JP2013170120A - Method for manufacturing faraday rotator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a Faraday rotator which is excellent in temperature characteristics and showing rectangular hysteresis, and a Faraday rotator.SOLUTION: In a bismuth-substituted rare earth iron garnet single crystal grown by the liquid phase epitaxial method which is shown by chemical formula Tb-HoBiFeGaO(0.4≤x≤0.7, 1.1≤y≤1.3, and 0.6≤z≤0.75 in the formula), by making the crystal growth temperature to be ≥870°C and ≤950°C, a temperature characteristic of ≤0.075 (deg/°C) is attained.

Description

  The present invention relates to a rare earth iron garnet single crystal used for a Faraday rotator such as an optical isolator or an optical circulator. Specifically, the present invention relates to a Faraday rotator made of a bismuth-substituted rare earth iron garnet single crystal that does not use a permanent magnet.

  In many cases of optical fiber communication and optical measurement, a semiconductor laser is used as a signal source. However, the semiconductor laser has a serious drawback that oscillation is unstable if there is so-called reflected return light that is reflected from the end face of the optical fiber and returns to the semiconductor laser itself. Therefore, an optical isolator is provided on the emission side of the semiconductor laser to block the reflected return light and stabilize the oscillation of the semiconductor laser.

  In general, an optical isolator includes a polarizer, an analyzer, a Faraday rotator, and a permanent magnet for magnetically saturating the Faraday rotator. The bismuth-substituted rare earth iron garnet single crystal used as this Faraday rotator has the ability to maintain a magnetic saturation state even if the external magnetic field is lost after the external magnetic field is once applied and magnetically saturated. There is something. When a Faraday rotator having this characteristic is used, there is no need for a permanent magnet in the optical isolator, so there are significant advantages in terms of downsizing and cost of the optical isolator.

Conventionally, when an optical component such as an optical isolator is configured in this way, a bismuth-substituted rare earth iron garnet single crystal having a performance that does not require a permanent magnet is (TbBi) ₃ (FeGaAl) ₅ O ₁₂ (Patent Document 1). , (TbHoBi) ₃ (FeGa) ₅ O ₁₂ (Patent Document 2), (EuHoBi) ₃ (FeGa) ₅ O ₁₂ (Patent Document 3), (TbYbBi) ₃ (FeGa) ₅ O ₁₂ (Patent Document 4), etc. It has been proposed and put into practical use.

In general, bismuth-substituted rare earth iron garnet single crystals (hereinafter abbreviated as RIG as appropriate) used as Faraday rotators are mainly produced by 3 inch (111) garnet by liquid phase epitaxial (hereinafter abbreviated as LPE) method. Growing using a single crystal (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrate. Thereafter, a manufacturing process is performed in which the substrate is cut to about 10 mm × 10 mm, both surfaces are polished to a mirror surface, and an antireflection film is attached to both surfaces.
After that, the RIG having the performance of eliminating the need for a permanent magnet after cutting into a product shape of a chip of about 1 mm × 1 mm is used as a Faraday rotator that does not require a permanent magnet by applying a magnetic field from the outside to magnetically saturate. .

  A problem with a Faraday rotator that does not require a permanent magnet is that the magnetic saturation state is broken under the influence of an external magnetic field and ambient temperature. The temperature environment in which the optical isolator is used is normally −40 to 85 ° C., and a case where an external magnetic field exists at the installation location of the optical isolator is also assumed. Therefore, when an external magnetic field is present in the temperature range of −40 to 85 ° C., for example, even when an external magnetic field of up to about 100 (Oe), preferably about 200 (Oe) is present, the Faraday rotation once saturated. It is desirable for these Faraday rotators that the angle be maintained. Here, a magnetic field that is opposite to an external magnetic field, particularly a magnetically saturated Faraday rotator, and that breaks this magnetically saturated state is defined as a coercive force Hc.

