Bismuth-substituted rare earth iron garnet single crystal, Faraday rotator, optical isolator and preparation method of bismuth-substituted rare earth iron garnet single crystal

Abstract

The present invention provides a bismuth-substituted rare earth iron garnet single crystal, which suppresses the valence change of Fe ions and is suitable for a Faraday rotator and an optical isolator in which the insertion loss is reduced. The bismuth-substituted rare earth iron garnet single crystal according to the present invention is characterized in that the composition formula is (Gd a Ln b Bi c Mg 3‑(a+b+c) )(Fe d Ga e Ti f Pt 5‑(d+e+f) ) 12 In the composition formula, 0.02≤f≤0.05, 0.02≤{3‑(a+b+c)}≤0.08, ‑0.01≤{3‑(a+b+c)}‑{f+5‑(d+e+f)}≤0.01. Ln is a rare earth element and can be selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu and Y.

Description

Technical Field

[0001] The present invention relates to an optical isolator for optical processing and optical measurement and a crystal for a Faraday rotator therein. Background Art

[0002] In laser light sources used in optical processing equipment and optical measuring machines, laser oscillation becomes unstable when emitted laser light reflects off the surface of a material placed in the propagation path and the reflected light returns to the laser light source. To block this reflected and returning light, optical isolators using Faraday rotators are used to prevent the plane of polarization from reciprocating.

[0003] Conventionally, bismuth-substituted rare earth iron garnet single crystals grown on a substrate crystal by liquid phase epitaxy are used as materials for magneto-optical components such as optical isolators. These garnet single crystals are contaminated with Pb (lead) ions from lead oxide, a flux component used in manufacturing, and Pt (platinum) ions from platinum, a crucible material. As a result, the valence of the Fe (iron) ions, which were present as trivalent ions in the garnet single crystals, shifts to divalent or tetravalent ions, reducing the light transmittance of the garnet single crystals.

[0004] When the garnet single crystal containing Fe ions with a valence other than trivalent as described above is used in optical isolators and magneto-optical elements, the light absorption at wavelengths commonly used in optical processing and optical measurement (0.8μm, 1.3μm, 1.55μm) increases, resulting in an increase in the insertion loss of the optical isolators and magneto-optical elements.

[0005] As a method for reducing insertion loss in optical isolators, it is known to place a metal oxide such as TiO2 along with other materials into a crucible material such as a platinum crucible during the manufacture of a garnet single crystal. A small amount of Ti (titanium) is then added to the garnet single crystal to balance the Pt ions dissolved from the platinum crucible and suppress valence fluctuations in the Fe ions. However, the amount of Ti required to balance the trace amounts of Pt ions dissolved from the platinum crucible is also very small. Consequently, the amount of TiO2 introduced into the garnet single crystal in the Pt crucible is also very small, and it is difficult to uniformly introduce Ti so that it balances the Pt ions in various regions of the garnet single crystal.

[0006] Prior Art Reference Documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent No. 3490143 Summary of the Invention

[0009] Problems to be solved by the present invention

[0010] An object of the present invention is to provide a bismuth-substituted rare earth iron garnet single crystal suitable for use in a Faraday rotator and an optical isolator, wherein the insertion loss is reduced by suppressing the valence fluctuation of Fe ions.

[0011] Means of solving the problem

[0012] The present inventors have found that insertion loss can be easily reduced by adding excess MgO and TiO2 to a raw material melt when growing a garnet single crystal and by balancing the composition ratio of the combination of Pt and Ti with the composition ratio of Mg.

[0013] In other words, in order to solve the above problems, the bismuth-substituted rare earth iron garnet single crystal of the present invention is characterized in that the composition formula is (Gd a Ln b Bi c Mg 3-(a+b+c) )(Fe d Ga e Ti f Pt 5-(d+e+f) )O 12 In the above composition formula, 0.02≤f≤0.05, 0.02≤{3-(a+b+c)}≤0.08, and -0.01≤{3-(a+b+c)}-{f+5-(d+e+f)}≤0.01. Ln is a rare earth element that can be selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu, and Y. It is particularly beneficial when Ln is Ho. Multiple of these rare earth elements can be used simultaneously. Bismuth-substituted rare earth iron garnet single crystals can be grown using a melt composition that does not contain PbO.

