CO_2 laser-based differential absorption lidar system for range-resolved and long-range detection of chemical vapor plumes

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<ul><li><p>Because of the fact that the output radiation from aCO2 laser lies in the middle of the 8-to-12-m atmo-spheric window, CO2 laser-based differential absorp-</p><p>requiring large fields of view that are difficult toobtain when one is using high-speed photodetectors1which are, of necessity, small2 and the large collectiontion lidar 1DIAL2 systems have been applied to a widerange of remote-sensing problems over the past 20years.16 Although atmospheric transmission is goodfor laser wavelengths that lie in this window, thevolume backscatter coefficient associated with natu-rally occurring aerosols is rather weak. As a result,direct-detection, range-resolvedDIAL systems operat-ing in the 8-to-12-m window require extremely highlaser energies to obtain sufficient backscatter signals.Large, transversely excited, atmospheric-pressure1TEA2 CO2 lasers are capable of transmitting severaljoules of energy 1on high-gain lines2 when operatedmultimode. However, the beam divergence is usu-ally high, with M2 factors as large as 4 to 5.7 This</p><p>apertures that are needed to detect theweak backscat-ter signals. To ensure high range resolution, notonly the photodetector but also all the processingreceiver electronics must possess the proper band-width. Thus a high-speed, low-noise preamplifierand a high-speed, high-accuracy digitizer must beincorporated into the DIAL receiver. One can readilyappreciate, in light of all these constraints, the chal-lenges involved in developing a sensitive, high-resolution DIAL system for operation in the 8-to-12-m spectral region. The approaches adopted indealing with all these constraints, along with datademonstrating the sensitivity and the long-rangecapability of direct-detection CO2 laser-based DIAL,are discussed in detail in this paper.We have designed, constructed, and field tested a</p><p>range-resolved CO2 laser-based DIAL system referredto as ADEDIS 1Appareil de DEtection a DIStance2.ADEDIS was developed to allow study and mappingof the evolution of chemical vapor plumes, especiallyplumes resulting from the evaporation of organophos-phate liquids deposited on the ground. Previous 2-and 4-laser CO2 DIAL systems have also been devel-</p><p>8,9</p><p>C. B. Carlisle, J. E. van der Laan, and L. W. Carr are with theOptical Sensing Program, SRI International, 333 RavenswoodAvenue, Menlo Park, California 94025. P. Adam and J-P. Chiaroniare with the Laboratoire de Teledetection, Le Centre dEtudes duBouchet, 91719 Vert-Le-Petit, France.Received 13 April 1994; revised manuscript received 15 March</p><p>1995.0003-6935@95@276187-14$06.00@0.CO2 laser-based differential absystem for range-resolved anddetection of chemical vapor p</p><p>Clinton B. Carlisle, Jan E. van der Laan, Lewis W. Caand Jean-Pierre Chiaroni</p><p>A dual CO2 laser-based differentiastrated for range-resolved mappinresolution through the use of plasCO2-laser output. Aprogrammablinterrogated field. A high-speed ddisplay vapor-concentration data inconcentrations of chemical plumesdetection mode, trace levels of seconDetection of an SF6 vapor plume relKey words: Lidar, DIAL, remote</p><p>1. Introductionr 1995 Optical Society of America.sorption lidarlong-range</p><p>lumes</p><p>rr, Philippe Adam,</p><p>l absorption lidar 1DIAL2 system has been constructed and demon-g of chemical vapor plumes. The system acquires high rangema-shutter pulse clippers that extinguish the nitrogen tail of thee servomotor-driven scanner allows full hemispherical coverage of theirect-detection receiver subsystem is used to gather, process, andnear real time. Data demonstrating range-resolved detection of lowfrom ranges of 1 to 2 km are presented. In the column-contentdary vapors from various organophosphate liquids were monitored.eased 16 km from the DIAL system is also adduced.sensing.