co_2 laser-based differential absorption lidar system for range-resolved and long-range detection of...

CO2 laser-based differential absorption lidarsystem for range-resolved and long-rangedetection of chemical vapor plumes

Clinton B. Carlisle, Jan E. van der Laan, Lewis W. Carr, Philippe Adam,and Jean-Pierre Chiaroni

A dual CO2 laser-based differential absorption lidar 1DIAL2 system has been constructed and demon-strated for range-resolved mapping of chemical vapor plumes. The system acquires high rangeresolution through the use of plasma-shutter pulse clippers that extinguish the nitrogen tail of theCO2-laser output. Aprogrammable servomotor-driven scanner allows full hemispherical coverage of theinterrogated field. A high-speed direct-detection receiver subsystem is used to gather, process, anddisplay vapor-concentration data in near real time. Data demonstrating range-resolved detection of lowconcentrations of chemical plumes from ranges of 1 to 2 km are presented. In the column-contentdetection mode, trace levels of secondary vapors from various organophosphate liquids were monitored.Detection of an SF6 vapor plume released 16 km from the DIAL system is also adduced.Key words: Lidar, DIAL, remote sensing.

1. Introduction

Because of the fact that the output radiation from aCO2 laser lies in the middle of the 8-to-12-µm atmo-spheric window, CO2 laser-based differential absorp-tion lidar 1DIAL2 systems have been applied to a widerange of remote-sensing problems over the past 20years.1–6 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-µm 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

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 d’Etudes duBouchet, 91719 Vert-Le-Petit, France.Received 13 April 1994; revised manuscript received 15 March

1995.0003-6935@95@276187-14$06.00@0.

r 1995 Optical Society of America.

creates significant problems for the DIAL receiver,requiring large fields of view that are difficult toobtain when one is using high-speed photodetectors1which are, of necessity, small2 and the large collectionapertures 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-µm 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

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-oped for mapping vapor plumes.8,9 However, they

20 September 1995 @ Vol. 34, No. 27 @ APPLIED OPTICS 6187

have operated with ill-defined range resolution be-cause of the presence of the nitrogen tail in thetransmitted laser beam. There are deconvolutiontechniques 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.

2. System Description

The 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

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 He–Ne 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

Fig. 1. ADEDIS DIAL system.

6188 APPLIED OPTICS @ Vol. 34, No. 27 @ 20 September 1995

determine where the CO2 beams are located at remotedistances from the system.The laser used in ADEDIS is a Laserbrand Model

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

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

Table 1. Specifications of the ADEDIS DIAL System

Parameter Specification

Vehicle and associatedequipment

Vehicle DAF DieselGenerator 55 kWClimate control 15 kW

Transmit and receiversubsystem

Transmit moduleLasers Two tunable, pulsed, TEACO2

lasersPulse width Clipped with plasma shutter to

130 to 160 ns 1FWHM2Energy 1.2 J in gain-switched spike on

10P1202Repetition rate 10 HzWavelength 9.2 to 10.8 µmBeam divergence 3 to 5 mrad 1full angle2Tuning Manual for two lasers

Receiver moduleTelescope 457 mm, [email protected] of view 5 mrad 1with [email protected] field lens2Detector HgCdTe photovoltaic, liquid

nitrogen cooledScanner moduleHorizontal travel 360°Vertical travel 180°Horizontal speed 30°@s

Data acquisition andrecording sub-

systemComputer VAXstation 3100Language FORTRAN 77Data acquisition Tektronix RTD710AInterfaces IEEE-488, RS-232, IEEE-488 to

SCSIDigitization 100 MHz, 10 bitRecorder Cipher 9-track magnetic tape

drive 1Digital Equipment Cor-poration supported2

Fig. 2. Floor plan of ADEDIS. M-1–M-7, mirrors; D-1–D-3, diagnostic path mirrors; G-1, G-2, green He–Ne beam mirrors; BS-1, BS-18,beam splitters; A-1, A-2, alignment diode laser beammirrors; R-1, R-2, red He–Ne beammirrors.

tail 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-µs pulse dura-tions with the full nitrogen tail2.

Fig. 3. Transmit optics layout in DIAL package.

