KEYWORDS: LIDAR, Filtering (signal processing), Semiconductor lasers, Sensors, Remote sensing, Digital filtering, Electronic filtering, Modulation, Diodes, Signal to noise ratio
The work on the development of compact diode laser-based lidar systems at SRI International is reviewed. Two systems, a pseudorandom modulation lidar, and a mobile remote sensor for natural gas pipeline leak detection are described in detail, and experimental results are presented. Methods to enhance signal detection by digital filtering are also reviewed.
We describe an integrated detection system based on upconverting phosphor particles bound to capture sites on the inside surfaces of rectangular wick capillaries. This deice can be used with either antibody or nucleic acid to detect specific micro-organisms. The system uses a high- power, 980 nm, semiconductor diode laser to illuminate 200 X 300 X 20 micrometers capture surfaces. The rectangular capillary wicks are held in a tray that is inserted into the detection system, positioning the capture surface at the object plane of the optical system. Phosphorescent light emitted from the capture surface is collected by a high numerical aperture microscope objective and directed through a series of filters onto either a CCD camera or a photomultiplier. A combination of band-reject filters attenuates the 980 nm laser excitation light and its harmonic at 490 nm, and a tunable liquid crystal filter provides for rapid scanning from 400 to 750 nm. The data acquisition and control is controlled by a laptop PC with a custom GUI interface developed using LabWindows/CVI. The system can detect a single phosphor particle bound to a capture surface.
SRI International has designed and built several instruments that use tunable diode lasers and frequency modulation spectroscopy. These instruments have been used for flux measurements of trace gases, explosives detection, and environmental monitoring. A detection sensitivity of 2X10-6 with a stability of 0.1% over 10 hours has been demonstrated using a GaAlAs laser and an oxygen absorption line at 760.56 nm.
Frequency modulations spectroscopy (FMS) with infrared lasers is an attractive technique for a number of environmental chemical sensing problems. The technique combines high detection sensitivity with high detection speed and, when implemented with tunable infrared laser sources, is capable of detecting numerous chemical species in the atmosphere. To date, the technique has been demonstrated with semiconductor diode lasers and carbon dioxide lasers, and absorptions at the 10-7 level have been detected. We will review the principles and status of FMS for chemical sensing and discuss applications in atmospheric and environmental monitoring.
A multiwavelength CO2 direct-detection DIAL system has been designed and developed to produce range-resolved vapor concentration contour plots of a 1 x 1 km grid at 20-m spatial resolution in 10 s intervals.
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