fiti

Cl

Tunable diode laser absorption spectroscopy ofcarbon monoxide around 2.35 mm

Jean-Christophe Nicolas, Alexei N. Baranov, Yvan Cuminal, Yves Rouillard, andClaude Alibert

Novel GaInSbAsyGaSb multiple-quantum-well lasers operating near room temperature have been suc-cessfully used for tunable diode laser absorption spectroscopy in the vicinity of 2.35 mm. Continuouscurrent tuning over a more than 150-GHz frequency range has been realized. Direct absorption mea-surements have been carried out on the R9, R10, R11, and R12 lines of carbon monoxide. Traces ofcarbon monoxide at the level of 0.3 part in 106 in volume at 100 Torr could be detected by the low-frequency wavelength-modulation technique and an astigmatic multipass cell. These results show apotential use of these diode lasers in portable low-cost trace-pollutant sensors. © 1998 Optical Societyof America

OCIS codes: 300.1030, 300.6340, 300.6380, 300.6260, 230.5590, 280.3420.

m

1. Introduction

Tunable diode laser absorption spectroscopy ~TD-LAS! is one of the most sensitive, selective, and fast-response techniques for gas analysis. The middleinfrared, from 2 to 15 mm, contains absorption lines ofalmost all gas species of interest for atmosphericmeasurements.1 In this region the absorption coef-

cients are so strong that only high-resolution spec-rometers are able to detect the chemical speciesnvolved with sufficient selectivity.2 Diode lasers

are attractive for molecular spectroscopy becausetheir emission bands are narrower than the Dopplerwidths of absorption lines. Besides, their tuningability permits continuous scanning of a large spec-tral interval, which makes it possible to detect simul-taneously several components of a gas mixture.Traditionally, lead-salt diode lasers emitting in the3–25-mm spectral range have been used in TDLAS.3However, these lasers can not work at room temper-ature ~RT! and should be cooled to 80–100 K, which

The authors are with the Centre d’Electronique et de Micro-optoelectronique de Montpellier, Unite Mixte de Recherche 5507,

entre National de la Recherche Scientifique, Ministere de’Education Nationale, Case Courrier 67, Universite Montpellier

II, 34095 Montpellier, France. The email address for J.-C. Nicolasis thesem2@univ-montp2.fr; for Y. Rouillard it is yves.rouillard@univ-monpt2.fr.

Received 26 March 1998; revised manuscript received 22 June,1998.

0003-6935y98y337906-06$15.00y0© 1998 Optical Society of America

7906 APPLIED OPTICS y Vol. 37, No. 33 y 20 November 1998

requires that they be expertly maintained. Becauseof material restrictions, their emission cannot be ex-tended to wavelengths shorter than 3 mm. Semicon-ductor lasers based on III–V compounds seem to beattractive for TDLAS because of their higher opticalpower and their ability to lase continuously near RTat wavelengths shorter than 3 mm. Despite the factthat absorption lines are in general weaker in the2–3-mm range, the availability of multipass cells,high-sensitivity detectors, and the possibility of RToperation for lasers and detectors open a way fordevelopment of high-sensitivity portable systems forgas analysis.

We performed TDLAS studies of carbon monoxidetransitions in the R branch of the first overtone ~2–0!band, using GaInSbAsyGaSb multiple-quantum-well~MQW! diode lasers operating near RT. Carbonmonoxide, one of the major atmospheric pollutants,has been extensively studied by TDLAS at shorterwavelengths in the vicinity of 1.6 mm, and we presentwhat we believe are the first results obtained in therange of stronger absorption near 2.35 mm with thesenovel lasers.

2. Experiment

A schematic of the experimental setup is shown inFig. 1. The setup includes a system for direct ab-sorption spectroscopy and equipment for analysis ofthe GaInSbAsyGaSb MQW laser diodes. The laserswere driven by a battery-powered ultralow-noise cur-rent source ~LDX-3620!. The driving current was

odulated by a synthesized waveform designed to

0cctl

amm

paoTw

eu

m

I

safl

mrte

cd2

diotT

provide the desired tuning range. The typical wave-form consisted of a 25–50-mA ramp modulated at1–100 Hz superimposed upon a constant-baselinecurrent within the range 100–400 mA.

The temperature of the lasers was maintainedfrom 230 °C to RT and stabilized with a precision of.01 °C by a single Peltier cell mounted upon a water-ooled heat sink to increase the heat-evacuation effi-iency. The temperature was monitored by ahermistor embedded between the cooler and theaser.

