Fibers with crystalline core and clad have high potential for high power applications. When compared with the photonic crystal fibers with glass core, the single-crystalline-core fiber has, in general, a higher melting point and larger dielectric breakdown field, that make it advantageous for high power light source applications. However, most of the crystal-core fibers use glass as a cladding, which are still affected by the fringe field. Air-cladding can avoid this damage, but it induces high losses. Using ceramic or crystal cladding, power can be scaled up without damaging the fiber while keeping a low transmission loss. For single-mode structures the laser intensity and the amplified spontaneous emission can be maintained at high level, whereas for optical amplification, it is essential to use single-mode waveguiding to achieve high signal gain.

In this study, we have developed single-mode Ti:sapphire crystalline fibers, cladded by dip coating and high-temperature sintering at 1750 °C. Solid-state single crystal growth was observed at the perimeter of the crystalline core, and the growth speed along (11 ̅0) is about 2.7 times faster than that of (001). As shown Fig. 1, the grown cladding was single-crystalline as evidenced by electron backscattered diffraction and scanning electron microscopy examinations. From optical transmission measurement at 1550 nm, the far-field distribution of the transmitted light matches well with that of the fundamental mode. With a core size of 30 μm, the refractive index difference between core and clad was measured to be 1.0x10^-4. From fluorescence mapping on the fiber end face, a very limited amount of the Ti³⁺ ions in core were diffused into the grown crystalline cladding during the solid-state growth. The realization of a fully crystalline single-mode fiber could be useful for applications where high power and/or high brightness light sources are needed.