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Characterization of a near-room-temperature, continuous-wave quantum cascade laser for long-term, unattended monitoring of nitric oxide in the atmosphere
Optics Letters
Abstract
We report on power, spectral linewidth, and mode purity for a cw 5.3 microm quantum cascade laser operated on a thermo-electric cooler. A totally noncryogenic nitric oxide monitor was constructed by integrating this laser with an astigmatic multipass cell and a thermo-electrically cooled infrared detector. The resulting instrument is capable of continuous unattended monitoring of ambient, atmospheric nitric oxide for several weeks with no operator intervention. The detection method was rapid sweep, direct absorption spectroscopy. A detection sensitivity of 0.03 parts in 10(9) is achieved with 30 s averaging time with a path length of 210 m, corresponding to an absorbance path length product of 1.5 x 10(-10) cm(-1).
... Recent advances in quantum cascade (QC) laser technology have extended the temperature range of these devices to permit continuous-wave (cw) operation near room temperature (RT) [1][2][3][4]. Measurements of atmospheric nitric oxide using these devices have recently been reported [5][6][7]. The advent of cw RT QC laser operation offers higher power and narrower laser line widths in the spectral region where molecular absorptions are strongest. ...
... We previously reported initial results using a cw RT QCL with nitric oxide absorption at 5.3 µm, and here expand on those findings. In that earlier paper [7], we described cw operation of the same TE-cooled laser in a spectrometer instrument with a TE-cooled detector. The laser showed narrow line width (≤ 0.0004 cm −1 HWHM) and high single-mode purity ≥ 99.99%. ...
... The line width in cw mode was determined previously to be 0.0004 ± 0.0002 cm −1 (HWHM) [7], by comparing experimental absorption spectra of the NO doublet at 1900.1 cm −1 to a simulation based on HITRAN [12] line positions and strengths. The observed cw laser line width is likely determined by electrical noise in the current controller or modulation circuit or by the temperature stability of the laser controller, and is therefore an upper limit to the instantaneous laser line width. ...
A quantum cascade laser operating near room temperature with thermoelectric (TE) cooling has been used in both continuous-wave
(cw) mode (-9°C) and pulsed mode (+45°C) to detect atmospheric nitric oxide using spectral lines at 1900.07cm-1 (5.3μm). The totally non-cryogenic spectrometer integrates the laser with a 69-m astigmatic multi-pass cell and a TE-cooled
infrared detector to enable operation for extended time periods without operator attention. The pattern of reflections on
the astigmatic cell mirrors has been designed to minimize optical interference fringes, which are substantially greater with
cw mode than with pulsed operation. The detection method uses direct absorption with rapid- scan sweep integration to achieve
sub-second time response. Detection precision for NO in air of 0.5 parts in 109Hz-1/2 (1σ) is obtained in pulsed mode with an Allan variance minimum corresponding to 0.1 parts in 109 after 30-s averaging time. The precision in cw mode improves to 0.1 parts in 109Hz-1/2 and 0.03 parts in 109 after 30-s averaging, corresponding to an absorbance per unit path length of 2×10-10cm-1. The advantages and disadvantages of cw compared to pulsed operation are discussed.
... Many small molecules have their strongest absorption lines in the spectral region between ~3 and 10 μm, the spectral region spanned by QCL's and ICL's. Trace gas instruments utilizing these mid-IR lasers are being designed at universities [9][10][11][12][13][14][15][16][17], government laboratories [18][19][20] and in corporations [21][22][23][24][25][26][27][28][29], in order to help answer pressing questions in atmospheric and environmental research. QCL's and ICL's are nearly ideal sources for trace gas instruments that must operate outside the laboratory, since they can operate at near room temperature, they are small and robust, and can be easily modulated in wavelength. ...
... While the first commercially available mid-IR QCL's were pulsed, now we use CW lasers almost exclusively. CW-QCL's give better results in instruments due to higher power, narrower linewidth and better stability [27,28]. Recently, ICL's operating near 3 μm have become commercially available [8] and help fill in the availability of laser wavelengths near the C-H stretch absorption bands of many hydrocarbon compounds. ...
