JPH04104232A - Waveguide conversion optical element and production thereof - Google Patents

Abstract

PURPOSE:To prevent the degradation in conversion efficiency by compensating the phase mismatching in the direction perpendicular to an optical waveguide layer by the polarization inversion of the optical waveguide layer. CONSTITUTION:The optical waveguide layer 20 having the refractive index larger than the refractive index of an untreated crystal substrate 10 consisting of an LiNbO3 crystal (LN) as a nonlinear optical crystal is formed on this crystal substrate 10. A second harmonic wave 32 of lambda2(=lambda1/2) wavelength is radiated as Cerenkov radiation light into the substrate 10 by the nonlinear optical effect when a basic wave 31 of a wavelength lambda1 is made incident on the optical waveguide layer 20. The polarization of a part of the optical waveguide layer 20 has an inverted structure. The degradation in the polarization efficiency occurring in the phase mismatching in the perpendicular direction of the Cerenkov radiation system SHG is prevented in this way and the large conversion efficiency is obtd.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a wavelength conversion optical element using a nonlinear optical material, and in particular to a wavelength conversion device that improves the polarization direction of an optical waveguide layer. The present invention relates to an optical element and its manufacturing method.

(Prior Art) In recent years, research on optical second harmonic generation (SHG) using nonlinear optical crystals has become active as a means of realizing short wavelength light sources, and with the aim of miniaturization and lower power consumption. Attempts have been made to use a semiconductor laser for the fundamental wave and to turn a nonlinear optical crystal into a waveguide. For example, a blue light source as the second harmonic of a semiconductor laser has been obtained using a proton-exchanged LINbOi waveguide as shown in FIG.
, Tanluchl et al.
tronics.

Vol.2. No, 1. PP, 53-58 (19
87)). In this method, a fundamental wave 3 is incident on an optical waveguide layer 2 formed on a crystal substrate 1, and a second wave is generated by Cerenkov radiation.
It radiates harmonic 4 into the waveguide substrate, which is similar to the conventional S
Compared to HG, it has the advantage that phase matching by angle control, temperature control, etc. is not required.

However, since this Cerenkov radiation method has a vertical phase mismatch in the waveguide layer, the conversion efficiency is not necessarily high. The phase mismatch in the direction perpendicular to the waveguide layer is greatly affected by the layer structure of the waveguide, but in conventional Cerenkov radiation SHG, the waveguide structure is determined to satisfy the Cerenkov radiation condition, so it is difficult to improve the conversion efficiency. There was a problem that there were few degrees of freedom in parameters for improvement, and high conversion efficiency could not be obtained.

(Problems to be Solved by the Invention) As described above, the conventional Cherenkov radiation type SHG has had a problem in that there is little freedom in parameters that can control conversion efficiency, and high conversion efficiency cannot be obtained.

The present invention has been made in consideration of the above circumstances, and its purpose is to prevent a reduction in conversion efficiency due to vertical phase mismatch in Cerenkov radiation SHG, and to improve conversion efficiency. An object of the present invention is to provide a wavelength conversion optical element that can be improved and a method for manufacturing the same.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is to provide a vertical phase mismatch in an optical waveguide layer.
The purpose is to improve conversion efficiency by compensating by polarization inversion of the optical waveguide layer.

That is, the present invention provides an optical waveguide-type wavelength conversion optical element in which an optical waveguide layer having a higher refractive index than the substrate is formed on a crystal substrate made of an optical crystal, and at least one of the substrate and the optical waveguide layer is made of a nonlinear optical crystal. In the optical waveguide layer, the polarization direction of at least a portion of the optical waveguide layer is reversed from the polarization direction of the substrate.

More specifically, the present invention provides the optical waveguide type wavelength conversion optical element described above, in which the optical waveguide layer is constructed by laminating two dielectric layers, and the polarization direction of the dielectric layer on the substrate side is set to the polarization direction of the substrate. Conversely, the polarization direction of the dielectric layer on the opposite side of the substrate is set to be the same as the polarization direction of the substrate.

