FBG inscription by a biprism interferometer: analysis and experiment

Abstract

In this paper, the effect of spatial variation of fringe visibility on the length, reflectivity and wavelength tuning of fiber Bragg gratings (FBGs), written by a biprism interferometer, is analyzed. The variation of fringe visibility and usable fringe area are the cumulative effect of spectral and spatial frequency spectrum of the laser source, beam profile and fringe stability. It is analytically shown that with the increase in the distance of FBG writing plane from the biprism, the saturation of UV fringes-induced refractive index modulation decreases, whereas the reflectivity of FBGs inscribed will pass through a maximum. Fiber Bragg gratings are written by a UV beam (255 nm, 5.6 kHz, 30 ns) at different distances from a 24o refraction angle biprism. The trends on experimental results matched the analysis. The maximum reflectivities observed are of 93.6 % (12 dB) for FBGs in Ge-B co-doped fiber and 99.8 % (29 dB) for FBGs in hydrogen-loaded SMF-28 fiber, attributed to the consideration of spatial variation of fringe visibility in the biprism fringe depth on the optimization of FBG reflectivity. These are incidentally the highest values, so far reported, for FBGs written by a biprism interferometer.