Observation of mode beating and self-frequency locking in a nearly single-mode semiconductor laser

The beat spectra between the adjacent modes of a semiconductor laser having no external cavity are reported. Passive beat-frequency locking is observed at high power without a saturable absorber. This self-induced locking is manifested by the dramatic narrowing (by four orders of magnitude) of the beat spectrum between the dominant and adjacent modes. It is accompanied by the transition from nearly single-mode behavior to a highly structured multifrequency optical spectrum