JPH06308340A - Production of periodic inversion type photorefractive waveguide - Google Patents

Abstract

PURPOSE:To provide the process for production of the periodic inversion type photorefractive waveguide capable of preventing the generation of cracks during working and lowering a defective rate, thereby decreasing the man-hours for mending, executing poling regardless of the sizes and size ratios of crystals and easily producing periodic inversion waveguides at low cost. CONSTITUTION:A BaTiO3 single crystal 10 worked to a prescribed shape is heated to >=130 deg.C in silicone oil 14 and is cooled while a uniform electric field is impressed thereto until the temp. passes the Curie point in a first electrical poling treatment stage. The single crystal is then brought into contact with periodically arranged plural electrodes and alternately inverted electric fields is impressed thereto between the adjacent electrodes at >=80 to <130 deg.C, by which the polarization directions of the single crystal are periodically inverted and the polarization inversion structure based on the electric field pattern is obtd. in a second electrical poling treatment stage. As a result, the periodic inversion type photorefractive waveguide is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a function of amplifying an optical signal by a photo-refractive effect and is also used for image processing such as hologram recording.
The present invention relates to an O ₃ ) optical single crystal, and more particularly to a method for manufacturing a period-inverted photorefractive waveguide having a structure in which the polarization direction is periodically inverted, which is composed of the barium titanate optical single crystal.

[0002]

2. Description of the Related Art In recent years, photorefractive (hereinafter referred to as P
Researches on optical amplification, hologram recording, and the like, which apply the (R) effect, have been actively conducted. Among them, a periodic polarization inversion type waveguide has been devised as a PR waveguide, which has a long effective working length and improves efficiency. In a LiNbO ₃ single crystal, such an inversion structure is obtained at an arbitrary inversion period by selectively diffusing an impurity in a part of the crystal by utilizing the property that polarization is inverted by diffusion of impurities and defects. (For example, Endo et al., 1992 Spring Symposium Proceedings 31a-G-6 and Webjorn et al. IEEE Photo
nics Technology Letters Vol.1, No.10, 1989).

In BaTiO ₃ single crystal as well, it has been reported that a waveguide having a periodically poled structure was produced by applying a spatially inverted electric field during poling (Ito et al. Appl. Phys. LeH. Vol.60, No.7, 19
1992).

Conventionally, such a polarization-inverted waveguide of BaTiO ₃ single crystal, as shown in FIGS. 6 (a) to 6 (c), combines mechanical poling and electrical poling to obtain the following structure. Created in process. First,
The BaTiO ₃ single crystal 1 formed into a predetermined size has 90 ° domains 2 and 180 ° domains 3 as shown in FIG. 6 (a).
As shown in (a), a uniaxial compressive stress 9 is applied to remove the 90 ° domain 2 (mechanical poling).

As shown in FIG. 6 (b), only 180 ° domains 5 remain in the crystal 4 after mechanical poling, but the crystals are arranged in the direction of the light propagation direction of this crystal. The plurality of electrodes 6 are brought into contact with each other with the crystal sandwiched therebetween, and a high voltage power source is connected so that the polarities of the adjacent electrodes are reversed. In this way, by applying an electric field that is alternately inverted in the light propagation direction to the crystal, a crystal 7 having a domain structure in which the polarization direction is inverted according to the direction of the electric field can be created as shown in FIG. 6C. .

[0006]

[Problems to be Solved by the Invention] However, BaT
Since the iO ₃ single crystal is a brittle material, it is easily broken during machining, and special attention is required especially in mechanical poling applying a large stress. Conventionally, when mechanical poling of crystals having a size of several mm square is performed, cracks 8 are generated in about half of the crystals as shown in FIG. 7. Therefore, after poling, polishing is performed to make cracks 8 Was being scraped off.

The occurrence of such a crack may cause a fatal defect when the waveguide becomes thinner. Moreover, when the ratio of the long side to the short side of the single crystal exceeds 10: 1, FIG.
The bending of the crystal makes it difficult to apply a compressive stress 9 along the axis of the crystal 1 and the 90 ° domains can no longer be removed by mechanical poling. Therefore, conventionally, it is necessary to first form a crystal to a large size (thickness) so that the size ratio is less than 10: 1, perform mechanical poling, and then re-polish it to the final size, which is very difficult to manufacture a waveguide. Was spending a lot of time.

The present invention has been made in view of the above problems, and prevents the occurrence of cracks during processing to reduce the defective rate, thereby reducing the number of repairing steps and
An object of the present invention is to provide a method for manufacturing a period-inverted photorefractive waveguide that can perform poling regardless of the size and size ratio of a crystal and can easily manufacture a period-inverted waveguide at low cost.

