Interference filters and their impact on FM-to-AM conversion: a method for investigation and characterization.

Abstract

High-power nanosecond laser pulses are usually spectrally broadened via temporal phase modulations to tackle the issue of stimulated Brillouin scattering and to achieve optical smoothing of the focal spot. While propagating along the beamline, such pulses can undergo frequency modulation to amplitude modulation (FM-to-AM) conversion. This phenomenon induces modulations of the optical power that can have a strong impact on laser performance. Interference filters are specific FM-to-AM conversion contributors that lead to high-frequency modulations that cannot be measured using conventional means. We propose an indirect method to investigate for such FM-to-AM contributors using spectral measurements. Further analysis of the collected data makes the quantification of the defining parameters of interference filters possible. In turn, we show that it is possible to estimate the range of power modulations induced by interference filters.