The output beam from the laser was collected by anoff-axis parabolic mirror ~ f 5 25 mm!. A gratingmonochromator with a resolution of 0.5 nm was usedto analyze emission spectra and to select a desiredlongitudinal mode for gas-absorption measurements.After the monochromator a part of the beam split bya semitransparent mirror was passed through a ger-manium Fabry–Perot etalon with a free spectralrange ~FSR! of 2.2 GHz used to analyze the tunabilitynd the spectral purity of the laser emission. Theain part of the beam was directed to a 100-m-longultipass gas cell ~New Focus 5612!.4 The optical

signals were detected by RT GaInAs photodiodes sen-sitive up to 2.6 mm ~Hamamatsu G 5853-01!. The

hotodiodes outputs were connected to transimped-nce preamplifiers, and voltage-output signals werebserved on a digital oscilloscope ~HP 54603B!.hen the averaged data stored by the digital scopeere processed by a personal computer.For pressure measurements a mechanical manom-

ter was used above 1 Torr and a Pirani gauge wassed at lower pressures.

3. Laser Characteristics

Figure 2 is a schematic of the structure of the GaInS-bAsyGaSb MQW laser. The structure was grown by

olecular beam epitaxy upon an n-GaSb substrate.The undoped active zone consisted of compressivelystrained 6-nm-thick Ga0.65In0.35As0.15Sb0.85 quantumwells embedded between 30-nm-thick GaSb barri-ers,5 which provided a maximum of the gain curvenear 2.35 mm at RT. The 1.8-mm-thick cladding lay-ers were made of Ga0.4Al0.6As0.05Sb0.95 latticematched to a substrate doped with tellurium and

Fig. 1. Experimental setup for diode laser analysis and gas-absorption experiments.

2

beryllium for n- and p-type doping, respectively.Details of the growth procedure were reported else-where.6 The threshold current density measured on200-mm-wide lasers fabricated from this wafer was aslow as 305 Aycm2. A standard photolithographyprocess was used to fabricate 80-mm-wide deep mesalasers. The laser Fabry–Perot cavity was formed be-tween two uncoated cleaved facets; the cavity lengthvaried from 400 to 900 mm. The threshold currentsth were in the range 200–300 mA, and the total

output power reached 40 mW at I 5 2Ith in a quasi-cwregime ~a 2% duty cycle, 2-ms pulse duration!.

We accomplished coarse and fine tuning of the la-er emission wavelength by varying the temperaturend the injection current, respectively. As is usualor Fabry–Perot cavity lasers, emission spectra of theasers contained several longitudinal modes ~as many

as five!. The temperature mode shift of dominatingodes was measured by a spectrometer. The cur-

ent tuning of a single longitudinal mode selected byhe monochromator was studied with the germanium

´talon.With the setup described, the laser temperature

ould be varied from 230 to 20 °C. The tested lasersemonstrated stable cw operation up to 3 °C. At0 °C, cw operation was achieved for the best devices.The spectral position of the longitudinal modes is

efined by the optical length of the Fabry–Perot cav-ty. As the refractive index and the physical lengthf the cavity increase with temperature, the resona-or modes usually move toward longer wavelengths.he temperature shift of lasing modes was 0.4 nmyK

~20 GHzyK! for our lasers ~Fig. 3!. A lasing modecould be tuned over the spectral interval of 1–3 nm bytemperature variations that did not exceed 12 K; oth-erwise, mode hopping occurred and another longitu-dinal mode started to lase. With heating from230 °C to RT the laser emission wavelength in-creased with several mode hops from 2.29 to 2.35 mmin accordance with the temperature shift of the gaincurve, which was ;1 nmyK ~50 GHzyK!.

The current tuning of a selected mode @Fig. 4~a!#was analyzed by the Fabry–Perot etalon; each oscil-lation of transmitted light corresponds to a frequencyshift equal to the free spectral range ~Fig. 4!. Theemission wavelength of the lasers increased with cur-rent, which indicates the thermal origin of the cur-

Fig. 2. Schematic structure and design of the GaInSbAsyGaSbMQW diode laser.

0 November 1998 y Vol. 37, No. 33 y APPLIED OPTICS 7907

~tsf4dtc

awclsrb

7

rent tuning. A current-induced redshift of the laseremission to 155 GHz without mode hopping was ob-tained @Fig. 4~a!#. The average tuning rate of20.9!–~21! GHzymA was nearly the same for theested devices. The etalon oscillations could be ob-erved even without a monochromator when theringes were produced by all longitudinal modes @Fig.~b!#. The high contrast of the observed fringes in-icates good spectral purity of the emission becausehe fringes generated by the side modes were weakompared with the dominant mode fringes.

