Performance comparison of phase mask interferometers for writing fiber Bragg gratings with femtosecond pulses

Using the Kostenbauder matrix formalism, we analyze the complex spatio-temporal behaviour of focused femtosecond pulses propagating in three tunable phase mask interferometers for writing fiber Bragg gratings. In interferometers that use a second half-period mask to reverse the phase front, we show that at zero detuning, and with a proper amount of pre-dispersion, the pulses can be perfectly focused on the fiber, whereas with a Talbot interferometer, the pulses suffer from large, unavoidable spatial and temporal broadening. We quantify the enhancement of grating writing efficiency with focusing as a function of beam diameter and interferometer detuning, for a photoinduced index change that is due to a two-photon or higher order process. Based on previously published experimental results, we estimate the potential improvement in grating writing speed. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement