Overlapped Pulsed Pumping of Tandem Pumped Fiber Amplifiers to Increase Achievable Pulse Energy

It has been reported previously that in the regime appropriate for amplifying femtosecond pulses using the chirped pulse amplification technique in Yb-fiber sources that sub-micro-second pulsed tandem pumping not only provides the thermal benefits of c.w. tandem pumping, but also enables strong suppression of ASE. In that case, the pump pulse preceded the signal pulse train. Here, we propose a tandem pumping scheme in rare-earth-doped fiber amplifiers, where a train of signal pulses is amplified by a pump pulse, which is almost exactly temporally overlapped. Simulations demonstrate that this can be used to create uniform gain across the signal pulse train, even at very high total pulse energies, where there would be significant gain shaping in the previous case. In addition, the pump is absorbed in a much shorter length, which increases the threshold for nonlinear effects and gain of greater than 26 dB is shown to be readily achievable in an amplifier as short as 1.5 m. This results in increased extractable energy before reaching the threshold for limiting nonlinear effects, such as stimulated Raman scattering. These attributes should be attractive for high energy, high average power, ultrashort pulse, coherently combined fiber laser systems.