The observation of the etalon fringes can give onlyn idea of the spectral characteristics, especiallyhen the finesse of the etalon is weak, as it was in the

ase described here. Absorption measurements atow pressure can give reliable information about thepectral properties of the lasers. In this spectralange, one of the gases of interest for TDLAS is car-on monoxide.

4. Direct Absorption Experiments

The absorption measurements were carried out withlasers operating in the cw regime, which provided

Fig. 3. Temperature shift of longitudinal modes at a current of300 mA.

Fig. 4. ~a! Current shift of one longitudinal mode near 4297 cm21

and oscillation fringes of the germanium etalon. ~b! Etalonfringes induced by all modes. Laser heat-sink temperature,217.3 °C; carbon monoxide concentration, 100 ppmv; optical pathlength, 100 m; gas temperature, 296 K.

908 APPLIED OPTICS y Vol. 37, No. 33 y 20 November 1998

good linearity of the emission wavelength versusscanning time. Some measurements were done inthe pulsed regime at millisecond pulse widths, but inthat case frequency calibration during the absorptionmeasurements was more difficult because of signifi-cant nonlinearity of the wavelength shift during thepulse duration. In cw regime the relative light fre-quency scale could be easily constructed by use of thegermanium Fabry–Perot etalon. The optical signalsobtained after the multipass cell were averaged dur-ing 64 scans. Then the data were normalized andcompared with the synthetic spectra provided by theHITRAN 92 database as shown in Fig. 5 for the R11CO absorption line. The line strength extractedfrom the absorption measurements was 4–15% lessthan expected from the HITRAN 92 database. Al-though the updated version of HITRAN in the 1996edition has increased line-strength values by 2%, thevalues obtained are still too low. We explain thisdifference as being due to a small power backgroundcaused by reflections beside the coupling hole of themultipass cell. This finding was proved by measure-ments at the high carbon monoxide concentrationsthat were necessary to provide total absorption at theresonance frequency. Nevertheless, even at the cen-ter of the absorption line we observed a nonnegligiblesignal, which resulted in a decrease in the measuredabsorption strength. We used the value of this back-ground signal at high carbon monoxide concentra-tions to correct data obtained for low absorbances.The rate of scattering depends on adjustments of thebeam alignment. During pressure changes the mul-tipass cell is strained and the beam alignment variesslightly. The multipass cell alignment was opti-mized for low pressure, which explains the betteragreement between experimental and calculatedcurves obtained at 10 Torr.

With the same laser some other carbon monoxideabsorption lines could be found: R12 at 4303.623cm21, R10 at 4297.70 cm21, and R9 at 4294.64 cm21.We reached these lines by adjusting laser tempera-ture to 225.1, 210.3, and 24 °C, respectively, at the

Fig. 5. Normalized experimental and calculated direct transmis-sion spectra of the R11 CO line for three pressures. Carbon mon-oxide concentration, 100 ppmv; gas temperature, 296 K; opticalpath length, 100 m; laser heat-sink temperature, 217.3 °C.

rs1smfHT2a

bciGp

same driving conditions. To provide the redshift of 9cm21 ~269 GHz! between R12 and R9 lines we neededa temperature increase of 21 K, which corresponds tothe temperature mode tuning rate measured previ-ously. This means that the same longitudinal modescanned all this spectral region as the temperatureincreased to 24 °C. The intensity of the mode de-creased with temperature, resulting in lower signal-to-noise ratio for the strongest R9 line ~Fig. 6!, andabove 24 °C this mode disappeared. Such atemperature-tuning result is exceptional for such la-sers, which generally can tune without mode hoppingover only 12 K. Only one laser diode exhibitedsingle-longitudinal-mode operation.

