JPH05173102A - Faraday rotor - Google Patents

Abstract

PURPOSE:To enable the formation of thicker films and to obtain the Faraday rotor having low light absorption loss by depositing specific crystal films by an ion beam sputtering method on both surfaces of a specific substrate. CONSTITUTION:Cracks and defects are tend to be generated by strains, etc., by mismatching and the thickness does not increase if the lattice constant difference DELTAa of the substrate and BiIG films increases. Then, the generation of strains and defect is controlled by using a single crystal plate 5 of nonmagnetic garnet having the lattice constant difference DELTAa within + or -0.03Angstrom , i.e., 12, 60 to 12, 66Angstrom lattice constants. Further, the front and rear surfaces of the substrate 5 are deposited respectively with Bi3Fe5O12 films 6 by the ion sputtering method at >=2.5mum which is the thickness of half the thickness to obtain 45 deg. Faraday rotating angle so as to have the structure to decrease the light absorption loss by increasing the thickness of the BiIG films as far as possible. The Bi3Fe5O12. films 6 are formed of a single crystal or polycrystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used for optical communication, optical measurement, optical recording, etc., particularly 600 to 850 nm.
The present invention relates to a Faraday rotator for an optical isolator used in the wavelength band of.

[0002]

2. Description of the Related Art A semiconductor laser used for optical communication and optical measurement suffers from mode hopping noise or the like when returning light is generated, which makes stable laser oscillation impossible. Also recently
A semiconductor laser is used as a light source even in optical recording of an optical disk, a magneto-optical disk, etc., and the return light causes a problem, so that the need for an optical isolator is increasing.

In general, an optical isolator, as shown in FIG. 2, includes a polarizer 1, an analyzer 2, a Faraday rotator 3 for rotating the plane of polarization of light by 45 °, and a permanent magnet 4 for magnetically saturating the Faraday rotator 3. Consists of. Then, the light emitted in the forward direction from the semiconductor laser is passed, and the light returned in the opposite direction is returned by the nonreciprocal action of the Faraday rotator.
By rotating it further by 45 ° from the plane of polarization in the forward direction, it becomes a crossed Nicol relationship with the polarizer 1, and the return light is blocked.

Most of the optical isolators that have been put into practical use have wavelengths in the near infrared region such as 1310 nm and 1550 nm. Fibers for optical communication have low optical loss at that wavelength, and are used for optical communication. This is because it is suitable, and a bulk YIG (Y ₃ Fe ₅ O ₁₂ ) crystal used as a Faraday rotator or a Bi-substituted YIG (BiYIG),
Furthermore, the light absorption loss of the Bi-substituted garnet film grown by the liquid phase epitaxial method (LPE method) is also low in that wavelength band.

On the other hand, an optical isolator for optical recording uses a wavelength in the 780 to 850 nm band for the purpose of improving recording density and facilitating semiconductor productivity. Recently, the wavelength of semiconductor lasers has been shortened, a visible light semiconductor laser that oscillates at a wavelength of around 670 nm has been developed, and a second harmonic oscillation (SHG) element has also been developed. As a result, an optical isolator with a shorter wavelength has been developed. The use of is suggested.

[0006]

However, in the above-mentioned bulk YIG, BiYIG or Bi-substituted garnet film grown by the LPE method, the light absorption loss is large in the visible light region (the light transmittance is poor), It is difficult to use as a Faraday rotator for isolators. This uses
It is considered that absorption by Fe ²⁺ becomes unavoidable due to the influence of impurities such as platinum (Pt) mixed from the Pt crucible and lead (Pb) used as flux.
It is also possible to use YIG or the like produced by the floating zone method (FZ method) in which impurities are not relatively mixed, but the Faraday rotation ability, that is, the angle (° / cm) of rotation per unit length is small, and it rotates 45 °. There is a drawback in that the optical path length required for this is long.

Therefore, in order to increase the Faraday effect as a solution to these problems, a nonequilibrium substance Bi ₃ Fe ₅ O ₁₂ (BiIG) having a large amount of Bi substitution and a small amount of impurities is mixed in by vapor phase growth.
Garnet was proposed in JP-A-1-93426. However, when this BiIG film is grown by a normal sputtering method, surface defects such as secondary nucleation are likely to occur,
Since it is difficult to make it thick, it is not possible to obtain a Faraday rotation angle of 45 °.

