JPH1031112A - Faraday rotator showing square hysteresis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an isolator which requires no permanent magnets with high reliability by using a Faraday rotator which is produced by magnetizing a specified bismuth-substd. rare earth iron garnet single crystal film. SOLUTION: The Faraday rotator 1 is obtd. by magnetizing a bismuth-substd. rare earth iron garnet single crystal film (G film-1) expressed by chemical compsn. of Tb3-x-y Hox Biy Fe5-z-w Gaz Alw O12 , wherein x, y, z, w satisfy 0.40<=x<=0.70, 1.30<=y<=1.55, 0.7<=z+w<=1.2, and 0<=w/z<=0.3 respectively. The Faraday rotator 1 is composed of a G film-1 in 2mm×2mm size which is preliminarily magnetized in 5000 Oe magnetic field at room temp. The Faraday rotator 1, a polarizer 2 and an analyzer 3 are fixed to metal jigs 4, 5 with an epoxy adhesive, and then the metal jigs 4, 5 are fixed with an epoxy adhesive to obtain an optical isolator which requires no magnet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bismuth-substituted rare earth iron garnet single crystal film having a very large magnetic hysteresis. More specifically, the present invention is a bismuth-substituted rare earth iron garnet single crystal film that has a nucleation magnetic field larger than the saturation magnetic field over the entire practical temperature range and can be suitably used as a Faraday rotator for an optical isolator without an external magnetic field.

[0002]

2. Description of the Related Art In recent years, optical isolators, optical switches, magneto-optical sensors, and the like using a bismuth-substituted rare earth iron garnet single crystal film having a large Faraday effect have been actively developed. The Faraday effect is a type of magneto-optical effect, and refers to a phenomenon in which the plane of polarization of light transmitted through a material exhibiting the Faraday effect, that is, a Faraday element (Faraday rotator) such as a rare-earth iron garnet single crystal film is rotated.

Generally, the Faraday rotation angle increases in accordance with the intensity of an external magnetic field applied to the Faraday rotator. However, in the bismuth-substituted rare earth iron garnet single crystal film, as shown in FIG. 1, the Faraday rotation angle is saturated by an external magnetic field having a certain magnitude or more and becomes a constant value. An external magnetic field having a constant Faraday rotation angle is referred to as a saturation magnetic field (Hs). Conversely, when the external magnetic field is reduced, the Faraday rotation angle gradually decreases. When the external magnetic field is 0, the Faraday rotation angle also becomes 0. That is, o (origin)
Follow the path of → a → b → c → b → a → o.

[0004] However, there is usually one showing hysteresis. In this case, in FIG. 1, o (origin) → a → b →
Follow the path of c → b → b ′ → a → o. Also, a certain bismuth-substituted rare earth iron garnet single crystal film [(YBi) ₃ (FeAl) ₅ O ₁₂ ]
, The once-saturated Faraday rotation angle is maintained as it is even when an external magnetic field having the same absolute value as the saturation magnetic field and the opposite direction is applied (FIG. 2; the path is o → a → b → c → b →
d → e → f → e) was discovered (Journal of
Applied Physics, Vol. 55 (1984), 1052-1061). In FIG. 2, the magnetic field strength when the Faraday rotation angle is reversed is called a nucleation magnetic field (Hn). The difference between Hs and Hn is the magnitude of the magnetic hysteresis.

The magnetic hysteresis curve which shows a large magnetic hysteresis exceeding the saturation magnetic field as shown in FIG. 2 is particularly called square hysteresis. A magneto-optical material exhibiting square hysteresis means having a function as a Faraday rotator even without an external magnetic field. Therefore, when a bismuth-substituted rare earth iron garnet single crystal film exhibiting square hysteresis is used as a Faraday rotator, an optical isolator that does not require a permanent magnet can be manufactured, and thus there is a great advantage in terms of miniaturization and cost.

