Dennis R. Pape

Multichannel acousto-optic spectrometer

Radiometer spectrometers are used in millimeter-wave radio astronomy for the spectral measurement of molecular rotational transitions. The spectrum of interest spans 10s of GHz and the measurement time is large in order to obtain useful signal-to-noise ratio. We describe here the design and performance of a multichannel acousto-optic spectrometer for radio astronomy spectroscopy applications. This instrument has 4 channels and can process signals from 4 separate sources simultaneously. The bandwidth of each channel is 1 GHz and the frequency resolution is 1 MHz, providing simultaneous processing over 4,000 1 MHz channels.

High-performance optical vector-matrix coprocessor

This paper describes the design and fabrication of a high performance optical vector-matrix coprocessor for optical computing research applications. The optical vector-matrix coprocessor is configured to multiply an 8-element vector by an 8 X 8 matrix with a throughput rate of 1 MHz--effectively achieving a processing rate of over 100 Mops. The Vector-Matrix Coprocessor interfaces to an industry standard Personal Computer with a single card and is controlled by software written and compiled in the ANSI C language. All data input and output to the coprocessor are in 8 bit digital words. An 8 to 12 bit look up table is provided for each input channel to provide real time linearization of analog optical data representing input values through the optical system. The optical signals representing calculation values are detected and received by a switched capacitor integrating filter to reduce detection bandwidth and reject broadband noise.

Multichannel acousto-optical spectrometer

Photonic Systems Incorporated is currently fabricating a Multichannel Acousto-Optical Spectrometer (MCAOS) for NASA Goddard Space Flight Center. This instrument will be used as a frequency channelized radiometer for radio astronomy spectroscopy. It will analyze the spectrum of four independent radio frequency (RF) channels simultaneously and has potential for eight to as many as sixteen channels. Each channel will resolve the RF spectrum to one megahertz within its 1000 megahertz band. Dynamic range exceeding 30 dB will be achieved by quantizing detector photo-charge to 12 bits and accumulating data for large periods of time. Long time integration requires an optical bench optimized for stability and the use of temperature stabilization. System drift due to speckle interference is minimized by using a novel polarization switching Bragg cell.

Analog performance characteristics of a VCSEL in an analog optical vector matrix processor

Analog optical vector matrix processors (AOVMP) have been implemented over the past three decades utilizing a variety of methodologies. Most of these methodologies were dependent on external modulation of the laser source. Photonic Systems Incorporated has furthered the development of the AOVMP by using a 64 channel analog modulated vertical cavity surface emitting laser diode (VCSEL). The novel analog modulation of the VCSEL is performed by linearizing the output of the VCSEL to 8 bits using real-time 12 bit look-up tables. VCSEL analog modulation characteristics and linearization techniques are discussed along with AOVMP performance.

Digital mirror device infrared scene projector

This paper describes the initial development of an IR scene projection system that will produce time-varying scenes using the Texas Instruments digital mirror device (DMD). We develop a dark field IR projector design in which light reflected from a DMD pixel is reflected into the aperture of the projector when the pixel is on and remains outside the aperture when the pixel is off. We apply an effective blackbody temperature model and a previously developed diffraction model to a projector design with an aperture stop smaller than the main lobe of the on pixel diffracted light. We show that this arrangement provides optimum blackbody temperature and image resolution performance. We calculate that a projector of this type could produce adequate resolution with effective temperatures of almost 400 degree(s)C. A breadboard DMD scene projector using a filtered quartz tungsten-halogen light source which provided IR emission at 1 micrometers and a 768 X 576 digital DMD device with 16 micrometers pixels was constructed. A contrast ratio of over 100 to 1 was observed with resolutions greater than one-half of the device resolution. A preliminary optical illumination and projection design is described for a brassboard DMD IR scene projector.

Photonic signal processing for inverse synthetic aperture radar imaging

A robust signal processor, capable of handling a multitude of signal processing functions over a wide instantaneous signal bandwidth is needed for future military systems where shared sensor and signal processing resources will be employed. We investigated a novel 2D acousto-optic (AO) processor capable of real-time multifunction signal processing. We developed the specifications for an optoelectronic ISAR image formation processor for ship imaging based upon the AN/APS-137(H) Airborne Radar. The baseline processor is designed for high resolution imaging- slant range resolution of 0.3 m and cross range resolution of 1.2 m over 30 m X 30 m window. The optical design of the processor is based on an optically efficient, in-line, high stability, 2D interferometer using four acousto-optic devices invested by Riza. We developed specifications for the processor components, including light source, lenses, photodetector array, and Bragg cells including a multichannel Bragg cell to improve processor bandwidth and reduce its size. We showed that all of these components are commercially available. We breadboarded a narrow bandwidth version of the processor and demonstrated its main operation features. We showed experimentally that the processor has simultaneous spatial carrier generation capability that is controllable with high phase stability and excellent fringe visibility.

Wideband multichannel acousto-optic spectrometer for millimeter and submillimeter wavelength radio astronomy

Radiometer spectrometers are used in millimeter-wave radio astronomy for the spectral measurement of molecular rotational transitions. The spectrum of interest spans 10s of GHz and the measurement time is large in order to obtain useful signal-to-noise ratio. The low power/channel and simplicity of acousto-optic technology has led to the current development of acousto-optic spectrometers (AOS) with 1 GHz bandwidth and 100 channels. Additional AOS bandwidth and channelization is needed to increase spectral coverage, reduce overall data acquisition time, and accommodate multibeam antennas. A multichannel acousto-optic spectrometer (MCAOS) for radio astronomy spectroscopy applications has been developed with 4 channels that can process signals form 4 separate sources simultaneously. The bandwidth of each channel is 1 GHz and the frequency resolution is 1 MHz, providing simultaneous processing over 4,000 1 MHz channels. The design and initial performance of this instrument is described. Design considerations for future wider bandwidth MCAOSs are also presented.

Optoelectronic radar receiver for real-time radar imaging

We have previously presented the architecture and basic analytic results for a functional 1D pipelined hybrid optical/digital processing concept capable of generating a target range- doppler profile in real time. Here we address the fundamental system processing algorithm and hardware development issues in some detail. The approach to performing real-time phase correction of the individual range profiles is outlined, along with the basic system operational runtime algorithms and system processing pipeline. A description of the receiver hardware and its component functionality in terms of the presented operational theory is given as well.

32-bit digital optical computer: a hardware update

Such state-of-the-art devices as multielement linear laser diode arrays, multichannel acoustooptic modulators, optical relays, and avalanche photodiode arrays, are presently applied to the implementation of a 32-bit supercomputers general-purpose optical central processing architecture. Shannons theorem, Morozovs control operator method (in conjunction with combinatorial arithmetic), and DeMorgans law have been used to design an architecture whose 100 MHz clock renders it fully competitive with emerging planar-semiconductor technology. Attention is given to the architectures multichannel Bragg cells, thermal design and RF crosstalk considerations, and the first and second anamorphic relay legs.