Title

Secure chaotic transmission of digital and analog signals under profiled beam propagation in acousto-optic Bragg cells with feedback

Date of Award

2015

Degree Name

Ph.D. in Electrical Engineering

Department

Department of Electrical and Computer Engineering

Advisor/Chair

Advisor: Monish Ranjan Chatterjee

Abstract

Modern day electronic communication is often prone to hacking within the communication system. Signal encryption using chaotic waves is a promising solution to circumvent this problem. While standard modulation can be readily intercepted, decoded and manipulated, modulation with chaotic signals is secure and cannot be decoded without knowledge of the chaos parameters. In recent work, acousto-optic (AO) Bragg diffraction has been shown to be a practical technique for chaotic modulation because the first-order intensity becomes effectively an amplitude-modulated chaotic wave. Previous research on this topic assumed the standard uniform light and sound model commonly applied to Bragg interactions. While these investigations clearly demonstrated the feasibility of encrypting, transmitting and recovering relatively low-bandwidth AC signals (up to about 1 MHz), the results were essentially based on the assumption of uniform optical and acoustic beams. Since practical optical beams are more likely to be non-uniform in profile, and chaos is extremely sensitive to amplitudes, it becomes necessary to examine the consequences of specific profiled light beams upon the feedback system under examination. The initial work completed for this research is an examination of profiled beam diffraction through an acousto-optic Bragg cell. This is a necessary first step because the scattered outputs from profiled beam propagation will impact the closed-loop behavior of the corresponding nonlinear dynamical system. Since such a feedback system is known to exhibit chaotic behavior, and the latter is highly sensitive to input amplitudes and initial conditions, it is necessary to characterize the scattered outputs for profiled beams for greater accuracy in our understanding of the dynamical system and improved realization of signal encryption. A previously-derived transfer function formalism is used to numerically evaluate the scattered first-order output of an acousto-optic Bragg cell with a profiled plane wave input. Subsequently, the numerical data for the output profile (as a function of the incident beamwidth, the sound pressure or optical phase shift parameter within the cell, and the applicable Klein-Cook parameter Q) is incorporated in a hybrid acousto-optic feedback model whereby the first-order output is detected, amplified and added to the bias input of the RF source of the sound cell. The result shows that the nonlinear dynamical thresholds for the hybrid cell are significantly different for non-uniform beams relative to the uniform case. The mono- and bistable- regimes do not coincide with the well-known uniform plane wave results and the chaotic thresholds, which are critical to understanding encryption applications, are altered noticeably. Encryption with a chaotic carrier wave is very sensitive to the closed-loop parameters involved in creating the chaos. Recovery of the original signal from the chaotically modulated signal requires the precise character set: the Klein-Cook parameter Q, the bias input, feedback gain and time delay Td. This set of parameters serves as a decoding key, providing data-security and reliability. Robustness tests indicate that chaotic encryption with profiled input beams is significantly stronger than encryption with uniform beams. In addition, the system is shown to operate successfully in the presence of channel noise. Chaotic encryption and decryption is implemented with closed-loop Bragg cells under profiled beam propagation, and is applied to high frequency data signals, including: audio, PCM, ECG signals, stationary images, and video streams. For each application, the sensitivity of the encryption to the Bragg parameters is studied along with the effect of channel noise. Additionally, the use of a dual Bragg cell is briefly explored as a method for asynchronous demodulation, along with zeroth-order modulation. The motivation for zeroth-order modulation is that spatial deflections of the first-order AO beam may potentially cause tracking problems at the receiver, a problem that is avoided by switching to the zeroth-order beam that remains spatially undeviated. Finally, for a medical image application, the combination of chaotic encryption with steganography is considered. Patient information is embedded as text within the medical image prior to encryption, creating a multi-layer scheme for greater security.

Keywords

Data encryption (Computer science) Testing, Acoustooptics Testing, Electrical Engineering, acousto-optics, Bragg regime, scattering, Gaussian, Klein-Cook, chaos, image, video, encryption, decryption, modulation, electrocardiography

Rights Statement

Copyright 2015, author

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