JPH05105593A - Single crystal of lithium tantalate and optical element - Google Patents

Abstract

PURPOSE:To obtain single crystal of lithium tantalate having excellent light damage resistance characteristics and to improve stability of SHG element and characteristics in rise of output by using the single crystal of lithium tantalate as a substrate for optical element utilizing short wavelength light and by taking advantage of a high nonlinear optical coefficient of the single crystal. CONSTITUTION:The objective single crystal of lithium tantalate has improved light damage resistance characteristics by adding <=10 atom % element of group IIB, IIIA and IIIB of the periodic table, and a strong light element using the single crystal is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single crystal used in the field of information processing using laser light or in the field of optical measurement control and communication, and particularly to a lithium tantalate single crystal excellent in light damage resistance. And an optical element using the same.

[0002]

2. Description of the Related Art A single crystal of lithium tantalate has a melting point of about 16
It is a ferroelectric crystal having a Curie temperature of 50 ° C. and a Curie temperature of about 600 ° C., and is usually grown from the melt by the Czochralski method using an iridium crucible in a reducing atmosphere or a reducing atmosphere containing oxygen. Since the grown single crystal is in a multi-domain state, it is subjected to a single domainization treatment by an electric field applied gradual cooling method in the air or an oxygen atmosphere while keeping the crystal temperature at the Curie temperature or higher. After that, the crystal is processed into a wafer and used in large quantities as a substrate for a surface acoustic wave device. Lithium tantalate crystals are relatively inexpensive and can grow large diameter crystals, and have relatively large nonlinear optical constants among inorganic oxide single crystals. It has been conventionally known that the lithium niobate single crystal is superior in light damage resistance, but no quantitative data has been reported. Further, it has been reported that the optical damage resistance of the crystal is improved by an electric field annealing method in which the grown crystal is single-domained near the Curie temperature (Journal of Applied Physics, Vol. 38, page 3109 (1967). )). Since it was recently reported that a low loss optical waveguide can be formed by the proton exchange method, tantalum has been used as a substrate material for a wavelength conversion element that inserts a fundamental wave into the optical waveguide and generates a second harmonic by the quasi-phase matching method. Lithium oxide single crystals have received attention.

[0003]

It has been said that the light damage resistance of the lithium tantalate single crystal produced by the above-mentioned conventional technique is higher than that of the lithium niobate single crystal, but this is not always the case and the wavelength conversion. When the device is used for optical applications, optical damage occurs, which is a serious problem that hinders its practical use. The optical damage referred to here is a phenomenon in which the refractive index of the crystal locally changes due to the incidence of laser light, and is called light-induced refractive index change. The cause of this optical damage is said to be due to impurities of the transition metal contained in the crystal, and the phenomenon is explained especially by the change in the valence of Fe ions in the crystal. That is, when light is incident in a direction that is not parallel to the Z axis (optical axis) of the crystal, Fe ²⁺ existing in the portion where the light intensity is strong in the light irradiation portion
Ions are excited to emit electrons and change to Fe ³⁺ . The electrons thus generated are generally trapped by other Fe ³⁺ in the non-irradiated part of the crystal or in the weakly irradiated region, and such ions are converted into Fe ²⁺ .
The overall effect due to such a phenomenon appears as a change in the distribution of Fe ²⁺ ions, and as a result, it appears as a non-uniformity in the local refractive index distribution due to the electro-optic effect of the crystal itself. When a crystal is used as a substrate for optical applications such as an optical modulator or a wavelength conversion element, the element does not operate stably due to such a change in the refractive index of the light irradiation part, and the characteristics originally possessed by the crystal are fully utilized. There is a very big problem of being unable to finish. 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. It is said that this photodamage occurs remarkably in the lithium niobate single crystal, and the light damage resistance strength of the lithium tantalate single crystal is stronger than that of the lithium niobate single crystal. So far, lithium tantalate single crystals have been used for surface acoustic wave device applications, and in these applications, a lot of subgrain boundaries and transition metal impurities contained in the crystal are included Did not pose a big problem. However, in optical applications, there are problems such as unstable operation of element characteristics due to occurrence of optical damage and light scattering at the subgrain boundary. There is also a problem that the light damage resistance of lithium tantalate is almost the same as the light damage resistance of the lithium niobate single crystal. Further, although the light damage resistance is improved by reducing the amount of transition metal impurities such as iron and the oxidation treatment by heat treatment of the grown crystal, there is a problem that it is insufficient for optical element application. As described above, it is said that the main cause of the occurrence of optical damage is the concentration of the selective metal impurities contained in the crystal, especially the Fe impurity concentration. Therefore, if it is reduced to, for example, 1 ppm or less, the optical damage resistance is certainly improved. Although effective, it is difficult to remove completely. The reason is that in the growth of oxide single crystals, the purity of the raw material that can be purchased is 4N to 5N.
Since it is not possible to purify it to the same level as semiconductors because impurities are taken into the grown crystal from the crucible material and the refractory heat insulating material in the furnace, there is a limit to the reduction of impurities. is there. Further, even by the electric field annealing method which has been conventionally said to be effective in improving the light damage resistance strength, optical damage occurs when the power density of the element used is increased, which is not sufficient. For this reason, there has been a problem that a crystal that sufficiently satisfies the light damage resistance strength cannot be obtained in the device application using short wavelength light. The present invention has been made to solve the problem of optical damage of the conventional lithium tantalate single crystal as described above, and by adding the elements 2b, 3a, and 3b, lithium tantalate excellent in the optical damage resistance is provided. SHG element that provides a single crystal and generates the second harmonic by passing the light emitted from the laser light source to the non-linear optical crystal as the fundamental wave, or the light emitted from the laser light source is incident on the electro-optical crystal to generate electricity. This single crystal is used as a substrate for an optical modulation element that controls the intensity and phase of light by the optical effect, and the optical element is manufactured and operated stably.

