate which is perpendicular to the substrate surface and extends along the polarization direction. Two or more domain-inverted portions extending from the surface in the polarization direction so as to form a layered structure in a cross section.

The fourth optical wavelength conversion element includes a concave portion formed on the surface of the ferroelectric crystal substrate, and a polarization inversion portion extending in the polarization direction of the ferroelectric crystal with the concave portion as one end. It is characterized by including.

The fifth light wavelength conversion element includes a ferroelectric crystal substrate, a first electrode formed on the surface of the substrate, and an insulating film formed on the surface including the first electrode. A second electrode formed on the insulating film at a position different from the position where the first electrode is formed, and a polarization direction of the ferroelectric crystal with the first electrode as one end. And a domain-inverted portion extending to the second electrode side along the line.

The sixth optical wavelength conversion element comprises a ferroelectric crystal substrate, a first electrode each having a comb shape, and a second electrode arranged in a direction in which the comb shape extends, and the comb shape is the same. And two or more pairs of electrodes formed on the surface of the substrate so as to face the second direction along the polarization direction of the ferroelectric crystal with the comb-shaped portion of the first electrode as one end.
And a domain-inverted portion extending toward the electrode side.

The optical wavelength conversion element including the domain-inverted portion, which can be manufactured by the above method, or the first to sixth optical wavelength conversion elements is preferably applied particularly to a light generation device, and generates such short wavelength light. A light generation device suitable for the optical pickup is preferably used for an optical pickup.

Further, according to the present invention, there is provided a ferroelectric crystal substrate having a polarization direction and a substrate surface inclined at 1 ° to 5 °, and a cross section perpendicular to the substrate surface and extending along the polarization direction. And a two or more domain-inverted portions extending from the surface in the polarization direction so as to form a layered structure. This diffractive element is an element capable of high-speed response using a domain-inverted portion that can be manufactured by the above method.

[0034]

Embodiments of the present invention will be described below. According to the present invention, domain-inverted nuclei (Domain Cor
A method is provided for forming a desired domain-inverted shape on a surface where e) does not occur. As a specific method of forming the domain-inverted portion, a monopolar ferroelectric crystal (eg, LiTa
A so-called electric field application method in which electrodes are formed on a substrate of an O ₃ plate and a high voltage is applied between the electrodes to form a domain-inverted portion under the electrodes can be applied. The domain-inverted portion is formed, for example, so as to form a periodic layered structure, and is used for an optical wavelength conversion element or the like.

As a method of forming a domain-inverted portion on a so-called C-plate ferroelectric crystal substrate whose polarization direction is substantially perpendicular to the substrate surface, a comb-shaped electrode is provided on the + C plane where domain-inverted nuclei are generated, and a -C plane is provided. A method in which a plate electrode is formed on the substrate, a pulse voltage (for example, about 21 kV / mm as an electric field) is applied between the two electrodes, and a periodically domain-inverted portion is grown from the comb portion of the comb electrode to the plate electrode. .

On the other hand, when a comb-shaped electrode is formed on the -C plane and a plane electrode is formed on the + C plane, and a voltage is applied between the two electrodes, the domain-inverted portions become flat and do not become periodic. This is because the domain-inverted portion grows from the domain-inverted nucleus in the −C plane direction. In order to form a domain-inverted portion having a striped pattern on the −C surface, it is necessary to grow the domain-inverted portion from the comb-shaped electrode on the + C surface. However, they are disturbed as they grow from the domain-inverted nuclei. Therefore, as described above, -C
It is difficult to form a domain-inverted portion having a finely controlled shape on a surface.

(Embodiment 1) An example of a method for manufacturing a domain-inverted portion that solves such a problem will be described below.

In this method, as shown in FIG.
The comb-shaped electrode 2 -C surface of the substrate 1 of the ferroelectric crystal (eg LiTaO _3), to form a flat electrode 3 to the + C surface (FIG. 1 (a)),
Next, a positive voltage is applied to the + C plane to the plate electrode 3 to grow a domain-inverted portion from this electrode (FIG. 1 (b); in this state, the direction of polarization of the crystal is reversed, and the surface on which the comb-shaped electrode is formed) Is +
The plane on which the plate electrode is formed on the C plane becomes the -C plane. Then, a + voltage is applied to the comb-shaped electrode 2 to invert the polarization from this electrode, thereby forming the polarization re-inversion portion 5 (FIG. 1 (c)). The polarization re-inversion unit 5 forms a pattern that draws a periodic stripe pattern corresponding to the comb-shaped electrode 2.

Observation of the formed domain-inverted portion shows that -C
It was found that a domain-inverted portion having the same shape as the comb-shaped electrode 2 formed on the surface was formed, and a uniform domain-inverted layer was also formed on the −C plane. As described above, by utilizing the re-inversion of the polarization, a domain-inverted portion that is finely controlled and has excellent uniformity can be formed on the -C plane.

However, when a domain-inverted portion was formed by the above method, in the step of FIG. 1C, a phenomenon was observed in which the inversion voltage was unstable or no domain inversion occurred at all. An experimental study of the cause revealed that an internal electric field was formed inside the crystal due to the application of the electric field in the step of FIG. 1 (b), and it was found that this prevented the re-inversion of the polarization.

Therefore, a method of reducing and preferably removing the internal electric field was studied. As a result, it was found that the internal electric field can be reduced by performing an annealing treatment between the steps shown in FIGS. 1 (b) and 1 (c). Was done. Further, the annealing temperature was also examined. The results are shown below.

Annealing temperature <50 ° C .: There is almost no effect of annealing. Annealing temperature = 100 ° C .: Re-inversion is easy, but the re-inversion portion is unstable. -Annealing temperature ≥ 150 ° C: Easy re-inversion, uniform and stable polarization inversion possible. -Annealing temperature> 500 ° C: The formed polarization is reduced or eliminated.

As described above, the annealing temperature is 150 to 500.
C is preferable, and annealing at a temperature in this range facilitates formation of a desired domain-inverted shape on the -C plane. The above annealing temperatures are all based on the substrate temperature.

Here, the preferable upper limit of the annealing temperature is as follows:
It is thought to be related to the Curie temperature of the crystal used. In addition, several minutes were sufficient for annealing time.

Next, a method for uniformly forming a periodically poled portion from the front surface to the back surface of the ferroelectric crystal by applying the above-described method will be described. This method
The polarization inversion by the application of an electric field is repeatedly performed as necessary while removing the internal electric field. Hereinafter, description will be made with reference to FIG.

In this method, first, a comb electrode 2a is formed on the + C plane of a ferroelectric substrate 1 such as LiTaO ₃ having a plane substantially perpendicular to the C axis, and the + C plane is formed on the + C plane. On the other hand, when the comb-shaped electrode 2b of the inverted pattern, that is, the electrode 2a on the + C plane is projected on the −C plane along the polarization direction, an electrode is formed in a portion where no electrode is provided (FIG. 2A). A voltage is applied between the electrodes 2b to form a domain-inverted portion under the electrode on the + C plane (FIG. 2).
(b) At this time, since the domain-inverted portion formed is generated from the + C plane, the non-uniformity of the domain-inverted shape increases near the −C plane. ), The substrate is annealed to remove the internal electric field (FIG. 2 (c)), and a voltage is applied between the electrodes (positive / negative in the step (b)) to reverse the polarization (FIG. 1 (c)). d))
Things.

In the step shown in FIG. 1D, the polarization is reversed again at a domain-inverted portion that does not correspond to the electrode on the + C plane, that is, a portion corresponding to disturbance. By this step, the non-uniform portion of the domain-inverted portion is removed, and a periodic domain-inverted portion corresponding to the electrode pattern is accurately formed from the front surface to the rear surface of the substrate.

If the above steps are repeated with the internal electric field removing step interposed therebetween, the uniformity is further improved.

In the above example, LiTaO ₃ of C plate was used, but the same effect can be obtained by using LiNbO ₃ and its mixture LiNb _x Ta _(1-x) O ₃ . Since LiNbO ₃ and its mixture have large nonlinear optical constants, highly efficient light wavelength conversion is possible.

Further, the same effect can be obtained with MgO-doped LiTaO ₃ , MgO-doped LiNbO ₃ and a mixture thereof, MgO: LiNb _x Ta _(1-x) O ₃ . When doped with MgO, the light damage resistance is improved, and stable characteristics can be obtained even with a high output SHG output.

In this embodiment, the C plate is used as the substrate having the nonlinear optical crystal. However, similar characteristics can be obtained by using an X plate or a Y plate having a surface parallel to the C axis. When these substrates are used, electrodes for polarization inversion are formed on the substrate surface at a distance in the C-axis direction, and polarization inversion is performed on the substrate surface. In this case, the electrodes are preferably, for example, a pair of comb-shaped electrodes having a comb shape fitted to each other. In addition, when such a substrate is used, SHG is excited by fundamental mode excitation of TE mode.
Since light can be generated, the coupling efficiency between the optical waveguide formed on the substrate surface and the semiconductor laser can be improved, and a small-sized, high-output, short-wavelength light generator can be provided.

The internal electric field can be reduced by applying an electric field in a direction opposite to the internal electric field, for example, without being limited to the annealing process. The voltage applied in this case is preferably 10 to 20 kW / mm.
This is because the internal electric field can be extinguished in a short time without causing polarization inversion.

Next, the stability of the domain-inverted portion will be examined. According to the electric field application method as described above, a deep domain inversion portion can be formed in a short period. However, when a heat resistance experiment was performed, it was found that the domain-inverted portions deteriorated during the heat treatment, the domain-inverted portions disappeared, and the like, and there was a problem in the stability of the domain-inverted portions themselves. It was also found that the domain-inverted layer may disappear in the waveguide forming process when forming the optical wavelength conversion element.

The results are summarized as follows: 1) Regarding the electric field application domain-inverted layer formed from the + C plane. Heat resistance: disappears by heat treatment at 500 ° C. or more. Waveguide formation: The domain-inverted portion in the waveguide disappears due to (proton exchange + annealing). 2) Regarding domain-inverted layer including re-inversion (Embodiment 1) formed from -C plane-Heat resistance: domain-inverted portion near -C surface disappears by heat treatment at 350 ° C. Waveguide formation: The domain-inverted portion in the waveguide disappears due to (proton exchange + annealing). Therefore, the improvement of the stability of the domain-inverted portion formed by applying an electric field was studied. The results are described below.

(Embodiment 2) The domain-inverted layer including the domain re-inverted portion formed by the method of the above-described embodiment can be subjected to -C even at a relatively low temperature heat treatment (for example, 350 ° C.).
The domain-inverted portions disappeared from the surface, and receding of the domain-inverted portions of 5 to 6 μm was observed. It is not conventionally known that the domain-inverted portion disappears from the −C plane and spreads toward the + C plane.
This is the phenomenon first revealed here.

Thus, as a result of various studies on methods for preventing the disappearance of inversion, it has been found that a method of performing ion exchange, preferably proton exchange, on the surface is effective. When the domain-inverted stabilizing portion was formed by performing proton exchange, the domain-inverted shape did not change even when heat treatment was performed at, for example, 500 ° C., and heat resistance was significantly improved.

The preferred proton exchange depth for stabilizing the domain-inverted portion was 0.01 to 0.2 μm, for example, about 0.1 μm. The disappearance of the domain-inverted portion on the -C plane can be prevented by suppressing the generation of domain-inverted nuclei on this plane. When proton exchange is performed, the asymmetry of the crystal is reduced and the ferroelectricity is deteriorated, so that the polarization is hardly inverted,
Generation of domain-inverted nuclei is suppressed. In this way, the stability of the domain-inverted portions, particularly the stability of the domain-inverted structure formed on the -C plane, could be significantly improved.

The proton exchange can be carried out using an acid such as pyrophosphoric acid. The depth of the proton exchange layer is determined by the processing temperature (the temperature of the acid) and the time.
In order to form a proton exchange layer having a depth of 2 μm, 26
The treatment may be performed at 0 ° C. for about 160 seconds and at 0.1 ° μm to form a 0.1 μm proton exchange layer at 260 ° C. for about 40 seconds.
In the case of forming a thinner proton exchange layer, it is preferable to lower the processing temperature to improve the controllability of the depth. For example, at a temperature of 200 ° C. for about 3 seconds, a depth of 0.01
A μm proton exchange layer is formed. If an exchange layer having this depth is to be formed at a temperature of 260 ° C., the processing time needs to be 1 second or less. Although the treatment temperature can be further lowered, the temperature is generally preferably 160 to 280 ° C. for forming the proton exchange layer. As the acid, benzoic acid, orthophosphoric acid, sulfuric acid, hydrochloric acid and the like can be used.

In this embodiment, a LiTaO ₃ substrate is used as the substrate, but other substrates as described above can be used. Such an improvement in the stability of the domain-inverted structure is effective for other crystals. For example, KNbO ₃ , TiP
bO ₃ Isopolarization is unstable, and polarization inversion is easily performed by external pressure or temperature change, and the stability of polarization inversion is greatly improved by ion-exchanging the crystal surface and changing the crystal structure of the surface. Can be improved. Also,
Although the C plate is used here, the same effect as described above can be obtained by forming the polarization inversion stabilizing portion also when using an X plate or a Y plate. In this case, it is preferable to form a polarization inversion stabilizing portion on at least the + C side of the substrate surface. Even when an oblique substrate is used, the formation of the domain inversion stabilizing portion is similarly effective.

As described above, the types of substrates that can be used are not limited unless otherwise specified, which is common to the embodiments in this specification.

(Embodiment 3) Here, a method for preventing the domain-inverted portion in the optical waveguide from disappearing when the optical waveguide is formed will be described.

