JPH0667233A - Waveguide type second higher harmonic generating element and its production - Google Patents

Abstract

PURPOSE:To provide the waveguide type 2nd higher harmonic generating element which is increased in 2nd higher harmonic conversion efficiency and lowered in wave front aberration and its manufacture. CONSTITUTION:An optical waveguide 52 which has a higher refractive index than a substrate 51 and is uniform in the refractive index is formed on the substrate 51 and the scattering of light in the optical waveguide is reduced; and rectangular self-polarization inversion areas 55 are periodically formed in the optical waveguide and then the phase matchability of a 2nd higher harmonic is improved to increase the conversion efficiency of the 2nd higher harmonic, thereby reducing the wave front aberration. For the purpose, an optical waveguide layer 52 of a single polarization area thin film is grown by an LPE method on the substrate 51 provided with a polarization inversion part 56 and then, the polarization inversion part 56 of the substrate is transferred into the optical waveguide layer 52 by a heat treatment to form the polarization inversion area 55 which is rectangularly sectioned.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the shortening of the wavelength of the light source of optical disk devices, laser printers, and other optical application devices, and in particular, semiconductor laser light having a wavelength of about 800 nm is blue with a wavelength of about 400 nm. Waveguide type second harmonic generation element (SHG, Seconnd Harmonic Generet)
ion) and its manufacturing method.

[0002]

2. Description of the Related Art At an early stage, as shown in FIG. 2, one end of an optical waveguide 22 formed by thermally diffusing titanium (Ti) on the surface of a single crystal 21 of lithium niobate (hereinafter referred to as LiNbO ₃ ). A method has been reported in which a fundamental wave 23 (power P ₁ ) of an ordinary ray polarized in the z direction is made incident and an extraordinary ray polarized in the y direction is used as the second harmonic wave 24 for phase matching.

Further, as shown in FIG. 3 of Japanese Patent Application Laid-Open No. 61-18964, an optical waveguide is formed on a LiNbO ₃ single crystal substrate 21 by a proton exchange method (a method of partially replacing Li ions of LiNbO ₃ with protons). There is disclosed a method of forming 31 and injecting a fundamental wave 23 polarized perpendicularly to the substrate surface from one end thereof and extracting a vertically polarized second harmonic wave 32 generated by Cherenkov radiation in the optical waveguide.

Also, electronics and letters (Elec
tronics, Letters) Vol. 25, pp. 731-732, as shown in FIG. 4, Ti film is periodically formed on LiNbO ₃ crystal substrate 21 and heated to about 1100 ° C. to form Ti film forming portion 41. Of the second harmonic wave 2 by producing the optical waveguide 42 by the proton exchange method.
4, a polarization inversion portion 55 in which the spontaneous polarization direction is inverted at an equal pitch and an optical waveguide 42 formed by a proton exchange method are provided on a ferroelectric material having a spontaneous polarization, and the z direction is formed from one end of the optical waveguide 42. The fundamental wave 23 polarized to
A method of extracting the second harmonic wave 24 polarized in the z direction from the other end has been proposed.

When lithium tantalate (LiTaO ₃ ) is used for the crystal substrate 21, a periodic proton exchange section 41 is produced by a proton exchange method instead of Ti diffusion and heated to about 600 ° C. to form a diffraction grating layer. Polarization of the optical waveguide 42 by the proton exchange method.
A method of producing a is also being tried.

[0006]

The above method of FIG. 2 includes:
Since the temperature coefficient of the refractive index for the ordinary light 23 and the temperature coefficient of the refractive index of the extraordinary light are largely different, there is a problem that the temperature control of 0.1 ° C. or less is required. Further, in the method using the Cherenkov radiation shown in FIG. 3, the beam shape of the second harmonic wave is crescent like 32, and the wavefront aberration is extremely large, and it is almost impossible to narrow it down to the diffraction limit.

On the other hand, in the method of FIG. 4, since the second harmonic wave 24 is collimated light, it is extremely easy to collect light as compared with Cherenkov radiation, but T which forms a polarization inversion grating is used.
Since the i-diffused portion and the Ti-non-diffused portion or the proton-exchanged portion and the non-proton-exchanged portion have different refractive indexes, the fundamental wave is lost due to Fresnel reflection loss at the boundary portion and the efficiency is reduced. There was a problem.

In addition, Ti is diffused in the polarization inversion lattice,
Since it is produced by proton exchange, the cross-sectional shape of the polarization inversion lattice depends on the shape of the Ti diffusion layer or the proton exchange layer, and it is essentially difficult to produce the polarization inversion lattice of rectangular cross section. Since the cross-sectional shape of the domain-inverted lattice produced by the Ti diffusion method is substantially triangular and the cross-sectional shape of the domain-inverted lattice produced by the proton exchange method is approximately semi-circular, the SHG having the domain-inverted lattice of an ideal rectangular section is provided. The problem was that the second harmonic could not be generated due to the original efficiency of the device.

