Key Laboratory of Film Electronics and Communication Devices. In order to obtain double-pulse with tunable pulse width, a scheme of laser structure based on semiconductor optical amplifier (SOA) is proposed. In this scheme, the SOA is put asymmetric in the nonlinear optical fiber loop mirror, pulse produced by nonlinear polarization rotation of the SOA. when the drive current of the SOA is up to 200 mA, through adjusting the polarization controller, double-pulses with pulse durations of 33.40 ns and 30.08 ns, with frequency repetitions of 10.05 MHz and 12.70 MHz are achieved, respectively. By changing the cavity length of the laser and the position of the SOA in the nonlinear amplifying loop mirror (NALM), double-pulse width changes accordingly due to the switch characteristics of the NALM.