Spectral properties and synchronization scenarios of two mutually delay-coupled semiconductor lasers

We experimentally study the intensity dynamics and the optical spectra of two similar mutually delay-coupled semiconductor lasers. While changing two relevant parameters of the coupled system, namely the bias currents and the coupling strength of the lasers, we observe significant modifications in the laser emission characterized via optical spectra, laser intensity dynamics, and cross-correlation functions. We find distinct synchronization scenarios for two bias currents: 1.02 and 1.25 times the threshold bias current of the solitary laser. Optical spectra in both cases strongly depend on the coupling strength between the lasers. For the low bias current and strong coupling, the lasers exhibit dynamical bistability, i.e. the coexistence of low-frequency fluctuations and stable continuous-wave emission.