A design and results for an instrument with a quantum cascade laser and an antimonide diode laser to measure simultaneously and with high precision seven isotopologues of carbon dioxide and water vapor. Methods and results for determining the effects that limit absorption noise at the level of 5x10⁻⁶ are presented and discussed.
... With the continuous development of laser technology, the quantum cascade laser (QCL) has extended the measurement band of gas to the mid-infrared region, where there is a stronger absorption strength and more spectral lines [23][24][25][26][27]. Based on these advantages, quantum cascade laser absorption spectroscopy (QCLAS) can provide faster and more accurate monitoring capabilities, and it has become a research hotspot [28]. It has been applied in the fields of environmental monitoring, industrial processes, respiratory gases, and so on [29][30][31][32]. ...
In this study, a method for double-beam quantum cascade laser absorption spectroscopy (DB-QCLAS) was developed. Two mid-infrared distributed feedback quantum cascade laser beams were coupled in an optical cavity for the monitoring of NO and NO2 (NO at 5.26 μm; NO2 at 6.13 μm). Appropriate lines in the absorption spectra were selected, and the influence of common gases in the atmosphere, such as H2O and CO2, was avoided. By analyzing the spectral lines under different pressure conditions, the appropriate measurement pressure of 111 mbar was selected. Under this pressure, the interference between adjacent spectral lines could be effectively distinguished. The experimental results show that the standard deviations for NO and NO2 were 1.57 ppm and 2.67 ppm, respectively. Moreover, in order to improve the feasibility of this technology for detecting chemical reactions between NO and O2, the standard gases of NO and O2 were used to fill the cavity. A chemical reaction instantaneously began, and the concentrations of the two gases were immediately changed. Through this experiment, we hope to develop new ideas for the accurate and rapid analysis of the process of NOx conversion and to lay a foundation for a deeper understanding of the chemical changes in atmospheric environments.
... These optical sensors are increasingly required in a number of elds, including environmental monitoring [1,2], marine science [3,4], biological studies [5,6], and breath analysis [7,8]. State-of-the-art laser absorption sensors using a multipass cell can achieve a noise equivalent absorption (NEA) as low as 10 -10 cm -1 with an effective path length of hundred meters [9]. To further enhance sensitivity, the absorption path length can be increased to kilometers by using a high-nesse optical cavity. ...
Gas sensors with high sensitivity, wide dynamic range, high selectivity, fast response, and small footprint are desirable across a broad range of applications in energy, environment, safety, and public health. However, designing a compact gas sensor with ultra-high sensitivity and ultra-wide dynamic range remains a challenge. Laser-based photoacoustic spectroscopy (PAS) is a promising candidate to fill this gap. Herein, we report a novel method to simultaneously enhance the acoustic and light waves for PAS using integrated optical and acoustic resonators. This increases sensitivity by more than two orders of magnitude and extends the dynamic range by more than three orders of magnitude, compared with the state-of-the-art photoacoustic gas sensors. We demonstrate the concept by exploiting a near-infrared absorption line of acetylene (C 2 H 2 ) at 1531.59 nm, achieving a detection limit of 0.5 parts-per-trillion (ppt), a noise equivalent absorption (NEA) of 5.7×10 ⁻¹³ cm ⁻¹ and a linear dynamic range of eight orders of magnitude. This study enables the realization of compact ultra-sensitive and ultra-wide-dynamic-range gas sensors in a number of different fields.
... This obstacle could be overcome using the rotation of the polarization plane of the free-induction radiation (FID) in magnetic field. [10,11] The technique of the Faraday rotation spectroscopy (FRS) in the frequency domain was used for very sensitive detection of several stable paramagnetic molecules (NO, [12] NO 2 , [13] and O 2 [14]). ...
