A. VanderLugt

Optimum sampling of Fresnel transforms.

The opportunity to process signals in domains other than the time or frequency domains arises naturally in coherently illuminated optical systems that produce Fourier transforms. It is well known that N samples are sufficient to represent the information content in the object, image, and Fourier planes. We extend these results to show that we can accurately represent the intensity signal in any Fresnel plane of a coherently illuminated optical system with exactly N samples, provided that we use a specified nonuniform sampling technique.

Role of photodetectors in optical signal processing

Optical signal processing applications place demands on photodetector arrays beyond those encountered in image sensing applications. We review the basic requirements and show that increased dynamic range and nonlinear decision operations that lead to reduced output data rates are the key improvements needed for both 1-and 2-D arrays. Arrays of high-speed photodetector elements with integrated postdetection circuitry are also needed. Although we suggest some possible methods for achieving these goals, our main objective is to stimulate the photodetector community to design and fabricate more useful devices.

Design relationships for acousto-optic scanning systems

We develop the general scanning relationships of an acousto-optic system by using both a purely geometric-optics and a physical-optics approach; each approach provides useful insights into the scanning relationships. The diffraction approach reveals that there are four basic scanning configurations: a long or short chirp scanner, either aperture or repetition-rate limited. The throughput rate for a scanner is always maximized if we use the short-chirp-scanning, repetition-rate-limited mode of operation. The maximum rate may be achieved with other configurations, but at the expense of a decrease in some of the other performance parameters. Examples are given of how these design relationships are used.

Multichannel acousto-optic crossbar switch with arbitrary signal fan-out

We propose an N x N acousto-optic switch architecture capable of arbitrary signal fan-out with O(N logN) hardware complexity. We also investigate the impact of signal fan-out on loss and cross talk.

Use of decimated photodetector arrays in spectrum analysis

Temporal heterodyne spectrum analysis requires the use of discrete photodetector arrays that have a large number of elements. Each element is generally followed by an amplifier, a bandpass filter, a demodulator, and nonlinear devices to handle the large dynamic range. When the number of elements in the array is of the order of 1000-2000, the readout hardware is difficult to implement. We consider decimating the array so that a much smaller number of elements are used. The spectrum is scanned across this array so that each element reads out o set of 3patial frequencies in a time division multiplexing fashion. In some cases there is no penalty in dynamic range; in others, the penalty is more strongly related to the reduction in the number of photodetectors. Similar techniques are applied to a cross-spectrum analyzer that uses temporal heterodyning to derive angle of arrival information from wideband signals.

Time-delay detection of short pulses by Fresnel and Fourier transformations

The Fresnel transform of a signal, when used as a preprocessing step to a subsequent spectrum analysis, helps to detect the time delay between short-pulse signals. Light from the short pulses disperses spatially as it propagates through free space to create an interference pattern. As the pulses move through an acousto-optic cell, a point photodetector produces a signal whose temporal frequency is directly proportional to the time delay between the pulses. When this signal is fed to a spectrum analyzer, the frequency domain represents a time-delay domain.

Signal distortion in an adaptive excision system

A narrowband signal excision system uses spatial light modulators in the spatial frequency domain to notch energy caused by narrowband interferers. We review the signal excision process and discuss the optical system developed to monitor the noise spectrum. A postdetection system searches for narrowband interferers in wideband spectra and provides control signals to implement the required notch; experimental results of the distortion imposed on short pulse signals are given.

Generalized filtering in acoustooptic systems using area modulation.

We report on a technique for implementing general filtering operations in acoustooptic signal processing systems. We use a binary recording method called area modulation to reduce linearity problems associated with spatial light modulators that operate by controlling the amplitude of light. We present an analysis of this method, we report on experiments with an acoustooptic system to verify the analysis, using photographic film and a liquid crystal display to implement the area modulation masks, and we discuss the limitations of the technique.

Acousto-Optic Photonic Switch: An Optical Crossbar Architecture

We describe a crossbar switch architecture based on acousto-optic beam deflection. The architecture has O(N) hardware complexity throughout, while exhibiting minimal insertion loss, low crosstalk, and fast reconfiguration. Because of the small amount of signal degradation imposed by this switch, it is suitable for nonregenerative applications within fiber-optic networks. By increasing hardware complexity, broadcasting can also be achieved within the framework of this architecture. We report experimental performance of a 1x4 switching element. Insertion loss ranges from 2 - 6 dB, worst-case signal-to-crosstalk ratios are in excess of 30 dB, and reconfiguration times are on the order of one microsecond.

Sampling of Fresnel transforms

Domains other than the time or frequency domains arises naturally in coherently illuminated optical systems that produce Fourier transforms. We extend the well known result that N samples are sufficient to represent the information content in the object, image, and Fourier planes to include any Fresnel plane of a coherently illuminated optical system, provided that we use a specified sampling technique.