The object of the present invention relates to the improvement of the second harmonic generating element shown in FIG. 4, and in particular, the refractive index of the optical waveguide is made uniform to reduce the Fresnel loss for the fundamental wave and the second harmonic, and It is an object of the present invention to provide a waveguide-type second harmonic generation element having a rectangular cross section of a domain-inverted grating to improve efficiency, small wavefront aberration, and easy to collect light, and a manufacturing method thereof.

[0010]

In order to solve the above-mentioned problems, a second embodiment of the present invention comprises a ferroelectric optical substrate and a ferroelectric optical waveguide layer having a refractive index higher than that of the substrate and periodically arranging domain inversion regions. In the harmonic generating element, the Curie point of the optical waveguide layer is set lower than the Curie point of the substrate, and a rectangular polarization inversion portion whose polarization direction is inverted is periodically provided on the surface of the substrate on the optical waveguide layer side. A polarization inversion region of the optical waveguide layer is provided on the polarization inversion part.

Therefore, the substrate and the optical waveguide layer are made of lithium niobate, and the optical waveguide layer has a composition ratio in which the ratio of lithium to niobium is lower than that of the substrate. Further, the substrate is made of magnesium-doped lithium niobate, and the optical waveguide layer is made of lithium niobate. Alternatively, the substrate and the optical waveguide layer are made of magnesium-doped lithium niobate, and the optical waveguide layer has a composition ratio in which the doping ratio of magnesium is lower than that of the substrate.

Further, a step of preparing a melt by heating and melting an optical waveguide layer on a substrate on which a domain-inverted portion is periodically provided in the presence of a flux of a raw material powder of a ferroelectric metal oxide; Lowering the temperature to the crystal precipitation temperature, contacting the domain-inverted surface of the substrate to perform liquid phase epitaxial growth of the metal oxide film, and then performing a heat treatment at a temperature near the Curie point of the metal oxide film. To be formed by. Further, a step of preparing a melt obtained by heating and melting an optical waveguide layer on a substrate provided with periodically domain-inverted parts in the presence of a flux of a raw material powder of a ferroelectric metal oxide, and curing the melt. At the crystal precipitation temperature above the point, the metal oxide film is brought into liquid phase epitaxial growth by contacting the polarization-inverted surface of the substrate.

Further, the above flux is added to vanadium pentoxide (V ₂ O ₅ ), boron trioxide (B ₂ O ₃ ), lithium fluoride (LiF), or potassium fluoride (K).
F) or boron trioxide (B ₂ O ₃ ) and molybdenum trioxide (MoO ₃ ) or a mixture of boron trioxide (B ₂ O ₃ ) and tungsten trioxide (WO ₃ ).

[0014]

By uniformly growing the above-mentioned epitaxial film, an optical waveguide layer having a uniform refractive index can be formed on the substrate, whereby the fundamental wave and the second harmonic in the domain inversion region in the optical waveguide layer can be formed. The Fresnel reflection loss of the waves is reduced. Further, the light can be confined in the optical waveguide layer by increasing the refractive index of the optical waveguide layer higher than that of the substrate. Further, by lowering the Curie point of the optical waveguide layer from the Curie point of the substrate, the polarization direction of the polarization inversion portion periodically provided on the substrate is transferred into the optical waveguide layer to form a rectangular polarization inversion region. Is formed. As a result, the phase variation of the second harmonic generated in the optical waveguide layer is reduced, and the second harmonic conversion efficiency is improved.

The substrate and the optical waveguide layer are made of lithium niobate, and the optical waveguide layer has a composition ratio in which the ratio of lithium to niobium is lower than that of the substrate, or the substrate is magnesium-doped niobate. lithium,
The optical waveguide layer is made of lithium niobate, or the substrate and the optical waveguide layer are made of magnesium-doped lithium niobate, and the optical waveguide layer has a composition ratio in which the doping ratio of magnesium is lower than that of the substrate. Due to
The Curie point of the optical waveguide layer is lower than that of the substrate.

The optical waveguide layer on the substrate is a substrate surface in which a polarization inversion portion is periodically provided at a crystal precipitation temperature of a melt obtained by heating and melting a raw material powder of a ferroelectric metal oxide in the presence of flux. To form a metal oxide film by liquid phase epitaxial growth, and then perform heat treatment in the vicinity of the Curie point of the metal oxide film. In the optical waveguide layer on the substrate, a polarization inversion portion is periodically provided at a crystal precipitation temperature of a Curie point or higher in a melt obtained by heating and melting a raw material powder of a ferroelectric metal oxide in the presence of a flux. It is formed by liquid phase epitaxial growth of a metal oxide film in contact with the substrate surface.

