University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2002—pp. 27-31

Novel Trace Gas Detection Techniques with Quantum-Cascade Lasers

Thomas L. Harman (UHCL), Frank K. Tittel (Rice U), John C. Graf (NASA-JSC), and Anatoliy A. Kosterev (UHCL and Rice U)

Abstract
There is an increasing need in many chemical sensing applications ranging from air quality in spacecraft habitats to environmental science and medical diagnostics for fast, sensitive, and selective gas detection based on laser spectroscopy. The recent availability of novel pulsed and cw quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared spectroscopic sources address this need. A number of spectroscopic techniques based on QC-DFB lasers have been employed by ISSO investigators to demonstrate the monitoring and quantification of trace gases and isotopic species in ambient air at ppmv and ppbv levels by means of direct absorption, wavelength modulation, cavity enhanced, and cavity ring-down spectroscopy.

SENSITIVE, COMPACT DEVICES FOR THE QUANTIFICATION OF trace gases are required for a number of applications that include air quality in spacecraft habitats, in environmental monitoring, and in non-invasive medical diagnostics. A well-established technique for detecting molecular species in the gas phase is high-resolution infrared absorption spectroscopy. Until recently, real-world applications of this method were limited because of the absence of convenient tunable coherent light sources in the mid-IR region (3 to 20 mm) where the fundamental absorption bands of most molecules are located. This situation is changing with the development of quantum cascade (QC),^1,2 especially single-frequency devices with an imbedded distributed feedback structure (QC-DFB).

QC-DFB lasers are capable of delivering tens, and for certain devices, even hundreds of milliwatts of narrow-linewidth mid-IR radiation. QC lasers can be operated in pulsed mode up to and above room temperature, which is of particular interest for real-world gas-sensing applications because it eliminates the bulkiness and cost of an optical cryostat and cryogenic consumables (liquid N₂). The first laboratory demonstration of the sensitive absorption spectroscopy with a pulsed QC-DFB laser was reported in Namjou et al.³ where wavelength modulation was used to improve signal-to-noise ratio (SNR). The application of the direct absorption technique with a pulsed QC-DFB laser for trace gas detection in ambient air was first described in Kosterev et al.⁴ In this report, we present details of the design and performance of a robust, portable ammonia sensor based on a pulsed QC-DFB laser operating at ~10 mm. This sensor has been successfully applied to dynamic ammonia concentration monitoring of bioreactor vent gases in the NASA-Johnson Space Center (JSC), Houston, TX.

Technical Plan and Equipment
To detect NH₃, the absorption lines ^aR₁(2) at 992.4503 cm^-1 and ^aR₀(2) at 992.6988 cm^-1 (~10.1 mm) in the n₂ fundamental absorption band were selected. These lines are strong, well resolved at pressures below 200 Torr, and free from interference by water absorption and absorption by other air components. The pulsed QC-DFB laser available for this work accessed this wavenumber region when operated at a temperature of -11.7°C.

Ultrasensitive ammonia detection in this spectral region with pulsed QC-DFB laser has been reported before in Kosterev et al.⁴ In this work, a multipass cell with a 209 m effective path-length and two IR detectors were used to achieve improved sensitivity in detection. The emphasis in the present work was to achieve moderate (sub-ppm) sensitivity levels with a simple and cost-effective sensor platform (i.e., a single detector and no multipass gas cell).

The NH₃ sensor design is shown schematically in Fig. 1. The QC-DFB laser was mounted on top of a three-stage thermo-electrical unit inside a vacuum-tight housing with overall dimensions of 100´160´180 mm³ (Fig. 2) assembled from commercially available vacuum and opto-mechanical components. To remove the heat generated by the operation of the Peltier cooler, the bottom of the thermo-electrical unit was soldered to a water-cooled housing base. A temperature controller (Wavelength Electronics LFI-3751 TE) was used to set and monitor the laser temperature. With this arrangement, the operating QC-DFB laser could be cooled to -55°C.

