SRS and interpulse power limits to the energy extraction from pulsed kW Yb-doped fiber amplifiers

A space-time finite difference model is exercised to examine the performance of large mode area (LMA), high-power, Yb-doped pulsed fiber amplifiers. The simulations include forward and backward amplified spontaneous emission (ASE) and stimulated Raman scattering (SRS) and exhibit good agreement with experimental measurements. It is found that the dominant pulse energy limiting factor is the SRS, which is quantified by a Raman ratio R = 0.05 equal to the ratio of the peak Stokes power to peak signal power. Interpulse power at the signal wavelength due to ASE generated by the preamplifier and incomplete extinction of the modulated seed laser diode contribute to the reduction in the pulse-background contrast. However, this is not the dominant pulse power limiting mechanism at moderate background levels, 35 dB down from the pulse energy.