[0017]

【Example】

[Embodiment 1] Embodiment 1 of the present invention will be described below with reference to FIGS. FIG. 1 is a cross-sectional view of the second harmonic generation element, FIG. 5 is an operation explanatory view thereof, and FIG. 6 is a manufacturing process drawing thereof. An object of the present invention is to increase the efficiency by making the cross sectional shape of the periodically poled grating of the second harmonic generation element rectangular. Further, in this embodiment, the optical waveguide layer 52 is formed on the + c surface of the substrate 51.

Therefore, consider a structure as shown in FIG. An optical waveguide layer 52 is provided on the + c surface of the substrate 51, and in the optical waveguide layer 52, spontaneous polarization upward portions 54 and downward polarization portions (polarization inversion regions) 55 are periodically arranged. 5
3 is a clad layer, which is usually an air layer. Reference numeral 56 denotes a polarization reversal portion provided in the substrate to form the polarization reversal region 55, and the polarization direction with respect to the substrate 51 is reversed. Λ
Is the period of 55.

The substrate 51 and the optical waveguide layer 52 are made of, for example, LiNb.
With a ferroelectric substance having a space group R3C such as O ₃ , the refractive index n in the optical waveguide layer 52 is constant regardless of the direction of spontaneous polarization (M. Didomenco Jr., Journal of Applied Phys.
ics magazine, Vol.40, No.2, pp720-734), optical waveguide layer 52
The fundamental wave (wavelength λ) incident on the
No Fresnel reflection occurs at the interface. That is, the reflection loss of the fundamental wave can be prevented.

FIG. 1B is a diagram showing such uniformity of the refractive index. The optical waveguide layer 52 can be formed by a vapor phase growth method such as a liquid phase epitaxial growth method or a sputtering method, or a solid phase growth method such as an epitaxial growth by melting method. As a result, the optical waveguide layer 52 having a Curie point lower than that of the substrate 51 can be formed, and thus the spontaneous polarization portion 55 can be formed in a rectangular shape.

Next, a method for forming the spontaneous polarization portion 55 into a rectangular shape will be described. In the present invention, the substrate 51
The polarization inverting portion 56 is provided on the optical waveguide layer 52 and then the optical waveguide layer 52 is formed. When the optical waveguide layer 52 is heat-treated at a temperature slightly lower than the Curie point, the spontaneous polarization of the optical waveguide layer 52 becomes smaller than that of the substrate 51. Therefore, the optical waveguide layer 52 is formed on the substrate 51.
An electric field in the vertical direction will be received from the surface of the domain-inverted portion 56. On the other hand, when an electric field in the opposite direction to the spontaneous polarization is applied to a single-domain crystal, first, buds in the opposite domain are generated on the surface, and the energy for maintaining the domain wall from the electric field is sufficient to overcome the energy. It is known that the supplied buds grow along the direction of the electric field and reach the edge of the crystal (see Chapter 8 in "Ferroelectrics" by Kazuo Kawabe).

Therefore, in the present invention, the polarization inversion portion 56 on the substrate 51 corresponds to the bud of the above domain, and an inversion electric field is applied from there to the optical waveguide layer 52. Therefore, the polarization direction on the polarization inversion portion 56 is the substrate. The polarization direction is reversed with respect to 51, and the other portions have the same polarization direction as the substrate 51. As described above, it is possible to form the optical waveguide layer 52 having the periodically poled grating having the rectangular cross section as shown in FIG.

The heat treatment temperature is set to the temperature of the optical waveguide layer 52
The temperature may be higher than the point and sufficiently lower than the Curie point of the substrate 51. In this case, the domain of the substrate is maintained, the optical waveguide layer becomes a paraelectric layer, and spontaneous polarization disappears, so that a periodically poled lattice having a rectangular cross section is formed during cooling after the heat treatment.

The above-mentioned optical waveguide layer 52 of the present invention can be produced by a combination of materials (BC Grabmaier, Journal.
al of Crystal Growth magazine, Vol. 110 (1991),
See pp 339-347). For example, LiNbO ₃ containing 5 mol% of magnesium is used for the substrate 51,
LiNbO ₃ having a congruent composition is used for the optical waveguide layer 52. The Curie point of LiNbO ₃ for the substrate 51 is about 1225 ° C.

