Abstract

We report the observation of enhanced photosensitivity in fibers exposed to a 12-atmosphere hydrogen pressure and 400C heat for 4 hours. Photosensitivity was measured with two different techniques. First, short Bragg gratings (1 cm long) were written with counterpropagating pulses from a mode- locked continuous-wave frequency-doubled Nd:YAG laser. The 80 ps pulses meet at the middle of a short strand of single-mode fiber, and the standing wave induces the formation of the grating. This portion of the fiber is maintained at a constant temperature. The grating reflectivity is measured by means of time-domain reflectometry with a fast photodiode and a sampling oscilloscope (resolution of 75 ps). Reflectivities as low as 0.1% are easily detectable. The rate of grating formation was found to be much faster in the treated fiber than in the untreated fiber. Our second technique uses the fact that the index change follows the polarization of the writing light. Thus, polarized light from the doubled Nd:YAG laser was sent through a short fiber strand, and we measured the transmission through an analyzer of a HeNe laser probe that was polarized at 45 with respect to the green beam. This technique minimizes the influence of thermal effects. Again, an increase by as much as a factor of five in the polarization rotation rate was observed in the treated fiber.

© 1990 Optical Society of America