Bismuth-substituted rare-earth iron garnet single crystal, faraday rotator, optical isolator and method for preparing bismuth-substituted rare-earth iron garnet single crystal

Abstract

The present invention provides a bismuth-substituted rare earth iron garnet single crystal which suppresses valence variation of Fe ions and is suitable for a Faraday rotor and an optical isolator having reduced insertion loss. The bismuth-substituted rare earth iron garnet single crystal according to the present invention is characterized by a composition formula of (Tb 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 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. Furthermore, Ln is a rare earth element and can be selected from Eu, Gd, Ho, Tm, Yb, Lu, and Y.

Description

TECHNICAL FIELD

[0001] The present application relates to an optical isolator for optical processing and optical measurement and a crystal for a Faraday rotator used therein. BACKGROUND

[0002] In a laser light source used in an optical processing machine or an optical measurement machine, when emitted laser light is reflected on a surface of a material provided in a propagation path and the reflected light returns to the laser light source, laser oscillation becomes unstable. In order to block such reflected light and return light, an optical isolator using a Faraday rotator which reciprocally rotates a polarization plane is used.

[0003] Conventionally, as a magneto-optical element such as an optical isolator, a bismuth-substituted rare-earth iron garnet single crystal grown on a substrate crystal by a liquid phase epitaxy method is used. Pb (lead) ions from lead oxide as a flux component used in the manufacture and Pt (platinum) ions from platinum as a crucible material are mixed in these garnet single crystals. As a result, the valence of Fe (iron) ions existing as trivalent ions in the garnet single crystal varies to divalent or tetravalent, which causes a decrease in the transmittance of the garnet single crystal.

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

[0005] As a method of reducing the insertion loss in the optical isolator, it is known that, at the time of manufacturing the garnet single crystal, a metal oxide such as TiO2 is put into the crucible material such as a platinum crucible together with other materials, and a small amount of Ti is added to the garnet single crystal to balance the Pt ions dissolved from the platinum crucible and suppress the valence variation of the Fe ions. However, the amount of Ti required to balance the trace amount of Pt ions dissolved from the platinum crucible is also very small. Therefore, the amount of TiO2 in which Ti is introduced to the garnet single crystal in the Pt crucible is also small, and it is difficult to uniformly introduce Ti so as to balance with the Pt ions in each region of the garnet single crystal.

[0006] PRIOR ART REFERENCE DOCUMENTS

[0007] PATENT DOCUMENTS

[0008] Patent Document 1: Japanese Patent No. 3490143 SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] An object of the present application is to provide a bismuth-substituted rare-earth iron garnet single crystal suitable for a Faraday rotator and an optical isolator, which reduces insertion loss due to suppression of valence variation of Fe ions.

[0011] Means for solving the problem

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

[0013] In other words, to solve the above problems, the bismuth-substituted rare-earth iron garnet single crystal of the present application is characterized in that the composition formula is (Tb 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, Gd, Ho, Tm, Yb, Lu, and Y. Multiple kinds of these rare earth elements can be used at the same time. The bismuth-substituted rare-earth iron garnet single crystal can be grown using a melt composition that does not contain PbO.

[0014] This configuration facilitates adjustment of the composition of the single crystal growth melt by adding excess amounts of Ti and Mg as valence variation elements, rather than simply suppressing valence variation of Fe caused by the amount of Pt mixed as an impurity with high precision. It also expands the adjustment range of divalent and tetravalent ions in the grown crystal, thereby suppressing generation of Fe 2+ and Fe 4+ ions as optical absorption factors. However, adding too much excess Ti and Mg reduces 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, the longer the crystal growth time. Furthermore, even if the insertion loss per unit length is equal, an increase in the length of the Faraday rotator results in an increase in insertion loss. In addition, a longer Faraday rotator length increases the optical isolator. This results in disadvantages such as an increase in the size of the isolator geometry. To solve these problems, 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 above-described bismuth-substituted rare-earth iron garnet single crystal. The optical isolator is characterized in that it is composed of the above-described Faraday rotator.

[0016] The method for producing a bismuth-substituted rare earth iron garnet single crystal according to the present application includes a step of producing a garnet single crystal substrate as a base substrate; a step of melting metal oxides containing at least TiO2and MgO as raw materials in a platinum crucible to produce a raw material melt; and a step of contacting the base substrate with the raw material melt and pulling up the base substrate, thereby growing a bismuth-substituted rare earth iron garnet single crystal film. In the production method, the raw material melt can have a composition not containing PbO.