[0004]

In order to achieve the above object, the present inventor has added lithium tantalate containing 2b, 3a and 3b elements in order to improve the light damage resistance of lithium tantalate single crystal. The means of growing a single crystal was adopted. The reason why the light damage resistance of the lithium tantalate single crystal according to the present invention is significantly improved is not clear yet, but the following reasons can be considered. That is, when elements 2b, 3a, 3b are added to a lithium tantalate single crystal, these elements form shallow impurity levels in the crystal. When light enters the crystal, electrons are excited from these impurity levels, and the photoconductivity of the crystal increases, which is the cause of improving the light damage resistance strength. Fe ² excited by light irradiation into the crystal an internal electric field created by the holes and Fe ³⁺ of electrons ⁺ is, be shielded or weakened by a large photoconductivity believed that the main cause. Further, the present inventor has processed the obtained lithium tantalate single crystal into a wafer or a block in order to improve various characteristics of an element which has been unstable in operation due to optical damage, and used as a substrate for various optical elements. An optical device was prepared by using. With the above structure, the light damage resistance of the crystal can be significantly improved,
Furthermore, since the obtained crystal has improved light damage resistance and homogeneity, it is possible to stably operate various optical elements such as wavelength conversion elements, optical modulators, and optical deflectors that use short wavelength light. Is. The addition amount of the elements 2b, 3a and 3b is preferably 10% or less in terms of atomic percentage. In the present invention, the addition of the elements 2b, 3a and 3b is effective in improving the light damage resistance, but when 10 at% or more is added, the light damage resistance is rather lowered, probably because the crystal quality is deteriorated. .. Further, in the present invention, the above 2b, 3
The combined addition of two or more kinds of elements a and 3b can be expected to have a synergistic effect by paying attention to the difference in the distribution coefficient of each element and devising the mixing ratio. In carrying out the present invention, the means for growing a single crystal is not limited, and it is generally the Czochralski method, and in some cases, the Bridgman method, the floating zone method or the fiber pedestal method can also be used for the growth. is there. In the present invention,
A double crucible containing a melt, a heating means provided on the outer periphery of the double crucible, and a means for obtaining a single crystal by pulling the seed crystal by bringing a seed crystal into contact with the melt in the double crucible. Using the lithium tantalate single crystal manufacturing apparatus as described above, it becomes easy to grow a homogeneous and large single crystal.
The compounding ratio of Li ₂ CO ₃ and Ta ₂ 0 ₅ as a raw material is generally congruent composition, is desirable when viewed from the plane of the single crystal growth to easily high-quality single crystals were obtained, the single crystal by the element application A different substrate refractive index may be needed. In such a case, Li ₂ CO ₃ and Ta ₂
A desired single crystal substrate can be obtained by changing the compounding ratio of O ₅ . It is desirable to add the element at the time of mixing in order to make the raw material uniform, but it may be added to the raw material melt.

