JP2002196381A - Optical wavelength converting element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-efficiency structure of an optical wavelength converting element in which inhomogeneity in the polarization inversion structure due to processes of forming an optical waveguide is prevented by repeating part of the periodical polarization inversion structure formed in a stoichiometric LN crystal to stabilize the structure. SOLUTION: The optical wavelength converting element has a stoichiometric LiNbO3 crystal substrate 1 having 49.5 to 50.2% molar ratio of Li2O/(Nb2O5+Li2 O) and a polarization inversion part 2 formed on the surface of the crystal. The polarization inversion part 2 is periodically arranged, at least part of which being formed in contact with each other by repetition. A comb-like electrode is formed on the surface of the substrate 1, a flat electrode is formed on the back face of the substrate and a voltage is applied between the electrodes to grow the polarization inversion from the comb-like electrode. The polarization inversed part and its continuous part are formed inward in the crystal. Then a proton exchange optical waveguide 3 is formed by proton exchanging and annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical wavelength conversion element utilizing a nonlinear optical effect and a method of manufacturing the same.

[0002]

2. Description of the Related Art A correlation diagram (phase diagram) of a temperature-composition ratio of a lithium niobate (LiNbO ₃ ) single crystal (hereinafter referred to as LN) has been known for a long time. Is produced from a melt having a mole fraction of 48.5% (a mole fraction of Li / Nb of 94%), which is a congruent melting composition in which the crystal and the melt have the same composition and equilibrium coexist. It was grown by the rotation pulling method. Since the grown as-grown LN single crystal is in a multi-domain state, a voltage is applied in the Z-axis direction of the crystal while the grown crystal is heated to a temperature higher than the Curie temperature of 1150 ° C. to form a single domain. After that, a poling treatment for cooling the crystal was performed. Crystals that have been subjected to single domain processing have been used in various applications after being processed to a predetermined size. LN has a high optical constant and is easy to grow a large crystal, so that it is applied to various optical elements.

[0003] As one of uses of the LN crystal, there is an optical wavelength conversion element utilizing a high nonlinear optical effect of the LN crystal. By forming a periodic domain-inverted structure in the crystal, it is possible to satisfy the phase matching condition, and it is possible to perform wavelength conversion of an arbitrary wavelength with high efficiency. Congruent LN
Various methods for forming a periodically poled structure in a crystal have been proposed. For example, there is a method using Ti diffusion. A stripe-shaped Ti metal is periodically formed on the surface of a Z-plate LN substrate having a congruent composition, and this is heat-treated near the Curie temperature of the substrate to form a periodically poled structure. The formed domain-inverted structure has an inverted triangular shape with the crystal surface as a bottom surface, and an optical wavelength conversion element is manufactured using this domain-inverted structure.

There is also a method of depositing SiO ₂ and heat treatment. In this method, a stripe-shaped SiO ₂ film is periodically formed on the surface of a Z-plate LN substrate having a congruent composition,
This is a method of heat treatment. By performing heat treatment near the Curie temperature of the substrate, an inverted triangular domain-inverted structure similar to the method using Ti diffusion is formed, and an optical wavelength conversion element is manufactured using this structure.

As another conventional method for manufacturing an optical wavelength conversion element, there is a method utilizing polarization inversion by applying an electric field. A comb-shaped electrode is formed on the surface of an off-cut plate LN crystal having a congruent composition, and an electric field is applied to the substrate so that the substrate has a Z-shape.
By forming the domain-inverted structure in the axial direction, a needle-shaped domain-inverted structure can be periodically formed in the crystal in an oblique direction. The polarization inversion is an inverted triangle close to a semicircle and is formed with the substrate surface as the bottom surface. An optical wavelength conversion element is manufactured using this method. A highly efficient wavelength conversion is performed by utilizing a nonlinear grating having a periodically poled structure and performing phase matching between a fundamental wave and a harmonic in a waveguide.

