JPH0656595A - Production of optical fiber made of barium titanate single crystal - Google Patents

Abstract

PURPOSE:To provide the process for production of a BaTiO3 single crystal of a tetragonal crystal system which can produce the fiber made of the BaTiO3 single crystal of the tetragonal crystal system by preventing the intrusion of impurities therein. CONSTITUTION:A raw material wire 1 having a compsn. composed of Ba-Ti-O is locally melted to obtain a molten part 2 having >=1.01 molar ratio of TiO2/BaO. The fiber of the barium titanate single crystal is then pulled up from this molten part 2 by using a seed crystal 3 consisting of the BaTiO3 single crystal of the tetragonal crystal system and is solidified. The cross-sectional area of the optical fiber of the barium titanate single crystal is <=1mm<2> in such a case. The raw material wire 1 is supplied to the molten part 2 at a speed at which the pulling up and solidifying amt. in the molten part 2 is replenished.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a tetragonal barium titanate optical single crystal fiber having a photorefractive property (photorefractive effect) and suitable for application to an optical computer or the like.

[0002]

2. Description of the Related Art Tetragonal BaTiO ₃ single crystals have a large photorefractive effect, and in recent years,
Applications to various fields such as optical computing have become popular. In these optical fields, fiber-shaped BaTiO ₃ single crystal fine wire has recently been applied [K. Kitayama, F. Ito, K. Tomomatsu; Optical.
fiber Communication Conference Technical Digest W
L8, Page 161 (1992)]. However, tetragonal BaTiO 3 having a practically sufficient photorefractive effect
_The current situation is that ₃ single crystal fibers have not yet been manufactured.

In order to obtain a good quality and large BaTiO ₃ single crystal, the TSSG method (Top Seeded Solution Growth) method [A.
Linz, V. Belruss and CS Nailman; J. Electro. C
hem.Soc. 60C (1965) 112] is generally used. The square BaTiO ₃ single crystal grown and processed by the TSSG method can also be cut into a wire rod [F. Ito, K. Kitayama and O. Nakao; App.
l. Phys. Lett. 60 (1992) 793]. However, this manufacturing method has a problem that it is difficult to manufacture a fibrous thin wire having a cross-sectional area of 1 mm ² or less.

Turner et al. Also reported that TSFZ (Traveling Solve)
nt ₂ is used as a solvent (Solven
t), a tetragonal BaTiO ₃ single crystal rod has been successfully prepared [CE Turner, NH Maso
n and AW Morris; J. Cryst. Growth 56 (1982) 13
7]. However, even with this method, the cross-sectional area is 1 mm.
It is difficult to fabricate fine fibrous wire rods of ² or less. Furthermore, Turner et al. Uses a heater with a Pt-Rh wire as a resistance wire to form a melting zone, and since this heater is in contact with the melt, the resulting BaTiO ₃ single crystal rod is Rh. There is also a problem that it is easily polluted by Pt.

Regarding the production of a fibrous thin wire having a cross-sectional area of 1 mm ² or less, which is made of a tetragonal BaTiO ₃ single crystal,
Saifi et al. Have succeeded by using the LHPG (Laser Heated Pedestal Growth) method [M. Saifi, B. Dubois,
EM Vogel and FA Thiel, J. Mater. Res. 1 (1986)
452]. However, in the production method of Saifi et al., Since SrTiO ₃ is used for the seed crystal, the Ba
The TiO ₃ single crystal fiber has a problem that contamination by Sr cannot be avoided.

The reason why contamination by a specific impurity element such as Sr becomes a problem will be described below. For bulk BaTiO ₃ single crystals, doping of transition metal elements [D. Rytz, BA
Wechsler, MH Garret, CC Nelson and RN Sc
hwartz; J. Opt. Soc. Am. B7 (1990) 2245] or by growing in an inert atmosphere (Japanese Patent Application No. 3-289076).
It has been reported that crystal defects such as oxygen vacancies are introduced into the iO ₃ single crystal, and the photorefractive effect thereof is increased. Therefore, in a BaTiO ₃ single crystal fiber having a low degree of contamination with a specific impurity element, it is possible to design the photorefractive effect to a desired size by applying these techniques.

