Theoretical analysis of time delay echoes in multimode semiconductor diode laser with optical feedback

Development of secure optical communication is based on external cavity semiconductor diode lasers. The signature of the external cavity is identified by time delay echoes (TDEs). Time delay echoes (TDEs) of the external cavity lasers are one of the critical parameters related to security of information exchange and its signature requires suppression. In this article, time delay echoes of a multimode semiconductor diode laser subjected to external optical feedback (EOF) are investigated. Lang-Kobayashi rate equations are adopted for multimode laser, and they are numerically solved to obtain temporal evolutions of laser field. Autocorrelation and mutual information of the intensity time series of the laser output are carried out to obtain the TDEs. The shifts in the position of time delay echoes are found to depend on the external cavity reflectivity, cavity delay time and multimode character. This dependence elucidates the provisions to enhance the security of the system consisting of semiconductor laser with feedback.