Although the output power is an important param-eter for a good signal-to-noise ratio, the spectral line-width of the laser diode determines the resolution ofTDLAS. One can estimate the laser emission line-width by measuring the spectral width of narrowabsorption lines. At low pressure, below 10 Torr,the carbon monoxide absorption lines are justDoppler-broadened lines whose width depends onlyon the temperature and the molar weight of the gasesinvolved in the mixture. For instance, the HWHMof the R12 CO absorption line is expected to be 0.005cm21 or 150 MHz in these conditions. The laserlinewidth can be deduced from comparison of thisvalue with the measured width of the absorptionline.7

Experiments were performed with a calibrated gasmixture containing 100 parts in 106 by volume~ppmv! of carbon monoxide in nitrogen at 1, 10, and100 Torr. The HWHM of the absorption line wasfound from Beer’s law: IsyIs

0 5 exp~2asl !, where Is

and Is0 are the transmitted and the nonabsorbed

laser intensities, respectively, at the wave number s,l is the optical path length inside the gas cell, and as

is the absorption coefficient.8 As expected, the mea-sured HWHM of carbon monoxide absorption lineswere larger than the values given by the HITRAN 92database. From comparison of these data the laserlinewidth was found to be less than 100 MHz. How-ever, these measurements give only the upper limitfor the laser linewidth because the estimated accu-racy is not better than 50 MHz. The possible

Fig. 6. Comparison of direct transmission spectra of R9, R10, andR11 CO absorption lines obtained at different laser temperatures.Gas temperature, 296 K; optical path length, 100 m; for 100-ppmvof carbon monoxide in nitrogen the total pressure is 100 Torr.

2

sources of errors are the poor finesse of the germa-nium etalon and small nonlinearities in frequencyversus current, which affect the frequency calibrationof experimental spectra.

More-accurate measurements were made by use ofthe steep transmission slope of a gas-absorptioncurve to convert frequency fluctuations into ampli-tude fluctuations, as described by Reid et al.9 Theesults obtained for the R12 CO absorption line arehown in Fig. 7 for 100 ppmv of carbon monoxide at00 Torr. The figure shows the calculated and ob-erved transmission curves, which are in good agree-ent, and the intensity fluctuations converted in

requency fluctuations. The calculations based onITRAN 92 include the baseline effect in this case.he FWHM of the laser emission line was 20 MHz at25.1 °C with an uncertainty of 5 MHz, in goodgreement with data obtained by Avetisov et al. for

liquid phase epitaxy–grown GaInSbAs lasers emit-ting near 2 mm.10 This value is typical for diodelasers with cleaved Fabry–Perot resonators at mod-erate output power levels. The contribution of thelaser linewidth to the measured carbon monoxide ab-sorption line profile should be taken into account formeasurements at low pressure. On the other hand,at pressures higher than 50 Torr the absorption line-width ~HWHM! exceeds 200 MHz, according to theVoigt profile, and the distortion of the line shape isnegligible ~Fig. 5!.11 At temperatures above 0 °C thelaser linewidth increases significantly, a result that isdue to lower optical power in each mode because ofrapid growth of the threshold current and worseningof the quantum efficiency.

5. Wavelength Modulation Experiments

To realize trace-gas detection we investigatedthe wavelength-modulation spectroscopy technique,which allows one to suppress laser excess noise and toachieve high sensitivity.12,13 The technique consistsin modulating the injection current with a sine wave

Fig. 7. Direct transmission signal ~dotted curve! of the R12 car-on monoxide absorption line compared with simulation ~solidurve with filled squares! from HITRAN database 92 and thentensity fluctuation ~jagged solid curve! converted into FWHM.as temperature, 296 K; carbon monoxide concentration, 100pmv; gas pressure, 100 Torr.

0 November 1998 y Vol. 37, No. 33 y APPLIED OPTICS 7909

as

Rc

R

7

modulation, v~t! 5 v0 1 m sin~2pft!, where f repre-sents the modulation frequency and m is the depthmodulation index. Typical modulation frequency iswithin the range 1 kHz–10 MHz. The absorptionsignal is demodulated by a standard lock-in amplifier~EG&G 7260! at a frequency of nf ~n 5 1, 2, 3 . . .!where the laser noise is minimal. The use of stan-dard lock-in amplifiers limits the modulation fre-quency to the range 1–150 kHz. If f is much higherthan the ramp repetition rate, the signal recovered at2f can be approximated by the second derivative ofthe scanned absorption line and by the fourth deriv-ative at 4f.14 As regards the modulation index, adeep amplitude modulation broadens the signal pro-file,15 which can worsen the sensitivity of the method,nd the index m is generally chosen to optimize theignal-to-noise ratio. The optimal values for m have

been shown to be 2Dv and 4Dv for the 2f and 4fsignals, respectively, where Dv represents theHWHM of the gas-absorption line.16