[0006]

The environment in which the optical isolator is used is usually an ambient temperature in the range of -20 ° C. to + 50 ° C. Also, it is assumed that an external magnetic field exists at the place where the optical isolator is installed. Is done. Therefore, the above-mentioned square hysteresis is maintained in a temperature range of −40 ° C. to + 70 ° C. that can be practically used, and when an external magnetic field of a certain magnitude is present, up to about 50 Oe, preferably about 100 Oe. It is required that the Faraday rotation angle once saturated is maintained even for an external magnetic field of up to.

[0007] In order to respond to such demands, the present inventors have made intensive studies and found that the chemical composition is Bi _x Tb _3-x Fe _5-yz Ga _y Al _z O ₁₂
(However, a range of 1.0 ≦ x ≦ 1.5, 0.65 ≦ y + z ≦ 1.2, z ≦ y) was developed, and a bismuth-substituted rare earth iron garnet single crystal film was developed, and a patent application was filed (Japanese Patent Application No. 08-14002).
0). Hereinafter, the bismuth-substituted rare earth iron garnet single crystal film is abbreviated as (BiTb) ₃ (FeGaAl) ₅ O ₁₂ . (BiTb) ₃ (FeGa
Al) ₅ O ₁₂ maintains square hysteresis in a temperature range of −40 ° C. to + 70 ° C., and also maintains a once-saturated Faraday rotation angle even in the presence of an external magnetic field of a certain magnitude. The Faraday rotation coefficient is very small, but on the other hand, the film thickness to achieve the 45 degree angle required for the Faraday rotator is smaller than that of the conventional bismuth-substituted rare earth iron garnet single crystal for the Faraday rotator. There was a problem that the film became thicker than the film.

For example, (HoTbBi) ₃ Fe ₅ O ₁₂ widely used as a Faraday rotator for an optical isolator has a thickness of 350 μm at a wavelength of 1.55 μm. However, (Bi
Tb) ₃ (FeGaAl) ₅ O ₁₂ has a thickness of 450 μm or more. In practice, a polishing thickness of at least 30 μm is required. However, in the liquid phase epitaxial method, when a bismuth-substituted rare earth iron garnet single crystal film having a thickness of 450 μm or more is grown, there are problems such as cracks being generated around the substrate and the substrate being cracked during crystal growth. Therefore, -40 ° C ~
Maintains square hysteresis in the temperature range of + 70 ° C.
Further, even if an external magnetic field of a certain magnitude is present, the once-saturated Faraday rotation angle is maintained, and at the same time, a wavelength of 1.55
A bismuth-substituted rare earth iron garnet single crystal having a thickness of 420 μm or less as a Faraday rotator in μm is desired.

[0009]

As a result of intensive studies to solve the above problems, the present invention has been completed. That is, the present invention is grown by a liquid phase epitaxial method,
This is a Faraday rotator having square hysteresis formed by magnetizing a bismuth-substituted rare earth iron garnet single crystal film represented by the chemical composition (1). _{Tb 3-xy Ho x Bi y} Fe 5-zw Ga z Al w O 12 (1) ( wherein, 0.40 ≦ x ≦ 0.70,1.30 ≦ y ≦ 1.55, 0.7 ≦ z + w ≦
1.2, 0 ≦ w / z ≦ 0.3)

In practicing the present invention, when the bismuth-substituted rare earth iron garnet single crystal film of the present invention is saturated to form square hysteresis, it is preferable to perform a magnetization process by applying an external magnetic field of 1,000 Oe or more. . Although the reason is not clear, as the applied magnetic field increases, the magnetic hysteresis increases, and this is a material suitable for an optical isolator that does not require a magnet. The operation of applying a strong magnetic field to magnetically saturate and increase the magnetic hysteresis is referred to as “magnetization processing”, and the external magnetic field is temporarily set to 0 after the magnetization processing, and then the external magnetic field is applied in a direction opposite to the magnetization processing. The operation is described as “reverse magnetization processing”.

