JPH05313033A - Optical waveguide, manufacture thereof and optical element - Google Patents

Abstract

PURPOSE:To provide a lithium niobate single crystal excellent in light-proof damage-proof characteristic, and an optical element using this single crystal. CONSTITUTION:An optical waveguide 5, formed of a lithium niobate single crystal Li2O/(Li2O+Nb2O5) the crystal composition of which contains less lithium component than congruent composition and the mole fraction of which is in a range of being larger than 0.45 and smaller than 0.486, is formed on a base made of a congruent composition lithium niobate single crystal or lithium tantalic acid single monocrystal, by an LPE method using either one kind or more than two kinds of flux among vanadium pentoxide (V2O5), boron trioxide (B2O3), (LiF), molybdenum pentoxide (Mo2O5) and tungsten pentoxide (W2O5), or by a gas phase growth method. This optical waveguide is used for an SGH element or a light modulation element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single crystal material used in the information processing field using laser light or in the optical application measurement control and communication field, and particularly to a lithium niobate single crystal excellent in light damage resistance. The present invention relates to a crystalline optical waveguide, a method for manufacturing the same, and an optical device using the same.

[0002]

2. Description of the Related Art A single crystal of lithium niobate has a melting point of about 125.
A ferroelectric crystal with a temperature of 0 ℃ and a Curie temperature of about 1150 ℃, which is grown from the melt by the Czochralski method using a platinum crucible in the air or an atmosphere containing oxygen. Widely used as a substrate for wave elements. In recent years, lithium niobate crystals have excellent optical quality, are relatively inexpensive, can grow large-diameter crystals, and can easily form low-loss optical waveguides. It is often used as a substrate material and optical waveguide material for various optical elements used. There are mainly two types of lithium niobate single crystal optical waveguides. One is a lithium niobate single crystal substrate with a large refractive index formed by thermal diffusion or ion exchange, and this is used as an optical waveguide. This is a method used, and for example, an optical waveguide is formed by diffusion of Ti or proton exchange on a substrate of a lithium niobate single crystal or a MgO-added lithium niobate single crystal. As another method, a thin film having a large refractive index is formed on a single crystal substrate by LPE.
It is a method of forming an optical waveguide by the method. In this case, since it is necessary that the refractive index of the substrate is smaller than that of the thin film to form the optical waveguide, the refractive index is generally smaller than the crystal of the congruent composition crystal, for example, the stoichiometric composition. Lithium niobate single crystal having an excessive composition of Li component such as lithium niobate single crystal or M
Using a crystal whose refractive index is lowered by adding gO as a substrate,
A lithium niobate single crystal thin film having a congruent composition is formed on a substrate.

[0003]

A major problem in the above-mentioned optical waveguide according to the prior art is that the single crystal of lithium niobate has a small optical damage resistance. When this material is used as a substrate for optical applications such as optical modulators and wavelength conversion elements when the light damage resistance strength is low, the refractive index of the light irradiation part changes and the element does not operate stably. There is a big problem that the existing characteristics cannot be fully utilized. This optical damage becomes more remarkable as the wavelength of light used becomes shorter, and therefore the problem of optical damage becomes more serious in device applications using light of shorter wavelength. The Ti diffusion optical waveguide, which is an optical waveguide formed by the conventional technique, has a problem because Ti deteriorates the light damage resistance strength of the LN single crystal. Also, in the proton exchange optical waveguide, the proton exchange part improves the light damage resistance strength, but there is a problem that the characteristics of the material cannot be used fully because the nonlinear optical constants and electro-optical constants inherent in the crystal are deteriorated by the proton exchange. .. In the conventional method of forming a thin film having a large refractive index on a single crystal substrate as an optical waveguide by the LPE method, a substrate having a small refractive index for forming an optical waveguide is required, and it is more refracted than a crystal of a congruent composition crystal. A substrate having a low refractive index, for example, a lithium niobate single crystal having an excessive Li component, such as a lithium niobate single crystal having a stoichiometry composition, or a crystal having a refractive index lowered by adding MgO is used as a substrate. .. Furthermore, it is said that LN single crystals added with MgO and LN single crystals of stoichiometry composition crystal are strong against optical damage, so that these crystals have a smaller refractive index than these crystals so as to form an optical waveguide. Attempts have also been made to form optical waveguides by using crystals as substrates. However, when MgO is added, the light damage resistance is improved, but when a crystal added with MgO is grown, the segregation coefficient of MgO is larger than 1, so that a uniform distribution of MgO cannot be obtained within the crystal, and the refraction of the LN crystal Since the refractive index has a large dependency on the MgO concentration, it is not possible to obtain a uniform refractive index for each wafer or lot. Also,
In a non-congruent composition crystal, the crystal composition changes in the crystal as it grows, and the refractive index also changes. For this reason,
Substrate materials having a uniform refractive index have not been obtained even for non-congruent composition crystal substrates such as stoichiometry composition crystals. An object of the present invention is to provide an optical waveguide of lithium niobate single crystal excellent in light damage resistance, solve the problems in the conventional lithium niobate single crystal light waveguide and the optical waveguide process as described above, and prevent light damage. An optical waveguide of lithium niobate single crystal excellent in manufacturing, a manufacturing method thereof, and an optical element using the same are stably manufactured and operated.

