In experiment of semiconductor optical amplifier based laser, the phenomenon of self-mode locking is observed. Pulse duration of 30 ns and 50 ns are achieved with repetition frequencies of 19.34 MHz and 9.67 MHz, respectively. The laser adopts a ring cavity configuration, with semiconductor optical amplifier used as key device of the self-mode locking. When the pump current adds to 200 mA, self-mode locking pulse is observed after adjusting the polarization controller. In addition, the phenomenon of dimidiate frequency is abserved, that is the repetition frequency of the pulses is equal to half of the cavity fundamental frequency. The effect of polarization rotation caused by the semiconductor optical amplifier leads to the change in output power, which is, the alternation of pulses with high power and low power, which is called the phenomenon of dimidiate frequency.