When the ratio of lithium to niobium is changed, the Curie point is changed to, for example, non-doped (non-added) L.
iNbO ₃ can be lowered to a range of 1150 ° C. to 1075 ° C. or higher, and the Curie point of the above-mentioned commercially available congruent composition material is about 1150 ° C., so that these are used for the optical waveguide layer 52. The refractive index of LiNbO ₃ for the substrate 51 is the optical waveguide layer 5
2 is lower than LiNbO _{3 having} a congruent composition for 2 and, for example, the extraordinary light refractive index for a wavelength of 6328Å
2.192 and 2.200.

FIG. 1 (c) is a diagram in which the coefficient d representing the generation performance of the second harmonic is entered in FIG. 1 (a). Further, since the guided light confined in the optical waveguide layer is hardly affected by the non-linearity of the substrate 51, FIG. 1 (c) can be approximated as shown in FIG. 1 (d).

FIG. 5 is a top view and a sectional view of the second harmonic generating element according to the present invention. The substrate 51 is a 5 mol% MgO-doped ZcutLiNbO ₃ single crystal whose surface is the + c plane, and the optical waveguide layer 52 is usually LiNb whose spontaneous polarization is upward.
It is an O ₃ single crystal thin film. Further, the nonlinear optics of the polarization inversion portion 56 and the polarization portion 55 are both downward. The ridge type optical waveguide 18 is formed on the optical waveguide layer 52. Ray
The outgoing light (fundamental wave) of the beam 11 is polarized by the optical system 12 in the direction perpendicular to the surface of the substrate and is condensed and incident.

The second harmonic is generated in the optical waveguide 18, and similarly polarized in the direction perpendicular to the substrate surface. Both the fundamental wave and the second harmonic wave are confined in the optical waveguide 18 and propagate. For reference, the theory of the target generation efficiency of the second harmonic is described at the end of this Example section.

FIG. 6 is a process diagram of the method for manufacturing the second harmonic generating element by the liquid phase epitaxial growth method and heat treatment. First, 5 mol% MgO-doped L shown in FIG.
The + c plane on the surface of the substrate 51 of iNbO ₃ has a fundamental wavelength λ of 1
After polishing to about / 10, ultrasonic cleaning in acetone, isopropyl alcohol, and pure water and drying are performed. Next, as shown in FIG. 6B, a 30Å Ti film 81 is formed on the + c surface by sputtering.

Next, as shown in FIG. 6C, a photoresist 82 is applied on the Ti film 81 by a spinner, and the photoresist 82 is patterned by using a photoresist having a domain-inverted portion 56 as a window (FIG. 6). (C)). Next, using the photoresist 82 as a mask, the Ti 81 is patterned by RIE using CF ₃ Cl gas, and the photoresist 82 is removed (FIG. 6E). Eleven kinds were prepared by changing the pattern period of the photomask from 2.5 to 3.5 μm by 0.1 μm.

Then, the above-mentioned substrate is put into an electric furnace, and about 80
Heat treatment is performed at about 1100 ° C. for about 10 minutes in an Ar atmosphere containing water vapor through a bubbler of warm water at 0 ° C. At the time of cooling, the atmosphere was changed to O ₂ containing water vapor. As a result, the domain inversion portion 56 is formed on the + c surface of the substrate 51 as shown in FIG.

Then, an optical waveguide layer 52 of a single-domain LiNbO ₃ single crystal thin film is grown on the + c surface of the substrate 51 by a liquid phase epitaxial crystal growth method. The melt during the epitaxial growth was made of 50 mol% lithium carbonate Li ₂ CO ₃ as a raw material and 10 mol% niobium pentoxide Nb.
₂ O ₅ and 40 mol% of vanadium pentoxide V ₂ O ₅ powders were weighed and mixed, put into a platinum crucible and melted at 900 ° C., and heated at 1200 ° C. in an electric furnace at a temperature of 1200 ° C. Maintain for time and make uniform.

Next, this melt was cooled to 930 ° C. at a cooling rate of 30 ° C./h, and the substrate of FIG.
When contacted for 30 minutes, 2.5 μm Li of the same figure (g)
An optical waveguide layer 52 of NbO ₃ thin film grows. Ten sheets of this sample were manufactured. Next, the substrate 51 is placed in an electric furnace at 30 ° C.
Slowly cool to room temperature at a cooling rate of / h.

Nitric acid: hydrofluoric acid = 1: 1 on the surface of the optical waveguide layer 52.
When the domain of the optical waveguide layer 52 was observed from the difference in the etching state by etching with the etching solution of No. 2, the substrate 51 before the polarization inversion portion 55 was formed regardless of the polarization pitch Λ.
It was found that a single domain film in which the polarization direction was reversed was formed. In addition to vanadium pentoxide (V ₂ O ₅ ), boron trioxide (B ₂ O ₃ ) is used as a flux material in the epitaxial growth of the optical waveguide layer 52.
Lithium fluoride (LiF), potassium fluoride (KF),
Boron trioxide (B ₂ O ₃ ) and molybdenum trioxide (Mo
O ₃ ), boron trioxide (B ₂ O ₃ ) and tungsten trioxide (WO ₃ ) may be used.