[0017] Effects of the Invention

[0018] The present application is a method for easily controlling variation in valence of Fe ions caused by dissolved Pt ions with a small amount of added elements when growing a garnet single crystal film in a Pt crucible, and a garnet single crystal film having low insertion loss can be easily obtained. BRIEF DESCRIPTION OF DRAWINGS

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

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

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

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

[0023] A bismuth-substituted rare earth iron garnet single crystal according to the present application will be described below. This bismuth-substituted rare earth iron garnet single crystal is suitable for a Faraday rotator and an optical isolator. The bismuth-substituted rare earth iron garnet single crystal is represented by the following composition formula (1).

[0024] (Tb 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] The Ln in the composition formula (1) is a rare earth element selected from Eu, Gd, Ho, Tm, Yb, Lu, and Y. A plurality of these rare earth elements can be used at the same time. In the composition formula (1), 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, 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 improve the Verde constant and the transmittance of the garnet single crystal. In particular, Fe is very effective in improving the Verde constant and can stably exist as a trivalent ion in the garnet single crystal.

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

[0028] By setting the lower limits of the composition ratios of Ti and Mg, respectively, as described above, the amount of the elements added to suppress the valence variation of Fe ions can be easily adjusted.

[0029] By setting the upper limits of the composition ratios of Ti and Mg, respectively, as described above, the proportion of Fe elements in the garnet single crystal can be prevented from being relatively reduced, and the transmittance can be prevented from being reduced. Since the reduction of the transmittance can be prevented, the thickness of the single crystal film required to rotate the polarization plane by a predetermined angle (for example, 45 degrees) can be made thinner, thereby having advantages such as reducing the manufacturing time of the single crystal film and miniaturizing the Faraday rotator and the optical isolator.

[0030] Further, by balancing the difference between the composition ratio of Mg in the above-described 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 variation of Fe ions can be suppressed while preventing the reduction of the transmittance of the garnet single crystal.

[0031] <Method of manufacturing a bismuth-substituted rare earth iron garnet single crystal>

[0032] The bismuth-substituted rare earth iron garnet single crystal of the present application can be grown with a melt composition containing no PbO. Hereinafter, the present application will be described in detail with reference to the accompanying drawings. Figure 1The flowchart shown illustrates one specific embodiment of a method for producing a bismuth-substituted rare-earth iron garnet single crystal.

[0033] First, a garnet single crystal substrate (hereinafter referred to as "base substrate") used as a substrate for growing a bismuth-substituted rare-earth iron garnet single crystal is prepared (step S10). The prepared substrate can be, for example, Gd3Ga5O12to which Ca, Mg, Zr, Y, etc. are added (Gd3Ga5O12:GdGa, garnet) single crystal substrate. Such a substrate can be obtained by pulling a single crystal by the Choklarsky method. 12 (GGG; gadolinium, gallium, garnet) single crystal substrate. Such a substrate can be obtained by pulling a single crystal by the Choklarsky method.

[0034] Next, metal oxides to be raw materials for a 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, Tb407, Eu203, Bi203, Fe203, Ga203, Ti02, and MgO. The raw material melt is prepared by preparing these metal oxides in a predetermined composition ratio and placing them in a platinum crucible to be heated and melted at a predetermined temperature. The amount of the metal oxides to be placed in the platinum crucible is determined by the molar weight ratio and converted into weight. The amount of Pt to be dissolved from the Pt crucible is experimentally determined by material analysis of a single crystal prepared with a composition not including Ti. The amount of Ti02and / or MgO to be added is adjusted to the balance according to the amount of Pt to be dissolved.

[0035] Then, a single crystal film is grown by bringing the base substrate into contact with the prepared raw material melt and pulling it upward (step S30). The single crystal film grown to a desired thickness is then cut and polished to obtain a bismuth-substituted rare-earth iron garnet single crystal that can be used for a Faraday rotator and an optical isolator (step S40).

[0036] Example

[0037] To confirm the effectiveness of the present application, a bismuth-substituted rare-earth iron garnet single crystal was prepared, and a Faraday rotator made of these crystals was evaluated as follows.

