frequency-doubled co_2 lidar measurement and diode laser spectroscopy of atmospheric co_2

Frequency-doubled CO2 lidar measurement and diode laserspectroscopy of atmospheric CO2

Jack L. Bufton, Toshikazu Itabe, L. Larrabee Strow, C. Laurence Korb,Bruce M. Gentry, and Chi Y. Weng

A lidar instrument based on pulsed frequency-doubled carbon-dioxide lasers has been used at 4.88 Am forremote sensing of atmospheric carbon dioxide. A tunable-diode laser spectrometer provided the high-reso-lution spectroscopic data on carbon-dioxide line strength and line broadening needed for an accurate differ-ential absorption measurement. Initial field measurements are presented, and instrument improvementsnecessary for accurate carbon dioxide measurement are discussed.

1. Introduction

A lidar instrument based on pulsed carbon-dioxide(CO2) lasers has been applied to the measurement ofatmospheric CO2 concentration in the earth's boundarylayer. Two lines of the 9-,um P branch CO2 laser arefrequency-doubled in a nonlinear infrared crystal to twowavelengths in the 4.88-,um region for this measure-ment. One frequency-doubled line is strongly absorbedby atmospheric CO2 , and the other is used as a referencein a conventional differential absorption lidar (DIAL)technique. Measurement of transmitted and receivedpulse energies at the two frequency-doubled source linesprovides the raw data necessary for computation of CO2differential absorption. Precise spectroscopic data onthe atmospheric CO2 line and interfering species, mainlywater (H20) vapor, are also required to invert absorp-tion data to obtain CO2 path-averaged concentration.Important supplemental information required is H20concentration and atmospheric temperature.

Verification of this DIAL technique was accom-plished first in the laboratory by passing the fre-quency-doubled radiation through a sample cell filled

When this work was done both Jack Bufton and Toshikazu Itabewere with NASA Goddard Space Flight Center, Instrument Elec-tro-Optics Branch, Greenbelt, Maryland 20771; the latter is now withMinistry of Posts & Telecommunications, Radio Research Labora-tories, Koganei, Tokyo, Japan. Chi Weng is with Science Systems& Applications, Inc., Seabrook, Maryland 20801; the other authorsare with NASA Goddard Space Flight Center, Laboratory for At-mospheric Sciences, Greenbelt, Maryland 20771.

Received 2 March 1983.

with CO2 and measuring on- and off-line absorption.The results were not precise but were in general agree-ment with spectroscopic data. In a parallel effort, theline strength and broadening coefficients of the atmo-spheric CO2 line at 4.88 gzm were measured with a tun-able-diode laser spectrometer. These results are be-lieved accurate to 3% or better and together with recentdata1 on the center frequency provide the requiredspectroscopic parameters for CO2.

The frequency-doubled DIAL instrument was thenused to acquire a set of demonstration CO2 measure-ments over a 600-m horizontal path near the earth'ssurface. The lidar results are presented here, and a fulldiscussion of data error sources is included to indicatehow these infrared DIAL measurements can be im-proved to yield accurate CO2 concentrations. The de-sire to make precise, rapid, remote-sensing measure-ments over areas of sources and sinks of atmosphericCO2 provides the motivation for this instrument de-velopment effort. These measurements are neededbecause of the significance of atmospheric CO2 as aclimate parameter. Short-term temporal and spatialchanges in atmospheric CO2 concentration are expectedto be 10% or less. As a result, much attention must bedevoted to the improvement of DIAL measurementaccuracy.

The DIAL measurement concept at 4.88 gim is de-tailed below followed by a report on CO2 spectroscopywith a tunable-diode laser spectrometer. We then givea description of frequency-doubled lidar instrumenta-tion and measurements over laboratory and fieldpropagation paths. This is followed by a detailedanalysis of DIAL error sources and recommendationsfor instrument improvement.

2592 APPLIED OPTICS / Vol. 22, No. 17 / 1 September 1983

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II. Dial Measurement Technique

The use of a CO2 lidar system operating near 10 gmwas first considered for this measurement since the CO2laser is efficient and has lines coincident with atmo-spheric CO2 absorption lines. Unfortunately, the 9.4-and 10.4-gm CO2 bands are hot band transitions, andthe resulting absorption is strongly temperature de-pendent. Murray et al. 2 have, in fact, reported mea-surements of atmospheric temperature with a pulsedC02 DIAL system at 10.6 gm. An analytical treatmentof temperature effects in DIAL measurement is givenby Korb and Weng.3 A 1-K uncertainty in temperatureproduces nearly a 3% change in absorption coefficientat this wavelength. Even an accurate knowledge oftemperature distribution along the propagation pathwould now allow accurate CO 2 measurement in the10-gm, region since the weak absorption, typically 0.1km-1 at line center, magnifies the effect of lidar mea-surement error. This effect is well covered in the lidaranalysis of Schotland 4 or Byer. 5

The CO2 Fermi diad band (1110, 0310)2 - 00°0centered at 2076.87 cm-' or 4.81 gm originates from theground state. As a result, absorption in this band ismuch less temperature sensitive. The change in ab-sorption coefficient is reduced to <0.3% for a 1-K tem-perature uncertainty. The absorption coefficients inthis band are also significantly stronger, which enhancesthe accuracy of a DIAL measurement. Korb et al.6described the near coincidence they found for thedoubled-frequency of the 9.75-gim P(42) line of the12 C16 02 laser and the 4.88-gim P(34) line of the Fermidiad band of CO2. Their plot of K, the optical depthper kilometer, for this line including the background ofinterfering water vapor lines and continuum has beenrevised and is shown here as Fig. 1. The CO2 lines ad-jacent to the P(34) line have been eliminated from thefigure for clarity, although their line centers are indi-cated along the top. The results of Fig. 1 are based onthe most recent (1982) AFGL line parameters7 and aline-by-line transmission program that includes all

Fig. 1. Concept for differential absorption lidarmeasurement of CO 2 near 4.8-,4m wavelength.