[0008]

Since the lattice constant of the BiIG film is 12.63Å, when the lattice constant difference Δa between the substrate and the BiIG film becomes large, cracks and defects are generated due to strain due to mismatch, etc. It was found experimentally that it tends to occur and that it does not become thick, and the occurrence of strain and defects is controlled by using a non-magnetic garnet single crystal substrate with Δa within ± 0.03Å, that is, a lattice constant of 12.60 to 12.66Å. There is. Furthermore, the film thickness of the BiIG film should be as thin as possible,
The BiIG film was deposited on the front and back sides of the substrate by ion beam sputtering to a thickness of 2.5 μm or more, which is about half the thickness to obtain a 45 ° Faraday rotation angle on each of the front and back sides of the substrate, so as to reduce the light absorption loss. Alternatively, it is characterized by being polycrystalline.

The Faraday rotatory power of the BiIG film is somewhat changed depending on the conditions at the time of film formation, but the maximum Faraday effect is in the wavelength region, and at the laser emission wavelength of 633 nm, a maximum of -90000 ° / cm can be obtained. Converted to a practical film thickness, it becomes -9 ° / μm. Therefore, in order to obtain 22.5 °, which is half of the 45 ° Faraday rotation angle, a film thickness of at least 2.5 μm or more is required, which is limited to this range. According to the present invention, since the lattice constant of the non-magnetic garnet single crystal substrate is almost the same as that of the BiIG film, it is possible to easily deposit a BiIG film having a large Faraday rotation ability on both surfaces of the substrate, and a wavelength of 600 to 850 nm. A Faraday rotator rotating at 45 ° can be obtained even in the band, and as a result, it becomes possible to manufacture an optical isolator in that wavelength band.

[0010]

Example 1 Bi ₃ Fe ₅ was formed on a samarium-scandium-gallium-garnet substrate {111} having a lattice constant of 12.63Å by an ion beam sputtering method using O ₂ as a reaction gas.
An O ₁₂ film was deposited on both sides of the substrate. The sputtering conditions at this time are: oxygen partial pressure 2 × 10 ⁻⁴ Torr, substrate temperature 550
The ion current density was 1.5 mA / cm ² at ℃. In the table below λ =
The present invention and a comparative example at the time of depositing a Bi ₃ Fe ₅ O ₁₂ film at 633 nm by the ion beam sputtering method are shown.

[0011]

As shown in the table above, the film thickness is 2.
The Faraday rotation angle was 44.8 ° when light with a wavelength of 633 nm passed through both sides with 5 μm and 3.5 μm on the other side. Since the saturation magnetization of the Bi ₃ Fe ₅ O ₁₂ film was 1600 gauss, the Faraday rotation angle at this time was measured by applying a magnetic field of 2 kilogauss or more so as to completely saturate the film. The optical absorption loss was 2.5 dB.

In the comparative example in which the difference between the lattice constant of the Bi ₃ Fe ₅ O ₁₂ film of 12.63Å and the lattice constant of the substrate is large, defects such as surface defects and cracks occur and the film does not become a single crystal or polycrystalline film. However, I could not get the Faraday rotation angle,
The light absorption loss also became a huge value and could not be measured.

[0014]

Example 2 A Faraday rotator rotating at 45 ° at a wavelength of 780 nm was prepared in the same manner as in Example 1 on a gadolinium-lutetium-gallium-garnet substrate {111} having a lattice constant of 12.64Å. When the wavelength is 780 nm, the Faraday rotation ability of the BiIG film is about -30 000 ° / cm, so about 8 μm on each side was deposited on both sides. In the table below, at λ = 780 nm, Bi
The present invention and a comparative example when a ₃ Fe ₅ O ₁₂ film is deposited by the ion beam sputtering method will be shown.

[0015]

The obtained BiIG film became a polycrystalline film,
As shown in the above table, when a substrate having a large lattice constant difference from the Faraday rotation was used for Faraday rotation, it was difficult to form a thick film and a Faraday rotator could not be obtained.

[0017]

As described above, by using the present invention, a BiIG film that rotates about 45 ° can be obtained by the ion beam sputtering method with good deposition controllability. Faraday rotator is possible.

[Brief description of drawings]

FIG. 1 is a block diagram of a 45 ° Faraday rotator of the present invention.

FIG. 2 is a configuration diagram of an optical isolator.

[Explanation of symbols]

1 Polarizer 2 Analyzer 3 Faraday rotator 4 Permanent magnet 5 Non-magnetic garnet substrate 6 Bi ₃ Fe ₅ O ₁₂ film

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Toshima 3-8-22 Nitta, Adachi-ku, Tokyo Namiki Seisetsu Gem Co., Ltd.

Claims (1)

[Claims] 1. A non-magnetic garnet single crystal substrate having a lattice constant in the range of 12.60 to 12.66Å is coated with a Bi ₃ Fe ₅ O ₁₂ single crystal or polycrystalline film by an ion beam sputtering method to a thickness of at least one side of 2.5. A Faraday rotator characterized by depositing on both sides of the substrate so that the thickness is at least μm.