Then, the substrate 51 shown in FIG. 9 (g) was placed in an electric furnace, passed through a bubbler of warm water at about 80 ° C., placed in an atmosphere of Ar containing water vapor, and heat-treated at about 1150 ° C. for about 10 minutes. At the time of cooling, the atmosphere was changed to O ₂ containing water vapor. When the domains of the optical waveguide layer 52 (single crystal thin film) and the substrate 51 were etched and observed with the above-mentioned etching solution, the polarization directions of the polarization portions 54 and 55 were irrespective of the period Λ. And its interface was almost perpendicular to the surface of the substrate 51.

By the way, 50 mol% of lithium carbonate Li
110 from a melt consisting of ₂ CO ₃ , 25 mol% niobium pentoxide Nb ₂ O ₅ , 25 mol% vanadium pentoxide V ₂ O _5.
When the LiNbO ₃ thin film was grown at a temperature of 0 ° C. and the domain of the optical waveguide layer 52 (single crystal thin film) and the substrate 51 was observed with the above etching solution without heat treatment, the polarization part 54 was observed regardless of the period Λ. The same direction 55 is the same as the polarization direction of the corresponding substrate 51, and its interface is substantially perpendicular to the surface of the substrate 51. The reason why the domain of the substrate could be transferred to the thin film without performing heat treatment was that the LiNbO ₃ thin film grew in the paraelectric phase, and when passing through the Curie point during cooling to room temperature after growth. This is because the polarization is transferred to. Further, after the substrate 51 was annealed in an ozone atmosphere to compensate the oxygen deficiency of the optical waveguide layer 52, the amount of niobium in the optical waveguide layer 52 (LiNbO ₃ thin film) was investigated by using EPMA. The same amount as that of the congruent LiNbO ₃ substrate.

Finally, a channel portion is prepared for a sample that has not been evaluated by etching or EPMA. First, the photoresist 82 is patterned as shown in FIG. 6I using a photomask having a channel portion as a light shielding portion, and then the optical waveguide layer 52 is etched by 2 μm by ion milling using this photoresist as a mask. The channel width is 3 μm. Then, the photoresist 82 is removed to obtain FIG. 6 (j).

The ion milling apparatus has a structure in which a plurality of permanent magnets are arranged on the outer periphery of a hollow vacuum chamber having a conical plasma chamber, and the ions generated in the plasma generating chamber are accelerating electrodes, decelerating electrodes, and ground. Since the structure is extracted by three electrodes, the spatial density distribution of ions is uniform, the directivity is extremely high, and etching can be performed with extremely high accuracy.

When Ti—S laser light having a wavelength λ = 830 nm was polarized and incident on the substrate surface through the prism coupler in a direction perpendicular to the substrate surface, an electric field was generated in the direction perpendicular to the substrate surface. A single TM mode having the main component of is excited and its effective refractive index N (ω) =
It was 2.1686. Further, when the same measurement was performed with a dye laser beam of λ = 415 nm, two modes were excited, and the effective refractive index of the low-order mode was N (2ω) = 2.30.
It was 16.

The optical propagation loss for light of 830 nm was measured by the cutback method and found to be 1 dB / cm.
That is a good value. The first reason for this is that a high-quality thin film could be grown by liquid phase epitaxial growth, and the second reason was that the sidewall of the channel part could be processed with extremely high precision by etching using an ion milling device with high directivity. Is based on.

When the polarization inversion period Λ in the case of M = 1 is calculated from Equation 11, it is about 3.1 μm, so Λ = 3.1 μ
A sample of m was cut with an optical waveguide length of 10 mm and a vertical length of 5 mm, and the vertical side was polished to perform a second harmonic generation experiment.
In the above experiment, the Ti-S laser light 11 was focused on the end face of the channel portion by the objective lens 12, the sample was placed on the copper block to which the Peltier element was connected, and its temperature was monitored by the thermocouple. The wavelength of the laser light source was set so that the generation efficiency of the second harmonic was maximized.

As a result, 4 mW at a fundamental wave input of 40 mW
The second harmonic output of was obtained, and the efficiency considering Fresnel reflection loss was 11.8%. This value is about 1/3 of the theoretical value of 40% calculated from the equation (18), but it is a sufficiently high value as compared with the conventional example. Further, estimating from the above results, when a high power semiconductor laser with an output of 200 mW is coupled to the optical waveguide with a coupling efficiency of 50%, the generation efficiency of the second harmonic becomes about 30%, and the second harmonic output of 30 mW is obtained. It can be sufficiently used as a light source for writing and reproducing on a magneto-optical disc or a phase change optical disc.