Approximately 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-

bly. The scanner module was developed by DFM,Inc., of Longmont, Colorado. The scanner specifica-tions are listed in Table 3. The coaligned televisioncamera 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-

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@[email protected] [email protected] field lens mounted in the Dewar approxi-


mately 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

Table 2. ADEDIS Laser Specifications


Laser medium CO2 TEA laserPulse width 130-ns gain-switched spike on

10P1202EnergyMultimode 1stablecavity2

5 J 3at 10P12024

Grating tuned 1stablecavity2

4 J 3at 10P12024multimode

Peak power 12 MWMaximum PRF 10 HzWavelength 9 to 11 µmPulse width 50 to 100 ns FWHM with 3.0-µs tailBeam cross section

1multimode230 mm 3 30 mm

Divergence 1multimode2 ,3.5 mrad 1full angle2 with a 30-mradius of curvature front optic1stable cavity2 and intracavityaperture

Jitter of dischargecircuit

65 ns for 90% of shots with laserfiring at 10 Hz with respect toexternal command

Dimensions 1200 mm 3 710 mm 3 495 mmWeight 160 kgInput services 110 VAC, 60 Hz, 16 Awith voltage

taps, making the laser suitable foroperation at 220 V, 50 Hz at afuture date

Water flow rate 2 L@minPremixed gas 1%2He 82CO2 8N2 8CO 2

Gas flow rate 40 cm3@min


Longwood, Florida, is used to amplify the lidar signalsbefore digitization.The data acquisition and processing subsystem is

illustrated schematically in Fig. 6. The waveformdigitizer is a Tektronics RTD 710A model 1dual chan-nel2 with a bandwidth of 100 MHz and up to 10 bits ofvertical resolution. The digitized lidar waveformsare transferred to the VAXstation over a standardIEEE-488 bus followed by an IEEE to SCSI converted.The VAX processes and displays the data in near realtime. In parallel with the processing, the formatteddata are stored to 9-track magnetic tape or the VAXhard disk, depending on operator choice.

3. Data and Test Results

The ADEDIS DIAL system was field tested at Dug-way Proving Ground 1DPG2, Utah, in June 1993. Theobjective of the field test was to establish the mini-mum concentrations of organophosphate vapor plumesdetectable byADEDIS. In addition, the system rangeresolution was independently verified in separateexperiments. The data pertaining to the perfor-mance of the ADEDIS DIAL system divide naturallyinto four categories: test results on pulse clippersand 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.

A. System Range-Resolution Tests

The 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

Fig. 4. Plasma-shutter pulse clipper.

Fig. 5. Scanner transceiver subassembly.

meters, unless pulse-deconvolution techniques areadopted.10–12 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 achievethe same result. However, the minimum permis-sible clear aperture due to the large laser beamdiameter is approximately 9 to 10 cm2, requiring anexcessively large and expensive electro-optic crystal;telescopic reduction of the beam diameter could beperformed to reduce the required aperture. Thiswould increase an already large 13 to 4 mrad fullangle2 beam divergence. Beam expansion subse-quent to the clipper could be performed to bring the

Table 3. Programmable Scanner Specifications


Clear aperture 457 mm diameterPosition motion Azimuth 0° to 360°, elevation 0° to 1180°Pointing resolution 0.01° 1174 µrad2Velocity 5 rpm 130°@s2Acceleration 15°@s2 1maximum2Drive dc motorControl Manual hand paddle 1CW, CCW, EL1, EL22

automatic computer-programmed scansa

Interface RS-232 16-bit parallel

aCW, clockwise; CCW, counterclockwise; EL1, increase elevation;EL2, decrease elevation.

