Jan E. van der Laan

Efficient second harmonic generation of 10‐μm radiation in AgGaSe2

AgGaSe2 crystals for nonlinear infrared applications are being grown reproducibly. Using high‐quality, 2‐cm‐long crystals, 14% energy and 60% peak intensity conversion efficiency have been demonstrated for second harmonic generation of the pulsed output of a CO2 miniature transversely excited atmospheric laser.

CO 2 laser-based differential absorption lidar system forrange-resolved and long-range detection of chemical vapor plumes

A dual CO(2) laser-based differential absorption lidar (DIAL) system has been constructed and demonstrated for range-resolved mapping of chemical vapor plumes. The system acquires high range resolution through the use of plasma-shutter pulse clippers that extinguish the nitrogen tail of the CO(2)-laser output. Aprogrammable servomotor-driven scanner allows full hemispherical coverage of the interrogated field. A high-speed direct-detection receiver subsystem is used to gather, process, and display vapor-concentration data in near real time. Data demonstrating range-resolved detection of low concentrations of chemical plumes from ranges of 1 to 2 km are presented. In the column-content detection mode, trace levels of secondary vapors from various organophosphate liquids were monitored. Detection of an SF(6) vapor plume released 16 km from the DIAL system is also adduced.

Carbon dioxide laser backscatter signatures from laboratory-generated dust

A dual CO2 laser system was used to measure aerosol backscatter spectral signatures from dust minerals (kaolin, illite, montmorillonite, colemanite, and limestone) as well as from a soil sample from Dugway Proving Ground, UT. Complex refractive indices measured from bulk samples of the materials and particle size distributions measured with a cascade impactor were used to compute theoretical backscatter spectra using Mie theory. The measured signatures agreed well with calculated signatures for most minerals and the soil sample. The experiment demonstrated the feasibility of detecting the compositional elements of dust using a CO2 laser-based system.

Lidar observations during dusty infrared Test-1.

Lidar measurements using ruby (0.7-microm) and CO(2) (10.6-microm) lidar systems during the dustry IR Test-1 are described. The test was conducted at the White Sands Missile Range (WSMR) in October 1978. Transmission comparisons are made between the two wavelengths through dust and smoke clouds generated by artillery barrages, TNT explosions, and oil-rubber fire in a test zone midway (1 km) along the lidar path. A target at the end of the lidar path provided a reference backscatter return for the transmission measurements. Results indicate that the broad particle size distribution present in the dust generated at WSMR produced little if any wavelength-dependent transmission effects.

Restoration and recognition of distant, blurry irises

Raw iris images collected outdoors at standoff distances exceeding 25 m are susceptible to noise and atmospheric blur and even under ideal imaging conditions are too degraded to carry out recognition with high accuracy. Traditionally, atmospherically distorted images have been corrected through the use of unique hardware components such as adaptive optics. Here we apply a pure digital image restoration approach to correct for optical aberrations. Image restoration was applied to both single images and image sequences. We propose both a single-frame denoising and deblurring approach, and a multiframe fusion and deblurring approach. To compare performance of the proposed methods, iris recognitions were carried out using the approach of Daugman. Hamming distances (HDs) of computed binary iris codes were measured before and after the restoration. We found the HD decreased from >0.46 prior to a mean value of <0.39 for random single images. The multiframe fusion approach produced the most robust restoration and achieved a mean HD for all subjects in our data set of 0.33 while known false matches remained at 0.44. These results show that, when used properly, image restoration approaches do significantly increase recognition performance for known true positives with low increase in false positive detections, and irises can be recognized in turbulent atmospheric conditions.

Diode-laser-based lidars: the next generation

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.

Frequency modulation spectroscopy with tunable diode lasers

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.

Compact upconverting phosphor detection system for wick assays

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.

Frequency modulation spectroscopy for chemical sensing of the environment

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.

Multiwavelength And Triple CO2 Lidars For Trace Gas Detection

SRI International has developed two infrared differential absorption lidar (IR DIAL) systems to detect infrared-absorbing trace gases and environmental monitoring applications. The multiwavelength DIAL system operates at 10.6 μm and performs both path-integrated (column-content) and range-resolved measurements. This van-mounted system contains four grating-tuned Tachisto 555-G (custom) TEA CO2 lasers, with a nominal 1 J of transmitted energy at 10P(20). The lasers are typically fired with a 100 μs spacing to freeze the atmosphere. The IR receiver uses a 16-in. f/2.5 telescope and a liquid-nitrogen-cooled HgCdTe quadrant detector. The scanner allows pointing over a full hemisphere. The lidar data system features two DEC PDP-11/73 microcomputers and a computer automated measurement and control (CAMAC) data acquisition system for real-time data collection, signal averaging, data analysis, color graphics display, and storage on magnetic tape. The multiwavelength DIAL system has demonstrated path-integrated measurements to a range of 9 km and range-resolved measurements to 4 km. The triple CO2 DIAL system operates at 3.4 μm and utilizes three mini-TEA CO2 lasers and nonlinear crystals in a novel frequency-mixing technique to detect and discriminate selective hydrocarbons.