[0038] (Comparative Example)

[0039] First, a garnet single crystal substrate was prepared as a base substrate for growing a bismuth-substituted rare-earth iron garnet crystal 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 Gd3Ga5O12to which Ca, Mg, Zr, Y, etc. are added. 12 Such a substrate can be obtained by pulling a single crystal by the Choklarsky method. The lattice constant on this substrate is

[0040] Then, a raw material melt was prepared as a raw material of a bismuth-substituted rare earth iron garnet single crystal. As metal oxides, Tb407: 100.1 g, Eu203: 10.7 g, Bi203: 6320 g, Fe203: 320.8 g, and Ga203: 20.6 g were prepared, put into a platinum crucible, and heated to 1100°C to be melted. Thus, a raw material melt was obtained.

[0041] Then, the raw material melt was set to 795 to 787°C, the substrate was contacted therewith and pulled up, and a garnet single crystal film having a thickness of 598 μm was obtained.

[0042] The single crystal film was analyzed by ICP, and was found to be (TbEuBi)3(FeGaPt)50i2 12 the compound shown by the formula. That is, a garnet single crystal film having a conventional composition was obtained, in which Pt derived from the platinum crucible was mixed without adding Ti and Mg to suppress the valence variation of Fe ions caused by Pt.

[0043] Then, the single crystal film was peeled from the substrate, the peeled single crystal film was cut and polished, an anti-reflection coating layer was applied to the film surface, and was cut into 1.5 x 1.5 x 0.465 mm. The magneto-optical properties of the single crystal film at a wavelength of 1.55 μm were investigated after the cutting process, and the results were as follows: the Faraday rotation angle was 45.0 degrees, the optical absorption loss was 0.37 dB, and the saturation magnetization was 350 G.

[0044] (Example)

[0045] Thirteen different raw material melts were prepared by placing oxides of Tb, Eu, Bi, Fe, and Ga in a platinum crucible in the same manner as in the comparative example, and further adding Ti02in a range of 0 to 2.8 g and MgO in a range of 0 to 12 g and heating to be melted. Using these raw material melts, thirteen different single crystal films of bismuth-substituted rare earth iron garnet (Composition Nos. 1 to 13) were prepared. The results of the magneto-optical properties of each single crystal film are shown in Table 1. The relationship between the difference in the composition ratio of Mg from the composition ratio of Ti and Pt (the difference in the amount of substitution) and the insertion loss is shown in Figure 2 .

[0046] [Table 1]

[0047]

[0048] As shown in Table 1 and Figure 2 , the addition of Mg and Ti tends to suppress the insertion loss. In particular, when the difference in the amount of substitution is in a range of -0.01 to 0.01 (Composition Nos. 3 to 9), it was found that the insertion loss of the optical isolator was particularly low, less than 0.15 dB.

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

[0050] Generally, it is difficult to control the crystal composition uniformly in the plane with a small amount of additive elements. However, according to the above description of the present application, by replacing the corresponding divalent, tetravalent, etc. elements, the range of growth conditions of the single crystal film can be relaxed, and a single crystal film of bismuth-substituted rare earth iron garnet having low insertion loss (less than 0.15 dB) can be easily obtained.

[0051] Although the embodiments are described above, the present application is not limited to these examples. For example, in the above-described Examples 1 and 2, Eu is used as Ln, but a rare earth element selected from Gd, Ho, Tm, Yb, Lu, Y other than Eu can also be used as Ln. Similar to the case of Eu, by adding TiO2and MgO, a single crystal film of bismuth-substituted rare earth iron garnet having low insertion loss can be obtained with these elements.

[0052] Furthermore, any appropriate addition, deletion, or design modification of the components of the above-described embodiments, and appropriate combination of the features of each embodiment, by those skilled in the art, are included in the scope of the present application, as long as they provide the gist of the present application.

Claims

1. A bismuth-substituted rare-earth iron garnet single crystal, characterized in that, The composition formula is (Tb 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 given 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, a>0, b>0, c>0, d>0, 5-(d+e+f) >0, and Ln is a rare earth element selected from Eu, Gd, Ho, Tm, Yb, Lu, and Y.

2. The bismuth-substituted rare-earth iron garnet single crystal according to claim 1, wherein, Crystals are grown using a PbO-free melt composition.

3. A Faraday rotor comprising a bismuth-substituted rare-earth iron garnet single crystal according to claim 1.

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

5. A method for preparing bismuth-substituted rare-earth iron garnet single crystals according to claim 1, comprising: Steps for preparing a garnet single crystal substrate as a base substrate; The step of preparing a raw material melt by melting a metal oxide containing at least TiO2 and MgO as raw materials in a platinum crucible; and The step of bringing the substrate into contact with the raw material melt and pulling up the 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 that does not contain PbO.

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