[0009]

According to the method of manufacturing a periodic inversion type photorefractive waveguide according to the present invention, a BaTiO ₃ single crystal processed into a predetermined shape is heated to 130 ° C. or higher and a uniform electric field is applied. Then, the first electric poling treatment step of cooling and passing through the Curie point is brought into contact with a plurality of electrodes arranged periodically, and an electric field is alternately inverted between adjacent electrodes at a temperature of 80 ° C or higher and lower than 130 ° C. Is applied to periodically invert the polarization direction of the single crystal to form a domain-inverted structure based on an electric field pattern.

[0010]

After processing the BaTiO ₃ single crystal into a predetermined waveguide shape, in the first electrical poling treatment step,
The wall mobility of the 90 ° domain and the 180 ° domain is improved by heating above 130 ° C, and a uniform DC electric field is applied to the entire crystal. And in a uniform DC electric field,
By slowly cooling the crystal from a temperature equal to or higher than the critical temperature Tc, both the 90 ° domain and the 180 ° domain are removed, and all the domains are aligned in the electric field direction. A suitable cooling rate is 1 ° C./hour.

In this way, an inversion electric field is applied to the single crystal that has been single-domained by the first electric poling at a temperature of 80 ° C. or higher and lower than 130 ° C. In this temperature range, only the 180 ° domain wall is easy to move, and by applying an inversion electric field in this temperature range, the 180 ° domain is locally generated and the polarization direction is reversed, and it has the desired periodic inversion domain structure. Crystals can be obtained. Thus, 90 ° domain and 180 °
The technique of electrically poling only the domain and 180 ° domain separately, and the combination of them, makes mechanical poling unnecessary, resulting in cracks and constraints on crystal size due to processing accuracy. It can be resolved. Further, by electrically poling under the temperature condition of the method of the present invention, a BaTiO ₃ single crystal waveguide having an ideal inversion structure can be manufactured without generating or leaving harmful 90 ° domains.

[0012]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples. Example 1 A 0.5 mm × 0.5 mm × 6 mm prismatic crystal 10 was cut out from the grown BaTiO ₃ single crystal along the orthogonal crystal main axis as shown in FIG. However, even if the planes that make up the prism are tilted within 30 ° from the crystal plane (100) or (001) plane,
The present invention can be applied. There was a fine domain structure in the as-cut crystal, and the crystal was opaque. First, as shown in FIG. 2, electrodes 11 are contacted from both sides to a pair of front and back surfaces of a crystal 10 cut out as a first poling step, leads 12 are connected to both electrodes 11, and then as shown in FIG. Then, it was dipped in silicone oil 14 to perform poling. Heat the silicon oil 14 to the heater 1
It is heated up to 140 ℃ by 5 and 2 between the electrodes 11 that sandwich the crystal 10.
A voltage of 50V was applied. Then, the temperature of the silicone oil was slowly lowered as it was, and when the temperature was lowered to 120 ° C., the power of the heater 15 was turned off, and then the voltage between the electrodes 11 was returned to zero. After cooling the crystal 10 to room temperature, the crystal 10
It was taken out and observed with a polarizing microscope.
The domain structure disappeared and the crystal 10 became transparent.

Next, as a second poling step, three divided electrodes were arranged on the crystal surface in order to form a periodically inverted polarization structure. The dimensions of the electrodes are
1.3mm, 2.6mm, 1.3mm, the distance between adjacent electrodes is 0.4mm. These divided electrodes were formed by sputtering gold. Then, after connecting the lead wire to each electrode, soak it in silicone oil and heat it to 125 ℃, connect the power supply to the electrodes so that the electric field with the polarity reversed between adjacent electrodes between each electrode is generated. Then, a voltage was applied. Up to 10
After applying a voltage of 0 V for 10 hours, the temperature was lowered to room temperature and the crystals were taken out. When the 180 ° domain structure of the crystal after poling was examined by the HCl etching method, it was clearly found that
It was confirmed that an inversion structure divided into two regions was formed. Also, when observed with a polarizing microscope, no visible 90 ° domain was generated in the crystal.

Further, when the two-wave mixing experiment was actually conducted to examine the amplification factor, an extremely high value of 3.0 cm ^−1, which is almost the same as the amplification factor obtained with the bulk crystal cut out from the same material, was obtained. It was The value of this optical amplification factor Γ (cm ⁻¹ ) is defined by the following mathematical formula 1, and was determined by the following measurement. Further, the intersection angle θ of the signal light and the pump light in the crystal at this time was set to 18 °.

