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 standard model and simulations are compared to experiments. An output coupler is designed to optimize the efficiency of coaxial fiber systems, and a path towards realizing the predicted 54% efficiency is presented.
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