First, a method of manufacturing an optical wavelength conversion device using a substrate such as LiTaO ₃ will be described. As shown in FIG. 3, a method of manufacturing an optical wavelength conversion element generally includes forming a periodically striped domain-inverted portion 4 on a crystal substrate 1 as shown in FIG.
(a)), a mask pattern 6 for forming an optical waveguide (for example, Ta
(FIG. 3B), and heat-treated in a solution containing an acid, for example, pyrophosphoric acid, to form a proton-exchanged portion 7 (having a thickness of about 0.5 to 2 μm) in the unmasked portion (FIG. 3B). 3 (c)),
After removing the mask, an annealing process (for example, at a temperature of 40
(About 0 ° C.) to form the optical waveguide 8 (FIG. 3D).

However, when the fabricated optical wavelength conversion element was cut along the optical waveguide section and the polarization inversion portion was observed,
As shown in FIG. 5, a phenomenon in which the domain-inverted portions 4 disappear inside the optical waveguide 8 is observed.

This phenomenon occurs in 1) the domain-inverted layers formed on either of the ± C planes. 2) It also occurs in the domain-inverted layer whose heat resistance has been improved by ion exchange treatment. 3) It occurs even when the annealing temperature of the optical waveguide is lowered to about 300 ° C. It had the characteristic of.

The disappearance of the domain-inverted portion is a problem that cannot be sufficiently solved by the method described in the second embodiment. This is considered to be because the domain-inverted structure disappears when protons thermally diffuse into the crystal.

To prevent this, it is necessary to form a domain-inverted portion that can withstand thermal diffusion of protons. Therefore, various studies were performed on stabilization of the domain-inverted portion. As a method of manufacturing the domain-inverted portion, proton exchange is selectively performed on the -C surface of LiTaO ₃ , and the proton exchange is performed near the Curie temperature of the substrate (530 ° C.).
) To form a periodic domain-inverted portion (proton exchange heat treatment method). However, such a method using the proton exchange heat treatment makes it difficult to form a deep domain-inverted portion, and can form only a domain-inverted portion having a depth of about 2 μm.

Then, when an attempt was made to improve the properties of the domain-inverted structure by applying a proton exchange heat treatment to the domain-inverted structure formed by applying an electric field, the following two new phenomena were observed.

1) When a proton exchange heat treatment is applied to the domain-inverted portion formed by applying an electric field, a domain-inverted portion is formed by the proton exchange heat treatment using the domain-inverted domain inversion as a nucleus. For this reason, it is possible to form a deep domain-inverted structure, which is impossible with the proton exchange heat treatment method. 2) By forming the domain-inverted structure by the proton exchange heat treatment near the surface of the domain-inverted domain-inverted domain, a domain-inverted structure that does not disappear by the optical waveguide forming process can be formed.

Hereinafter, an example of a method of forming a stable domain-inverted structure having excellent resistance by applying a proton exchange heat treatment method to an electric field application method will be described.

In this method, as shown in FIG. 5, a periodic domain-inverted portion 4 is formed on the −C plane by the domain re-inversion method as described in the first embodiment (FIG. 5A). Then, the substrate surface is heat-treated in an acid such as pyrophosphoric acid to form a proton exchange portion 7 on the surface (FIG. 5 (b)). Apply the method (Fig. 5 (c)).

When the domain-inverted portions were formed by the above method, it was confirmed that the domain-inverted shape was almost equal to the shape formed by applying an electric field, and that a deep domain-inverted portion could be formed uniformly in a short period.

Next, an optical waveguide was formed in the domain-inverted portion manufactured by the above method, and the disappearance of the domain-inverted portion in the waveguide was observed. As a result, it was found that the heat treatment temperature in the step of FIG. 5C has a great effect on the annihilation characteristics of the domain-inverted portions. Therefore, the relationship between the distance at which the polarization inversion disappears from the -C plane and the heat treatment temperature was determined. FIG. 6 shows the result.

When the heat treatment temperature was 400 ° C. or lower, the domain-inverted portions were not formed by the proton exchange heat treatment and disappeared over a depth of 1 μm or more from the surface. 50
As the temperature approaches 0 ° C., the depth at which the domain-inverted layer disappears decreases, and at 480 ° C. or more, the receding depth becomes 0.35 μm or less. A preferable temperature range in which the domain-inverted portions do not substantially disappear is 500 to 520 ° C. A value of 0.5 μm or less is required as the inversion extinction depth that does not affect the conversion efficiency of the light wavelength conversion element. From such a viewpoint, it is appropriate to perform the heat treatment at 480 to 530 ° C. When the heat treatment temperature was 540 ° C. or higher, the periodic domain-inverted portions disappeared, slab-shaped domain-inverted portions were formed, and the optical wavelength conversion element could not be manufactured.

From the above results, it is understood that the structure including the domain-inverted portions formed by applying an electric field is unstable as a crystal structure. This is because in the original crystal structure, the polarization direction of the crystal does not change due to a low-temperature heat treatment or an ion exchange treatment. On the other hand, the reason why a domain-inverted stabilization part can be formed by forming a domain-inverted part by applying an electric field, performing ion exchange on the surface thereof by proton exchange or the like, and then performing a heat treatment at a temperature lower than the Curie temperature of the substrate, is possible. It is considered that ions are diffused into the substrate by heat-treating the layer having the changed crystal structure in the vicinity of the Curie temperature, and the ions are fixed in a stable state when the ion-exchange portion approaches the original crystal structure. That is, an unstable crystal structure formed by applying an electric field changes to a stable crystal structure through ion exchange and heat treatment.

However, the heat treatment temperature at this time is preferably lower than the Curie temperature of the substrate and near the Curie temperature. For example, in the case of LiTaO ₃ having a Curie temperature of about 600 ° C., the heat treatment temperature is preferably 480 to 530 ° C., and more preferably around 500 ° C. The reason is that the potential at which the polarization changes (inverts) near the Curie temperature decreases, and the inversion becomes easy. Also, when the temperature exceeds the Curie temperature, the polarization is dispersed, and the crystal is not a single-polarized crystal. From such a viewpoint, generally, the heat treatment temperature is
The temperature is preferably between (Curie temperature-150 ° C) and (Curie temperature).

Next, an optical waveguide was formed on the domain-inverted structure that had been heat-treated (temperature: 510 ° C.), an optical wavelength conversion element was fabricated, and its characteristics were evaluated. FIG. 7 shows the configuration of the manufactured SHG element. As shown in FIG. 7, an optical waveguide 8 is formed on a LiTaO ₃ substrate 1 so as to periodically cross a periodically formed layered domain-inverted portion 4. In addition, the layer 9 in which the proton exchange layer has been annealed is less likely to cause polarization inversion than LiTaO ₃ . In addition, it was also confirmed that the electrical resistance was reduced by performing the ion exchange and the heat treatment.
Further, for example, the polarization inversion period is 3.5 μm, and the optical waveguide 8
Has a width of about 5 μm and a depth of about 2 μm.

In such an optical waveguide 8, as a fundamental wave,
When the light (wavelength 850 nm, 70 mW) from the semiconductor laser was input, the output of the second harmonic was 10 mW. When the conventional method of manufacturing the domain-inverted portion is used, the domain-inverted portion in the optical waveguide tends to disappear, so that the conversion efficiency is extremely reduced, and an output of only about 1 mW is obtained in a similar experiment. Inside.

Further, the optical wavelength conversion element obtained by the above method has greatly improved the resistance to optical damage. In the conventional method, since the domain-inverted portion near the surface disappears, optical damage occurs at this portion, causing an output fluctuation of SHG, and a stable output of only several mW was obtained. However, according to the above method, a deep domain-inverted portion can be formed up to the substrate surface, so that it is possible to suppress the occurrence of optical damage. As a result, a stable output characteristic was obtained for an SHG output of 20 mW, and instability due to optical damage was almost completely prevented.

The domain-inverted portion manufactured by the above method has a short cycle and a deep ideal structure. In addition, excellent resistance with little change in characteristics during heat treatment or chemical treatment at the time of forming the optical waveguide could be realized. As a result, it becomes possible to form a periodically poled structure from the surface of the optical waveguide to a deep portion of the substrate in the optical waveguide, which has been difficult in the past, and has high efficiency and excellent stability (light damage resistance). SHG elements can now be manufactured.

In the above embodiment, the optical waveguide type S
Although the HG element has been described, a domain-inverted structure having a domain-inverted stabilizing portion is also effective for a bulk-type SHG element. In a bulk-type device, the stability of the domain-inverted layer becomes the stability of the device. Therefore, by using the domain-inverted structure, a device excellent in heat resistance and reliability can be configured.

Further, in the above embodiment, proton exchange using an acid was used as ion exchange for forming the polarization inversion stabilizing portion, but Nd, K, Ag, Rb, Cu, Zn, Cd, T
Ion exchange by l or the like may be used. By mixing ions in an acid and performing heat treatment, ion exchange can be performed simultaneously with proton exchange, and ions can be implanted into the substrate. Also in this case, a heat treatment of the substrate at a temperature lower than the Curie temperature and in the vicinity thereof can change the crystal structure and realize a structure in which instability of polarization inversion hardly occurs.

(Embodiment 4) According to the present invention,
Another method is provided for forming a domain inversion uniformly and preferably over a large area.

Here, another method of forming a domain-inverted portion by applying an electric field will be described. By forming the domain-inverted portions on the X-plate and the Y-plate, it becomes possible to manufacture a waveguide-type SHG element capable of coupling with a semiconductor laser with high efficiency.
This is because the polarization direction of the waveguide mode is the same between the light propagating in the waveguide and the semiconductor laser. Conventionally, a method of forming a domain-inverted portion on an X plate or a Y plate is to form a comb-shaped electrode and a plate electrode separated in the C-axis direction of a substrate surface, and to apply a voltage between the electrodes. This is a method of inverting the polarization. However, the conventional method has the following problems.

1) The interval between the electrodes is limited to several hundred μm at most, the area for forming the SHG element is limited, and the efficiency of mass production is low. 2) The formed domain-inverted portion is shallow, and the overlap with the waveguide is not sufficient.

Therefore, as shown in FIG. 8, the method of manufacturing the domain-inverted portion according to the present embodiment uses the comb-shaped electrodes 11 and 2 in the C-axis direction of the ferroelectric crystal substrate 1 (eg, LiTaO _{3 of a} Y plate). The above-mentioned plate electrodes 12a, 12b, 12c... Are formed.
(a); the comb-shaped electrode 11 is arranged on the + C side), and a pulse-like voltage is applied between the comb-shaped electrode 11 and the first plate electrode 12a to form a domain-inverted portion between both electrodes (FIG. 8 ( b)) Further, a voltage is applied between the comb-shaped electrode 11 and the second plate electrode 12b to extend the domain-inverted portion to the electrode 12b (FIG. 8 (c)). The applied voltage is, for example, 20 to 30 kV / m
m and the application time are, for example, about 20 ms. The applied voltage waveform can be inverted even with a rectangular applied voltage,
If a pulsed high voltage such as the above-mentioned voltage is applied in the initial stage and then a CW voltage of, for example, about 5 kV / mm is continuously applied, a uniform domain-inverted portion can be formed.

According to this method, a periodic domain-inverted structure over 200 μm can be formed. Further, by applying a voltage between the comb-shaped electrode and the electrode 12c, a periodic domain-inverted portion can be formed in a wider area. The distance between adjacent electrodes in this method is not particularly limited, but may be, for example, 50 μm.
m to 1 mm, preferably 200 to 400 μm.

Furthermore, it should be noted that it was confirmed that the method of manufacturing the domain-inverted portion also improved the depth of the domain-inverted portion. First polarization reversal (comb-shaped electrode 11 and electrode 1
2a), the inversion depth is at most 1
However, when a voltage was applied to the electrodes 12b and 12c to grow the domain-inverted portion, a domain-inverted portion of about 1.5 μm or more was formed, and the depth of the domain-inverted portion increased. .

In addition, in addition to the electrode structure used in the present embodiment, an electrode structure that improves the uniformity of the domain-inverted structure has been found. This is an electrode structure in which an electrode and an area between the electrodes are covered with an insulator film. For example, when a resist and SiO ₂ were deposited to a thickness of about 1 μm and an electric field was applied, the uniformity of the voltage distribution between the electrodes was improved, and a uniform domain-inverted portion could be formed. The insulator film was particularly effective when the thickness was 0.1 μm or more.

Although a large number of flat electrodes are formed in the present embodiment, other methods can be used in the same manner. For example,
A similar domain-inverted portion may be formed by gradually etching the plate electrode and performing domain-inversion while widening the interval between the comb-shaped electrode and the plane electrode.

(Embodiment 5) Further, according to the present invention,
Another method is provided for forming a uniform domain inversion on a ferroelectric crystal substrate having a surface substantially perpendicular to the C-axis.

Here, a method of manufacturing a domain-inverted portion using an insulating film will be described. As shown in FIG. 9, first, a comb-shaped electrode 13 is formed on a + C plane of a ferroelectric crystal (for example, LiNbO ₃ ) of a C plate, and an insulating film (for example, an SiO ₂ film) 14 is formed on a −C plane.
Then, a flat plate electrode 15 is formed on the insulating film (FIG. 9B), and an insulating film (for example, SiO ₂ film) 16 is deposited on the comb electrode 13 on the + C plane (FIG. 9C). )), And a voltage is applied between the electrodes to invert the polarization (FIG. 9D).

According to such a method, the applied electric field can be made uniform by depositing the insulating film 14 on the -C plane, and uniform periodic domain inversion can be formed. Further, by depositing the insulating film 16 on the + C plane, a short-period domain-inverted structure can be formed. Therefore, it becomes possible to form a short-period domain-inverted portion over a larger area than before.

