JP2965644B2 - Manufacturing method of wavelength conversion optical element - Google Patents

Description

Description: Object of the Invention (Industrial application field) The present invention relates to a wavelength conversion optical element using a non-linear optical material, and in particular, to wavelength conversion by improving the polarization direction of an optical waveguide layer. The present invention relates to a method for manufacturing an optical element.

(Prior Art) In recent years, research on optical second harmonic generation (SHG) using a nonlinear optical crystal as a means for realizing a short wavelength light source has been actively conducted, and for the purpose of miniaturization and low power consumption, Attempts have been made to use a semiconductor laser as a fundamental wave to make a nonlinear optical crystal into a waveguide. For example, using a proton-exchanged LiNbO ₃ waveguide as shown in FIG.
A blue light source as a harmonic has been obtained (T. Taniuchi e
t al. Optoelectronics, Vol. 2, No. 1, PP. 53-58 (198
7)). In this method, a fundamental wave 3 is incident on an optical waveguide layer 2 formed on a crystal substrate 1, and a second harmonic 4 is emitted into the waveguide substrate by Cherenkov radiation.
Compared with the above, there is an advantage that phase matching by angle control, temperature control, and the like is unnecessary.

However, in this Cherenkov radiation system, the conversion efficiency is not always large because there is a phase mismatch in the vertical direction in the waveguide layer. The phase mismatch in the direction perpendicular to the waveguide layer is greatly affected by the layer structure of the waveguide.However, in the conventional Cherenkov radiation type SHG, the waveguide structure is determined so as to satisfy the Cherenkov radiation condition. There is a problem that the degree of freedom of the parameter for improving is small and a large conversion efficiency cannot be obtained.

(Problems to be Solved by the Invention) As described above, in the conventional Cherenkov radiation system SHG, there is a problem that the degree of freedom of parameters for controlling the conversion efficiency is small and a large conversion efficiency cannot be obtained.

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to prevent a decrease in conversion efficiency due to vertical phase mismatch in a Cherenkov radiation type SHG, and to reduce the conversion efficiency. An object of the present invention is to provide a method of manufacturing a wavelength conversion optical element that can be improved.

[Constitution of the Invention] (Means for Solving the Problems) The gist of the present invention is to improve the conversion efficiency by compensating the phase mismatch in the direction perpendicular to the optical waveguide layer by reversing the polarization of the optical waveguide layer. is there.

That is, the present invention provides a method for manufacturing an optical waveguide type wavelength conversion optical element in which an optical waveguide layer having a higher refractive index than a substrate is formed on a crystal substrate made of an optical crystal, and the substrate and the optical waveguide layer are formed of a nonlinear optical crystal. In this method, a polarization inversion layer having a direction opposite to the polarization direction of the substrate is formed on at least a part of the optical waveguide layer in a direction perpendicular to the substrate by utilizing a difference in Curie point between the optical waveguide layer and the substrate.

More specifically, according to the present invention, in the method for manufacturing an optical waveguide type wavelength conversion optical element, a first dielectric layer having a lower Curie point than the substrate is formed on the substrate as a part of the optical waveguide layer. Thereafter, an electric field is applied to the first dielectric layer, and a 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 to change the polarization direction of the first dielectric layer to the substrate. Then, a second dielectric layer having a lower Curie point than the dielectric layer is formed on the first dielectric layer as a part of the optical waveguide layer, and then the second dielectric layer is formed.
And applying a heat treatment 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 matched with the polarization direction of the substrate.

Further, the present invention is characterized in that, as means for controlling the Curie point of the optical waveguide layer, the composition ratio of the constituent materials of the optical waveguide layer is made different, or the amount of impurities doped into the optical waveguide layer is made different.

(Operation) According to the present invention, by providing a portion where the polarization is inverted in the optical waveguide layer, it is possible to prevent a decrease in conversion efficiency due to a phase mismatch in the vertical direction in the Cherenkov radiation type SHG, A wavelength conversion element having a large conversion efficiency can be realized. Also, by utilizing the difference in Curie point between the substrate and the optical waveguide layer, a domain-inverted layer can be easily formed only in the optical waveguide layer. Further, by forming layer structures having different Curie points, polarization inversion can be easily realized.

