Abstract

Optical chaos is easily observed in a laser diode with either conventional (COF) or phase-conjugate optical feedback (PCF). We focus here on the so-called chaotic low-frequency fluctuations (LFF) where the laser exhibits slow power dropouts together with fast picosecond dynamics. We report here on a systematic study of LFF in a laser diode when subjected to PCF for which the bifurcations leading to those LFF remain to be elucidated. Qualitatively different dynamics are observed when increasing the phase-conjugate mirror (PCM) reflectivity or the injection current level. A combination of spectral and temporal measurements unveils the transition to LFF dynamics. The statistical properties of the time between power dropouts -mean dropout time and standard deviation- are studied as a function of the laser and feedback parameters and show that the laser may undergo what we call here a deterministic coherence resonance for a particular value of the PCF ratio. Interestingly, the phase conjugation build-up time here in SPS crystal is around 3 ms, hence is three orders of magnitude faster than what has been reported in previous dynamical studies of chaos in lasers with PCF using BaTiO<sub>3</sub> photorefractive crystals. How the phase conjugation time-scale impacts on the laser dynamics remains an interesting question to which we contribute here.

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