We grew transparent SrB4O7 (SBO) crystal fibers by the micro-pulling-down (μ-PD) method at pulling rates (VP) of 0.01-0.40 mm/min. We observed the SBO crystals by optical microscopy. The SBO crystal fiber at VP of 0.40 mm/min was cloudy due to the incorporation of voids. In contrast, transparent single-crystal fibers were obtained at VP of less than 0.02 mm/min. This is the first time that transparent SBO crystal fibers have been grown with a length of several centimeters. These results demonstrate that it is crucial to grow SBO crystals at a low VP to obtain transparent crystal fibers. In addition, we considered the generation mechanism of voids in the SBO crystals by using the relation between the growth rate of SBO crystals and the diffusion rate of voids. Among these noble borate crystals, SrB4O7 (SBO) has attracted much attention as a nonlinear crystal in the DUV region because of its superior transparency in the vacuum UV region, high radiation damage threshold, and high nonlinearity [4][5][6]. This crystal belongs to the Pnm21 space group and has unit cell parameters a = 4.4255 Å, b = 10.709 Å, C = 4.231 Å, and Z = 2 [7]. Crystal growth of SBO has been achieved by the Czochralski (CZ) method [8], whereas Fan et al. [9] reported crystal growth by the Kyropoulos method. The nonlinear properties in the DUV region include the wavelength conversion of femtosecond second-harmonic generation pulses down to 125 nm using an SBO crystal [10].One of most serious and common problems in borate crystal growth is macroscopic opaque defects, which make the crystal cloudy [11]. Cloudiness in SBO borate crystals grown by the CZ method has also been reported and it was revealed that reducing the growth rate produced transparent SBO crystals without cloudiness [12].We have examined growth of SBO crystal fibers by the micro-pulling-down (μ-PD) method because crystal fibers can be easily used as nonlinear devices for lasers. In this paper, we investigated growth conditions to suppress the appearance of cloudiness during growth, and transparent SBO crystal fibers were grown by the μ-PD method. The generation mechanism of cloudiness in crystals is also discussed.

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