Passive and active mode locking of a semiconductor laser without an external cavity

This letter describes the first attempt to passively and actively mode lock a discrete semiconductor laser, i.e., one not coupled to an external cavity. Beat notes of the longitudinal modes of a 1.97-mm-long GaAlAs laser have been observed at 17.7 GHz. The spectral width of the beat note was approximately 100 kHz. Stable passive mode locking has been observed under appropriate operating conditions. Active mode locking by an externally injected microwave signal was also achieved.