26K

2MIl/

water lines within +10 cm-' of each frequency, an at-mospheric temperature of 286 K, and sea level pressure.Water vapor concentration is fixed at 1% specific hu-midity (0.01 atm). The center of the frequency-dou-bled CO2 laser line is at 2050.596 cm-', just 0.031 cm-1

from the P(34) line center at 2050.565 cm-. The CO2laser line position is that reported by Freed et al.,8 andthe frequency for the P(34) line is the recently deter-mined value of Rinsland et al. 1 Despite this near co-incidence, the frequency offset and stability of the fre-quency-doubled CO2 laser radiation are important ef-fects which could ultimately determine measurementaccuracy. This point is discussed in Sec. V.

Korb et al. 6 identified the frequency-doubled 9.72-,m R(14) line of the 1 3 C1 6 02 laser as the best candidatefor the reference lidar line. Its location at 2057.024cm-1 appeared from prior AFGL line parameters to belargely free from CO2 absorption and well-matched inH20 absorption to that at the signal line. The newAFGL line parameters7 show significant revisions inH20 absorption line centers and strengths, with theresult that there is now a weak H20 line near the P(34)CO2 line center and a poor match in H20 absorption toRef. 1. For the initial measurement reported here wehave used the frequency-doubled 9.73-gm P(40) line ofthe 12C160 2 laser at 2054.764 cm-1 as a practical choicefor the lidar reference measurement. It has the ad-vantage of being available from the natural isotopiclaser. It is also closer to the signal line center, whichwould reduce the effect of any differential reflectancefrom topographical targets. Use of the 9.72-gm R(14)line would require sealed-off laser operation because ofthe C13 isotope expense. Both reference lines are in-dicated in the spectrum of Fig. 1 and absorption coef-ficient values in (cm atm)- 1 for H2 0 interference at thelidar signal line, and the two references are listed inTable I. Temperature dependence of this interferenceis illustrated by values for 286 and 296 K. The H20absorption coefficient at Ref. 2 shows a better match tothe coefficient at the signal line but is still a factor of 2

1 September 1983 / Vol. 22, No. 17 / APPLIED OPTICS 2593

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Table 1. Absorption Coefficients for DIAL Measurements of CO2

TEA Doubled Wave- Temper- Absorption coefficientslaser frequency length ature (cm atm)Y'line (cm-,) (Am) (K) CO2 H 2 0

9P(42) 2050.596 4.877 286 0.0118 AFGL 7.8 X 10-5

0.0106 GSFC

296 0.0121 AFGL 1.1 X 10-4

0.0108 GSFC9P(40) 2054.764 4.867 286 7.6 X 10-4 3.8 X 10-5

296 7.6 X 10-4 4.3 X 10-5

9R(14)a 2057.024 4.861 286 1.2 X 10-1 2.1 X 10-5296 1.2 X 10-3 2.2 X 10-5

a 3 CI6 0 2 laser.

less. The net H20 differential absorption effect is anoptical depth of 0.04 km-' or -10% of the CO2 differ-ential absorption of 0.432 km-' for 286 K and 1% spe-cific humidity. A 10% H2 0 concentration error wouldresult in a 1% CO2 concentration error, which indicatesthe need for independent H20 concentration data foraccurate CO2 measurement. Note also the strongtemperature dependence for H20 absorption.

The lidar equation which expresses received laserenergy ER in terms of transmitted energy ET, targetproperties, range, atmospheric transmission, and lidaroptical parameters is given for the case of a discretetarget of reflectance (p/Q) at range Z by the expres-sion

ERA = ETAR - LoX exp[-2 Jo 1 i(,7,)Mi(1tX)dnl, (1)

where AR is the receiver telescope area, Lo; is the opticalsystem loss factor at wavelength X, and cxi and Mi rep-resent, respectively, the absorption coefficient andconcentration of atmospheric specie (i), which in generaldepend on X and on space and time over the propagationpath. In this expression the more complete analysis ofMeasures9 has already been simplified to assume novariations in equation parameters or laser parameterswithin the laser pulse length and no effect of the at-mosphere other than absorption by trace species. Inthe DIAL technique measurements of ETX and ERX atthe signal and reference X values provide a determina-tion of the differential absorption ratio as

? =(ERET) = exp[-2Z(Aaco 2 Mco2 + AaH2OMH 2 )I (2)(ERY/ETY)

where the subscript x refers to on-line and y refers tooff-line lidar lines, and the path-averaged differentialabsorption coefficients are denoted by AaCo2 andALaH20 for the two principal species CO2 and H2 0 in ourapplication. The concentration of CO2 is then givenby

Mco2 = - (n? + AaH2O -MH2 O)/Aaco2 . (3)

In the discussion of our data in Sec. IV the measuredvalues of YR will be compared with predicted valuesbased on AFGL line parameters, our CO2 spectroscopy,measured H20 concentrations, and an assumed value(340 ppm) for CO2 concentration.

III. Laboratory Spectroscopy of the P(34) CO2 LidarLine

A. Tunable-Diode Laser Spectrometer

The parameters required for the interpretation of thelidar data are the CO2 absorption line strength, air-broadened width, and center frequency. We presenthere measurements of the strength, N2-broadeningcoefficient, and air-broadening coefficient of the P(34)line made with a tunable-diode laser spectrometer. Thenarrow tuning range of our diode laser mode (0.2 cm- 1)did not allow a measurement of the line-center fre-quency relative to a calibration standard (such as carbonmonoxide (CO) or carbonyl sulfide (OCS). Fortunately,Rinsland et al. 1 recently measured the center frequencyof the P(34) line, and their result can be used in theanalysis of the lidar results. Their preliminary valuefor the line center is 2050.5654 + 0.0004 cm-1. This is<0.001 cm-' from the AFGL line parameter compila-tion7 value of 2050.566 cm-.

Features of the tunable-diode laser spectrometer havebeen described in detail elsewhere.10 A lead(Pb) saltdiode laser is mounted in a closed cycle refrigeratorcontained within a Laser Analytics model LS-3 lasersource spectrometer. An off-axis parabola collimatesthe radiation emerging from the exit slit of the mode'selecting grating monochromator in the LS-3. Thecollimated radiation is split into two beams of ap-proximately equal intensity with an uncoated piece ofgermanium wedged slightly to eliminate interferencefringes. One beam then passes through a 7.65-cm longtemperature-stabilized (1 mK) solid germanium etalonfor relative frequency calibration and is focused ontoa photoconductive HgCdTe detector with an off-axisparabola. The second beam traverses a 1-m base-pathmultiple-pass (White) cell and is focused with a lensonto a second HgCdTe detector.

