A femtosecond fiber system based on nonlinear chirped-pulse amplification is investigated. It is the first investigation on
pulse properties from all-normal-dispersion fiber laser after amplification. Nonlinearities due to fibers are carefully
managed. The system generates up to 5-μJ pulses, and delivers near diffraction-limit beam (M2 < 1.1), polarization
extinction ratio (40 dB) and polarization extinction ratio (36 dB).
The compensation of nonlinear phase shifts by dispersion in femtosecond fiber amplifiers is explained. Contrary to previous understanding, a chirped-pulse fiber amplifier with mismatched stretcher and compressor can out-perform a matched system when the pulse acquires a significant nonlinear phase shift.