Spontaneous phase-locking of transverse modes in a broad-area laser

Broad-area edge-emitting semiconductor lasers are well known for their spatio-temporal instabilities caused by the interaction of multiple transverse modes. We perform high-resolution spatio-spectral measurements using heterodyning of a 50 µm wide broad-area laser to obtain the mode profiles and linewidths of all transverse modes. First, we find that the profiles of certain transverse modes depend significantly on the pump current, which we attribute to carrier-induced index changes and thermal lensing effects. Second, we observe the formation of multiplets of 1st and 2nd order transverse modes which generate a narrow RF beat note. Since the linewidths of the involved modes are much larger than that of the beat note, we conclude that these transverse modes are phase locked. The analysis of the phase fluctuations of the heterodyning time traces confirms the observation of spontaneous phase-locking of transverse modes in a broad-area laser.