Total Performance of Magneto-Optical Ceramics with a Bixbyite Structure

High-quality magneto-optical ceramics (TbxY1−x)2O3 (x = 0.5–1.0) with a Bixbyite structure were extensively investigated for the first time. The total performances of these ceramics were far superior to those of commercial TGG (Tb3Ga5O12) crystal, which is regarded as the highest class of Faraday rotator material. In particular, the Verdet constant of Tb2O3 (when x = 1.0) ceramic was the largest—495 to 154 rad·T−1·m−1 in the wavelength range of 633 to 1064 nm, respectively. It was possible to further minimize the Faraday isolator device. The insertion loss of this ceramic was equivalent to that of the commercial TGG single crystal (0.04 dB), and its extinction ratio reached more than 42 dB, which is higher than the value for TGG crystal (35 dB). The thermal lens effect (1/f) was as small as 0.40 m−1 as measured by a 50 W fiber laser. The laser damage threshold of this ceramic was 18 J/cm2, which is 1.8 times larger than that of TGG, and it was not damaged during a power handling test using a pulsed laser (pulse width 50 ps, power density 78 MW/cm2) irradiated at 2 MHz for 7000 h.

11 S1(a). Firstly, powder compact having a composition of Tb4O7 (50 mol%)-Y2O3 (50 mol%) was sintered 12 under Ar-3%H2 atmosphere for 2 h at 1500 °C, and then it was grown by the FZ (floating zone) method 13 (crystal growth rate 5 mm/h, rotation speed 30 rpm, and atmosphere Ar-8%H2). Internal 14 microstructure was observed under transmission polarized optical microscope (see Figure S1(b)). It 15 was not homogeneous. Voids, cracks, double refractions and inclusions were observed in all positions 16 of the crystal. The crystal structure of this material at room temperature is a cubic system. However, 17 during cooling process after melting at 2400 °C phase transition occurred from hexagonal ⇒ 18 orthorhombic ⇒ cubic crystal system. Therefore, some parts were not confirmed as dark-field under 19 cross nicols due to the formation of optically anisotropic phases. Insertion loss (I.L.) and extinction 20 ratio (E.R.) were measured (sample thickness: 5mm). The average values of insertion loss (I.L.) and 21 extinction ratio (E.R.) were 2.57 dB and 10.6 dB, respectively, which imply very high optical loss and 22 very small extinction ratio. Therefore, it is noteworthy that even a single crystal TYO produced by 23 melt-growth method cannot provide a good optical quality with practical size for this kind of isolator

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Verdet constants were determined by the following formula: θF = VHL, where is Faraday rotation 58 angle, H is magnetic field intensity, and L is length of the Faraday rotator.

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Thermal conductivities of each ceramic sample were measured by laser flash method using an 60 Advance-Riko TC-7000. Triangular prisms were used and minimum angle of deviation method was 61 applied to calculate the refractive index (Möller-Wedel Gmbh, Gonio-Spectrometer Type II). Output 62 power of 50 W laser (1070nm wavelength, cw single mode ytterbium fiber laser manufactured by IPG 63 photonics corp.) was used as a light source to evaluate the thermal lens effect of the materials. Due to 64 thermal lens effect, generally beam shape is slightly deformed after passing through a sample.

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Change in beam waist of laser beam after passing through each sample was measured as thermal 66 lens effect index by using a beam profiler (Coherent Inc.). In power handling test, pulsed laser (pulse 67 width 50 ps, peak power 0.3 MW, beam spot Φ 0.7 mm, power density 78 MW/cm 2 ) was irradiated 68 into the optical polished sample at 2 MHz for 7000 h, and inspected the condition of the irradiated 69 sample.

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To evaluate Faraday rotation performance, a continuous wave (cw) laser diode (FiberLabs Inc.

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FPLD-1060-24) was used as an incident laser source (1064nm, max. output 10 mW). Laser was 72 irradiated onto the sample, which is placed between input polarizer and output analyzer made of    Figure S2 shows the relationships between the concentration of Tb ions in TYO ceramics and the 84 refractive index, and the thermal conductivity. The thermal conductivity for Tb = 50~100 % regions is 85 about 3.3-4.6 Wm -1 K -1 , which is comparable to that of the commercial TGG or TAG single crystals. 86 87 Figure S2. Relationships between the Tb ion concentration and the refractive index, and the thermal 88 conductivity.

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Prototype of optical isolator using TYO (Tb-60%) ceramic is shown in Figure S3

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and an output polarizer. An AR-coated Faraday rotator element is placed inside an Nd-Fe-B 94 permanent magnet (a hollow cylinder magnet) such that the element is located at the center of light 95 axis. The angle between the input polarizer and the output polarizer is set to 45°. The Faraday rotator 96 is selected to provide a 45° rotation angle with a certain length. As for TGG crystal, it requires 20.0mm 97 length. As for TYO (Tb = 60%) ceramic sample, it requires 8.0mm length in the same magnetic field.

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As illustrated in Figure S3(c), magnetic flux density decreased with the distance from the center line.

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Therefore, magnetic field can be more effectively used by placing a shorter element with larger Verdet 100 constant in the case of same magnet house. In other words, as shown in the Figure S3

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Good reproducibility and productivity were achieved in this work, which are better than the 118 case of single crystal, with ceramic fabrication technology. For example, in the case of sample with Φ 119 6 mm × 10 mm dimension, it is possible to produce several thousands to ten thousands of pieces per 120 batch. Samples with 5 mm diameter described above are normally usable for laser power up to 100 121 W class. For kW class high-power laser operations, Faraday rotator element with large aperture 122 (Φ10~15mm) are required. For application in nuclear fusion and high energy physics in the future, 123 samples with larger aperture (Φ 20-50 mm) will become indispensable. With the invention from this 124 work, it was successful to produce large samples with good transparency (see Figure S4). The work 125 on the development of large scaled samples with improved optical quality is in progress, and it is 126 still necessary to achieve good laser damage performance of large samples higher than the TGG 127 reference. We have confirmed that the laser damage property of the TYO ceramics significantly 128 exceeded the value of TGG but the details of their laser damage properties will be reported in another 129 paper in near future.

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Production style of ceramic is different from that of single crystal. In the case of single crystal, a 131 relatively large size crystal is produced and then it is cut and machined to get required smaller size 132 elements. In the case of ceramic, they can be produced in near net shaping to the required size and in 133 large quantity. Therefore, ceramic production style is more favorable than that of single crystal.