Quantum-Cascade Laser Measurements of Stratospheric Methane and Nitrous Oxide

Quantum-cascade laser measurements of stratosphericmethane and nitrous oxide

Christopher R. Webster, Gregory J. Flesch, David C. Scott, James E. Swanson, Randy D. May,W. Stephen Woodward, Claire Gmachl, Federico Capasso, Deborah L. Sivco,James N. Baillargeon, Albert L. Hutchinson, and Alfred Y. Cho

A tunable quantum-cascade ~QC! laser has been flown on NASA’s ER-2 high-altitude aircraft to producethe first atmospheric gas measurements with this newly invented device, an important milestone in theQC laser’s future planetary, industrial, and commercial applications. Using a cryogenically cooled QClaser during a series of 20 aircraft flights beginning in September 1999 and extending through March2000, we took measurements of methane ~CH4! and nitrous oxide ~N2O! gas up to ;20 km in thestratosphere over North America, Scandinavia, and Russia. The QC laser operating near an 8-mmwavelength was produced by the groups of Capasso and Cho of Bell Laboratories, Lucent Technologies,where QC lasers were invented in 1994. Compared with its companion lead salt diode lasers that werealso flown on these flights, the single-mode QC laser cooled to 82 K and produced higher output power~10 mW!, narrower laser linewidth ~17 MHz!, increased measurement precision ~a factor of 3!, and betterspectral stability ~;0.1 cm21 K!. The sensitivity of the QC laser channel was estimated to correspondto a minimum-detectable mixing ratio for methane of approximately 2 parts per billion by volume.© 2001 Optical Society of America

OCIS codes: 010.0010, 120.0120, 140.0140, 300.0300.

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1. Introduction

A. In Situ Laser Spectrometers for Earth and PlanetaryMeasurements

Both Earth and planetary atmospheric sciences havelong awaited the development of single-mode tunablelaser sources that operate in the mid-IR region atroom temperature. For over a decade, tunable lasersources in this wavelength region have relied on leadsalt tunable diode lasers ~TDL’s! that required cool-ing typically to 80 K or below with liquid cryogens orJoule–Thompson or Sterling-cycle coolers.1 Exceptfor limited-duration applications ~e.g., descendingprobe measuring vertical profiles2 that need only a

C. R. Webster [email protected]!, G. J. Flesch,D. C. Scott, J. E. Swanson, R. D. May, and W. S. Woodward arewith the Jet Propulsion Laboratory, MS 183 401, 4800 Oak GroveDrive, Pasadena, California 91109-8001. C. Gmachl, F. Capasso,D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho arewith Bell Laboratories, Lucent Technologies, 700 Mountain Ave-nue, Murray Hill, New Jersey 07974.

Received 19 November 1999; revised manuscript received 17August 2000.

0003-6935y01y030321-06$15.00y0© 2001 Optical Society of America

few hours of operation!, this approach is not practicalfor longer-duration missions and has inhibited theminiaturization of the spectrometers. Lead saltTDL’s also suffer from spectral degradation and reli-ability issues associated with thermal recycling.

For most planetary applications, whether a rover,lander, aerobot, or descending subsurface probe, se-vere power, mass, and volume limitations apply.For this reason, although difference-frequency gen-eration from various mixing schemes ~e.g., two differ-nt wavelength near-IR TDL’s! is possible,3 ideally,ingle-device tunable laser sources are needed.rogress in raising the operating temperature of leadalt TDL devices to room temperature has been pain-ully slow1 and has not been achieved in two decades

of development. The disappointment in the leadsalt TDL development has been partially replaced bythe development of room-temperature near-IR diodelasers in the 1–2-mm wavelength region.4 These de-vices have had a large effect on Mars experiments,where H2O and CO2 are present in sufficient quantityto offset the weakness of absorption in this near-IRregion. Room-temperature ~TE cooler! TDL sourcesf high spectral purity ~single mode! and high outputowers ~5–50 mW! are now available in the near-IRegion where molecules such as H2O and CO2 have

