Quantum-cascade laser measurements of stratospheric methane 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 共QC兲 laser has been flown on NASA’s ER-2 high-altitude aircraft to produce the first atmospheric gas measurements with this newly invented device, an important milestone in the QC laser’s future planetary, industrial, and commercial applications. Using a cryogenically cooled QC laser during a series of 20 aircraft flights beginning in September 1999 and extending through March 2000, we took measurements of methane 共CH4兲 and nitrous oxide 共N2O兲 gas up to ⬃20 km in the stratosphere over North America, Scandinavia, and Russia. The QC laser operating near an 8-m wavelength 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 were also flown on these flights, the single-mode QC laser cooled to 82 K and produced higher output power 共10 mW兲, narrower laser linewidth 共17 MHz兲, increased measurement precision 共a factor of 3兲, and better spectral stability 共⬃0.1 cm⫺1 K兲. The sensitivity of the QC laser channel was estimated to correspond to 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.
1. Introduction A. In Situ Laser Spectrometers for Earth and Planetary Measurements
Both Earth and planetary atmospheric sciences have long awaited the development of single-mode tunable laser sources that operate in the mid-IR region at room temperature. For over a decade, tunable laser sources in this wavelength region have relied on lead salt tunable diode lasers 共TDL’s兲 that required cooling typically to 80 K or below with liquid cryogens or Joule–Thompson or Sterling-cycle coolers.1 Except for limited-duration applications 共e.g., descending probe measuring vertical profiles2 that need only a C. R. Webster 共
[email protected]兲, G. J. Flesch, D. C. Scott, J. E. Swanson, R. D. May, and W. S. Woodward are with the Jet Propulsion Laboratory, MS 183 401, 4800 Oak Grove Drive, Pasadena, California 91109-8001. C. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho are with Bell Laboratories, Lucent Technologies, 700 Mountain Avenue, Murray Hill, New Jersey 07974. Received 19 November 1999; revised manuscript received 17 August 2000. 0003-6935兾01兾030321-06$15.00兾0 © 2001 Optical Society of America
few hours of operation兲, this approach is not practical for longer-duration missions and has inhibited the miniaturization of the spectrometers. Lead salt TDL’s also suffer from spectral degradation and reliability issues associated with thermal recycling. For most planetary applications, whether a rover, lander, aerobot, or descending subsurface probe, severe power, mass, and volume limitations apply. For this reason, although difference-frequency generation from various mixing schemes 共e.g., two different wavelength near-IR TDL’s兲 is possible,3 ideally, single-device tunable laser sources are needed. Progress in raising the operating temperature of lead salt TDL devices to room temperature has been painfully slow1 and has not been achieved in two decades of development. The disappointment in the lead salt TDL development has been partially replaced by the development of room-temperature near-IR diode lasers in the 1–2-m wavelength region.4 These devices have had a large effect on Mars experiments, where H2O and CO2 are present in sufficient quantity to offset the weakness of absorption in this near-IR region. Room-temperature 共TE cooler兲 TDL sources of high spectral purity 共single mode兲 and high output powers 共5–50 mW兲 are now available in the near-IR region where molecules such as H2O and CO2 have 20 January 2001 兾 Vol. 40, No. 3 兾 APPLIED OPTICS
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sufficiently strong IR absorption cross sections. For other gas-phase species, systems with increased optical path lengths offer a means of offsetting the loss in molecular absorption cross sections. For wavelengths in the 1–2-m range, the Jet Propulsion Laboratory’s 共JPL’s兲 Microdevices Laboratory have produced single-mode distributed feedback 共DFB兲 devices that were tested and integrated into the Mars Volatile and Climate Surveyor lander payload of the failed Mars 98 Surveyor mission: one for measurement of atmospheric H2O at 1.37 m and isotopic CO2 at 2.04 m and a second as an analyzer for evolved H2O and CO2 from heated soil. However, more sensitivity-limited applications remain inaccessible to near-IR sources because increasing path lengths by factors of 100 are not compatible with miniaturization efforts and are not possible in applications with restricted space 共e.g., subsurface probe兲. Tunable lasers operating near room temperature 共above 220 K for TE cooler operation兲 would promise a new generation of miniature, tunable laser mid-IR spectrometers for in situ measurement of atmospheric and evolved planetary gases. Such an allsolid-state spectrometer would have immediate applications to Mars, Titan, Venus, and Europa missions; could be operated on a descending or penetrating probe, lander, rover, or aerobot; would use only a few watts of power; and would weigh less than 1 kg. Because it directly accesses the wavelength region of strong vibration–rotation spectral lines, a mid-IR laser spectrometer has wide-ranging and immediate application to the measurement of concentrations of several planetary gases such as H2O, CH4, CO, CO2, C2H2, HCN, C2H6, C2N2, HC3N, O3, OCS, H2S, and SO2 and numerous stable isotopes. Such measurements could be made to study 共i兲 atmospheric photochemistry and transport of Mars, Titan,2 and Venus; 共ii兲 Mars and Europa mineralogical and biological experiments 共e.g., quantification of adsorbed or evolved gases and their isotopic fractionation from thermal decomposition of minerals or ice兲; and 共iii兲 respiratory and hazardous gas monitoring for human exploration of the solar system. Commercial and industrial applications are numerous. Mid-IR quantum-cascade 共QC兲 laser spectrometers that operate at TE cooler temperatures 共220 K to room temperature兲 would require suitable mid-IR detectors. Currently, HgCdZnTe photovoltaic detectors offer the best detectivities D*, with D* values in excess of 1 ⫻ 109 cm Hz1兾2 W⫺1 over the range of 2–10 m. However, D* values fall with increasing wavelength, and two or three tailored detectors are needed to cover this range. B.
