BISMUTH-SUBSTITUTED RARE EARTH IRON GARNET SINGLE CRYSTAL, FARADAY ROTATOR, OPTICAL ISOLATOR, AND PRODUCTION METHOD FOR BISMUTH-SUBSTITUTED RARE EARTH IRON GARNET SINGLE CRYSTAL

Abstract

A bismuth-substituted rare earth iron garnet single crystal suitable for Faraday rotators and optical isolators with reduced insertion loss due to suppressed valence fluctuation of Fe ions is provided. The bismuth-substituted rare earth iron garnet single crystal of the present invention is characterized by the composition formula (Gd.sub.aLn.sub.bBi.sub.cMg.sub.3−(a+b+c))(Fe.sub.dGa.sub.eTi.sub.fPt.sub.5−(d+e+f))O.sub.12. In the composition formula above, 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 and may be selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu, and Y.

Claims

1. A Bismuth-substituted rare earth iron garnet single crystal characterized by the composition formula (Gd.sub.aLn.sub.bBi.sub.cMg.sub.3−(a+b+c))(Fe.sub.dGa.sub.eTi.sub.fPt.sub.5−(d+e+f))O.sub.12, wherein in the composition formula above, 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, and Ln is a rare earth element selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu, and Y.

2. The bismuth-substituted rare earth iron garnet single crystal as claimed in claim 1, wherein the crystal is grown with a PbO-free melt composition.

3. A Faraday rotator comprising the bismuth-substituted rare earth iron garnet single crystal as claimed in claim 1.

4. An optical isolator comprising the Faraday rotator as claimed in claim 3.

5. A production method for bismuth-substituted rare earth iron garnet single crystal as claimed in claim 1 comprising: a step of preparing a garnet single-crystal substrate as a base substrate; a step of melting a metal oxide containing at least TiO.sub.2 and MgO as raw materials in a platinum crucible to prepare a raw material melt; and a step of bringing the base substrate into contact with the raw material melt and pulling up the base substrate, thereby growing a bismuth-substituted rare earth iron garnet single crystal film.

6. The production method according to claim 5, wherein the raw material melt has PbO-free composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 shows a flowchart of the production method for bismuth-substituted rare earth iron garnet single crystal.

[0015] FIG. 2 shows the corresponding relationship between the difference between the composition ratio of Mg and the composition ratio of Pt and Ti combined and the insertion loss of the optical isolator.

DESCRIPTION OF EMBODIMENTS

[0016] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited thereto.

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

[0017] The following is a description of the bismuth-substituted rare earth iron garnet single crystal of the present invention. This bismuth-substituted rare earth iron garnet single crystal is suitable for use in a Faraday rotator and an optical isolator. The bismuth-substituted rare earth iron garnet single crystal is represented by the following composition formula (1).

(Gd.sub.a Ln.sub.bBi.sub.cMg.sub.3−(a+b+c))(Fe.sub.dGa.sub.eTi.sub.fPt.sub.5−(d+e+f))O.sub.12  (1)

[0018] Ln in the composition formula (1) is a rare earth element selected from Eu, Dy, Gd, Ho, Tm, Yb, Lu, and Y. A plurality of these rare earth elements may be used simultaneously. It is especially good if Ln is Ho among these. In 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 and Ti composition ratios 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.

[0019] In composition formula (1), Ln, Bi, and Fe are elements added to improve the Verde constant and light transmittance of the garnet single crystal. In particular, Feis highly effective in improving the Verde constant and can be stably present as a trivalent ion in the garnet single crystal.

[0020] In composition formula (1), Pt is taken into the crystal by dissolving out of the platinum crucible used to the produce garnet single crystal. In composition formula (1), Ti and Mg are elements added to suppress valence fluctuations of Fe ions caused by Pt ions dissolved from the platinum crucible. As mentioned above, the range of f, which indicates the composition ratio of Ti, is 0.02≤f≤0.05, and the range of {3−(a+b+c)}, which indicates the composition ratio of Mg, is 0.02≤{3−(a+b+c)}≤0.08.

