JPH0642026B2 - Magneto-optical element material - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magnetic garnet material for a magneto-optical element used in an optical isolator, a circulator, a switch, or the like that utilizes the Faraday rotation effect.

(Prior art and its problems) Recently, the progress of optical fiber communication technology is remarkable. With the development of a low-loss fiber and a semiconductor laser capable of continuously oscillating for a long time, optical fiber communication technology is expected as a means for providing an inexpensive and high-quality communication means in response to an increase in communication volume. However, there is a serious problem that the stability of laser oscillation is impaired when the return light reflected from components such as a switch provided in the middle of the optical transmission path enters the semiconductor laser which is the light source.

In order to solve this problem, it has been proposed to provide an optical isolator after the laser light source. As an optical isolator for the long wavelength band of 1.3 to 1.8 μm, yttrium, iron, garnet (Y ₃ Fe ₅ O ₁₂ , YIG), which is a ferromagnetic material, has been reported, as reported in IEICE Technical Report OQE78-133. )
The one using the Faraday effect of is proposed. The YIG used in this report is a bulk single crystal grown by the flux method.

On the other hand, in order for the incident deflecting surface to rotate when light passes through the YIG, the YIG must be magnetically saturated in parallel with the incident direction of the light. When a YIG bulk single crystal processed into a cylindrical shape is used, the external magnetic field for saturation becomes extremely large and reaches 2000 Oe. In order to solve this problem, YIG bulk single crystal and the 7th Annual Meeting of the Japan Society for Applied Magnetics, pp.163-164 (Lecture number 8pC-1, as shown in Technical Report OQE80-53 of IEICE) -2) 19
As shown in 1983, it is proposed to use bismuth-substituted gadolinium, iron garnet, or bulk single crystal polished into a thin plate. However, as long as a bulk single crystal is used, it is extremely difficult to obtain a high-quality bulk single crystal of YIG or Bi-substituted gadolinium iron garnet. Therefore, the raw material cost of the optical isolator becomes high, which prevents the spread of the optical isolator. . To solve this problem, as disclosed in Japanese Patent Application No. 55-126239, it has been proposed to employ a garnet thick film epitaxially grown on a non-magnetic garnet substrate. This makes it possible to reduce the raw material cost and obtain a garnet material of higher quality than bulk crystals.

When the epitaxially grown garnet film is used as such a magneto-optical element, if a material having a large Faraday rotation coefficient is used, the optical path length can be shortened, and the element can be further downsized. Garnet substituting Bi is a material with a large Faraday rotation coefficient, and especially when Gd ³⁺ is used as a rare earth ion, the Journal of Applied Physics (Journal of App
lied Physics, Vol. 44, page 4789 (1973) and Japanese Journal of Applied Physics, Vol. 13, 166.
Large amounts of Bi, as disclosed on page 3 (1974)
Can be used as a solid bath, and exhibits an excellent figure of merit (ratio between Faraday rotation coefficient and absorption coefficient). Further, if a garnet film having a large film thickness can be obtained, it becomes an excellent magneto-optical element material having a large Farade rotation angle.

When the optical isolator using this Bi-substituted garnet thick film is used in optical communication, etc., the Faraday rotation is 45
It is necessary to do this step by step to make a good reflection and cut back the light. In the 0.8 μm band, the film thickness needs to be at least 40 μm, and in the 1.3 μm band, at least 200 μm. However, the film thickness that can be grown with conventional technology is
The limit is about 10 μm, and increasing the film thickness of Bi-substituted garnet has been an important issue.

Problems in thickening Bi-substituted garnets include:
(1) Since Bi ³⁺ has a large ionic radius, when the substitution amount of Bi ³⁺ changes in the film thickness direction, the crystal lattice constant changes significantly, and cracks and crystal defects occur due to the difference in the lattice constant between the substrate and the film thickness. To do. (ii) Journal of Crystal Growth, Volume 56, pp. 132-136 (1982)
As shown in (3), pb, which is the flux for growing the Bi-substituted garnet film, enters the film and increases the light absorption.

