Study of spectral and annealing properties of fiber Bragg gratings written in H₂-free and H₂- loaded fibers by use of femtosecond laser pulses

Abstract

The spectral and annealing properties of a series of fiber Bragg gratings (FBGs) written in both H₂-loaded and H₂-free fibers by use of 800nm femtosecond laser pulse irradiation and created through a phase mask, have been investigated. It is found that type II FBGs inscribed in H₂-loaded fibers exhibit superior spectral quality when compared with those written in H₂-free fibers. Isochronal annealing tests shows that type II FBGs written in H₂-free fibers have the highest thermal stability, followed (in order of stability) by H₂-loaded type II, H₂-free type I and then H₂-loaded type I FBGs. The thermal stability of the H₂-loaded type II FBGs can effectively be increased by using a high temperature pre-annealing treatment. After the treatment, type II FBGs written into both H₂-free and H₂-loaded fibers can sustain long-term annealing (for more than 12 hours) at temperatures of more than 1000°C while their high reflectivities can still be maintained. This demonstrates the real potential of the FBGs developed and investigated in this work to be used as the ideal sensing elements for a series of high temperature measurement applications.