The present invention also provides an optical waveguide type wavelength conversion optical element in which an optical waveguide layer having a higher refractive index than the substrate is formed on a crystal substrate made of an optical crystal, and at least one of the substrate and the optical waveguide layer is formed of a nonlinear optical crystal. In this manufacturing method, the difference in Curie points between the optical waveguide layer and the substrate is utilized to form a polarization inversion layer, which is reversed in the polarization direction of the substrate, on at least a portion of the optical waveguide layer.

More specifically, the present invention provides the method for manufacturing the optical waveguide type wavelength conversion optical element described above, in which a first dielectric layer having a Curie point lower than that of the substrate is formed on the substrate as part of the optical waveguide layer. Thereafter, an electric field is applied to this first dielectric layer, and heat treatment is performed at a temperature higher than the Curie point of the dielectric layer and lower than the Curie point of the substrate, so that the polarization direction of the first dielectric layer is changed to the direction of the substrate. Then, a second dielectric layer having a Curie point lower than that of the dielectric layer is formed on the first dielectric layer as a part of the optical waveguide layer, and then this second dielectric layer is While applying an electric field to the layer, heat treatment is performed at a temperature higher than the Curie point of the second dielectric layer and lower than the Curie point of the first dielectric layer, thereby changing the polarization direction of the second dielectric layer to that of the substrate. This method is made to match the polarization direction.

Furthermore, the present invention provides a means for controlling the Curie point of the optical waveguide layer, which includes varying the composition ratio of the constituent materials of the optical waveguide layer.
Alternatively, the optical waveguide layer may be doped with different amounts of impurities.

(Function) According to the present invention, by providing a portion with reversed polarization in the optical waveguide layer, it is possible to prevent a reduction in conversion efficiency due to vertical phase mismatch in Cerenkov radiation SHG. It becomes possible to realize a wavelength conversion element with high conversion efficiency. Further, by utilizing the difference in Curie point between the substrate and the optical waveguide layer, the polarization inversion layer can be easily formed only on the optical waveguide layer. Furthermore, by forming a layered structure with different Curie points, polarization inversion can be easily achieved.

(Example) Before describing the example, the basic principle of the present invention will be explained.

First, as shown in FIG. 14, the wavelength conversion optical element is constructed by forming an optical waveguide layer 2 having a higher refractive index than the substrate 1 on a crystal substrate 1 made of a nonlinear optical crystal. When a fundamental wave 3 with a wavelength λ1 is incident on the optical waveguide layer 2, a second harmonic wave 4 with a wavelength λ2 (−λ, /2) is emitted into the substrate 1 as Cherenkov radiation due to a nonlinear optical effect.

The refractive index of the substrate 1 for wavelengths 21 and λ2 is n
When lr is 02, the effective refractive index n EPP of the waveguide consisting of the substrate 1 and the optical waveguide layer 2 is n+ <nEFP <n
The refractive index n6 of the optical waveguide layer 2 is selected so as to satisfy 2'''■.

In general, in a Cherenkov radiation type SHG element, when it is designed to satisfy the condition of equation (2), the conversion efficiency η at each location in the vertical direction is calculated as shown by the solid line in FIG. 13. As can be seen from this figure, the sign of η is reversed depending on the location, and the contributions to the conversion efficiency cancel each other out as a whole. Therefore, if a structure in which the polarization is reversed in the portion where the sign is reversed is used, the conversion efficiency will be improved. The present invention has been made with this point in mind, and the polarization is inverted so as to obtain the characteristics shown by broken lines A and B in FIG. 13.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing a schematic structure of a wavelength conversion optical element according to a first embodiment of the present invention.

In the figure, 10 is L i N b O3 as a nonlinear optical crystal.
This is a crystal substrate made of crystal (hereinafter abbreviated as LN), and an optical waveguide layer 2o having a higher refractive index than the untreated substrate 1o is formed on this substrate 10.