[0004]

In order to achieve the above object, the present inventor has found that the composition of the optical waveguide is Li ₂ O / (Li ₂ O + Nb) containing less lithium than the congruent composition.
It has been found that a lithium niobate single crystal having a molar fraction of ₂ O ₅ ) of more than 0.45 and less than 0.486 improves the optical damage resistance and further increases the refractive index, and is therefore useful as an optical waveguide. It was The optical waveguide of the above composition is L
It is formed on a substrate of lithium niobate single crystal or lithium tantalate single crystal with a congruent composition by PE method or vapor phase growth method. In the method of manufacturing an optical waveguide, the flux used in the LPE method is vanadium pentoxide (V2
O5), or boron trioxide (B2O3), (LiF), molybdenum pentoxide (Mo2O5), or tungsten pentoxide (W2O5), or any one or more fluxes may be contained to obtain good results. In the SHG element, a highly-efficient quasi-phase matching SHG element is prepared by using a periodically poled lithium niobate single crystal as a crystal substrate and forming an optical waveguide thereon. Furthermore, since the obtained optical waveguide has excellent resistance to light damage, it is possible to stably operate various optical elements such as a wavelength conversion element that uses short-wavelength light and an optical modulator and an optical deflector, in addition to the wavelength conversion element. It is possible.

[0005]

The present invention will be described in more detail based on the following examples.

Example 1 A sample was prepared by the following manufacturing method. First, by the Czochralski method, single crystals of lithium niobate having different compositions were grown. Diameter 100mm depth 1
Put the raw material powder in a platinum crucible of 00 mm and melt it by high frequency heating to make a melt, and then perform seeding. Approximately 3 days in a predetermined direction, diameter 60 mm, length 80 m
m single crystal was grown. The growth rate at this time is 1 to 4 mm
/ H, the rotation speed is 10 to 30 rpm. The crystal grown by the above-mentioned pulling method is, for example, P so as to face in the Z-axis direction of the crystal through a conductive powder which is non-reactive with the crystal.
A t-electrode plate was provided and inserted into an electric furnace for single-domain division processing. Then, from each crystal, each edge is x-axis oriented,
10x10x10mm parallel to y-axis and z-axis
^Three square blocks were cut out and each surface was mirror-polished. The light damage was measured on the above-mentioned polished sample with a wavelength of 0.488 μm.
This was performed by irradiating an Argon laser of No. 1 and measuring the amount of change in refractive index caused by this. The result is shown in FIG. Irradiation with an argon laser causes optical damage to the conventional non-doped lithium niobate single crystal within a few seconds after irradiation, and the refractive index changes significantly. On the other hand, in the lithium niobate single crystal grown by the present invention and having a smaller lithium component than the congruent composition, no optical damage was observed when an argon laser having a power density of 100 W / cm ² was incident. As a result of evaluating the composition dependence of the photodamage property in this way, unlike the previous reports, a crystal having a stoichiometric composition that is a stoichiometric composition without Li deficiency and excess Nb is vulnerable to photodamage and has a crystal composition The light damage resistance strength was improved as the amount of Li was decreased, and the result that the light damage resistance strength was the largest in the crystal having less lithium component than the congruent composition was obtained. In optical device applications, it is necessary to form a crystal thin film having a larger refractive index than the substrate on a single crystal substrate and use it as an optical waveguide. FIG. 2 shows the relationship between the refractive index and the composition of the lithium niobate single crystal. Li ₂ O /
A composition crystal having a molar fraction of (Li ₂ O + Nb ₂ O ₅ ) larger than 0.45 and smaller than 0.486 is excellent in light damage resistance and has a larger refractive index than a lithium niobate single crystal having a congruent composition. Can be formed on these substrates by a vapor deposition method or a vapor phase growth method and used as an optical waveguide.