  In order to make RIG that does not require a permanent magnet, that is, in order to give RIG a large Hc, the magnetic anisotropy of RIG is increased and the saturation magnetic field (hereinafter abbreviated as Hs as appropriate) is lowered. Is effective. Therefore, it is effective to lower the saturation magnetic field by selecting rare earth elements that contribute to an increase in magnetic anisotropy or a combination thereof, and replacing iron at the tetrahedral site with a nonmagnetic element. Generally, Ga or Al is used as a nonmagnetic element that replaces iron at the tetrahedral site. This is because Ga and Al are selectively substituted in the tetrahedral site out of the tetrahedral and octahedral two-site iron, and have the effect of lowering the saturation magnetic field of the ferrimagnetic RIG. In particular, the site selectivity to the tetrahedron of Ga is generally said to be about 90% and is suitable for the purpose of lowering the saturation magnetic field. In the RIGs of Patent Documents 1 to 4, the RIG having the performance of reducing the saturation magnetic field and making the permanent magnet unnecessary is realized.

  However, when iron is replaced with these nonmagnetic elements, the change in the Faraday rotation angle per 1 ° C. (hereinafter referred to as temperature characteristics) in a Faraday rotator with a Faraday rotation angle of 45 deg becomes worse as the amount of nonmagnetic elements increases. It is known to do. Usually, the temperature characteristics of a Faraday rotator that is not substituted with non-magnetic ions is an absolute value, which varies depending on the type of rare earth constituent, and is 0.045 to 0.07 (deg / ° C.) (hereinafter, the temperature characteristic value is an absolute value) In the RIG described in Patent Document 2 or Patent Document 3 that does not require a permanent magnet, the temperature characteristic is 0.09 to 0.1 (indicated by the value of the value). deg / ° C). The large temperature characteristics of RIG cause performance degradation due to external temperature changes of the optical isolator.

On the other hand, the RIG described in Patent Document 1 has a relatively good temperature characteristic of 0.07 to 0.08 (deg / ° C.), but the guaranteed value of Hc of a commercially available Faraday rotator made of this RIG is It is 200 (Oe) (product catalog value) at room temperature, and is small compared with other commercially available products (400 to 500 (Oe)). Furthermore, the present inventors have confirmed that the decrease in Hc is large at high temperatures. Increasing the magnetic anisotropy constant, which is the magnetic property of RIG, increases Hc. It is generally known that the magnetic anisotropy constant is related to the type of rare earth ion and its ion radius. Patent Document 2 proposes improvement of Hc by adding Ho having a small ionic radius and magnetism to RIG described in Patent Document 1. In Patent Document 3, Eu and Ho having magnetism and different ionic radii are disclosed. It is combined. It has been found in Patent Document 4 that RIG described in Patent Document 1 is combined with Yb having a small ionic radius and magnetism to have a large Hc, and its temperature characteristic is 0.075 to 0.08, which is smaller than the conventional one. A RIG of (deg / ° C.) has been proposed.
Furthermore, in Patent Document 5, the temperature characteristics of the RIG described in Patent Document 2 and Patent Document 4 are 0.075 by using gold as the crucible material for growing the RIG and adding Ca to the RIG. It has been found that (deg / ° C.) or less. However, in order to improve temperature characteristics, it is essential to use gold for the crucible material, and a platinum crucible that has been used in the past and has a higher melting point than gold and is excellent in productivity cannot be used. There is a restriction.
Moreover, although it is known that the site selectivity of Ga substituting the iron site is changed by heat treatment after growth (Non-Patent Document 1), in this case, a large-scale apparatus called a heat treatment furnace for controlling the atmosphere is used. In addition, man-hours for the heat treatment process are required.

Japanese Patent No. 3520889 JP-A-10-31112 Japanese Patent No. 3198053 JP 2010-72263 A JP2010-256588

Bubble Technology Handbook by the Institute of Electrical Engineers Ohmsha

  The present inventors manufactured the Faraday rotator having excellent temperature characteristics even if the RIG proposed in Patent Document 2 with a large Hc is produced using a proven and highly productive platinum crucible. The problem was to provide a method.

