Flint-glass-clad crystalline YAG-core single-mode fiber

Since the advent of single-mode silica fibers, their combination of waveguiding properties and high surface-area-to-volume ratio has enabled various laser advancements ranging from ultra-low-threshold laser operation to high-power double-clad step-index and micro-structured fiber laser operation. Laser brightness can be further scaled up with a crystalline core, provided that the transverse mode quality is maintained. Here, we report the realization of lanthanum-dense-flint-glass-clad crystalline-YAG core fiber with a broad single-mode wavelength range. Lanthanum-dense flint glasses have indexes of refraction from 1.8 to 1.9, depending on the actual glass composition and thermal treatment conditions. With the slow treatment and precision control using the co-drawing laser-heated pedestal growth method, the glass-clad YAG-core fiber can be engineered to fine-tune the fiber V number, thus ensuring the lowest order mode. Utilizing the unique specifics of the mutual positioning of the dispersion curves of the glass and the crystalline YAG core, the cut-off wavelength of the core waveguiding can be tuned from 950 nm to 1380 nm with single-mode operation in the wavelength range from 1.5 µm to 3 µm. The single-mode glass-clad crystalline fibers have high potential for lasers and medical uses in the eye-safe wavelength ranges.