Measurement of the induced refractive index in a photothermorefractive glass by a liquid-cell shearing interferometer

A liquid-cell shearing interferometer was developed to measure refractive-index variations (Δn) in transparent materials. The cell was filled with a liquid having a matched refractive index. The achieved resolution was better than 1/1000 of a fringe shift and resulted in a Δn measurement sensitivity down to 10-7 for 1-mm-thick samples. A refractive-index increment in photothermorefractive glass of up to 5 x 10-6 was observed after UV exposure at 325 nm. A refractive-index decrement of up to 1 x 10-3 was observed after thermal development of the exposed sample. It was proved that photothermorefractive glass obeys the reciprocity law; i.e., Δn depends on the UV dosage but does not depend on the irradiance