A two-stage fiber-based femtosecond amplification system is presented, based on chirped-pulse amplification in highly nonlinear regime. The amount of self-phase modulation is separately adjusted in each stage selecting the proper stretching ratio in order to compensate gain narrowing. Analytical design rules are validated using numerical simulations. Our experimental implementation leads to the generation of high temporal quality 20 µJ 202 fs pulses at repetition rate of 200 kHz, a record duration at this energy level.
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