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

First, as shown in FIG. 14, the wavelength conversion optical element is formed by forming an optical waveguide layer 2 having a larger refractive index than the substrate 1 on a crystal substrate 1 made of a nonlinear optical crystal. When this optical waveguide layer 2 is the wavelength lambda ₁ of the fundamental wave 3 is incident, the wavelength _{λ 2 (= λ 1/2} ) second harmonic 4 of the nonlinear optical effect
Is emitted into the substrate 1 as Cherenkov radiation. The refractive indices of the substrate 1 for the wavelengths λ ₁ and λ ₂ are n ₁ and n, respectively.
₂ to the effective refractive index of the waveguide consisting of the substrate 1 and the optical waveguide layer 2 n _EFF is _{n 1 <n EFF <n 2} ... refractive index n _G of the optical waveguide layer 2 so as to satisfy the is selected.

In general, in a Cherenkov radiation type SHG element, when the setting is made so as to satisfy the condition of the equation, when the conversion efficiency η at each location in the vertical direction is calculated, it becomes as shown by a solid line in FIG. As can be seen from this figure, the sign of η is inverted depending on the location, and the contribution to the conversion efficiency is canceled as a whole. Therefore, if a structure in which the polarity is inverted is used in the portion where the sign is inverted, the conversion efficiency is improved. The present invention has been made in consideration of this point, and has performed polarization inversion so as to obtain characteristics as shown by broken lines A and B in FIG.

 Hereinafter, embodiments of the present invention will be described 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, reference numeral 10 denotes a crystal substrate made of a LiNbO ₃ crystal (hereinafter abbreviated as LN) as a nonlinear optical crystal, and an optical waveguide layer 20 having a higher refractive index than the unprocessed substrate 10 is formed on the substrate 10. This optical waveguide layer
The refractive index n _G of 20, the formula have been chosen to establish, when the fundamental wave 31 having a wavelength lambda ₁ enters into the optical waveguide layer 20, the wavelength lambda ₂ by the nonlinear optical effect ₍₌ λ _1/2) Second harmonic 3 of
2 is emitted into the substrate 10 as Cherenkov radiation.

Here, a feature of this embodiment is that the optical waveguide layer 20 has a structure in which a part of the polarization is inverted. That is, the optical waveguide layer 20
Is formed by laminating first and second dielectric layers 21 and 22 having different Curie points, and the polarization direction of the first dielectric layer 21 is
The direction of polarization of the second dielectric layer 22 is opposite to that of the substrate 10.
It is in the same direction as.

With such a configuration, since the polarization direction of the first dielectric layer 21 is opposite to that of the substrate 10, the contribution coefficient η to the conversion efficiency shown in FIG. The portion where the sign is inverted can be made positive as shown by the broken line A. For this reason, the contribution to the conversion efficiency as a whole can be increased, and the conversion efficiency can be improved.

 Next, a method for manufacturing the element of this embodiment will be described.

It is known that the Curie point of a dielectric crystal varies depending on the difference in crystal composition and the presence of impurities (Bergman et al.).
al. Appl. Phys Lett., 12, 92, (1968)). 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, as shown in FIG. 2 (a), a liquid phase epitaxial method using MgO-added LN crystal as a substrate 10 and V ₂ O ₅ as a flux at, for example, about 0.2 μm at 800 ° C.
The first dielectric layer 21 is grown to a thickness of m. At this point, the directions of polarization of the dielectric layer 21 are different as shown in FIG. 2 (a). The LN used as the substrate 10 has a high Curie point, and the 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 the substrate 1 is
A voltage is applied between 0 and the dielectric layer 21. In this state, heat treatment is performed at a temperature higher than the Curie point of the dielectric layer 21 and lower than the Curie point of the substrate 10, 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 changed to an Nb-excess crystal by adjusting the Li / Nb ratio in the flux in advance. Accordingly, the Curie point of the dielectric layer 21 is low, and the polarization is easily made uniform by the heat treatment at 500 ° C. as shown in FIG. 2C. Moreover, the direction is opposite to the direction of the substrate 10.

Next, as shown in FIG. 2 (d), the first dielectric layer
A second dielectric layer 22 is grown on 21 by a liquid phase epitaxial method. This second dielectric layer 22 is more Nb than the first layer 21.
It is excessive and the Curie point is lower than that of the first dielectric layer 21.