Quantum-Cascade Laser
A huge leap in laser technology has been made in the past few years by Federico Capasso’s and Alfred Cho’s groups at Bell Laboratories, Lucent Technologies, with the invention of the QC laser.5–7 This device, operating pulsed at room temperature and continuous wave 共cw兲 at lower temperatures, pro322
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Fig. 1. Schematic of the geometry of a QC DFB laser.
duces tunable mid-IR laser output from a revolutionary new approach to laser design, that of quantum engineering of electronic energy levels 共see Fig. 1兲. QC lasers are fundamentally different from diode lasers in that the wavelength is determined essentially by quantum confinement, i.e., by the thickness of active layers rather than the energy bandgap of the material. By tailoring the active layer thickness, one can select the laser wavelength over a wide range 共3–12 m兲 of the IR spectrum by using the same material.5–7 In addition, QC lasers have much higher power than diode lasers at the same wavelength because an electron, after it emits a laser photon in the first active region stage of the device, is recycled and reinjected into the following stage where it emits another photon. A typical QC laser has N ⫽ 25–75 stages so that N laser photons are emitted per injected electron. These new devices produce singlemode laser light that is tunable over 10 –20 cm⫺1 of output power 共fractions of a watt兲 hundreds of times greater than that of lead salt lasers at cryogenic temperatures. Furthermore, these devices are highly reliable, with long-duration spectral integrity. Progress in QC laser development has been rapid. Room-temperature 共300 K兲 operation has been demonstrated in pulsed operation from 3.6 to 11.5 m, with extremely high output peak powers up to a half of a watt.8,9 In the 5– 8-m wavelength region cw operation at temperatures above 120 K produces output power levels of 2–20 mW and approximately 200 mW at 80 K 共Ref. 9兲 共note that cw lead salt TDL’s produce1 only 0.2 mW at 80 K兲. QC lasers operate near room temperature in a pulsed mode, but not in a cw mode, and this is a thermal problem. High electron temperatures destroy the electron population inversion inside the cascade and lasing stops. If a laser is operated in the cw mode, then the core of the laser ridge heats up considerably with respect to the heat-sink temperature because of the limited thermal conductivity of the semiconductor materials and the large amount of electrical power dissipated inside the device. A laser operated in cw at a heat sink of ⬃150 K has a measured core temperature of ⬃100 K higher, and the discrepancy increases with temperature. On the other hand, in the pulsed mode the heat-sink and laser core temperatures are approximately the same, and the laser has time to cool down
Fig. 2. NASA’s ER-2 high-altitude aircraft takes off from Dryden Flight Research Center 共NASA photo courtesy of Tony Landis兲. The ALIAS is located in the superpod on the right wing. This payload carries numerous other NASA, National Oceanic and Atmospheric Administration, and university experiments.