On the contrary, when the degree of contamination by a specific impurity element is high, it is impossible to apply these techniques to design the photorefractive effect to a desired size. From the above, at present, it is desired to produce a tetragonal BaTiO ₃ single crystal which is not contaminated with a specific impurity element.

The present invention was made in view of the above problems, production of tetragonal BaTiO ₃ single crystal of BaTiO ₃ single crystal fiber tetragonal can be produced by preventing entry of impurities The purpose is to provide a method.

[0009]

Means for Solving the Problems Tetragonal system B according to the present invention
The manufacturing method of the aTiO ₃ single crystal fiber is Ba-Ti-
The raw material wire rod of O composition is locally melted and TiO ₂ / Ba
A step of obtaining a molten part having a molar ratio of O of 1.01 or more,
A step of pulling up and solidifying the barium titanate single crystal fiber by using a seed crystal from the melting part, and supplying the raw material wire rod to the melting part at a rate of supplementing the amount of pulling and solidifying in the melting part .

[0010]

In the present invention, the raw material wire rod having a Ba—Ti—O composition is locally melted by condensing laser light or infrared light onto the raw material wire rod, and a seed crystal is used from the upper end of the melted portion. And pulling the single crystal to solidify
An aTiO ₃ optical single crystal is obtained. As described above, since the crystal is grown from the minute molten portion obtained by locally heating the raw material wire rod, it is possible to obtain a fiber-shaped thin wire rod having a small diameter. Further, since the raw material wire material of the amount for replenishing the amount of pull-up and solidification is supplied to the melting portion, the amount of melt in the melting portion is always kept constant, and the fiber thin wire material can be continuously manufactured.

In the present invention, laser light and infrared light are used as the heating means for forming the melted portion because the heat source and the melt are not in contact with each other to contaminate the melted portion. This is to prevent

Further, in the method of the present invention, the composition of the melted portion is such that the molar ratio of TiO ₂ / BaO is 1.01 or more (TiO ₂
Is more than 50 mol%). Figure 2
Shows an equilibrium phase diagram of BaO-TiO ₂ system (reported by Rase et al.). BaTiO ₃ is orthorhombic at 5 to 10 ° C. or lower, tetragonal at 130 ° C. or lower, and cubic at 1460 ° C. or lower,
It has a hexagonal crystal structure at 1460 ° C or higher. And B
aTiO ₃ exhibits an excellent photorefractive effect only in the tetragonal system.

In this case, as can be seen from FIG.
When a single crystal is crystallized from a melt having a BaO / TiO ₂ molar ratio of 1.0 (TiO ₂ is 50 mol%), a hexagonal crystal is crystallized [DE Rase amd
R. Roy: J. Am. Ceram. Soc. 38 (1955) 110]. This hexagonal crystal does not undergo crystal transformation even at 1460 ° C., and a tetragonal single crystal cannot be obtained even when the temperature is lowered to room temperature.

The variation between cubic and tetragonal is reversible, and BaTiO ₃ which is cubic above the Curie point temperature around 130 ° C. becomes tetragonal below the Curie point temperature. Therefore, the BaTiO ₃ single crystal for optical use needs to be a tetragonal crystal, and in order to produce this tetragonal single crystal, a hexagonal single crystal is not crystallized but a cubic single crystal is produced. It is necessary to crystallize the crystals directly from the melt.

The bulk BaTiO ₃ single crystal is mainly produced by the TSSG method described below. TSSG
Like the CZ (Chochralski) method, this method is one of the crystal growth methods by pulling up, and a desired single crystal cannot be grown from a congruent composition melt (Congruent Melt) having the same composition as that of the desired single crystal, and If the melting point is high, the seed crystal is immersed in a melt having a composition different from the desired single crystal composition, and the concentration of solute components in the composition of the melt is gradually reduced to form a single crystal having the desired composition in the seed crystal. It is crystallized and grown.