The conventional LN single crystal has a high homogeneity of the conventional composition.
In order to produce a large LN, the crystal and the melt are
Li is a congruent composition that coexists
_TwoO / (Nb_TwoO_Five+ Li_TwoO) having a molar fraction of 48.5%
It was grown from the melt by the spin-up method. It is formed
The molar fraction of crystals is equal to the composition of the solution, 48.5% (Li
/ Nb mole fraction is 94%) and the Curie temperature is about
1150 ° C. On the other hand, Li_TwoO / (Nb_TwoO
_Five+ Li_TwoO) with a molar fraction of 49.5 to 50.2%
Growth of stoichiometric LN crystals close to the stoichiometric ratio
(Japanese Patent Application Laid-Open No. 10-45498). Work
The production method is based on the double crucible method, when pulling crystals.
When the composition of the lithium niobate solution is
Li _TwoO / (Nb_TwoO_Five+ Li_TwoO) molar fraction of 56 to
The solution composition is maintained within a specific range of 60%.
Is used. Steak
Ometric LN is as-grown single domain
No need for poling after growth, crystal growth and optical
It is characterized by good homogeneity. In addition, congruen
Curie temperature of 1185 to 121
Features 5 ° C higher.

[0007] Stoichiometric LN is a slight change in mole fraction relative to the congruent composition LN.
As they approach the stoichiometric ratio, their crystal properties differ significantly. In particular, when the molar fraction of the crystal is 49.5 to 50.2% (Li
/ Nb molar ratio is in the range of 95 to 101%), and has optical characteristics greatly different from those of the conventional congruent composition crystal.

[0008]

With respect to the formation of a periodic domain-inverted structure in a congruent composition and an optical wavelength conversion device using the same, various methods of manufacturing a domain-inverted device and the configuration of the optical wavelength conversion device have been reported. . However, in a crystal having a stoichiometric composition in which the Li / Nb ratio in the crystal is controlled, there is a problem that the formation of a periodic domain-inverted structure and its characteristics have not been clarified.

In addition, the polarization inversion characteristics of the LN crystal having a stoichiometric composition have not been clarified yet, and the polarization inversion characteristics are significantly different from those of the conventional congruent composition. Is difficult to form.

A wavelength conversion element using LN having a congruent composition has achieved high conversion efficiency, and it has been confirmed that converted light in the blue and green wavelength regions with high efficiency is obtained. However, in a light wavelength conversion element using an LN crystal having a congruent composition, output instability due to occurrence of optical damage is a serious problem. To stabilize the output of the optical wavelength conversion element,
It is desired to improve the light damage resistance. The value of the light damage resistance (maximum light intensity at which no light damage occurs) of the LN depends on the waveguide shape, but is 1 mW or less for light in the wavelength band of 400 nm. In order to realize a high-output optical wavelength conversion element with a power of about several tens of mW, improvement in light damage resistance is required.

In order to solve the above-mentioned conventional problem, the present invention provides a stable domain-inverted structure in a periodic domain-inverted structure formed in a stoichiometric LN crystal, and uses this domain-inverted structure to produce an optical waveguide process. In order to suppress the occurrence of the non-uniformity of the domain-inverted structure due to the above-mentioned factors, a highly efficient optical wavelength conversion element structure is realized, and the position of domain-inverted formation can be precisely controlled, thereby forming the optical wavelength conversion element. Yield and the second harmonic (SH
G) It is an object of the present invention to provide an optical wavelength conversion element having improved element characteristics and a method for manufacturing the same.

[0012]

In order to achieve the above object, an optical wavelength conversion device according to the present invention comprises Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O) having a stoichiometric LiNbO ₃ crystal having a molar fraction of 49.5 to 50.2%, and domain-inverted portions formed on the crystal surface, wherein the domain-inverted portions are periodically arranged. And at least a part of the domain-inverted portion is continuous.

Next, the method for manufacturing an optical wavelength conversion device of the present invention is characterized in that the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 4
9.5 to 50.2% stoichiometric LiNbO
₍₃₎ forming a comb-shaped electrode on one surface of the crystal, forming a planar electrode on the other surface of the crystal, and bringing a part of the comb-shaped electrode into contact with the crystal surface via an insulating film so as to be continuous; An electric field is applied between them to form a periodically poled structure.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the finding of a stable periodically poled structure in a stoichiometric LN crystal. A uniform periodic domain-inverted structure is indispensable for configuring a highly efficient optical wavelength conversion element in a stoichiometric LN crystal, and a crystallographically stable periodic domain-inverted structure is required.

Further, a new formation method for forming a periodically poled structure in a stoichiometric LN crystal is proposed. In the stoichiometric LN crystal, it has been found that it is difficult to accurately determine the position where the domain-inverted structure is formed. Therefore, by proposing a polarization inversion formation method that solves this problem, it is possible to control the formation location of the polarization inversion structure with high accuracy, and to realize a highly efficient light wavelength conversion element from blue to ultraviolet. did.