Figure 1. Schematic of QC-DFB Laser Based Ammonia Sensor

The laser emission was collimated using an aspheric AR coated ZnSe lens with a focal length of 3 mm and a diameter of 6 mm mounted inside the housing. The lens position could be adjusted externally. The collimated laser light emerged from the housing through a 30 ft. wedged AR coated ZnSe window and was directed into a 0.5 m-long optical gas cell. This cell consisted of a glass tube fitted with Teflon valves and stainless steel window holders based on commercial 1-1/3 in. vacuum flanges. One end of the cell was equipped with a 30 ft. wedged AR coated ZnSe window and the other one with a SiO protected flat Al mirror (both Æ25.4 mm). This resulted in a two-pass configuration with a total optical path-length of 1 m. The exiting beam was focused onto a liquid nitrogen cooled HgCdTe detector with a built-in preamplifier (Kolmar KMPV10-1-J1/DC, 20 MHz bandwidth) by means of a 25.4 mm diameter off-axis parabolic mirror.

The utilization of a cryogenic detector does not imply strong limitations on the sensor performance because it uses only a small amount of liquid nitrogen and has a holding time of ~ 15 hours. In the future, researchers envision the use of a thermoelectrically cooled detector. The air to be analyzed flowed continuously through the absorption cell at a low controlled flow rate. The relatively slow flow was necessary because the bioreactor vent system permitted gas sampling only at a rate of 10 sccm or lower. A pressure controller was used to maintain the pressure inside the cell at 95 Torr. The laser housing, optical gas cell, gas flow meter, pressure controller, IR detector, visible diode laser for alignment, and optical components were mounted on a 30.5 cm ´ 61 cm aluminum breadboard.

Experimental Activity
The laser current was supplied in 5 ns long, ~4A peak current pulses at a 20 kHz repetition rate, as described in previous publication.⁴ A compact driver (Directed Energy PCO-7110 model 40-4) was used for the QC-DFB laser excitation and a computer-controlled sub-threshold current pedestal was added to each pulse for setting the optical pulse frequency. By appropriate ramping of the sub-threshold current, 512 laser pulses at a 20 kHz repetition rate scanned the desired spectral range. The tuning voltage that defines the sub-threshold current was applied in the shape of a linear ramp with an offset to compensate for the initial bias of the QC laser. A DAQ-1200 data acquisition card and LabView software (National Instruments) was used to trigger the laser pulser, set the sub-threshold current, and acquire spectral data. The number of pulses in a scan was limited by the capabilities of this card. The repetition rate was limited by the gated integrator (Stanford Research Systems, model SR250) used as an interface between the fast IR detector and data acquisition card. The scans followed each other with ~50% duty cycle because of computer software/hardware limitations. In most sensor runs, data were analyzed to determine the NH₃ concentration after acquiring and averaging 400 scans. The measurement sequence was repeated every four minutes.

The position of NH₃ absorption lines provided an absolute frequency reference for frequency calibration. The frequency scale was linearized by means of interference fringes from two air-separated uncoated ZnSe surfaces. The scan length was set to 0.75 cm^-1 in order to observe simultaneously the 992.4503 and 992.6988 cm^{- 1} lines. By using data acquired at relatively high NH₃ concentration, researchers determined that the QC laser possessed a linewidth of ~0.02 cm^{- 1} with an asymmetric shape. This broad, asymmetric laser line shape makes traditional analysis of spectra by fitting the lines with either Lorentz or Voigt functions inapplicable. Instead, the following procedure was used:

1. A reference spectrum in terms of base e absorbance was acquired using a N₂:NH₃ mixture containing ~100 ppm of NH₃. The actual concentration of NH₃ in this reference sample was determined by comparing the area of the absorption lines (adn, where [cm^-1] is the absorption coefficient) to the area predicted by the HITRAN database for the same temperature. The spectrum was then stored in the computer memory as a function yi = f(i), where i is a data point number (i.e., a number of the corresponding laser pulse in the frequency scan). This function results from the convolution of the real absorption spectrum and the laser line-shape.

