In the experiment of laser based on semiconductor optical amplifier (SOA), when the drive current of the SOA is up to 220 mA, by adjusting the polarization controller (PC), dark pulse is observed. The pulse durations are 74 ns and 20 ns with frequency repetition rates of 9 MHz (fundamental frequency) and 33.4 MHz (double frequency), respectively. By changing the position of the SOA in the nonlinear amplifying loop mirror (NALM), dark pulse width change accordingly.