Therefore, the inventors of the present invention have T
Focusing on the fact that the crystallization temperature of the crystal can be controlled and the tetragonal BaTiO ₃ single crystal can be grown by changing the molar ratio of iO ₂ / BaO, the composition of the melted portion in the above-mentioned method is variously changed to obtain a single crystal. An attempt was made to grow a crystal fiber. As a result, it was found that a tetragonal BaTiO ₃ single crystal fiber was crystallized when the composition of the melted portion was 1.01 or more in terms of the molar ratio of TiO ₂ / BaO. Therefore, in the present invention, the molar ratio of TiO ₂ / BaO is 1.01.
Set above.

In the method of the present invention, hexagonal BaTi is used.
In order to prevent crystallization of O ₃ single crystal, it is desirable to use tetragonal BaTiO ₃ single crystal as a seed crystal. In this case, since the seed crystal also has a Ba-Ti-O composition, it is possible to avoid contamination by the impurity element.

Further, in the method of the present invention, the melting portion is formed at the tip of the raw material wire, and its shape is maintained by the surface tension of the melt. Further, the variation in fiber diameter greatly depends on the shape of the fusion zone. Therefore, when trying to produce a fiber with a large wire diameter, it is necessary to enlarge the entire fusion zone, and if the fusion zone is enlarged, it becomes difficult to maintain its shape in a good state, and in the grown fiber Has a large variation in wire diameter. The inventors of the present invention obtained the relationship between the size of the fused part and the variation in the wire diameter of the BaTiO ₃ single crystal by various experiments, and as a result, when the cross-sectional area of the fiber was 1 mm ² or less, the wire diameter of the fiber was The variation could be reduced to a range sufficient for optical applications. Therefore, the cross-sectional area of the single crystal fiber in the present invention is preferably 1 mm ² or less.

[0019]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples.

FIG. 1 shows a growing portion of a single crystal fiber in the method of the embodiment of the present invention. As shown in FIG.
The raw material wire rod 1 is arranged vertically with its tip end facing upward. Then, the tip of the raw material wire rod 1 is irradiated with a laser beam to be heated to locally melt this portion. As a result, the fusion zone 2 is formed at the tip of the raw material wire rod 1.

Thereafter, as shown in FIG. 1 (b), the seed crystal 3 is lowered, and the lower end of the seed crystal 3 is immersed in the molten portion. Next, when the seed crystal 3 is moved upward at a predetermined speed by a pinch roll (not shown), a part of the melt of the melting part 2 is pulled up by the seed crystal 3 and the pulled up portion is solidified. Single crystal grows.

On the other hand, the raw material wire 1 is also raised in association with the pulling of the seed crystal 3. As a result, the raw material wire rod 1 is continuously supplied to the melting portion 2 and is irradiated with laser light to be heated and melted. In this way, the reduced amount of the melt in the melting portion accompanying the pulling of the single crystal is replenished, and the melting portion 2
The liquid surface of is always kept constant. Therefore, the single crystal fiber grows continuously.

Next, according to the above-mentioned method of this embodiment, characteristics of tetragonal BaTiO ₃ produced under the conditions specified in the present invention and conditions deviating from the specified conditions are shown. The result of comparison will be described.

Example 1 In this example, BaCO ₃ powder and TiO ₂ powder were mixed with Ti / B.
The powder mixed so as to have a molar ratio of a = 1.85 was molded and sintered to obtain a raw material wire rod having a wire diameter of 1.5 mm.
Then, a CO ₂ laser beam was focused on the upper end portion of this raw material wire rod to form a fused portion. After that, as a seed crystal,
A 5 mm long tetragonal (room temperature) BaTiO ₃ single crystal having a square cross section with a side length of 0.5 mm was used. The seed crystal was raised to grow a single crystal fiber having a diameter of 0.5 mm.

Example 2 A single crystal fiber was grown in the same manner as in Example 1 except that the raw material wire rod had a wire diameter of 3 mm and the grown fiber had an average wire diameter of 1 mm.

Example 3 A fusion zone was formed by condensing infrared rays emitted from a halogen lamp. Other conditions were the same as in Example 1 to grow fibers.

Example 4 A single crystal fiber was grown in the same manner as in Example 1 except that the composition of the raw material wire rod was Ti / Ba = 1.01.