20 January 2001 y Vol. 40, No. 3 y APPLIED OPTICS 321

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sufficiently strong IR absorption cross sections.For other gas-phase species, systems with in-creased optical path lengths offer a means of offset-ting the loss in molecular absorption cross sections.For wavelengths in the 1–2-mm range, the JetPropulsion Laboratory’s ~JPL’s! Microdevices Lab-oratory have produced single-mode distributedfeedback ~DFB! devices that were tested and inte-grated into the Mars Volatile and Climate Surveyorlander payload of the failed Mars 98 Surveyor mis-sion: one for measurement of atmospheric H2O at1.37 mm and isotopic CO2 at 2.04 mm and a second

s an analyzer for evolved H2O and CO2 fromheated soil. However, more sensitivity-limited ap-plications remain inaccessible to near-IR sourcesbecause increasing path lengths by factors of 100are not compatible with miniaturization efforts andare not possible in applications with restrictedspace ~e.g., subsurface probe!.

Tunable lasers operating near room temperatureabove 220 K for TE cooler operation! would promisenew generation of miniature, tunable laser mid-IR

pectrometers for in situ measurement of atmo-pheric and evolved planetary gases. Such an all-olid-state spectrometer would have immediatepplications to Mars, Titan, Venus, and Europa mis-ions; could be operated on a descending or penetrat-ng probe, lander, rover, or aerobot; would use only aew watts of power; and would weigh less than 1 kg.ecause it directly accesses the wavelength region oftrong vibration–rotation spectral lines, a mid-IR la-er spectrometer has wide-ranging and immediatepplication to the measurement of concentrations ofeveral planetary gases such as H2O, CH4, CO, CO2,2H2, HCN, C2H6, C2N2, HC3N, O3, OCS, H2S, and

SO2 and numerous stable isotopes. Such measure-ents could be made to study ~i! atmospheric photo-

chemistry and transport of Mars, Titan,2 and Venus;ii! Mars and Europa mineralogical and biologicalxperiments ~e.g., quantification of adsorbed orvolved gases and their isotopic fractionation fromhermal decomposition of minerals or ice!; and ~iii!espiratory and hazardous gas monitoring for hu-an exploration of the solar system. Commercial

nd industrial applications are numerous. Mid-IRuantum-cascade ~QC! laser spectrometers that op-rate at TE cooler temperatures ~220 K to room tem-erature! would require suitable mid-IR detectors.urrently, HgCdZnTe photovoltaic detectors offer theest detectivities D*, with D* values in excess of 1 309 cm Hz1y2 W21 over the range of 2–10 mm. How-

ever, D* values fall with increasing wavelength, andwo or three tailored detectors are needed to coverhis range.

B. Quantum-Cascade Laser

A huge leap in laser technology has been made in thepast few years by Federico Capasso’s and AlfredCho’s groups at Bell Laboratories, Lucent Technolo-gies, with the invention of the QC laser.5–7 This

evice, operating pulsed at room temperature andontinuous wave ~cw! at lower temperatures, pro-

22 APPLIED OPTICS y Vol. 40, No. 3 y 20 January 2001

duces tunable mid-IR laser output from a revolution-ary new approach to laser design, that of quantumengineering of electronic energy levels ~see Fig. 1!.QC lasers are fundamentally different from diode la-sers in that the wavelength is determined essentiallyby quantum confinement, i.e., by the thickness ofactive layers rather than the energy bandgap of thematerial. By tailoring the active layer thickness,one can select the laser wavelength over a wide range~3–12 mm! of the IR spectrum by using the samematerial.5–7 In addition, QC lasers have muchhigher power than diode lasers at the same wave-length because an electron, after it emits a laser pho-ton in the first active region stage of the device, isrecycled and reinjected into the following stage whereit emits another photon. A typical QC laser has N 525–75 stages so that N laser photons are emitted perinjected electron. These new devices produce single-mode laser light that is tunable over 10–20 cm21 ofoutput power ~fractions of a watt! hundreds of timesreater than that of lead salt lasers at cryogenic tem-eratures. Furthermore, these devices are highlyeliable, with long-duration spectral integrity.

