Cavity Requirements for Realizing High-Efficiency, Tm/Ho-doped, Coaxial Fiber Laser Systems.

Coaxial fiber lasers, consisting of a Ho-doped core surrounded by a Tm-doped ring, are studied via experiments and numerical simulations. Previous simulations indicated that coaxial fiber lasers have the potential to reach power conversion efficiencies of up to 54%, but experiments have yielded much lower efficiencies. To understand this difference, a wavelength dimension is added to the previous model. The new simulations explain the discrepancies with the experiments via spectral gain competition and lead to a path to optimize experimental efficiencies. Specifically, an output coupler is spectrally designed to optimize the efficiency of coaxial fiber systems, and a path towards realizing the predicted 54% efficiency is presented. The results indicate that the Tm/Ho coaxial fiber laser has the potential to be a compact and efficient means of producing 2100 nm radiation.