Stoichiometric LN is a slight change in mole fraction relative to the congruent composition LN.
As they approach the stoichiometric ratio, their crystal properties differ significantly. In particular, when the mole fraction of the crystal is in the range of 49.5 to 50.2%, the crystal has optical characteristics that are significantly different from those of the conventional congruent composition crystal, and the polarization inversion characteristics found this time also have this mole fraction. This is an effect unique to the stoichiometric LN.

Stoichiometric LN is a crystal that can be manufactured recently, and all of its optical characteristics and polarization reversal characteristics have not been clarified yet. In particular, the polarization inversion characteristics have been clarified for the first time by the present inventors. Further, improvement of optical element characteristics utilizing this characteristic has been an unexplored field.

In the present invention, it is preferable that the contact portion is the bottom of the domain-inverted portion from the viewpoint of the stability of the domain-inverted structure.

In the present invention, it is preferable that the C axis of the crystal is inclined at 0.3 to 10 ° with respect to the crystal surface, since a deep domain-inverted structure can be formed. .

In the present invention, the stoichiometric LiNbO ₃ crystal has a Curie temperature of 1185.
It is preferable that the temperature is in the range of -1205 ° C., since the nonlinear optical design constant can be improved and a highly efficient wavelength conversion element can be formed.

Further, in the present invention, the crystal is Mg,
0.03% by weight of any additive of Zn, Sc and In
It is preferable that the content is in the range of 1% by weight or less, since the light damage resistance of the crystal is increased and the high output characteristics are improved. If the content is less than 0.03% by weight, the above-mentioned effects are unlikely to be exhibited, and if it exceeds 1% by weight, the crystal structure becomes non-uniform, and the SHG characteristics tend to deteriorate.

In the present invention, if the period of the domain-inverted portion is 3.5 μm or less, the S
It is preferable because HG light can be generated.

Next, in the method of the present invention, the comb-shaped electrode includes a tooth portion having a periodic grating structure,
It is preferable that a stripe portion connecting the tooth portions to each other is formed, and the stripe portion is in contact with the crystal surface via an insulating film, since a formed domain-inverted structure becomes uniform.

In addition, it is preferable that the C axis of the crystal is inclined within a range of 0.3 to 10 ° with respect to the crystal surface, since the propagation loss of the waveguide is reduced.

The continuous portion of the periodically poled portion has a thickness of 5 to 5 times the thickness of the periodically poled portion as viewed from the cross section.
The range of 200% is preferable because the stability of the domain-inverted structure is improved.

[0026]

(Example 1) An LN crystal having a stoichiometric composition for the purpose of improving the optical constant of the LN crystal has been proposed (Japanese Patent Application Laid-Open No. Hei 10-45498). Li ₂ O
/ (Nb ₂ O ₅ + Li ₂ O) mole fraction of 49.5 to 5
By using LN of 0.2%, the defect density in the crystal can be reduced, and it has been confirmed that various constants such as an electro-optical constant and a nonlinear optical constant are increased. Further light scattering, wavelength 4
It also has excellent light transmission characteristics in the visible light range of 00 to 600 nm. The inventors have tried to form a periodically poled structure in this stoichiometric LN crystal. LN having a congruent composition can have a periodically poled structure with a pattern electrode. The applied voltage needs a very high applied voltage of about 20 kV / mm. The formed domain-inverted structure is stable and stable without being changed by heat treatment at about 500 ° C. and proton exchange necessary for forming a waveguide. On the other hand, in the stoichiometric LN, the applied voltage required for polarization reversal is extremely reduced. The voltage required for the polarization reversal was 5 kV / mm or less. This is Li, Nb
This is because the stoichiometric LN whose composition is close to a perfect crystal has few crystal defects in the crystal, and therefore has a small internal resistance against polarization inversion. Since polarization inversion can be performed at a low voltage, dielectric breakdown does not easily occur and a polarization inversion structure can be easily formed even in a thick crystal.

The present inventors have determined that stoichiometric LN
A detailed study of the domain reversal characteristics revealed that the domain reversal formed by the disturbance was re-inverted, increasing nonuniformity in the domain reversal structure. It is considered that the instability of the domain-inverted structure increased due to the extremely low domain-inverted voltage. In order to generate light of the second harmonic (SHG) from blue to ultraviolet with a wavelength of 450 nm or less, a short-period domain-inverted structure with a period of 3.5 μm or less is required. However, when the domain-inverted structure is a short period structure of 3.5 μm or less, the phenomenon of the instability appears remarkably. It is considered that the cause was that the polarization inversion was re-inverted by the pyroelectric electric field generated during the heat treatment.