between current pulses, thus allowing higher operating heat-sink temperatures.10 The cw operation of tunable lasers offers the most sensitive means of gas detection because cw devices are associated with very small laser linewidths 共typically tens of megahertz兲, and phase-sensitive detection can be employed. Although QC DFB lasers operate in cw at 80 K with fractions of a watt of output power, room-temperature operation is currently achieved only in a pulsed mode. In the pulsed configuration, QC DFB lasers are driven with pulses of approximately a 10-ns duration, with up to 1-MHz repetition rates. Despite the low duty cycle of approximately 1%, the average laser powers demonstrated with room-temperature pulsed QC DFB lasers 共⬎10 W兲 are still high enough to compete favorably with even cooled lead salt devices that produce typically 100-W cw single-mode power. For high-resolution spectroscopic applications, single-mode operation with narrow linewidth and high tunability is required. Recently, the DFB principle was applied to QC lasers. Single-mode tuning ranges of 100 and 150 nm were demonstrated with a tuning coefficient of ⬃0.35 nm兾K at 5.2 m and 0.55 nm兾K near 8 m,9 respectively. To date, QC DFB lasers have been fabricated at various wavelengths from 5 to 11.5 m. QC DFB lasers have been used in the laboratory to demonstrate detection of N2O at 7.8 m close to room temperature. In the first spectroscopic measurements made with a room-temperature QC DFB laser, Namjou and co-workers11 used wavelength modulation to detect both N2O and CH4 near
8 m. Sensitivities achieved were equivalent to minimum-detectable absorptances of 5 parts in 105, within a factor of 10 of that demonstrated for TDL measurements.12 Laser linewidth is of more concern. Namjou and co-workers11 measured a laser linewidth of 720 MHz, compared with typical molecular linewidths 共HWHM at 6 – 8 m兲 of approximately 50 MHz 共0.0017 cm⫺1兲, at very low pressures 共Doppler broadened兲 and approximately 3000 MHz 共0.1 cm⫺1兲 at atmospheric pressure. More recently, kilohertz-level linewidths have been measured13 from frequency-stabilized cw DFB QC lasers, and spectroscopic measurements have included Doppler-limited14 and photoacoustic15 spectroscopy. 2. Stratospheric Measurements from the ER-2 Aircraft
Over the past 16 years, Webster’s JPL group has pioneered the development of tunable laser spectrometers for Earth and planetary applications. Currently, the group has nine laser spectrometers for aircraft, balloon, and spacecraft, including one designed for Saturn’s moon Titan, two that were part of the Mars lander payload that failed to communicate with Earth in 1999, and one selected for the Mars 2005 lander payload. In over 250 aircraft and balloon flights, the JPL group has demonstrated the high sensitivity of tunable laser absorption spectroscopy for in situ measurement of atmospheric gas concentrations in the mid-IR 共3– 8-m兲 region, including tracers such as N2O, CO, H2O, and CH4; radicals 20 January 2001 兾 Vol. 40, No. 3 兾 APPLIED OPTICS
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Fig. 3. Chip comprising seven QC DFB lasers mounted onto a gold-coated, oxygen-free copper heat sink designed for compatibility with typical lead salt TDL packages. Only two lasers are wire bonded to connections for use 共one as an unused spare兲.
such as NO and NO2; and reservoir gases such as HCl and HNO3.2,16 –18 In preparation for a series of aircraft flights from Kiruna, Sweden, in spring 2000 as part of NASA’s Stratospheric Aerosol and Gas Experiment 共SAGE兲 III Ozone Loss and Validation Experiment 共SOLVE兲 mission, JPL’s aircraft laser infrared absorption spectrometer 共ALIAS兲17 was taken to Edwards Air Force Base in September 1999 for test flights on NASA’s ER-2 aircraft. The ER-2 aircraft 共see Fig. 2兲 is a single-engine, high-altitude aircraft that is a modified U2 aircraft, capable of flying to altitudes greater than 20 km. On 23, 25, and 28 September 1999, NASA pilots Ken Broda, Jan Nystrom, and Jim Barrilleaux each flew a single flight from the NASA Dryden Flight Research Center at Edwards Air Force Base on 2– 8 h sorties that made up a science measurement intercomparison series. With four-laser capability in the liquid-nitrogen Dewar of the ALIAS instrument, a cryogenically cooled QC laser operating near an 8-m wavelength 共1261 cm⫺1兲 was incorporated to measure the N2O and the methane CH4. The QC laser was mounted on a special package designed by the JPL group 共see Fig. 3兲 that was mechanically compatible with the commercial lead salt TDL packaging. All four lasers 共one QC and three lead salt TDL’s兲 in the ALIAS Dewar are held at a fixed temperature by the liquid-nitrogen reservoir pressure and individual passive heaters. The laser wavelengths are 324
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Fig. 4. QC laser spectral scans of a reference gas cell, a Ge etalon with a free spectral range of 0.015 cm⫺1, and the actual flight spectrum showing second-harmonic line shapes from CH4 and N2O. Four spectra such as this are recorded in-flight each 1.3 s, produced from absorption in 80 passes of the 1-m-long multipass cell of ALIAS. During flight, the cell is kept at a constant temperature of 280 K. With increasing current, the QC laser scans to a lower wave number 共opposite the lead salt TDL’s, but similar to near-IR TDL’s兲. Over the full range shown, the frequency tuning exhibits a small nonlinearity, the tuning rate increases by ⬃10% with an increase in current. The reference gas cell, which is 5 cm long and contains enough pure gas to absorb fully at line center, is used only for line identification and a mode-purity check, and the small interference fringes that can be seen in the upper trace result from an etalon effect with the cell in the beam.