</p><p>creates significant problems for the DIAL receiver,oped for mapping vapor plumes. However, they</p><p>20 September 1995 @ Vol. 34, No. 27 @ APPLIED OPTICS 6187</p></li><li><p>have operated with ill-defined range resolution be-cause of the presence of the nitrogen tail in thetransmitted laser beam. There are deconvolution</p><p>Table 1. Specifications of the ADEDIS DIAL System</p><p>Parameter Specificationtechniques that address this problem10,11; we haveadopted the direct approach of extinguishing the tailwith a plasma-shutter pulse clipper. The specifica-tions of ADEDIS are presented in Section 2. InSection 3 we discuss the data gathered by ADEDIS inrange-resolved and column-content modes of opera-tion. Data on some interesting aspects of long-range1i.e., greater than 10 km2 DIAL applications are alsopresented.</p><p>2. System DescriptionThe ADEDIS DIAL system 1Fig. 12 operates with thegeneral specifications listed in Table 1. It was con-ceived as a device for detecting and mapping 1withhigh-spatial resolution2 chemical vapor plumes overtest grids of approximately one square kilometer.ADEDIS comprises three distinct subsystems: thetransmit optics package 1including the lasers andpulse clippers2, the programmable scanner trans-ceiver subassembly, and the data acquisition andprocessing electronics, each of which is discussedbelow.The floor plan of the complete ADEDIS system is</p><p>shown in Fig. 2, which provides the approximatedimensions of the shelter that encloses the DIALpackage. The DIAL package in the ADEDIS systemis shown in Fig. 3. Each laser beam exits horizon-tally, is directed by steering mirrors through theafocal pulse-clipper assembly, then through an antire-flection-coated ZnSe plate used to sample a smallfraction of the beam for power-monitoring purposes,and after is sent to the scanner via a series of mirrors1see Fig. 3 for details2. The insertable beam splitteris used to direct approximately 20% of the beam to theoptical spectrum analyzer to determine the line onwhich the CO2 laser is operating. 1This function isused for setup purposes only, and the beam splitter isalways removed before data are gathered.2 The redand green HeNe lasers are carefully coaligned in thefar field 1i.e., at the focus of the 1.82-m focusingmirror2with the CO2 laser so that they may be used to</p><p>Fig. 1. ADEDIS DIAL system.</p><p>6188 APPLIED OPTICS @ Vol. 34, No. 27 @ 20 September 1995determine where the CO2 beams are located at remotedistances from the system.The laser used in ADEDIS is a Laserbrand Model</p><p>XL-750 TS fitted with a manually tuned diffractiongrating, manufactured by Coherent Hull, Ltd., ofGreat Britain. These devices produce high outputenergies; they find their most common application inindustrial marking. Table 2 lists the general laserspecifications, along with weight and power require-ments.The plasma-shutter pulse clipper used in ADEDIS</p><p>is shown in Fig. 4. The collimated laser beam entersthe afocal lens pair and is focused at a point in spacebetween two tungsten electrodes, producing an inten-sity of approximately 5 3 1011 W@cm2. In dry flow-ing air, this intensity is close to the level required tocause breakdown and create a plasma. A smallelectrical discharge produced in the focal region dur-ing the laser pulse can thus provide the means toinitiate the plasma. Once formed, the plasma acts asa strongly negative lens and effectively disperses anylaser light that passes. In this manner, the nitrogen</p><p>Vehicle and associatedequipment</p><p>Vehicle DAF DieselGenerator 55 kWClimate control 15 kW</p><p>Transmit and receiversubsystem</p><p>Transmit moduleLasers Two tunable, pulsed, TEACO2</p><p>lasersPulse width Clipped with plasma shutter to</p><p>130 to 160 ns 1FWHM2Energy 1.2 J in gain-switched spike on</p><p>10P1202Repetition rate 10 HzWavelength 9.2 to 10.8 mBeam divergence 3 to 5 mrad 1full angle2Tuning