Since the stoichiometric LN crystal has a low inversion voltage, the electric field generated by the pyroelectric effect affects the domain-inverted structure. When the temperature of the stoichiometric LN substrate having the periodic domain-inverted structure having a period of 3 to 3.5 μm was rapidly changed, the periodic domain-inverted structure was partially re-inverted. Uniformity deteriorated. This is a phenomenon that was not observed in the conventional LN having a congruent composition. Further, the phenomenon that the uniformity of the domain inversion is deteriorated also occurs when the heat treatment is performed at a high temperature.

The present inventors conducted various studies on stabilization of a short-period domain-inverted structure, and found that the occurrence of reinversion differs depending on the domain-inverted structure. That is, it has been found that in some domain-inverted structures, the domain-inverted structure can be stably maintained even with rapid temperature changes and high-temperature heat treatment. Usually, since the domain-inverted structure forms a nonlinear grating, domain-inverted portions and non-inverted portions are alternately present. When the period becomes short, the cross-sectional area of the domain-inverted portion formed becomes small. This increased the instability of the domain-inverted portion.

In a short-period structure having a period of 3.5 μm or less, a domain-inverted structure formed by heat treatment or the like changed. On the other hand, the stability of the domain-inverted structure was greatly improved by making the adjacent domain-inverted structures contact each other to be continuous. In the stoichiometric LN, the period is 10
There is no problem in the stability of the structure in the inverted structure of μm or more. However, in the short-period structure having a period of 3.5 μm or less, the instability of the structure became remarkable. On the other hand, it was found that the stability of the domain-inverted structure was greatly improved when the cross-sectional area of domain-inverted structure was increased by bringing a part of the domain-inverted structure into contact with the difference.

Next, the orientation of the substrate crystal forming the domain inversion was examined. On the Z-plate substrate, the formed domain-inverted structure was relatively stable. On the other hand, in the domain-inverted structure formed on the off-cut substrate, a re-inversion phenomenon caused by disturbance was remarkable, and the instability increased. This is because the cross-sectional area of each domain-inverted portion formed on the off-cut substrate is restricted. Polarization reversal formed in the off-cut substrate occurs at the tip of the electrode and is formed inside the substrate along the crystal axis. Since the adjacent domain-inverted portions are not in contact with each other and each domain-inverted portion is independent, the cross-sectional area of each domain-inverted portion is as small as about 10 μm ² . Therefore, a stable domain-inverted structure in the off-cut stoichiometric LN was examined. The crystal is a stoichiometric LN crystal of an off-cut substrate, and the X-axis of the crystal is inclined by 3 ° with respect to the normal to the substrate surface.

FIG. 1A is a perspective view of an optical wavelength conversion device according to one embodiment of the present invention, and FIG. 1B is a plan view of the same.

As shown in FIG. 1A, a comb-shaped electrode 4 is formed on the front surface of the substrate 1 and a plane electrode 5 is formed on the back surface of the substrate. By applying a voltage between the electrodes, domain inversion grows from the comb-shaped electrode. Then, the domain-inverted portions 6 (FIGS.
(C)) was formed. The formed domain-inverted structure was observed from a cross section (cross section taken along line XX in FIG. 1B). This cross section is shown in FIGS. 2 (a), 2 (b) and 2 (c).

