Mode instability thresholds for Tm-doped fiber amplifiers pumped at 790 nm

We use a detailed numerical model of stimulated thermal Rayleigh scattering to compute mode instability thresholds in Tm(3+)-doped fiber amplifiers. The fiber amplifies 2040 nm light using a 790 nm pump. The cross-relaxation process is strong, permitting power efficiencies of 60%. The predicted instability thresholds are compared with those in similar Yb(3+)-doped fiber amplifiers with 976 nm pump and 1060 nm signal, and are found to be higher, even though the heat load is much higher in Tm-doped amplifiers. The higher threshold in the Tm-doped fiber is attributed to its longer signal wavelength, and to stronger gain saturation, due in part to cross-relaxation heating.