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention have a site selectivity to a tetrahedral site that has an effect of lowering a saturation magnetic field in Ga substituted for an RIG iron site with an increase in growth temperature. As a result, the inventors have found that it is possible to reduce the saturation magnetic field without deteriorating the temperature characteristics of RIG with a smaller amount of Ga substitution than before, and completed the present invention. It was.
That is, the formula _{Tb 3-x-y Ho x} Bi y Fe 5-z Ga z O 12 ( wherein, 0.4 ≦ x ≦ 0.7,1.1 ≦ y ≦ 1.3,0.6 ≦ z ≦ 0.75), a bismuth-substituted rare earth iron garnet single crystal grown by a liquid phase epitaxial method and a method for producing a Faraday rotator showing a square hysteresis formed by magnetizing the bismuth-substituted rare earth iron garnet single crystal, Temperature characteristics of 0.075 (deg / ° C.) or less were achieved by setting the temperature to 870 ° C. or more and 950 ° C. or less.

The graph which showed the amount of Ga substitution required in order to adjust growth temperature and a saturation magnetic field, and the relationship of a temperature characteristic.

  Table 1 summarizes the evaluation results of the Faraday rotators produced by the production methods of Examples 1 to 5 and Comparative Examples 1 and 2.

In RIG manufactured by the method described in each Example and each comparative example in Table 1, the relationship between the growth temperature at that time, the substitution amount of Ga in RIG, and the temperature characteristic is shown in FIG. In Table 1, as the definition of the value of Hc, in the RIG obtained as described in the following examples, the average value in Hc of 100 chips processed into 1 mm × 1 mm chips is obtained, and further sufficient Hc is obtained. The criterion for the determination was that the average value of Hc of the 100 chips was 1000 (Oe) or more. Since Hc is inversely proportional to Hs, when the saturation magnetic field in Table 1 exceeds 70 (Oe), Hc decreases. On the other hand, if it is less than 30 (Oe), the magnetic compensation temperature is near room temperature, so that Hc becomes too high, and troubles such as the need for a large magnetizing magnetic field when producing the product occur.
As is clear from FIG. 1, the present inventors have found that the amount of Ga substitution necessary for adjusting the saturation magnetic field to obtain sufficient Hc as the growth temperature rises decreases, and the temperature characteristics improve accordingly. It was. The reason why the amount of Ga necessary for adjusting the saturation magnetic field has decreased is that the site selectivity to the tetrahedral site of the iron site that has the effect of lowering the saturation magnetic field in Ga replaced with the iron site of RIG as the growth temperature increases. The present inventors think that this has increased. From FIG. 1, the temperature characteristic of 0.075 (deg / ° C.) or less can be achieved by setting the growth temperature to 870 ° C. or higher.
Further, the present invention is also effective in a manufacturing method using a gold crucible because there is no restriction on the crucible material. Since gold has a low melting point of 1064 ° C., the growth temperature is desirably 950 ° C. or lower, but when a platinum crucible is used, a higher growth temperature may be used.

The substitution amount x of Ho is preferably 0.4 or more and 0.7 or less. If x is less than 0.4, the amount of Bi substitution is reduced, the Faraday effect is lowered, and the required film thickness is increased, which is not preferable. On the contrary, if x exceeds 0.7, a large amount of Bi will be contained, but if a large amount of Bi is substituted, the quality and productivity of the crystals are lowered, which is not preferable. By setting the amount of substitution x of Ho as described above, the amount of substitution of Tb and Bi depends on the consistency between the ionic radii of Ho, Tb, and Bi at the rare earth site in the RIG and the lattice constant of the substrate to be used. The Bi substitution amount y is 1.1 or more and 1.3 or less.
It is effective for the temperature characteristic of the Faraday rotation angle that the substitution amount z of Ga is as small as possible. When the present invention is used, the saturation magnetic field can be obtained when the Ga substitution amount z is in the range of 0.6 to 0.75.

