Ultrafast high energy amplifiers beyond the B-integral limit

High average power single-mode fiber lasers have attracted significant attention as alternatives to conventional solidstate lasers owing to their relative high brightness, compactness and robustness. Likewise the turn-key operation of industrially qualified ultrafast fiber oscillators is well established. In recent years the convergence of reliable ultrafast fiber oscillators, high brightness pump diodes and high power fiber amplifiers has enabled ultrafast fiber lasers to surpass ultrafast solid-state lasers in terms of average power. While fiber lasers have generally not been able to match the ultrashort pulse energies produced by solid-state lasers, careful management of nonlinearities can overcome the conventional B-integral limit of π thereby permitting stable operation of practical ultrafast fiber lasers with pulse energies approaching the milli-Joule level. Here we review modes of nonlinear propagation in fibers which have enabled increases in ultrashort pulse energies from nano-Joule to milli-Joule levels, namely: solitons, similaritons and cubicons. As an example of a practical high energy ultrafast fiber laser, we demonstrate a cubicon Yb fiber chirped pulse amplification system producing 550 fs pulses with 50 μJ at >15 W.