Grazing-incidence-based hollow taper for infrared laser-to-fiber coupling

Based on grazing incidence, a simple uncoated hollow glass taper as an alternative technique for efficient infrared laser-to-fiber coupling is presented. Because of the mutual action of the direct parallel laser excitation, the mode-coupling process, and mode filtering, the hollow taper serves as a mode converter that transforms the highly multimode intensity distribution of the input laser emission into a high-quality smooth profile at the taper output. When an uncoated Pyrex-glass hollow taper is used for direct launching, without any intermediate focusing elements, of a powerful infrared Er:YAG laser emission (λ=2.94 μm), we obtain high laser-to-taper (81%), taper-to-fiber (96%), and total laser-to-fiber (78%) coupling efficiencies.