We recorded 2f and 4f spectra of the R10 absorptionline of carbon monoxide at a constant pressure of 100Torr ~Fig. 8!. The lock-in amplifier’s time constantwas 5 ms, and the signal was averaged 64 times.For small absorbances, the 2f signal was accompa-nied by fringes generated by the small nonlinearitiesof the power versus current. We could minimize thiseffect by choosing m equal to Dv. When the signalwas recovered at 4f, we observed fewer fringes andthe best signal-to-noise ratio with a higher depthmodulation of 5 Dv. Measurements at low pressurein the range 50–100 Torr were preferred toatmospheric-pressure measurements because theformer are more selective and less sensitive to am-plitude modulation. To obtain a low carbon monox-ide concentration we diluted the calibrated CO–N2mixture several times in nitrogen. We could easilydetect 300 parts in 109 by volume ~ppbv! of carbonmonoxide at 100 Torr, corresponding to an absor-bance of 9 3 1024. For a 100-m path length thedetection level would correspond to 39.5 ppbv of car-bon monoxide at ambient air, or 3.95 ppmvym. The

10 CO line strength is 3.04 3 10221ycm21y~mole-ules cm22! ~HITRAN 96!, which is two decades

Fig. 8. Wavelength modulation spectroscopy signals of the R10carbon monoxide line at 100 Torr recovered at 2f and 4f, where f 54 kHz. Time constant, 5 ms; gas pressure, 100 Torr; gas temper-ature, 296 K.

910 APPLIED OPTICS y Vol. 37, No. 33 y 20 November 1998

higher than the strongest carbon monoxide absorp-tion line near 1.57 mm. The InGaAs lasers operat-ing near this wavelength could reach only 40ppmvym for an associated absorbance of 1 3 1025 atatmospheric pressure,17 which means that the GaIn-AsSb lasers are more promising for use in carbonmonoxide detection.

6. Discussion and Conclusions

We have investigated semiconductor lasers based ona GaInSbAsyGaSb multiple-quantum-well structurefrom the point of view of their use in molecular spec-troscopy. The temperature and current tuningcharacteristics were measured near room tempera-ture. A wavelength zone from 2.29 mm ~4366 cm21!to 2.35 mm ~4255 cm21! could be scanned with thesame laser operating in the cw regime from 230 °C toroom temperature. Current tuning of the laseremission up to 155 GHz without mode hopping wasobtained.

Direct absorption experiments with carbon monox-ide were carried out with these lasers. R9, R10,

11, and R12 CO absorption lines were observed atdifferent laser temperatures in a CO–N2 mixture con-taining 100 ppmv of CO at pressures in the range1–100 Torr. At typical driving conditions the laseremission linewidth was 20 MHz. Carbon monoxideabsorption spectra were also recorded by the low-frequency wavelength-modulation technique. With4-kHz frequency modulation of the slow ramp currentwe were able to detect 0.3 ppmv of carbon monoxidemixed with nitrogen at 100 Torr in a 100-m-longmultipass optical cell. The sensitivity of thewavelength-modulation spectroscopy can be furtherimproved by use of a higher modulation frequency forwhich the 1yf laser excess noise is less and by im-provement of laser quality.

The lasers studied demonstrated stable cw opera-tion at temperatures slightly below RT. The bestdevices were able to work in the cw regime withoutany cooling, but in this case their parameters such asoutput power, emission linewidth, and mode stabilitywere too poor to permit detailed studies of carbonmonoxide absorption at low concentrations to be per-formed. It should be possible to achieve some im-provement in RT operation by reducing the laserwidth to 10 mm, which should provide lower thresh-old current and reliable single-spatial-mode opera-tion. On the other hand, the ability of lasers to workat temperatures higher than RT is much more attrac-tive for TDLAS than any below-RT operation. Tem-perature control by heating is simpler, more effective,and much less expensive. Besides, in this case thereis no need to protect laser facets from water conden-sation. To achieve this goal the laser structure itselfshould be improved to decrease the threshold currentdensity.

Another problem to be solved is to provide single-frequency operation. Competition of several longi-tudinal modes creates excess noise and reducesoptical power in each mode, which significantlybroadens the laser emission line and worsens the

trw

7. D. C. Hovde and C. A. Parsons, “Wavelength modulation de-

sensitivity of gas detection. Research is in progressto develop distributed-feedback lasers based onGaInSbAsyGaSb MQW structures. The disadvan-age of this approach is the much narrower spectralange accessible for scanning by one laser comparedith that for the Fabry–Perot geometry.In conclusion, GaInSbAsyGaSb MQW lasers oper-

ating near RT have been successfully used for molec-ular spectroscopy of carbon monoxide. The resultsobtained show a potential for use of these lasers inportable low-cost trace-pollutant sensors.