The present invention is concerned with the ₅ O composition _(TbHoBi) 3 _(FeGa), described in Patent Document 1 _{(TbBi) 3 (FeGaAl) 5} O 12, Patent Documents 4 and 5, wherein the _(TbYbBi) 3 ( In FeGa) ₅ O _{12 and the} like, it can be expected that the temperature characteristics are improved by increasing the growth temperature to reduce the amount of Ga and Al necessary for adjusting the saturation magnetic field.

A known substrate can be used as the seed crystal substrate used for manufacturing the RIG film used in the present invention. In general, it is appropriately selected from non-magnetic garnet (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrates having a lattice constant of 1.2490 nm to 1.2515 nm that are commercially available as SGGG substrates.
The details of the RIG production methods and evaluation results described in Table 1 are described below. In Examples and Comparative Examples, a high purity reagent of 3N or more is used. In addition, during the crystal growth in the liquid phase epitaxial method by any manufacturing method, no special atmosphere control is performed, and all are performed in the air.

Hereinafter, the present invention will be specifically described by way of examples.
Example 1
Raw materials, bismuth oxide (Bi ₂ O ₃ ), holmium oxide (Ho ₂ O ₃ ), terbium oxide (Tb ₄ O ₇ ), gallium oxide (Ga ₂ O ₃ ), ferric oxide (Fe ₂ O) in a platinum crucible ₃ ) In boron oxide (B ₂ O ₃ ) and lead oxide (PbO), the molar ratio [Ga ₂ O ₃ ] / [Fe ₂ O ₃ ] of gallium oxide to ferric oxide is 0.11, and further oxidation A melt was prepared so that the sum of the molar fractions of holmium, terbium oxide, ferric oxide, and gallium oxide was 0.19.
This melt was placed at a predetermined position of a precision vertical tubular electric furnace, heated and melted to 1000 ° C., and sufficiently mixed with sufficient stirring to obtain a RIG growth melt. After the temperature of the melt obtained here was lowered to 878 ° C., a 3 inch (111) garnet single crystal (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrate having a lattice constant of 1.2496 nm was formed on the melt surface. One side was brought into contact and epitaxial growth was performed while rotating the substrate to obtain RIG (hereinafter referred to as RIG-1). As a result of measuring the saturation magnetic field Hs of RIG-1, the value was 40 (Oe). As a result of analyzing this RIG-1 by ICP emission spectrometry, the composition was Tb _1.21 Ho _0.54 Bi _1.25 Fe _4.27 Ga _0.73 O ₁₂ . After dividing the obtained RIG-1 into 11 mm × 11 mm, the substrate was removed, and the thickness was adjusted so that the Faraday rotation angle was 45 degrees. Thereafter, an antireflection film centered on a wavelength of 1550 nm was applied.
Next, one RIG-1 having an arbitrary size of 11 mm × 11 mm was selected, and the temperature characteristic of the Faraday rotation angle was measured. As a result, the value was 0.075 (deg / ° C.).
One piece of this RIG-1 was cut into a size of 1 mm × 1 mm. As a result of measuring the coercive force Hc of 100 obtained chips, the average value of 100 Hc was 1384 (Oe).