Next, the SiO ₂ film thickness was optimized. The SiO ₂ film was deposited by a sputtering method, and the polarization reversal characteristics with respect to the film thickness were investigated. First, the thickness of the SiO ₂ film on the −C plane was examined. When the film thickness is less than 0.5 μm, the contribution to the uniformity of the polarization inversion is small, but when the thickness is 0.7 μm or more, the uniformity of the inversion is improved, and the polarization inversion is formed uniformly over an area of 10 × 10 mm or more. Became possible. However, when the film thickness is larger than 2.3 μm, the polarization inversion hardly occurs, and the reproducibility of the inversion is reduced. Therefore, it became clear that the preferable thickness of the SiO ₂ film 14 for forming a uniform domain-inverted portion is about 0.7 to 2.3 μm.

Further, SiO ₂ deposited on the comb electrode on the + C plane
The thickness of the film was studied. When the film thickness is less than 0.5 μm, only the reversal period of 3 μm or more is formed, and the non-uniformity of the reversal structure is increased, so that the characteristics are deteriorated when used as an SHG element. When the SiO ₂ film thickness is 1 μm or more, the uniformity is improved, and for example, it is possible to form a domain-inverted portion with a period of 2 μm or less. Therefore, the thickness of the SiO ₂ film is 1 μm
The above is preferred. Thus, by depositing the SiO ₂ films 14 and 16 with an appropriate thickness on the ± C planes respectively, the period of 2 μm
The following polarization inversion can be formed up to 10 × 10 mm or more.

(Embodiment 6) The domain-inverted portion manufactured according to the above-described embodiment has characteristics suitable for use as an optical wavelength conversion element. Such an optical wavelength conversion element generates, for example, short wavelength light. It is used as a light generator.
Here, an example of the light generation device of the present invention will be described.

As shown in FIG. 10, the short-wavelength light generating device includes a light wavelength conversion element 22 and a semiconductor laser 21, and the fundamental wave emitted from the semiconductor laser 21 is wavelength-converted by the light wavelength conversion element 22, The light is emitted as SHG23. According to such a short wavelength light generator, for example, when a semiconductor laser having a wavelength of 800 nm is used, blue SHG light having a wavelength of 400 nm can be obtained. This short wavelength light generator,
Since the preferred domain-inverted portion as described above is used, it can be used as a stable blue light source. Such a small-sized short-wavelength light generation device can be applied to a wide range of fields such as medical and biotechnology applications such as high-density optical recording, a color laser printer, and a fluorescence microscope, for example.

Further, by using as a fundamental wavelength of a red semiconductor laser having a wavelength of 680 nm, ultraviolet light having a wavelength of 340 nm can be generated, and a compact ultraviolet light source can be realized. Such a small ultraviolet light source can be applied particularly to biotechnology, fluorescence lifetime measurement, special measurement, and the like. Also, when the laser is pulsed, a fundamental wave with high peak power can be obtained.
Highly efficient wavelength conversion becomes possible. For example, in a CW drive, even a semiconductor laser having a maximum output of about 40 mW can generate a high peak power of several 100 mW by pulse driving, and a SHG output of several tens mW can be obtained.
SHG light having a high peak power can be used for detecting impurities by applying it to fluorescence lifetime measurement and the like. In addition, by driving the semiconductor laser at high frequency RF, pulse train oscillation with high peak power can be achieved, and the conversion efficiency can be improved by a factor of 5 or more compared to the CW driven semiconductor laser at the average power.

When high output SHG light is generated, output instability due to optical damage becomes a problem. In the device shown in this embodiment, since a deep domain-inverted structure is formed, the device has excellent light damage resistance and stable characteristics even when an SHG output exceeding 10 mW is generated.

(Embodiment 7) Here, an optical pickup using the above short wavelength light generating device as a light source for pickup of an optical disk will be described.

[0100] High density recording is desired for optical discs, and for that purpose, the realization of a small, short wavelength light source is indispensable. An optical pickup for reading an optical disk includes a light source, a condensing optical system, and a light receiving portion. When the light wavelength conversion element as described above is used as a light source, a wavelength of 400 nm
Since the blue light in the band can be used as a reading light source for an optical disk, it has become possible to double the recording density. Furthermore, since high-output blue light can be generated,
In addition to reading, it has become possible to write information on an optical disc.

FIG. 11 shows an optical pickup of this embodiment. As shown in FIG. 11, a beam having an output of, for example, about 10 mW, emitted from the short-wavelength light generator 25, passes through a beam splitter 26 and is irradiated on an optical disk 28 as an information recording medium by a lens 27. Optical disk 28
Is reflected by the beam splitter 26 after being collimated by the lens 27, and the signal is read by the detector 29. Further, information can be written on the optical disk 28 by intensity-modulating the output of the short-wavelength light generator. As a result, the light-collecting characteristics by short-wavelength light can be improved, and the recording density can be doubled as compared with the related art.

By using the semiconductor laser as the fundamental wave light source, the optical pickup can be downsized.
It can also be used as a small consumer optical disk reading and recording device. Further, for the optical wavelength conversion element, the aspect ratio of the output beam can be optimized by optimizing the optical waveguide width. This eliminates the need for a beam shaping prism or the like for improving the light-collecting characteristics of the optical pickup, and can achieve high transmission efficiency, excellent light-collecting characteristics, and low cost. Further, noise of scattered light generated during beam shaping can be reduced, and simplification of the pickup can be realized.

(Embodiment 8) Further, according to the present invention,
A method is provided for forming a deep domain-inverted portion on a ferroelectric substrate in which the polarization direction is substantially parallel to the substrate surface or has a constant inclination.

Here, a method of manufacturing a domain-inverted portion having a characteristic in the direction of application of an electric field will be described. First, similarly to the conventional polarization inversion method, only the portion between the comb-shaped electrode 32 and the rod-shaped electrode 33 formed on one surface of the X-plate ferroelectric substrate (for example, MgO: LiNbO ₃ substrate) 31 in FIG. Voltage source 35
The voltage was applied from the above to try to form a domain-inverted portion. The comb-shaped electrode 32 is arranged in the + C direction of the substrate rather than the rod-shaped electrode 33 so that the comb-shaped electrode 32 faces the rod-shaped electrode 33. The electrode spacing was 100 μm and the electrode length was 10 mm. When applying a voltage, the entire substrate was immersed in an insulating liquid to prevent discharge. When a voltage of about 0.4 kV was applied (that is, when the electric field was about 4 kV / mm), a domain-inverted portion was formed. Observing the domain-inverted part from the cross section,
m depth. Observation of the domain-inverted portion formed from the surface revealed that a portion was not formed uniformly over an electrode length of 10 mm, and that a portion where no domain inversion was formed was found in some places. As described above, the domain-inverted portion formed by applying an electric field in a direction parallel to the substrate has a shallow depth of about 1 μm and does not have sufficient in-plane uniformity.

Then, in order to form a deep polarization inversion, the characteristics of the polarization inversion portion formed were examined.
The following became clear.

1) The depth of the domain-inverted portion largely depends on the direction of the electric field generated at the tip (the tip of the electrode finger) of the comb-shaped portion (striped pattern portion) of the comb-shaped electrode. That is, since the electric field generated between the electrodes formed on the substrate surface is localized near the surface of the substrate, the electric field is not distributed in the depth direction of the substrate, and the domain-inverted portion does not become deep. 2) The depth of the domain-inverted portion largely depends on the depth of the domain-inverted portion formed at the tip of the + C-side electrode finger. Generally, the polarization inversion depth at the tip of the electrode finger on the + C side where the domain inversion nuclei are generated is the deepest, and the depth of the domain inversion portion decreases toward the + C side.

From the above results, it has become clear that in order to form a deep domain-inverted portion, it is necessary to form a deep domain-inverted portion at the tip of the electrode finger of the comb-shaped electrode. To achieve this, it is necessary to strongly tilt the direction of the electric field at the tip of the electrode finger in the depth direction of the substrate.

Therefore, when forming the domain-inverted portion, an electric field is applied in the direction perpendicular to the substrate surface in addition to the electric field in the direction parallel to the substrate surface, so that the direction of the electric field at the tip of the electrode finger is reduced by the depth of the substrate. A flat electrode 34 is formed on the other surface of the substrate 31 so as to face the direction, and while a voltage is applied from a voltage source 36 between the comb electrode 32 and the flat electrode 34,
A voltage was applied between 2 and the rod-shaped electrode 33 in the same manner as described above. However, a DC voltage was applied between the electrodes 32 and 34, a pulse voltage having a pulse width of 100 ms was applied between the electrodes 32 and 33 so that the electric field between the electrodes 32 and 33 became 4 kV / mm.

FIG. 13 shows the relationship between the voltage between the electrodes 32 and 34 and the polarization inversion depth at this time. The electric field between the electrodes 32 and 34 was calculated from the substrate thickness (0.5 mm). As shown in FIG. 13, when the voltage between the electrodes 32 and 34 is 2 kV / mm or less (about 50% of the voltage between the electrodes 32 and 33), the depth of the domain-inverted portion is At about 1 μm, which is almost the same as when no voltage is applied, no increase in the polarization inversion depth was observed. However, when the voltage was higher than 4 kV / mm (about the same as the voltage between the electrodes 2 and 3), the polarization inversion depth increased almost in proportion to the applied voltage.

Further, 3 kV (electric field: 6 kV / mm, electrode 3
When the electric field exceeds about 1.2 times the electric field between 2 and 33), the effect of improving the uniformity of the domain-inverted portion is observed, and it is confirmed that the domain-inverted portion having excellent uniformity is formed over an electrode length of 10 mm. Was done. That is, as the applied electric field, the electrodes 32, 33
It is preferable that the inter-substrate voltage between the electrodes 32 and 34 be higher than the inversion voltage between them, and if the voltage is 1.2 times or more, the uniformity is improved.

As a result of forming a domain-inverted portion by applying a voltage of 4 kV between the electrodes 32 and 34 by the above-described method, a domain-inverted (period: 3.2 μm) having a depth of 1.5 μm was obtained with a working length of 40 μm.
It could be formed uniformly over mm or more.

Next, a method of applying a voltage was examined.
As a method of applying a voltage to the front and back surfaces of the substrate, electrodes 33,
It is also possible to apply a voltage between. However, this method has a limitation in increasing the polarization inversion depth. This is considered because the increase in the polarization inversion depth greatly depends on the generation depth of the domain inversion nuclei. The domain-inverted portion starts with the generation of domain-inverted nuclei at the + C-side electrode, and the domain-inverted portion grows from the domain-inverted nucleus toward the −C side. Therefore, the depth of the domain-inverted portion is almost determined by the depth near the + C-side electrode. Therefore, as described above, a method of applying a voltage between the comb-shaped electrode 32, which is the electrode on which the domain-inverted nuclei are generated, and the electrode 34 on the back surface of the substrate is effective.

Next, the waveform of the applied voltage was examined.
As described above, by applying a pulse-like voltage between the electrodes 32 and 33 while applying a DC voltage between the electrodes 32 and 34 on the front and back surfaces of the substrate, deep polarization inversion can be formed. However, in order to further increase the depth of the domain-inverted portion, if the voltage is set to about 6 kV (electric field: 12 kV / mm) or more in order to increase the voltage between the electrodes 32 and 34, It has been found that dielectric breakdown may occur and it is difficult to apply a high voltage.

Therefore, as a result of examining the voltage waveform, it was found that the dielectric breakdown can be prevented by shortening the voltage application time. Regarding the application time, if the applied time exceeds 1 second, only about 5 kV can be applied, but the voltage applied between the electrodes 32 and 34 can be increased to about 6 kV by setting the voltage applied waveform to a pulse waveform of 1 second or less. Becomes
It is possible to form a domain-inverted portion having a depth of 1.8 μm.
Further, when the pulse width is reduced to 100 ms or less, a voltage of about 8 kV can be applied, and a domain-inverted portion having a depth of 2 μm or more can be formed.

As the pulse voltage in the thickness direction of the substrate was increased, when the voltage exceeded 9 kV, cracks sometimes occurred in the substrate due to dielectric breakdown. As described above, it is preferable that the electric field in the thickness direction of the substrate be 4 kV / mm or more and less than the voltage that causes dielectric breakdown of the substrate. This electric field is preferably applied by a pulse voltage. In this case, the electric field is 8 kV / mm or more, particularly 12 kV / mm or more, for example, 8 to 10 kV / mm.
It was effective to set it to 18 kV / mm.

The voltage at which the dielectric breakdown of the substrate is caused by LiNb
In the case of a substrate such as O ₃ and LiTaO ₃ , the CW voltage was 5 to 6 kV / mm, but the pulse voltage of the order of ms did not break even at a voltage of 10 kV / mm.

When the voltage is applied in the polarization direction of the substrate by a pulse voltage, the pulse width of the pulse voltage in the thickness direction of the substrate is set to be larger than the pulse width of the pulse voltage in the polarization direction of the substrate. It was confirmed that it was effective. This is considered to be due to the fact that the depth of the domain-inverted portion depends on the depth of domain-inverted nuclei generated at the tip of the electrode finger as described above.

The application of the pulse voltage in the thickness direction of the substrate is substantially simultaneous with the start of the voltage application in the substrate polarization direction.
Or it was effective to apply it earlier. On the other hand, the application of the pulse voltage in the polarization direction of the substrate was effective even immediately before the application of the voltage in the thickness direction of the substrate. this is,
When a voltage is applied to the electrodes 32 and 34, domain-inverted nuclei are formed at the tips of the electrode fingers of the comb-shaped electrode 32 in the thickness direction of the substrate, and a domain-inverted portion is formed between the electrodes 32 and 33 starting from the domain-inverted nuclei. it is conceivable that. As described above, in the present embodiment, it is not always necessary to apply the electric field components in two directions at the same time.

As described above, in the present embodiment, the electric field is extended in the polarization direction by the electric field including the electric field component in the polarization direction (for example, the substrate surface direction) and the electric field component in the direction perpendicular to this direction (for example, the substrate thickness direction). The domain-inverted portion is formed so as to expand also in the direction perpendicular to the polarization direction.