Further, the square hysteresis is -40 ° C. to +
It is preferable that the sign of the Faraday rotation angle does not change even if the temperature is maintained in a temperature range of 70 ° C. and a magnetic field opposite to the magnetization direction up to at least 50 Oe. These can be reliably used in a normal use environment of the Faraday rotator.

The composition of the bismuth-substituted rare earth iron garnet single crystal film of the present invention is represented by the following formula (1).
Is preferably 0.4 or more and 0.7 or less. If x is less than 0.4, the Faraday effect decreases, and the required film thickness increases, which is not preferable. Conversely, if x exceeds 0.7, problems such as the inability to maintain square hysteresis in the operating temperature range of the optical isolator and the inability to maintain the Faraday rotation angle saturated by the reverse magnetization treatment occur.

The bismuth substitution amount y is preferably 1.3 or more and 1.55 or less. If y is less than 1.3, the Faraday effect is reduced, and the film thickness required for the Faraday rotator is undesirably large. On the other hand, if it exceeds 1.55, problems such as the inability to maintain the square hysteresis in the operating temperature range of the optical isolator and the inability to maintain the Faraday rotation angle saturated by the reverse magnetization treatment will occur.

The substitution amount (z + w) between gallium and aluminum is
It is preferably from 0.7 to 1.2. If (z + w) is less than 0.7, problems such as the inability to maintain square hysteresis in the operating temperature range of the optical isolator and the inability to maintain the Faraday rotation angle saturated by the reverse magnetization process occur. If (z + w) exceeds 1.2, the Faraday effect decreases, and the thickness of the Faraday rotator increases, which is not preferable.

The ratio of aluminum to gallium (w /
z) is preferably 0.3 or less. If it exceeds 0.3, since the ionic radius of aluminum is small, lattice matching with a substrate which is easily available at present cannot be achieved, and production becomes difficult.

As the garnet substrate used for growing the bismuth-substituted rare earth iron garnet single crystal film of the present invention, a (111) garnet single crystal having a lattice constant of 1.2497 ± 0.0002 nm [(Gd
There is a Ca) ₃ (GaMgZr) ₅ O ₁₂ ] substrate and a (509) (111) garnet single crystal [Nd ₃ Ga ₅ O ₁₂ ] substrate, which is preferable from the viewpoint of lattice matching.

[0017]

The present invention will be specifically described below with reference to examples and comparative examples. Example 1 Lead oxide (PbO, 4N) 3,559 g, bismuth oxide (Bi ₂ O ₃ , 4N) 3,
715 g, ferric oxide (Fe ₂ O ₃ , 4N) 434 g, boron oxide (B ₂ O ₃ , 5
N) 171 g, terbium oxide (Tb ₂ O ₃ , 3N) 28.3 g, holmium oxide (Ho ₂ O ₃ , 3N) 19.2 g, gallium oxide (Ga ₂ O ₃ , 3
76.3 g of N) was heated and melted in a platinum crucible having a capacity of 2,000 ml (milliliter) to obtain a bismuth-substituted rare earth iron garnet single crystal growing melt.

According to a conventional method, the surface of the obtained melt is 500 μm thick and has a lattice constant of 1.2497 ± 0.0002 nm.
One surface of the (111) garnet single crystal [(GdCa) ₃ (GaMgZr) ₅ O ₁₂ ] substrate was brought into contact with the substrate to perform epitaxial growth. And Tb _0.98 Ho _0.59 Bi _1.43 Fe _3.95 Ga _1.05 O _{12 with a} thickness of 447 μm
A single crystal film (hereinafter referred to as "G film-1") was obtained, and the composition was analyzed by a plasma emission analysis.

The G film-1 was polished to a thickness of 406 μm. However, the substrate was removed during the polishing process. After that, an anti-reflection coating for 1.55μm wavelength
m Faraday rotator (hereinafter referred to as “Faraday rotator −1
") And its magnetic properties were examined.