Example 2
Raw materials, bismuth oxide (Bi ₂ O ₃ ), holmium oxide (Ho ₂ O ₃ ), terbium oxide (Tb ₄ O ₇ ), gallium oxide (Ga ₂ O ₃ ), ferric oxide (Fe ₂ O) in a platinum crucible ₃ ) In boron oxide (B ₂ O ₃ ) and lead oxide (PbO), the molar ratio [Ga ₂ O ₃ ] / [Fe ₂ O ₃ ] of gallium oxide to ferric oxide is 0.11, and further oxidation A melt was prepared by adding the molar fractions of holmium, terbium oxide, ferric oxide, and gallium oxide to 0.20.
This melt was placed at a predetermined position of a precision vertical tubular electric furnace, heated and melted to 1000 ° C., and sufficiently mixed with sufficient stirring to obtain a RIG growth melt. After the temperature of the melt obtained here was lowered to 891 ° C., a 3 inch (111) garnet single crystal (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrate having a lattice constant of 1.2496 nm was formed on the melt surface. One side was brought into contact and epitaxial growth was performed while rotating the substrate to obtain RIG (hereinafter referred to as RIG-2). As a result of measuring the saturation magnetic field Hs of RIG-2, the value was 35 (Oe). As a result of analyzing this RIG-2 by the ICP emission spectrometry, the composition was Tb _1.25 Ho _0.53 Bi _1.22 Fe _4.28 Ga _0.72 O ₁₂ . After dividing the obtained RIG-2 into 11 mm × 11 mm, the substrate was removed and the thickness was adjusted so that the Faraday rotation angle was 45 degrees. Thereafter, an antireflection film centered on a wavelength of 1550 nm was applied.
Next, as a result of selecting one arbitrary RIG-2 of 11 mm × 11 mm and measuring the temperature characteristics of the Faraday rotation angle, the value was 0.074 (deg / ° C.).
One piece of this RIG-2 was cut into a size of 1 mm × 1 mm. As a result of measuring each coercive force Hc in 100 obtained chips, the average value of 100 Hc was 1516 (Oe).

Example 3
Raw materials, bismuth oxide (Bi ₂ O ₃ ), holmium oxide (Ho ₂ O ₃ ), terbium oxide (Tb ₄ O ₇ ), gallium oxide (Ga ₂ O ₃ ), ferric oxide (Fe ₂ O) in a platinum crucible ₃ ) In boron oxide (B ₂ O ₃ ) and lead oxide (PbO), the molar ratio [Ga ₂ O ₃ ] / [Fe ₂ O ₃ ] of gallium oxide to ferric oxide is 0.11, and further oxidation A melt was prepared so that the sum of the molar fractions of holmium, terbium oxide, ferric oxide, and gallium oxide was 0.21.
This melt was placed at a predetermined position of a precision vertical tubular electric furnace, heated and melted to 1000 ° C., and sufficiently mixed with sufficient stirring to obtain a RIG growth melt. After the temperature of the melt obtained here was lowered to 901 ° C., a 3 inch (111) garnet single crystal (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrate having a lattice constant of 1.2496 nm was formed on the melt surface. One side was brought into contact and epitaxial growth was performed while rotating the substrate to obtain RIG (hereinafter referred to as RIG-3). As a result of measuring the saturation magnetic field Hs of RIG-3, the value was 65 (Oe). The RIG-3 the results of analysis by ICP emission spectrometry, the composition was _{Tb 1.24 Ho 0.51 Bi 1.25 Fe 4.30} Ga 0.70 O 12. The obtained RIG-3 was divided into 11 mm × 11 mm, the substrate was removed, and the thickness was adjusted so that the Faraday rotation angle was 45 degrees. Thereafter, an antireflection film centered on a wavelength of 1550 nm was applied.
Next, as a result of selecting one arbitrary RIG-3 of 11 mm × 11 mm and measuring the temperature characteristics of the Faraday rotation angle, the value was 0.072 (deg / ° C.).
One piece of this RIG-3 was cut into a size of 1 mm × 1 mm. As a result of measuring the coercive force Hc of 100 obtained chips, the average value of 100 Hc was 1008 (Oe).

