Ravindra A. Athale

Flexible multimodal camera using a light field architecture

We present a modified conventional camera that is able to collect multimodal images in a single exposure. Utilizing a light field architecture in conjunction with multiple filters placed in the pupil plane of a main lens, we are able to digitally reconstruct synthetic images containing specific spectral, polarimetric, and other optically filtered data. The ease with which these filters can be exchanged and reconfigured provides a high degree of flexibility in the type of information that can be collected with each image. This paper explores the various tradeoffs involved in implementing a pinhole array in parallel with a pupil-plane filter array to measure multi-dimensional optical data from a scene. It also examines the design space of a pupil-plane filter array layout. Images are shown from different multimodal filter layouts, and techniques to maximize resolution and minimize error in the synthetic images are proposed.

Optical implementation of associative memory with controlled nonlinearity in the correlation domain

A mathematical model for incorporating controllable nonlinearity in the correlation domain of a conventional associative memory is described. Such a mechanism provides the flexibility of rapidly and arbitrarily changing the strengths of the stored states in an associative memory. Such a feature corresponds to shifting of attention in psychological terms. This attentive associative memory can be implemented optically. Results obtained with computer simulation and a design for a compact optical implementation are discussed.

Optical matrix-matrix multiplier based on outer product decomposition

An optical matrix multiplier using two linear modulating arrays in which the columns of the first matrix to be multiplied control the modulation of one array and the rows of the second matrix control the other array. Light is directed through all combinations of elements on the two arrays and the resultant beams measured by individual elements on a two-dimensional detector array. The detecting elements time integrates the intensity of light falling on each of them, which value corresponds to an element of the product of the two matrices. The invention may be implemented among other ways with two linear electrooptical arrays, a linear array of light emitting diodes and a linear electrooptical array, or two Bragg cells and a pulsed light source.

High accuracy computation with linear analog optical systems: a critical study

High accuracy optical processors based on the algorithm of digital multiplication by analog convolution (DMAC) are studied for ultimate performance limitations. Variations of optical processors that perform high accuracy vector-vector inner products are studied in abstract and with specific examples. It is concluded that the use of linear analog optical processors in performing digital computations with DMAC leads to impractical requirements for the accuracy of analog optical systems and the complexity of postprocessing electronics.

Modified light field architecture for reconfigurable multimode imaging

Light field cameras can simultaneously capture the spatial location and angular direction of light rays emanating from a scene. By placing a variable bandpass filter in the aperture of a light field camera, we demonstrate the ability to multiplex the visible spectrum over this captured angular dimension. The result is a novel design for a single-snapshot multispectral imager, with digitally reconstructed images exhibiting reduced spatial resolution proportional to the number of captured spectral channels. This paper explores the effect of this spatial-spectral resolution tradeoff on camera design. It also examines the concept of utilizing a non-uniform pinhole array to achieve varying spectral and spatial capture over the extent of the sensor. Images are presented from several different light field - variable bandpass filter designs, and limitations and sources of error are discussed.

Sorting with optical compare-and-exchange modules

Sorting is central to the solution of many knowledge-based and switching problems in advanced computation and communication systems. Parallel-pipelined sorting algorithms are appropriate for applications that demand high throughput, low delay, and many data channels. One such algorithm, the bitonic sort, can be implemented with passive perfect shuffle interconnects between active stages of compare-and-exchange (C&E) elements. In this paper we focus on optical hardware to implement the C&E operation and show that, by taking advantage of a distinctive feature of optical logic, namely, bistability, comparison circuits of remarkable simplicity are attainable. We describe implementations of C&E in a variety of optical device technologies capable of performing latching and nonlatching logic. Based on the device characteristics we outline potential application areas for each technology.

Photonic crystal filters for multi-band optical filtering on a monolithic substrate

Many applications require the ability to image a scene in several different narrow spectral bands simultaneously. Conventional multi-layer dielectric filters require control of film thickness to change the resonant wavelength. This makes it difficult to fabricate a mosaic of multiple narrow spectral band transmission filters monolithically. We adjusted the spectral transmission of a multi-layer dielectric filter by drilling a periodic array of subwavelength holes through the stack. Multi-band photonic crystal filters were modeled and optimized for a specific case of filtering six optical bands on a single substrate. Numerical simulations showed that there exists a particular air hole periodicity which maximizes the minimum hole diameter. Specifically for a stack of SiO2 and Si3N4 with the set of filtered wavelengths (nm): 560, 576, 600, 630, 650, and 660, the optimal hole periodicity was 282 nm. This resulted in a minimum hole diameter of 90 nm and a maximum diameter of 226 nm. Realistic fabrication tolerances were considered such as dielectric layer thickness and refractive index fluctuations, as well as vertical air hole taper. Our results provide a reproducible methodology for similar multi-band monolithic filters in either the optical or infrared regimes.

Pupil plane multiplexing for multi-domain imaging sensors

We describe an approach to polarimetric imaging based on a unique folded imaging system with an annular aperture. The novelty of this approach lies in the systems collection architecture, which segments the pupil plane to measure the individual polarimetric components contributing to the Stokes vectors. Conventional approaches rely on time sequential measurements (time-multiplexed) using a conventional imaging architecture with a reconfigurable polarization filter, or measurements that segment the focal plane array (spatial multiplexing) by super-imposing an array of polarizers. Our approach achieves spatial multiplexing within the aperture in a compact, lightweight design. The aperture can be configured for sequential collection of the four polarization components required for Stokes vector calculation or in any linear combination of those components on a common focal plane array. Errors in calculating the degree of polarization caused by the manner in which the aperture is partitioned are analyzed, and approaches for reducing that error are investigated. It is shown that reconstructing individual polarization filtered images prior to calculating the Stokes parameters can reduce the error significantly.

Acousto-optic processing with electronic image feedback for morphological filtering.

An optical morphological processor is described and demonstrated in which the structuring element is generated by means of an acousto-optic cell in the Fourier plane of a coherent optical correlator. A magneto-optic spatial light modulator is employed in the input plane. A single PC generates the drive signals for the acousto-optic cell, processes the output image, and controls the input magneto-optic spatial light modulator. Complex morphological operations of opening and closing are experimentally demonstrated by using electronic image feedback. The results of a morphological noise-removal algorithm implemented by using the optical processor are compared with computer simulations, possible sources of discrepancies are proposed, and potential remedies are discussed.

Fourier-plane filtering by a thick grating: a space–bandwidth analysis

The space–bandwidth requirements of thick gratings in the Fourier plane of a coherent optical image processor are analyzed. Based on Kogelnik’s relationship for the diffraction efficiency, relations for the space–bandwidth products of the input image and the point-spread function are obtained in terms of grating parameters. It is concluded that thick gratings, such as photorefractive crystals, are not well suited for implementing shift-invariant operations through Fourier-plane filtering.