By changing the electrode structure and electric field application conditions, FIG.
(A), (b) and (c), the cross section of the domain-inverted structure was formed. 2A is a structure in which adjacent domain inversions are not in contact, FIG. 2B is a structure in which adjacent domain inversions are in contact near the top surface, and FIG. This is a structure that is in close contact. FIG. 2A shows a conventional domain-inverted structure, which has a problem in the stability of the domain-inverted structure. On the other hand, FIG. 2B to FIG.
By adopting the structure of (c), the stability was greatly improved even with temperature changes and high-temperature treatment. FIG. 2A shows an ideal shape. However, in the stoichiometric crystal, it was necessary to limit the domain-inverted portion to a relatively small size in order to form the structure shown in FIG. That is, the shape of FIG. 2A can be formed by extremely limiting the amount of applied current. However, the polarization inversion thickness in this case was 0.5 μm or less, and it was difficult to form a highly efficient light wavelength conversion element. As you add more current,
A domain-inverted structure having the shape shown in FIG. 2B was formed. However, when the current amount was further increased, the adjacent domain-inverted portions contacted each other, became partially continuous, and the periodic structure was not observed. In order to form a domain-inverted structure as shown in FIG. 2C, it is necessary to shorten the voltage to be applied. In the method shown in FIGS.
It has an application time of 1 to 1 s or more. On the other hand, several ms
By applying the following short pulse voltage, the domain-inverted structure shown in FIG. 2C can be formed. In particular, the applied pulse voltage is preferably a short pulse of 5 ms or less. This trend is
It was observed as a phenomenon peculiar to stoichiometric LN. It was revealed that the domain inversion formation was significantly different between the stoichiometric LN and the congruent LN.

Next, an optical waveguide was formed inside the formed domain-inverted structure to form an optical wavelength conversion element shown in FIG. FIG.
2 shows the configuration of the optical wavelength conversion element of the present invention, that is, the optical wavelength conversion element having the domain-inverted structure shown in FIG.
In FIG. 3, 1 is an LN substrate having a stoichiometric composition, 2 is a periodically poled structure, and 3 is a proton exchange optical waveguide. After forming the domain-inverted structure 2, the optical waveguide 3 was formed by proton exchange and annealing. By forming the proton exchange layer in a stripe shape and performing annealing, a low-loss optical waveguide having high nonlinearity was formed. Light having a wavelength of 820 nm was incident on the waveguide, and the wavelength was converted by a domain-inverted structure to form a light wavelength conversion element that generates violet light having a wavelength of 410 nm.

When an optical wavelength conversion element is formed using the domain-inverted structure shown in FIG. 2A, the conversion efficiency of the second harmonic (SHG) is 5% or less, and the production of a highly efficient wavelength conversion element is not possible. was difficult. This is considered to be due to the non-uniformity of the domain-inverted structure caused by the process of forming the optical waveguide.
The optical wavelength conversion element having the domain-inverted structure shown in FIG.
A violet light of 20 mW can be generated with respect to a semiconductor laser of 00 mW, and an efficient wavelength conversion of 20% was achieved. On the other hand, in the domain-inverted structure of FIG. 2B, the conversion efficiency was reduced to 1/10 of 2%. This is because the function as a nonlinear grating did not work efficiently because the polarization inversion was in contact near the surface of the optical waveguide.

The optical waveguide has a thickness of about 2 to 3 μm,
The highest power density of light propagating through the optical waveguide is about 1 μm from the waveguide surface. The overlap between the fundamental wave and the harmonic is also maximum in this part. When the domain-inverted structures contact each other near the surface, wavelength conversion does not occur near the surface of the optical waveguide, so that the conversion efficiency is greatly reduced. On the other hand, in the structure of FIG. 2C, the polarization reversal is in contact and continuous at the bottom surface of the waveguide, so that high-efficiency wavelength conversion is achieved without hindering the wavelength conversion near the waveguide surface. I was able to. In the structure of FIG. 2C, the continuous portion 8 of the periodically poled portion 6 is about 10% of the thickness of the periodically poled portion 6. Table 1 summarizes the stability and conversion efficiency characteristics of the domain-inverted structure. As for the thickness of the electrode portion, 5% or more of the domain-inverted portion is necessary for stabilization. However, if the thickness exceeds twice the thickness of the periodic portion, the thickness of the periodic inversion portion decreases, so that the thickness is preferably suppressed to twice or less.

[0038]

[Table 1]

In Table 1, the stability of the domain-inverted structure
Indicates that there is no problem in stability. Specifically, 400
It was confirmed that the heat treatment process at about ℃ did not affect the domain-inverted shape. In the case of ×, a change in the domain-inverted shape was observed by a heat treatment process at about 400 ° C. Regarding the conversion efficiency ○, a conversion efficiency of 20% or more was obtained for a fundamental wave of 100 mW. In the case of ×, a conversion efficiency of only several% was obtained for a fundamental wave of 100 mW.

It is in the structure of FIG. 2C that the stability of the domain-inverted structure and the highly efficient wavelength conversion can be realized.