The following method was used for the magnetic measurement. First, the Faraday rotator-1 was arranged at the center of a magnetic field generator comprising a Helmholtz coil manufactured by Magnetic Tech. The semiconductor laser light of 1310 nm was applied to the Faraday rotator-1 while applying a magnetic field. From the measurement of the rotation angle of the plane of polarization of the laser beam transmitted through the Faraday rotator, a saturation magnetic field was obtained, and the saturation magnetic field was measured while changing the ambient temperature of the Faraday rotator-1. The saturation magnetic field of the Faraday rotator-1 was 163 Oe at 25 ° C and 218 Oe at a maximum in the range of -40 ° C to + 70 ° C.

Next, the Faraday rotator-1 was heated at 25.degree.
The magnetic field of 5,000 Oe was applied perpendicularly to the film surface, and the magnetic field was saturated. After saturation, the magnetic field was reduced to 0, but the Faraday rotator-1 maintained the Faraday rotation angle at the time of saturation. The Faraday rotation angle was 45.3 degrees. Again, the Faraday rotator-1
Was returned to the Helmholtz coil, and a magnetic field was applied in the direction opposite to the external magnetic field previously applied by the electromagnet, and the nucleation magnetic field was examined. That is, it was determined how much external magnetic field the magnetically saturated state can withstand. As a result, the temperature range
It was found that it can withstand an external magnetic field up to 87 Oe in the range of 40 ° C. to + 70 ° C.

Next, an optical isolator for 1.55 μm was actually manufactured using the Faraday rotator-1. FIG. 3 shows the configuration of this optical isolator. In FIG. 3, reference numeral 1 denotes a Faraday rotator composed of a G film-1 having a size of 2 mm × 2 mm and magnetized in advance with a magnetic field of 5,000 Oe at room temperature, and reference numeral 2 denotes a Corning 1.55 μm. A polarizer made of a glass polarizing element and a polar core (trade name), reference numeral 3 is an analyzer made of a Corning 1.55 μm glass polarizer and a polar core (trade name), and reference numeral 4 is a Faraday rotator. , Polarizer, and metal jig for fixing the analyzer. Reference numeral 5 denotes a metal jig for fixing the analyzer. Faraday rotator, polarizer,
Each analyzer was fixed to a metal jig (4,5) with an epoxy-based adhesive, and then the metal jigs (4,5) were similarly fixed with an epoxy-based adhesive to obtain an optical isolator without a magnet.

Next, the isolation was measured while heating and cooling from −40 ° C. to 70 ° C. without applying any external magnetic field to the optical isolator. As a result, isolation of 26 dB or more was obtained in the temperature range of -10 ° C to 40 ° C, and 21 dB or more in the temperature range of -40 ° C to 70 ° C. The reason that the isolation differs in the temperature range is due to the temperature dependence of the Faraday rotation angle. Further, while applying a magnetic field of 50 Oe in a direction opposite to the magnetic field at the time of the magnetization treatment to the optical isolator, the isolation was measured while heating and cooling from −40 ° C. to 70 ° C. As a result, an isolation of 27 dB or more was obtained in a temperature range of -10 ° C to 40 ° C, and an isolation of 20 dB or more was obtained in a temperature range of -40 ° C to 70 ° C.

Example 2 3,559 g of lead oxide (PbO, 4N), bismuth oxide (Bi_TwoO_Three, 4N) 3,
715 g, ferric oxide (Fe_TwoO_Three, 4N) 434g, boron oxide (B_TwoO_Three, Five
N) 171 g, terbium oxide (Tb_TwoO_Three, 3N) 32.00 g,
Lumium (Ho_TwoO_Three, 3N) 15.2 g, gallium oxide (Ga_TwoO_Three, 3
N) 61.9 g, aluminum oxide (Al_TwoO_Three, 3N) 4.21g
Heat and melt in a 2,000 ml platinum crucible and replace with bismuth
A melt for growing a rare earth iron garnet single crystal was used. Got
A 438 μm-thick Tb according to Example 1 except that a melt was used.
_1.08Ho_0.44Bi _1.48Fe_4.09Ga_0.77Al_0.14O₁₂Single crystal film (hereinafter
Below, referred to as "G film-2").

