Yb fiber vortex laser using an interferometric mode converting output coupler

Abstract

We apply mode transformation to an Yb fiber laser for direct generation of a first order vortex mode (LG 01 ), yielding LG 01 power of 5 W at 96 % purity (from modal decomposition) with 16 W pumping. The laser used standard single mode Yb doped fibers operating at 1064 nm. A free-space Sagnac interferometer formed one reflector of the cavity by feeding back the internal Gaussian mode of the fiber laser and output coupling a LG 01 via interferometric mode transformation. It was stable over hours of operation and days of inactivity, and was insensitive to polarisation. The maximum output power was only limited through heating of a optical element, which could be mitigated with thermal management. We also show that additional spiral phase plates (SPPs) are a route to higher purity, higher order vortex modes than with SPPs alone due to improved intensity matching between LG 01 and higher order states.