Effect of external longitudinal magnetic field on the optical Free Induction Decay from a free radical is observed for the first time. The experiments were performed on a rotational transition of hydroxyl radical, OH (23/2(J=1) 23/2(J=0) at 83.8 cm−1) using Terahertz Free Electron Laser. In contrast to the results of the experiments with a stable paramagnetic molecule, NO, the observed effect of external magnetic field on the Free Induction Decay from hydroxyl radicals is more complicated. Longitudinal magnetic field leads to the rotation of the polarization plane of the FID radiation as well as to an additional modulation of the signal intensity. The angle of the rotation of the plane of polarization is large, in agreement with the theoretical predictions. The observed FID kinetics in the time domain are in semi-quantitative agreement with the modeling. This observation opens an opportunity of selective detection of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species by polarization discrimination.
... Lead-salt diode lasers were the first successful semiconductor based mid-IR diode lasers [4][5][6][7][8][9][10][11], but only a decent number of gas sensors, working in the mid-IR, have been demonstrated and published [12][13][14][15][16][17][18][19]. This has changed with the development of the quantum cascade laser (QCL) [20]. ...
A quantum cascade laser-based sensor for ambient air monitoring is presented and five gases, affecting the air quality, can be quantified. The light sources are selected to measure CO, NO, NO2, N2O and SO2. The footprint of the measurement setup is designed to fit in two standard 19” rack (48 cm × 65 cm) with 4 height units (18 cm) whereas one is holding the optical components and the other one contains the electronics and data processing unit. The concentrations of the individual analytes are measured using 2f-Wavelength Modulation Spectroscopy (2f-WMS) and a commercially available multipass gas cell defines the optical path. In addition, CO can also be measured with a dispersion-based technique, which allows one to cover a wider concentration range than 2f-WMS. The performance of this prototype has been evaluated in the lab and detection limits in the range of 1ppbv have been achieved. Finally, the applicability of this prototype for ambient air monitoring is shown in a five-week measurement campaign in cooperation with the Municipal Department for Environmental Protection (MA 22) of Vienna, Austria.
... Particularly, the 2f/1f method introduced by Hanson et al. [13][14][15] that overcomes the complicated calibration issue has further promoted the TDLAS technique toward practical applications. Meanwhile, Siemens, Aerodyne Research, and other companies have developed numerous online monitoring equipment based on TDLAS [16][17][18], including the LDS6 analyzer for NH3, CO, and other exhaust gases. In recent years, more demands for improved anti-noise ability, high signal-to-noise ratio (SNR) and high temporal resolution are put forward for TDLAS. ...
... Particularly, the 2f/1f method introduced by Hanson et al. [13][14][15] that overcomes the complicated calibration issue has further promoted the TDLAS technique toward practical applications. Meanwhile, Siemens, Aerodyne Research, and other companies have developed numerous online monitoring equipment based on TDLAS [16][17][18], including the LDS6 analyzer for NH3, CO, and other exhaust gases. In recent years, more demands for improved anti-noise ability, high signal-to-noise ratio (SNR) and high temporal resolution are put forward for TDLAS. ...
Following the theoretical work in Part I, in this experimental study, the robustness, temporal resolution, and the narrow scan performance of the proposed wavelength modulation-direct absorption spectroscopy (WM-DAS) method are experimentally validated in a high-temperature tube furnace. The electromagnetic and other random-frequency noises can be effectively eliminated by extracting the characteristic spectra of the light intensity. The performance of WM-DAS with modulation frequencies from 0.1 to 100 kHz and scan indexes from 3.3 to 11.1 are also investigated at atmospheric pressure. The proposed method produces accurate line profile and high SNR over 500 consistently even with a weak absorption. As for real applications, the spectral line parameters of CO at 4300.6999 cm−1 including the collisional broadening, Dicke narrowing, and their dependence on temperature are measured. Furthermore, the high-speed measurement (1 ms) of the temperature and CO concentration of a McKenna flat flame are demonstrated.