The spectra are collected using the method of sweepintegration as described by Jennings." 1 The laser ismodulated by a sawtooth waveform phase-locked to a24-Hz chopper placed before the entrance slit of thegrating monochromator. The detector signals areamplified and fed to a dual-channel signal averagerwhich is phase locked to the laser modulation. Thechopper has a duty cycle of 15%, so that during eachsweep of the laser frequency the beam is blocked 15%of the time, thereby producing a 0% transmission levelduring a portion of each sweep. The digitized spectraof 1024 points/channel are read into a computer fromthe signal averager and stored on magnetic tape.

The CO2 gas had a stated purity of 99.995% and wasof natural isotopic composition. Pressures were mea-sured with MKS Baratron capacitance pressure gaugeshaving full-scale ranges of 1000 and 10 Torr. The 10-Torr gauge was calibrated at the National Bureau ofStandards before the start of this experiment. At 0.3Torr, the pressure at which the line strength was mea-sured, the 10-Torr gauge is conservatively estimated tobe accurate to +1.5%. The 1000-Torr gauge was cali-brated within the past year by the manufacturer and isestimated to be accurate to at least +0.5% for the pres-


frequency scale. A least-squares fit of the 100%transmission region surrounding the absorption line toa cubic polynomial is used to interpolate the 100%transmission level in the region of the line.

The equivalent width is defined by

W = f[1 - exp(-k(v)X)]dv,

Fig. 2. Low-pressure (0.309 Torr) spectrum of the P(34) CO2 linenear 4.88 ,m averaged over 2500 tunable-diode laser scans for a White

cell path of 20.26 m.

sures used in the broadening measurements. A plati-num resistance thermometer monitored the gas celltemperature to better than +0.5 K.

As previously mentioned, the laser mode used torecord the P(34) line could be continuously tuned only0.2 cm-1, and, therefore, no OCS or CO calibration linescould be reached with this laser mode. However, lasermodes operating at the P(32) and P(36) lines of CO2could be continuously tuned to nearby OCS lines. Thisallowed us to positively identify both of these CO2 linesas well as the P(34) line.

B. Line-Strength Measurement

The spectrum of the P(34) line was recorded at apressure of 0.309 Torr and path lengths of 16, 20, and36 m for determination of the line strength. The linewas recorded three times at each path length. Eachspectrum was averaged for either 1.7 (2500 sweeps) or2.6 min (3800 sweeps). The 0% transmission level de-tected during the closed portion of the chopper cyclewas checked by comparison with a spectrum taken withthe CO2 absorption saturated. This saturated ab-sorption level deviated from the chopper 0% transmis-sion level by 0.6% of the full-scale intensity. The in-tensity scale of all the spectra were subsequently ref-erenced to the 0% transmission level given by the satu-rated absorption. A sample of one of the low-pressurespectra is shown in Fig. 2.

The line strength was determined from the observedspectra using the method of equivalent widths.'2 Thehigh signal-to-noise and near-linear 100% transmissionbase line of the spectra allowed us to use this methodwith good accuracy. The observed low-pressure line-widths were on the average -1.5% (or 3 X 10-5 cm-1)larger than the calculated width,' 3 indicating that thespectra are slightly distorted by the laser linewidth orby smearing in the averaging process. The dispersionwas determined from the etalon fringes using a calcu-lated value for the free spectral range.'0 The etalonfringe peak positions are found from a fit of the regionaround each peak to a cubic polynomial. A fit of thesefringe peaks to cubic spline functions gives the relative

(4)

where X is the pressure path-length product, and k (v)is the absorption coefficient at frequency v. For aDoppler profile, W is related to the line strength Sby'2

W= SX= (-1)n(ln2/r)n/ 2 (SXYD)n (5)n=O (n + 1)! (n + )1/2

where YD is the Doppler halfwidth. The line strengthis determined from W by iteratively solving Eq. (5).The measured values of W ranged from 0.0017 to 0.0031cm-'. Even at 0.3 Torr, residual pressure broadeningcan introduce errors into strengths determined usingEq. (5). The table of Jansson and Korb,14 which tab-ulates W for Voigt lines, was used to correct ourstrengths as determined from Eq. (5) for the effects ofresidual pressure broadening. These corrections de-pend on the value of W and ranged from 0.5 to 1.5%. Aself-broadening coefficient' 6 of 0.092 (cm atm)-' wasused to determine these corrections. The resulting linestrengths were then normalized to 296 K, the temper-ature at which line strengths are tabulated in the AFGLline parameter compilation.7

The average of the nine measurements is S = (1.191+ 0.029) X 10-22 cM-1/Mol cm2 . The error in S is theaddition of the statistical error and the maximum pos-sible systematic error of 1.5% in the pressure measure-ments. The AFGL line compilation7 lists S = 1.331 X10-22 cm-1/mol cm2, almost 12% higher than our mea-sured value. Measurements of individual line strengthsin this band have been recently reported by Arcas etal. 17 Using a grating spectrometer with a resolution of0.025-0.030 cm-' they measured the strengths of anumber of lines in this band to determine the Coriolisinteraction constant. They report a measured value forS of 1.246 X 10-22 cm-1/mol cm2 and a fitted value of1.323 X 10-22 cm-1/mol cm2 but no absolute error barsfor the measured strengths. These two values for S are4.6 and 11% larger than ours, respectively.

C. Line-Broadening Measurements

Several measurements of the N2 broadening coeffi-cients of CO2 exist for various lines of other bands.' 8"19

These coefficients should vary little from band to bandcompared to their variation with rotational quantumnumber. However, for the highest accuracy in atmo-spheric applications, the air-broadened width shouldbe used. No measurements of CO2 air-broadenedwidths could be found in the literature.

