Effect of the spatial coherence of ultraviolet radiation (255 nm) on the fabrication efficiency of phase mask based fiber Bragg gratings

In this paper, we present a comparative study on fiber Bragg gratings fabrication efficiency namely the peak reflectivity and rate of growth in non-hydrogenated boron/germanium co-doped fiber for three writing UV beams (255 nm) of spatial coherence width varying from 30% to 100% of the full beam cross-section. All other UV-beam parameters viz. fluence, average power, spot size as well as the phase mask to fiber distance of 0.8 mm and grating length of 1 cm were kept same. The maximum reflectivity of about 31 dB was achieved, in about 210 s, for the UV beam having full spatial coherence. To the best of our knowledge, this is one of the best results for UV copper vapor laser (CVL) based system for non-hydrogenated FBGs writing. This high reflectivity is observed at about five times lower UV fluence than that normally reported. It is attributable to the vastly improved spatial coherence of UV beam, generated from the high spatial coherence, generalized diffraction filtered resonator based CVL.