[0021] By setting the lower limits of the composition ratio of Ti and the composition ratio of Mg as shown above, respectively, the amount of the element added to suppress the valence fluctuation of Fe ions can be easily adjusted.

[0022] By setting the upper limits of the composition ratios of Ti and Mg as described above, respectively, the relative decrease in the ratio of Fe elements in the garnet single crystal can be prevented and the light transmittance can be prevented from decreasing. Since a decrease in light transmittance can be prevented, the thickness of the single-crystal film required to rotate the polarization plane by a predetermined angle (e.g., 45 degrees) can be made thinner, resulting in advantages such as reduced manufacturing time for the single-crystal film and miniaturization of the Faraday rotator and optical isolator.

[0023] Furthermore, by balancing the difference between the composition ratio of Mg and the composition ratio of Ti and Pt combined in the composition formula (1) as described above as −0.01≤{3−(a+b+c)}−{f+5−(d+e+f)}≤0.01, the valence fluctuation of Fe ions can be suppressed while preventing a decrease in the light transmittance of garnet single crystals.

<Production Method for Bismuth-Substituted Rare Earth Iron Garnet Single Crystal>

[0024] The bismuth-substituted rare earth iron garnet single crystal of the present invention may be grown with a PbO-free melt composition. A specific example of the production method for bismuth-substituted rare earth iron garnet single crystal is explained below with reference to the flowchart shown in FIG. 1.

[0025] First, a garnet single-crystal substrate (hereinafter referred to as “base substrate”) used as a substrate for the growth of bismuth-substituted rare earth iron garnet single crystal is prepared (Step S10). The substrate to be prepared may be, for example, a Gd.sub.3Ga.sub.5O.sub.12 (GGG; gadolinium, gallium, garnet) single-crystal substrate to which Ca, Mg, Zr, Y, etc. is added. Such a substrate can be obtained by pulling single crystal by the Choklarsky method.

[0026] Next, the metal oxide that will be the raw material for the bismuth-substituted rare earth iron garnet single crystal is melted in a platinum crucible to prepare the raw material melt (Step S20). The raw metal oxides include, for example, Gd.sub.2O.sub.3, Ho.sub.2O.sub.3, Bi.sub.2O.sub.3, Fe.sub.2O.sub.3, Ga.sub.2O.sub.3, TiO.sub.2, and MgO. The raw material melt is prepared by preparing these metal oxides in predetermined molar weight ratios, putting them in a platinum crucible, and heating and melting them at a predetermined temperature. The amount of Pt that will be dissolved out of the Pt crucible is determined experimentally by material analysis of a single crystal prepared with a composition that does not include Ti. The amount of TiO.sub.2 and/or MgO to be added is adjusted to balance based on the amount of Pt to be dissolved.

[0027] Then, a single-crystal film is grown by bringing the base substrate in contact with the prepared raw material melt and pulling it up (step S30). The grown single-crystal film 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).

EXAMPLES

[0028] 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.

Comparative Example

[0029] First, a garnet single-crystal substrate was prepared as a base substrate for growing a single-crystal film of bismuth-substituted rare-earth iron garnet crystals. The base substrate may be NOG (product name of Shin-Etsu Chemical Co., Ltd.) or SGGG (product name of Saint-Gobain), which is Gd.sub.3Ga.sub.5O.sub.12 to which Ca, Mg, Zr, Y, etc. is added. Such substrates can be obtained by pulling single crystal by the Choklarsky method. The lattice constant on this substrate was 12.496±0.004 Å.

[0030] The raw material melt was then prepared as the raw material for bismuth-substituted rare earth iron garnet single crystals. As metal oxides, Gd.sub.2O.sub.3: 46.9 g, Ho.sub.2O.sub.3: 48.8 g, Bi.sub.2O.sub.3: 5810 g, Fe.sub.2O.sub.3: 363.5 g, and Ga.sub.2O.sub.3: 12.1 g were prepared, and these were placed in a platinum crucible and heated to 1100° C. Thereby the raw melt was obtained.

