Title

Secure encryption and decryption by aperture variations of a photodetector in an acousto-optic Bragg cell

Date of Award

2016

Degree Name

M.S. in Electrical Engineering

Department

Department of Electrical and Computer Engineering

Advisor/Chair

Advisor: Monish Ranjan Chatterjee

Abstract

Security of information transfer has a higher priority in this era of communication, and hence the importance of an acousto-optic (A-O) Bragg cell under feedback for generating chaotic carriers. In this research, the aperture area à of the photodetector (PD) is assumed to vary with time under uniform plane wave incidence which may arise in a hybrid A-O Bragg cell under first order feedback (HAOF) due to PD efficiency, intensity of the incident light, and disturbances in the medium between exit plane of the A-O Bragg cell and the PD. Bifurcation maps over varying à of a PD show a wide range of chaotic passbands with streaking transitional stopbands. A chaotic passband with sufficient signal dynamic range and stability with specific values of key parameters including input bias a₀, feedback gain ß, time delay TD, and the effective aperture cross section area à is chosen for the transmission of message signals. Initial simulations in the ideal scenario for the transmission of square and falling sawtooth waveforms at 10 KHz with à equal to 1, along with quiescent values of feedback gain set at 3 and 4 reveals superior encryption at higher orders of ß. Despite noisy recoveries at higher ß, turns out that square waves can be perfectly reconstructed by thresholding techniques which would be an added advantage for digital transmission. If the intensity of the light beam is increased, the system shows wider chaotic passbands suitable for accommodating larger amplitudes of message signals. For an aperture area varying linearly with time, an optimal passband (which is both wide and stable) is chosen for the transmission of data using text messages and gray and color images as input. For the study of the level of mismatch between the message and the recovered signals (to assess the robustness of the encryption) correlation plots are generated; it is found that the simulated results for an RGB image of size 512 X 512 indicate a mismatch less than 0.083%. Parameter tolerance for disruption of recovered data is verified as a means to test the viability of system security.

Keywords

Data encryption (Computer science), Signal processing, Optoelectronic devices, Square waves, Telecommunication systems Security measures, Digital communications, Electrical Engineering, Bragg cell, Acousto-Optics, Modulation, Chaos, Text and Image Transmission, Digital data transmissions

Rights Statement

Copyright 2016, author

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