... The fundamental technique we use for measuring ambient CO 2 mixing ratio is Tunable Infrared Laser Differential Absorption Spectrometry (TILDAS). Details of the instrument design as well as the data acquisition and analysis have been described before Nelson et al., 2006). In general, we scan a tunable IR laser over the desired wavelength region and pass the laser light through a multi-pass sample cell at reduced pressure. ...
We present a novel spectral method to measure atmospheric carbon dioxide (CO2) with high precision and stability without resorting to calibration tanks during long-term operation. This spectral null method improves precision by reducing spectral proportional noise associated with laser emission instabilities. We employ sealed quartz cells with known CO2 column densities to serve as the permanent internal references in the null method, which improve the instrument's stability and accuracy. A prototype instrument – ABsolute Carbon dioxide (ABC) is developed using this new approach. The instrument has a one-second precision of 0.02 ppm, which averages down to 0.007 ppm within one minute. Long-term stability of within 0.1 ppm is achieved without any calibrations for over a one-month period. These results have the potential for eliminating the need for calibration cylinders for high accuracy field measurements of carbon dioxide.
... In this work typically 1-2 cm' 1 of frequency tuning was used to probe one of more CO transitions. The laser line width was not measured but is expected to be less than 10 MHz based on previous line width studies for room-temperature cw-QCLs [44][45]; in this work the transition half widths, typically around 1.5 GHz, are significantly larger than the laser line width. ...
... Thus, knowledge of the surface chemistry of Table 2 Predicted levels in void spaces (see text for detailed conditions), exemplary urban levels, possibilities of detection by portable instruments and classification of potential volatile markers of human presence. 30 × 10 6 (b) 408 ppm (Dallas) [122] (c) 390 ppm (Phenix) [123] (d) 469 ppm (Wrocław) [124] (e) 403-408 ppm (Portland) [125] Vehicle emissions, (a) 0.25% optical sensor [126] (b) 0.23% Solvatochromic probe [127] A NO 10102-43-9 7.7 (a) 24.5 ppb (Hong Kong) [128] (b) 11.7 ppb (A Coruna) [129] (c) 127/35 ppb (Seoul) [130] Vehicle exhaust (a) 6 ppb chemiresistor (PEDOT:PSS/TiO2) [131] (b) 5 ppb electrochemical (WO3/Pt) [132] (c) 18 ppb chemiresistor (WO3/Cr2O3) [133] (d) 3.6 ppb ICOS [134] (e) 0.03 ppb lase QCL [135] (f) 4 ppb electrochemical sensor [136] C CO 630-08-0 1350 (a) 1.6 ppm (Karachi) [137] (b) 0.592 ppm (Hong Kong) [128] (c) 1.7 ppm (Rio de Janeiro) [138] (d) 0.53 ppm (London) [139] (e) 1.2 ppm (Seul) [130] Coal burning Vehicular exhaust Cigarette smoke (a) 1 ppm chemiresistor (Ca-SnO2) [140] (b) 1 ppm electrochemical sensor [141] (c) 0.1 ppm controlled potential electrolysis [142] (d) 4 ppb electrochemical sensor [136] C Ammonia [153] (e) 13.5 ppb (average from towns) [154] (f) 7 ppb (Beijing) [155] (g) 1.45 ppb (Zurich, Switzerland) [156] Solvents, oxidation of NMHCs (a) 0.02 ppb MCC-IMS [147] (b) 14 ppb AIMS [157] (c) 500 ppb AIMS [148] (d) 20 ppb Si:WO3 chemiresistor [158] (e) 120 ppb Pt-WO3 chemiresistor [159] (f) 130 ppb optical spectroscopy [160] (g) 170 1.5 (a) 0.3 ppb (Seul) [170] (b) 1.1 ppb (Karachi) [137] (c) 1.2 ppb (Rio de Janeiro) [175] (d) 0.1 ppb (Lille) [171] (e) 0.79 ppb (Guangzhou) [173] (f) 0.3 ppb (A Coruna) [129] (g) 0.04 ppb (London) [139] Petrol evaporation, vehicle exhaust C (continued on next page) 99 0.14 C building materials can determine the applicability of markers of human presence. The interactions of VOCs forming the human scent with debris materials have already received some attention. ...