A short-wavelength light source was realized using an AlGaAs-based tunable DBR semiconductor laser (wavelength 820 nm). The tunable DBR semiconductor laser has an active region and a DBR.
The oscillation wavelength can be adjusted by adjusting the injection current into the DBR region, which is composed of two electrodes in the region. By directly coupling the semiconductor laser and the waveguide-type optical wavelength conversion device, a compact short-wavelength light source was realized. A semiconductor laser beam of 60 mW was coupled to the optical waveguide for a semiconductor laser output of 100 mW.

The waveguide loss was -0.5 dB / cm, which is 1/2 of the conventional waveguide loss, and a low-loss optical waveguide was realized. By using the stoichiometric LN, it is possible to increase the transmittance of the crystal, and at the same time, polarization inversion is caused by application of a low-voltage electric field. Thus, a low-loss optical waveguide can be formed.

Furthermore, this is because the increase in the propagation loss of the optical waveguide due to a slight change in the refractive index caused at the boundary between the domain-inverted portions due to the contact between the adjacent domain-inverted portions is reduced. Therefore, it is possible to form an optical waveguide with very low loss. Further, the conversion efficiency of the optical wavelength conversion element is about 5
0%, which is more than double the conventional value.
This is because the nonlinear optical constant of the substrate is improved in addition to the reduction of the waveguide loss, and the stoichiometric LN crystal has a nonlinear optical constant that is 1.2 times or more as large as the congruent composition LN crystal. Since the proton concentration in the optical waveguide can be reduced by utilizing a higher refractive index change, the nonlinear optical constant in the waveguide is increased, and a highly efficient optical wavelength conversion element can be formed.

In this embodiment, an X-plate stoichiometric LN with a 3 ° off-cut was examined as the substrate. However, the off-cut angle of the substrate (the angle between the normal to the substrate surface and the X-axis of the crystal) is 0.3-10 degrees is desirable. When the off-cut angle is slightly present, the domain inversion grows along the Z axis of the crystal, so that a deep domain inversion structure is formed.
However, as the off-cut angle becomes shallower, the thickness of the domain inversion decreases, and when it becomes 0.3 ° or less, only a domain-inverted structure of about 1 μm formed on a normal X plate is formed.

On the other hand, when the off-cut angle is large, a deep domain-inverted structure is formed, and a highly efficient optical wavelength conversion element can be manufactured. However, as the off-cut angle increases, the propagation loss of the proton exchange optical waveguide formed increases. 10
At an angle of more than °, the propagation loss becomes more than 3 dB / cm,
It is not preferable because the conversion efficiency of the light wavelength conversion element is significantly reduced.

In this embodiment, the polarization inversion structure in the stoichiometric LN was examined. However, an additive of Mg, Zn, Sc, or In is added to the stoichiometric LN in an amount of 0.03% by weight or more. This has made it possible to greatly improve the characteristics of the optical wavelength conversion element. Output characteristics are improved by increasing the light damage resistance. In the case of crystals containing no additive, an unstable phenomenon due to optical damage was observed even for an SHG output of about 1 mW, but the SHG output of 50 mW or more can be output stably by adding an additive. It became.

In this embodiment, the application to the optical wavelength conversion element has been described. However, the same applies to the application to the optical switch, the optical deflector, the modulator and the like using the domain-inverted structure. Was improved. For example, a periodic domain-inverted structure is formed in a three-dimensional waveguide, and an electric field is applied thereto to form a grating structure in the waveguide.
It can be erased. Functions such as a directional coupler using a grating, a TE / TM mode converter, and a DBR grating can be controlled by applying an electric field. Even when such a grating structure is formed, a stable shape can be maintained by adopting the configuration of the present invention.

Also, there are optical deflectors that use a DBR by a grating and those that use a triangular inversion shape to realize polarization by the prism effect. However, when a fine polarization inversion is used, Also, a stable shape can be realized by contacting a part of the domain inversion. In these devices utilizing the change in the refractive index due to the application of an electric field, the use of a stoichiometric LN enables the use of an optical waveguide having a high refractive index, and the increase in the electro-optical constant allows the optical device to be used. The characteristics of the device were greatly improved.

Embodiment 2 Here, a method of manufacturing a periodically poled structure in a stoichiometric LN will be described. In the method of forming the periodic domain-inverted structure on the off-cut substrate, domain-inverted was formed by the electrode structure shown in FIG. Polarization reversal occurs near the tip of the electrode finger,
Formed inside the substrate along the axis. This is because when a voltage is applied between the comb-shaped electrode and the plane electrode, the electric field strength is maximized at the tip of the electrode finger.