Example 2 The substrate 5 in Example 1
The optical waveguide layer 52 was formed on the + c plane of 1. However, -c
It can also be formed on the surface. Further, the domain-inverted portion on the substrate 51 can be formed by electron beam irradiation, and similarly good performance can be obtained. In this embodiment, the −c of the zcutLiNbO ₃ single crystal substrate 51 having a stoichiometric composition is used.
An optical waveguide layer 52 of a LiNbO ₃ single crystal thin film containing less lithium than the congruent composition in which spontaneous polarization is normally downward is formed on the surface. Therefore, the lower surface of the substrate 51 is +
It becomes c-plane.

FIG. 7 is a manufacturing process drawing of the second harmonic generating element. First, the domain-inverted portion 65 is formed on the surface of the substrate 51. Both sides of the substrate 51 have a wavelength λ of 1 with respect to the laser light.
After polishing to about / 10, ultrasonic cleaning in acetone, isopropyl alcohol, and pure water, followed by drying, and as shown in FIG. 7 (b), a Cr film 91 is formed on the + c surface (lower surface) to a thickness of 2000 Å by sputtering. To film.

Then, the Cr film 91 is grounded at room temperature, and an electron beam drawing apparatus for a semiconductor manufacturing process is used to obtain an acceleration voltage of 25 kV and an electron flow velocity of 2 × 10 ⁹ electrons / sec.
The surface is irradiated with an electron beam in the shape of a lattice pattern, as shown in FIG.
As shown in Fig. 11, 11 kinds of domain inversion domains 65 from 2.5 to 3.5 µm were formed on the surface of -c by changing the pattern period Λ by 0.1 µm. Collection (Autumn 1991) 11p-M-8 (p953)).

Next, the Cr film 91 is removed by etching with a ceric ammonium nitrate solution as shown in FIG.
A single-domain LiNbO ₃ single crystal thin film was grown on the −c plane of the substrate by the liquid phase epitaxial crystal growth method as follows. The melt during the epitaxial growth was measured by mixing powders of 48 mol% of lithium carbonate Li ₂ CO ₃ , 12 mol% of niobium pentoxide Nb ₂ O ₅ and 40 mol% of vanadium pentoxide V ₂ O ₅ . Then, put it in a platinum crucible at 900 ℃
Melted in an electric furnace in an air atmosphere at 1200 ° C
It is maintained at the temperature of 10 hours for uniform production.

Next, this melt was cooled to 940 ° C. at a cooling rate of 30 ° C./h, and the substrate 51 was brought into contact with this for 3 minutes to grow a 2.2 μm LiNbO ₃ thin film, and the melt was separated, and electricity was applied. The optical waveguide layer 66 was formed by gradually cooling to a room temperature in the furnace at a cooling rate of 30 ° C./h. Note that the number of prepared samples is 10. Further, when the domain in the optical waveguide layer 66 is etched with an etching solution of nitric acid: hydrofluoric acid = 1: 2 and observed from the difference in etching state, the polarization direction is a domain inversion region regardless of the pattern period Λ. It was found that the same monopolar film as the substrate 51 before the formation of 65 was formed.

In addition to vanadium pentoxide (V ₂ O ₅ ) as the flux material of the melt, boron trioxide (B ₂ O ₃ ), lithium fluoride (LiF), potassium fluoride (KF), Boron trioxide (B ₂ O ₃ ) and molybdenum trioxide (MoO ₃ ), boron trioxide (B ₂ O ₃ ) and 3
Tungsten oxide (WO ₃ ) or the like may be used. Next, the substrate 51 was put in an electric furnace, passed through a bubbler of warm water at about 80 ° C., heat-treated at about 1070 ° C. for about 10 minutes in an atmosphere of Ar containing steam, and cooled in a steam atmosphere containing O ₂ . .