... Thus, knowledge of the surface chemistry of Table 2 Predicted levels in void spaces (see text for detailed conditions), exemplary urban levels, possibilities of detection by portable instruments and classification of potential volatile markers of human presence. 30 × 10 6 (b) 408 ppm (Dallas) [122] (c) 390 ppm (Phenix) [123] (d) 469 ppm (Wrocław) [124] (e) 403-408 ppm (Portland) [125] Vehicle emissions, (a) 0.25% optical sensor [126] (b) 0.23% Solvatochromic probe [127] A NO 10102-43-9 7.7 (a) 24.5 ppb (Hong Kong) [128] (b) 11.7 ppb (A Coruna) [129] (c) 127/35 ppb (Seoul) [130] Vehicle exhaust (a) 6 ppb chemiresistor (PEDOT:PSS/TiO2) [131] (b) 5 ppb electrochemical (WO3/Pt) [132] (c) 18 ppb chemiresistor (WO3/Cr2O3) [133] (d) 3.6 ppb ICOS [134] (e) 0.03 ppb lase QCL [135] (f) 4 ppb electrochemical sensor [136] C CO 630-08-0 1350 (a) 1.6 ppm (Karachi) [137] (b) 0.592 ppm (Hong Kong) [128] (c) 1.7 ppm (Rio de Janeiro) [138] (d) 0.53 ppm (London) [139] (e) 1.2 ppm (Seul) [130] Coal burning Vehicular exhaust Cigarette smoke (a) 1 ppm chemiresistor (Ca-SnO2) [140] (b) 1 ppm electrochemical sensor [141] (c) 0.1 ppm controlled potential electrolysis [142] (d) 4 ppb electrochemical sensor [136] C Ammonia [153] (e) 13.5 ppb (average from towns) [154] (f) 7 ppb (Beijing) [155] (g) 1.45 ppb (Zurich, Switzerland) [156] Solvents, oxidation of NMHCs (a) 0.02 ppb MCC-IMS [147] (b) 14 ppb AIMS [157] (c) 500 ppb AIMS [148] (d) 20 ppb Si:WO3 chemiresistor [158] (e) 120 ppb Pt-WO3 chemiresistor [159] (f) 130 ppb optical spectroscopy [160] (g) 170 1.5 (a) 0.3 ppb (Seul) [170] (b) 1.1 ppb (Karachi) [137] (c) 1.2 ppb (Rio de Janeiro) [175] (d) 0.1 ppb (Lille) [171] (e) 0.79 ppb (Guangzhou) [173] (f) 0.3 ppb (A Coruna) [129] (g) 0.04 ppb (London) [139] Petrol evaporation, vehicle exhaust C (continued on next page) 99 0.14 C building materials can determine the applicability of markers of human presence. The interactions of VOCs forming the human scent with debris materials have already received some attention. ...
... Spectral region around 5.3 mm containing the strongest fundamental n 2 ro-vibrational band of NO is usually targeted to assure the highest sensitivity. In addition to that, a variety of sensitivity enhancement schemes such as multi-pass cells 16 or high finesse cavity enhanced detection 17,18 have been applied to achieve NO detection at sub-ppbv/Hz 1/2 levels. All absorption based techniques require careful selection of the target transition to minimize spectral interference from other species, but in case of complex gas mixtures a possibility of unintended spectral interference still exist. ...