[0014] This configuration facilitates the adjustment of the composition of the melt for single crystal growth by adding excess amounts of Ti and Mg as valence-variable elements, rather than simply suppressing with high precision the valence fluctuation of Fe caused by the amount of Pt mixed in as an impurity. It also expands the adjustment range of divalent and tetravalent ions in the growing crystal, thereby suppressing the Fe ions that are light absorption factors. 2+ and Fe 4+ ions. However, adding too much excess Ti and Mg will reduce the proportion of Fe elements in the single crystal, resulting in a decrease in Faraday rotation ability per unit length. In other words, the longer the Faraday rotator length, the longer the crystal growth time. In addition, even if the insertion loss per unit length is equal, an increase in the Faraday rotator length will lead to an increase in insertion loss. In addition, a longer Faraday rotator length enlarges the optical isolator. This can lead to disadvantages such as an enlarged isolator geometry. To address these issues, the upper limit of the Ti composition ratio is 0.08 or less, and the upper limit of the Mg composition ratio is 0.05 or less.

[0015] The Faraday rotator is characterized in that it is composed of the bismuth-substituted rare earth iron garnet single crystal. The optical isolator is characterized in that it is composed of the Faraday rotator.

[0016] The method for producing a bismuth-substituted rare earth iron garnet single crystal according to the present invention includes: preparing a garnet single crystal substrate as a base substrate; melting a metal oxide containing at least TiO2 and MgO as raw materials in a platinum crucible to prepare a raw material melt; and bringing the base substrate into contact with the raw material melt and pulling up the base substrate to grow a bismuth-substituted rare earth iron garnet single crystal film. In this production method, the raw material melt may have a composition that does not contain PbO.

[0017] Effects of the Invention

[0018] The present invention is a method for easily controlling the valence change of Fe ions caused by dissolved Pt ions when growing a garnet single crystal film in a Pt crucible using a small amount of additive elements, and can easily obtain a garnet single crystal film with low insertion loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A flow chart showing a method for preparing a bismuth-substituted rare earth iron garnet single crystal is shown.

[0020] Figure 2 The corresponding relationship between the difference between the composition ratio of Mg and the composition ratio of a combination of Pt and Ti and the insertion loss of the optical isolator is shown. DETAILED DESCRIPTION

[0021] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited thereto.

[0022] <Composition of Bismuth-Substituted Rare Earth Iron Garnet Single Crystal>

[0023] The bismuth-substituted rare earth iron garnet single crystal of the present invention is described below. This bismuth-substituted rare earth iron garnet single crystal is suitable for use in Faraday rotators and optical isolators. The bismuth-substituted rare earth iron garnet single crystal is represented by the following composition formula (1).

[0024] (Gd a Ln b Bi c Mg 3-(a+b+c) )(Fe d Ga e Ti f Pt 5-(d+e+f) )O 12 ...(1)

[0025] Ln in the composition formula (1) is a rare earth element selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu, and Y. It is particularly beneficial when Ln is Ho. Multiple rare earth elements can be used simultaneously. In the composition formula (1), the Ti composition ratio is 0.02≤f≤0.05, the Mg composition ratio is 0.02≤{3-(a+b+c)}≤0.08, the difference between the Mg composition ratio and the Ti composition ratio is -0.01≤{3-(a+b+c)}-{f+5-(d+e+f)}≤0.01, a>0, b>0, c>0, d>0, e>0, and 0 <d+e+f<5.0。

[0026] In the composition formula (1), Ln, Bi, and Fe are elements added to increase the Verde constant and transmittance of the garnet single crystal. In particular, Fe is very effective in increasing the Verde constant and can be stably present as a trivalent ion in the garnet single crystal.

[0027] In composition formula (1), Pt is incorporated into the crystal by dissolving from a platinum crucible used to produce a garnet single crystal. In composition formula (1), Ti and Mg are elements added to suppress the valence fluctuation of Fe ions caused by Pt ions dissolving from the platinum crucible. As described above, the range of f representing the composition ratio of Ti is 0.02 ≤ f ≤ 0.05, and the range of {3-(a+b+c)} representing the composition ratio of Mg is 0.02 ≤ {3-(a+b+c)} ≤ 0.08.

