Directly photoinscribed refractive index change and Bragg gratings in Ohara WMS-15 glass ceramic

Abstract

We inscribed thick volume gratings in WMS-15 glass ceramic by ultraviolet light at 193 and 248 nm. Unlike earlier work in ceramic materials, the inscription process modified the optical properties of the material without the need for any additional chemical or thermal processing. Experimental evidence from measurements of grating growth, thermal annealing, and spectral absorption indicates that two distinct physical mechanisms are responsible for the grating formation. Weak, easily thermally bleached gratings resulted from exposure fluences below 0:3 kJ=cm 2 . Optical absorption measurements suggest that these low fluence gratings are predominantly absorption gratings. More thermally stable gratings, found to be refractive index gratings with unsaturated refractive index modulation amplitude as large as 6 × 10 −5 were formed at cumulative fluences of 1 kJ=cm 2 and above.

Effect of the refractive index change kinetics of photosensitive materials on the diffraction efficiency of reflecting Bragg gratings

“…It was shown theoretically and confirmed experimentally that the reflection coefficient of such VBGs is completely determined by amplitude of refractive index modulation (RIM), thickness, and incident angle. Based on these results, people have commonly used the measured evolution of diffraction efficiency of Bragg gratings recorded in photosensitive materials (bulk or fibers) to characterize its RIC kinetics [10,11]. It is obvious that while a linear response of the photosensitive medium results in a RIM profile identical to that of the interference pattern (i.e., perfectly sinusoidal), a nonlinear response of the recording medium should cause distortions in the spatial profile of RIM.…”