Demonstration of negative refractive index photosensitivity mechanism in a phosphate glass optical fibre using 248nm, 500fs laser radiation

摘要

Several studies have been presented on the photosensitivity and index engineering of phosphate glasses using high intensity ultraviolet [1, 2] lasers. These studies are of particular interest due to the increased number of applications of rare-earth codoped phosphate glasses in the development of high efficiency planar and fibre active components. We present results on the Bragg grating recording behavior and photosensitivity of an all-phosphate glass rare earth doped optical fibre, utilizing 248nm, 500fs laser radiation. In the grating inscription results presented, non-monotonic refractive index changes with respect to accumulated energy density are observed. This unique behavior resembles that of the Type IIA photosensitivity mechanism that usually occurs during grating recording in high-Ge doped silicate glass optical fibres using deep ultraviolet laser sources; however, never been observed before in grating recording in soft glasses, including phosphate glass fibres. Moreover, the origin of the negative refractive index changes induced, is investigated utilizing Knoop micro-hardness (HK) measurements, performed in irradiated and side-polished fibres [3]. The Bragg grating inscription was performed using a 248nm, 500-fs KrF excimer-dye laser system, at a 10Hz repetition rate. The experimental set-up used for grating recording was a double phase mask interferometer, being described elsewhere [4]. A single mode phosphate glass, Er/Yb codoped phosphate glass optical fibre, manufactured by INO-Canada, was exposed. The concentration of the rare earth dopants in the fibre are 0.90wt% for Er, and 7.0wt% for Yb.