Non-monotonous refractive index changes recorded in a phosphate glass optical fibre using 248nm, 500fs laser radiation

Results are presented on the photosensitivity behavior of a phosphate glass optical fibre using 248nm, 500fs laser radiation. Bragg grating exposures performed using a double phase mask interferometer and peak intensities of 0.37TW/cm super(2), revealed that grating growth becomes non-monotonic in terms of average and modulated refractive index changes, resembling the Type IIA photosensitivity behavior. Average refractive index changes greater than 10 super(-3) were measured after accumulated energy density doses of 6.5KJ/cm super(2). The Bragg gratings inscribed maintained significant part of their strength up to 377 degree C. Exposed and side-polished fibre samples were subjected to Knoop micro-hardness measurements for revealing that the irradiated glass undergoes significant volume dilation.