As a method for solving the problem of (1), JP-A-58-5
By examining the crystal orientation of the non-magnetic garnet substrate as disclosed in Japanese Patent No. 4314, it is possible to obtain a Bi-substituted garnet thick film having a thickness of 40 μm or more. However, since there is a large change in the lattice constant in the film thickness direction, birefringence occurs, and there is a problem that the extinction ratio, which is an important characteristic as a garnet thick film for optical isolators, cannot be sufficiently obtained.

(Object of the Invention) The object of the present invention is to replace Al ^3+, which has a good lattice index matching between the substrate and the film, and therefore exhibits a mirror surface state, and has an excellent figure of merit due to its small light absorption and large film thickness. Another object of the present invention is to provide a magneto-optical garnet material.

(Experimental facts that are the basis of the invention) The present inventors have found that Gd ₃ -xBixFe from the PbO-Bi ₂ O ₃ -B ₂ O ₃ -based melt.
₅ -y-zAlyGa _z O ₁₂ (however, x = 0.9 to 1.3, y = 0.07 to 0.08, z =
0.25-0.30) was grown on a Nd ₃ Ga ₅ O ₁₂ single crystal {111} substrate. Al ₂ O ₃ was added to the melt in order to make uniform the lattice constant change in the film thickness direction and in the crystal plane. As a result, even if a thick film having a film thickness of 50 to 300 μm is grown, the crystal surface is a mirror surface and the extinction ratio of 40 dB can be obtained because the lattice constant change in the film thickness direction and in the plane is small due to the effect of Al ³⁺ . And 1
The inventors have found that a performance index of 00 deg / dB or more can be obtained, and completed the present invention.

Example 1 A 210 μm-thick Gd _1.9 film grown on a {111} Nd ₃ Ga ₅ O ₁₂ substrate from a PbO—Bi ₂ O ₃ —B ₂ O ₃ -based flux by a liquid phase epitaxial method.
The Bi _1.1 Fe _4.63 Al _0.07 Ga _0.30 O ₁₂ garnet thick film had a lattice constant of 12.518 Å, and did not show any defects due to the lattice constant mismatch with the substrate having a lattice constant of 12.509 Å, and exhibited a mirror surface.
The Faraday rotation coefficient and light absorption coefficient of this material are -1850 deg / cm and 1.6 cm ^-1 , respectively, and the figure of merit (ratio between Faraday rotation coefficient and absorption loss) is 267 deg / dB. showed that.

Example 2 A 300 μm-thick Gd _2.1 film grown on a {111} Nd ₃ Ga ₅ O ₁₂ substrate from a PbO—Bi ₂ O ₃ —B ₂ O ₃ -based flux by a liquid phase epitaxial method.
The Bi _0.9 Fe _4.64 Al _0.08 Ga _0.28 O ₁₂ garnet thick film had a lattice constant of 12.508 Å, and did not show defects due to the lattice constant mismatch with the substrate, and exhibited a mirror surface. The Faraday rotation coefficient and light absorption coefficient of this material are -1872deg / cm and 1.3cm, respectively.
^-1 , the figure of merit is 332deg / dB, which shows excellent characteristics as a magneto-optical material.

It should be noted that when Bi is replaced by more than 1.3 per garnet molecular formula, the lattice constant becomes too large and lattice constant mismatch causes facets on the garnet thick film surface to deteriorate the crystallinity. The limit for substituting Al and Ga was 0.38 per molecular formula. Beyond this, the Faraday rotation coefficient becomes smaller even if Bi is increased,
Rather, light absorption due to the increase of Pb with the increase of Bi became a problem.

If the amount of Ga or Al is too small, the lattice constant cannot be matched, so at least 0.25 or more of Ga should be included per molecular formula.

(Effects of the Invention) As described above, by using the present invention, a Bi-substituted gadolinium garnet thick film having a film thickness of 40 μm or more and a mirror-like surface and a performance index of 100 deg / dB or more is obtained, and an optical isolator, It can be used as a magneto-optical element such as a circulator and a switch.

Claims (1)

[Claims] 1. A magneto-optical element material comprising a non-magnetic garnet {111} single crystal substrate on which a garnet liquid layer epitaxial thick film is formed, having a film thickness of 40 μm or more and having a composition formula of Gd _3-x Bi _x. Fe _5-yz Al _y Ga _z O ₁₂ (however, x = 0.9
.About.1.3, y = 0.07 to 0.08, and z = 0.25 to 0.30).