In the above, it has been confirmed that a deep domain-inverted portion can be formed by applying a high voltage between the electrodes 2 and 4 using a substrate having a thickness of 0.5 mm. However, there is a certain limit to the increase in applied voltage due to the decrease in pulse width.
Therefore, when various studies were made to increase the applied electric field, it was confirmed that the voltage at which dielectric breakdown occurred was basically unchanged even when the substrate thickness was reduced. That is, it has been found that the applied electric field can be greatly increased by reducing the thickness of the substrate even when the same voltage is applied.

For example, the substrate is set to a half of 0.5 mm of 0.25.
mm, the voltage that can be applied is substantially the same,
The applied electric field is doubled. Therefore, the relationship between the substrate thickness and the formed polarization inversion depth was examined. The result is shown in FIG. As shown in FIG. 14, when the substrate thickness is around 0.4 mm, the polarization inversion depth is 2.0 μm, and when the substrate thickness is 0.3 mm, the polarization inversion depth is 2.5 μm or more. Therefore, it has been found that a domain-inverted portion having a thickness of about the thickness of a normal optical waveguide or more can be formed.

As described above, a waveguide type optical wavelength conversion element is formed by forming an optical waveguide on the surface of a substrate having a domain inversion portion. When the fundamental wave guided through the optical waveguide propagates through the waveguide, the wavelength is converted by the domain-inverted portion formed so as to periodically cross the waveguide.
The conversion efficiency at this time is affected by the degree of overlap between the light propagating through the optical waveguide and the polarization inversion portion. This overlap can be maximized by setting the depth of the domain-inverted portion to 2.0 μm or more. Therefore,
By setting the substrate thickness to 0.4 mm or less, the conversion efficiency of the optical wavelength conversion element having the optical waveguide can be greatly improved.

In this embodiment, a substrate made of a ferroelectric crystal is used. However, a similar domain-inverted portion can be formed by using a ferroelectric crystal film manufactured by vapor or liquid layer growth. it can. Since a film manufactured by crystal growth has a low impurity concentration and high crystallinity, a highly efficient light wavelength conversion element can be formed. Such a crystal film can be used in other embodiments.

(Embodiment 9) Here, Embodiment 8 will be described.
A method for manufacturing another domain-inverted portion related to the above will be described. As described above, as a method of forming a domain-inverted portion on the surface of a ferroelectric crystal of an X or Y plate, for example, MgO: LiNbO ₃ , an electric field in a polarization direction is applied by applying a voltage between electrodes formed on the surface. A method for inverting the polarization by inducing the polarization has been conventionally known.

However, it has been found that a similar domain-inverted portion is generated on the substrate surface by newly applying a voltage between the pair of electrodes formed on the front and rear surfaces of the substrate.
An example of this manufacturing method will be described with reference to FIG. As shown in FIG. 15, a comb-shaped electrode 37 and a plate electrode 38 were formed on opposing substrate surfaces of an X-plate MgO: LiNbO ₃ substrate 36, respectively. Then, when a voltage was applied from the voltage source 39 between the comb-shaped electrode 37 and the plate electrode 38, it was confirmed that a domain-inverted portion was formed at the tip of the electrode finger of the comb-shaped electrode 37. The substrate was 0.5 mm, and the applied voltage was about 8 kV.

It has been confirmed that the length of the domain-inverted portion formed at this time depends on the electrode shape. Specifically, by increasing the ratio of the width of the back plate electrode 38 in the polarization direction to the width of the front comb electrode 37 in the polarization direction,
It has been found that the length of polarization reversal can be increased. For example, when the electrode width ratio is less than twice, the length of the domain-inverted portion is 1
0 μm or less, but when the ratio is 4 times or more, 100 μm
It was confirmed that a domain-inverted portion was formed to a certain extent. Further, by making it 10 times or more, the uniformity of the domain-inverted portion was also increased.

Although the reason is not clear, it is considered that an electric field in the direction of polarization is generated when an electric field at the tip of the electrode finger is applied to the edge of the back electrode of the substrate.

Then, the position of the back electrode was examined.
By forming an electric field component generated between the front electrode and the back electrode in the polarization direction of the substrate, a domain-inverted portion is formed. Therefore, as shown in FIG. 16, it is considered that the polarization inversion may be caused by shifting the position of the flat plate electrode on the back surface in the polarization direction with respect to the comb-shaped electrode 37 on the front surface. Therefore, the bar-shaped electrode on the back side is formed so as to be shifted in the −C plane direction with respect to the comb-shaped electrode on the front side. The shift distance is 5, 10, 2
0 and 100 μm. When a voltage was applied between the electrodes, no polarization reversal portion was formed at 10 μm or less, but at 20 μm or more (the angle between the line connecting the electrodes and the substrate surface was about 88 °), the polarization was not changed at all. An inversion was formed. The formed domain-inverted portion had a deep shape of about 2 μm and was excellent in uniformity. In this case, it was also confirmed that the angle formed between the line connecting the electrodes and the substrate surface was preferably 2 to 80 °. By making it 2 ° or more,
This is because a substrate thickness capable of securing a substrate strength necessary for practical use is ensured, and by setting the angle to 80 ° or less, a domain-inverted portion can be formed stably. In addition,
To be precise, the angle is defined as an angle formed by a line segment connecting the near ends 37a and 37b of a pair of electrodes formed to be shifted on the opposing surfaces and the substrate surface.

The method of applying such an electric field is shown in FIG.
As shown in FIG. 7, it was also effective when the polarization direction was shifted from the substrate surface by a predetermined angle θ. Angle θ in this case
Is preferably 1 to 5 °. By making it 1 ° or more,
This is because the depth of the domain-inverted portion can be increased, and by setting it to 5 ° or less, uniformity of the domain-inverted portion can be ensured.

(Embodiment 10) Here, an example of an optical wavelength conversion element using a domain-inverted portion formed according to the above embodiment will be described. As shown in FIG. 18, the optical wavelength conversion element is a ferroelectric crystal of X plate (for example, MgO: LiNbO ₃ ).
On the surface of the substrate 41 having a period of 3.2 μm.
Is formed, and an optical waveguide 42 formed by proton exchange is formed so as to be orthogonal to the domain-inverted portion. The optical waveguide 42 forms a stripe-shaped mask pattern on the surface of the substrate after the formation of the domain-inverted portions 43, and heat-treats this in an acid, so that Li in the substrate and protons in the acid are exchanged in the unmasked portion. Formed by

Next, Ti: Al ₂ O ₃ was added to the manufactured optical wavelength conversion element.
Laser light was incident and the conversion efficiency was measured. FIG. 19 shows the result of examining the relationship between the substrate thickness and the conversion efficiency. When the substrate thickness is 0.3mm or less, the conversion efficiency is maximized, about 400%
/ W value.

Further, by controlling the substrate thickness 44 to 0.3 mm or less, the temperature control of the light wavelength conversion element was facilitated. An optical wavelength conversion device using a periodically poled structure can perform high-efficiency conversion, but the phase matching conditions are severe and the wavelength tolerance of the convertible fundamental wave is limited to a narrow range of about 0.1 nm. Problem. Therefore, when the wavelength of the fundamental wave fluctuates, it is necessary to adjust the wavelength of the phase matching to the wavelength of the fundamental wave by controlling the temperature of the substrate. However, when the substrate is thick, a large amount of electric power is required for temperature control because the heat capacity of the substrate is high. Also, if a rapid temperature change is attempted to perform high-speed control, a temperature difference occurs between the front and back surfaces of the substrate, causing distortion in the substrate and causing deterioration in characteristics. Since the substrate distortion due to the temperature difference increases depending on the thickness of the substrate, for example, for a 0.5 mm-thick substrate, about 0.1 second is required to stabilize the output with temperature while preventing deterioration of characteristics. Needed time. However, when the thickness of the substrate is reduced to 0.3 mm or less, the deterioration of the characteristics due to the temperature change is almost eliminated, and even if the temperature is stabilized at a speed of 0.01 seconds or less, the deterioration of the characteristics is not observed. Stabilization could be performed fast enough.

(Embodiment 11) Here, a light generator using the optical wavelength conversion element of Embodiment 10 will be described. This light generation device is suitable for generating short-wavelength light as in the case of the sixth embodiment, and can be used for the optical pickup as described in the seventh embodiment.

FIG. 20 shows a short-wavelength light generator according to the present embodiment. The fundamental wave emitted from the semiconductor laser 46 is wavelength-converted by the optical wavelength conversion element 47, and is emitted as SHG 45. For example, if a semiconductor laser having a wavelength of 800 nm is used, blue SHG light having a wavelength of 400 nm can be obtained, and a compact blue light source can be realized. The light wavelength conversion element of this embodiment is Mg.
Since an X plate of a crystal having a nonlinear optical effect such as O: LiNbO ₃ is used, the optical waveguide is TE-polarized. Therefore, the polarization direction is the same as that of the optical waveguides constituting the semiconductor laser, and the electric field distribution and the polarization direction between the waveguides are maintained in substantially the same shape, so that highly efficient coupling is possible. Therefore, the output light from the semiconductor laser is reduced to 7
Since the light can be coupled into the optical wavelength conversion element with a coupling efficiency of 0% or more, a high-output small-wavelength light source can be realized.

Such a light generating device can be applied in a wide range of fields as in the device described in the sixth embodiment. When the laser is pulse-driven, a fundamental wave having a high peak power can be obtained, and the light damage resistance can be reduced. It was excellent also from a viewpoint.

(Embodiment 12) In addition to the method described above, according to the present invention, the polarization direction and the substrate surface are substantially parallel to each other, or the ferroelectric substrate having a constant inclination is uniformly formed on the ferroelectric substrate. As a method for forming a deep domain-inverted portion, a method for forming a domain-inverted portion using a low-resistance portion formed between electrodes arranged on a substrate surface of a ferroelectric crystal is provided.

First, the polarization inversion characteristics by the conventional polarization inversion method were examined. As shown in FIG. 21, a comb-shaped electrode 112 is formed on the surface of an X-plate ferroelectric crystal (MgO: LiNbO ₃ ) substrate 111.
And a bar-shaped electrode 113, and a high voltage is applied between the two electrodes from a power supply 115 to reverse the polarization between the electrodes. In addition,
The comb-shaped electrode 112 is formed in the + C direction of the substrate, and the bar-shaped electrode 113 is formed in the -C direction. When applying a voltage, the entire substrate was immersed in an insulating liquid to prevent discharge.

When the relationship between the distance between the electrodes 112 and 113 and the polarization reversal depth was examined, the results shown in FIG. 22 were obtained. As the electrode interval became smaller, the polarization inversion depth became deeper, and the electrode interval became about 100 μm and the polarization inversion depth became about 1 μm. This is because, as shown in FIG. 23, the electric field distribution when the electrode interval is narrow (FIG. 23 (b)) is smaller than the electric field distribution when the electrode interval is wide (FIG. 23 (b)). This is because the electric field component in the depth direction increases, so that a deep polarization inversion is formed. However, in order to form a deeper polarization inversion, if the electrode interval is set to 100 μm or less, dielectric breakdown may occur between the electrodes, and a polarization inversion portion may not be formed. Therefore, in order to control the direction of the electric field applied to the crystal without reducing the distance between the electrodes, a low resistance portion is arranged between the electrodes in the present embodiment.

That is, as shown in FIG. 24, a comb-shaped electrode 117 and a rod-shaped electrode 118 are formed on the surface of an X-plate ferroelectric crystal substrate (MgO: LiNbO ₃ ) 116,
7, a low resistance portion 120 was formed. As shown, the low-resistance portion 120 was formed so as to correspond to the comb-shaped portion of the comb-shaped electrode 117, and the distance between the electrodes 117 and 118 and the low-resistance portion 120 was 30 μm. Further, the low resistance portion 120 is a metal film here. According to this method, when a voltage is applied between the electrodes from the voltage source 119, a domain-inverted portion having a depth of 1.5 μm or more is formed in the depth direction, and a domain-inverted portion having a depth 1.5 times that in the related art can be formed. It was confirmed that.

The reason why such a deep polarization inversion portion can be formed is that the electric field distribution when a voltage is applied between the electrodes 117 and 118 by arranging the low resistance portion 120 is shown in FIG. As described above, it has a larger component in the depth direction than the conventional one (FIG. 23).

Next, the shape of the low resistance portion 120 was examined. The electric field component at the tip of the electrode depends on the shape of the low resistance portion 120. For example, when the length of the low resistance portion in the polarization direction is less than 5 μm, only the domain inversion having almost the same depth as that without the low resistance portion was formed. When the low resistance portion is set to 10 μm or more, the effect of increasing the polarization inversion depth by the low resistance portion appears. However, the length of the low resistance part is 30 μm.
The polarization inversion depth became maximum at about m, and the depth did not change even if it was longer. Therefore, the low resistance portion is 10 to
Preferably, the thickness is about 30 μm.

When only one low-resistance portion is provided, only two polarization inversion portions are formed between the electrodes (the tip end of the comb-shaped electrode and one end of the low-resistance portion). Can not do. Therefore, a method of forming a plurality of low resistance portions between the electrodes as a method of effectively using a crystal was studied.

More specifically, as shown in FIG. 26, the low-resistance portions 121a and 121b were arranged almost evenly between the electrodes 117 and 118. When a voltage is applied between the electrodes 117 and 118, a voltage is evenly applied between the low resistance portions 121a and 121b. The shape of the applied electric field was the same between the low-resistance portions, and a deep domain-inverted portion was formed uniformly. Further, it has been confirmed that the use of this method can greatly increase the interval between the electrodes, so that a domain-inverted portion can be formed in a wide area.

That is, in the conventional method, the electrode interval is 100
When the domain-inverted portion is optimally formed on the order of μm and the domain-inverted portion is to be formed in a wider area, there is a problem that the domain-inverted portion becomes shallow as shown in FIG. However, if a plurality of low resistance portions are provided between the electrodes, the distance between the electrodes can be increased while avoiding such a problem. For example, by setting the distance between the electrodes 117 and 118 to 500 μm and arranging the low-resistance portions evenly between the electrodes, the conventional 100 μm
A uniform domain-inverted portion could be formed in an area five times as large as about m.