The refractive index nQ of this optical waveguide layer 20 is selected so that the above-mentioned formula (2) holds true, and when the fundamental wave 31 with a wavelength λ1 is incident on the optical waveguide layer 2o, a nonlinear optical effect causes a wavelength λ2 (
-λ1/2) is radiated into the substrate 10 as Cerenkov radiation.

Here, the feature of this embodiment is that the optical waveguide layer 20 has a structure in which the polarization of a part of the optical waveguide layer 20 is reversed.

That is, the optical waveguide layer 20 has first and second layers having different Curie points.
The first dielectric layer 21 is polarized in the opposite direction to the substrate 10, and the second dielectric layer 22 is polarized in the same direction as the substrate 10. It has become.

With such a configuration, since the polarization direction of the first dielectric layer 21 is opposite to that of the substrate 10, the thirteenth
At the contribution coefficient η to the conversion efficiency shown in the figure, the optical waveguide layer 2
The part whose sign is inverted at 0 can be made positive as shown by the broken line A. Therefore, the contribution to the overall conversion efficiency can be increased, and the conversion efficiency can be improved.

Next, a method of manufacturing the device of this example will be explained.

It is known that the Curie point of a dielectric crystal varies depending on the composition ratio of the crystal and the presence of impurities (Bergwa et al.
net at, Appl, Phys Lett,
, 12°92, (1988)). For example, in an LN crystal, the Curie point changes by changing the ratio of the number of Li and Nb atoms in the crystal.

Therefore, in this embodiment, first, as shown in FIG. 2(a),
By liquid phase epitaxial method using MgO-doped LN crystal as substrate 10 and V2O5 as flux, the temperature is, for example, 800°C.
The first dielectric layer 21 is grown to a thickness of about 0.2 μm. At this point, the polarization directions of the dielectric layer 21 are varied as shown in FIG. 2(a). L as the substrate 10
N has a high Curie point, and its polarization does not change even when exposed to this temperature.

Next, as shown in FIG. 2(b), metal films 41 and 42 are formed on the substrate 10 and the dielectric layer 21, and a voltage is applied between the substrate 10 and the dielectric layer 21 using the DC power source 43. In this state, the temperature of the substrate 10 is higher than the Curie point of the dielectric layer 21.
Heat treatment is performed at a temperature lower than the Curie point of, for example, 500° C. for 3 to 5 hours. After that, the metal films 41 and 42 are removed. The Li/Nb atomic ratio of the dielectric layer 21 can be easily made into crystals containing excessive Nb by adjusting the LL/Nb ratio in the flux in advance. Therefore, the Curie point of the dielectric layer 21 is low, and the polarization can be easily aligned by heat treatment at 500° C. as shown in FIG. 2(C). Moreover, its direction is opposite to that of the substrate 10.

Next, as shown in FIG. 2(d), the first dielectric layer 2
1, a second dielectric layer 22 is formed by a liquid phase epitaxial method.
grow. The second dielectric layer 22 has an excess of Nb than the first layer 21 and has a Curie point lower than that of the first dielectric layer 21 .

Next, as shown in FIG. 2(e), the substrate 10 and the second
A metal film 44, 45 is again formed on the dielectric layer 22, and a voltage is applied between the substrate 10 and the dielectric layer 22 using the DC power supply 46. In this state, for example, 400°C (first layer 21
The heat treatment is performed at a temperature lower than the Curie point of the second layer 22 and higher than the Curie point of the second layer 22 for 3 to 5 hours. At this time, the polarization of the first reading body layer 21 does not change, and the polarization of the second dielectric layer 21 does not change.
The polarizations of 2 are easily aligned. Moreover, its direction is the same as that of the substrate 10.

Thereafter, a proton conversion method or the like is carried out at about 230° C. so that the change in refractive index meets condition 0, thereby forming the optical waveguide layer 20 as shown in FIG. Fabricate the element.

