Design of a viable radiation-balanced fiber laser or amplifier

Recent advances in power scaling of fiber lasers are hindered by the transverse mode instability (TMI), which deteriorates the output beam quality. TMI is blamed on the overheating of the fibers. Anti-Stokes fluorescence (ASF) cooling of the rare-earth-doped gain material has been suggested as a potentially viable heat removal scheme. In this scheme, the fiber is pumped at a pump wavelength, which is longer than the mean fluorescence wavelength of the active ions. Therefore, the fluorescence removes some of the excess heat. It may be possible even to balance the ASF cooling against all other sources of heating and make a non-heating radiation-balanced laser (RBL). I will focus on the general scaling laws that govern the design of RBLs. I will show that the magnitude of the parasitic absorption plays an essential role in the feasibility of such designs, especially in a double-cladding setting, and the measures that need to be taken to achieve net heat-balancing in a viable RBL.

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