[0028] By setting the lower limits of the composition ratio of Ti and the composition ratio of Mg as described above, the amount of the element added for suppressing the valence fluctuation of Fe ions can be easily adjusted.

[0029] By setting the upper limits of the composition ratios of Ti and Mg as described above, it is possible to prevent a relative decrease in the Fe content in the garnet single crystal and to prevent a decrease in light transmittance. This prevents a decrease in light transmittance, allowing the thickness of the single crystal film required to rotate the plane of polarization by a predetermined angle (e.g., 45 degrees) to be thinner. This offers advantages such as reduced single crystal film manufacturing time and the ability to miniaturize the Faraday rotator and optical isolator.

[0030] In addition, by balancing the difference between the composition ratio of Mg in the above composition formula (1) and the composition ratio of the combination of Ti and Pt to -0.01≤{3-(a+b+c)}-{f+5-(d+e+f)}≤0.01, the valence fluctuation of Fe ions can be suppressed while preventing the transmittance of the garnet single crystal from decreasing.

[0031] <Method for preparing bismuth-substituted rare earth iron garnet single crystal>

[0032] The bismuth-substituted rare earth iron garnet single crystal of the present invention can be grown using a melt composition that does not contain PbO. Figure 1 The flowchart shown illustrates a specific embodiment of the method for preparing a bismuth-substituted rare earth iron garnet single crystal.

[0033] First, a garnet single crystal substrate (hereinafter referred to as "base substrate") is prepared as a substrate for growing a bismuth-substituted rare earth iron garnet single crystal (step S10). The prepared substrate may be, for example, a Gd3Ga5O 12 (GGG; gadolinium, gallium, garnet) single crystal substrate. Such a substrate can be obtained by pulling a single crystal using the Choklarsky method.

[0034] Next, the metal oxides that will become the raw materials for the bismuth-substituted rare earth iron garnet single crystal are melted in a platinum crucible to prepare a raw material melt (step S20). The raw material metal oxides include, for example, Gd2O3, Ho2O3, Bi2O3, Fe2O3, Ga2O3, TiO2, and MgO. The raw material melt is prepared by preparing these metal oxides in a predetermined molar weight ratio, placing them in a platinum crucible, and heating and melting them at a predetermined temperature. The amount of Pt dissolved from the Pt crucible is experimentally determined by material analysis of a single crystal prepared from a composition that does not include Ti. The amount of TiO2 and / or MgO to be added is adjusted to a balance based on the amount of Pt to be dissolved.

[0035] Next, a single crystal film is grown by contacting the base substrate with the prepared raw material melt and pulling it upward (step S30). The single crystal film grown to the desired thickness is then cut and polished to obtain a bismuth-substituted rare earth iron garnet single crystal suitable for use in Faraday rotators and optical isolators (step S40).

[0036] Example

[0037] In order to confirm the effectiveness of the present invention, bismuth-substituted rare earth iron garnet single crystals were prepared, and Faraday rotators made from these crystals were evaluated as follows.

[0038] (Comparative Example)

[0039] First, a garnet single crystal substrate is prepared as a base substrate for growing a bismuth-substituted rare earth iron garnet single crystal film. The base substrate can be NOG (product name of Shin-Etsu Chemical Co., Ltd.) or SGGG (product name of Saint-Gobain), which is Gd3Ga5O with added Ca, Mg, Zr, Y, etc. 12 Such a substrate can be obtained by pulling a single crystal using the Choklarsky method. The lattice constant on this substrate is

[0040] Next, a raw material melt was prepared as the raw material for the bismuth-substituted rare earth iron garnet single crystal. Metal oxides: 46.9g of Gd2O3, 48.8g of Ho2O3, 5810g ​​of Bi2O3, 363.5g of Fe2O3, and 12.1g of Ga2O3 were prepared. These were placed in a platinum crucible and heated to 1100°C to melt. This yielded a raw material melt.

[0041] The raw material melt was then set to 790 to 785°C, and the substrate was brought into contact with it and pulled up to obtain a garnet single crystal film with a thickness of 612 μm.

[0042] The single crystal film was analyzed by ICP and found to be composed of (GdHoBi)3(FeGaPt5O 12 That is, a garnet single crystal film having a conventional composition in which Pt from a platinum crucible was mixed without adding Ti and Mg to suppress the valence fluctuation of Fe ions caused by Pt was obtained.