Measurement of NO and/or its metabolites in the various body compartments has transformed our understanding of biology. The inability of the current NO measurement methods to account for naturally occurring and experimental NO isotopes, however, has prevented the scientific community from fully understating NO metabolism in vivo. Here we present a mid-IR Faraday rotation spectrometer (FRS) for detection of NO isotopes. The instrument utilizes a novel dual modulation/demodulation (DM) FRS method which exhibits noise performance at only 2 times the fundamental quantum shot-noise level and provides the record sensitivity in its class. This is achieved with a system that is fully autonomous, robust, transportable, and does not require cryogenic cooling. The DM-FRS enables continuous monitoring of nitric oxide isotopes with the detection limits of 3.72 ppbv/Hz(1/2) to(14)NO and 0.53 ppbv/Hz(1/2) to(15)NO using only 45 cm active optical path. This DM-FRS measurement method can be used to improve the performance of conventional FRS sensors targeting other radical species. The feasibility of the instrument to perform measurements relevant to studies of NO metabolism in humans is demonstrated.
... Thus, knowledge of the surface chemistry of Table 2 Predicted levels in void spaces (see text for detailed conditions), exemplary urban levels, possibilities of detection by portable instruments and classification of potential volatile markers of human presence. 30 × 10 6 (b) 408 ppm (Dallas) [122] (c) 390 ppm (Phenix) [123] (d) 469 ppm (Wrocław) [124] (e) 403-408 ppm (Portland) [125] Vehicle emissions, (a) 0.25% optical sensor [126] (b) 0.23% Solvatochromic probe [127] A NO 10102-43-9 7.7 (a) 24.5 ppb (Hong Kong) [128] (b) 11.7 ppb (A Coruna) [129] (c) 127/35 ppb (Seoul) [130] Vehicle exhaust (a) 6 ppb chemiresistor (PEDOT:PSS/TiO2) [131] (b) 5 ppb electrochemical (WO3/Pt) [132] (c) 18 ppb chemiresistor (WO3/Cr2O3) [133] (d) 3.6 ppb ICOS [134] (e) 0.03 ppb lase QCL [135] (f) 4 ppb electrochemical sensor [136] C CO 630-08-0 1350 (a) 1.6 ppm (Karachi) [137] (b) 0.592 ppm (Hong Kong) [128] (c) 1.7 ppm (Rio de Janeiro) [138] (d) 0.53 ppm (London) [139] (e) 1.2 ppm (Seul) [130] Coal burning Vehicular exhaust Cigarette smoke (a) 1 ppm chemiresistor (Ca-SnO2) [140] (b) 1 ppm electrochemical sensor [141] (c) 0.1 ppm controlled potential electrolysis [142] (d) 4 ppb electrochemical sensor [136] C Ammonia [153] (e) 13.5 ppb (average from towns) [154] (f) 7 ppb (Beijing) [155] (g) 1.45 ppb (Zurich, Switzerland) [156] Solvents, oxidation of NMHCs (a) 0.02 ppb MCC-IMS [147] (b) 14 ppb AIMS [157] (c) 500 ppb AIMS [148] (d) 20 ppb Si:WO3 chemiresistor [158] (e) 120 ppb Pt-WO3 chemiresistor [159] (f) 130 ppb optical spectroscopy [160] (g) 170 1.5 (a) 0.3 ppb (Seul) [170] (b) 1.1 ppb (Karachi) [137] (c) 1.2 ppb (Rio de Janeiro) [175] (d) 0.1 ppb (Lille) [171] (e) 0.79 ppb (Guangzhou) [173] (f) 0.3 ppb (A Coruna) [129] (g) 0.04 ppb (London) [139] Petrol evaporation, vehicle exhaust C (continued on next page) 99 0.14 C building materials can determine the applicability of markers of human presence. The interactions of VOCs forming the human scent with debris materials have already received some attention. ...
Volatile organic compounds emitted by a human body form a chemical signature
capable of providing invaluable information on the physiological status of an
individual and, thereby, could serve as signs-of-life for detecting victims
after natural or man-made disasters. In this review a database of potential
biomarkers of human presence was created on the basis of existing literature
reports on volatiles in human breath, skin emanation, blood, and urine.
