Temporal thermal response of Type II-IR fiber Bragg gratings.

We use the phase mask method to investigate both experimentally and theoretically the temporal thermal response of Type II-IR fiber Bragg gratings inscribed by a femtosecond laser. A fast testing system is developed to measure the thermal response time by means of periodic CO2 laser irradiation, which creates a rapid temperature change environment. The temporal thermal response is found to be independent of the heat power and the heat direction, although the grating produced destroys the axial symmetry of the fiber. The measured values of the temporal thermal response are approximately 230 ms for heating and approximately 275 ms for cooling, which different from the simulation results obtained from a lumped system equation. The causes of such differences are investigated in detail.