Effect of Gain Nonlinearity on Time Delay Signature of Chaos in External-Cavity Semiconductor Lasers

The effects of gain nonlinearity on the time delay signature (TDS) of both intensity and phase chaos in semiconductor lasers (SLs) with optical feedback are investigated numerically. The SLs subject to single-path feedback are mainly considered, and SLs with dual-path feedback are also discussed for the sake of generality. The TDS is evaluated via both auto-correlation function (ACF) and permutation entropy function. It is found that the TDS is more sensitive to nonlinear gain factor (NGF) for large injection current and low feedback strength. For given feedback strength, the peak heights of ACF for both intensity and phase chaos around TDS can be decreased by using SL with low NGF, especially when the injection current is relative high. Furthermore, with low NGF, successful TDS concealment can be achieved in wider region in the parameter planes of feedback strength and injection current. Hence, careful selection of an internal parameter is an effective candidate for TDS concealment in wider parameter region, which is complementary to the existing techniques by controlling the external parameters, and thus is highly desirable for expanding the key space for chaos-based secure communication.