Next, as shown in FIG.
Metal films 44 and 45 are formed again on the dielectric layer 22 of
As a result, a voltage is applied between the substrate 10 and the dielectric layer 22. In this state, for example, at 400 ° C. (a temperature lower than the Curie point of the first layer 21 and higher than the Curie point of the second layer 22), 3 to 5
Heat treatment is performed for a time. At this time, the polarization of the first dielectric layer 21 does not change, and the polarization of the second dielectric layer 22 is easily aligned. Moreover, the direction is the same as that of the substrate 10.

Thereafter, a proton conversion method or the like is performed at about 230 ° C. so that the change in the refractive index satisfies the conditional expression, thereby forming the optical waveguide layer 20 as shown in FIG. Is prepared. In the above example, two dielectric layers are used. However, as shown in FIG. 3B, only one dielectric layer may be used and the polarization of the dielectric layer 21 may be reversed. Good. In this case, the conversion efficiency contribution coefficient of the waveguide layer portion in FIG. 13 is generally inverted, but the conversion efficiency is improved by inverting the conversion efficiency contribution factor since the conversion efficiency contribution coefficient is larger in the portion than in the positive portion. become. Further, in this embodiment, the liquid phase epitaxial method is used for forming the dielectric layers 21 and 22, but other growth methods such as the MOCVD method and the MBE method can be used.

As described above, according to the present embodiment, it is possible to form a domain-inverted layer in the optical waveguide layer 20 by utilizing the difference between the Curie points of the dielectric layers 21 and 22 constituting the optical waveguide layer 20,
The contribution coefficient to the conversion efficiency in the optical waveguide layer 20 can be increased. Therefore, a wavelength conversion optical element having high conversion efficiency can be manufactured.

FIG. 4 is a sectional view showing a schematic configuration of a second embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

This embodiment differs from the first embodiment described above in that an inversion layer is formed not only on the optical waveguide layer but also on the substrate side. That is, the buffer layers 11 and 12 made of a dielectric are formed on the substrate 10, and the dielectric layers 21 and 22 constituting the optical waveguide layer 20 are formed in the same manner as in the previous embodiment. Then, the polarization directions 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 order from the layer on the substrate 10 side, and executing the polarization inversion process described in the previous embodiment at a predetermined temperature.

In the wavelength conversion optical element thus obtained, the substrate
Polarization inversion also occurs on the 10 side, and in FIG.
At the same time, as shown by the broken line B, the contribution coefficient to the conversion efficiency can be inverted, so that the conversion efficiency can be improved more than the first embodiment.

FIG. 5 is a perspective view showing a schematic configuration of a third embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

This embodiment differs from the first embodiment in that a dielectric layer as an optical waveguide layer is formed by ion implantation of impurities. That is, a first dielectric layer 51 and a second dielectric layer 52 are formed on the substrate 10 by impurity ion implantation, 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 of manufacturing the device shown in FIG. 5 will be described with reference to FIGS. 6 and 7. In addition to the difference in crystal composition,
The Curie temperature of a dielectric crystal also changes depending on the type and amount of impurities contained in the crystal. In this embodiment, a polarization inversion method using an ion implantation method is described.

First, as shown in FIG. 6A, portions of the substrate 10 other than the domain-inverted locations are covered with a resist mask 61, and Ti ions are ion-implanted into the domain-inverted locations (for example, at a peak concentration of 1 × 10 4).
¹⁸ cm ^-3), carried recrystallization, diffusion by heat treatment at about 800 ° C. 1 hour. As a result, a first dielectric layer 51 is formed. At this time, the direction of polarization of the ion implantation layer is different, and the Curie point of the dielectric layer 51 is about 900 ° C.
It is.

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

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

Thereafter, by performing a proton conversion method or the like so that the change in the refractive index satisfies the conditional expression, the optical waveguide layer 50 is formed to manufacture a wavelength conversion optical element. If the change in the refractive index of the dielectric layers 51 and 52 due to the ion implantation satisfies the conditional expression, this proton conversion method is unnecessary.

As described above, according to the present embodiment, it is possible to form a domain-inverted layer in the optical waveguide layer 50 by utilizing the difference between the impurity doping amounts of the dielectric layers 51 and 52 constituting the optical waveguide layer 50. Thus, the contribution coefficient to the conversion efficiency in the optical waveguide layer 50 can be increased. Therefore, it becomes possible to produce a wavelength conversion optical element having high conversion efficiency as in the first embodiment.

