Spectral line broadening in depolarized, high-power narrow-linewidth fiber lasers

High power, narrow-linewidth fiber lasers with depolarized output have emerged as an important source for beam-combining. Depolarization assists in enhancing output power per module by reducing Brillouin and modal instability effects. However, it has been observed that in depolarized lasers there is Kerr-type linewidth broadening with increasing power which is not common in polarized lasers. The source of intensity noise necessary to create this Kerr linewidth broadening is unclear. Here we demonstrate that polarization dependent loss in the amplifier chain is a main source of intensity noise and show its effects theoretically and experimentally in a 2kW, 10GHz, packaged laser module.