Further, as a result of studying another low-resistance body as the low-resistance portion, it was confirmed that a material having an electric resistance lower than that of the substrate was effective as the low-resistance portion. Further, it was confirmed that a material having a resistance value that is at least one digit lower than the electric resistance of the substrate is effective for uniformizing the domain-inverted portions.

Next, an attempt was made to use an ion exchange portion as the low resistance portion. Here, proton exchange was used as ion exchange. According to the proton exchange, it is possible to reduce the electric resistance of the substrate by one digit or more. FIG.
The low-resistance portion 121 is formed by a method substantially similar to the method shown in FIG.
When a and 121b were formed by proton exchange and a voltage was applied between the electrodes, the polarization inversion was 1.2 times deeper than when the metal film was used for the low resistance portion. This is presumably because the distribution of the applied electric field became deeper because the proton exchange portion was formed inside the substrate while the metal film was formed on the substrate surface.

Although the proton exchange is used as the low resistance part, the method of ion exchange with other ions or metal diffusion as described above is also effective as a method of reducing the resistance of the substrate.

(Thirteenth Embodiment) Next, a monopolar ferroelectric crystal substrate (MgO: LiNbO ₃ oblique substrate) having a polarization direction at an angle of 0 ° or more with respect to the substrate surface is formed. Tried to apply the method.

First, as shown in FIG. 27, a comb-shaped electrode 127 is formed on an oblique substrate 126 in which the X-axis of the crystal is inclined by 3 ° with respect to the substrate normal, according to a method conventionally reported.
And the rod-shaped electrode 128 were formed at an electrode interval of 400 μm, and a voltage was applied from a voltage source 129 in an insulating liquid.

As a result, as shown in FIGS. 27 (b) and 27 (c), a rod-shaped domain-inverted portion sunk into the substrate along the direction of polarization was formed. When the polarization inversion period is 3.2 μm,
The size of the domain-inverted portion in the depth direction was about 2 μm.

As described above, according to the conventional method, it is possible to form a deep domain-inverted portion by using an oblique substrate, but the domain-inverted portion grows at an angle with respect to the substrate surface. As you move away from
The domain-inverted portion has sunk into the substrate, and a problem has arisen that the overlap with the optical waveguide formed on the substrate surface is reduced. For example, when the crystal axis is tilted by 3 ° with respect to the substrate surface, a domain-inverted portion with a depth of 2 μm is formed from the surface near the tip of the electrode finger, but at a distance of 20 μm, the domain-inverted portion extends below 1 μm. Therefore, it overlaps only about half with the optical waveguide. If it is 20 μm away, it will sink into 2 μm and will not almost overlap the optical waveguide. Therefore, the portion where the optical waveguide is formed is substantially limited to a part of the substrate surface, and only one or two optical waveguides can be formed on the substrate.

As shown in FIG. 28, as in the twelfth embodiment, a low resistance portion is used. A comb-shaped electrode is formed on an MgO: LiNbO ₃ oblique substrate 126 whose polarization direction is inclined by 3 ° with respect to the substrate surface (the X axis of the crystal is inclined by 3 ° with respect to the substrate normal as shown in the figure, and the Y axis is parallel to the substrate surface). 127 and rod-shaped electrode 1
28, and low resistance portions (metal films) 122a and 122b were formed between the electrodes 127 and 128. Comb electrode 12
The distance between the electrode 7 and the rod-shaped electrode 128 is 400 μm, the low-resistance parts 122 a and 122 b are formed so as to correspond to the comb-shaped part of the comb-shaped electrode 127, the length between the electrodes is 20 μm, and the distance between the electrodes 127 and 128 is equal. Was placed.

When a voltage is applied between the electrodes 127 and 128 to invert the polarization in this manner, as shown in a cross-sectional view of FIG. But also grown from the low resistance portion 122. The polarization inversion depth grown from the comb-shaped electrode 127 is about 2.
The polarization reversal depth grown from the low-resistance portions 122a and 122b was 2 μm, and the depth was about 1.8 μm. As described above, according to the present embodiment, a plurality of domain-inverted portions that sufficiently overlap with the optical waveguide over the electrode interval of 400 μm are formed by acting as a so-called intermediate electrode due to the electric field applied to the low-resistance portion. The area has improved several times.

The domain-inverted portions 131 formed by this method are:
This is a deep periodic structure that cannot be formed by the conventional method, and is also formed periodically in the substrate depth direction as shown in FIG.

Further, by narrowing the interval between the intermediate electrodes 122a and 122b, it becomes possible to bring the domain-inverted portions into contact in the depth direction. It was confirmed that such a method can form a periodically domain-inverted portion over a depth of 10 μm or more from the surface. The polarization inversion depth, which was conventionally at most about 2 μm, can be increased to 10 μm or more, and the efficiency of the optical wavelength conversion element can be dramatically improved.

Next, the inclination between the crystal axis of the substrate crystal and the plane of the substrate was examined. The tilt of the crystal axis affects the polarization inversion depth. When manufacturing a domain-inverted portion, as shown in FIG. 29, the angle was 0.8 μm at an angle of 0 ° and 2 μm at an angle of 3 °. However, when the angle was 10 ° or more, it was difficult to form a domain-inverted portion. When the crystal angle is reduced, the domain-inverted portion formed tends to be shallower.Therefore, when a domain-inverted portion is formed only on a diagonal substrate with a comb-shaped electrode and a rod-shaped electrode, a domain-inverted depth similar to that of an optical waveguide is required. 1-5 °, especially 2
~ 5 ° is preferred.

However, in the case where the domain-inverted portions are formed by contacting the domain-inverted portions in the depth direction as described above, deep domain-inversion can be formed even with a substrate having a small crystal axis tilt angle. For example, even for a substrate having an inclination of 2 ° or less, 10
A deep domain inversion of at least μm can be formed. When the inclination of the crystal axis becomes 2 ° or less, the inclination of the domain-inverted portion becomes small,
There is an advantage that the uniformity of the domain-inverted portions in the plane is improved.

Further, it has been found that the length of the intermediate electrode 122 affects the depth of the domain-inverted portion growing from this portion, and further affects the domain-inverted portion growing from the comb electrode or another intermediate electrode. did.

As shown in FIG. 28B, the intermediate electrode 12
The domain-inverted portions formed below the domain-inverted domain invert over substantially the length of the low-resistance portion and sunk into the substrate from the tip of the electrode finger, so that the electrode length affects the domain-inverted depth. FIG.
9 shows the relationship between the electrode length of the intermediate electrode 122 and the depth of the domain-inverted portion formed thereunder. When the electrode length is 5 μm or less, polarization inversion in the depth direction is hardly substantially formed. on the other hand,
From about 10 μm, the domain-inverted portion is formed in the depth direction and increases in proportion to the electrode length, but saturates at about 30 μm, and the domain-inverted depth does not increase even if the electrode length is further increased.
Therefore, the electrode length is preferably set to 10 to 30 μm.

On the other hand, if the length of the intermediate electrode is long, it affects the polarization reversal formed by the other electrodes. Comb electrode 12
The polarization inversion portion grows from 7 toward the rod-shaped electrode 128, but when the intermediate electrode 122 existing between the electrodes becomes longer,
It has been found that the length of the growing domain inversion is limited. That is, when the electrode length is 30 μm or less, the polarization reversal is formed from the tip of the electrode finger of the comb-shaped electrode 127 to the end of the rod-shaped electrode 128. However, when the electrode length exceeds 30 μm, the polarization reversal part is formed only in the middle of the intermediate electrode. It is not formed, and the growth of the domain-inverted portion becomes less than half. This is because the electric field distribution between the electrodes differs depending on the electrode structure. Accordingly, it is clear from this point that the length of the intermediate electrode is preferably 10 to 30 μm. To form a deeper polarization inversion, the thickness is preferably 20 to 30 μm.

The shape of the electrode affects the uniformity of the polarization reversal. When observing the domain-inverted portion 131 formed below the electrode from the upper surface, as shown in FIG. 31, the domain-inverted portion 131 becomes thinner at first and becomes deeper as it goes to the tip of the electrode finger. In order to improve the uniformity of the domain-inverted portion, it was effective to make the electrode structure similar to the domain-inverted shape. For example, when the uniformity of the domain-inverted shape was examined when the shape of the intermediate electrode 122 was substantially triangular, the uniformity was improved about 1.5 times as compared with the case where the rectangular electrode was used. confirmed. This is probably because the electrode structure was brought closer to the domain-inverted shape.

Further, as described above, a crystal thin film formed by crystal growth can be used. Since the crystal-grown thin film layer has excellent crystallinity and high nonlinearity, a highly efficient light wavelength conversion element can be formed. Further, since the crystal layer can be used as an optical waveguide, deterioration of nonlinearity due to the formation of the waveguide can be prevented.

(Embodiment 14) In addition to the above embodiments, according to the present invention, a uniform and deep polarization inversion is performed on a ferroelectric substrate whose polarization direction is substantially parallel to the substrate surface or whose inclination is constant. As another method for forming the portion, there is provided a method in which a concave portion is formed on a substrate surface of a ferroelectric crystal, and a domain-inverted portion is formed using an electrode formed in the concave portion.

Next, another method for forming a deep domain inversion structure will be described. As shown in FIG. 32, a concave portion 140 is periodically formed on one side (+ C side) of the X-plate substrate, and a first electrode 142 is formed in a region including the concave portion. Further, a rod-shaped electrode 143 is formed on the −C side of the substrate surface, and a voltage is applied between both electrodes to form a periodically poled portion. Note that FIG.
As shown in FIG. 2, the crystal surface between the concave portions 140 was covered with an insulating film (for example, SiO ₂ ) 141.

By forming an electrode in the recess 140 engraved in the substrate, a domain-inverted nucleus can be generated in a deep portion of the substrate, and a deep domain-inverted portion 145 can be grown from the domain-inverted nucleus. For example, when the depth of the concave portion is about 1 μm, the depth of the domain-inverted portion is 2 μm,
A polarization inversion of twice the depth was formed.

The effect of the insulating film 141 between the recesses 140 lies in the formation of a uniform periodic structure. When not covered with the insulating film 141, adjacent domain-inverted portions generated between the concave portions 140 may come into contact with each other, making it difficult to form a short-period domain-inverted configuration. In particular, it was difficult to form an inversion structure having a polarization inversion cycle of 5 μm or less. However, it has been found that by covering with an insulating film, insulation between the electrodes is increased, and a short-period domain-inverted portion can be formed. As a result, a domain-inverted structure having a period of 3 μm or less can be formed.

The depth of the domain-inverted portion depends on the depth of the electrode 142. Since the domain-inverted portion grows up and down about 1 μm from the bottom of the concave portion, the depth is 0.5 μm.
A recess having the above depth is formed, and the polarization inversion depth is set to 1.5 μm.
Thus, a highly efficient optical wavelength conversion element was formed. However, when the depth of the concave portion is set to 3 μm or more, a domain-inverted portion is formed inside the substrate. Therefore, the depth is preferably set to 3 μm or less.

A method for easily forming such an engraved electrode has been newly found. In order to form the electrode structure as shown in FIG. 32, it is necessary to selectively form a metal film on the engraved portion, which requires a complicated manufacturing process such as mask alignment. Therefore, a method capable of self-alignment will be described with reference to FIG.

As shown in FIG. 33, according to this method, an insulating film (for example, SiO ₂ film) 147 is deposited to a thickness of, for example, 0.2 μm on the surface of an X-plate substrate (for example, MgO: LiNbO ₃ ) 146. (FIG. 33 (a)), the insulating film 147 and the substrate 146 are simultaneously etched by photolithography and etching, and a concave portion 148 having a depth of, for example, about 2 μm is formed periodically (FIG. 33 (b)). A first electrode 149 is formed by evaporating a metal thereon (FIG. 33 (c)), and a rod-shaped electrode 150 is formed on the substrate surface (FIG. 33 (d)).
By applying a voltage between 48 and 150,
A domain-inverted nucleus is generated from eight side surfaces and grown on the rod-shaped electrode 150 side. By using this method, it was not necessary to selectively form an electrode in the carved portion, and the electrode structure could be selectively formed in the recess by self-alignment.

In this case, a domain-inverted portion having a depth of about 2 μm was formed. As shown in FIG. 44 (b), the cross section of the concave portion has the substrate exposed at the bottom surface of the concave portion, but the convex portion is insulated by the SiO ₂ film. Therefore, the metal is in direct contact with the substrate surface on the bottom surface of the concave portion. Since the occurrence of the polarization inversion is remarkable at the metal contact portion, the polarization inversion occurs only at the bottom surface of the concave portion and grows toward the rod-shaped electrode. On the other hand, since the convex portion is covered with the insulating film 141, the occurrence of polarization is suppressed.

Since the bottom surface of the concave portion is located at a depth of about 2 μm from the substrate surface, a domain-inverted structure having a depth of 3 μm can be formed. This is about three times the depth of the conventional one, and a highly efficient optical wavelength conversion element can be manufactured.

An optical wavelength conversion element using a periodically poled portion formed by the method of manufacturing a poled portion according to the above embodiment was manufactured. As shown in FIG. 26, this optical wavelength conversion element has a polarization inversion section 153 formed on the surface of a ferroelectric crystal substrate 151 such as MgO: LiNbO ₃ so as to periodically cross the optical waveguide 154 and the optical waveguide. The most remarkable feature is that a concave portion 152 exists at one end of the domain-inverted portion 153.