In the present embodiment, the description has been made using the ion implantation method of Ti ions.However, in addition to this, diffusion of impurities of Li and Nb atoms may be used, and further, neutron irradiation and radiation irradiation may be used. Similar results can be expected. Further, as shown in the second embodiment, the present method can be used repeatedly.

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

As shown in FIG. 8, a flux and an LN raw material 72 are mixed and held in a platinum crucible 71. Usually, the LN substrate 73 is immersed in the solution 72, and the temperature is gradually lowered to form a single crystal layer. When a single crystal layer is formed from this liquid phase, an electric field is applied to the LN substrate 73 via the substrate support rod 75 by the power supply 74 to form a concentration difference between the positive ions 76 and the negative ions 77 immediately below the LN substrate 73. . In this state, the temperature of the liquid phase is gradually lowered to form a single crystal layer. In the state shown in FIG. 8, the positive ion concentration of Nb increases immediately below the substrate 73, and an LN crystal layer having a high Nb ratio grows. Accordingly, the Curie point is low, and the polarization inversion shown in the first embodiment can be performed with good reproducibility.

It is needless to say that the method according to the present embodiment can be applied to the removal of impurities that do not need to be mixed into the LN waveguide layer. Further, by alternately changing the direction of the electric field during the crystal growth, it becomes possible to form a single crystal layer in which the Curie point is variously changed. Further, by using a substrate having a high Curie point during the growth of the epitaxial layer, the polarization of the epitaxially grown layer can be reversed from that of the substrate.

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

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

FIG. 10 is a schematic diagram showing the purification of the melt of the present invention. An LN melt 82 is accommodated in a platinum crucible 81. In this state, for example, a rod 85 of platinum is immersed in the
To apply an electric field to the melt 82. With this device, platinum rod 85
, Many impurities of the positive ions 86 are collected. By gradually cooling the temperature in the LN melt 82, an LN polycrystal containing many impurities is crystallized around the platinum rod. By removing the platinum rod 85 and growing a single crystal, a high-purity LN single crystal can be grown.

Further, another platinum rod is inserted into the melt, and a positive electric field is applied, thereby performing the same operation as above to crystallize the LN polycrystal mixed with a large amount of negative ions around the platinum rod, and form the platinum rod. After removing the rod, it is possible to grow a single crystal. By applying positive and negative electric fields separately using two platinum rods, it is possible to produce a highly pure LN melt, and to produce an LN single crystal that can be used as an optical crystal Becomes possible.

Next, a sixth embodiment of the present invention will be described. In this embodiment, a method for manufacturing an LN substrate having a high Curie point will be described. As shown in FIG. 1, the generated second harmonic passes through the substrate crystal and is emitted to the outside of the wavelength conversion element. Therefore, when there is a non-uniform portion such as a fluctuation of the refractive index in the substrate crystal corresponding to the path of the second harmonic, the second harmonic is scattered or refracted and the optical lens is used even after being emitted out of the substrate crystal. It becomes impossible to collect light. In particular, in the case of producing an LN crystal having a high laser damage value by adding MgO or the like, unevenness in the concentration of Mg causes unevenness in the refractive index as it is and cannot be used as an optical substrate. The biggest cause of uneven Mg concentration is convection in the melt. MgO in LN melt
When added, the viscosity of the LN melt increases, and not a small-scale thermal oscillation but a large thermal oscillation occurs every several tens to several hours. The magnitude of the thermal vibration is in the range of 5 to 15 ° C. In the crystal grown as a result of this thermal vibration, growth fringes are generated at intervals of several 100 μm to several mm based on Mg concentration unevenness.

For such growth stripes based on the convection of the melt, a method of suppressing a convection by applying a magnetic field or the like from the outside in a semiconductor crystal or the like is known, but in an oxide material such as LN, the electric resistance of the melt is low. High and not effective. Therefore, a uniform LN single crystal can be produced by applying an electric field to the LN melt and using a deterrent by the electric field to grow a single crystal in which thermal oscillation is suppressed.