The G film-2 was polished to a thickness of 408 μm. However, the substrate was removed during the polishing process. Thereafter, an antireflection film corresponding to a wavelength of 1.55 μm was applied to both sides to produce a Faraday rotator for 1.55 μm (hereinafter referred to as “Faraday rotator-2”), and its magnetic properties were examined.

Next, the Faraday rotator-2 was placed at the center of the electromagnet at room temperature, and a magnetic field of 3,000 Oe was applied vertically to the film surface to magnetically saturate the film. After saturation, the magnetic field
However, the Faraday rotator-2 maintained the Faraday rotation angle when saturated. The Faraday rotation angle was 44.7 degrees. Again, the Faraday rotator-2 was returned to the Helmholtz coil, and a magnetic field was applied in a direction opposite to the external magnetic field previously applied by the electromagnet to check the nucleation magnetic field. That is, it was determined how much external magnetic field the magnetically saturated state can withstand. As a result, the temperature range -40 ° C to +
It was found that it could withstand an external magnetic field of up to 65 Oe in the range of 70 ° C.

Example 3 The G film-1 obtained in Example 1 was polished to 260 μm.
In addition, anti-reflection coatings for wavelengths of 1.31 μm are applied to both sides, and 1.
A Faraday rotator for 31 μm (hereinafter referred to as “Faraday rotator-3”) was produced. Next, the Faraday rotator
3 is placed at the center of the electromagnet at room temperature and perpendicular to the membrane surface.
A magnetic field of 000 Oe was applied to magnetically saturate.

After saturation, the magnetic field was reduced to 0, but the Faraday rotator-3 maintained the Faraday rotation angle at the time of saturation. The Faraday rotation angle is 1.31 μm
Was 44.1 degrees. This Faraday rotator-3 was installed in a Helmholtz coil, and a magnetic field was applied in a direction opposite to the external magnetic field previously applied by an electromagnet, and the nucleation magnetic field was examined. That is, it was determined how much external magnetic field the magnetically saturated state can withstand. As a result, the temperature range -40
It was found that the film could withstand an external magnetic field of up to 126 Oe in the range of ° C to + 70 ° C.

Comparative Example 1 Lead oxide (PbO, 4N) 3,559 g, bismuth oxide (Bi_TwoO_Three, 4N) 3,
715 g, ferric oxide (Fe_TwoO_Three, 4N) 434g, boron oxide (B_TwoO_Three,Five
N) 171 g, terbium oxide (Tb_TwoO_Three, 3N) 22.70 g, oxide oxide
Lumium (Ho_TwoO_Three, 3N) 25.7 g, gallium oxide (Ga_TwoO_Three, 3N) 7
Heat and melt 6.3 g in a platinum crucible with a capacity of 2,000 ml.
A melt for growing a mass-substituted rare earth iron garnet single crystal was prepared.
Except for using the obtained melt, the thickness was 444 according to Example 1.
μm Tb_0.84Ho_0.76Bi _1.40Fe_4.17Ga_0.83O₁₂Single crystal film
(Hereinafter referred to as "G film-C1"). This G film-C1
Faraday for 1.55 μm with a thickness of 411 μm according to Example 1.
Rotor (hereinafter referred to as “Faraday rotator-C1”),
The magnetic properties were examined. As a result, the saturation field is 3 at 25 ° C.
76 Oe, maximum 413 Oe in the temperature range of -40 ℃ to + 70 ℃
there were.