Comparative Example 1
Raw materials, bismuth oxide (Bi ₂ O ₃ ), holmium oxide (Ho ₂ O ₃ ), terbium oxide (Tb ₄ O ₇ ), gallium oxide (Ga ₂ O ₃ ), ferric oxide (Fe ₂ O) in a platinum crucible ₃ ) In boron oxide (B ₂ O ₃ ) and lead oxide (PbO), the molar ratio [Ga ₂ O ₃ ] / [Fe ₂ O ₃ ] of gallium oxide to ferric oxide is 0.13, and further oxidation A melt was prepared by adding the molar fractions of holmium, terbium oxide, ferric oxide, and gallium oxide to 0.13.
This melt was placed at a predetermined position of a precision vertical tubular electric furnace, heated and melted to 1000 ° C., and sufficiently mixed with sufficient stirring to obtain a RIG growth melt. After the temperature of the melt obtained here was lowered to 790 ° C., a 3 inch (111) garnet single crystal (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrate having a lattice constant of 1.2496 nm was formed on the melt surface. One side was brought into contact and epitaxial growth was performed while rotating the substrate to obtain RIG (hereinafter referred to as RIG-4). As a result of measuring the saturation magnetic field Hs of RIG-4, the value was 50 (Oe). As a result of analyzing this RIG-4 by the ICP emission spectrometry, the composition was Tb _1.17 Ho _0.58 Bi _1.25 Fe _4.17 Ga _0.83 O ₁₂ . After dividing the obtained RIG-4 into 11 mm × 11 mm, the substrate was removed and the thickness was adjusted so that the Faraday rotation angle was 45 degrees. Thereafter, an antireflection film centered on a wavelength of 1550 nm was applied.
Next, as a result of selecting one arbitrary 11 mm × 11 mm RIG-4 and measuring the temperature characteristics of the Faraday rotation angle, the value was 0.085 (deg / ° C.).
One piece of this RIG-4 was cut into a size of 1 mm × 1 mm. As a result of measuring each coercive force Hc in 100 obtained chips, the average value of 100 Hc was 1574 (Oe).

Comparative Example 2
Raw materials, bismuth oxide (Bi ₂ O ₃ ), holmium oxide (Ho ₂ O ₃ ), terbium oxide (Tb ₄ O ₇ ), gallium oxide (Ga ₂ O ₃ ), ferric oxide (Fe ₂ O) in a platinum crucible ₃ ) In boron oxide (B ₂ O ₃ ) and lead oxide (PbO), the molar ratio [Ga ₂ O ₃ ] / [Fe ₂ O ₃ ] of gallium oxide to ferric oxide is 0.12, and further oxidation A melt was prepared so that the sum of the molar fractions of holmium, terbium oxide, ferric oxide, and gallium oxide was 0.15.
This melt was placed at a predetermined position of a precision vertical tubular electric furnace, heated and melted to 1000 ° C., and sufficiently mixed with sufficient stirring to obtain a RIG growth melt. After the temperature of the melt obtained here was lowered to 823 ° C., a 3 inch (111) garnet single crystal (GdCa) ₃ (GaMgZr) ₅ O ₁₂ substrate having a lattice constant of 1.2496 nm was formed on the melt surface. One side was brought into contact and epitaxial growth was performed while rotating the substrate to obtain RIG (hereinafter referred to as RIG-5). As a result of measuring the saturation magnetic field Hs of RIG-5, the value was 55 (Oe). As a result of analyzing this RIG-5 by the ICP emission spectrometry, the composition was Tb _1.19 Ho _0.59 Bi _1.22 Fe _4.23 Ga _0.77 O ₁₂ . After the obtained RIG-5 was divided into 11 mm × 11 mm, the substrate was removed and the thickness was adjusted so that the Faraday rotation angle was 45 degrees. Thereafter, an antireflection film centered on a wavelength of 1550 nm was applied.
Next, as a result of selecting one arbitrary 11 mm × 11 mm RIG-5 and measuring the temperature characteristics of the Faraday rotation angle, the value was 0.080 (deg / ° C.).
One piece of this RIG-5 was cut into a size of 1 mm × 1 mm. As a result of measuring each coercive force Hc in 100 obtained chips, the average value of 100 Hc was 1313 (Oe).

Claims (1)

Chemical formula Tb _3-xy Ho _x Bi _y Fe _5-z Ga _z O ₁₂ (where 0.4 ≦ x ≦ 0.7, 1.1 ≦ y ≦ 1.3, 0.6 ≦ z ≦ 0) .75), a method for producing a bismuth-substituted rare earth iron garnet single crystal grown by liquid phase epitaxy and a Faraday rotator having a square hysteresis formed by magnetizing the bismuth-substituted rare earth iron garnet single crystal. The manufacturing method characterized by being 950 degreeC or less above.

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