2

Fig. 6. Data acquisition and processing subsystem layout. PRF,pulsed repetition frequency.


divergence back down to an acceptable value. How-ever, the complexity of the processing optical trainwould then be fairly high; loss and near-field scatterfrom all the optical elements would also be signifi-cant.For these and other reasons, the plasma-shutter

clipper was chosen for ADEDIS. To examine theefficacy of clipping, a room-temperature HgCdTe pho-toconductive detector with a bandwidth of 100 MHzwas used to observe a small fraction of the laser beamreflected from a carbon block after transmissionthrough the clipper assembly. An example of clip-ping is illustrated in Fig. 7. The laser line used forthis study was the 9P1202. However, many otherlines were clipped with equal facility, including the9P1442. The resolution in time for the clipping wasapproximately 1 ns and was set by the resolution ofthe digital delay generator used to generate thelaser-fire and pulse-clipper pulses. The degree ofextinction of the clipped portion of the beam is, ofcourse, finite. Careful examination of the baselinereveals that the attenuation of the clipped portion ofthe laser pulse within 500 ns of the breakdown isapproximately 104; the degree of attenuation is some-

Fig. 7. Examples of pulse clipping of 9P1202 laser line.


what sensitive to optical alignment. Some 2 to 4 µsafter breakdown, a small recovery effect occurs, andthe attenuation factor drops to approximately 103.If there is some low level of near-field scatter into thereceiver this effect can be quite significant, as theaerosol backscatter return is of the order of 10214 ofthe transmitted beam power. In this case, the insuf-ficiently clipped, near-field scattered signal 1which isseen in the receiver several microseconds after thepeak laser pulse is transmitted2 can be confused withthe weak signal backscattered from atmospheric aero-sols lying hundreds of meters from the lidar system,thus appearing coincident in time with the near-fieldscatter.To verify the range resolution of ADEDIS with the

pulse clippers operational, the test setup in Fig. 8 wasused. The semitransparent targets consisted of anet of nylon wires strung between two poles. Thesetargets provide a sufficiently strong lidar return sothat accurate measurements can be performed.However, the signals were not strong enough tosaturate the photodetector and thus distort the resolu-tion measurements. By the use of a fully clippedoutput pulse on the 9P1202 line, the lidar waveformshown in Fig. 9 was detected. It is apparent fromthe data that lidar returns from targets separated by20 m can be resolved. Thus the range resolution ofthe system is at least 20 m when it is operating onhigh-gain laser lines. For very weak lines such as9P1442, the risetime of the output pulse is signifi-cantly slower; consequently, the range resolution of

Fig. 8. Range-resolution test setup.

the system is reduced by as much as 25% when it isoperating on those lines

B. Column-Content Sensitivity Measurements

The sensitivity of ADEDIS in the column-contentmode was established by means of measurementsagainst calibrated vapor concentrations of triethyl-phosphate 1TEP2, diethylmethylphosphonate 1DEMP2,and di-isopropylmethylphosphonate 1DIMP2. The di-mensions and details of the vapor chamber used areillustrated in Fig. 10. A flame-ionization-detection1FID2 point sampler sensitive to phosphorous vaporwas used to monitor the concentration in the cham-ber; the location of the FID sensor is shown in Fig. 10.

Fig. 9. Lidar returns from semitransparent targets separated by20 m.

Fig. 10. Layout of vapor chamber.

The procedure observed for the vapor chambertrials was as follows. ADEDIS was configured andbackground data were gathered with the chamberclosed and empty. The background data were typi-cally acquired over a period of 5min. Once a satisfac-tory concentration–path-length 1CL2 baseline had beenestablished, a series of injections of the simulantunder study wasmade. The injection was performedthrough a glove box to avoid opening the chamber.Aseries of syringes filled with the appropriate amountof liquid simulant were used to administer the simu-lant onto a hot plate 1maintained at approximately400 °C2. The evaporated simulant then settled intoan equilibrium vapor concentration in the chamber.The individual injections in the series were carriedout every 5 min, increasing the amount injected witheach step. At the end of the injections, a 5-min airwash of the chamberwas conductedwith fansmountedon the chamber walls to evacuate the chamber and todetermine if the DIAL systemwould reliably return toits zero-concentration baseline.The vapor concentration multiplied by the length of

the chamber 1in this case 6 m2 is the quantity mea-sured by a DIAL system when operated in column-content mode. A very simple calculation establishesthe relation between the concentration–path-length-product 1CL2 value that was measured and the in-jected simulant:

CL 5rliqVinj

A1mg@m22, 112

where C is the concentration of injected simulantvapor 1in milligrams per cubic meter2, L is the pathlength of the vapor chamber 1in meters2, rliq is thedensity of the injected simulant liquid 1in milligramsper liter2, Vinj is the volume of injected simulant liquid1in liters2, andA is the cross-sectional area of the vaporchamber 1in square meters2.If the CL values measured withADEDIS agree well

with the numbers derived by means of Eq. 112, then itis clear that the DIAL system responds correctly andreproducibly. In all the vapor-chamber trials, thelisted CL measured with ADEDIS for each injectionrepresents the change in CL on injection of thesimulant and not the cumulative quantity in thechamber.The CL values detected by the point samplers and

the DIAL system can be compared during a giventrial. Historically, the level of agreement betweenpoint sampling and infrared DIAL technologies hasbeen modest at best.13,14 However, the comparisonbetween the two detection techniques is interestingscientifically. Therefore the data sets from ADEDISand the FID are compared for the vapor-chambertests discussed below.A critical performance parameter for ADEDIS is

the standard deviation of its CL baseline, sCL. Byconvention, sCL is equated with the minimum detect-able CL value 1also termed the system sensitivity2 fora DIAL operating in column-content mode. It is well


established that, for the case of uncorrelated fluctua-tions in the two DIAL channels, sCL can be expressedas15,16

sCL 51

Œ21Da2SNR, 122

where Da is the differential absorption coefficient forthe absorbing vapor being detected 1in square metersper milligram2 and SNR is the mean of the signal-to-noise ratios of the two lidar echoes from the hardtarget.Column-content trials were conducted on TEP,

DEMP, and DIMP. The performance of ADEDIS inmonitoring calibrated concentrations of TEP second-ary vapors is discussed in detail below. The absorp-tion spectra for TEP 1along with DEMP and DIMPfor comparison2 is shown in Fig. 11, along with theabsorbed and reference laser lines used. These ab-sorption spectra were gathered with a Fourier-transform infrared spectrometer with 0.1-cm21 spec-tral resolution.A TEP vapor-chamber trial was conducted using

the test setup in Fig. 12. Liquid TEP has a densityrliq 5 1.07 3 106 mg@L and a molecular weight of 182.The absorbed and unabsorbed laser wavelengths cho-sen for this trial were 9P1202 and 9R1262, respectively,resulting in a Da 5 1023 m2@mg. Table 4 presentsthe sequence of events for this TEP trial. Figure 13is a plot of the CL versus time data recorded byADEDIS. The numbers on the plot correspond to

Fig. 11. Absorption spectra of TEP, DEMP, and DIMP.


the events listed in Table 4. In all the vapor-chamber trials, background data collection was initi-ated exactly 5 min before the first injection. The

Fig. 12. Test setup for the vapor-chamber trials.

Table 4. TEP Vapor-Chamber Trial Sequence of Events

Event Time Description

1 14:18 ADEDIS begins collecting data2 14:28 Begin 5-min background data3 14:33 Injection of 20 µL of TEP4 14:38 Injection of 50 µL of TEP5 14:43 Injection of 100 µL of TEP6 14:48 Injection of 250 µL of TEP7 14:53 Begin 5-min air wash of chamber8 15:09 Conclude data collection

baseline mean and standard deviation values werecalculated over that period to remain consistent withthe data gathered by the other detection devices inuse.Several qualitative observations can be made on

examination of the CL versus time plot. First, thereis baseline drift with a very long 1tens of minutes2 timeconstant that is apparent from the data. This is notuncommon in DIAL data and can be the result of avariety of physical mechanisms both in the chamberand in the optical train of the lidar. This drift issubtracted 1to the extent allowed by the inherentsignal accuracy2 in the data analysis. Second, thesensitivity of the lidar is such that the injection of 20µL of TEP produces a noticeable rise in the CL versustime curve. The signal level is approximately equalto the baseline sCL for this injection, indicating 1cor-rectly2 that this CL product produces a thresholdsignal in the lidar.Table 5 summarizes the results of the TEP vapor-

chamber trial. The agreement between the mea-sured CL values 1listed in column 22 and the CLvalues calculated from Eq. 112 1listed in column 32 isadequate, with increasing accord between them asthe injection dosage is increased. The CL valuesmeasured by the FID 1listed in column 42 agree atleast roughly with the values from ADEDIS. Thereare many possible explanations as to why the twotechniques are not in even better agreement. Firstand foremost, ADEDIS 1when operated in column-content mode2 measures a CL and the FID measures

Fig. 13. CL versus time for TEP vapor-chamber trial.