In the above example, the reading body layer is made of two layers, but as shown in FIG. good. In this case, the conversion efficiency contribution coefficient of the waveguide layer portion in FIG. 13 is completely reversed, but since the negative value is larger than the positive value in this portion, the conversion efficiency can be improved by reversing this. become. Furthermore, in this embodiment, the dielectric layer 2
A liquid phase epitaxial method was used to form 1.22, but M
It is also possible to use other growth methods such as the OCvD method or the MBE method.

As described above, according to this embodiment, by utilizing the difference in the Curie points of the dielectric layers 21 and 22 constituting the optical waveguide layer 20, a polarization inversion layer can be formed within the optical waveguide layer 20. The contribution coefficient to the conversion efficiency within the optical waveguide layer 20 can be increased. Therefore, it becomes possible to produce a wavelength conversion optical element with high conversion efficiency.

FIG. 4 is a sectional view showing a schematic configuration of a second embodiment of the present invention. Note that the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted.

The difference between this embodiment and the first embodiment described above is as follows.
The reason is that an inversion layer is formed not only on the optical waveguide layer but also on the substrate side. That is, a buffer layer 11.12 made of a dielectric material is formed on the substrate 10, and a dielectric layer 21.22 constituting the optical waveguide layer 20 is formed in the same manner as in the previous embodiment. And the polarization direction of these layers 11, 12, 21, 22,
are set as shown in FIG. This polarization direction can be set by lowering the Curie point in the layers starting from the substrate 10 side and performing the polarization inversion process described in the previous embodiment at a predetermined temperature.

In the wavelength conversion optical element thus obtained, the substrate 1
Polarization reversal also occurs on the 0 side, and the contribution coefficient to the conversion efficiency can be reversed as shown by the broken line A and the broken line B in FIG. 13, and the conversion efficiency is improved more than in the first embodiment. can be measured.

FIG. 5 is a perspective view showing a schematic configuration of a third embodiment of the present invention. Note that the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted.

This embodiment differs from the first embodiment in that the dielectric layer serving as the optical waveguide layer is formed by implanting impurity ions. That is, a first dielectric layer 51 and a second dielectric layer 52 are formed on the substrate 10 by implanting impurity ions, and the polarization direction of the first dielectric layer 51 is opposite to that of the substrate 10. The polarization direction of the second dielectric layer 52 is the same as that of the substrate 10.

Next, a method for manufacturing the device shown in FIG. 5 will be explained with reference to FIGS. 6 and 7. In addition to the difference in composition ratio of the crystal, the Curie temperature of the dielectric crystal also changes depending on the type and amount of impurities contained in the crystal. Therefore, in this embodiment, a polarization inversion method using an ion implantation method is shown.

First, as shown in FIG. 6(a), parts of the substrate 10 other than the polarization inversion locations are covered with a resist mask 61, and Ti ions are implanted into the polarization inversion locations.
X 1018c■-3), recrystallization and diffusion are carried out by heat treatment at about 800° C. for 1 hour. This forms the first dielectric layer 51. At this point, the direction of polarization of the ion implantation layer is different, and the direction of polarization of the dielectric layer 51 is different.
The Curie point of is approximately 900°C.

Next, a metal film (not shown) is formed on the surface region including at least the first dielectric layer 51, and a voltage is applied from the DC power supply 62 as shown in FIG. The heat treatment is performed under a temperature condition (1000° C.) that is higher than the Curie point of and does not affect the polarization fluctuation of the substrate 10. This operation causes polarization inversion in the first dielectric layer 51.

Next, as shown in FIG. 6(C), after forming a resist mask 63 having an opening width narrower than that of the resist mask 61, Ti ions are implanted at a peak concentration of 1 x 10.
20c-3), forming the second dielectric layer 52; Next, as shown in FIG. 6(d), a voltage is applied from the DC power supply 64, and the temperature is lower than the Curie point of the first dielectric layer 51 and higher than the Curie point of the second dielectric layer 52 ( Heat treatment is performed at 820°C). As a result, the polarization direction of the second dielectric layer 52 is the same as that of the substrate 10.