2. A spectrum of the gas sample containing an unknown concentration of NH₃ was acquired at the same pressure and temperature.

3. Using a high concentration reference spectrum, three (almost) absorption-free parts of the spectral scan were defined (i.e., between and to both sides from the two NH₃ absorption lines). The baseline of the low concentration, unknown gas sample spectrum was determined by fitting a sixth order polynomial to the corresponding segments of its spectrum. The fractional absorption, yi, of the unknown sample was computed extending the baseline under the two absorption lines. Because the absorbance was small for the unknown spectrum, conversion to base e absorbance was not needed.

4. The best fit of the sample fractional absorption by the function
        y_i = Bf(i-x) + b                                                                                                                          (1)
   was found. The parameter x is introduced to compensate for slow drift of the line positions on the scan because of slight variations of the laser temperature. The b parameter helps to partially correct for an error in the baseline interpolation. Usually the fit resulted in b » 0. The parameter B yields the concentration in the test gas sample relative to the reference gas sample.

In order to perform the fitting in step 4, we modified a LabView procedure "Nonlinear Lev-Mar Fit.vi" so that it can use a tabulated function instead of an analytic expression. An example of the fitted data is presented in Fig. 3. This spectrum corresponds to 6.7 ppm of NH₃ and was acquired with 400 frequency scans. The standard single-point deviation of the fit is s = 7.14´10^-4 fractional absorbance (with a l0 s data acquisition time).

The 1 Mhz-equivalent sensitivity was 3.2´10^-4 Hz^-1/2 after applying normalization to the repetition frequency (i.e., the square root of the frequency ratio). The primary source of noise in our measurements is attributed to fluctuations of the laser pulses energy. Two-channel detection can be used if improved sensitivity is required for a specific application. In a separate experiment, we could obtain at least one order of improved sensitivity by normalization to a reference channel.

Real-time determination of the actual trace gas concentration in a system such as the atmosphere or bioreactor, as reported in this work, can be made more accurate by taking into account intrinsic system properties. In our case, the short time fluctuations in the measured concentration reflect the sensor properties and not the behavior of the gas concentration in the system. For this system, the actual NH₃ concentration is not expected to exhibit stochastic fluctuations every four minutes as measurements are performed. Changes in the bioreactor occur on a time scale of hours. Ammonia absorption and desorption from the walls of the tubing connecting the sensor to the system add further inertia to the measurements. Therefore, mathematical filtering that removes part of the high-frequency variations from one measurement to the next should improve the measurement accuracy.

To discern slow trends in concentration in real time, a Kalman filtering technique⁵ can be applied to trace gas measurements for various slow-changing systems. With an appropriate choice of filter parameters, such filtering has been shown to reduce the scatter greatly.^6,7 An important advantage of the Kalman filter over more common averaging techniques is that it keeps track of the whole measurement history without retaining all the previous data in memory and acts in real time, thereby providing a corrected concentration value after each measurement. It is especially useful if the measured concentration is used to control the system via a feedback loop.

The QC-DFB laser based NH₃ sensor described here was used to continuously monitor NH₃ concentration levels in bioreactor vent gases for more than 72 hours at NASA-JSC. The detected NH₃ concentration never exceeded a 0.5 ppm level, which was considered satisfactory in terms of bioreactor performance.

Conclusions
We have designed a compact portable ammonio sensor based on a thermoelectrically cooled pulsed QC-DFB laser operating at ~10 mm. This device was applied to NH₃ concentration measurements. Furthermore, the configuration of this sensor will serve as the basis for the design of future portable QC-DFB laser based gas sensors for single or multiple trace gas quantification. The present sensitivity can be considerably improved by adding a second infrared detector. Such a two-beam configuration will cancel the noise caused by laser shot-to-shot energy fluctuations.