When a concave portion was formed in the substrate to form a domain-inverted portion, a domain-inverted portion deeper than in the conventional method using an oblique substrate could be formed. For example, the domain-inverted portion of the LiNbO ₃ substrate doped with MgO expands in the periphery of the electrode, but expands in the depth direction more than in the width direction. For this reason, the domain-inverted portions formed on the oblique substrate are substantially semicircular domain-inverted portions.
On the other hand, when the above-mentioned electrode structure is used, the domain-inverted portion becomes a substantially circular inversion, and is 3 μm, which is 1.5 times as large as that when the oblique substrate is used.
It has been clarified that polarization inversion having the above thickness can be formed. For this reason, the optical wavelength conversion element was twice as efficient as when the oblique substrate was used.

Further, since the domain-inverted portions are formed substantially parallel to the substrate surface, there is an advantage that all the domain-inverted portions formed have a sufficient overlap with the optical waveguide. In the conventional method using an oblique substrate, the effective area of the domain-inverted portion that can be used is small. However, the use of the engraved electrode has the advantage that the effective area of the domain-inverted portion can be greatly increased.

Further, the concave portion formed at one end of the domain inversion can be effectively used for forming a light wavelength conversion element and a short wavelength light generation device using the light wavelength conversion element. In order to form an optical wavelength conversion element, it is necessary to form an optical waveguide in alignment with the fabricated domain-inverted structure. However, since the domain-inverted structure cannot be visually observed, it is necessary to form a positioning marker or the like. The fabrication process is complicated. However, if a concave portion is formed at one end of the domain-inverted portion, alignment with the optical waveguide can be easily performed by adjusting the position to the concave portion. Similarly, the short-wavelength light generator fixes the optical wavelength conversion element and the semiconductor laser by directly coupling them. For this reason, it is necessary to align the optical waveguide of the optical wavelength conversion element with the optical waveguide of the semiconductor laser with submicron accuracy. For this purpose, marks for alignment are formed in each optical waveguide, and each mark is image-detected to perform alignment. If a concave portion is formed at one end of the domain-inverted portion, it is very easy to detect an image of the concave portion. Further, since the optical waveguide is formed in accordance with the concave portion, there is an advantage that the position of the optical waveguide can be detected with submicron accuracy from the position of the concave portion. In addition, since the concave portion engraved on the substrate cannot be easily removed, it can be reliably detected without being damaged by other manufacturing processes or the like.

In this embodiment, the insulating film is made of Si.
Although O ₂ was used, other resists can be used. When a resist is used, a similar domain-inverted structure can be formed by forming an electrode on the resist using the resist used for etching as it is, so that the manufacturing process can be simplified.
The point that a resist can be used as the insulating film is the same in other embodiments. In the present embodiment, the X plate is used, but it goes without saying that a Y plate or the like can be used. Further, the effects of the present embodiment can be obtained even when an oblique substrate or the like is used as described above.

(Embodiment 15) Still another embodiment of the optical wavelength conversion element of the present invention will be described. This optical wavelength conversion element uses a domain-inverted structure that can be manufactured according to the thirteenth embodiment. As shown in FIG. 35 (a), a ferroelectric material whose crystal axis is cut out from the substrate surface is inclined. On the surface of a crystal (MgO: LiNbO ₃ ) substrate plate 161, a domain-inverted portion 162 and an optical waveguide 163 are formed. As shown in FIG. 35 (b), the domain-inverted portions 162 form a layered structure in a cross section perpendicular to the substrate surface and along the polarization direction of the substrate, and the optical waveguide extends. It is also formed periodically in the direction. As described above, the domain-inverted portion 162 has a periodic structure in the extending (progressing) direction of the optical waveguide and the substrate depth direction. The thickness of the domain-inverted portions is, for example, 2 μm, and the interval between domain-inverted portions is, for example, 0.3 μm
It is said. The thickness of the optical waveguide is, for example, 4 μm.

In such an optical wavelength conversion device, the power density of the optical waveguide was reduced, and the shape of the waveguide was increased so that high-output harmonics could be generated. Even in this case, by forming the domain-inverted portions 162 periodically in the depth direction, it was possible to form a deep domain-inverted structure, which was difficult in the related art, and a sufficient overlap with a deep waveguide was secured.

When the characteristics of this light wavelength conversion element were evaluated, 100 mW of blue output light 165 was obtained with 200 mW of incident light 164, and a high output more than 5 times that of the conventional device was realized.
The output was stable, and output fluctuation due to optical damage was not substantially observed. The deep waveguide structure improves the efficiency, and by increasing the cross-sectional area of the waveguide, it is possible to reduce the occurrence of optical damage due to the increase in power density. became.

Conventionally, it was difficult to form a deep domain-inverted portion in a short period, partly because the domain-inverted portion expanded in the width direction simultaneously with the depth direction. In order to convert the wavelength into light having a short wavelength from blue to ultraviolet, a polarization inversion period of 3.5 μm is required.
A short period structure of m or less is required. For this reason, the width of the domain-inverted portion must be reduced to 2 μm or less. on the other hand,
In order for the depth of the domain-inverted portion to sufficiently overlap with the optical waveguide, 2 μm is required.
A deep structure of m or more is required. Conventionally, it has been difficult to form a deep domain-inverted structure by limiting only the spreading speed of the domain-inverted portion in the width direction. However, as mentioned above,
By periodically overlapping the domain-inverted structures in the depth direction,
A substantially deep domain inversion structure can be formed.

In addition, the presence of such a domain-inverted structure significantly improved the light damage resistance. When optical damage occurs, the impurity is excited by the light and has a distribution in the polarization direction, so that an electric field is generated, and the refractive index fluctuates through the electro-optic effect. When the polarization is reversed, the movement of the excited impurity is reversed in the polarization reversal part, so that the generated electric field is canceled out and the occurrence of optical damage is suppressed. By forming the domain-inverted portions periodically in the depth direction, the electric field canceling effect was further enhanced. As a result, a light damage resistance of about twice that of the related art was obtained.

In order to use the domain-inverted structure as a deep inversion, it is necessary to reduce the thickness of the inversion portion with respect to the period in the depth direction. This is because if the domain-inverted parts are separated,
This is because the portion overlapping the optical waveguide is reduced. In order to obtain an appropriate overlap with the optical waveguide, it is preferable that the thickness D of the domain-inverted portion be equal to or less than 1/2 of the domain-inverted period Λ in the depth direction. Further, in order to increase the efficiency, it is desirable to set it to about Λ / 4.

(Embodiment 16) As described in the above embodiment, by utilizing the property that the domain-inverted portions are formed along the direction of spontaneous polarization, a bulk-type domain-inverted structure can be formed by using an oblique substrate. Can be formed. This domain-inverted structure can be used as a bulk type device. The present embodiment provides an example of a method of manufacturing such a domain-inverted structure and an element. As shown in FIG. 36, a ferroelectric crystal (MgO: LiNb) whose X axis is inclined by 3 ° from the normal line of the substrate 166
O ₃ ) Electrodes 167 and 168 are formed on a substrate 166, and a rod-like intermediate electrode 16 is provided between the electrodes 167 and 168.
9 are formed. As the intermediate electrode, a plurality of electrodes may be arranged at equal intervals. When a voltage is applied between the electrodes 167 and 168, a domain-inverted portion is formed below the electrode 167 and the intermediate electrode 169, and the domain-inverted portion grows in the polarization direction of the crystal of the substrate 166. By using such a method, it is possible to form a structure in which a plurality of domain-inverted portions 170 are in a layer shape parallel to each other, as shown in FIG.

The domain-inverted structure as shown in FIG. 36 (b) is used as a bulk-type optical wavelength conversion element that allows light to enter almost vertically. The features of this optical wavelength conversion element are:
The point is that the short-period domain-inverted portions 170 can be easily formed. As is clear from FIG. 36 (b), the period of the domain-inverted portions in the direction perpendicular to the substrate surface (depth direction) can be shorter than the period of the electrodes. For example, when a substrate having a crystal axis inclined by 3 ° is used, the period in the depth direction of the domain-inverted portion is about 1/20 of the period of the electrode. That is, when the rod-shaped electrode is
When formed at intervals of 0 μm, the period of the domain-inverted portion is about 1/20, that is, 1 μm. Thus, a very short-period domain-inverted portion can be easily formed. If a short-period domain-inverted structure can be formed, the wavelength to be converted can be shortened.

Although a rod-shaped electrode is used as the electrode 167 in this embodiment, a comb-shaped electrode is more preferably used instead. When a comb-shaped electrode is used, cracks generated around the electrode due to distortion of the crystal due to a piezoelectric effect when an electric field is applied can be suppressed. However, when a comb-shaped electrode is used, the domain-inverted portions become periodic in the plane. In order to avoid this, the width of the comb of the comb-shaped electrode is set to 周期 of the period.
With the above, the same domain-inverted portions can be formed by connecting the domain-inverted portions to each other and using a rod-shaped electrode.

(Embodiment 17) The present invention also provides an optical diffraction element using a bulk domain-inverted structure that can be manufactured according to the above-described embodiment 16. This diffraction element is, as shown in FIG. 37, an oblique substrate 1 made of MgO: LiNbO ₃ or the like.
A domain-inverted portion 172 is periodically formed on the substrate 71, and electrodes 173 and 174 are formed on the front and back surfaces of the substrate. When a voltage is applied between the electrodes when light 175 is incident on the substrate, the domain-inverted portion and the non-inverted portion each undergo a change in refractive index due to an electro-optic effect, and a grating having a domain-inverted structure is formed. This makes it possible to change the light transmitted through the substrate. In the optical diffraction element of the present embodiment, the polarization inversion portion can have a two-dimensional periodic structure in both the in-plane direction and the depth direction, so that a two-dimensional diffraction element can be formed. Further, by applying a voltage, the diffraction intensity can be modulated.

In this embodiment, the diffraction by the domain-inverted portion is used, but a diffraction element by etching can be further integrated on the substrate surface. When a diffraction element is formed on the substrate surface, a complex function can be integrated into one element, so that a more complicated diffraction function can be realized.

In this embodiment, a pair of electrodes are formed on the front surface and the back surface as electrodes for refractive index modulation. However, a plurality of electrodes may be provided on the front surface or the back surface. It is preferable to control the applied electric field with a plurality of electrodes because the efficiency of the diffraction element can be two-dimensionally distributed in a plane. In this case, when a transparent electrode is used, the influence of the electrode on the diffracted light can be avoided, so that there is no restriction on the place where the electrode is formed.

Further, an example of an optical information processing apparatus using the diffraction element of the present embodiment will be described with reference to FIG. FIG.
At 8, the beam emitted from the semiconductor laser 180 passes through the beam splitter 181, and is irradiated on the optical disc 183 as an information recording medium by the diffraction element 185 and the lens 182. Conversely, the reflected light is collimated by a lens 182 and reflected by a beam splitter 181, and a signal is read by a detector 184. Diffraction element 185
Controls the focused spot focused on the optical disk 183. In order to read the signal of the optical disk 183 stably, it is necessary to focus the focal spot at a predetermined position. Therefore, the condensing spot is controlled by using the diffraction element 185. Since the diffraction element 185 can respond at high speed, it is possible to control the diffraction at a speed of 500 MHz or more. Thus, a high-speed and stable optical information processing device can be configured.

(Embodiment 18) Further, according to the present invention, using a substrate in which the polarization direction of a crystal is inclined with respect to the surface, an insulating film is provided between two electrodes spaced apart from the surface of the crystal. And applying a voltage to provide a method of manufacturing the domain-inverted portion.

The substrate was made of a ferroelectric crystal (L doped with MgO).
iNbO ₃ ) whose polarization direction was inclined by 3 ° with respect to the substrate surface was used. First, as shown in FIG.
Electrodes were formed directly on the substrate surface, and a voltage was applied between the electrodes. The first electrode 192 is a comb-shaped electrode, and the second electrode 1
93 is a rod-shaped electrode. The voltage was applied in an insulating liquid to avoid discharge. However, in many cases, high voltage cannot be applied due to dielectric breakdown between the electrodes at the time of domain inversion formation, and it has been found that the rate of forming a uniform domain inversion portion is low.

The thickness of the domain-inverted portion was as thin as about 1.2 μm. It is considered that this is because a voltage drop occurred between the electrodes due to the occurrence of the domain-inverted portions, and the applied voltage temporarily decreased. The electrode spacing was 400 μm and the applied voltage was 3 kV. When a higher voltage was applied, dielectric breakdown occurred, so that no voltage could be applied.

Further, when the domain inversion was formed over 10 mm, many non-uniform domain-inverted portions as shown in FIG. 45 came to exist. This is because, when a high voltage is applied due to polarization inversion, current is generated in a part of the comb-shaped electrode due to non-uniformity of the crystal, or the domain-inverted portion grows partially large.

Next, when a voltage was applied while covering the entire electrode as shown in FIG. 40 with the insulating film 195, the insulation between the electrodes was increased, and the uniformity of the domain-inverted portions was improved. But,
Non-uniform portions still existed, and it was difficult to form a domain-inverted portion over the entire 10 mm. This is presumably because both electrodes were in contact with the crystal surface, and the resistance value changed at non-uniform portions of the crystal, causing non-uniformity in the applied voltage. Although SiO ₂ was used as the insulating film, the inversion characteristics did not change even if the SiO ₂ film was deposited at a thickness of 50 nm or more.

As described above, the problem that the domain-inverted portions are formed non-uniformly was examined. As a result, it was confirmed that as a cause of the non-uniformity, the non-uniformity of the crystal also became a problem, but the leakage current between the electrodes, which was confirmed as a voltage drop, became a serious problem. Further, as a result of a detailed study of the leakage current, it was confirmed that the leakage current at the following two locations was a problem.

1) A portion where the electrode interval is partially narrow, or a portion where dust, dirt, etc. are attached between the electrodes. 2) Between the vicinity of the tip of the domain-inverted portion growing obliquely downward from one electrode and the other electrode above it.