Approximate fluxes of these species from the human body were estimated and used
to predict their concentrations in the vicinity of victims. The proposed
markers were classified into groups of different potential for victim
detection. The major classification discriminants were the capability of
detection by portable, real-time analytical instruments and background levels
in urban environment. The data summarized in this review are intended to assist
studies on the detection of humans via chemical analysis and accelerate
investigations in this area of knowledge.
... Control of polarization and isolation of the laser from feedback are of vital importance for many applications of lasers, especially in spectroscopy and communication. As a high gain laser source, Quantum Cascade lasers are especially susceptible to external feedback [1,2]. However, currently there are very limited number of optical materials that are suitable for the control of polarization and consequently isolation in the mid-Infrared (MIR). ...
There have been limited choices of optical materials in the Mid-Infrared for polarization control and
subsequent isolation. We show several combinations of existing materials and optics that could realize
polarization control and isolation for quantum cascade lasers in the MIR. Improvements in signal to noise
ratio in MIR laser spectroscopy, as well as saturated absorption spectroscopy utilizing the isolation
achieved, will be discussed.
... The addition of a germanium etalon determines the laser tuning rate. With a fixed path length of 76 m, an astigmatic Herriott cell provides long absorption path lengths (Nelson et al., 2006). ...
Long-term time series of the atmospheric composition are essential for
environmental research and thus require compatible, multi-decadal
monitoring activities. The current data quality objectives of the World
Meteorological Organization (WMO) for carbon monoxide (CO) in the
atmosphere are very challenging to meet with the measurement techniques
that have been used until recently. During the past few years, new
spectroscopic techniques came to market with promising properties for
trace gas analytics. The current study compares three instruments that
have recently become commercially available (since 2011) with the best
currently available technique (Vacuum UV Fluorescence) and provides a
link to previous comparison studies. The instruments were investigated
for their performance regarding repeatability, reproducibility, drift,
temperature dependence, water vapour interference and linearity.
Finally, all instruments were examined during a short measurement
campaign to assess their applicability for long-term field measurements.
It could be shown that the new techniques perform considerably better
compared to previous techniques, although some issues, such as
temperature influence and cross sensitivities, need further attention.
... The TEC and RTD array allows for a higher control of temperature fluctuations, with the platinum RTD acting as a high precision thermostat and the TEC acting as the thermoregulator within the device [14]. The thin-film deposition of the platinum RTD is a more reliable and smaller-scale method for creating a sensitive thermocouple capable of the required sensitivity for the TEC to function properly [15]. ...
We demonstrate a temperature stabilized CMOS-compatible frequency comb based on an integrated optical micro-ring resonator. The instrument operates in the wavelength interval 1520-1600 nm with a wide free spectral range (FSR) of 200 GHz. By embedding a highly sensitive “resistive thermal device” (RTD) on the surface of the chip to provide temperature feedback to the thermal electric cooler, the bench top unit achieves wavelength stability of ~1 pm over a 24 hour period with good power stability. The new frequency comb is designed to be robust, compact and portable. Our approach reduces the cost and complexity of existing high precision frequency combs currently used in the fields of metrology, remote sensing and stellar spectroscopy where high stability is required for prolonged periods of time.
... Mode noise is absent in QCL spectrometers employing conventional long path cells which are today increasingly based on TE cooled lasers and detectors and hence exhibit higher sensitivities even down to 3 × 10 -10 cm -1 Hz -1/2 [30]. However, the volume of commercial multiple pass cells covering effective path lengths up to 210 m, typically ranges from 0.5 to 5 l which may increase the dimensions of the system 6 Value estimated by the authors from the cited data. ...
Absorption spectroscopy based on optical resonators is known to be a very sensitive diagnostic technique. For many years the infrared spectral range could not be employed, because of the lack of suitable radiation sources with the required power and tunability. Recent advances in semiconductor laser technology, in particular the advent of quantum cascade lasers (QCL) provides new possibilities for highly sensitive and selective detection of molecular species. Cavity Enhanced Absorption Spectroscopy (CEAS) with a thermoelectrically (TE) cooled cw QCL emitting at 7.66 μm and employing a ~0.5 m long cavity yielded effective path lengths of 1080 m and a sensitivity of 2 x 10-7 cm-1 Hz-1/2, mainly limited by incomplete averaging over cavity resonances. The molecular concentration detection limit with a 20 s integration time was found to be 6 x 108 molecules/cm3 for N2O and 2 x 109 molecules/cm3 for CH4 which is good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar.