Comparative Example 1 As a seed crystal, 5 having a square cross section of 0.5 mm on each side.
A single crystal fiber was produced in the same manner as in Example 1 except that a hexagonal BaTiO ₃ single crystal having a length of mm was used.

Comparative Example 2 BaCO ₃ powder and TiO ₂ powder were mixed with Ti / Ba = 1.0.
A powder mixed with a molar ratio of 0 was molded and sintered to obtain a raw material wire rod having a wire diameter of 1 mm, and a single crystal fiber was produced by the same method as in Example 1 except that this raw material wire rod was used. .

Comparative Example 3 A single crystal was grown from a raw material wire having a wire diameter of 5 mm using a tetragonal BaTiO ₃ single crystal having a side length of 2 mm and a length of 5 mm as a seed crystal. Other conditions were the same as in Example 1 to produce a single crystal fiber having an average wire diameter of 2 mmφ.

Regarding the single crystal fibers produced in the examples and the comparative examples, the crystal structure confirmed by powder X-ray diffraction and the minimum and maximum values of the fiber diameter within the length of 50 mm are shown in Table 1 below.

[0032]

[Table 1]

In Examples 1 to 4, tetragonal BaTiO 3 was used.
_Three single crystal fibers were grown, and the variation in the wire diameter was as small as 0.03 to 0.08 mm at a length of 50 mm, and fibers with a sufficiently uniform wire diameter were obtained. On the other hand, in Comparative Example 1 and Comparative Example 2, the fiber diameter was sufficiently uniform as in Examples 1 and 2, but the crystal structure of the obtained fiber was hexagonal. Further, in Comparative Example 3, although the crystal structure was tetragonal, an attempt was made to obtain a fiber having a wire diameter of 2 mm. The fiber diameter after growth was 1.64 mm at the minimum and at the maximum.
There was a large variation of 2.31 mm.

[0034]

As described above, according to the method of the present invention, it is possible to produce a fiber-shaped single crystal,
A tetragonal BaTiO ₃ single crystal fiber having an excellent photorefractive effect is obtained by using a tetragonal BaTiO ₃ single crystal as a seed crystal and setting the composition of the melted portion at a molar ratio of TiO ₂ / BaO of 1.01 or more. It can be reliably manufactured. Further, in this method, BaTiO 3 is used as a seed crystal.
₃ is used, a raw material wire having a composition of Ba-Ti-O is used, and a fused part is formed by a non-contact method by condensing laser light or infrared light. It is possible to prevent impurities from being mixed in, it is easy to control trace impurities that bring about a photorefractive effect, and it is possible to obtain a BaTiO ₃ optical single crystal fiber having an excellent photorefractive effect. Therefore, the present invention makes a great contribution to the field of optical computing and the like.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an embodiment method of the present invention.

FIG. 2 is a phase diagram of a BaO—TiO ₂ system.

[Explanation of symbols]

 1; Raw material wire 2; Melting part 3; Seed crystal

Front page continuation (72) Inventor Akito Kurosaka 1-5-1 Kiba, Koto-ku, Tokyo Fujikura Densen Co., Ltd. (72) Inventor Shoji Ajimura 1-1-5 Kiba, Koto-ku, Tokyo Fujikura Den Line Co., Ltd. (72) Inventor Satoshi Nakao 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Electric Line Co., Ltd. (72) Inventor Kenichi Kitayama 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Japan Telegraph and Telephone Incorporated (72) Inventor Fumihiko Ito 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A step of locally melting a raw material wire rod having a Ba—Ti—O composition to obtain a melted part having a TiO ₂ / BaO molar ratio of 1.01 or more, and using a seed crystal from the melted part. A step of pulling and solidifying the barium titanate single crystal fiber, and a barium titanate optical single crystal fiber characterized in that the raw material wire is supplied to the melting portion at a rate of supplementing the amount of pulling and solidifying in the melting portion. Production method. 2. The method for producing a barium titanate optical single crystal fiber according to claim 1, wherein the seed crystal is a tetragonal BaTiO ₃ single crystal. 3. The method for producing a barium titanate optical single crystal fiber according to claim 1, wherein a cross-sectional area of the barium titanate optical single crystal fiber is 1 mm ² or less.