scanned with a current ramp and superposed sinusoidal modulation for second-harmonic detection. The scanning ramp frequency is 10 Hz, and individual scans are coadded to produce a 1.3-s spectral average. These 1.3-s spectral averages are written to a large hard drive for postflight processing. The QC laser is driven by the same electronics as the lead salt TDL’s, except that it has an additional circuit between the laser and the drive that provides for a ⫺8-V bias and larger current sweep capability. For a constant Dewar temperature, the QC laser could be scanned continuously through the maximum current ramp available, which corresponded to a sweep range of ⬃200 mA. Figure 4 shows the QC laser scan over the N2O and CH4 lines used in this study. The QC laser worked extremely well during the aircraft mission. Compared with its companion lead salt TDL’s in the other three channels that showed laser linewidths of 50 –100 MHz, the QC laser linewidth was significantly smaller. A fit to direct absorption molecular line shapes recorded from low-pressure 共Doppler-limited兲 lines produced a measured laser linewidth 共HWHM兲 of ⬃17 MHz, which was better represented by a Gaussian rather than Lorentzian shape. Current drive fluctuations are believed to be the limiting contribution to this laser linewidth. The Gaussian nature of the observed QC laser linewidth is in agreement with earlier studies.14 For frequency-stabilized QC DFB
lasers, Lorentzian laser linewidths have been reported.13 Additional advantages observed for the cryogenically cooled QC laser over the lead salt TDL’s included output power and mode purity. Although the TDL’s put out only 0.1-mW output power with mode purities varying from 75 to 96%, the QC laser delivered approximately 10 mW of output power with a constant 96% single-mode purity. The QC laser demonstrated several additional practical advantages over the traditional lead salt TDL’s. First, the device was thermally recycled numerous times without performance degradation. Second, the QC laser exhibited slow tuning with temperature and current that proved to be a blessing in the flight. The slow current tuning rate 共⬃115 MHz兾mA, which is approximately ten times less than typical lead salt TDL’s兲 means that the current noise contribution is smaller, producing narrower laser linewidths. However, the operating voltage of QC lasers is considerably higher than that of lead salt devices, and as a result they dissipate more power for the same tuning coefficient. In fact, because of the fundamentally different carrier distribution 共parallel bands of the same symmetry兲 and carrier dynamics in the active regions, which are due to their anticipated lack of a noticeable alpha parameter, QC lasers in general are expected to display an intrinsically narrower linewidth than diode lasers.14 The slower temperature tuning 共⬃0.1 cm⫺1 K兲 of the QC laser made the selected scan range much less dependent on changes in the exact cold-finger temperature. During the flight of 28 September, a heater failure near the end of the flight caused a partial failure of the O rings in the Dewar liquid-nitrogen pressure seal, causing the spectral lines from all three lead salt laser channels to move quickly off the screen as the loss of pressure cooled the cold finger by a few degrees. The QC laser spectral scan, however, barely moved because it was so much less sensitive to the cryogen temperature, and its scan of N2O and CH4 lines remained on screen for the entire flight. The sensitivity of the QC laser channel can be estimated from the background noise level of the spectra shown in Fig. 4. For a 2-min average, a minimum-detectable absorptance of only ⬃5 ⫻ 10⫺5 was observed, corresponding to a minimum-detectable mixing ratio for methane of approximately 2 parts per billion by volume 共ppbv兲. Following the September 1999 test flights, the ALIAS instrument continued on as part of the SOLVE mission aircraft payload that made 20 flights from Kiruna, Sweden. The QC laser worked extremely well on each flight with flawless reliability. Figure 5 shows in-flight measurements of atmospheric CH4 taken on two flights, 16 December 1999 关Fig. 5共a兲兴 and 11 March 2000 关Fig. 5共b兲兴 as examples of flight data. The final volume mixing ratios are produced from the peak-to-peak 2f signal that can be seen in each 1.3-s spectrum 共with corrections for laser power, pressure, temperature, etc.兲. In Fig. 5共a兲, we made an intercomparison between simultaneous measurements of CH4 using a lead salt TDL at 2926.7001 cm⫺1 and the
