Optical short pulses are generated by active mode-locking with a laser that is not coated for anti-reflection. The optimum condition to generate the shortest pulse is that the resonance frequency of laser diode coincides with the external modulation frequency. Optical pulses with 70ps FWHM is generated under the optimum condition. Resonance frequency shift and multiple feedback effects are observed. The amount of optical feedback is estimated to be small, by measuring the variation of threshold current.