Trace-gas sensing is a rapidly developing field, in demand for applications such as breath diagnostics, process and air-quality monitoring, atmospheric measurements, and security and surveillance. Quantum cascade lasers have dramatically affected the field of trace-gas sensing, increasing the sensitivities of detectors that are becoming smaller and more convenient almost monthly. And the development of CW versions of the lasers promises to enable more advances. Breath analysis is a noninvasive medical diagnostic method that addresses more than 200 compounds in exhaled breath.1 Many of these compounds can be used to identify particular medical conditions — if they can be measured accurately at very low concentration levels, typically in the range of a few parts per billion. Moreover, this must be accomplished using a small sample volume and a simple-to-use device. Applications in atmospheric monitoring, security and workplace surveillance may require molecular-specific trace detection of gases with a degree of spatial resolution. The goal here is to detect perturbations in the chemical composition of the atmosphere — particularly carbon-containing gases — that either affect air quality or pose a hazard.Frequently, trace-gas monitoring is performed using unmanned aerial vehicles for applications such as surveillance and plume tracing in the atmospheric boundary layer above urban areas. Currently, such analyses are performed with tunable diode laser spectrometers, which are relatively compact and provide the required sensitivity. The analytical challenge in sensing atmospheric gases, however, is more than achieving the required sensitivity; it also involves sensing minute fluctuations in concentration, because small changes may be highly relevant.Quantum cascade laserMost organic molecules have distinct vibrational/rotational spectra in the 3- to 20-μm mid-infrared range, so techniques to detect trace gases usually concentrate on this region. The most advanced infrared source for these wavelengths is the quantum cascade laser.2 These lasers take advantage of intersubband transitions rather than electron-hole recombination for generating coherent light (Figure 1). Figure 1. The dark blue line is the potential-energy structure of the conduction band in the quantum cascade laser. Electrons (yellow) are pumped to a given energy level of one quantum well. They relax to lower energy levels in another well (light blue), releasing a photon (red). Electrons then tunnel out of the low-energy state into the next injector region, where the process is repeated, resulting in a cascade effect. Designing the structure of the quantum potential wells — that is, “band structure engineering” — enables deliberate selection of the emission wavelength.The dimensions of the quantum heterostructure created by molecular beam epitaxial growth of semiconductor layers (i.e., the thickness of the layer) enables the tailoring of the emission wavelength of the laser via band structure engineering. This allows almost any emission wavelength in the mid-IR to be achieved by changing the feature size (again, the layer thickness) rather than the entire materials system, as in conventional diode lasers. Consequently, quantum cascade lasers are much more flexible and, thus, more attractive for sensing applications than are other laser sources. Distributed feedback quantum cascade lasers incorporate a grating to provide single-mode emission.3 The narrow linewidth of these lasers facilitates overlap with a particular absorption band, enabling the construction of systems with inherent molecular selectivity.Despite their comparative youth — the first operating device was reported by Federico Capasso’s team at AT&T Bell Laboratories in Murray Hill, N.J., in 1994 — quantum cascade lasers already have found application in various gas-sensing formats because of their wide range of available emission frequencies (see “State-of-the-Art Laser-Based Gas Sensing,” page 66). Sensitive detection has been reported by photoacoustic spectroscopy and quartz-enhanced photoacoustic spectroscopy, with the latter demonstrating the detection of N2O in concentrations as low as 4 ppb.4 Quantum cascade lasers applied to cavity ringdown spectroscopy have led to detection limits of 0.7 ppb of NO in N2.5 Integration of these laser sources into cavity output spectroscopy yielded detection of less than 1 ppb of NO in human breath samples.6,7Quantum cascade lasers also have been applied in gas sensing by transmission spectroscopy, including applications such as atmospheric monitoring, NO detection, and cigarette smoke and vehicle emission analysis. Most of these measurements use a comparatively voluminous multipass gas cell to achieve the desired sensitivities, but hollow waveguides, which serve as both a waveguide and as a miniaturized gas cell, have been demonstrated as a viable and compact alternative for portable sensing systems. These waveguides essentially are an air core fiber, which provides transmission over a wide range of frequencies and can be filled with the gas sample, resulting in a small-volume absorption gas cell.8Hollow waveguides coupled to quantum cascade lasers for gas sensing were first reported in 2000 by Lubos Hvozdara and colleagues at Technische Universität Wien in Vienna, Austria, and detected ethylene gas at concentration levels of 250 ppm.9 This system used a waveguide with an inner diameter of 2 mm as well as a Fourier transform IR spectrometer coupled to the distal end of the waveguide to resolve the obtained signal by wave number. Gregory J. Fetzer and researchers at Areté Associates in Tucson, Ariz., and the