The N2 pressure-broadened spectra of the P(34) linewere taken at eight N2 pressures ranging from 87 to 380Torr. The air-broadened spectra (room air was used)were taken at four pressures that ranged from 91 to 227Torr. In both sets of data the CO2 partial pressure waskept close to 1.8% of the total pressure. The line was


Z 0.8

z 0.6ccI-

J 0.4

3 0.2

2050.550 2050.575 2050.600FREQUENCY (cm-')

Fig. 3. Nitrogen-broadened P(34) CO2 line shapes for a CO2/N 2mixing ratio of 1.8% and a White cell path of 44.2 m.

E

I-

-a

-J

N

0A:

0.A

100 200TOTAL PRESSURE (Torr)

Fig. 4.

recorded three times at each pressure followed by ameasurement of the background intensity level with thecell empty. Each spectrum was averaged for 41 sec(1000 sweeps) and taken at a path length of 44.2 m.

A computer program is used to ratio each spectrumto the appropriate background scan and determine thehalfwidth from least-squares fits of portions of the lineprofile to a cubic polynomial. The Lorentz pressure-broadened widths are determined from the observedVoigt widths using an expression given by Olivero andLongbothum.16 When inverted, this expressionyields

TL = yv 17.7254 - 6.7254 [1 + 03195(YD/YV)] 21,

0.04EI-or

2 0.03a:

N 0.02?-

0J 0.01

(6)

Nitrogen-broadened P(34) CO2 linewidth vs total pressurein cell.

100 150TOTAL PRESSURE (Torr)

where YL, yv, and YD are the Lorentz pressure-broad-ened, the Voigt, and the Doppler widths, respectively.The resulting values for YL were adjusted to account forthe slight amount of self-broadening present and thenaveraged for each pressure group. This adjustmentassumed a self-broadening coefficient'6 of 0.092 (cm -atm)-'. The pressure-broadening coefficient was thendetermined from a least-squares fit of the Lorentz widthvs pressure to a straight line through the origin. Figure3 shows several ratioed scans of the nitrogen-broadenedP(34) line. The line-center absorptions are all aboutthe same since the CO2 mixing ratios at each pressurewere almost equal. The pressure-broadening coeffi-cients derived from the least-squares fits are shown inFigs. 4 and 5. The pressure-broadening coefficientsderived from these fits are

7= 0.0732 0.0009 (cm . atm)1, (7)

TAIM = 0.0700 0.0010 (cm atm)l.

The error bars are the addition of the statistical errorfrom the fits and a maximum possible systematic errorof 0.5% in the pressure measurements. Eng andMantz18 reported a value of 0.0752 + 0.0038 (cm -atm)-1 for the N2-broadening coefficient of the P(34)line of 2 of CO2. Their value and ours overlap withinthe stated error bounds. Our measurements indicatethat the air-broadened width of this line is slightly lowerthan the N2-broadened width. Both the air and N2-

Fig. 5. Air-broadened P(34) CO2 linewidth vscell.

total pressure in

broadened coefficients measured in this study are largerthan the 0.068 (cm -atm)-' coefficient listed for this linein the AFGL line parameter compilation.7

IV. Lidar Measurements

A. 4.88-Am DIAL Instrument

The lidar instrument we developed is based on twoCO2 transversely excited atmospheric (TEA) lasersystems. It is diagrammed in Fig. 6. The TEA lasersemit pulses with a characteristic initial spike of'100-nsec width and total pulse energy of -100 mJ atthe 9.73-gim P(40) and 9.75-gm P(42) lines used in thisexperiment. This DIAL instrument is a modificationof that reported by Bufton and Stewart20 by the addi-tion of a second TEA laser for near-simultaneous on-lineand reference measurements and the use of a fre-quency-doubling crystal. The lidar system is similarto the frequency-doubled TEA laser direct-detectionDIAL system used by Killinger et al. 21 and Menyuk etal. 22 for measurements of carbon monoxide and nitricoxide near 5-gtm wavelength. The nonlinear infraredcrystal which we use for frequency-doubling is cadmiumgermanium di-arsenide (CdGeAs2) grown at MIT Lin-


= 0.0732 ± 0.0009 cm-' a16 -

4 -

0 _ /

0.0

OXc

0.c

-0 300 400

Fig. 6. Frequency-doubled CO2 DIALinstrument.

The authors wish to express their ap-preciation to Norman Menyuk of MITLincoln Laboratory for loan of aCdGeAs2 frequency-doubling crystalwhich made these lidar measurementspossible. We also wish to thank bothNorman Menyuk and his colleagueDennis Killinger for many helpful dis-cussions of CO2 DIAL technology.

coln Laboratory. In our final instrument configurationthe TEA laser outputs are sent through a beam con-denser to yield a collimated beam cross section of-2-minm diam and an energy fluence of -0.3 J/cm2 at the

crystal input aperture. This results in measured dou-bling efficiencies of 1% in the 4 X 6 X 10-mm crystal.The crystal is angle-tuned for maximum output on theadjacent P(40) and P(42) lines. Both lines are simul-taneously within the phase-matching acceptance angleof the crystal.

Each TEA laser is grating tuned to one of the two(signal or reference) lines, and the lasers are pulsed se-quentially with a time separation of 50 sec. This latterfeature provides a near-simultaneous DIAL measure-ment while using the same detectors and electronics formeasurements on both wavelengths. The laser beamsare combined at a 50/50 beam splitter and pass colli-nearly through the doubling crystal. The second sur-face of the beam splitter is AR coated to prevent inter-ference effects. We felt the use of the same transmitterand receiver optical paths and data electronics for bothlaser wavelengths would be essential for an accurateDIAL measurement. Quarterwave plates (not shown)are inserted into the beam of each TEA laser to producecircularly polarized beams prior to the beam splitter.This is done simply as an expedient method of providingpolarization components matched to the doublingcrystal input requirements. A seven-power beam ex-pander telescope consisting of a negative zinc selenide(ZnSe) lens and off-axis parabola mirror reduces4.88-,gm beam divergence to 1 mrad. The outputpulses are directed collinearly to the receiver optical axisby means of a flat mirror mounted above the receiversecondary. A large optical flat directs transmittedbeams to and collects backscattered energy from theremote target board at 600-m horizontal range. A0.75-m square flame-sprayed aluminum sheet serves asthe remote target board. This material has a high re-

flectively (80%) and a backscatter pattern that is similarin angular width to a Lambertian pattern. Recentmeasurements of this material were reported by Kavayaet al. 23