The above-mentioned 1) can be solved by securing the electrode interval and reducing dust during the process. However, the problem 2) could not be avoided by the electrode design. The domain-inverted portion grows from one electrode and extends to a position immediately below the other electrode. Thereafter, the domain-inverted portions continue to grow in the thickness direction. However, if the leakage current of the above 2) exists, it becomes difficult to form a thick domain-inverted portion.

The leakage current between the domain-inverted portion and the electrode is such that the domain-inverted portion has a lower electric resistance than the substrate, and the distance between the tip of the domain-inverted portion and the electrode is considerably narrow. It is thought to be. For example, if the electrode interval is set to 200 μm using a 3 ° -cut X plate, this interval is about 10 μm. Even if the electrode interval is 400 μm, the above interval is about 10 μm. Therefore, the electrode interval is set to 200
When a domain-inverted portion is formed by the method shown in FIGS. 39 and 40, the leakage current is likely to occur, and the voltage that can be applied is 2 kV or less. No improvement was seen.

Therefore, as a method for completely preventing such a leakage current between the electrodes, a method as shown in FIG. 41 was studied. According to this method, the electrode 193 and the substrate 191
Are insulated by the insulating film 195, and a uniform and deep domain-inverted portion as shown in FIG. 42 can be formed. In this case, the voltage that can be applied without dielectric breakdown is 3 in the case of the form shown in FIGS.
Although it was kV, it increased to 4 kV or more. Due to such an increase in the applicable voltage, the thickness of the domain-inverted portion that can be formed also increased to 2 μm or more.

It was also found that the use of the method as shown in FIG. 41 greatly improved the uniformity of the domain-inverted portions. By forming an electrode via an insulating film,
This is because the non-uniformity of the applied voltage due to the non-uniformity of the crystal could be effectively suppressed. For this reason, it was possible to form a uniform domain-inverted portion over 30 mm or more.

The above SiO ₂ insulating film was formed by a sputtering method. This film thickness is 100 nm.
If less than 200 nm, there is almost no effect, and if 200 nm or more, a uniform and deep domain-inverted portion can be formed. Even if the deposition is 1 μm or more, there is no change in the reversal characteristics, but the deposition takes a long time.

When forming a domain-inverted portion by the method shown in FIG. 41, it is preferable to accurately control the amount of charge flowing between the electrodes. This is because there is substantially no leakage current, and the current flowing between the electrodes becomes the polarization reversal current. Therefore, the control of the charge amount becomes insufficient. For example, if the charge amount becomes too large, the adjacent domain-inverted portions may come into contact with each other, making it difficult to form a periodic domain-inverted structure. The preferred charge amount is the sum of the electric capacity (C) between the electrodes 192 and 193 and the domain-inverted charge (Ps × S). Here, Ps is the spontaneous polarization of the substrate crystal, and S is the sum of the cross-sectional areas of the domain-inverted portions. Note that the charge amount (2
Ps × S) is based on the fact that the value of the capacitance C between the electrodes and the value of the domain-inverted charge are not equal.

Next, a method of depositing an insulating film as shown in FIG. 43 under the electrode was examined. According to this method, it is possible to isolate the electrode from the crystal surface, to avoid the non-uniformity of the applied voltage due to the non-uniformity of the crystal, and to form the domain-inverted portion with excellent uniformity. It became something. However, when the thickness of the insulating film is increased, the electric field components between the electrode fingers of the comb-shaped electrode 194 overlap, so that the domain-inverted portions do not become periodic, and the adjacent domain-inverted portions are connected to each other. Was observed. The thickness of the insulating film in which the periodic domain-inverted portions are uniformly formed was ₂₀ to 100 nm when SiO ₂ was used as the insulating film. If it is less than 20 nm, the domain inversion becomes non-uniform, and if it exceeds 100 nm, no periodic domain inversion is formed. In order to prevent discharge, a SiO ₂ film thickness of 10 nm or more is required. However, if this film thickness exceeds 100 nm, a periodic domain-inverted portion cannot be obtained. In order to obtain a periodically domain-inverted portion while preventing discharge, the film thickness needs to be 10 to 100 nm. Further, when the film thickness was 20 to 60 nm, good characteristics could be obtained.

(Embodiment 19) Further, a method of forming a domain-inverted portion in a large area of the substrate surface was studied. FIG.
FIG. 4 is a plan view showing the configuration of the present embodiment. FIG.
As shown in (1), by forming a domain-inverted portion with two or more electrode pairs on a ferroelectric crystal substrate 196, the production efficiency was greatly improved. As shown, each electrode pair 1
Reference numeral 99 denotes a comb-shaped electrode 197 and a bar-shaped electrode 198 formed in a direction in which the comb-shaped portion of the comb-shaped electrode is extended. The individual electrode pairs are arranged such that the combs of the comb electrodes are in the same direction.

Further, it was confirmed that the presence of the rod-shaped electrodes 198 in front of and behind the comb-shaped electrode 197 improved the uniformity and the depth of the domain-inverted portions. A pair of electrodes 199
When the domain-inverted portion is formed only by applying a voltage between the electrodes, the electric field applied to the tip of the comb-shaped electrode 197 is affected only by the bar-shaped electrode 198 paired with the electrode.
However, by arranging a plurality of electrode pairs 199, the electric field applied to the tip of the comb-shaped electrode is also affected by the bar-shaped electrode 200 behind it. The electric field component in the thickness direction of the substrate at the tip of the comb-shaped electrode is increased by the bar-shaped electrode behind the electrode. Therefore, according to the present embodiment, a deeper domain-inverted portion can be formed.

Further, the distance L between the electrodes in the electrode pair is
₁ was investigated the effect of inter-electrode distance L ₂ in the adjacent electrode pair. As the substrate 196 at this time, MgO: LiNbO ₃ whose polarization direction was inclined by 3 ° with respect to the substrate surface was used.

[0207] L ₁ is, 100~1000μm was appropriate. If the thickness is less than 100 μm, a discharge may occur between the electrodes and the formation of the domain-inverted portion may not be achieved. If the thickness exceeds 1000 μm, the applied voltage may become too high (for example, 10 kV) and dielectric breakdown may occur. When the thickness is 200 to 800 μm, the uniformity of the polarization inversion is improved, and more preferable results are obtained. L ₂ is, 100~200μm was appropriate.
If the thickness is less than 100 μm, discharge may occur in the same manner as described above, and the thickness of the domain-inverted portion formed is limited to about 1 μm. On the other hand, if it exceeds 200 μm, it becomes difficult to form a periodically domain-inverted portion.

However, the comb-shaped electrode 197 is covered with an insulating film,
When a rod-shaped electrode 198 was formed on this insulating film and a voltage was applied, no problem was observed in the periodically poled portion even when L ₂ exceeded 200 μm. This is the comb-shaped electrode 19
It is considered that this is because the electrical isolation between the electrode fingers of the comb-shaped electrode was secured by covering the insulating film 7 with the insulating film. It is also considered that the leakage current between the bar-shaped electrode 200 and the comb-shaped electrode 197 at the back was suppressed. In this configuration, L _{2 is set} to 100 to 1
It has been found that when the thickness is 000 μm, the uniformity of the domain-inverted portion is improved. Further, the depth of the domain-inverted portion to be formed was about 2 μm, which was about twice the conventional value.

Further, the uniformity of the domain-inverted portions to be formed is as follows:
It was also confirmed that it depends on the total length of the comb-shaped electrode (the sum of L ₃ in FIG. 26). As the total length increases, the uniformity of the domain-inverted portions tends to deteriorate. This is considered to be because the current value increases almost in proportion to the total length. As a result of the experiment, the total length was 10 to 200 mm, especially 10
It was confirmed that it is preferable to set the thickness to 100 mm from the viewpoint of improving the uniformity of the domain-inverted portions.

The substrate used in each of the above embodiments includes:
It is preferable to use one utilizing liquid phase growth. This is because a uniform optical crystal can be manufactured at low cost.
In particular, when forming an optical waveguide, since the optical waveguide uses only a range of several μm from the crystal surface, it is sufficient to use a portion epitaxially grown from a liquid phase. Also, by utilizing liquid phase growth, for example, Li
It becomes easy to dope NbO ₃ with MgO. Doping with MgO can improve the light damage resistance of the substrate. If Nd is doped into the substrate, it can be used as a laser medium.

[0211]

As has been described in detail, according to the present invention, the polarization inversion is improved in terms of depth, uniformity, effective area, etc., and is suitable for an optical wavelength conversion element and the like. Department is provided. Further, according to the present invention, for example, an optical wavelength conversion element with improved characteristics, which can generate a short wavelength more stably than before, is provided, and a light generation device and an optical pickup using such an element are provided. Is done. further,
According to the present invention, a diffraction element capable of high-speed response is provided.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a method for manufacturing a domain-inverted portion according to the present invention from a cross-sectional direction of a substrate.

FIG. 2 is a view showing another example of the method of manufacturing the domain-inverted portion according to the present invention from a cross-sectional direction of a substrate.

FIG. 3 is a diagram illustrating an example of a method for manufacturing an optical wavelength conversion element.

FIG. 4 is a sectional view of an optical wavelength conversion element manufactured by the method of FIG.

FIG. 5 is a diagram showing a method of stabilizing a domain-inverted portion according to the present invention from a cross-sectional direction of a substrate.

FIG. 6 is a diagram showing a relationship between a heat treatment after proton exchange and a depth at which the polarization inversion disappears.

FIG. 7 is a perspective view showing an example of a light wavelength conversion element of the present invention.

FIG. 8 is a diagram illustrating an example of a method for manufacturing a domain-inverted portion according to the present invention from a plane direction of a substrate.

FIG. 9 is a view showing another example of the method of manufacturing the domain-inverted portion according to the present invention from a cross-sectional direction of the substrate.

FIG. 10 is a perspective view of an example of a short-wavelength light generation device according to the present invention.

FIG. 11 is a diagram illustrating an example of a configuration of an optical pickup of the present invention.

FIG. 12 is a diagram showing another example of the method for manufacturing the domain-inverted portion of the present invention.

13 is a diagram showing the relationship between the electric field in the thickness direction of the substrate and the polarization inversion depth in the method shown in FIG.

FIG. 14 is a diagram showing a relationship between a substrate thickness and a polarization inversion depth in the method shown in FIG.

FIG. 15 is a diagram showing another example of the method for manufacturing the domain-inverted portion of the present invention.

FIG. 16 is a view showing another example of the method of manufacturing the domain-inverted portion according to the present invention, as viewed from the cross-sectional direction of the substrate.

FIG. 17 is a view showing another example of the method of manufacturing the domain-inverted portion according to the present invention from a cross-sectional direction of the substrate.

FIG. 18 is a perspective view showing another example of the optical wavelength conversion element of the present invention.

FIG. 19 is a diagram showing a relationship between substrate thickness and light wavelength conversion efficiency in the light wavelength conversion device shown in FIG.

FIG. 20 is a perspective view of another example of the short wavelength light generation device of the present invention.

FIG. 21 is a perspective view illustrating a conventional method for manufacturing a domain-inverted portion.

FIG. 22 is a diagram showing a relationship between an electrode interval and a depth of polarization inversion in the method shown in FIG. 21;

FIG. 23 is a sectional view showing an electric field distribution between electrodes in the method shown in FIG. 21;

FIG. 24 is a perspective view showing another example of the method for manufacturing the domain-inverted portion of the present invention.

25 is a sectional view showing an electric field distribution between electrodes in the method shown in FIG.

FIG. 26 is a perspective view showing another example of the method of manufacturing the domain-inverted portion of the present invention.

FIG. 27 is a perspective view (a) and cross-sectional views (b) and (c) showing another example of the method for manufacturing the domain-inverted portion of the present invention.

FIG. 28 is a perspective view (a) and cross-sectional views (b) and (c) showing another example of the method for manufacturing the domain-inverted portion of the present invention.

FIG. 29 is a diagram showing the relationship between the inclination of the crystal axis of the substrate with respect to the substrate surface and the depth of the domain-inverted portion.

FIG. 30 is a diagram showing the relationship between the length of the intermediate electrode in the inter-electrode direction and the depth of the domain-inverted portion.

FIG. 31 is a plan view showing a shape of a domain-inverted portion formed by using an intermediate electrode.

FIG. 32 is a perspective view (a) and a cross-sectional view (b) showing another example of the method for manufacturing the domain-inverted portion of the present invention.

FIG. 33 is a diagram showing another example of the method for manufacturing the domain-inverted portion of the present invention.

FIG. 34 is a perspective view showing another example of the optical wavelength conversion element of the present invention.

FIG. 35 is a perspective view (a) and a sectional view (b) showing another example of the optical wavelength conversion element of the present invention.

FIG. 36 is a perspective view (a) showing another example of a method for manufacturing a domain-inverted portion of the present invention, and a cross-sectional view (b) of an example of an optical wavelength conversion element manufactured by using this method.

FIG. 37 is a sectional view showing an example of the diffraction element of the present invention.

FIG. 38 is a diagram showing an example of the configuration of an optical information processing device using the diffraction element of the present invention.

FIG. 39 is a cross-sectional view showing a positional relationship between a pair of electrodes and an insulating film for forming a domain-inverted portion.

FIG. 40 is a cross-sectional view showing a positional relationship between a pair of electrodes and an insulating film for forming a domain-inverted portion.

FIG. 41 is a cross-sectional view showing a positional relationship between a pair of electrodes and an insulating film in another example of a method for manufacturing a domain-inverted portion according to the present invention.

FIG. 42 is a sectional view showing a domain-inverted portion manufactured by the method shown in FIG. 41;

FIG. 43 is a cross-sectional view showing a positional relationship between a pair of electrodes and an insulating film for forming a domain-inverted portion.

FIG. 44 is a plan view showing another example of the method for manufacturing the domain-inverted portion of the present invention.

FIG. 45 is a plan view showing a domain-inverted portion formed by the method shown in FIG. 39.

FIG. 46 is a perspective view illustrating a conventional method for manufacturing a domain-inverted portion.

