Time and Space Integrating Acousto-Optic Signal Processing

Citation

Wagner, Kelvin H. (1987) Time and Space Integrating Acousto-Optic Signal Processing. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4WH0-H941. https://resolver.caltech.edu/CaltechETD:etd-05052006-155412

Abstract

One dimensional acousto-optic signal processing techniques are examined from the systems and functional viewpoint, and are then used as building blocks to synthesize multidimensional time and space integrating architectures. Time and space integrating signal processing systems are capable of performing 2-dimensional linear transformations upon images or matrices, by sequentially entering rows of the image with a travelling wave acousto-optic Bragg cell. The travelling rows are frozen by a pulsed laser diode, and the stationary diffracted fields are spatially processed by an optical system. The successively transformed rows are sequentially multiplied by a time varying reference wavefront, and accumulated on a time integrating CCD detector array to complete the two dimensional processing. Long 1-dimensional signals can also be linearly transformed by a time and space integrating system, by using a similar strategy upon a folded, or rastered, version of the high time bandwidth product signal. Small pieces of the long signal are slid into the system with an acousto-optic devices, and are spatially transformed over the device aperture. Then, successively transformed portions of the long signal are multiplied by a reference, and appropriately delayed and accumulated on a 2-D CCD in order to perform multichannel time integrations in the orthogonal dimension. The desired high time bandwidth one dimensional linear transformation is represented in the folded coordinate space of the 2-dimensional output detector.

The operational characteristics of the principal active devices used in these time and space integrating systems are examined from the viewpoint of the system architect. The effects of the devices on the overall system operation are discussed, and device designs intended for application in a time and space integrating system operating environment are proposed.

The final chapter is a detailed theoretical and experimental investigation into the particular operating characteristics of systems designed to perform a folded spectrum analysis of very high time bandwidth signals. This spectrum analysis problem has a shift variant transformation kernel, which can be broken down into a succession of smaller temporal and spatial sub transformations. The 1-dimensional space integrating spectrum analysis operation performed by a lens is used to produce a coarse spectral channelization of the input signal, displayed as a one dimensional spatial profile. Each resolvable spectral channel is fine frequency analyzed by temporal integration, producing a resulting intensity variation of each channel in the orthogonal direction, thereby forming a folded representation of the desired high time bandwidth spectrum analysis. The information which is needed to perform the fine frequency analysis is carried on the optical phase, so interferometric techniques are employed in order to detect the phase and transform it to an optical intensity modulation. Various bias terms are produced on the detector by the interferometric detection operation, and techniques for removing the unwanted bias are investigated. These include spatial carrier encoding of the interferometric terms combined with bandpass filtering, and direct bias subtraction techniques.

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