The receiver optical system is a 0.75-m diam New-tonian telescope, but the size of the folding flat permitsonly 50% of the receiver area to be used for the hori-zontal mode of operation. The transmitted pulse en-ergy is monitored by a 1-mm diam indium antimonide(InSb) detector with a 1-MHz bandwidth located at thefocus of a ZnSe lens. A small reflection from a bariumfluoride (BaF2 ) window in the crystal output pathprovides this signal. A 9.5-mm thick sapphire window(not shown) in front of the InSb detector and a thinnersapphire window in the detector Dewar isolate the de-tector from strong 9.7-gim radiation. We use a 2-mmdiam sandwich detector of InSb over mercury cadmiumtelluride (HgGdTe). The latter detector material isused to facilitate optical alignment by use of the strong9.7-um radiation that passes through the crystalundoubled. A 2.54-cm focal length ZnSe lens serves asa field lens in the telescope focal plane. Measued SNRat 4.8 m for the 600-m target board link was 30 dB onthe average. This agrees within an order of magnitudewith a standard SNR analysis for this path and the,quoted detector noise equivalent power NEP of 2 X10-11 W/Hz 1 2 . Bandwidth is only 200 kHz at 4.8 gimin this detector. This is far from optimum as the fre-quency-doubled laser pulse has a 100 nsec or less du-ration. Prior to digitization the transmitted and re-ceived pulse waveforms are amplified and level shiftedin two dc to 1-MHz amplifier modules. They are thensampled at a 10-MHz rate in two 8-bit waveform digi-tizers. All processing electronics are mounted in aCAMAC crate, where a microcomputer acting as cratecontroller is programmed to acquire data from thedigitizers and perform statistical analysis of the re-sults.


B. Laboratory Tests

Prior to actual horizontal path measurements in theatmosphere a laboratory test of the lidar technique wasperformed. In this test the output of a single TEA laserwas first attenuated, then focused into the CdGeAs2crystal for 1% efficient frequency doubling. The crystaloutput was recollimated with a lens and then blockedbeyond 8 gim by a sapphire window. The 4.8-gm ra-diation was passed through a 1-m sample cell thatcontained CO2 at a 1-atm pressure. Laser pulses inputto the cell were sampled with a BaF2 beam splitter andInSb detector. Cell output was monitored with a sec-ond InSb detector, and the in/out ratio or cell trans-mission was computed. The cell was a vacuum-tightglass cylinder with calcium fluoride windows expoxiedto both ends. A vacuum pump and CO2 gas supplymanifold permitted evacuation of the cell and back-filling to any desired CO2 pressure up to 1 atm. Thelaser was grating tuned to the desired 9.4-gim P branchline, and cell transmission was measured, first for thecell evacuated and then for a 1-atm CO2 pressure. EachCO2 transmission data run was normalized by resultsof the corresponding evacuated cell data run to accountfor variation in optical and electronic gain between thecell input and output measurements.

Results of these tests for a number of frequency-doubled lines of the 9.4-gim P branch of the CO2 laserare listed in Table II for comparison with calculationsof cell transmission from the AFGL line parameters.The difference between AFGL and GSFC predictionsfor the frequency-doubled 9.75-gm P(42) laser line isthe use of our measured line strength and a self-broadened linewidth for the 4.88-gm P(34) CO2 line.We estimate our transmission measurement error as+5% due in part to very coarse optical alignment andomission of focusing lenses in front of either detector.This estimate comes from the scatter in repeated trialsfor a fixed frequency-doubled wavelength. Despite thedata uncertainty and the presence of self-broadeningin the sample cell rather than air-broadening (assumedin the AFGL line parameters), there is generally quitegood correlation among measured and predicted samplecell transmission. For 1-atm CO2 pressure in the cellthe CO2 concentration is 100 atm cm in one passthrough the cell. This compares with an ambient levelof -340 ppm in the atmosphere or 34 atm cm/km ofhorizontal path. Results for the 9.73-gim P(40) and9.75-gim P(42) lines verify the practicality of the lidarconcept by virtue of their substantial differential ab-

Table 11. Laboratory Test of CO2 Measurement Concept

TEA Doubled Transmission of 1-m celllaser frequency Wavelength Predicted Measuredline (cm-') (Am) AFGL GSFC (±5%)

9P(34) 2066.98 4.838 0.93 - 0.939P(36) 2062.95 4.847 0.38 - 0.359P(38) 2058.88 4.857 0.95 - 0.869P(40) 2054.76 4.867 0.93 - 0.879P(42) 2050.60 4.877 0.27 0.40 0.32

sorption and correspondence to theory. Note that thereis also strong absorption near 2063 cm-' because of thenear coincidence between the 4.85-gm P(18) CO2 lineand the frequency-doubled 9.69-,um P(36) laser line.This line is less suitable for lidar operation because ofa relatively large offset from the 4.85-gim atmosphericCO2 line center and significant H20 interference.

C. Field Measurements

The lidar instrument shown in Fig. 6, with laserstuned to the 9.75-gtm P(42) signal line and the 9.73-gmP(40) reference line, was used with frequency-doublingfor the remote sensing of atmospheric CO2 over a 600-mhorizontal path. Field measurements were conductedduring summer and fall of 1982 at the Goddard OpticalResearch Facility. The path is -10 m above partiallywooded terrain and lies -70 m above mean sea level.Data were acquired in a number of 1.5-min data runs inthe afternoon on several different days. Laser repeti-tion rate was -1 Hz, and data were recorded on mag-netic diskettes. The raw data were 8-bit digital samplesof the pulse waveforms and the detector/digitizer elec-tronic offsets (background and pedestal) just prior (2gsec) to laser firing. Data were initially collected se-quentially by tuning one laser to the signal wavelengthand then tuning it to the reference wavelength. Resultsof this initial measurement attempt, while clearly ex-hibiting differential absorption due to atmospheric C0 2 ,were highly variable due to the constant tuning of thelaser source. The dual-laser concept of Fig. 6 was thenimplemented, and data reported here were recorded.