When the domain of this substrate was etched by the above etching solution and observed, a polarization inversion region 67 whose polarization direction was the same as that of the substrate 51 was formed regardless of the pattern period Λ, and each polarization inversion region 67 was formed. It was found that the interface of was almost perpendicular to the substrate interface. By the way, 46 mol% lithium carbonate Li ₂ CO ₃ , 24 mol% niobium pentoxide Nb ₂ O ₅ , 3
A LiNbO ₃ thin film was grown at a temperature of 1100 ° C. from a melt composed of 0 mol% vanadium pentoxide V ₂ O _5, and the optical waveguide layer 52 (single crystal thin film) and the substrate 51 were subjected to no heat treatment.
When the domain of is observed with the above etching solution, the period Λ
Regardless of this, the directions of the polarization parts 54 and 55 are the same as the polarization direction of the corresponding substrate 51, and the interface thereof is substantially perpendicular to the surface of the substrate 51. The reason why the domain of the substrate could be transferred to the thin film without performing heat treatment was that the LiNbO ₃ thin film grew in the paraelectric phase, and when passing through the Curie point during cooling to room temperature after growth. This is because the polarization is transferred to. Then, this substrate is annealed in an ozone atmosphere to compensate the oxygen deficiency of the optical waveguide layer 66,
When the amount of niobium in the optical waveguide layer (LiNbO ₃ thin film) 66 was investigated using EPMA, a commercially available congruent LiN
It was about 1.05 times that of the bO ₃ substrate. Finally, the optical waveguide layer 6
An optical waveguide is etched in 6 and a sample not evaluated by EPMA is prepared.

First, as shown in FIG. 6F, the photoresist is patterned using a photomask that shields the optical waveguide, and the optical waveguide layer 66 having a width of 3 μm is etched by 1.8 μm by ion milling using the photoresist as a mask. The photoresist was removed and an optical waveguide as shown in FIG.

In the optical waveguide manufactured as described above, λ =
When 830 nm Ti-S laser light was polarized in a direction perpendicular to the substrate surface and was incident through a prism coupler,
One T that has the main component of the electric field in the direction perpendicular to the surface of the substrate
The M mode is excited and its effective refractive index N (ω) = 2.1
It was 688. In addition, 830n by the cutback method
When the optical propagation loss for m light is measured, it is 1 dB /
A good value of cm was obtained. Further, when the same measurement was carried out by injecting a dye laser beam of λ = 415 nm, two modes were excited and the effective refractive index of the low order mode was N.
(2ω) = 2.3018.

The polarization inversion period Λ obtained from the equation (11) is M
In case of = 1, it is about 3.1 μm. Therefore, the sample of Λ is cut into the optical waveguide length = 10 mm, the length in the vertical direction is cut to 5 mm, and the side of 5 mm is polished to perform the second harmonic generation experiment. went. The sample is placed on a copper block connected to a Peltier device, the temperature is monitored with a thermocouple, the temperature is set to 25 ° C., the Ti-S laser light is focused on the end face of the optical waveguide by the objective lens, and the second harmonic The generation efficiency is maximized.
As a result, a second harmonic output of 3.8 mW was obtained with a fundamental wave input of 40 mW.

[Reference Formula] In FIG. 1D, TM wave polarized in a direction (z direction) perpendicular to the substrate surface is used as incident light,
Assuming that the propagation direction is the x direction, the z component of the incident optical electric field is represented by Expression (1).

[Equation 1] Similarly, assuming that the second harmonic wave in the optical waveguide is also a TM wave polarized in the direction perpendicular to the substrate, the z component of the electric field is expressed by equation (2).

[Equation 2]

The non-linear polarization for generating the second harmonic is expressed by the equation (3).

[Equation 3] On the other hand, Maxwell's equation that takes nonlinear polarization into consideration is represented by equation (4).

[Equation 4]

Substituting equation (3) into equation (4) and using the wave equation of equation (5) and the approximation of equation (6), equation (7)
Is obtained.

[Equation 5]

[Equation 6]

[Equation 7] By multiplying Equation (7) by Equation (8) and integrating from Z = −∞ to Z = ∞, Equation (9) governing the power transfer from the fundamental wave to the second harmonic is obtained.

[Equation 8]

[Equation 9]

In FIG. 1, since the nonlinear optical coefficient d ₃₃₃ of each polarization region in the optical waveguide changes its direction periodically, the expression (1
It can be expanded to Fourier series like 0).

[Equation 10] The period Λ is expressed by equation (1) so as to satisfy the phase matching condition.
Determine as in 1).

[Equation 11] As a result, the phase matching of the fundamental wave and the second harmonic is satisfactorily performed, and the equation (9) becomes the equation (12).

[Equation 12]

Equation (12) is integrated from x = 0 to x = 1 (however, it is assumed that the power of the fundamental wave is almost unchanged and constant), and the amplitude of the second harmonic wave is zero when x = 0. When the boundary condition is used, the conversion efficiency η from the fundamental wave to the second harmonic becomes as shown in Expression (13).

[Equation 13]

[0058]

According to the present invention, an optical waveguide having a refractive index higher than that of the substrate and a uniform refractive index can be formed on the substrate to reduce light scattering in the optical waveguide. Since a rectangular spontaneous polarization inversion region can be periodically formed in the direction of the phase matching property of the second harmonic, the excellent second harmonic with high conversion efficiency of the second harmonic and low wavefront aberration can be obtained. It is possible to provide a generating element and a manufacturing method thereof.

