Title

Examination of acousto-optic chaos and application to RF signal encryption and recovery

Date of Award

2012

Degree Name

Ph.D. in Electrical Engineering

Department

Department of Electrical and Computer Engineering

Advisor/Chair

Advisor: Monish Chatterjee

Abstract

In communication systems, there are different coding schemes such as linear predictive coding, block coding, and veterbi coding that can be used to encode data for different purposes. One of the techniques to encode/encrypt data for security purposes is to use chaos. Chaotic systems can be manipulated, via arbitrarily small time-dependent perturbations, to generate controlled chaotic orbits whose symbolic representation corresponds to the encoding of a desirable message. An advantage of this type of communication strategy is that the nonlinear chaotic oscillator that generates the wave form for transmission can remain simple and efficient, while all the necessary electronics controlling encoding of the signal remain at the low-powered microelectronic level. Moreover, since the chaotic dynamics can be recovered from a chaotic signal, which in principle can be noisy, by using standard dynamical data analysis techniques, communicating with chaos is also more robust and better behaved against channel noise. Signal encryption and recovery using chaotic optical waves has been a subject of active research in the past 10 years. Since an acousto-optic Bragg cell with zeroth- and first-order feedback exhibits chaotic behavior past the threshold for bistability, such a system was examined for possible chaotic encryption using a low-amplitude sinusoidal signal applied via the bias input of the sound cell driver. Subsequent recovery of the message signal was carried out via a heterodyne strategy employing a locally generated chaotic carrier, with threshold parameters matched to the transmitting Bragg cell. The simulation results, though encouraging and extend to the following (i) increasing the chaos frequency using appropriate parameter control; (ii) carefully examining the system sensitivity to three system parameters, viz., feedback delay, feedback gain, and dc bias level; (iii) examine signal recoverability relative to shifts in the three parameters mentioned above relative to the transmitter; and (iv) determining the robustness of such a system relative to the primary transmitter parameters. Additionally, we consider also the effect of the additive bandpass noise (obtained from white Gaussian noise in the simulator) on signal recovery in such a system from a performance standpoint. It is also conjectured that signal recovery can be effected by passing the modulated light through a second sound cell in a matched chaotic regime. Since an acousto-optic Bragg cell with zeroth- and first-order feedback exhibits chaotic behavior past the threshold for bistability, such a system was recently examined for possible chaotic encryption of simple messages such as a low-amplitude sinusoidal signal applied via the bias input of the sound cell driver. Also, the nonlinear dynamics of the A-O feedback including the effect of the parameters such as feedback gain, dc bias, and time delay are examined in some detail taking into consideration that the intensity amplitude equals 1 and I1(0) = 0. The results obtained via computer simulation reveal variety of interesting dynamics including bistability, bifurcation, and chaos. Also, Lyapunov exponents have been generated for variety of parameters. However, Lyapunov and Bifurcation maps with varying the three parameters are going to vary if the light intensity and I1(0) were varied.

Keywords

Data encryption (Computer science) Research, Chaotic behavior in systems Research, Acoustooptics Security measures Research, Signal processing Digital techniques, Signal processing Data processing, Signal processing Research

Rights Statement

Copyright 2012, author

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