In dual-laser data each pulse waveform area is com-puted by first adding up all digitizer samples in a gateinterval larger than the pulse, dividing by the totalnumber of samples, and then subtracting the measuredoffset or pedestal. Each receiver pulse area is normal-ized by its corresponding transmitter pulse area andthen the on-line pulse area is divided by the off-line areato obtain the differential absorption ratio of Eq. (2) foreach pulse pair. Optical and electronic gain variationsin the lidar system affecting each wavelength separatelyare canceled in this ratio calculation because the samedetector is used for both wavelengths; one detector fortransmitted pulses and one for received pulses. Weassume that each pulse waveform area, despite thedegradation in bandwidth introduced by the InSb de-tectors, is linearly proportional to pulse energy. Pri-mary statistics are the mean and standard deviation ofthe differential absorption ratio for each 100 pulse-pairdata runs. Mean and standard deviations are alsocomputed for the on- and off-line pulse waveform areas,and correlation coefficients are calculated betweentransmitted and received data and between on- andoff-line data.

The computed values of mean differential absorptionratio for five dates from 26 Aug. 1982 to 9 Nov. 1982 arelisted in Table III along with measured H20 vaporconcentrations for the data acquisition time interval.All data for a particular day are grouped together forthese calculations, and the total number of pulse pairsis listed in the table. The exception to this rule is the


Table Ill. Lidar Measurement of CO2 Over a 600-m Horizontal Path

Measured Tempera- Number ofwater vapor ature pulse pairs Differential absorption ratio

Date (atm) (K) (N) Predicted Measured

08/26/82 0.012 301 900 0.63 0.5609/03/82 0.012 301 1020 0.63 0.7010/28/82 0.0079 294 240 0.64 0.6511/03/82 0.020 299 600 0.60 0.5011/09/82 0.010 291 333 0.63 0.61 first half

- - - 387 - 0.50 second half

1.

0.

0.

0.

0.

u0 1 2 3 4 5 6 7 8

ELAPSED TIME (min)

9 10 11

data record for 9 Nov. 1982, where a stability test fordifferential absorption data resulted in mean values forthe first and second half of the data set. We havechosen to present the data without inverting SR to getthe CO2 concentration because the present data qualitydoes not warrant a claim of accurate CO2 measurementby the lidar technique. In addition, no accurate orground-truth CO2 concentration was available for pre-cise comparison. Our main motivations were lidardemonstration and assessment of relative data qualityrather than calibrated measurement. The data can becompared with a differential absorption ratio predictedfrom AFGL line parameters on H2 0 and our recent CO2spectroscopy data with an assumption of an atmo-spheric CO2 mixing ratio of 340 ppm. Measurementsof relative humidity taken during the data runs are usedto estimate an H20 concentration for a first-order cor-rection to the predicted SR. All absorption coefficientsused in these calculations were those for frequency-doubled laser line centers. Comparison of differentialabsorption ratios with predictions shows good agree-ment in a few cases and relatively large (up to 27%)disparity on other days. While some variability mightbe assigned to CO2 concentration near ground level, webelieve DIAL instrument effects are largely respon-sible.

It was obvious, even during data acquisition, thatsome of this variability resulted from short-term fluc-tuations within a 1.5-min data run (100 pulse-pairmeasurements of BR). Large fluctuations were apparentin pulse-to-pulse amplitude for each wavelength. Al-though there was partial correlation between the pulsesreceived on the two separate wavelengths, the ratiocalculation did not remove all this variability. It wasalso obvious that significant shifts in SR resulted fromthe realignment of the two CO2 lasers between data

Fig. 7. Differential absorption ratio data for CO2measurement at 4.88 m 9 Nov. 1982, 11:30 a.m.

EDT.

12

runs. Realignment was found to be necessary becauseof drifts in the spatial mode pattern and overall align-ment of each CO2 laser beam. The effect of thesealignment drifts and the resultant changes in laser beampower, while important to the performance of a 9.7-gumlidar system, are even more important at 4.8 m becauseof the nonlinear (square-law) action of the doublingcrystal.

For the data collected on 9 Nov. 1982, a long warmuptime (1 h) was used in an attempt to have the lasersreach thermal equilibrium. Data were then collectedin a series of eight runs spanning a 12-min time period.These data are displayed in Fig. 7 in the form of meanratios (for nine pulse-pairs) as a function of time. Al-though the initial mean ratio was very close to predic-tions (see Table III), there is an apparent drift with timethat results in a significantly lower ratio for the secondhalf of the data set. The two values for SR in Table IIIrefer to the first and second half of this data set. Noattempt was made to realign the lasers or any otheroptical components during this 12-min period. At theend of this period the original collinear circularly sym-metric laser beams were found to have drifted slightlyapart at the entrance to the doubling crystal and hadchanged mode pattern shapes.

The short-term variability in these data noted by the+1 standard deviation error bars and the scatter of

adjacent 9-point mean ratios in Fig. 7 is -25-30%. Thisis the uncertainty one would assign to a measurementof SR provided by a single pulse pair. If the long-termdrift is removed, we expect a significant decrease in theuncertainty of SR by averaging over many pulse pairs.If the distribution of S values is Gaussian, the uncer-tainty should vary as N-'1 2. This is illustrated in Fig.8, where our data of 9 Nov. 1982 starting at time zero areplotted as ER, the standard deviation of the differential


8

0 0

}s~~ ~~ F* sO e 1.i * 000erb

* 9 point mean ratio)2

i ± 1 S short term variation

1.02 2 I+ 2E2 EX 2 EY

N

Fig. 8. Comparison of measured () and predicted standard de-viation (es) of differential absorption ratio vs number (N) of lidarpulse pairs averaged. Predictions based on models of lidar datatemporal correlation: (-) p = 0.10 - 0.025 lnj, (- - -) p = 0.05, and--- ) p = exp(1.75 j); where j is the integer that represents the

temporal lag value between ratio measurements.

absorption ratios normalized by the mean ratio, as afunction of number N of pulse pairs averaged. For thiscalculation the data are grouped sequentially intogroups of N members, each group mean ratio () iscomputed, and then Ey?(N) is computed for the numberof groups available in the first 4 min of data. There isno apparent drift in SR over this segment of the data.Also plotted are the N-/ 2 line and three analyticalmodels which take into account residual temporal cor-relation in fluctuations of measured SR. These modelsare suggested by Menyuk et al. 2 4 (MKM) in an analysisof their 10-gM CO2 lidar data. All analytical curvesoriginate from the measured ER (1) value. Our datashow best agreement with the MKM model of p = 0.10- 0.025 lnj, where the two constants have been deter-mined by a least-squares fit to our data and j is the in-teger representation of time lag between ratio mea-surements. These values compare favorably with theMKM results and indicate the slower than N-112 im-provement in differential absorption ratio achieved inactual lidar experiments. Our net result for this oneparticularly good data set is an El of 6% (precision inknowledge of SR) after 1.5 min of data.