[0043] The single crystal film was then peeled from the substrate, cut and polished, coated with an air-repellent antireflection coating, and cut into 1.5 × 1.5 × 0.51 mm sections. The magneto-optical properties of the post-cut single crystal film at a wavelength of 1.55 μm were investigated, revealing a Faraday rotation angle of 46.3 degrees, an optical absorption loss of 0.35 dB, and a saturation magnetization of 650 G.

[0044] (Example)

[0045] Thirteen different raw material melts were prepared by placing oxides of Gd, Ho, Bi, Fe, and Ga in a platinum crucible in the same manner as in the comparative example, and further adding TiO2 in the range of 0 to 5.0 g and MgO in the range of 0 to 15 g and heating them to melt. These raw material melts were used to prepare 13 different single crystal films of bismuth-substituted rare earth iron garnet (compositions No. 1 to 13). The results of the magneto-optical properties of each single crystal film are shown in Table 1. The relationship between the difference between the composition ratio of Mg and the composition ratio of the combination of Ti and Pt (substitution amount difference) and the insertion loss is shown in Table 1. Figure 2 shown.

[0046] [Table 1]

[0047]

[0048] As shown in Table 1, the addition of Mg and Ti tends to suppress insertion loss. In particular, when the difference between the composition ratio of Mg and the composition ratio of the combination of Ti and Pt is within the range of -0.01 to 0.01 (Compositions Nos. 3 to 9), the insertion loss of the optical isolator is found to be particularly low, less than 0.15 dB.

[0049] Even if the difference between the Mg composition ratio and the combined composition ratio of Ti and Pt is within the range of -0.01 to 0.01, the crystal thickness required to rotate the polarization by 45 degrees increases as the amount of substitution increases because the Fe composition ratio in the crystal decreases relatively. To achieve miniaturization and reduced insertion loss in optical isolators, the difference between the Mg composition ratio and the combined composition ratio of Ti and Pt should be within the range of -0.01 to 0.01, the Ti composition ratio should be 0.05 or less, and the Mg composition ratio should be 0.08 or less.

[0050] Generally, it is difficult to uniformly control the crystal composition within a plane using only a small amount of additive elements. However, according to the present invention, by replacing the corresponding divalent or tetravalent elements, the growth conditions for single crystal films can be relaxed, and single crystal films of bismuth-substituted rare earth iron garnet with low insertion loss (less than 0.05 dB) can be easily obtained.

[0051] Although the embodiments are described above, the present invention is not limited to these embodiments. Any appropriate component addition, deletion or design modification to the above embodiments by those skilled in the art, as well as the appropriate combination of the features of each embodiment, are all included in the scope of the present invention, as long as they provide the gist of the present invention.

Claims

1. A bismuth-substituted rare earth iron garnet single crystal, characterized in that: The composition formula is (Gd a Ln b Bi c Mg 3-(a+b+c) )(Fe d Ga e Ti f Pt 5-(d+e+f) ) 12 , Wherein, in the composition formula, 0.02≤f≤0.05, 0.02≤{3-(a+b+c)}≤0.08, -0.01≤{3-(a+b+c)}-{f+5-(d+e+f)}≤0.01, and Ln is a rare earth element selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu and Y, d>0, 5-(d+e+f)>0.

2. The bismuth-substituted rare earth iron garnet single crystal according to claim 1, wherein: The crystals were grown with a melt composition that did not contain PbO.

3. A Faraday rotator, comprising the bismuth-substituted rare earth iron garnet single crystal according to claim 1.

4. An optical isolator comprising the Faraday rotator according to claim 3.

5. A method for preparing the bismuth-substituted rare earth iron garnet single crystal according to claim 1, comprising: a step of preparing a garnet single crystal substrate as a base substrate; The step of melting metal oxides containing at least TiO2 and MgO as raw materials in a platinum crucible to prepare a raw material melt; and The step of bringing the base substrate into contact with the raw material melt and pulling up the base substrate to grow a bismuth-substituted rare earth iron garnet single crystal film.

6. The method according to claim 5, wherein: The raw material melt has a composition free of PbO.

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