Fig. 5. 共a兲 Intercomparison between lead salt TDL and QC final flight data for CH4 measurements over California during the cruise and descent portions of the 16 December 1999 flight. For both channels, data points result from the peak-to-peak magnitude of the second-harmonic line in a 1.3-s spectral average, with appropriate correction. For clarity of comparison, the QC values were displaced to lower values by 50 ppbv. The ER-2 aircraft pressure altitude is also plotted. For this flight, outside air temperatures were typically 212 K during cruise, decreasing to 204 K at the tropopause, and increasing to 270 K on descent. 共b兲 Final 1.3-s flight data for CH4 measurements over Sweden and Russia during ascent, dive, climb, and descent for the 11 March 2000 flight. The lower mixing ratios recorded during the latter one third of the flight are characteristic of sampling the Arctic polar vortex into which air from higher altitudes has descended. The ER-2 aircraft pressure altitude is also plotted. For this flight, outside air temperatures were typically 197 K during cruise and climb, increasing to 270 K on descent.
QC laser at 1256.6018 cm⫺1 共the QC values were displaced to lower values by 50 ppbv for clarity of comparison兲. Both tracers result from each 1.3-s measurement. Within the uncertainty in the calibration gas standards that were used 共1%兲, the two measurements agreed well in absolute accuracy. The QC laser produced significantly more output power 共⬃10 mW兲 than the lead salt TDL 共⬃200 W兲, and this results in a significant improvement of the measurement precision by a factor of approximately 3. At this level, other sources of noise still contribute to the achieved precision limit, which is limited principally by interference fringes. In Fig. 5共b兲, a full flight data set is shown for the 11 March 2000 flight, where structure in the mixing ratio is indicative of a dynamic exchange, and the low mixing ratios near the latter part of the flight are typical for the large descent of air associated with the polar vortex. This flight demonstrates the high precision of measurement achieved 20 January 2001 兾 Vol. 40, No. 3 兾 APPLIED OPTICS
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over the large changes in methane observed in and out of the Arctic polar vortex. Historically, both CH4 and N2O have been used as important long-lived tracers of large-scale atmospheric circulation. They are both produced at the Earth’s surface and photochemically removed in the stratosphere. In situ observations of N2O and CH4 show distinct compact relationships for the tropics and extratropics.18 The variation of their vertical profiles with latitude has been used to infer processes that entrain mid-latitude air into the tropics.19 Although by themselves the stratospheric measurements made here do not reveal new insights into these processes, they are nevertheless fully consistent with these earlier studies. In addition to numerous industrial and commercial applications, the development of room-temperature QC lasers is expected to have a significant effect on our ability to produce miniaturized spectrometers that perform highly sensitive in situ identification and quantitative characterization of Earth and planetary atmospheres, aerosols, and their surface and subsurface mineralogy. In collaboration with the groups at Bell Laboratories, the JPL group is currently building a prototype QC laser spectrometer for Mars. The prototype QC laser spectrometer is a four-laser miniature spectrometer that operates at room temperature and uses HgCdZnTe detectors. The entire instrument will weigh less than 1 kg and have the capability for six gas species measurements at the parts per billion level. QC lasers onboard would take measurements of biogenic and geothermal gases such as methane and sulfur dioxide, which could be present in the Martian atmosphere, and determine their isotope ratios. Part of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding from NASA’s Upper Atmospheric Research Program 共UARP兲 and Planetary Instrument Definition and Development Program is gratefully acknowledged. The aircraft flights were made as part of UARP’s SAGE III Ozone Loss Validation Experiment 共SOLVE兲 mission 共pilots are named in the text兲. The research performed at Bell Laboratories is supported in part by the Defense Advanced Research Projects Agency, U.S. Army Research Office, under contract DAAG55-98-C-0050. References 1. M. Tacke, “New developments and applications of tunable IR lead salt lasers,” Infrared Phys. Technol. 