Femtosecond laser processing of fiber Bragg gratings with photo-induced gradient-index assisted focusing

摘要

Gradient-index modulation prefabrication in the cladding of standard telecom fibers was proposed for femtosecond laser processing of Type II-IR fiber Bragg gratings. The refractive index of the prefabricated region in the cladding had a large gradient across the laser beam, which could act as a cylindrical lens to enhance the focusing of laser beam when writing Type II-IR gratings. With the help of prefabrication, the threshold pulse energy for processing Type II-IR FBGs was lowered from 750μJ to 520μJ. The fabricated FBGs showed good thermal stability at temperatures over 900℃.