Manual for two lasers</p><p>Receiver moduleTelescope 457 mm, f@2.5Field of view 5 mrad 1with f@0.57 field lens2Detector HgCdTe photovoltaic, liquid</p><p>nitrogen cooledScanner moduleHorizontal travel 360Vertical travel 180Horizontal speed 30@s</p><p>Data acquisition andrecording sub-</p><p>systemComputer VAXstation 3100Language FORTRAN 77Data acquisition Tektronix RTD710AInterfaces IEEE-488, RS-232, IEEE-488 to</p><p>SCSIDigitization 100 MHz, 10 bitRecorder Cipher 9-track magnetic tape</p><p>drive 1Digital Equipment Cor-poration supported2</p></li><li><p>camera provides a visual scene of the field interro-gated by the DIAL system to assist the operator. Thescanner is controlled by a dedicated Apple computer1which in turn is controlled and configured from theVAXstation 3100 operated by the system user2. Fullhemispherical coverage is allowed by the scannersystem. As the scanner is completely program-mable, any arbitrary scan pattern can be chosen bythe operator. This is an attractive feature when oneis repetitively probing limited, specific areas.The photodetector used in ADEDIS is a liquid-</p><p>nitrogen-cooled HgCdTe photovoltaic device manufac-tured by SAT Groupe SAGEM of Paris, France. Thephotovoltaic detector was preferred over a photocon-ductive detector because of its lower noise characteris-tics combined with a high electronic bandwidth.The detector possesses an electronic bandwidth ofapproximately 10 MHz when appropriately reversebiased and a detectivity 1D*2 5 6 3 1010 1cm Hz1@22@W.An f@0.6 field lens mounted in the Dewar approxi-Fig. 3. Transmit optics layout in DIAL package.</p><p>20 September 1995 @ Vol. 34, No. 27 @ APPLIED OPTICS 6189tail of the TEA CO2 laser beam can be clipped, thusshortening the output laser pulse to some 130 to 170ns in duration 1as opposed to 3-to-4-s pulse dura-tions with the full nitrogen tail2.</p><p>Fig. 2. Floor plan of ADEDIS. M-1M-7, mirrors; D-1D-3, diagnobeam splitters; A-1, A-2, alignment diode laser beammirrors; R-1, R-2Approximately two thirds of the laser energy iscontained in the nitrogen tail. However, because thelaser energy is distributed over such long time inter-vals, it severely reduces the range resolution of theDIAL system if it is transmitted. When the tail wasextinguished, range resolutions of 20 to 25 m wereobtained from ADEDIS. It is of course also possibleto reduce the nitrogen tail by the use of low nitrogenlaser gas mixture. However, this approach reducesthe energy in the gain-switched spike and does noteliminate the tail as fully as clipping does.10 Dataverifying the system resolution are presented in Sub-section 3.A.Figure 5 shows the scanner transceiver subassem-</p><p>bly. The scanner module was developed by DFM,Inc., of Longmont, Colorado. The scanner specifica-tions are listed in Table 3. The coaligned television</p><p>stic path mirrors; G-1, G-2, green HeNe beam mirrors; BS-1, BS-18,, red HeNe beammirrors.</p></li><li><p>Longwood, Florida, is used to amplify the lidar signalsbefore digitization.</p><p>m isformhan-its oformsdardrted.realttedVAX</p><p>Dug-Theini-mesangeraterfor-rally</p><p>into four categories: test results on pulse clippers</p><p>Table 2. ADEDIS Laser Specifications</p><p>LasPul</p><p>EneM</p><p>G</p><p>PeaMaxWavPulBea</p><p>Div</p><p>Jitt</p><p>DimWeiInput services 110 VAC, 60 Hz, 16 Awith voltagemately 0.5 mm from the 2-mm-diameter detectorelement was installed to increase the field of view.A low-noise 1noise figure 2 dB2 RF preamplifier with60-dB gain, manufactured by Analog Modules of</p><p>taps, making the laser suitable foroperation at 220 V, 50 Hz at afuture date</p><p>Water flow rate 2 L@minPremixed gas 1%2He 82CO2 8N2 8CO 2</p><p>Gas flow rate 40 cm3@min</p><p>Fig. 4. Plasma-sh</p><p>6190 APPLIED OPTICS @ Vol. 34, No. 27 @ 20 September 1995and the system range resolution, system sensitivity1in column-content mode2 to releases of calibratedconcentrations of vapor into a test chamber, systemsensitivity 1in range-resolvedmode2 to open releases ofchemical vapors, and long-range detection 1in column-content mode2 of open releases of a test vapor. Thedata obtained in these tests are presented and dis-cussed in detail below.