Christopher Scott Anderson

Fringe visibility, irradiance, and accuracy in common path interferometers for visualization of phase disturbances.

Common-path interferometers have been used to perform phase visualization for over 40 years. A number of techniques have been proposed, including dark central ground, phase contrast (π/2 and π), and field-absorption interferometers. The merits of the interferometers have been judged ad hoc by either tests with a small number of phase objects or by computer simulation. Three standardized criteria, which consolidate the work of others, are proposed to evaluate common-path interferometers: fringe visibility, fringe irradiance, and fringe accuracy. The interferometers can be described as one generic class of Fourier-plane filters and can be analyzed for all input conditions. Closed-form expressions are obtained for visibility and irradiance under the forced condition that little inaccuracy is tolerated. This analysis finds that the π-phase-contrast interferometer is a good choice if the optical phase disturbance is at least 2π; for smaller disturbances, the Π/2 filter selected by Zernike is near optimum. It is shown mathematically that the resulting fringe visibility is highly object dependent, and good results are not ensured. By allowing the optical beam to be 50% larger than the phase object, the interferometer performs well under all conditions. With this approach and a combination π-phase/field-absorption filter, interference fringe visibility is greater than 0.8 for all phase objects.

Design and characterization of a long time aperture acousto-optic delay line

The design and characterization of a 54-μs, continuously variable, acousto-optic delay line developed for radar testing applications is documented. The operating principles of this new common-path delay line architecture are reviewed, and key component selection issues are discussed. Ultimately, the characteristics of these key components limit the achievable performance of the completed delay line. For example, the laser diode introduces high-frequency noise at the output of the delay line. This noise, which is a direct function of the relative intensity noise of the laser, is the limit to the output noise floor. Bragg cell design is presented with emphasis on the minimization of multiple time-delayed signals caused by acoustic reflections. A computer design and analysis tool is introduced that predicts delay line performance in terms of bandwidth, insertion loss, and acoustic attenuation for this slow shear, tellurium dioxide (TeO 2 ) based delay line. Experimental results are presented showing 10-MHz operating bandwidth at a 33-MHz center frequency. The 3-dB bandwidth is maintained over delays ranging from 0.75 to 54 μs.

Comparison Of Phase-Only And Classical Matched Filter Scale Sensitivity

Expectation values in the frequency domain are used to calculate the correlation peak of an optical matched filter. These calculations show that the phase-only and classical matched filters are equally sensitive to scale changes when modest high-pass filtering is used. Computer simulations were done for verification.

Laser/rf personnel identification system

This paper documents the design of a Laser/RF Personnel Identification System developed for the US Army Communications and Electronics Command (CECOM) for soldier identification. The system has dual use applications, including law enforcement officer protection, and includes a laser interrogation unit with a programmable activation code. The interrogation unit is very directive for low probability of intercept (LPI), which is of interest during covert operations. A responder unit, worn by the law enforcement personnel or soldier, transmits an LPI radio frequency (RF) response only after receiving the proper interrogation code. The basic subsystems for the identification system are a laser interrogation unit, an RF responder unit, and a programming/synchronization unit. In this paper, the operating principles for the subsystems are reviewed and design issues are discussed. In addition to the design performed for CECOM, a breadboard system was developed to validate the concept. Hardware implementation is reviewed and field testing of the breadboard is presented.

Wideband acousto-optic processor for ESM applications

This paper describes an acousto-optic (AO) processor that offers a small, lightweight solution to detecting and analyzing wide-bandwidth, spread-spectrum signals. The processor is being developed for insertion into an existing electronic support measure (ESM) test-bed. The correlator will have a processing bandwidth of 500 MHz and will be used to detect direct- sequence phase modulated (PM) signals, frequency-hopped signals, chirps, and impulse signals. An in-line AO correlator is the heart of the processor and is used for detecting wideband activity. Subsequent digital processing, including Fourier transformation, will be used to determine center frequencies, bandwidths, and band shape. Theoretical operation of the correlator is discussed along with descriptions of the radio frequency (RF) interfaces and digital post-processing.