Up to this point we have dealt only with mean andstandard deviations of SR. There is additional infor-mation on lidar data quality and error sources containedin the statistics of single wavelength laser pulse data anddual-wavelength pulse correlation. Killinger andMenyuk2 5 and more recently Bufton et al.26 analyzeddual-wavelength CO2 TEA laser data variability interms of an uncorrelated noise component whose vari-ance 2 is attributed to random fluctuations in TEAlaser pointing angle or far-field pattern from pulse topulse. Values of cr2 for a particular data set are com-puted by first forming the dual-wavelength correlationcoefficient Pxy, where x and y are the signal and refer-ence wavelengths in the DIAL measurement. Thecross-covariance Cxy of fluctuations in these two pulsetrains is then given26 by 2pxy o- oy/( (Ex ) (Ey ) ) and

(Relative Units)

Fig. 9. Comparison of uncorrelated noise variance a 2 of the differ-ential absorption ratio, before (0) and after (0) normalization by4 .8 -Am output pulse energy vs inverse-squared DIAL signal strength

E-2 = E + E-2

2u is just the residual variance obtained by subtractingcross covariance from the sum of variance at bothwavelengths:

2 2

+ 2

Y C(E. )

2(Ey) 2 ,Y. (8)

Note that all variances are normalized by mean valuessquared and that cr/E' = Ex, for example. In Fig. 9 e2is plotted as a function of the inverse squared DIALsignal strength, E 2 = Ex 2 + EY2. Data for areplotted before and after normalization of Ex and Ey bythe laser transmitter energy. Note that, in our newnotation, Ex and Ey refer to mean received laser pulseenergies on wavelengths x and y. The lines are least-squares fits of the two data sets, They reveal that atlarge signal strength (the intercept at E- 2 = 0), thevalues of 2 are, respectively, 0.18 and 0.027 for un-normalized and normalized data. This means that thepercent variability (Ex or Ey) in lidar signals is 42% beforenormalization by transmitter energy and 17% afternormalization. The combination of two lidar signalsto form the ratio SR = Ex/Ey gives an uncertainty in SRof 34% after normalization. This is in agreement withthe single pulse pair E given in Fig. 8. The least-squares fits of Fig. 9 also show, because of their positiveslope, that a 2> increase is associated in part with lowsignal levels in the backscattered 4.8-gim radiation.

Even with high signal level lidar detection the resid-ual y2u values are considerably larger than those reportedin the literature25'2 6 for 10-gm CO2 TEA laser DIALsystems. We attribute this to the doubling crystal,which increases sensitivity of the measurement to TEAlaser variability because of the proportionality of fre-quency-doubling to the square of the input 10-,um in-tensity. Although we normalize by total 4.8-gm pulseenergy that is transmitted, the far-field pattern and itsposition are expected to be more variable at 4.8 gm thanthat at 10 gim. As a result of the near-total indepen-dence of one laser pattern from that of the other, wemight expect these larger values of o,.


aU2

100

-0

3 _ 1

.1 _ -

nvl )~

O.

0,

0,

2.0

V. Lidar Instrumentation for High AccuracyMeasurement of CO 2

A. Frequency-Doubled DIAL Error Sources

, The acquisition and analysis of 4.8-gim DIAL datasets has given us indications of the presence of a numberof serious error sources in frequency-doubled CO2 TEAlaser data. We will review these and other importanterror sources prior to proposing instrumentation andmethods necessary to achieve high accuracy in CO2concentration measurements.

The short-term variability in the lidar data reportedhere amounts to as much as 30% of the mean differentialabsorption ratio even after normalization by transmitterpulse energy. In Sec. IV we explained some of the un-correlated noise as the result of TEA laser pulse-to-pulse fluctuations in pointing angle and spatial modepatterns. The typical 10% variability for 10-gumwavelength laser output becomes a 20% variability at4.8 gm due to the nonlinear infrared crystal. While wemight expect that the transmitter energy monitor wouldmeasure this increased variability, the independentpointing angle and spatial mode pattern for each laserresult in distinctly different far-field patterns andslightly different propagation paths for the two wave-lengths through the optics and the atmosphere. Thiscould give rise to a significant level of uncorrelatednoise. Target-induced speckle could also account forsome u 2 since speckle is uncorrelated between on- andoff-line wavelengths. 2 6 We believe, however, thatspeckle is almost negligible in our system due to spatialaveraging in the receiver aperture. This belief is sup-ported by the 25:1 ratio of receiver and transmitterareas.

Interaction of the laser pulses with the refractive-index fluctuations of atmospheric turbulence could alsocause uncorrelated noise. Normally turbulence effectsare well-correlated with wavelength and with the small(50-,usec) separations between dual-wavelength pulses.When the two laser pulses are separated in angle andhave different beam cross sections, however, uncorre-lated scintillation could result. The total magnitudeof scintillation on any one wavelength might be signif-icant despite the 4.8-gm wavelength and some receiveraperture averaging because of the strong midafternoonturbulence levels expected for this horizontal propa-gation path.

Detector SNR is an important consideration in un-correlated noise at low signal levels. Thermal back-ground noise, detector dark current, digitizer quanti-zation, and a limited detection dynamic range could actindependently on the two pulsed wavelengths andwould be more significant at low signal levels. In ad-dition, the small (2-mm diam) detector element andspatial nonuniformities in detector response couldcouple with independent angular fluctuations of thelidar pulses to produce contributions to a 2. Evidencefor these effects is contained in the data of Fig. 9.