Acknowledgments
We acknowledge the assistance of Dr. Darrin Leleux in providing the Kalman filtering program. Financial support of the work performed by the Rice group was provided by the Institute for Space Systems Operations (ISSO), the National Aeronautics and Space Administration (NASA), the Texas Advanced Technology Program, the National Science Foundation, and the Welch Foundation.

Special Note
After serving several years as a post-doctoral fellow on the trace gas detection project supported by ISSO, Dr. Anatoliy Kosterev joined the Rice Quantum Institute as a full-time employee. Dr. Kosterev will be a research scientist at Rice University and he will continue to contribute to the advancement of technology in Houston.

References
¹F. Capasso, C. Gmachl, R. Paiella, A. Tredicucci, A. L. Hutchinson, D. L. Sivco, J. N. Baillargeon, A. Y. Cho, and H. C. Liu. "New Frontiers in Quantum Cascade Lasers and Applications," IEEE J. on Selected Topics in Quantum Electronics 6 (2000): 931-46.
²C. Gmachl, F. Capasso, R. Köhler, A. Tredicucci, A. L. Hutchinson, D. L. Sivco, J. N. Baillargeon, and A. Y. Cho. "The Sense-Ability of Semiconductor Lasers," Circuits & Devices (May 2000): 10-18.
³K. 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 (1997): 219-21.
⁴A. A. Kosterev, F. K. Tittel, C. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho. "Trace-Gas Detection in Ambient Air with a Thermoelectrically Cooled, Pulsed Quantum-Cascade Distributed Feedback Laser," Appl. Opt. 39 (2000) 6866-72.
⁵R. E. Kalman. "A New Approach to Linear Filtering and Prediction Problems," J. Basic Eng. 82 (1960): 35-42.
⁶H. Riris, C. B. Carlisle, and R. E. Warren. "Kalman Filtering of Tunable Diode Laser Spectrometer Absorbance Measurements," Appl. Opt. 33 (1994): 5506-08.
⁷D. P. Leleux, R. Claps, W. Chen, F. K. Tittel, and T. L. Harman. "Adaptive Filtering Applications to Realtime Simultaneous CO₂ and NH₃ Measurements," Appl. Phys. B (June 2001).

Publications
Kosterev, A. A., F. K. Tittel, W. Durante, M. Allen, R. Köhler, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho. "Detection of Biogenic CO Production above Cell Cultures Using a Near-Room Temperature QC-DFB laser," Appl. Phys. B 74 (2002): 95-99.
Kosterev, A. A., R. F. Curl, F. K. Tittel, R. Köhler, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho. "Transportable Automated Ammonia Sensor Based on a Pulsed Thermoelectrically Cooled QC-DFB Laser," Applied Optics 41 (2002): 573-78.
Kosterev, A. A., R. F. Curl, F. K. Tittel, R. Kochler, C. Gmachl, F. Capasso, D. L. Sivco, A. Y. Cho, S. Wehe, and M. Allen. "Room Temperature Quantum Cascade Laser Based Gas Sensor for Continuous Monitoring of Atmospheric CO, " Applied Optics 41 (2002): 1169-73.
Kosterev, A. A. and F. K. Tittel. "Chemical Sensors Based on Quantum Cascade Lasers," IEEE JQE Special Issue on QC Lasers 38 (2002): 582-91.
Kosterev, A. A., F. K. Tittel, R. F. Curl, M. Rochat, and J. Faist. "Chemical Sensing Applications with Long Wavelength Midinfrared QC-Lasers," Applied Physics B 75 (2002): 351-57.
Kosterev, A. A., Y. A. Bakhirkin, R. F. Curl, and F. K. Tittel. "Quartz-Enhanced Photoacoustic Spectroscopy," Optics Letters 27 (2002): 1902-04.