</p><p>A. System Range-Resolution TestsThe dominant time constant in ADEDIS is attribut-able to the unclipped laser pulse, whose nitrogen tailcan extend for several microseconds; this limits therange resolution of the lidar to some hundreds of</p><p>utter pulse clipper.The data acquisition and processing subsysteillustrated schematically in Fig. 6. The wavedigitizer is a Tektronics RTD 710A model 1dual cnel2 with a bandwidth of 100 MHz and up to 10 bvertical resolution. The digitized lidar wavefare transferred to the VAXstation over a stanIEEE-488 bus followed by an IEEE to SCSI conveThe VAX processes and displays the data in neartime. In parallel with the processing, the formadata are stored to 9-track magnetic tape or thehard disk, depending on operator choice.</p><p>3. Data and Test ResultsThe ADEDIS DIAL system was field tested atway Proving Ground 1DPG2, Utah, in June 1993.objective of the field test was to establish the mmumconcentrations of organophosphate vapor pludetectable byADEDIS. In addition, the system rresolution was independently verified in sepaexperiments. The data pertaining to the pemance of the ADEDIS DIAL system divide natu</p><p>Parameter Specification</p><p>er medium CO2 TEA laserse width 130-ns gain-switched spike on</p><p>10P1202rgyultimode 1stablecavity2</p><p>5 J 3at 10P12024</p><p>rating tuned 1stablecavity2</p><p>4 J 3at 10P12024multimode</p><p>k power 12 MWimum PRF 10 Hzelength 9 to 11 mse width 50 to 100 ns FWHM with 3.0-s tailm cross section1multimode2</p><p>30 mm 3 30 mm</p><p>ergence 1multimode2 ,3.5 mrad 1full angle2 with a 30-mradius of curvature front optic1stable cavity2 and intracavityaperture</p><p>er of dischargecircuit</p><p>65 ns for 90% of shots with laserfiring at 10 Hz with respect toexternal command</p><p>ensions 1200 mm 3 710 mm 3 495 mmght 160 kg</p></li><li><p>the same result. However, the minimum permis-sibdiextepewoanqu</p><p>s</p><p>ClPoPoVeAcDCo</p><p>In</p><p>a</p><p>ELPRF,</p><p>6191le clear aperture due to the large laser beamameter is approximately 9 to 10 cm2, requiring ancessively large and expensive electro-optic crystal;lescopic reduction of the beam diameter could berformed to reduce the required aperture. Thisuld increase an already large 13 to 4 mrad fullgle2 beam divergence. Beam expansion subse-ent to the clipper could be performed to bring the</p><p>Table 3. Programmable Scanner Specifications</p><p>Parameter Specification</p><p>ear aperture 457 mm diametersition motion Azimuth 0 to 360, elevation 0 to 1180inting resolution 0.01 1174 rad2locity 5 rpm 130@s2celeration 15@s2 1maximum2rive dc motorntrol Manual hand paddle 1CW, CCW, EL1, EL22</p><p>automatic computer-programmed scansa</p><p>terface RS-232 16-bit parallel</p><p>CW, clockwise; CCW, counterclockwise; EL1, increase elevation;2, decrease elevation.</p><p>Fig. 6. Data acquisition and processing subsystem layout.pulsed repetition frequency.</p><p>20 September 1995 @ Vol. 34, No. 27 @ APPLIED OPTICSmeters, unless pulse-deconvolution techniques areadopted.1012 The simplest and most straightfor-ward method of attaining higher resolution is to clipthe long-lived tail and only allow the short gain-switched spike to be transmitted. This approachwas followed forADEDIS. The plasma-shutter pulseclippers, which are described above and shown in Fig.4, allow adjustable degrees of clipping of the laserpulses. Of course, electro-optic crystals such as CdTeand CdSe could be used between polarizers to achieve</p><p>Fig. 5. Scanner tran ceiver subassembly.</p></li><li><p>divergence back down to an acceptable value. How-ever, the complexity of the processing optical train</p><p>what sensitive to optical alignment. So...</p></li></ul>