National Jewish Medical and Research Center in Denver have demonstrated hollow waveguide quantum cascade laser sensing with a 1-mm-inner-diameter waveguide, building on their work with tunable diode lasers coupled to a hollow waveguide.10 This system, based on a 9-m-long coiled waveguide, could detect NO to a concentration of 60 ppb.Most recently, our team at Georgia Institute of Technology in Atlanta demonstrated gas sensing with a quantum cascade laser coupled to a silica hollow waveguide.11 This was followed by an improved system, in which a quantum cascade laser was coupled to a photonic bandgap hollow waveguide.12 These studies used a silica hollow waveguide with a 700-μm inner diameter and a length of 4 m for the detection of C2H5Cl gas at concentrations as low as 500 ppb. System improvements, including the replacement of the silica waveguide with a photonic bandgap hollow waveguide with an inner diameter of 700 μm and a length of 1 m (Figure 2), improved the sensitivity to 30 ppb. Figure 2. The first demonstration of gas sensing inside a photonic bandgap material employed a fiber developed by OmniGuide Inc. CW advancesThe gas-sensing systems described above are routinely capable of measuring concentrations in the range of parts per million to parts per billion — and occasionally parts per trillion. But sensing at the level of a few parts per trillion is required for applications such as clinical breath diagnostics and atmospheric monitoring. The currently achievable sensitivity levels must be improved.Besides the development of new detection schemes, further evolution of the laser technology will play a key role in increasing the sensitivity of these quantum-cascade-laser-based systems. The majority of quantum cascade lasers in gas sensing studies to date have been operated in pulsed mode. However, CW lasers have been reported13 and are in the initial phase of commercialization.Because CW quantum cascade lasers increase average power, facilitating more sophisticated detection schemes, this advance in technology has the potential for dramatically improving the limits of detection of compact laser-based gas-sensing devices.Meet the authorsChristy M. Charlton performed her doctoral studies on quantum-cascade-laser-based sensors in Georgia Institute of Technology’s Applied Sensors Laboratory in Atlanta and graduated from the institute in December; e-mail: firstname.lastname@example.org.Boris Mizaikoff is an associate professor at Georgia Tech’s School of Chemistry and Biochemistry and is head of the Applied Sensors Laboratory; e-mail: email@example.com. References1. W. Miekisch et al (September 2004). Diagnostic potential of breath analysis — focus on volatile organic compounds. CLIN CHIM ACTA, pp. 25-39.2. J. Faist et al (April 22, 1994). Quantum cascade laser. SCIENCE, pp. 553-556.3. D. Hofstetter et al (Dec. 13, 1999). Surface-emitting 10.1 μm quantum-cascade distributed feedback lasers. APPL PHYS LETT, pp. 3769-3771.4. A.A. Kosterev, Y.A. Bakhirkin and F.K. Tittel (January 2005). Ultrasensitive gas detection by quartz-enhanced photoacoustic spectroscopy in the fundamental molecular absorption bands region. APPL PHYS B:LASERS O, pp. 133-138.5. A.A. Kosterev et al (October 2001). Cavity ringdown spectroscopic detection of nitric oxide with a continuous-wave quantum-cascade laser. APPL OPTICS, pp. 5522-5529.6. Y.A. Bakhirkin et al (April 2004). Mid-infrared quantum cascade laser based off-axis integrated cavity output spectroscopy for biogenic nitric oxide detection. APPL OPTICS, pp. 2257-2266.7. M.L. Silva et al (September 2005). Integrated cavity output spectroscopy measurements of NO levels in breath with a pulsed room-temperature QCL. APPL PHYS B:LASERS O, pp. 705-710.8. J.A. Harrington (July 1, 2000). A review of IR transmitting, hollow waveguides. FIBER & INTEGRATED OPT, pp. 211-227.9. L. Hvozdara et al (December 2000). Spectroscopy in the gas phase with GaAs/AlGaAs quantum-cascade lasers. APPL OPTICS, pp. 6926-6930.10. G.J. Fetzer, A.S. Pittner and P.E. Silkoff (July 2003). Midinfrared laser absorption spectroscopy in coiled hollow optical waveguides. Proc. SPIE, Vol. 4957, Optical Fibers and Sensors for Medical Applications III, pp. 124-133.11. C. Charlton et al (Aug. 18, 2003). Hollow-waveguide gas sensing with room-temperature quantum cascade lasers. IEE P-OPTOELECTRON, pp. 306-309.12. C. Charlton et al (May 9, 2005). Midinfrared sensors meet nanotechnology: Trace gas sensing with quantum cascade lasers inside photonic band-gap hollow waveguides. APPL PHYS LETT, 194102.13. M. Beck et al (Jan. 11, 2002). Continuous wave operation of a mid-infrared semiconductor laser at room temperature. SCIENCE, pp. 301-305.Commercially Available SystemsSeveral quantum-cascade-laser-based sensing systems are commercially available for a limited set of analytes:• Aerodyne Research Inc. of Billerica, Mass., produces a gas analyzer that incorporates room-temperature pulsed quantum cascade lasers coupled to a multipass gas cell with a 56-m path length. It can detect N2O, CH4 and NH3 at levels of a few parts per billion.• Physical Sciences Inc. of Andover, Mass., offers two quantum cascade laser systems with wavelengths at 5.2 and 4.6 μm that detect 10-ppb concentrations of NO and CO gas, respectively. The instruments have path lengths of approximately 1 m.• Cascade Technologies Ltd. of Glasgow, UK, is producing the Micro Sensor, which can detect levels below the parts-per-million range, and the Ultra Gas Sensor, which offers parts-per-trillion detection.• Laser Components GmbH of Olching, Germany, provides a quantum cascade laser housing with an integrated laser control. The component eases integration into gas-sensing systems.