Response characteristics of thin-film-heated tunable fiber Bragg gratings

We investigate the thermal response of a tunable fiber Bragg grating (FBG) with a metal coating on bare fiber surface as a heater both theoretically and experimentally. By solving a differential equation of temperature as a function of time, together with some reasonable approximation, we obtained an explicit description of the thermal response characteristics with the structure´s heat capacity and a time constant as its parameters. Using a squared wave modulation current and a tunable laser, we measured the temporal response of the tunable FBG. The obtained response curves match our solution. A time constant in the range of subseconds was deduced. Discussions on the tuned FBG spectra are also given at the end to explain the linewidth broadening effect at higher temperature.