FIG. 47 is a perspective view showing a conventional method for manufacturing a domain-inverted portion.

FIG. 48 is a perspective view showing a conventional method for manufacturing a domain-inverted portion.

FIG. 49 is a perspective view showing a conventional light wavelength conversion element.

FIG. 50 is a perspective view showing a conventional light wavelength conversion element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ferroelectric crystal substrate 2 Comb-shaped electrode 3 Plate electrode 4 Polarization inversion part 5 Polarization re-inversion part 7 Proton exchange part 8 Optical waveguide 11 Comb-shaped electrode 12a, 12b, 12c Plate electrode 14, 16 Insulating film 21 Semiconductor laser 23 Optical waveguide 25 Short wavelength light generator 26 Beam splitter 28 Optical disk 29 Detector 31 Ferroelectric crystal substrate 32 Comb electrode 33 Bar electrode 34 Plate electrode 41 Ferroelectric crystal substrate 42 Optical waveguide 43 Polarization inversion section 116 Ferroelectric crystal substrate 117 Comb electrode 118 Rod electrode 120 Low resistance part 121a, 121b Intermediate electrode 131 Polarization inversion part 140 Concave part 141 Insulating film 145 Polarization inversion part 153 Polarization inversion part 154 Optical waveguide 155 Fundamental wave 156 Second harmonic 162 Polarization inversion part 163 Optical waveguide 167, 168 Electrode 169 Intermediate electrode 170 Polarization inversion section 172 Polarization inversion section 173, 174 electrode 180 Semiconductor laser 181 Beam splitter 183 Optical disk 184 Detector 185 Diffraction element 191 Ferroelectric crystal substrate 192 Comb electrode 193 Rod electrode 195 Insulating film

Claims (57)

[Claims] A step of forming a domain-inverted portion by applying a voltage in a polarization direction of a substrate of a ferroelectric crystal; and a step of performing a process of reducing an internal electric field of the substrate caused by the application of the voltage. Applying a voltage in a direction opposite to the voltage in the polarization direction to re-invert the polarization of at least a part of the domain-inverted portion. 2. A process for reducing an internal electric field, comprising the steps of:
2. The domain-inverted portion according to claim 1, which is at least one process selected from heat treatment of the substrate at a temperature equal to or higher than the Curie temperature of the substrate and application of an electric field to the substrate by an electric field in a direction opposite to the internal electric field. Production method. 3. The method according to claim 1, further comprising, after the step of re-inverting the polarization, a step of forming a polarization inversion stabilizing portion in a region near the surface of the substrate where the polarization has been inverted at least once. A method for manufacturing a domain-inverted portion. 4. The method according to claim 3, wherein the polarization inversion stabilizing section is formed by a step including ion exchange in the region. 5. The method according to claim 1, wherein the polarization inversion stabilizing section is performed after the ion exchange and the ion exchange, and is performed at a temperature lower than the Curie temperature of the substrate and 150 ° C. from the Curie temperature.
The method for manufacturing a domain-inverted portion according to claim 4, wherein the method is formed by a process including a heat treatment at a temperature equal to or higher than a low temperature. 6. A first electrode and a second electrode each having a striped pattern portion on the surface of the substrate, wherein one electrode is projected on the other electrode by projecting the striped pattern portion in the direction of polarization of the substrate. Formed so as to fit with the striped pattern portion,
The method of claim 1, wherein the domain-inverted portion is grown from the first electrode side, and the domain-inverted portion is re-inverted from the second electrode side. 7. A projection of a second electrode having a first electrode and a striped pattern portion on a surface of the substrate in a direction of polarization of the striped pattern portion of the second electrode in the polarization direction of the substrate. 2. The polarization according to claim 1, wherein the polarization is formed so as to be included in one electrode, the domain-inverted portion is grown from the side of the first electrode, and the domain-inverted portion is re-inverted from the side of the second electrode. 3. Manufacturing method of the reversing part. 8. A step of applying a voltage in the direction of polarization of the ferroelectric crystal substrate to form a domain-inverted portion, and a step of forming a domain-inverted stabilizing portion near the surface of the substrate in the domain-inverted portion. A method for manufacturing a domain-inverted portion, comprising: 9. The method according to claim 8, wherein the polarization inversion stabilizing section is formed by a step including ion exchange of the polarization inversion section. 10. The polarization inversion stabilizing section, which is performed after the ion exchange and the ion exchange, is performed at a temperature lower than the Curie temperature of the substrate and higher than the Curie temperature by 15%.
The method for manufacturing a domain-inverted portion according to claim 9, wherein the method is formed by a process including a heat treatment at a temperature of 0 ° C. or lower. 11. A step of forming a first electrode having a striped pattern portion on the first surface of a ferroelectric crystal substrate having a first surface and a second surface facing each other; A step of covering one electrode with a first insulating film; a step of forming a second insulating film on the second surface; and the second surface defined by projecting the first electrode in a polarization direction. Forming a second electrode on the second insulating film so as to cover an upper region with the second insulating film interposed therebetween; and between the first electrode and the second electrode. Applying a voltage to grow a domain-inverted portion from the striped pattern portion of the first electrode to the second electrode. 12. A step of forming first, second and third electrodes in this order in a polarization direction of a ferroelectric crystal substrate whose polarization direction is substantially parallel to the substrate surface; Applying a voltage between an electrode and the second electrode to form a striped domain-inverted portion along the polarization direction between the first electrode and the second electrode; Applying a voltage between the first electrode and the third electrode to grow the stripe-shaped domain-inverted portion from the second electrode to the third electrode. Manufacturing method of the domain-inverted portion. 13. An electric field having a first electric field component parallel to the polarization direction of the ferroelectric crystal and a second electric field component perpendicular to the polarization direction is applied to the ferroelectric crystal substrate. And a step of growing a domain-inverted portion in the polarization direction. 14. The method according to claim 13, wherein the second electric field component is larger than the first electric field component. 15. The method for manufacturing a domain-inverted part according to claim 13, wherein the second electric field component is 4 kV / mm or more and less than an electric field at which dielectric breakdown of the substrate occurs. 16. The method according to claim 13, wherein the second electric field component is an electric field generated by a pulse voltage. 17. The method according to claim 16, wherein the first electric field component is generated by a pulse voltage having a pulse width narrower than a pulse voltage that generates the second electric field component. 18. The method according to claim 13, wherein the polarization direction is substantially parallel to a surface of the substrate. 19. The thickness of the substrate is 0.1 mm to 0.4 mm
The method for manufacturing a domain-inverted portion according to claim 18, wherein 20. The method according to claim 19, wherein the electric field is 2 °
The method for manufacturing a domain-inverted portion according to claim 18, wherein the method is generated by a pair of electrodes arranged in a direction forming an angle of 80 °. 21. The polarization direction and the surface of the substrate are 1
The method for manufacturing a domain-inverted portion according to claim 13, wherein the angle has an angle of 5 to 5 degrees. 22. A step of forming a low resistance portion having a lower resistance value than the ferroelectric crystal on the surface between the first electrode and the second electrode formed on the surface of the ferroelectric crystal substrate. And applying a voltage between the first electrode and the second electrode to grow a domain-inverted portion in the direction of polarization of the ferroelectric crystal. How to manufacture the part. 23. The method according to claim 22, wherein the low resistance portion is formed by a process including ion exchange. 24. The method according to claim 22, wherein the two or more low-resistance portions are arranged at substantially equal intervals between the first electrode and the second electrode. 25. The method according to claim 22, wherein the polarization direction and the surface are substantially parallel. 26. The method according to claim 22, wherein the polarization direction of the ferroelectric crystal and the surface have an angle of 1 ° to 5 °. 27. The method according to claim 26, wherein the low resistance portion is formed of a metal. 28. The method according to claim 27, wherein the low resistance portion has a length of 10 μm to 30 μm in a direction along the polarization direction on the surface. 29. A step of forming a concave portion on the surface of the ferroelectric crystal substrate, a step of forming a first electrode in the concave portion, and a step of forming the first electrode and the second electrode formed on the surface. Growing a domain-inverted part from the first electrode in the polarization direction of the ferroelectric crystal by applying a voltage between the electrode and the electrode. 30. The method according to claim 29, wherein the depth of the concave portion is 0.1 μm or more. 31. A step of forming an insulating film on the surface of the ferroelectric crystal substrate, a step of forming a concave portion penetrating the insulating film on the surface, and forming a conductive film on at least a region of the surface including the concave portion. Forming a film, and applying a voltage between the first electrode and the second electrode formed on the surface using the conductive film in the recess as a first electrode to form the first electrode Growing a domain-inverted portion in the direction of polarization of the ferroelectric crystal. 32. The method according to claim 29, wherein the polarization direction and the surface are substantially parallel to each other. 33. The angle between the polarization direction and the surface is 1 ° to 5 °.
The method for producing a domain-inverted portion according to any one of claims 29 to 31, which has an angle of °. 34. A step of forming a first electrode on a surface of a ferroelectric crystal substrate, a step of forming an insulating film on the surface including the first electrode, and a step of forming a first electrode on the surface of the insulating film. Forming a second electrode at a position different from the position at which the first electrode is formed, and applying a voltage between the first electrode and the second electrode to form the first electrode. Growing a domain-inverted portion from an electrode in the direction of polarization of the ferroelectric crystal. 35. The insulating film has a thickness of 0.2 μm to 1 μm.
The method for manufacturing a domain-inverted portion according to claim 34. 36. The polarization direction and the surface may have an angle of 1 ° or more.
35. The method according to claim 34, wherein the polarization inversion portion has an angle of 5 [deg.]. 37. A first electrode in the form of a comb and a second electrode arranged in a direction in which the comb extends in a surface of the substrate of the ferroelectric crystal.
Forming two or more electrode pairs each including the first electrode and the second electrode such that the comb shape faces in the same direction; and applying a voltage between the first electrode and the second electrode to form the first electrode.
Growing a domain-inverted portion from the comb shape of the electrode to the second electrode side. 38. The method according to claim 37, wherein an interval between the first electrode and the second electrode in the electrode pair is 100 μm to 1000 μm. 39. An interval between adjacent electrode pairs is 100 μm.
The method for producing a domain-inverted portion according to claim 37, wherein the thickness is from m to 200 m. 40. The method according to claim 40, further comprising:
38. The method according to claim 37, further comprising forming an insulating film below the electrode. 41. The method according to claim 1, wherein two or more domain-inverted portions are formed on the substrate of the ferroelectric crystal so as to form substantially parallel layers. Of manufacturing an optical wavelength conversion element. 42. A ferroelectric crystal substrate comprising at least two domain-inverted portions forming substantially parallel layers formed by the method according to claim 1. Description: Light wavelength conversion element. 43. A ferroelectric crystal substrate, a domain-inverted portion formed in the substrate so as to form layers substantially parallel to each other, and a domain-inverted portion formed in the domain-inverted portion near the surface of the substrate. An optical wavelength conversion element comprising a polarization inversion stabilizing section. 44. The polarization inversion stabilizing section is formed by a step including ion exchange of the polarization inversion section.
4. The optical wavelength conversion element according to 3. 45. A substrate of a ferroelectric crystal having a polarization direction substantially parallel to the substrate surface, an optical waveguide formed in the substrate along the surface, and periodically traversing the optical waveguide. An optical wavelength conversion element, comprising: two or more domain-inverted portions having a depth of 2 μm or more formed in the substrate. 46. The optical wavelength conversion device according to claim 45, wherein the thickness of the substrate is 0.4 mm or less. 47. A ferroelectric crystal substrate having a polarization direction and an inclination of 1 ° to 5 ° between the substrate surface and a layered structure formed in a cross section perpendicular to the substrate surface and along the polarization direction. An optical wavelength conversion element comprising two or more polarization inversion portions extending from the surface in the polarization direction. 48. The optical wavelength conversion device according to claim 47, wherein an optical waveguide is formed on the surface of the substrate so as to periodically cross the cross section. 49. A semiconductor device comprising: a concave portion formed on a surface of a ferroelectric crystal substrate; and a domain-inverted portion having the concave portion as one end and extending in a polarization direction of the ferroelectric crystal. Light wavelength conversion element. 50. The optical wavelength conversion device according to claim 49, wherein the polarization direction and the surface are substantially parallel. 51. The polarization direction and the surface are 1 ° to 5 °.
The light wavelength conversion element according to claim 50, wherein the light wavelength conversion element has an angle of °. 52. A ferroelectric crystal substrate, a first electrode formed on a surface of the substrate, an insulating film formed on the surface including the first electrode, and A second electrode formed at a position different from the position where the first electrode is formed, and the second electrode along the polarization direction of the ferroelectric crystal with the first electrode as one end. An optical wavelength conversion element comprising: a polarization reversal part extending to an electrode side. 53. The insulating film has a thickness of 0.1 μm to 1 μm.
The optical wavelength conversion device according to claim 52, wherein: 54. A ferroelectric crystal substrate, a first electrode each having a comb shape, and a second electrode arranged in a direction in which the comb shape extends.
And two or more electrode pairs formed on the surface of the substrate so that the comb shape faces in the same direction.
A polarization inversion portion extending to the second electrode side along the polarization direction of the ferroelectric crystal with the comb-shaped portion of the electrode as one end. 55. A light wavelength conversion device comprising: the light wavelength conversion device according to claim 42; and a semiconductor laser, wherein light emitted from the semiconductor laser is wavelength-converted by the light wavelength conversion device. Light generator. 56. An optical pickup comprising the light generating device according to claim 55, a light collecting optical system, and a recording medium, wherein the light emitted from the light generating device is recorded on the recording medium by the light collecting optical system. An optical pickup characterized by focusing light on the top. 57. A ferroelectric crystal substrate having a polarization direction and a substrate surface inclined at 1 ° to 5 °, and a layered structure is formed in a cross section perpendicular to the substrate surface and along the polarization direction. And at least two domain-inverted portions extending from the surface in the polarization direction.