Quasi-parabolic pulses in a coherent nonlinear optical amplifier.

We show that, in a coherent two-level optical amplifier with Kerr nonlinearity and linear loss, any weak seed pulse evolves into a fixed powerful linearly chirped pulse with quasi-parabolic shape. This process is associated with a transition from the incoherent into the coherent amplification regime, thus enabling in practice the generation of pulses with a spectrum wider than the linear gain bandwidth.