Table 5. Summary of Results from TEP Vapor-Chamber Trial

Injectionof TEP 1µL2

CLMeasuredby ADEDIS1mg@m22

CL Calculatedfrom Eq. 1121mg@m22

CLMeasuredby FID1mg@m22

20 2.4 2.4 2.550 8.3 6.0 5.4100 15.3 12.0 11.9250 31.9 30.0 21.6

the concentration of phosphorous at a given point inspace. Therefore, if the vapor does not mix homoge-neously throughout the chamber, the results willdiffer. Analysis of the background fluctuations re-veals that sCL 5 2.0mg@m2. FromEq. 122 this impliesan operational SNR of 340. These data were gath-ered by the use of 16-shot averaging along withKalman filtering17 of the time series.

C. Range-Resolved Sensitivity Measurements

Figure 14 shows the setup used for the range-resolvedtrials of ADEDIS. TEP was the only simulant testedin the open field because of the cost of the othersimulants as well as environmental considerations.These trials were designed to test the ability ofADEDIS to detect and map clouds of evaporated TEPin a range-resolved mode with 20-m range cells.Simulant disseminations in the open field are inher-ently difficult to calibrate. The volume in spaceinterrogated by the DIAL is so large that only anenormous array of point sensors would be capable ofproviding comparison data. The evolution of anevaporated simulant cloud is highly dependent on theprevailingmeteorological conditions, as previous stud-ies by the Centre d’Etudes du Bouchet14 have shown.Unfortunately, the conditions at DPG 1intense directsunlight, high winds, and low atmospheric pressure2

Fig. 14. Test setup for range-resolved detection trials.


tend to induce irregular evaporation of the simulantcloud followed by dispersal upward of the cloud by thecombined effects of high winds and low atmosphericpressure. Data were gathered byADEDIS in a scan-ning mode over the test grid for several open-fieldtrials. The TEP was disseminated by truck in themanner illustrated in Fig. 15. The disseminationlocation and lidar line-of-sight scan pattern are de-picted in Fig. 14. The wind velocity during the testwas approximately 2 to 3 m@s. The prevailing winddirection during dissemination is indicated in Fig. 14.The pulse clippers were used throughout the test,yielding a range resolution of 20 m. From the clear-air returns of the range-resolved lidar signals beforedissemination, the standard deviation of the concen-tration estimates 1sc2 was measured to be 0.13 mg@m3

1at a range of 1 km from the lidar2. sc is calculated bymeans of the expression

sc 51

Œ2DaDRSNR, 132

where DR is the range resolution of the system.Note that the SNR is now range dependent and, for

the case of thermal-background-limited detection1which generally holds in the weak-signal limit in themid-infrared2, is determined from the relation

SNR 5ŒND*ETcbho exp122d2A

2R2ŒAdB, 142

where N is the number of pulses averaged, D* is thephotodetector detectivity, ET is the transmitted laserenergy, b is the atmospheric volume backscattercoefficient, ho is the optical transfer efficiency, exp122d2is the atmospheric-transmission factor, A is the re-ceiver telescope area, Ad is the photodetector area, Bis the electronic detection bandwidth, and R is therange to the sampled volume.Inserting the system parameter values forADEDIS

during the open-field tests 1listed in Table 62 into Eq.142 yields an operational SNR 1at 1 km2,

SNR . 350, 152

which in turn allows calculation of the anticipated scand comparison with the experimentally determined

Fig. 15. Dissemination technique for TEP in range-resolvedtrials.