FIG. 11 shows a schematic diagram of this embodiment. Platinum crucible (φ
50) Store the LN melt 92 (400 g) in 91 and heat it at 1200 ° C or higher to melt the 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 by a power supply 94. A hole was formed in the center of the electrode 95, and growth was performed in the C-axis direction through the hole at a pulling speed of 5 mm / h and a rotation speed of 5 rpm.
Although the diameter of the pulled crystal 93 was about 25 mm, there was no intense growth stripes in the crystal, and it was possible to obtain an optically excellent single crystal.

In this embodiment, the electrode 95 was in contact with the melt 92, but it did not have to be in contact with the melt 92 as shown in FIG. Thermal vibration is reduced, and a good effect can be obtained.

Note that the present invention is not limited to the above-described embodiments. In the embodiments, LN is mainly described, but the present invention can 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 types of growth and the like. In addition, various modifications can be made without departing from the scope of the present invention.

[Effects of the Invention] As described above in detail, according to the present invention, the phase mismatch in the direction perpendicular to the waveguide layer is compensated by the polarization reversal of the optical waveguide layer, and the conversion efficiency is improved. It is possible to realize a wavelength conversion optical element having a large conversion efficiency in the system SHG.

[Brief description of the drawings]

FIG. 1 is a cross-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 cross-sectional view showing a manufacturing process of the above-mentioned embodiment element, and FIG. FIG. 4 is a sectional view showing a schematic configuration of a second embodiment of the present invention, FIG. 5 is a perspective view showing a schematic configuration of a third embodiment of the present invention, and FIG. FIG. 7 is a cross-sectional view showing a manufacturing process of the device of the third embodiment, FIG. 7 is a schematic diagram showing the relationship between the Curie point and the heating temperature in the third embodiment, and FIGS. FIG. 10 is a schematic view showing an apparatus configuration for performing the growth method and the vapor phase growth method, FIG. 10 is a schematic view for explaining the purification of the melt, and FIGS. FIG. 13 is a schematic view 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. It is. 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.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yutaka Uematsu 1 Toshiba-cho, Komukai, Koyuki-ku, Kawasaki-shi, Kanagawa Prefecture (58) Investigated field (Int.Cl. ⁶ , DB name) G02F 1 / 37

Claims (5)

(57) [Claims] 1. A method for manufacturing an optical waveguide type wavelength conversion optical element, comprising: forming an optical waveguide layer having a higher refractive index than a substrate on a crystal substrate made of an optical crystal; and forming the substrate and the optical waveguide layer from a nonlinear optical crystal. In the wavelength conversion, a polarization inversion layer in which a polarization direction of a substrate is reversed is formed on at least a part of the optical waveguide layer in a direction perpendicular to the substrate by utilizing a difference in Curie points between the optical waveguide layer and the substrate. A method for manufacturing an optical element. 2. A method of manufacturing an optical waveguide type wavelength conversion optical element, comprising: forming an optical waveguide layer having a larger refractive index than a substrate on a crystal substrate made of an optical crystal, and comprising the substrate and the optical waveguide layer by a nonlinear optical crystal. Forming a first dielectric layer having a lower Curie point than the substrate on the substrate as a part of the optical waveguide layer, and then applying an electric field to the first dielectric layer, Heat treatment at a temperature higher than the Curie point of the body layer and lower than the Curie point of the substrate to invert the polarization direction of the first dielectric layer with the polarization direction of the substrate, and then as a part of the optical waveguide layer Forming a second dielectric layer having a lower Curie point than the dielectric layer on the first dielectric layer;
And applying a heat treatment 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. Making the wavelength coincide with the polarization direction of the substrate. 3. The production of a wavelength conversion optical element according to claim 1, wherein the means for controlling the Curie point of the optical waveguide layer has different composition ratios of constituent materials of the optical waveguide layer. Method. 4. The method for manufacturing a wavelength conversion optical element according to claim 1, wherein the means for controlling the Curie point of the optical waveguide layer differs in the amount of impurities doped into the optical waveguide layer. . 5. An optical waveguide type wherein an optical waveguide layer composed of a plurality of dielectric layers having a higher refractive index than the substrate is formed on a crystal substrate composed of an optical crystal, and each dielectric layer is composed of a nonlinear optical crystal. In the method of manufacturing a wavelength conversion optical element, the polarization direction of one dielectric layer and the polarization direction of the other dielectric layer are set to be opposite by utilizing the difference between the Curie points of the dielectric layers. A method for producing a wavelength conversion optical element, comprising: