A Lyot depolarizer for depolarizing a laser with multiple longitudinal modes comprises two segments of PM fiber optimized for length as a function of the longitudinal mode spacing of the laser source, and not as taught in the prior art as a function of the laser coherence length. The present invention has found secondary coherence peaks in the coherence function of a multiple longitudinal mode source, which can significantly worsen the DoP if the PM fiber lengths are significantly extended beyond optimum, contrary to the teaching in the prior art. The present invention makes use of the periodic structure of the source spectrum by converting half the longitudinal modes to an orthogonal polarization state. As a result a significantly shorter fiber depolarizer is needed than for a single longitudinal mode source. Greater temperature stability is achieved by properly adjusting the lengths of both segments of PM fiber.