Chaotic time-delay signature suppression in a semiconductor laser with frequency-detuned grating feedback

Chaotic dynamics of a semiconductor laser subject to optical feedback from a frequency-detuned fiber Bragg grating (FBG) is investigated experimentally and numerically. Although the FBG is similar to a mirror in perturbing the laser into chaos, it is not the same as a mirror because it provides spatially distributed reflections. Such distributed reflections effectively suppress the undesirable time-delay signature (TDS) contained in the autocorrelation function of the chaotic intensity time-series. The investigation shows that the best suppression of TDS is attained when the FBG is positively detuned from the free-running laser frequency. The TDS suppression is due to dispersion at frequencies near an edge of the main lobe of the FBG reflectivity spectrum. The suppression prefers the FBG at a positive detuning frequency because the laser cavity is red-shifted by the antiguidance effect. Numerically, the dynamical behavior is mapped in the parameter space of detuning frequency and feedback strength, where wide regions of chaos are identified. Experimentally, the TDS suppression by FBG feedback is demonstrated for the first time. The positively detuned FBG suppresses the TDS by over ten times to below 0.04 in the experiments.