value,

sc 5 0.10 mg@m3 1calculated2,

sc 5 0.13 mg@m3 1measured2. 162

The agreement between the measured and the calcu-lated sensitivities is quite good, given the manydegrees of freedom in such a DIALmeasurement.Figure 16 presents the evolution of the TEP second-

ary vapor cloud during the course of the trial. Threecomments can bemade concerning these data. First,the noise level of the DIAL is quite low in the rangecells of interest. Only in the near-field scatter, ex-tending to 300 to 400 m from the system 1because ofincomplete extinction of the nitrogen tail2 are thefluctuations in the backscatter returns significant.Second, structure of dimensions less than 100 m isevident in the TEP cloud data, indicating that ADE-DIS was indeed operating in a high-resolution mode.Third, the concentration of the secondary vapor cloudsproduced during the trial was in general low. This isattributable to the atmospheric conditions at DPG,where the atmospheric pressure is low and the sun-light intense.

D. Long-Range Vapor Detection

A long-range column-content measurement of SF6was conducted with the test scenario shown in Fig.17. Approximately 14 min into the trial the bottle ofSF6 was opened for approximately 3 min. Figure 18shows the CL versus time plot of the trial. Althoughthe SNR of the lidar returns at this target distance islow, it is obviously sufficient to produce a measurableCL value, as Fig. 18 clearly indicates. The largedifferential absorption coefficient of SF6 1Da . 1022

m2@mg or 633 atm21 cm212 certainly facilitates detec-tion.18 1For comparison, SF6 has a differential ab-sorption coefficient that is approximately 10 timesthat of TEP.2 However, the plume produced byopening a bottle of SF6 in high winds 16 km awayfrom the DIAL system was small and doubtlessinteracted with only a fraction of the ADEDIS laserbeams, each of which was approximately 50 m indiameter at the vapor-release point. This effect actsto reduce the sensitivity of the DIAL system and

Table 6. ADEDIS System Parameters for Open-Field TEP Trials

Parameter Symbol Value

Laser output energy1clipped2

ET 0.7 J

Number of effective pulseaverages per line of sight

N 100

Photodetector detectivity D* 6 3 1010 1cm Hz1@22@WVolume backscatter coeffi-cient

b 3 3 1028 m21 sr21

Optical transfer efficiency ho 0.65 1measured2Atmospheric transmission exp122d2 0.9Receiver telescope area A 0.15 m2

Photodetector area Ad 3.1 3 1022 cm2

Electronic detection band-width

B 8 3 106 Hz

make long-rangeDIALdetection all themore challeng-ing. Nonetheless, this experiment demonstrates thepowerful ranging andDIAL detection capability of theADEDIS system.As a final experiment, the nature of the reflection

surface on lidar signal returns from 16 km wasstudied. Figure 19 illustrates schematically the ex-perimental scenario. An oscilloscope photographwastaken of the raw lidar signal return when a laseroperating on line 10P1202 with Granite Mountainitself as the target was used. This is shown in Fig.20a. Next, a 15-cm-diameter retroreflector wasplaced in the beam and, with everything else identi-cal, another oscilloscope photograph was taken of thelidar return, with the result shown in Fig. 20b. Thedifference in signal levels, although at first surpris-

Fig. 16. Range-resolved monitoring of the evolution of a TEPvapor cloud.

ing, can be easily explained by means of some simplecalculations.The lidar equation for a hard-target return from a

Lambertian scattering surface can be written as

Pr 5 PtrA

R2ho exp122d2, 172

where Pr is the received backscattered signal power1in watts2, Pt is the peak transmitted signal power 1inwatts2, r is the surface reflectivity 1in inverse sterudi-ans2, A is the receiver telescope area 1in squaremeters2, R is the range to the target 1in meters2, ho isthe optical efficiency of the transmission system, andexp122d2 is the atmospheric-transmission factor.

Fig. 17. Test scenario for long-range vapor detection.