[Brief description of drawings]

FIG. 1 is various sectional views of a second harmonic generation element according to the present invention.

FIG. 2 is a perspective view of a second harmonic generation element using a conventional temperature phase matching method.

FIG. 3 is a perspective view of a second harmonic generation element using conventional Cherenkov radiation.

FIG. 4 is a perspective view of a conventional second harmonic generation element using polarization inversion.

FIG. 5 is a configuration diagram of a light source using the second harmonic generation element of the present invention.

FIG. 6 is a manufacturing process diagram of a second harmonic generation element of the present invention.

FIG. 7 is another manufacturing process diagram of the second harmonic generation element of the present invention.

[Explanation of symbols]

11 ... Laser, 21, 51 ... Substrate, 52, 66 ... Optical waveguide layer, 55, 67 ... Polarization inversion region, 56, 65 ... Polarization inversion part, 81 ... Ti film, 82 ... Photoresist, 91 ... Cr
layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazutomi Kawamoto 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa, Ltd.Institute of Industrial Science and Technology, Hitachi, Ltd. Formula company Magnetic Materials Research Laboratory (72) Inventor Hisao Kurosawa 5200 Sankejiri, Kumagaya, Saitama Prefecture Hitachi Metals Co., Ltd.Formula Magnetic Materials Research Laboratory (72) Inventor Masazumi Sato 5200 Mikageri, Kumagaya, Saitama Hitachi Metals Co., Ltd. In the laboratory

Claims (8)

[Claims] 1. A second harmonic generation device comprising a ferroelectric optical substrate and a ferroelectric optical waveguide layer having a refractive index higher than that of the substrate and periodically arranging domain inversion regions. A waveguide type second harmonic generation element, wherein the − point is lower than the Curie point of the substrate. 2. The polarization reversal part for reversing the polarization direction is periodically provided on the surface of the substrate on the optical waveguide layer side, and the polarization reversal region of the optical waveguide layer is provided on the polarization reversal part. A waveguide-type second harmonic generation element characterized by being provided. 3. The substrate according to claim 1 or 2, wherein the substrate and the optical waveguide layer are made of lithium niobate, and the optical waveguide layer has a composition ratio in which the ratio of lithium to niobium is lower than that of the substrate. And a waveguide type second harmonic generation element. 4. The waveguide type second harmonic wave generating element according to claim 1, wherein the substrate is made of magnesium-doped lithium niobate, and the optical waveguide layer is made of lithium niobate. 5. The composition according to claim 1, wherein the substrate and the optical waveguide layer are made of magnesium-doped lithium niobate, and the optical waveguide layer has a composition ratio in which the magnesium doping ratio is lower than that of the substrate. Characteristic Waveguide type second harmonic generation element. 6. A method of manufacturing a second harmonic generation device comprising a ferroelectric optical substrate and a ferroelectric optical waveguide layer having a refractive index higher than that of the substrate and periodically arranging domain inversion regions, A step of periodically providing a polarization inversion portion and preparing a melt obtained by heating and melting the optical waveguide layer on the raw material powder of the ferroelectric metal oxide in the presence of flux, and crystallizing the temperature of the melt. It is formed by a step of lowering the temperature and contacting the polarization-inverted surface of the substrate to grow a metal oxide film by liquid phase epitaxial growth, and then performing a heat treatment at a temperature near the Curie point of the metal oxide film. A method of manufacturing a waveguide type second harmonic wave generating element, characterized in that. 7. A method of manufacturing a second harmonic generation element comprising a ferroelectric optical substrate and a ferroelectric optical waveguide layer having a refractive index higher than that of the substrate and periodically arranged domain inversion regions, A step of periodically providing a polarization inversion portion and preparing a melt obtained by heating and melting the optical waveguide layer on the raw material powder of the ferroelectric metal oxide in the presence of flux, and setting the temperature of the melt to Waveguide-type second harmonic generation, characterized in that it is formed by a step of lowering the crystal precipitation temperature above the point and contacting the polarization-inverted surface of the substrate to perform liquid phase epitaxial growth of the metal oxide film. Device manufacturing method. 8. The method according to claim 6, wherein the flux is vanadium pentoxide (V ₂ O ₅ ), boron trioxide (B ₂ O ₃ ), lithium fluoride (LiF), or potassium fluoride (KF). ), Or boron trioxide (B ₂
O ₃ ) and molybdenum trioxide (MoO ₃ ), or a mixture of boron trioxide (B ₂ O ₃ ) and tungsten trioxide (WO ₃ ), which is a waveguide type second harmonic generation device. Production method.