Thereafter, the optical waveguide layer 50 is formed by performing a proton conversion method or the like so that the change in refractive index satisfies Condition 0, thereby producing a wavelength conversion optical element. Note that if the change in the refractive index of the dielectric layers 51 and 52 due to ion implantation satisfies condition 0, this proton conversion method is unnecessary.

As described above, according to this embodiment, a polarization inversion layer can be formed in the optical waveguide layer 50 by utilizing the difference in the amount of impurity doping between the dielectric layers 51 and 52 constituting the optical waveguide layer 50. , the contribution coefficient to the conversion efficiency within the optical waveguide layer 50 can be increased. Therefore, as in the first embodiment, it is possible to manufacture a wavelength conversion optical element with high conversion efficiency.

In addition, in this example, explanation was given using the ion implantation method of Ti ions, but in addition to this, Li ion implantation method was used.

The impurity diffusion of Nb atoms may be used, and further,
Similar results can be expected using neutron irradiation or radiation irradiation. It is also possible to use this method repeatedly as shown in the second embodiment.

Next, a fourth embodiment of the present invention will be described. This example is a method for efficiently controlling the Li/Nb ratio when growing an epitaxial layer in manufacturing the device shown in FIG.

As shown in FIG. 8, flux and LN raw material 72 are mixed and melted and held in a platinum crucible 71. Usually, the LN substrate 73 is immersed in this solution 72 and the temperature is gradually lowered to form a single crystal layer. When forming a single crystal layer from this liquid phase, an electric field is applied to the LN substrate 73 by the power supply 74 via the substrate support rod 75 to form a concentration difference between positive ions 76 and negative ions 77 directly under the LN substrate 73. .

A single crystal layer is formed by gradually lowering the temperature of the liquid phase in this state. In the state shown in FIG. 8, the concentration of positive Nb ions is high directly below the substrate 73, and an LN crystal layer with a high Nb ratio grows. Therefore, the Curie point is also low, and the
The polarization inversion shown in the example can be carried out with good reproducibility.

It goes without saying that the method of this embodiment can also be applied to the removal of impurities that would be problematic if they were mixed into the LN waveguide layer. Furthermore, by alternately changing the direction of the electric field during crystal growth, it is possible to form single crystal layers with various Curie points. Furthermore, by using a substrate with a high Curie point during the growth of the epitaxial layer, it is possible to reverse the polarization of the epitaxially grown layer with that of the substrate.

Further, the method of this embodiment is applicable not only to liquid phase growth method but also to vapor phase growth method. FIG. 9 shows a schematic diagram thereof. By placing the LN substrate 73 on a substrate support 78 and forcibly applying an electric field to the substrate surface using a platinum electrode 79 during crystal growth, it becomes possible to control the adsorption of molecules with a polarization moment, and the basic It can bring about the same effect as liquid phase growth method.

Next, a fifth embodiment of the present invention will be described. In this example, purification of a melt for growing crystals will be described.

FIG. 10 is a schematic diagram showing melt purification according to the present invention. A platinum melt 81 contains an LN melt 82 . In this state, a platinum rod 85, for example, is immersed in the melt 82, and an electric field is applied to the melt 82 from the power source 84. Due to this device, a large amount of impurities such as positive ions 86 gather around the platinum rod 85. By gradually cooling the temperature inside the LN melt 82, LN polycrystals containing many impurities crystallize around the platinum rod.

By removing this platinum rod 85 and growing a single crystal, it becomes possible to grow a high purity LN single crystal.

