Acousto-optic scanning and reflection sensing for large area object search and recovery

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

Acousto-optic beam steering has been used over the years for a variety of applications [1, 2]. The ability to steer laser beams at high frequencies (tens of MHz to a few GHz) electronically with high angular resolution (given Bragg angles in the mrad range) makes the Bragg cell an ideal device for angular and spatial steering. Lasers are extensively used in many applications such as target detection [3, 4] as well as in ocean depth measurement [5]. In this research, we describe a simple sector-based angular scanning system intended to cover a large surface area in order to identify and spatially locate relatively small objects scattered over the terrain [6]. The scanning system is modeled as a planar surface on the horizontal (XY) plane, with an acousto-optic Bragg cell on board an unmanned aerial vehicle (UAV) operating in the XZ plane. The Bragg cell is excited by a chirped RF signal with frequency ramping from low-to-high or high to-low. As the scanning beam reflects off the horizontal surface, a detector placed on the UAV picks up the reflected wave (shown to be effective over the scan range), and thereby evaluates the refractive index of the material at the location on the basis of the corresponding Fresnel reflection coefficient [7]. If the surface is, say, primarily sea water, then the detection is considered negative" unless a material different from sea water is detected. Following each horizontal scan (about 374.15 m) within a sector, the return path is a blank. The Bragg cell, mounted on a stepper motor, is then rotated in the horizontal plane by a small angle, and the second scan run is carried out from the rotated position. Following this process with only L-to-R or R-to-L active scans and interleaving blanks, a "unit" circular sector is scanned with physical dimensions approximately 374.15 m x 300 m. Any "positive" refractive index returned by the sensor is stored in the system in terms of the spatial coordinates of the scanned point. Since the unit sector does not entirely cover a rectangular area or grid, the coverage efficiency can be increased by rotating the Bragg cell from its nominal XZ plane by approximately 900 such that it then moves to the YZ plane. Under this configuration, another series of horizontal scans in the XY is carried out, providing additional scanning area that includes portions of the grid previously left out. In reality (as will be discussed) the actual rotation angle is chosen to be 1070 which allows the scan coverage to be maximal. In this manner, incidentally, some portions of the unit grid may be scanned more than once. This would lead to possible "positive" detection of objects multiple times. The scanning scheme consists of a horizontal row (along Z) reached via multiple grid scans along the Z axis until a chosen scanning distance (about 30 km in this design) is reached, following which the UAV is instructed to reverse direction of flight, and a new row scan is carried out in the reverse direction along an incremental change in Y along the horizontal plane. In this manner, a series of horizontal scans covering an arbitrarily large surface area is carried out (technically over a few hundred square km or the equivalent of a moderate-sized city). The scheme is shown in the numerical simulation to yield coordinate locations of any arbitrary distribution of non-sea-water materials randomly scattered over the scanned surface. This scanning system may be useful in search and rescue applications over large (relatively uniform, homogeneous and planar) surface areas (especially in unreachable terrains directly below the UAV) using the A-O scanning methodology via a Bragg cell mounted on board at an altitude of approximately 8 km."

Keywords

Frequency modulation detectors, Beam optics, Scanning systems, Acoustooptical devices, Electrical Engineering, acousto-optics, beam steering, Gaussian beam, sector-based laser scanning, Bragg cell, Fresnel coefficients, UAV

Rights Statement

Copyright © 2016, author

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