[0015]

[Number 1] ^{Γ (cm -1) = ln (} I / I 0) / L where, I _0: incident intensity of the signal light I: amplified signal light intensity L: length of the optical propagation direction of the crystal.

First, as shown in FIG. 4, the signal light 17 is incident on the waveguide 16, and the intensity of the light that has passed through the waveguide crystal is received by the detector 18 to determine the incident light intensity I ₀ . taking measurement. Next, as shown in FIG. 5, the pump light 19 is incident on the waveguide, and the intensity I of the amplified signal light, that is, the intensity I of the amplified signal light is increased by combining with the pump light in the crystal due to the PR effect. The measurement was performed, and the optical amplification factor Γ was obtained by the above equation 1 with the length L of the waveguide set to 6 mm.

(Comparative Example 1) As a first poling step, poling was carried out under the same conditions as in Example 1 except that the heating temperature of the silicon oil was 125 ° C., and then observed with a polarizing microscope. The 90 ° domain remained in the first poling, but the second poling was performed under the same conditions as in Example 1, but the 90 ° domain remaining in the first poling did not disappear. When the optical amplification factor by the two-wave mixing was measured, the optical amplification factor Γ was about 0 cm ⁻¹ . Thus, the reason why the optical amplification factor is extremely small is considered to be that most of the propagating light was scattered due to the existence of the 90 ° domain.

(Comparative Example 2) In the second poling step, when the heating temperature of the silicon oil was set to 135 ° C and poling was carried out under the same conditions as in Example 1, except that crystals between adjacent electrodes were formed. A 90 ° domain occurred inside.

The amplification factor Γ of this crystal by two-wave mixing is about
It was 0 cm ^-1 . (Comparative Example 3) In the second poling step, poling was performed under the same conditions as in Example 1 except that the heating temperature of the silicone oil was 60 ° C. 180 by HCl etching method
° When we investigated the domain structure, we found that there were many fine domains, and it did not have the perfect three-part structure. The optical amplification factor of this crystal was measured by two-wave mixing, and was Γ = 1.8 cm ⁻¹ , which was considerably smaller than that of the example.

[0020]

As described above, according to the present invention,
Both the 180 ° domain and the 90 ° domain can be removed or generated in the first electrical poling step, and only the 180 ° domain can be removed or generated in the second electrical poling step. Because you can
A domain-inverted structure with 180 ° domains can be formed reliably and efficiently. Since mechanical polling is not performed, defects due to cracks generated during mechanical polling are eliminated. Further, there is an advantage that there is no restriction due to the size and shape of the crystal that can be poled.

[Brief description of drawings]

FIG. 1 is a diagram showing one step of an embodiment method of the present invention.

FIG. 2 is a diagram showing the next one step of the method according to the embodiment of the present invention.

FIG. 3 is a diagram showing a further next step of the method according to the embodiment of the present invention.

FIG. 4 is a diagram showing a method for measuring an optical amplification factor P.

FIG. 5 is a diagram showing a method of measuring the optical amplification factor P of the same.

FIG. 6 is a diagram showing one step of a conventional method.

FIG. 7 is a diagram illustrating a drawback of the conventional technique.

FIG. 8 is a diagram for explaining a drawback of the conventional technique.

[Explanation of symbols]

 1, 4, 7, 10; Single crystal 2; 90 ° domain 3, 5; 180 ° domain 6, 11; Electrode 14; Silicon oil 15; Heater 16; Waveguide 17; Signal light 19; Pump light

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoji Ajimura 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Stock Company (72) Inventor Akito Kurosaka 1-5-1, Kiba, Koto-ku, Tokyo In Fujikura, Ltd. (72) Haruo Tominaga, Inventor Haruo Tominaga 1-5-1, Kiba, Koto-ku, Tokyo In-house Fujikura (72) Kenichi Kitayama 1-1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Incorporated (72) Inventor Fumihiko Ito 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A first electrical poling treatment step in which a BaTiO ₃ single crystal processed into a predetermined shape is heated to 130 ° C. or higher, a uniform electric field is applied and cooled to pass a Curie point, and a cycle. A plurality of electrodes arranged in an array,
A second electric field for periodically reversing the polarization direction of the single crystal by applying an electric field which is alternately inverted between adjacent electrodes at a temperature of 80 ° C. or higher and lower than 130 ° C. to form a polarization inversion structure based on the electric field pattern. And a periodic inversion type photorefractive waveguide manufacturing method.