[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,
In a crucible made of iridium having a diameter of 100 mm and a depth of 120 mm, 5 kg of LiTaO ₃ raw material powder (the raw material used for the growth was 4 N purity of Li ₂ O and Ta ₂ O ₅ ) was added to ₂ b (purity of ₃ N to 4 N Zn, Cd, Hg powder), 3a (purity 3
N-4N Sc, Y, lanthanoid powder), 3b (pure 3N-4N B, Al, Ga, In, Tl powder) elements are added and dissolved to form a melt, and then seeding is performed. A 2-inch single crystal was grown in a predetermined orientation for about 3 days by the Czochralski method. The growth rate at this time is 2
-4 mm / h, the rotation speed is 10-30 rpm. Next, the crystal grown by the above method was subjected to a single domainization treatment. For example, a Pt electrode plate is provided so as to face the crystal in the Z-axis direction through a conductive powder that is non-reactive with the crystal, and the crystal is inserted into an electric furnace to perform a single domainization process. Then, from each crystal, each ridge has an x-axis orientation, a y-axis orientation,
And a square block of 10 × 10 × 10 mm ³ parallel to the z-axis direction was cut out and each surface thereof was mirror-polished. Alternatively, a 2-inch wafer was prepared from each crystal. The content of each additive element in these wafers was 5 at%. Thus, a lithium tantalate single crystal was prepared and examined for improvement in light damage resistance. The light damage resistance strength was measured by two kinds of measuring methods. One measurement method was to input an argon laser having a wavelength of 0.488 μm into a crystal or an optical waveguide and evaluate the input / output characteristics. If the optical damage does not occur, the input / output characteristics are on a straight line, but if the optical damage occurs, the output beam after passing through the crystal is deformed from the spot shape, and therefore the optical damage can be detected.
Using these various crystals, the laser power at which the shape of the emitted light beam began to be deformed was determined as the optical damage threshold of the crystal, and the results are shown in Table 1.

[Table 1] By irradiation with an argon laser, the conventional undoped lithium tantalate single crystal undergoes optical damage several seconds after irradiation when an argon laser with a power density of 0.1 kW / cm ² is incident, the refractive index changes significantly, and the shape of the emitted beam changes. Deform. On the other hand, in the case of the lithium tantalate single crystal containing the elements 2b, 3a and 3b according to the present invention, the power density is 10 KW.
No photodamage was observed with an argon laser incidence of / cm ² . (Embodiment 2) The present inventors generate a second harmonic by passing a crystal whose light damage resistance is improved by adding 2b, 3a, and 3b elements to a nonlinear optical crystal using light emitted from a laser light source as a fundamental wave. It was confirmed that the SHG output of about 2 mW was obtained when used as the substrate of the SHG element, and the operation was stable without causing optical damage. In the future, SH of higher output will be achieved by optimizing the device structure.
It seems that G light can be obtained. (Embodiment 3) An optical modulator for changing the phase of light emitted from a laser light source into an electro-optic crystal by using a crystal having a light damage resistance improved by adding 2b, 3a and 3b elements of the present invention as a substrate. When a prototype was made, it was confirmed that the operation was stable with no optical damage.

[0006]

According to the present invention, a lithium tantalate single crystal excellent in light damage resistance can be obtained for the first time. As a result, a lithium tantalate single crystal can be used as a substrate for an optical device that uses short-wavelength light, and it is possible to improve the stability and high output characteristics of the SHG device by utilizing the large nonlinear optical constant of the lithium tantalate single crystal. it can.

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

Claims (4)

[Claims] 1. A lithium tantalate single crystal characterized by having improved optical damage resistance by containing one or more elements of Periodic Tables 2b, 3a and 3b. 2. The lithium tantalate single crystal according to claim 1, which contains 10% or less of one or two or more elements of the periodic tables 2b, 3a and 3b in terms of atomic percentage. 3. A non-linear optical element that generates harmonics by passing light emitted from a laser light source as a fundamental wave to a non-linear optical crystal, wherein the non-linear optical crystal is lithium tantalate. A non-linear optical element characterized by using a single crystal. 4. An optical element for controlling the intensity and phase of light by making light emitted from a laser light source incident on an optical crystal by an electro-optic effect or an acousto-optic effect, wherein the optical crystal is the optical element. An optical device using the lithium tantalate single crystal of.