[0031] The raw material melt was then set to 790-785° C., and the substrate was brought into contact and pulled up to obtain a garnet single-crystal film with a thickness of 612 μm.

[0032] The single crystal was analyzed by ICP and found to be the compound shown as (GdHoBi).sub.3(FeGaPt).sub.5O.sub.12. That is, a garnet single-crystal film with conventional compositions were obtained, in which Pt derived from the platinum crucible was mixed in, while Ti and Mg were not added to suppress valence fluctuations of Fe ions due to Pt.

[0033] The single-crystal film was then peeled off from the substrate, the peeled single-crystal film was cut and polished, and the film surface was coated with an antireflection coating against air and cut to 1.5×1.5×0.51 mm. The magneto-optical properties at a wavelength of 1.55 μm of the single-crystal film after the cutting process were investigated, and the results were as follows: Faraday rotation angle of 46.3 degrees, optical absorption loss of 0.35 dB, and saturation magnetization of 650 G.

Example

[0034] Thirteen different raw material melts were prepared by placing oxides of Gd, Ho, Bi, Fe, and Ga in a platinum crucible as in the comparative case and further adding TiO.sub.2 in the range of 0 to 5.0 g and MgO in the range of 0 to 15 g, and heating to melt them. Thirteen different single-crystal films of bismuth-substituted rare earth iron garnet (composition No. 1-13) were prepared using these raw material melts. The results of the magneto-optical properties of each single-crystal film are shown in Table 1. A graph showing the relationship between the difference between the composition ratio of Mg and the composition ratio of Ti and Pt combined (substitution amount difference) and insertion loss is shown in FIG. 2.

TABLE-US-00001 TABLE 1 Mg Composition Ti Composition Pt Composition {Mg − (Ti + Pt)} 45 deg ratio ratio rat̆io substitution amount Insertion loss thickness No. 3 − (a + b + c) f 5 − (d + e + f) difference [dB] [μm] 1 0.080 0.030 0.024 0.026 0.24 501 2 0.050 0.015 0.020 0.015 0.10 499 3 0.050 0.020 0.020 0.010 0.05 499 4 0.080 0.050 0.022 0.008 0.04 503 5 0.075 0.050 0.023 0.002 0.03 503 6 0.045 0.015 0.030 0.000 0.02 500 7 0.042 0.025 0.020 −0.003 0.02 500 8 0.050 0.025 0.028 −0.003 0.02 501 9 0.060 0.045 0.024 −0.009 0.04 502 10 0.065 0.055 0.022 −0.012 0.07 503 11 0.015 0.008 0.022 −0.015 0.13 498 12 0.020 0.015 0.024 −0.019 0.14 499 13 0.045 0.045 0.020 −0.020 0.18 502

[0035] As shown in Table 1, the addition of Mg and Ti tends to suppress insertion loss. In particular, the insertion loss of the optical isolator was found to be particularly low, less than 0.05 dB, when the difference between the composition ratio of Mg and the composition ratio of Ti and Pt combined was in the range of −0.01 to 0.01 (compositions No. 3 to 9).

[0036] Even if the difference between the composition ratio of Mg and the composition ratio of Ti and Pt combined 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 both miniaturization of the optical isolator and reduction of insertion loss, the difference between the composition ratio of Mg and the composition ratio of Ti and Pt combined should be in the range of −0.01 to 0.01, and the composition ratio of Ti should be 0.05 or less, and the composition ratio of Mg should be 0.08 or less.

[0037] In general, it is very difficult to control the crystal composition uniformly in the plane with a small amount of additive elements. However, according to the present invention described above, by replacing the corresponding elements such as divalent and tetravalent, the growth condition range of single-crystal films can be relaxed, and it is easy to obtain single-crystal films of bismuth-substituted rare earth iron garnet with low insertion loss below 0.05 dB.

[0038] Although embodiments are described above, the present invention is not limited to these examples. Any addition, deletion, or design modification of components as appropriate by those skilled in the art to the aforementioned embodiments, as well as any combination of features of each embodiment as appropriate, are included within the scope of the invention as long as they provide the gist of the invention.