Progress in QC laser development has been rapid.oom-temperature ~300 K! operation has been dem-nstrated in pulsed operation from 3.6 to 11.5 mm,ith extremely high output peak powers up to a halff a watt.8,9 In the 5–8-mm wavelength region cw

operation at temperatures above 120 K produces out-put power levels of 2–20 mW and approximately 200mW at 80 K ~Ref. 9! ~note that cw lead salt TDL’sproduce1 only 0.2 mW at 80 K!. QC lasers operatenear room temperature in a pulsed mode, but not ina cw mode, and this is a thermal problem. Highelectron temperatures destroy the electron popula-tion inversion inside the cascade and lasing stops. Ifa laser is operated in the cw mode, then the core of thelaser ridge heats up considerably with respect to theheat-sink temperature because of the limited thermalconductivity of the semiconductor materials and thelarge amount of electrical power dissipated inside thedevice. A laser operated in cw at a heat sink of ;150K has a measured core temperature of ;100 Khigher, and the discrepancy increases with tempera-ture. On the other hand, in the pulsed mode theheat-sink and laser core temperatures are approxi-mately the same, and the laser has time to cool down

Fig. 1. Schematic of the geometry of a QC DFB laser.

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between current pulses, thus allowing higher operat-ing heat-sink temperatures.10

The cw operation of tunable lasers offers the mostsensitive means of gas detection because cw devicesare associated with very small laser linewidths ~typ-cally tens of megahertz!, and phase-sensitive detec-ion can be employed. Although QC DFB lasersperate in cw at 80 K with fractions of a watt ofutput power, room-temperature operation is cur-ently achieved only in a pulsed mode. In the pulsedonfiguration, QC DFB lasers are driven with pulsesf approximately a 10-ns duration, with up to 1-MHzepetition rates. Despite the low duty cycle of ap-roximately 1%, the average laser powers demon-trated with room-temperature pulsed QC DFBasers ~.10 mW! are still high enough to compete

favorably with even cooled lead salt devices that pro-duce typically 100-mW cw single-mode power.

For high-resolution spectroscopic applications,single-mode operation with narrow linewidth andhigh tunability is required. Recently, the DFB prin-ciple was applied to QC lasers. Single-mode tuningranges of 100 and 150 nm were demonstrated with atuning coefficient of ;0.35 nmyK at 5.2 mm and 0.55nmyK near 8 mm,9 respectively. To date, QC DFBlasers have been fabricated at various wavelengthsfrom 5 to 11.5 mm. QC DFB lasers have been used inthe laboratory to demonstrate detection of N2O at 7.8mm close to room temperature. In the first spectro-scopic measurements made with a room-temperatureQC DFB laser, Namjou and co-workers11 used wave-length modulation to detect both N2O and CH4 near

Fig. 2. NASA’s ER-2 high-altitude aircraft takes off from DrydenLIAS is located in the superpod on the right wing. This paylodministration, and university experiments.

8 mm. Sensitivities achieved were equivalent toinimum-detectable absorptances of 5 parts in 105,

within a factor of 10 of that demonstrated for TDLmeasurements.12

Laser linewidth is of more concern. Namjou andco-workers11 measured a laser linewidth of 720 MHz,compared with typical molecular linewidths ~HWHMt 6–8 mm! of approximately 50 MHz ~0.0017 cm21!,t very low pressures ~Doppler broadened! and ap-roximately 3000 MHz ~0.1 cm21! at atmospheric

pressure. More recently, kilohertz-level linewidthshave been measured13 from frequency-stabilized cwDFB QC lasers, and spectroscopic measurementshave included Doppler-limited14 and photoacoustic15

spectroscopy.

2. Stratospheric Measurements from the ER-2 Aircraft

Over the past 16 years, Webster’s JPL group haspioneered the development of tunable laser spectrom-eters for Earth and planetary applications. Cur-rently, the group has nine laser spectrometers foraircraft, balloon, and spacecraft, including one de-signed for Saturn’s moon Titan, two that were part ofthe Mars lander payload that failed to communicatewith Earth in 1999, and one selected for the Mars2005 lander payload. In over 250 aircraft and bal-loon flights, the JPL group has demonstrated thehigh sensitivity of tunable laser absorption spectros-copy for in situ measurement of atmospheric gas con-centrations in the mid-IR ~3–8-mm! region, includingracers such as N2O, CO, H2O, and CH4; radicals

ht Research Center ~NASA photo courtesy of Tony Landis!. Therries numerous other NASA, National Oceanic and Atmospheric

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such as NO and NO2; and reservoir gases such as HCland HNO3.2,16–18

In preparation for a series of aircraft flights fromKiruna, Sweden, in spring 2000 as part of NASA’sStratospheric Aerosol and Gas Experiment ~SAGE!II Ozone Loss and Validation Experiment ~SOLVE!ission, JPL’s aircraft laser infrared absorption spec-

rometer ~ALIAS!17 was taken to Edwards Air ForceBase in September 1999 for test flights on NASA’sER-2 aircraft. The ER-2 aircraft ~see Fig. 2! is aingle-engine, high-altitude aircraft that is a modi-ed U2 aircraft, capable of flying to altitudes greaterhan 20 km. On 23, 25, and 28 September 1999,ASA pilots Ken Broda, Jan Nystrom, and Jim Bar-

illeaux each flew a single flight from the NASA Dry-en Flight Research Center at Edwards Air Forcease on 2–8 h sorties that made up a science mea-urement intercomparison series. With four-laserapability in the liquid-nitrogen Dewar of the ALIASnstrument, a cryogenically cooled QC laser operatingear an 8-mm wavelength ~1261 cm21! was incorpo-

rated to measure the N2O and the methane CH4.The QC laser was mounted on a special package de-signed by the JPL group ~see Fig. 3! that was me-chanically compatible with the commercial lead saltTDL packaging.

All four lasers ~one QC and three lead salt TDL’s!in the ALIAS Dewar are held at a fixed temperatureby the liquid-nitrogen reservoir pressure and individ-ual passive heaters. The laser wavelengths are

Fig. 3. Chip comprising seven QC DFB lasers mounted onto agold-coated, oxygen-free copper heat sink designed for compatibil-ity with typical lead salt TDL packages. Only two lasers are wirebonded to connections for use ~one as an unused spare!.


scanned with a current ramp and superposed sinu-soidal modulation for second-harmonic detection.The scanning ramp frequency is 10 Hz, and individ-ual scans are coadded to produce a 1.3-s spectralaverage. These 1.3-s spectral averages are writtento a large hard drive for postflight processing. TheQC laser is driven by the same electronics as the leadsalt TDL’s, except that it has an additional circuitbetween the laser and the drive that provides for a28-V bias and larger current sweep capability. Fora constant Dewar temperature, the QC laser could bescanned continuously through the maximum currentramp available, which corresponded to a sweep rangeof ;200 mA. Figure 4 shows the QC laser scan overthe N2O and CH4 lines used in this study.

The QC laser worked extremely well during theaircraft mission. Compared with its companionlead salt TDL’s in the other three channels thatshowed laser linewidths of 50–100 MHz, the QClaser linewidth was significantly smaller. A fit todirect absorption molecular line shapes recordedfrom low-pressure ~Doppler-limited! lines produced a

easured laser linewidth ~HWHM! of ;17 MHz,which was better represented by a Gaussian ratherthan Lorentzian shape. Current drive fluctuationsare believed to be the limiting contribution to thislaser linewidth. The Gaussian nature of the ob-served QC laser linewidth is in agreement with ear-lier studies.14 For frequency-stabilized QC DFB

Fig. 4. QC laser spectral scans of a reference gas cell, a Ge etalonwith a free spectral range of 0.015 cm21, and the actual flightspectrum showing second-harmonic line shapes from CH4 andN2O. Four spectra such as this are recorded in-flight each 1.3 s,produced from absorption in 80 passes of the 1-m-long multipasscell of ALIAS. During flight, the cell is kept at a constant tem-perature of 280 K. With increasing current, the QC laser scans toa lower wave number ~opposite the lead salt TDL’s, but similar tonear-IR TDL’s!. Over the full range shown, the frequency tuningexhibits a small nonlinearity, the tuning rate increases by ;10%with an increase in current. The reference gas cell, which is 5 cmlong and contains enough pure gas to absorb fully at line center, isused only for line identification and a mode-purity check, and thesmall interference fringes that can be seen in the upper trace resultfrom an etalon effect with the cell in the beam.

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lasers, Lorentzian laser linewidths have been report-ed.13

Additional advantages observed for the cryogeni-cally cooled QC laser over the lead salt TDL’s in-cluded output power and mode purity. Although theTDL’s put out only 0.1-mW output power with modepurities varying from 75 to 96%, the QC laser deliv-ered approximately 10 mW of output power with aconstant 96% single-mode purity. The QC laserdemonstrated several additional practical advan-tages over the traditional lead salt TDL’s. First, thedevice was thermally recycled numerous times with-out performance degradation. Second, the QC laserexhibited slow tuning with temperature and currentthat proved to be a blessing in the flight. The slowcurrent tuning rate ~;115 MHzymA, which is ap-proximately ten times less than typical lead saltTDL’s! means that the current noise contribution issmaller, producing narrower laser linewidths. How-ever, the operating voltage of QC lasers is consider-ably higher than that of lead salt devices, and as aresult they dissipate more power for the same tuningcoefficient. In fact, because of the fundamentallydifferent carrier distribution ~parallel bands of thesame symmetry! and carrier dynamics in the activeegions, which are due to their anticipated lack of aoticeable alpha parameter, QC lasers in general arexpected to display an intrinsically narrower line-idth than diode lasers.14 The slower temperature

tuning ~;0.1 cm21 K! of the QC laser made the se-lected scan range much less dependent on changes inthe exact cold-finger temperature. During the flightof 28 September, a heater failure near the end of theflight caused a partial failure of the O rings in theDewar liquid-nitrogen pressure seal, causing thespectral lines from all three lead salt laser channelsto move quickly off the screen as the loss of pressurecooled the cold finger by a few degrees. The QC laserspectral scan, however, barely moved because it wasso much less sensitive to the cryogen temperature,and its scan of N2O and CH4 lines remained on screenfor the entire flight. The sensitivity of the QC laserchannel can be estimated from the background noiselevel of the spectra shown in Fig. 4. For a 2-minaverage, a minimum-detectable absorptance of only;5 3 1025 was observed, corresponding to aminimum-detectable mixing ratio for methane of ap-proximately 2 parts per billion by volume ~ppbv!.

Following the September 1999 test flights, theLIAS instrument continued on as part of the SOLVEission aircraft payload that made 20 flights fromiruna, Sweden. The QC laser worked extremelyell on each flight with flawless reliability. Figure 5

hows in-flight measurements of atmospheric CH4taken on two flights, 16 December 1999 @Fig. 5~a!# and11 March 2000 @Fig. 5~b!# as examples of flight data.The final volume mixing ratios are produced from thepeak-to-peak 2f signal that can be seen in each 1.3-sspectrum ~with corrections for laser power, pressure,temperature, etc.!. In Fig. 5~a!, we made an inter-comparison between simultaneous measurements ofCH4 using a lead salt TDL at 2926.7001 cm21 and the

QC laser at 1256.6018 cm21 ~the QC values were dis-placed to lower values by 50 ppbv for clarity of com-parison!. Both tracers result from each 1.3-smeasurement. Within the uncertainty in the calibra-tion gas standards that were used ~1%!, the two mea-surements agreed well in absolute accuracy. The QClaser produced significantly more output power ~;10mW! than the lead salt TDL ~;200 mW!, and thisresults in a significant improvement of the measure-ment precision by a factor of approximately 3. At thislevel, other sources of noise still contribute to theachieved precision limit, which is limited principallyby interference fringes. In Fig. 5~b!, a full flight dataset is shown for the 11 March 2000 flight, where struc-ture in the mixing ratio is indicative of a dynamicexchange, and the low mixing ratios near the latterpart of the flight are typical for the large descent of airassociated with the polar vortex. This flight demon-strates the high precision of measurement achieved

Fig. 5. ~a! Intercomparison between lead salt TDL and QC finalflight data for CH4 measurements over California during the cruiseand descent portions of the 16 December 1999 flight. For bothchannels, data points result from the peak-to-peak magnitude ofthe second-harmonic line in a 1.3-s spectral average, with appro-priate correction. For clarity of comparison, the QC values weredisplaced to lower values by 50 ppbv. The ER-2 aircraft pressurealtitude is also plotted. For this flight, outside air temperatureswere typically 212 K during cruise, decreasing to 204 K at thetropopause, and increasing to 270 K on descent. ~b! Final 1.3-sflight data for CH4 measurements over Sweden and Russia duringascent, dive, climb, and descent for the 11 March 2000 flight. Thelower mixing ratios recorded during the latter one third of theflight are characteristic of sampling the Arctic polar vortex intowhich air from higher altitudes has descended. The ER-2 aircraftpressure altitude is also plotted. For this flight, outside air tem-peratures were typically 197 K during cruise and climb, increasingto 270 K on descent.


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4. S. Forouhar, S. Keo, A. Larsson, A. Ksendzov, and H. Temkin,

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over the large changes in methane observed in and outof the Arctic polar vortex.

Historically, both CH4 and N2O have been used asimportant long-lived tracers of large-scale atmo-spheric circulation. They are both produced at theEarth’s surface and photochemically removed in thestratosphere. In situ observations of N2O and CH4show distinct compact relationships for the tropicsand extratropics.18 The variation of their verticalprofiles with latitude has been used to infer processesthat entrain mid-latitude air into the tropics.19 Al-though by themselves the stratospheric measure-ments made here do not reveal new insights intothese processes, they are nevertheless fully consis-tent with these earlier studies.

In addition to numerous industrial and commercialapplications, the development of room-temperatureQC lasers is expected to have a significant effect onour ability to produce miniaturized spectrometersthat perform highly sensitive in situ identificationand quantitative characterization of Earth and plan-etary atmospheres, aerosols, and their surface andsubsurface mineralogy. In collaboration with thegroups at Bell Laboratories, the JPL group is cur-rently building a prototype QC laser spectrometer forMars. The prototype QC laser spectrometer is afour-laser miniature spectrometer that operates atroom temperature and uses HgCdZnTe detectors.The entire instrument will weigh less than 1 kg andhave the capability for six gas species measurementsat the parts per billion level. QC lasers onboardwould take measurements of biogenic and geother-mal gases such as methane and sulfur dioxide, whichcould be present in the Martian atmosphere, anddetermine their isotope ratios.

Part of the research described in this paper wascarried out at the Jet Propulsion Laboratory, Califor-nia Institute of Technology, under contract with theNational Aeronautics and Space Administration.Funding from NASA’s Upper Atmospheric ResearchProgram ~UARP! and Planetary Instrument Defini-ion and Development Program is gratefully acknowl-dged. The aircraft flights were made as part ofARP’s SAGE III Ozone Loss Validation Experiment

SOLVE! mission ~pilots are named in the text!. Theesearch performed at Bell Laboratories is supportedn part by the Defense Advanced Research Projectsgency, U.S. Army Research Office, under contractAAG55-98-C-0050.