All the factors other than the transmitted powercan be grouped together to form a single factor f,representing the fraction of the transmitted powerthat is returned to the lidar receiver. Inserting thevalues from Table 7 into Eq. 172 yields the result that

f 5 4 3 10214 1Lambertian surface2, 182

with the received power then being

Pr 5 2 3 1027 W, 192

and a single-shot SNR being equal to

SNR 5 20, 1102

which is roughly consistent with the value measuredbyADEDIS.To calculate the strength of the return signal from

the retroreflector, one must first determine the frac-tion of the lidar beam that it intercepts. This issimply the ratio of the area of the retroreflector1Ar 5 2 3 1022 m22 to the area of the transmit beam1Ab2 at 16 km. The laser line used in this experimenthad a full-angle divergence of 3.2 mrad; Ab is thenquickly calculated to be 2 3 103 m2. Therefore thefraction of the transmit beam intercepted by theretroreflector, finter, > 1025. This fraction of thetransmit beamnow returns toward the lidar, propagat-ing as a diffraction-limited source with output aper-ture equal to 15 cm. The return beam then occupiesan area of approximately 6 m2 when it reaches thelidar receiver 1area 0.15 m22. Including the atmo-spheric-propagation constant value from Table 7 andthe reflectivity of the retroreflector, which was 0.5 1theretroreflector mirrors were badly weather worn2, thetotal fraction of the return beam received by the lidaris then calculated to be

f 5 finter10.156 2exp122d20.5 > 1029, 1112

which is some 4 orders of magnitude larger than theLambertian scattered signal. If the pointing field ofview for the retroreflector is misaligned, the differ-

Fig. 18. CL versus time of SF6 detection from 16 km.


ence in signal levels between the two cases may bereduced slightly.It would have been preferable to insert sufficient

optical attenuation into the lidar receiver for theretroreflector return signal so that an exact, quantita-tive comparison could be made between the twotarget scenarios. Nevertheless, this example force-fully illustrates the enormous loss in lidar signalwhen receiving isotropically 1or even quasi-isotropi-cally2 scattered signals due to the infinitesimallysmall solid angle occupied by the lidar receiver at longranges. 1In this example, the ADEDIS receiver occu-

Fig. 19. Test setup for study of surface reflectivity effects inlong-range DIAL detection.

pied a solid angle of 5 3 10210 sr, as seen from thetarget.2 Furthermore, for those interested in ultra-long-range 1i.e., at distances of 100 km or more2 DIALmonitoring of certain fixed installations, this exercisedemonstrates that the use of a few strategicallyplaced cooperative targets can make an impossibledetection problem quite feasible. The strong retrore-flector signal return relative to the normal Lamber-tian-scattered returns would also greatly facilitatealignment and target acquisition 1which are not

Fig. 20. Lidar signal returns from 16 km.

Table 7. ADEDIS System Parameters for Detection of Hard-TargetReturns from Lambertian and Specular Scatterer at Long Distances

Parameter Symbol Value

Transmitted peak laser output Pt 5 3 106 WLaser divergence 1full angle2 uD 3.2 mradSurface reflectivity r 1023 sr21

Receiver telescope area A 0.15 m2

Optical efficiency of transmitter ho 0.65Atmospheric-transmission factor exp122d2 0.1Range to target R 16 km

trivial tasks when operating at 100 km or greaterdistances2 for the lidar. It should be noted thatspeckle noise and turbulence effects increase enor-mously if a single, small retroreflector is used. Theretroreflector signal shown in Fig. 20b exhibitedshot-to-shot fluctuations of approximately 100%.However, a small group of retroreflectors could beplaced to allow spatial averaging and thus greatlyreduce the fluctuations.

4. Conclusions

A high-power, high-resolution, CO2 laser-based DIALsystem called ADEDIS has been demonstrated as asensitive device for monitoring organophosphate va-por plumes in both range-resolved and column-contentmodes. In the column-contentmode of opera-tion, the vapor produced by as little as 20 µL of TEPliquid injected into a 54 m3 vapor chamber wasdetected. Secondary vapor plumes of TEP were de-tected and mapped at concentrations of 0.2 mg@m3 atranges between 1 and 2 km. A release of SF6 vaporsome 16 km from ADEDIS was also detected incolumn-content mode, demonstrating the long-rangedetection capability of the system. Furthermore,initial experiments indicate that, if a cooperativetarget is used, DIAL monitoring of vapor plumes atranges of 100 km byADEDIS appears feasible.The support of Steven Gotoff of the U.S. Army

Edgewood Research, Development, and EngineeringCenter of Edgewood, Maryland, in the planning andsponsoring of the tests at DPG is gratefully acknowl-edged.

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