Type 1A fibre Bragg grating photosensitivity and the development of optimum temperature invariant type I - type 1A strain sensors

Type 1A fibre Bragg gratings (FBG) form only after the erasure of a standard grating in hydrogenated germanosilicate fibre, under prolonged UV exposure. They are distinct from other grating types as they exhibit a uniquely large increase in the mean index of the core, readily identifiable by a large red shift in the Bragg wavelength. Type 1A gratings can surpass the conventional mean index change by a factor of six with a typical red shift of up to 20nm, interpreted as a mean index increase of up to 1.9x10(-2). Importantly, 1A gratings have been shown to exhibit the lowest temperature coefficient of all FBG, which makes them ideal for use as temperature compensated, dual grating sensors. We report on the formation of Type 1A gratings and the correlation between the mean index change of the grating and the growth of a loss band at close to 1400nm that is associated with the formation of OH centres within the fibre. We present annealing data comparing the decay of Type I and Type 1A gratings. Finally, we demonstrate a dual temperature compensated strain sensor system, based on two adjoining Type 1 and Type 1A gratings, which have been formed using a common phase mask, yet with central wavelengths many nm apart.