• Alpes Lasers SA of Neuchâtel, Switzerland, is the leading commercial manufacturer of quantum cascade lasers. It offers a starter kit that can interface with gas- or liquid-phase sensing modules.State-of-the-Art Laser-Based Gas SensingState-of-the-art measurement techniques already enable trace-gas sensing in the mid-infrared spectral range at levels of sensitivity of parts per billion to parts per trillion. The most prevalent sensing concepts, their advantages and disadvantages, and any variations are:Multipass transmission absorption spectroscopy — The most common IR gas sensing technique uses a multipass transmission cell: a chamber filled with analyte gas with mirrors at each end. The beam is folded back and forth through the cell, creating an extended yet defined optical path length in a confined space.• Advantages: High sensitivity and a relatively compact system footprint.• Disadvantage: Relatively high volumes of sample are required (typically several hundreds of milliliters to liters), leading to extended sample residence time and slow system response to concentration fluctuations.Hollow waveguide transmission absorption spectroscopy — Hollow waveguides usually consist of a metal-coated tube with an air-filled core, which transmits radiation by reflection off the inner wall. In sensing applications, the waveguide simultaneously guides the propagating radiation and serves as a miniaturized gas cell for direct absorption measurements. • Advantage: An extended optical path length, yielding high sensitivity from low sample volumes (typically several milliliters).• Disadvantage: Substantial optical reflection losses limit system miniaturization.• Variation: Photonic bandgap fibers are a fundamentally new type of hollow waveguide that employs photonic crystals for light guiding. In particular, the omnidirectional guide from OmniGuide Inc. of Cambridge, Mass., features a one-dimensional photonic crystal wrapped into a cylindrical shape with a hollow center, which can be extruded into a hollow waveguide.1 This configuration allows the photonic-bandgap guide to serve as a hollow waveguide gas cell with omnidirectional reflectivity, reducing optical reflective losses.Photoacoustic spectroscopy — Photoacoustic spectroscopy relies on the photoacoustic effect for the detection of absorbing analytes. The sample gas is in a confined (resonant or nonresonant) chamber, where modulated (e.g., chopped) radiation enters via an IR-transparent window and is absorbed by IR-active molecular species. The temperature of the gas thereby increases, leading to a periodic expansion and contraction of the gas volume synchronous with the modulation frequency of the radiation. This produces a pressure wave that can be acoustically detected via simple microphones.• Advantages: High sensitivity and small sample volume, and the acoustic measurement avoids the need for optical detection.• Disadvantage: Sensitivity to vibrational noise because measurements are based on an acoustic signal.• Variation: Quartz-enhanced photoacoustic spectroscopy incorporates a tuning fork rather than a microphone.2 A simple quartz tuning fork extracted from a watch is used as a resonant transducer (at approximately 32 kHz) for the acoustic signal, providing a Q-factor of more than 10,000. The output of the excitation laser, which is modulated at half the resonant frequency of the tuning fork, is focused between the two legs of the fork, and “2f” wavelength modulation is used to detect the photoacoustic signal monitoring the piezoelectric current generated by the tuning fork. In comparison with conventional photoacoustic detection, similar sensitivity is achieved with substantially improved noise immunity and reduced size. Cavity ringdown spectroscopy — Cavity ringdown spectroscopy confines gas in an optically reflective cavity where laser radiation is introduced. The amplitude of the radiation decays at a certain rate in the absence of absorption. An absorbing sample gas in the cavity increases the rate of decay, indicating the presence of an absorbing species. • Advantages: High sensitivity and a small sample volume.• Disadvantage: Indirect measurement as the measured parameter is the rate of decay of the light intensity, requiring that the decay caused by absorption from the analyte of interest be separated from decay caused by mirror and other cavity-dependent losses. • Variation: In integrated cavity output spectroscopy, the cavity output is integrated to provide an absorption spectrum as the source is scanned across wavelengths. This technique provides sensitivity levels comparable to cavity ringdown spectroscopy, but with a significantly reduced data analysis effort. Light detection and ranging (lidar) — Lidar is a technique commonly used in atmospheric sensing, in which light from a source is transmitted over large distances to a target and is partially reflected to a detector. Changes in the properties of the light return information about the target or about compounds in the path between the source and the target. • Variation: Differential absorption lidar can be used to detect atmospheric gases or pollutants by the ratio of the intensity of a beam exposed to the absorbing sample to that of a reference beam.References1. B. Temelkuran et al (Dec. 12, 2002). Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission. NATURE, pp. 650-653.2. A.A. Kosterev, Y.A. Bakhirkin and F.K. Tittel (January 2005). Ultrasensitive gas detection by quartz-enhanced photoacoustic spectroscopy in the fundamental molecular absorption bands region. APPL PHYS B:LASERS O, pp. 133-138.