A high pulse energy femtosecond laser amplifier with Yb³⁺-doped single polarization large-mode-area photonic crystal fibers (LMA-PCF) is demonstrated.Based on the nonlinear amplifying configuration,amplification of laser pulses in the LMA-PCF is accompanied by spectral broadening and nonlinear phase-shift accumulation due to the self-phase modulation.This configuration can often help to produce high-peak-power ultrashort laser pulses,since the self-phase modulation induced nonlinear phase shift can be compensated by an appropriate negative-dispersion component following the amplification stage.This strategy of ultrashort laser pulse generation requires,however,a precise balance between the gain,nonlinearity,and high-order dispersion in the oscillator-amplifier system.An acousto-optical modulator is applied to reduce the repetition rate to 1 MHz.Laser pulses with 124 fs duration(the shortest pulse) are generated with 1.56 μJ pulse energy,corresponding to 12.6 MW peak power.