Presentations
Allan, M., F. K. Tittel, and T. O’Keefe. "Photonic Techniques for Early Detection of Human Disease," Joint NASA-NCI BPC Meeting, Monterey, CA, July 30-Aug. 1, 2002.
Kosterev, A. A. and F. K. Tittel. "Trace Gas Sensing Applications of Quantum Cascade Lasers," invited, OSA Annual Meeting, Orlando, FL, Oct. 14-18, 2002.
Kosterev, A. A., F. K. Tittel, C. Gmachl, and F. Capasso. "Trace Gas Concentration Measurements Using Quantum Cascade Lasers," IQEC/LAT, Moscow, Russia, June 22-28, 2002.
Kosterev, A. A., Y. Bakhirkin, F. K. Tittel, and R. F. Curl. "Photoacoustic Spectroscopy in Gases with High-Finesse Solid-State Resonators," OSA Annual Meeting, Orlando, FL, Oct. 14-18, 2002.
Kosterev, A. A., F. K. Tittel, M. Rochat, M. Beck, and J. Faist. "Chemical Sensing with a Pulsed 16 Microns QC-DFB Laser," Conference on Lasers and Electro-Optics (CLEO), Long Beach, CA, May 19-24, 2002.
Kosterev, A. A., F. K. Tittel, S. Wehe, D. M. Sonnenfroh, M. G. Allen, R. Köhler, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho. "Spectroscopic Trace Gas Detection with Pulsed Quantum Cascade Lasers," Laser Applications to Chemical and Environmental Analysis (LACEA), Boulder, CO, Feb. 7-10, 2002.
Paige, M. E., S. M. Massick, D. Bomse, A. A. Kosterev, and F. K. Tittel. "Wavelength Modulated Photoacoustic Spectroscopy Using Quantum Cascade Lasers," MIOMD-V, Annapolis 2002, Annapolis, MD, Sept. 8-11, 2002.
Tittel, F. K. "Chemical Sensing Applications of Laser Based Spectroscopic Sources," Physics Department Colloquium, Jena University, Germany, June 24, 2002.
Tittel, F. K. "Chemical Sensors Using Quantum Cascade Lasers," 3rd Quantum Cascade Laser Workshop, Fraunhofer Institute, Freiburg, Germany, Sept. 19-20, 2002.
Tittel, F. K. and A.A. Kosterev. "Trace Gas Detection using Laser Spectroscopy, Air Quality Measurements in Spacecraft Habitats," Environmental Sentinels 2002, NASA-Johnson Space Center, Houston, TX, Sept. 17-18, 2002.
Tittel, F. K., A. A. Kosterev, C. Gmachl, and F. Capasso. "Compact Gas Sensors Based on Pulsed Quantum Cascade Lasers for Industrial Applications," International Symposium on Gas Flow and Chemical Lasers and High Power Laser Conference, Wroclaw, Poland, Aug. 26-30, 2002.
Tittel, F. K., A. A. Kosterev, M. Rochat, M. Beck, and J. Faist. "Chemical Sensors Based on Quantum Cascade Lasers," 2002 SPIE International Symposium on Optical Science and Technology, Diode Lasers and Applications, Seattle, WA, July 7-11, 2002.

Funding and proposals
"Diode-Based Trace Gas Monitors for Semiconductor Manufacturing Process Control." ATP, 2001, Rice University, $99,260, UHCL, $131,850 (not funded).
"Diode-Based Trace Gas Monitors for Semiconductor Manufacturing Process Control." NSF, 2001, Rice U, $143,393, UHCL, $558,422, three years (not funded).
"Early Detection of Human Diseases Based on Breath Analysis Using Photonic Technologies." AT P, June, 2003, Rice University, $94,880, UHCL, $94,880 (pending).

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Institute for Space Systems Operations - Y2002 Annual Report
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