36, 447– 463 共1995兲. 2. C. R. Webster, S. P. Sander, R. Beer, R. D. May, R. G. Knollenberg, D. M. Hunten, and J. Ballard, “Tunable diode laser infrared spectrometer for in-situ measurements of the gas phase composition and particle size distribution of Titan’s atmosphere,” Appl. Opt. 29, 907–917 共1990兲. 3. K. P. Petrov, A. T. Ryan, T. L. Patterson, L. Huang, and S. J. Field, “Mid-IR spectroscopic detection of trace gases using guided-wave difference-frequency generation,” Appl. Phys. B 67, 357–361 共1998兲. 326
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4. S. Forouhar, S. Keo, A. Larsson, A. Ksendzov, and H. Temkin, “Low-threshold continuous operation of InGaAs兾InGaAsP quantum well lasers at 2 microns,” Electron. Lett. 29, 574 –576 共1993兲. 5. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science 264, 553–555 共1994兲. 6. F. Capasso, C. Gmachl, D. L. Sivco, and A. Y. Cho, “Quantum cascade lasers,” Phys. World 12, 27–33 共1999兲. 7. F. Capasso, C. Gmachl, A. Tredicucci, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade lasers,” Opt. Photon. News 10共10兲, 31–37 共1999兲. 8. C. Gmachl, J. Faist, J. N. Baillargeon, F. Capasso, C. Sirtori, D. L. Sivco, S. N. G. Chu, and A. Y. Cho, “Complex-coupled quantum cascade distributed-feedback laser,” IEEE Photon. Technol. Lett. 9, 1090 –1092 共1997兲. 9. C. Gmachl, F. Capasso, J. Faist, A. L. Hutchinson, A. Tredicucci, D. L. Sivco, J. N. Baillargeon, S. N. G. Chu, and A. Y. Cho, “Continuous-wave and high-power pulsed operation of index-coupled distributed feedback quantum cascade laser at ⬇ 8.5 m,” Appl. Phys. Lett. 72, 1430 –1432 共1998兲. 10. C. Gmachl, A. M. Sergent, A. Tredicucci, F. Capasso, A. L. Hutchinson, D. L. Sivco, J. N. Baillargeon, S. N. G. Chu, and A. Y. Cho, “Improved cw operation of quantum cascade lasers with epitaxial-side heat-sinking,” IEEE Photon. Technol. Lett. 11, 1369 –1371 共1999兲. 11. K. Namjou, S. Cai, E. A. Whittaker, J. Faist, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser,” Opt. Lett. 23, 219 –221 共1998兲. 12. R. D. May and C. R. Webster, “Balloon-borne laser spectrometer measurements of NO2 with gas absorption sensitivities below 10⫺5,” Appl. Opt. 29, 5042–5044 共1990兲. 13. 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 frequency stabilized mid-infrared quantum cascade lasers,” Opt. Lett. 24, 1844 –1846 共1999兲. 14. S. W. Sharpe, J. F. Kelly, J. S. Hartman, C. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, and A. Y. Cho, “Highresolution 共Doppler-limited兲 spectroscopy using quantumcascade distributed-feedback lasers,” Opt. Lett. 23, 1396 –1398 共1998兲. 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, “Photoacoustic spectroscopy using quantum-cascade lasers,” Opt. Lett. 24, 178 –180 共1999兲. 16. C. R. Webster, R. D. May, R. Toumi, and J. Pyle, “Active nitrogen partitioning and the nighttime formation of N2O5 in the stratosphere: simultaneous in-situ measurements of NO, NO2, HNO3, O3, N2O, and jNO2 using the BLISS diode laser spectrometer,” J. Geophys. Res. 95, 13851–13866 共1990兲. 17. C. R. Webster, R. D. May, C. A. Trimble, R. G. Chave, and J. Kendall, “Aircraft 共ER-2兲 laser infrared absorption spectrometer 共ALIAS兲 for in-situ stratospheric measurements of HCl, N2O, CH4, NO2, and HNO3,” Appl. Opt. 33, 454 – 472 共1994兲. 18. D. C. Scott, R. L. Herman, C. R. Webster, R. D. May, G. J. Flesch, and E. J. Moyer, “Airborne Laser Infrared Absorption Spectrometer 共ALIAS-II兲 for in situ atmospheric measurements of N2O, CH4, CO, HCl, and NO2 from balloon or remotely piloted aircraft platforms,” Appl. Opt. 38, 4609 – 4622 共1999兲. 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 stratospheric N2O and CH4 observations,” Geophys. Res. Lett. 25, 2781– 2784 共1998兲.