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9. C. Gmachl, F. Capasso, J. Faist, A. L. Hutchinson, A.Tredicucci, D. L. Sivco, J. N. Baillargeon, S. N. G. Chu, andA. Y. Cho, “Continuous-wave and high-power pulsed operationof index-coupled distributed feedback quantum cascade laserat l ' 8.5 mm,” Appl. Phys. Lett. 72, 1430–1432 ~1998!.

10. C. Gmachl, A. M. Sergent, A. Tredicucci, F. Capasso, A. L.Hutchinson, D. L. Sivco, J. N. Baillargeon, S. N. G. Chu, andA. Y. Cho, “Improved cw operation of quantum cascade laserswith epitaxial-side heat-sinking,” IEEE Photon. Technol. Lett.11, 1369–1371 ~1999!.

11. K. Namjou, S. Cai, E. A. Whittaker, J. Faist, C. Gmachl, F.Capasso, D. L. Sivco, and A. Y. Cho, “Sensitive absorptionspectroscopy with a room-temperature distributed-feedbackquantum-cascade laser,” Opt. Lett. 23, 219–221 ~1998!.

12. R. D. May and C. R. Webster, “Balloon-borne laser spectrom-eter measurements of NO2 with gas absorption sensitivitiesbelow 1025,” Appl. Opt. 29, 5042–5044 ~1990!.

3. R. M. Williams, J. F. Kelly, J. S. Hartman, S. W. Sharpe, M. S.Taubman, J. L. Hall, F. Capasso, C. Gmachl, D. L. Sivco, J. N.Baillargeon, and A. Y. Cho, “Kilo-hertz linewidth from fre-quency stabilized mid-infrared quantum cascade lasers,” Opt.Lett. 24, 1844–1846 ~1999!.

4. S. W. Sharpe, J. F. Kelly, J. S. Hartman, C. Gmachl, F.Capasso, D. L. Sivco, J. N. Baillargeon, and A. Y. Cho, “High-resolution ~Doppler-limited! spectroscopy using quantum-cascade distributed-feedback lasers,” Opt. Lett. 23, 1396–1398~1998!.

15. B. A. Paldus, T. G. Spence, R. N. Zare, J. Oomens, F. M. J.Harren, D. H. Parker, C. Gmachl, F. Capasso, D. L. Sivco, J. N.Baillargeon, A. L. Hutchinson, and A. Y. Cho, “Photoacousticspectroscopy using quantum-cascade lasers,” Opt. Lett. 24,178–180 ~1999!.

16. C. R. Webster, R. D. May, R. Toumi, and J. Pyle, “Activenitrogen partitioning and the nighttime formation of N2O5 inthe stratosphere: simultaneous in-situ measurements of NO,NO2, HNO3, O3, N2O, and jNO2 using the BLISS diode laserspectrometer,” J. Geophys. Res. 95, 13851–13866 ~1990!.

17. C. R. Webster, R. D. May, C. A. Trimble, R. G. Chave, and J.Kendall, “Aircraft ~ER-2! laser infrared absorption spectrom-eter ~ALIAS! for in-situ stratospheric measurements of HCl,N2O, CH4, NO2, and HNO3,” Appl. Opt. 33, 454–472 ~1994!.

18. D. C. Scott, R. L. Herman, C. R. Webster, R. D. May, G. J. Flesch,and E. J. Moyer, “Airborne Laser Infrared Absorption Spectrom-eter ~ALIAS-II! for in situ atmospheric measurements of N2O,CH4, CO, HCl, and NO2 from balloon or remotely piloted aircraftplatforms,” Appl. Opt. 38, 4609–4622 ~1999!.

19. R. L. Herman, D. C. Scott, C. R. Webster, R. D. May, E. J.Moyer, R. J. Salawitch, Y. L. Yung, G. C. Toon, B. Sen, J. J.Margitan, K. H. Rosenlof, H. A. Michelsen, and J. W. Elkins,“Tropical entrainment timescales inferred from stratosphericN2O and CH4 observations,” Geophys. Res. Lett. 25, 2781–2784 ~1998!.

Documents

Quantum-Cascade Laser Measurements of Stratospheric Methane and Nitrous Oxide