Complex spatio-temporal polarization dynamics of a broad-area VCSEL in continuous-wave operation

Abstract

The dynamics of vertical-cavity surface emitting lasers (VCSELs) is interesting since they exhibit modes with different polarization states and can exhibit ultrafast dynamics. Chaotic dynamics and polarization switching have been investigated in VCSELs with a single spatial mode, as well as their use for dual-task reservoir computing. Broad-area VCSELs feature additionally multiple transverse modes. Both the formation of static spatial structures in free-running broad-area VCSELs and cavity solitons in an external cavity have been studied. There are, however, very few experimental works on the spatio-temporal dynamics of free-running broad-area VCSELs. We present an experimental study of the complex ultrafast dynamics of a broad-area VCSEL under continuous-wave electrical pumping in the spatial, spectral and polarization domains. The VCSEL has a 15 µm wide circular aperture and exhibits multi-mode lasing in different transverse modes. The lasing modes feature two orthogonal linear polarization states, and the dominant polarization as well as the active transverse modes change multiple times with increasing pump current. The switching of the polarization and the transverse modes are thermally induced and correlated with one another, hinting at coupling of the spatial and polarization degrees of freedom via the active medium. The polarization switching points are accompanied by changes in the temporal dynamics, with series of bifurcations yielding increasingly complex temporal dynamics. The dominant frequency of the unstable temporal dynamics is similar to the frequency splitting between modes of different polarization, suggesting an important role of the birefringence in the laser dynamics. The combined analysis of the spatial, spectral and polarization data shows a complex interplay of the different spatial and polarization modes and opens the way for possible applications for example in neuromorphic computing.