Fiber Bragg gratings in carbon-coated glass fibers for monitoring in harsh hydrogen environments

Carbon-coated optical glass fibers were designed to reduce the hydrogen-induced light attenuation (hydrogen darkening) in harsh environments. The carbon layer reduces the hydrogen permeability, but makes the post-treatment for direct point-by-point inscription of fiber Bragg gratings (FBG) more challenging. Moreover, the laser pulse treatment can greatly affect the tensile strength of the fiber. In this paper, we present direct point-by-point inscription of FBG through carbon-coated fiber layers with femtosecond laser pulses with a center wavelength of 400 nm, achieved via second harmonic generation of Ti:sapphire laser pulses. An array of four FBGs are successfully inscribed in a single-mode fiber. A direct comparison between the 400 nm and 800 nm inscribed FBGs in the carbon fibers is presented. The polarization dependence was examined of FBGs written with both laser processing wavelengths, as well as the mechanical stability of the processed fibers via tensile tests.