Optimal operating conditions and security considerations for optical chaos communications

This thesis details research on chaos-based optical communications using semiconductor lasers. Theoretical work has identified optimal operating condition of optical chaos communications systems without the need for re-adjustment of laser operating conditions in the field. In addition, an experimental investigation has been undertaken of one aspect of the security of message transmission using chaotic semiconductor lasers and specifically using Vertical-Cavity Surface-Emitting Laser (VCSEL). In optical chaos communications a message is masked in the noise-like broadband output of a chaotic transmitter laser, and message recovery is enabled through the synchronization of the transmitter and the (chaotic) receiver laser. Key issues are to identify the laser operating conditions which provide the highest quality synchronization conditions and those which provide optimized message extraction. In general such operating conditions are not coincident. In this thesis numerical simulations are performed with the aim of identifying a regime of operation where the highest quality synchronization and optimizing message extraction efficiency are achieved simultaneously for the analogue and digital message modulation. Use of such an operating regime will facilitate practical deployment of optical chaos communications systems without the need for re-adjustment of laser operating conditions in the field. In this way it has been found, for example, that in an optical chaos communication system an optimal configuration for 1 GHz modulation frequency may utilize a 2% modulation depth and 20% coupling rate from transmitter laser to receiver laser for analogue message modulation. Ideal operating conditions for digital message modulation, identified using the laser bias current as the control parameter, are found for 2GB/s message transmission and a modulation depth of about 2%. An experimental study has also been undertaken to determine the effect of time delay and concealment in the external cavity semiconductor laser systems. The time delay (TD) signature arises due to the external cavity round trip time of feedback in the external cavity semiconductor laser system. The identification of the time delay may affect the security of encoded messages. An external cavity VCSEL with variable optical-polarization-angle feedback has been explored experimentally in order to identify conditions for time delay concealment. In this work the VCSEL was subject not only to variable optical polarization angle of feedback but also variable optical feedback strength and bias current. The TD signature concealment is evaluated through the use of an autocorrelation function and an information theory-based permutation entropy function. It was found that the TD signature is concealed at low feedback strength of order –18 dB. At moderate feedback strengths, the TD signature is sensitive to the rotation of the polarization angle of the optical feedback with TD concealment being observed for polarization angles in the range from 45◦ to 90◦