Theoretical analysis of effect of pump and signal wavelengths on modal instabilities in Yb-doped fiber amplifiers

摘要

We present, using numerical simulations, investigations of the modal instability thresholds in high-power Yb-doped fiber amplifiers. We use a time-dependent temperature solver coupled to the optical fields and population inversion equations to determine the temporal dynamics of the modal content of the signal as well as the modal instability threshold. Our numerical code is optimized to achieve fast computations; thus allowing us to perform efficient detailed numerical studies of fiber amplifiers ranging in lengths from 1-20 meters using various pump and seeding wavelengths. Simulation results indicate promising modal instability suppression through gain tailoring, tandem pumping, or through seeding at an appropriate wavelength. We examine the threshold of an amplifier pumped using fiber lasers operating at 1018 nm; similar to the multi-kilowatt single-mode fiber laser demonstrated by IPG. In this case, we show an increase in threshold of 370%. By simply seeding at other wavelengths, as low as 1030 nm, a 60% suppression of the modal instability threshold can also be realized. Furthermore, we show that gain tailoring is an effective mitigation technique leading to an appreciable suppression of the instability in a fiber design that has already been experimentally tested.