On the other hand, it has been found that in the stoichiometric LN, the position where the polarization inversion is formed is not limited to the tip of the electrode finger, but occurs randomly at the base of the electrode finger or at the stripe portion. This is presumably because the inversion voltage of the stoichiometric LN is low, the electric field intensity difference in the electric field distribution in the electrode is small, and the location of the domain inversion is relatively random. If the position where the polarization inversion is formed is not determined with good reproducibility, the position of the optical waveguide is not determined, and high-efficiency design of the optical wavelength conversion element cannot be performed with good reproducibility. Therefore, a method was found in which the electrode was brought into contact with the substrate only at the portion where the domain inversion was formed.

FIG. 4 shows a configuration of a method for manufacturing an optical wavelength conversion element according to the present invention. A striped electrical insulating film 12 was formed on the surface of a stoichiometric composition LN off-cut substrate 11. As the electric insulating film 12, a commercially available photosensitive resin was used. Next, a comb-shaped electrode 13 was formed via the electric insulating film 12. The plane electrode 14 was formed on the back surface of the substrate. The comb-shaped electrode is composed of a tooth portion 15 in which electrode portions are arranged periodically and a stripe portion 16 connecting the tooth portions. Only the electrode 15 necessary for forming the domain-inverted structure is brought into contact with the substrate to be continuous. The part was insulated from the substrate by an electric insulating film.
When a voltage is applied between the electrodes, the electrode portion 15 in contact with the substrate
From this, domain inversion occurred, and a periodic domain inversion structure was formed. In the conventional method, the position where the domain inversion is formed is several tens μm.
That was all. On the other hand, in the domain-inverted structure manufactured by the method of the present embodiment, the position of the domain-inverted structure can be determined with a high reproducibility with an accuracy of 1 μm or less. The continuous portion of the periodically poled portion was about 10% of the thickness of the periodically poled portion as in Example 1.

An optical wavelength conversion element was manufactured using the domain-inverted structure manufactured by the conventional method and the method of the present invention. As a manufacturing method, an optical waveguide was formed so as to cross the periodically poled structure. The optical waveguide was manufactured by performing proton exchange in a stripe shape using a selection mask. When the conversion efficiency of the manufactured light wavelength conversion device was measured, the yield of the light wavelength conversion device manufactured by the conventional method was as low as 5% or less. This is because the position where the domain-inverted portion is formed cannot be determined, and the probability that the optical waveguide and the domain-inverted portion overlap efficiently is low.

On the other hand, the optical wavelength conversion device manufactured by the manufacturing method of the present invention could be manufactured with a high yield of 80% or more. Further, the conversion efficiency of the manufactured light wavelength conversion element was more than twice as high as that of the conventional method. This is because a uniform periodic polarization inversion structure can be formed in the optical waveguide by the manufacturing method of the present invention.

As for the high concentration addition of Mg, 0.2 mol
The light damage resistance is significantly improved by adding 1% or more,
By adding 2 mol% or more, the damage resistance increased to 70 mW or more with respect to blue light, and by adding 3 mol% or more, it became possible to generate 100 mW of blue light. No crystal defects and no increase in scattering loss were observed even with an increase in the Mg doping amount, and good optical characteristics were obtained.

[0055]

As described above, according to the present invention, in the periodically poled structure formed in the stoichiometric LN crystal, a stable poled structure can be obtained by partially contacting adjacent poled parts. Became possible. By using this domain-inverted structure, the occurrence of non-uniformity in the domain-inverted structure due to an optical waveguide process or the like is suppressed, and a highly efficient optical wavelength conversion element structure can be realized.

In forming the domain-inverted structure, it is possible to precisely control the domain-inverted formation position by insulating a part of the electrode from the substrate. This makes it possible to improve the yield and the SHG element characteristics when forming the optical wavelength conversion element, and the practical effect is large.

[Brief description of the drawings]

FIG. 1A is a perspective view of an optical wavelength conversion device according to an embodiment of the present invention, and FIG. 1B is a plan view of the same.

FIGS. 2A and 2B are cross-sectional views of a domain-inverted structure of an embodiment of the present invention and a conventional optical wavelength conversion element, where FIG. 2A shows a domain-inverted structure of a conventional optical wavelength conversion element, and FIG. A diagram showing a domain-inverted structure in which the upper surface of one embodiment of the present invention is in contact,
(C) is a diagram showing a domain-inverted structure in which the lower surface of one embodiment of the present invention is in contact.

