Spatial Mode Selection in a Passively Coupled Fiber Laser: Theory and Experiment

Abstract

In this thesis, self-phasing due to spatial mode selection in a two-element passively coupled fiber laser is studied. The fields emitted by a two-core ytterbium doped fiber are coherently combined with a Dammann grating in an external cavity. We measure the combined beam power and supermode relative phase in the presence of phase errors between the gain elements and find that implementation of spatial mode selection via beam recycling results in a 90% increase in the average output power and nearly π/2 radians of passive phase adjustment. We show that these results require a phase of zero (modulo 2π) between the beams in the external cavity. Otherwise, the average output power and the coherence of the laser decreases. These findings are supported by the results of an eigenmode analysis of the resonator. These results show that beam recycling is a useful resonator design feature but must be appropriately implemented to obtain beneficial results.