Furthermore, by inserting another platinum rod into the melt and applying a positive electric field, the same operation as above is performed to crystallize LN polycrystals mixed with a large amount of negative ions around the platinum rod. After removing the rod, it is possible to grow single crystals. By using two platinum rods and applying a positive and a separate electric field to each, it became possible to create a highly pure LN melt, which could be used as an optical crystal.
It becomes possible to produce an N single crystal.

Next, a sixth embodiment of the present invention will be described. In this example, a method for manufacturing an LN substrate with a high Curie point will be described. As can be seen from FIG. 1, the generated second harmonic passes through the substrate crystal and is emitted to the outside of the wavelength conversion element. Therefore, if there are non-uniform parts such as fluctuations in the refractive index in the substrate crystal that is the path of the second harmonic, the second harmonic will be scattered or refracted, and even after being emitted outside the substrate crystal, an optical lens will be used. It becomes impossible to focus the light. In particular, when adding MgO or the like to produce an LN crystal with a high laser damage value, unevenness in Mg concentration directly causes unevenness in the refractive index, making it impossible to use as an optical substrate. The biggest cause of uneven Mg concentration is convection within the melt. When MgO is added to the LN melt, the viscosity of the LN melt increases, and instead of small-scale thermal vibrations, large thermal vibrations occur every several tens of minutes to several hours. The magnitude of thermal vibration is in the range of 5 to 15°C.

Within the crystal grown as a result of this thermal vibration, there are
Growth stripes occur at intervals of several meters due to uneven Mg concentration.

In semiconductor crystals, etc., it is known to suppress the convection by applying an external magnetic field, etc., to prevent growth streaks caused by convection of the melt, but in the case of oxide materials such as LN, the electrical resistance of the melt increases. Expensive and not effective. Therefore, a uniform LN single crystal can be produced by applying an electric field to the LN melt and using the suppressing force of the electric field to grow a single crystal with suppressed thermal vibrations.

FIG. 11 shows a schematic diagram of this embodiment. Platinum crucible (φ
50) Store LN melt 92 (400g) in 91 and
Heat to 200°C or higher to melt LN.

Thereafter, a platinum electrode 95 was brought into contact with the surface of the LN melt, and a DC voltage of 100 V was applied from a power source 94.

There is a hole in the center of the electrode 95, and through the hole,
Growth was performed in the C-axis direction at a pulling speed of 511 m/h% and a rotational speed of 5 rp. Although the diameter of the pulled crystal 93 was approximately 25 m5, there were no severe growth striations within the crystal, making it possible to obtain an optically excellent single crystal.

In this example, the electrode 95 was in contact with the melt 92, but the
As shown in Figure 2, the electrodes 95 do not necessarily need to be in contact with each other, and even if the electrode 95 is placed 1 to 2@II away from the surface, thermal vibrations on the melt surface will be reduced and a good effect can be obtained. .

Note that the present invention is not limited to the embodiments described above. In the embodiments, LN was mainly described, but the present invention can also be applied to other dielectric crystals and oxide nonlinear optical crystals. Further, as a means for forming the optical waveguide layer, dielectric thin films having different polarization directions may be bonded together instead of using various growth methods. In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As detailed above, according to the present invention, the phase mismatch in the direction perpendicular to the waveguide layer is compensated for by polarization inversion of the optical waveguide layer.
By improving the conversion efficiency, it becomes possible to realize a wavelength conversion optical element with high conversion efficiency in Cerenkov radiation SHG.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a schematic configuration of a wavelength conversion optical element according to a first embodiment of the present invention, FIG. 2 is a sectional view showing the manufacturing process of the above embodiment element, and FIG. 3 is a sectional view of the above embodiment element. 4 is a sectional view showing a schematic structure of a second embodiment of the present invention. FIG. 5 is a perspective view showing a schematic structure of a third embodiment of the present invention. The figure is a cross-sectional view showing the manufacturing process of the device of the third example, FIG. 7 is a schematic diagram showing the relationship between the Curie point and the heating temperature in the third example, and FIGS. 8 and 9 are the liquid phase A schematic diagram showing the equipment configuration when carrying out the growth method and the vapor phase growth method, FIG. 10 is a schematic diagram for explaining the purification of the melt, and FIGS. 11 and 12 show the electric field application state in the pulling method. FIG. 13 is a schematic diagram for explaining the basic principle of the present invention, and FIG. 14 is a perspective view showing an example of a conventional wavelength conversion optical element. 10... Crystal substrate, 11.12... Buffer layer, 20.50... Optical waveguide layer, 21.22, 51.52... Dielectric layer, 41.42,
44.45...Metal film, 43.46,62.64...
・Power supply, 61.63...Resist mask. Fig. 1 Mimihosae A Sairiyan Tsuduni Shishi = 2 (door m) Fig. Fig. 7 Fig. 10 Fig. 11 Fig. 8 Fig. Fig. 12