Radiometers, autocorrelators, and matched-filter receivers by means of acousto-optic spectrum analysis

Acousto-optic spectrum analyzers provide a convenient means of separating wide-bandwidth signals into their frequency components. By a change in the rf input signal into the spectrum analyzer and by the provision of additional digital postprocessing, it is possible to perform radiometry, signal autocorrelation, and matched-filter reception. Although the acousto-optic device has a space-integrating architecture, the matched-filter receiver can be implemented for signals having time durations much longer than the acousto-optic cell. The resulting signal-to-noise ratio improvements from the receiver are consistent with the time-bandwidth product of the waveform, rather than the time-bandwidth product of the acousto-optic device. A mathematical foundation of the processor is presented along with specific receiver implementations. Computer-simulation and experimental results demonstrate key findings. In one experimental example, a linear-frequency-modulated waveform is matched-filter processed to recover a signal that is -24 dB with respect to the input noise floor.

Design and characterization of a 54-μs acousto-optic delay line

This paper documents the design and characterization of a 54-microsecond(s) , continuously variable, acousto-optic delay line developed for radar testing applications. The operating principles of this new common-path delay line architecture are reviewed, and key component selection issues are discussed. Ultimately, the characteristics of these key components limit the achievable performance of the completed delay line. For example, the laser diode introduces high frequency noise at the output of the delay line. This noise, which is a direct function of the relative intensity noise of the laser, is the limit to the output noise floor. Bragg cell design is presented with emphasis on the minimization of multiple time-delayed signals caused by acoustic reflections. A computer design and analysis tool is introduced that predicts delay line performance in terms of bandwidth, insertion loss, and acoustic attenuation for this slow shear, tellurium dioxide (TeO2) based delay line. Experimental results are presented showing 10-MHz operating bandwidth at a 33-MHz center frequency. The 3-dB bandwidth is maintained over delays ranging from 0.75 to 54 microsecond(s) .

Acousto-optic electronic support measures (ESM) receiver upgrade

The acousto-optic (AO) module described in this paper is an in-line, time- integrating correlator architecture that detects and analyzes inherently wide bandwidth signals in a small and lightweight package. The correlator processes a 500 MHz instantaneous bandwidth to provide enhanced detection capability for broadband signals. The existing electronic support measures (ESM) testbed processes a wide bandwidth but can only detect the presence of narrowband signals. This paper will describe the AO correlator design and the radio frequency and digital interface required for the insertion into the ESM testbed.

Wideband acousto-optic correlator for an ESM receiver upgrade

An acousto-optic (AO) correlator is being constructed that offers a small, lightweight solution to detecting and analyzing wide-bandwidth, spread-spectrum signals. The processor is being inserted into an existing electronic support measure (ESM) test-bed under the Defense Advanced Research Projects Agency (DARPA) Transition of Optical Processors into Systems (TOPS) program. The correlator has a processing bandwidth of 500 MHz and will be used to detect direct-sequence, phase-modulated signals, frequency-hopped signals, chirps, and impulses. A description of the processor is provided along with experimental results obtained from an interim developmental breadboard. Subsequent digital processing, which includes nonlinear detection and Fourier transformation, is used to determine center frequencies, bandwidths, and band shape. Theoretical descriptions of the post-processing are provided and simulations results are discussed.

Soldier identification system for dismounted soldier application: prototype development and testing

This paper documents the design and testing of a prototype laser/radio frequency (RF) Soldier Identification (ID) System developed by Dynetics, Inc., Harris, Corp., and the U.S. Army Communications and Electronics Command (CECOM). The Soldier ID system consists of an Interrogation Unit, a Responder Unit, and a Programming Unit. The Interrogation Unit consists of a directive, eyesafe laser and a spread-spectrum RF transceiver. This allows for a low probability of intercept (LPI) interrogation, which is of interest during covert operations. A Responder Unit is worn, for example, by a soldier and transmits an LPI spread-spectrum RF response, only after receiving the proper interrogation codes. The operating principles for the subsystem are reviewed, and key design issues are presented. In addition, both breadboard and prototype test results are presented.