This research was supported in part by the RegionLanguedoc Roussillon.

References1. H. I. Schiff, G. I. Mackay, and J. Bechara, “The use of tunable

diode laser absorption spectroscopy for atmospheric measure-ments,” in Air Monitoring by Spectroscopic Techniques, M. W.Sigrist, ed. Vol. 127 of Chemical Analysis Series ~Wiley, NewYork, 1994!, pp. 239–318.

2. A. I. Nadezhdinskii and A. M. Prokhorov, “Modern trends indiode laser spectroscopy,” in Tunable Diode Laser Applica-tions, A. I. Nadezhdinskii and A. M. Prokhorov, eds., Proc.SPIE 1724, 2–17 ~1992!.

3. H. Preier, “Physics and applications of IV–VI compound semi-conductor lasers,” Semicond. Sci. Technol. 5, S12–S20 ~1990!.

4. J. B. McManus, P. L. Kebabian, and M. S. Zahniser, “Astig-matic mirror multipass absorption cells for long-path lengthspectroscopy,” Appl. Opt. 34, 3336–3348 ~1995!.

5. A. N. Baranov, Y. Cuminal, G. Boissier, C. Alibert, and A.Joullie, “Low-threshold laser diodes based on type-II GaIn-AsSbyGaSb quantum-wells operating at 2.36 microns at roomtemperature,” Electron. Lett. 32, 2279–2280 ~1996!.

6. A. N. Baranov, Y. Cuminal, G. Boissier, J. C. Nicolas, J. L.Lazzari, C. Alibert, and A. Joullie, “Electroluminescence ofGaInSbyGaSb strained quantum well structures grown by mo-lecular epitaxy,” Semicond. Sci. Technol. 11, 1185–1188 ~1996!.

2

tection of water vapor using a vertical cavity emitting laser,”Appl. Opt. 36, 1135–1138 ~1997!.

8. B. Rosier, P. Gicquel, D. Henry, and A. Coppalle, “Carbonmonoxide concentrations and temperature measurements in alow pressure CH4–O2–NH3 flame,” Appl. Opt. 27, 360–364~1988!.

9. J. Reid, D. T. Cassidy, and R. T. Menzies, “Linewidth mea-surements of tunable diode lasers using heterodyne and etalontechniques,” Appl. Opt. 21, 3961–3965 ~1982!.

10. V. G. Avetisov, A. N. Baranov, A. N. Imenkov, A. I. Nadezh-dinskii, A. N. Khusnutdinov, and Yu. P. Yakovlev, “Measure-ments of the emission linewidth of long-wavelength injectionlasers based on GaInAsSb,” Sov. Tech. Phys. Lett. 16, 549–550~1990!.

11. J. J. Olivero and R. L. Longbothum, “Empirical fits to the Voigtline width: a brief review,” J. Quant. Spectrosc. Radiat.Transfer 17, 233–236 ~1977!.

12. P. Werle, F. Slemr, M. Gehrtz, and C. Brauchle, “Widebandnoise characteristics of a lead-salt diode laser: possibility ofquantum noise limited TDLAS performance,” Appl. Opt. 28,1638–1642 ~1989!.

13. D. S. Bomse, A. C. Stanton, and J. A. Silver, “Frequency mod-ulation and wavelength modulation spectroscopies: compar-ison of experimental methods using lead-salt diode laser,”Appl. Opt. 31, 718–731 ~1992!.

14. R. Arndt, “Analytical line shapes for Lorentzian signals broad-ened by modulation,” J. Appl. Phys. 36, 2522–2524 ~1965!.

15. A. P. Larson, L. Sanstrom, and S. Hojer, “Evaluation of dis-tributed Bragg reflector lasers for high-sensitivity near infra-red gas analysis,” Opt. Eng. 36, 117–123 ~1997!.

16. D. C. Hovde, A. C. Stanton, T. P. Meyers, and D. R. Matt,“Methane emissions from a landfill measured by eddy corre-lation using a fast response diode laser sensor,” J. Atmos.Chem. 20, 141–162 ~1993!.

17. Mihalcea, D. S. Baer, and R. K. Hanson, “Diode laser sensor formeasurements of CO, CO2, and CH4 in combustion flows,”Appl. Opt. 36, 8745–8752 ~1997!.

0 November 1998 y Vol. 37, No. 33 y APPLIED OPTICS 7911