Low-loss pilot-beam delivery in hollow optical fiber for high-energy infrared laser light

New techniques were established in fabricating cyclic olefin polymer-coated silver (COP/Ag) hollow optical fiber. Low-loss properties in visible region were obtained owing to the smoother layers of silver and polymer. In silver layer plating, a SnCl 2 solution pre-treatment was used to sensitize the inner surface of the glass capillary. A silver layer with an RMS surface roughness of 8 nm was obtained comparing with 18 nm RMS roughness without the pre-treatment. In the liquid-phase coating of the polymer layer, an ambient air of organic solvent was used in the whole process. A smoother polymer layer with RMS roughness of 10 nm was obtained comparing with 20 nm roughness when a normal nitrogen gas-flow was used. The straight and bending loss properties for the hollow fibers have been measured by using a CO 2 laser and a red laser diode at the wavelengths of 10.6 μm and 0.63 μm. The straight losses were 0.4 dB and 1 dB for the hollow fiber (700 μmφ×1 m) made by the new developed fabrication techniques.