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Acousto-optic spectrometers.

Introduction

The AOS-1000 and 1000X4 use state-of-the-art acousto-optic Bragg cell technology to measure instantaneous RF power spectral density for signal analysis (including electronic intelligence reception), spectroscopic and, test and measurement equipment applications. The AOS-1000 operates over a 1 GHz instantaneous frequency range, while the AOS-1000X4 operates over a 4 GHz range using four separate 1 GHz bandwidth channels. Each system has a frequency resolution of 1 MHz. These spectrometers use frequency channelization to achieve 100 percent probability of detection. The power spectral density output from each unit is displayed on a front-panel display and available in a 12-bit digital word format on an IEEE 488 bus interface. The real-time output data rate is 10 MS/s for the AOS-1000 and 1 MS/s for the AOS-1000X4. Each spectrometer also contains a digital signal processing board and PC/AT computer. Each unit is housed in a rack mount.

Functional Description

The AOS-1000 and 1000X4 are parallel multichannel-filter RF spectrometers implemented with acousto-optic signal processing technology. Figure 1 is a block diagram of the PSI AOS-1000. A wideband RF signal is split into a bank of contiguous bandpass filters. Each filter output from each square-law detector is integrated in time, producing a DC voltage proportional to the integrated power density of the input RF signal over the bandpass of the filter. The DC voltage in each channel is sampled in parallel and converted to a serial output using a parallel-to-serial converter. The serial output represents the instantaneous power spectral density of the input RF signal. The AOS-1000X4 implements the functionality of four AOS-1000 spectrometers in a single package.

Optoelectronic Implementation

The AOS-1000 and 1000X4 are implemented with state-of-the-art acousto-optic signal processing technology. A schematic diagram of the spectrometers optical portion is shown in Figure 2. Coherent light from a laser is collimated and shaped to illuminate an acousto-optic Bragg cell, which consists of a transparent crystal with a piezo-electric transducer bonded to one end. RF signals input to the transducer are converted to sound waves that propagate through the crystal. Light incident on the cell interacts with the sound wave and is partially diffracted at an angle proportional to the frequency of the original RF signal, the acousto-optic effect. The spatial location of the diffracted light, when brought to focus by a lens, is proportional to the frequency of the original RF signal. Different RF frequencies present simultaneously in the Bragg cell will be displayed simultaneously as multiple diffracted light beams. A photodetector array in the focal plane of the lens samples the multiple diffracted beams and produces discrete electronic charge packets whose array position and intensity are proportional to the original RF signal frequency and power, respectively. The video output of the photodetector array represents the instantaneous power spectral density of the input RF signal.
TABLE I
AOS-1000 AND 1000X4
SPECTROMETER SPECIFICATIONS
                                      1000/1000X4
IF center frequency
(GHz)                                     1.55
Instantaneous bandwidth
(GHz)                                      1/4
Number of channels                      1000/4000
Channel bandwidth (MHz)                     1
Spurious-free
dynamic range (dB)                        70/30
Sensitivity (dBm)                        -90/-50
Integration time (min) (|mu~s)           50/3200
(max) (ms)                                  10
Power (W)                                  200


Mechanical Assembly

The AOS-1000 and 1000X4 are arranged in two assemblies inside the rack mount package. The optical assembly contains the laser source, optics, Bragg cell and photodetector array. The electronic assembly contains the laser drive and laser cooling control, photodetector array drive electronics, a 12-bit A/D converter, digital signal processing board display, PC/AT and an IEEE 488 bus interface. Size: 19" X 25" X 10.5". Table 1 lists the spectrometers' specifications.
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No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Photonic Systems Inc.'s AOS-1000 and 1000X4 spectrometers
Publication:Microwave Journal
Date:Apr 1, 1993
Words:591
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