Another very significant factor in uncorrelated noiseproduction and also in DIAL systematic error is thefrequency-space shape of the frequency-doubled lidar

pulse and its interaction with the atmospheric CO2absorption line. Since the CO2 TEA laser operates ata 1-atm pressure, there is significant pressure broad-ening of the 9.7-gim pulse. Wood27 gives a value of 3.95GHz for the full width at half-maximum (FWHM) ofthis pulse. In wave-number units this FWHM is -0.13cm-1, which is about the same as our results in Eq. (7).When the 9.7-gim laser pulse is frequency-doubled, thislinewidth is compressed because the infrared doublingcrystal output is proportional to the square of crystalinput pulse intensity. As a result, the linewidth is re-duced by a factor of the square root of 2. The CO2 TEAlaser output contains only frequencies where laser gainis above threshold; not the whole pressure-broadenedenvelope. This leads to further frequency-space nar-rowing of the pulse.

Note that, despite line narrowing, the 4.88-gim signalsource is an appreciable fraction of the P(34) line.Laser operation can occur on several longitudinal modessimultaneously and mode hopping (chirping) duringeach laser pulse is also possible. The net effect of afrequency-doubled laser line shape that is off centerfrom the CO2 absorption line, is an appreciable fractionof the absorption linewidth, and has multiple longitu-dinal modes, is significant uncertainty and variabilityin the applicable absorption coefficient. Korb andWeng28 report an analytical description of laser band-width effects in DIAL measurements. Additionalmaterial on the effect can be found in the work of Cohenand Megie.2 9

The effect of interfering species, primarily watervapor, but also carbon monoxide and ozone must beconsidered. Likewise, the differential reflectance oftargets are potential error sources.30 Usually these bothare systematic errors, but they might vary on short timescales and be a significant contribution to u2. We havetried to estimate these and the other error sources in ourpresent DIAL measurement and list the results in TableIV.

Table IV. Estimates of Error Sources and Their Magnitudes in Frequency-Doubled CO2 DIAL Measurements

Differential absorptionratio error per

Error source pulse pair (%)

TEA laser short-term instabilities in 15pointing angle and spatial modepattern 30

Effect of TEA laser instabilities afterfrequency-doubling

Long-term laser alignment drift 20 (typical over10 min)

Detector noise 15Dynamic range and digitizer 10

quantizationTEA laser linewidth and longitudinal 12

mode hoppingUncertainty in H20 interference 12Other interfering species 1Uncertainty in temperature 1CO2 spectroscopy 3Speckle noise 1Uncorrelated turbulence effects 5


B. Lidar Improvements

It is apparent that significant improvements in ourCO2 DIAL instrument are necessary for high-accuracyfrequency-doubled DIAL measurements. Majorsources of error are the dynamic instabilities in CO2TEA laser pointing angle and mode patterns (spatialand temporal), drift in TEA laser alignment, and widthof the TEA laser line shape. The TEA laser can beimproved by use of an unstable resonator and injectionlocking for single longitudinal mode operation; but thisis done at the cost of a large, complex, and expensivedevice of the type required for heterodyne detection.An alternate approach, that retains a small, simple, andinexpensive laser, complimentary to the direct-detec-tion receiver, is the use of a low-pressure single modepulsed-discharge CO2 laser.

Further improvements can be made by optimizing thedirect-detection receiver for higher SNR operation andby using high repetition rates in DIAL measurements.Detector improvements that are easy to implement are(1) photoconductive HgCdTe detector elements at 4.8gim rather than photovoltaic InSb in order to get a largedetection bandwidth; (2) immersion optics for apparentincrease in detector size; and (3) narrow bandpass coldfiltering and shielding for optimum detectivity. Weestimate that specific detectivity (D*) approaching 1012

cm Hz 1/2 /W with a 5-MHz bandwidth is achievable.This is -1 order of magnitude improvement over thedetector we use now and would result in a factor of 2larger lidar field of view. Laser repetition rate and datacollection can be increased at least 1 order of magnitudeto -10-20 Hz. This would enable averaging over anyremaining a2 in a time short compared to changes in theoverall properties of target reflectance and atmospherictransmission.

The effects of systematic errors can be greatly re-duced by an accurate calibration of the lidar sensor within situ infrared gas analyzers, which are now accurateto 0.1% of atmospheric CO2 concentration. This, infact, may be the only way to reduce uncertainty in DIALmeasurement due to residual uncertainties in CO2 lineparameters, differential reflectance effects, laser-linefrequency offset, and effects of interfering species.

VI. Conclusion

We have described a DIAL measurement techniquebased on frequency-doubled CO2 lasers for remotesensing of atmospheric carbon dioxide at 4.8-gimwavelength. We have reported detailed laboratoryspectroscopy of the relevant CO2 line, laboratory testsof the lidar concept, and demonstration field mea-surements with a lidar system based on conventionalCO2 TEA lasers and a single CdGeAs 2 frequency-dou-bling crystal. Sources of uncorrelated noise in mea-surements made at the signal and reference lidarwavelengths and sources of systematic error in theselidar data have been reviewed in detail and found toseriously limit current DIAL measurement accuracy.Steps to improve measurement accuracy have beenoutlined.

References

1. C. P. Rinsland, D. C. Benner, M. A. H. Smith, R. K. Seals, Jr., andJ. M. Russell III, NASA Langley Research Center; private com-munication (Jan. 1983).

2. E. R. Murray, D. D. Powell, and J. E. van der Laan, Appl. Opt.19, 1794 (1980).

3. C. L. Korb and C. Y. Weng, Appl. Meteorol. 21, 134 (1982).4. R. M. Schotland, J. Appl. Meteorol. 13, 71 (1974).5. R. L. Byer, Opt. Quantum Electron. 7, 147 (1975).6. C. L. Korb, L. D. Kaplan, J. L. Bufton, and C. Y. Weng, "A Lidar

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Transfer 22, 349 (1979).20. J. L. Bufton and R. W. Stewart, "Measurement of Ozone Vertical

Profiles in the Boundary Layer with a CO2 Differential Absorp-tion Lidar," in Proceedings, Tenth International Lidar Con-ference, Silver Spring, Md., 6-9 Oct. 1980, paper 75.

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frequency-doubled co_2 lidar measurement and diode laser spectroscopy of atmospheric co_2

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