Example 2 A lithium niobate single crystal wafer substrate having a diameter of 2 inches was placed in a lithium niobate-based flux, and a thin film having a thickness of several to 10 μm was formed while rotating the wafer at a melt temperature of about 900 ° C. Formed. After slowly cooling the sample, the surface was mirror-polished for the purpose of aligning the flux adhering to the thin film surface and the flatness of the surface. The refractive index of the surface is He
When measured with a -Ne laser, it was confirmed that the refractive index increased with respect to the substrate. Next, an argon laser having a wavelength of 0.488 μm and a He—Ne laser having a wavelength of 0.633 μm and a wavelength of 0.
When semiconductor lasers of 83 μm and 1.55 μm were incident, good optical waveguide was confirmed for each wavelength depending on the film thickness. Furthermore, when the light damage intensity was measured,
In the optical waveguide prepared by the present invention, the power density is 1 kW
No photodamage was observed for laser incidence of / cm ² .

Example 3 A lithium niobate single crystal wafer was thoroughly washed, and then Ti was deposited on the lithium niobate single crystal substrate by sputtering to a film thickness of about 30 Å. A photoresist was applied onto the Ti film by a spinner and patterned by photolithography using a photomask having a window opened to form a domain-inverted portion. Further, Ti was patterned using the photoresist as a mask, and then the photoresist was removed. This was inserted into a resistance heating electric furnace and heat-treated at a temperature of about 1100 ° C. for about 10 minutes. The atmosphere was an Ar gas atmosphere containing water vapor. Oxygen gas was passed at the end of the heat treatment process to prevent oxygen deficiency of the crystal. As a result, a domain-inverted part was formed on the c-plane of the substrate. A single crystal thin film was formed by the LPE method.
Lithium carbonate, niobium pentoxide, and vanadium pentoxide were used as raw materials for the melt. After weighing them, they were put into a platinum crucible and homogenized and melted at about 1180 ° C in the atmosphere, and then gradually cooled to about 935 ° C and the flux melted. The liquid was prepared. The lithium niobate single crystal wafer substrate prepared above was put into this lithium niobate-based flux, and a thin film having a thickness of several μm was formed while rotating the wafer at a melt temperature of about 920 ° C. After slowly cooling the sample, the surface was mirror-polished for the purpose of aligning the flux adhering to the thin film surface and the flatness of the surface. When the composition of this lithium niobate thin film was examined, it was found that the Li component was smaller than the congruent composition. Furthermore, when the refractive index of the surface was measured by using a He-Ne laser, it was confirmed that the refractive index increased with respect to the substrate. Further, after the prepared thin film was placed in an electric furnace and heat-treated at about 1140 ° C. for about 10 minutes, the surface was examined with an etching solution of hydrofluoric acid and nitric acid. .. Next, a channel waveguide was formed by photolithography and ion milling. When titanium sapphire laser light having a wavelength of 830 nm was made to enter the optical waveguide thus created, it was confirmed that the optical waveguide was formed and that a good optical waveguide was formed. In addition, the occurrence of photodamage was evaluated, but no photodamage was detected.

