GENERATION AND CONTROL OF CHAOS FOR SECURE OPTICAL COMMUNICATION USING EDFRL

Abstract

Secure communication is the need of the time and chaos, due to its complexity of waveform and long time unpredictability, is used to hide message signal in electrical and optical domain. Optical Chaos can be generated using fiber laser, semiconductor laser and microchip lasers. In this thesis EDFRL parameter optimization and trend analysis has been performed to drive EDFRL into chaos from its other three possible regions of operation i.e. stable, periodic and quasi-periodic. Phase plot direct-observation method has been employed for qualitative validation of existence of chaos and Lyapunov exponents have been calculated for its quantitative measurement. The ability to generate large variety of chaotic sequences from same EDFRL configuration has been demonstrated with 10 sample sets of chaos at higher modulating frequencies. It is shown that modulation index, pump power and cavity gain are required to be increased and cavity loss to be decreased for generating chaos at higher modulating frequency. EDFRL Chaos waveform analysis has been done in a time expanded mode which will help in implementing EDFRL chaos based secure optical communication system. A square modulating signal with varying duty cycle has been successfully tested and found to show 16 times better Lyapunov exponent for a duty cycle of 60% as compared to that of pure sine modulating signal. An intermittency route to chaos in EDFRL is observed for the first time by variation of duty cycle. The capability to generate an infinite number of different chaotic sequences just by electrically changing the harmonic combinations of the modulating waveform is demonstrated and with a better LE than that of sine wave. The electrical flexibility to generate new chaos without optically modifying the cavity enhances the security at the field level where it is not easy to optically change the cavity frequently and the number of unknown parameters also increases. The comparative study of degree of security provided by EDFRL and semiconductor lasers based chaos generators is performed by calculating the average of positive Lyapunov Exponent spectrum and semiconductor laser is found to be 10 times better than EDFRL. Similarly within EDFRL, loss modulation scheme based pulsed chaos is better than the nonlinearities based non-pulsed chaos. The synchronisation properties of chaos propagation thru a 120 km link with dispersion compensation are studied with lumped and distributed EDFA and distributed amplification is found superior to lumped amplification in terms of Noise Figure. A new hybrid scheme based on polarisation interleaving with unequal channel spacing is proposed which reduces the number of FWM mixing terms falling on allocated DWDM channels from 124 to only 4 and also offsets the impairment made by Raman effect in a 8 channel DWDM chaos scheme. The step by step derivation of EDFRL rate equations used in this thesis is given in Appendix A. The equations for calculations of Lyapunov exponents are given in Appendix B. Appendix C gives the list of pertinent publications.