Claims (6)

[Claims] (1) An optical waveguide type wavelength conversion optical element in which an optical waveguide layer having a refractive index higher than that of the substrate is formed on a crystal substrate made of an optical crystal, and at least one of the substrate and the optical waveguide layer is made of a nonlinear optical crystal, A wavelength conversion optical element characterized in that the polarization direction of at least a portion of the optical waveguide layer is reversed to the polarization direction of the substrate. (2) In an optical waveguide type wavelength conversion optical element in which an optical waveguide layer having a refractive index higher than that of the substrate is formed on a crystal substrate made of an optical crystal, and at least one of the substrate and the optical waveguide layer is made of a nonlinear optical crystal, The optical waveguide layer is constructed by laminating two dielectric layers, and the polarization direction of the dielectric layer on the substrate side is opposite to the polarization direction of the substrate, and the polarization direction of the dielectric layer on the opposite side to the substrate is the polarization direction of the substrate. A wavelength conversion optical element characterized by being set in the same direction as the wavelength conversion optical element. (3) Manufacturing an optical waveguide type wavelength conversion optical element in which an optical waveguide layer having a higher refractive index than the substrate is formed on a crystal substrate made of an optical crystal, and at least one of the substrate and the optical waveguide layer is made of a nonlinear optical crystal. In the method, a polarization inversion layer whose polarization direction is reversed with the polarization direction of the substrate is formed in at least a part of the optical waveguide layer by utilizing a difference in Curie point between the optical waveguide layer and the substrate. Production method. (4) Manufacturing an optical waveguide type wavelength conversion optical element in which an optical waveguide layer having a higher refractive index than the substrate is formed on a crystal substrate made of an optical crystal, and at least one of the substrate and the optical waveguide layer is made of a nonlinear optical crystal. The method includes the steps of: forming a first dielectric layer having a Curie point lower than that of the substrate on the substrate as part of the optical waveguide layer; and then applying an electric field to the first dielectric layer; A step of performing heat treatment at a temperature higher than the Curie point of the dielectric layer and lower than the Curie point of the substrate to reverse the polarization direction of the first dielectric layer with the polarization direction of the substrate, and then a part of the optical waveguide layer. a step of forming a second dielectric layer having a Curie point lower than that of the first dielectric layer on the first dielectric layer;
While applying an electric field to the dielectric layer, heat treatment is performed at a temperature higher than the Curie point of the second dielectric layer and lower than the Curie point of the first dielectric layer, and the polarization direction of the second dielectric layer is changed. A method for manufacturing a wavelength conversion optical element, comprising the step of matching the polarization direction of the substrate with the polarization direction of the substrate. (5) The method for manufacturing a wavelength conversion optical element according to claim 3 or 4, wherein the means for controlling the Curie point of the optical waveguide layer is to vary the composition ratio of the constituent materials of the optical waveguide layer. (6) The method for manufacturing a wavelength conversion optical element according to claim 3 or 4, characterized in that the means for controlling the Curie point of the optical waveguide layer includes varying the amount of impurities doped into the optical waveguide layer.