Abstract
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.
Highlights
Kilowatt signal power levels have been achieved in Yb3+-doped fiber amplifiers operating near 1.0 μm and in Tm3+-doped fiber amplifiers operating near 2.0 μm
Our model indicates that a partial cancellation between the cross relaxation and nonradiative decay heat terms contribute to the strong saturation in the Tm-doped fiber
We have extended our model of stimulated thermal Rayleigh scattering (STRS) in fiber amplifiers to Tm-doped fibers pumped at 790 nm, and found mode instability thresholds to be significantly higher than those of similar Yb-doped fibers despite several times greater heatload
Summary
Kilowatt signal power levels have been achieved in Yb3+-doped fiber amplifiers operating near 1.0 μm and in Tm3+-doped fiber amplifiers operating near 2.0 μm. High power Tm amplifiers pumped at 790 nm have been demonstrated by Ehrenreich et al [6] who achieved 1 kW at 2045 nm in a 12 m long 20:400 (core:pump cladding diameters) Tm-doped silicate fiber with numerical aperture (NA) of 0.08 (V=3.1) Measured power efficiencies are greater than 60%, and this is attributed to a cross-relaxation process This process must be included in an accurate Tmdoped fiber amplifier STRS model. This reduces the antisymmetric part of the heat and pushes it toward the outer edge of the doped region. The concentration of Tm can be as high as 7%, 4-6% seems to be the most common level [18,19,20]
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