Next, the Faraday rotator-C1 was placed at the center of the electromagnet at room temperature, and a magnetic field of 5,000 Oe was applied vertically to the film surface to magnetically saturate the film. After saturation, the magnetic field
However, the Faraday rotator-C1 maintained the Faraday rotation angle when saturated. The Faraday rotation angle was 43.8 degrees. Again, the Faraday rotator-C1 was returned to the Helmholtz coil, and a magnetic field was applied in a direction opposite to the external magnetic field previously applied by the electromagnet to check the nucleation magnetic field.
That is, it was determined how much external magnetic field the magnetically saturated state can withstand. As a result, it was found that in the range of −40 ° C. to + 70 ° C., an external magnetic field of up to 15 Oe could be tolerated.

Comparative Example 2 The Faraday rotator-1 obtained in Example 1 was placed at the center of an electromagnet, a magnetic field of 500 Oe was applied, and the magnetic field was saturated at 25 ° C. After saturation, the magnetic field was reduced to 0, but the Faraday rotator-1 maintained the Faraday rotation angle at the time of saturation. Again, the Faraday rotator-1 was returned to the Helmholtz coil, and a magnetic field was applied in a direction opposite to the external magnetic field previously applied by the electromagnet to check the nucleation magnetic field. That is, it was determined how much external magnetic field the magnetically saturated state can withstand. As a result, it was found that it could withstand an external magnetic field of up to 27 Oe in a temperature range of −40 ° C. to + 70 ° C.

[0032]

According to the present invention, the Faraday rotator having a square hysteresis obtained by magnetizing the bismuth-substituted rare earth iron garnet single crystal film has a square hysteresis of -40 ° C.
Maintained at a temperature range of + 70 ° C and at least 50
Since the sign of the Faraday rotation angle does not change even with a magnetic field opposite to the magnetization direction up to Oe, an isolator without a magnet can be reliably formed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an example of magnetic properties of a bismuth-substituted rare earth iron garnet single crystal that does not exhibit magnetic hysteresis.

FIG. 2 is a schematic diagram showing an example of magnetic properties of a bismuth-substituted rare earth iron garnet single crystal exhibiting magnetic hysteresis. It is a schematic diagram which shows an example of the magnetic characteristic of the bismuth substitution rare earth iron garnet single crystal which forms a square hysteresis because magnetic hysteresis is very large.

FIG. 3 is a schematic diagram showing a structure of an optical isolator according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Bismuth substituted rare earth iron garnet single crystal film made of the present invention 2 ... Polarizer composed of a glass polarizing element (trade name; polar core) 3 ... Polarized element formed of glass polarizer (trade name: polar core) Analyzer 4 ・ ・ ・ Metal jig 5 ・ ・ ・ Metal jig

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Norio Takeda 6-1-1 Shinjuku, Katsushika-ku, Tokyo Mitsubishi Gas Chemical Co., Ltd. Tokyo Research Laboratory

Claims (4)

[Claims] 1. A Faraday rotator grown by a liquid phase epitaxial method and having a square hysteresis obtained by magnetizing a bismuth-substituted rare earth iron garnet single crystal film represented by the chemical composition represented by the following formula (1). _{Tb 3-xy Ho x Bi y} Fe 5-zw Ga z Al w O 12 (1) ( wherein, 0.40 ≦ x ≦ 0.70,1.30 ≦ y ≦ 1.55, 0.7 ≦ z + w ≦
1.2, 0 ≦ w / z ≦ 0.3) 2. The method according to claim 1, wherein the magnetization is performed by applying a magnetic field of 1,000 Oe or more perpendicularly to the surface of the bismuth-substituted rare earth iron garnet single crystal film and then removing the external magnetic field. Faraday rotator showing square hysteresis. 3. The method according to claim 1, wherein said square hysteresis is from -40 ° C. to + 70 ° C.
The Faraday rotator exhibiting square hysteresis according to claim 1, wherein the temperature range is maintained. 4. The Faraday rotator exhibiting square hysteresis according to claim 1, wherein the sign of the Faraday rotation angle does not change even in a magnetic field opposite to the magnetization direction up to at least 50 Oe.