Spatiotemporal complexity of chaos in a phase-conjugate feedback laser system

An 852 nm semiconductor laser is experimentally subjected. to phase-conjugate time-delayed feedback achieved through four-wave mixing in a photorefractive (BaTiO3) crystal. Permutation entropy (PE) is used to uncover distinctive temporal signatures corresponding to the sub-harmonics of the round-trip time and the relaxation oscillations. Complex spatiotemporal outputs with high PE mostly upwards of similar to 0.85 and chaos bandwidth (BW) up to similar to 31 GHz are observed over feedback strengths up to 7%. The low-feedback region counterintuitively exhibits spatiotemporal reorganization, and the variation in the chaos BW is restricted within a small range of 1.66 GHz, marking the transition between the dynamics driven by the relaxation oscillations and the external cavity round-trip time. The immunity of the chaos BW and. the complexity against such spatiotemporal reorganization show promise as an excellent candidate for secure communication applications. (C) 2020 Optical Society of America