FIG. 3 is a perspective view of an optical wavelength conversion element according to one embodiment of the present invention.

FIG. 4 is a perspective view illustrating a method for manufacturing an optical wavelength conversion element according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 LN substrate of stoichiometric composition 2 Periodic polarization inversion structure 3 Proton exchange optical waveguide 4, 13 Comb electrode 5, 14 Planar electrode 6 Polarization inversion part 7 Polarization non-inversion part 8 Continuous part of polarization inversion part 11 Stoichiometric composition LN off-cut substrate 12 Striped electrical insulating film 15 Electrode tooth portion 16 Electrode stripe portion

Continuing from the front page (71) Applicant 500121621 Yasunori Furukawa 4-17-17 Nishi, Kamishiba-cho, Fukaya-shi, Saitama (71) Applicant 500121643 Shunji Takekawa 2-1-111, Azuma, Tsukuba-shi, Ibaraki 801-403 (71) Application Person 500583151 Yu Nakamura 1-18-1 Azuma, Tsukuba-shi, Ibaraki Pref. 406 Building 402 (72) Inventor Kiminori Mizuuchi 1006 Ojidoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (72) Inventor Kazuhisa Yamamoto Osaka 1006 Kadoma, Kadoma City Matsushita Electric Industrial Co., Ltd. (72) Inventor Kenji Kitamura 4-13-61, Azuma, Tsukuba-shi, Ibaraki Prefecture ) Inventor Shunji Takegawa 2-Chome, 11-11, Azuma, Tsukuba-shi, Ibaraki Pref. EA04 FA26 FA27 GA05 GA07 HA20

Claims (11)

[Claims] 1. A stoichiometric Li having a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 49.5 to 50.2%.
An optical wavelength conversion element having an NbO ₃ crystal and a domain-inverted portion formed on the crystal surface, wherein the domain-inverted portion is periodically arranged, and at least a part of the domain-inverted portion is continuous. An optical wavelength conversion element, comprising: 2. The optical wavelength conversion device according to claim 1, wherein the continuous portion exists at a bottom of the domain-inverted portion. 3. The crystal according to claim 1, wherein the C axis of the crystal is aligned with the crystal surface.
3. The light wavelength conversion device according to claim 1, wherein the light wavelength conversion device is inclined in a range of 0.3 to 10 degrees. 4. The light wavelength conversion device according to claim 1, wherein the Curie temperature of the stoichiometric LiNbO ₃ crystal is in the range of 1185 to 1205 ° C. 5. The crystal according to claim 1, wherein said crystal contains at least 0.03% by weight of an additive of Mg, Zn, Sc or In.
5. The optical wavelength conversion element according to any one of 4. 6. The optical wavelength conversion element according to claim 1, wherein a period of said domain inversion section is 3.5 μm or less. 7. A continuous portion of said periodically poled portion has a thickness of 5 to 200 times the thickness of said periodically poled portion as viewed from the cross section.
%. The optical wavelength conversion device according to claim 1, wherein the ratio is in the range of%. 8. A stoichiometric Li having a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 49.5 to 50.2%.
Forming a comb-shaped electrode on one surface of the NbO ₃ crystal, forming a planar electrode on the other surface of the crystal, and bringing a part of the comb-shaped electrode into contact with the crystal surface via an insulating film so as to be continuous; A method for manufacturing an optical wavelength conversion element in which an electric field is applied between electrodes to form a periodically poled structure. 9. The comb-shaped electrode includes teeth having a periodic grating structure and stripes connecting the teeth to each other, and the stripes are brought into contact with the crystal surface via an insulating film. The method for manufacturing an optical wavelength conversion element according to claim 7, wherein the light wavelength conversion element is continuous. 10. The crystal according to claim 1, wherein a C axis of said crystal is aligned with said crystal surface.
The method for manufacturing a light wavelength conversion element according to claim 8 or 9, wherein the light wavelength conversion element is inclined in a range of 0.3 to 10 °. 11. A continuous portion of said periodic domain-inverted portion,
5-20 of the thickness of the periodically poled portion as viewed from the cross section
The method for producing an optical wavelength conversion element according to any one of claims 8 to 10, wherein the range is 0%.