(Embodiment 4) The optical waveguide manufactured in the above-mentioned embodiment is used as a substrate of an SHG element which generates a second harmonic by passing the light emitted from the laser light source as a fundamental wave to the nonlinear optical crystal optical waveguide. Using. Titanium sapphire laser light is focused on the end face of the optical waveguide with a polarization inversion period of about 3 μm by the objective lens, and the substrate temperature is 28 ° C by the Peltier device.
Then, the wavelength of the laser light source was changed and the output of the SHG light was monitored to find the wavelength with the maximum power. Fundamental wave input 3
It was confirmed that an SHG output of about 3 mW was obtained at 5 mW, and that the operation was stable without causing optical damage. In the future, by optimizing the element structure and coupling high-power fundamental wave laser light with high efficiency, it is possible to obtain SHG light of higher output and use it as a light source for optical disks.

(Embodiment 5) A prototype of an optical modulator, which uses the lithium niobate single crystal optical waveguide of the present invention to change the phase of light emitted from a laser light source into an electro-optic crystal, was produced. It was confirmed that the operation was stable with no optical damage.

[0006]

According to the present invention, an optical waveguide of lithium niobate single crystal excellent in optical damage resistance can be obtained for the first time. As a result, an optical waveguide of lithium niobate single crystal can be used for an optical element using short-wavelength light, and the stability and high output characteristics of the SHG element are improved by utilizing the large non-linear optical constant of the lithium niobate single crystal. You can

[Brief description of drawings]

FIG. 1 is a diagram showing a measurement result of light damage (refractive index change) induced by argon laser irradiation.

FIG. 2 is a diagram showing the relationship between the refractive index and the composition of a lithium niobate single crystal.

FIG. 3 is a diagram showing an optical element manufactured by using an optical waveguide of lithium niobate.

[Explanation of symbols]

1 Lithium niobate single crystal substrate 2 Fundamental wave 3 SHG wave 4 Polarization inversion part 5 Optical waveguide of lithium niobate with less Li component than congruent composition

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kohei Ito 5200 Mikkajiri, Kumagaya City, Saitama Hitachi Metals Co., Ltd.

Claims (8)

[Claims] 1. Li ₂ O / (Li ₂ O + Nb ₂ ) in which the composition of the optical waveguide contains less lithium than the congruent composition.
A lithium niobate single crystal optical waveguide having excellent resistance to optical damage due to the composition of the lithium niobate single crystal having a molar fraction of O ₅ ) of more than 0.45 and less than 0.486. .. 2. The optical waveguide according to claim 1, which is formed on a substrate of congruent composition lithium niobate single crystal or lithium tantalate single crystal. 3. The method of manufacturing an optical waveguide according to claim 1, wherein the method of forming the optical waveguide according to claim 1 on a substrate is an LPE method. 4. The method of manufacturing an optical waveguide according to claim 1, wherein the method of forming the optical waveguide of claim 1 on a substrate is a vapor phase growth method. 5. The method for manufacturing an optical waveguide according to claim 1, wherein the substrate according to any one of claims 3 to 4 is a lithium niobate single crystal in which polarization is periodically inverted on the substrate surface. 6. The method of manufacturing an optical waveguide according to claim 3, wherein the flux used is vanadium pentoxide (V2O5) or boron trioxide (B).
2O3) or (LiF) or molybdenum pentoxide (Mo2O5) or tungsten pentoxide (W2O)
5. The method for producing an optical waveguide according to claim 1, wherein the flux contains one or more fluxes of any one of 5). 7. An SHG element for generating a second harmonic by passing light emitted from a laser light source as a fundamental wave to an optical waveguide of a nonlinear optical crystal, wherein the optical waveguide of the nonlinear optical crystal is used as an optical waveguide. An SHG device using the optical waveguide according to any one of items. 8. An optical modulator for controlling the intensity and phase of light emitted from a laser light source into an optical waveguide of an electro-optical crystal and controlling the intensity and phase of the light by an electro-optical effect. An optical modulator using the optical waveguide according to any one of items 5.