... Note the sharpening of the spectral lines and the improved separation between the water and formaldehyde lines. This improved resolution may be achievable in the future as cw QC lasers, which are presently available at 1900 cm À1 Nelson et al., 2006], become available in the 1765 cm À1 spectral region. Until recently, most cw QC lasers required cryogenic cooling but near room temperature operation is now being reported by several groups at many different wavelengths [Bakhirkin et al., 2006; Beck et al., 2002; Blaser et al., 2005; Moeskops et al., 2006; Yu et al., 2005].Figure 3c also shows a dashed trace. ...
... 4.8. More recently (Nelson et al. 2006), a detection sensitivity of 0.03 ppbv was achieved with 30 s averaging time with a path length of 210 m, corresponding to an absorbance path length product of 1.5 × 10 −10 cm −1 . Detection of formaldehyde using off-axis ICOS with a 12 mW ICL was recently demonstrated (Miller et al. 2006). ...
There is an increasing need in many chemical sensing applications ranging from environmental science to industrial process control as well as medical diagnostics for fast, sensitive, and selective trace gas detection based on laser spectroscopy. The recent availability of continuous wave near infrared diode lasers-, mid-infrared quantum cascade and interband cascade distributed feedback (QC and IC DFB) lasers as mid-infrared spectroscopic sources addresses this need. A number of spectroscopic techniques have been demonstrated. For example, the authors have employed infrared DFB QC and IC lasers for the detection and quantification of trace gases and isotopic species in ambient air by means of direct absorption, cavity-enhanced, and photoacoustic spectroscopy. These spectroscopic techniques offer an alternative to non-spectroscopic techniques such as mass spectrometry (MS), gas chromatography (GC) and electrochemical sensors. The sensitivity and selectivity that can be achieved by both techniques (excluding electrochemical sensors) are similar, but the sensor response time, instrumentation size and cost of ownership for spectroscopic techniques can be advantageous as compared to MS-GC spectrometry.
Keywords: Laser absorption spectroscopy, cavity-enhanced and photoacoustic spectroscopy, near infrared diode lasers, mid infrared quantum cascade lasers, chemical sensing of trace gases
... Lead salt laser-based systems have achieved 1.5 ppbv and better and are commercially available [13]. A quantum cascade laser (QCL) spectrometer using a multipass cell has achieved a sensitivity of < 0.1 ppbv [14]. The cavity ring-down spectroscopy (CRDS) technique uses ultra-high-reflectivity mirrors (R > 99.99%) to reach noise-equivalent sensitivity at the sub-ppbv level in several seconds using a comparatively small sample volume [15]. ...
A nitric oxide (NO) sensor employing a thermoelectrically cooled, continuous-wave, distributed feedback quantum cascade laser
operating at 5.47μm (1828cm-1) and off-axis integrated cavity output spectroscopy was used to measure NO concentrations in exhaled breath. A minimum measurable
concentration (3σ) of 3.6 parts-per-billion by volume (ppbv) of NO with a data-acquisition time of 4s was demonstrated. Five
prepared gas mixtures and 15 exhaled breath samples were measured with both the NO sensor and for intercomparison with a chemiluminescence-based
NO analyzer and were found to be in agreement within 0.6ppbv. Exhaled NO flow-independent parameters, which may provide diagnostic
and therapeutic information in respiratory diseases where single-breath measurements are equivocal, were estimated from end-tidal
NO concentration measurements collected at various flow rates. The results of this work indicate that a laser-based exhaled
NO sensor can be used to measure exhaled nitric oxide at a range of exhalation flow rates to determine flow-independent parameters
in human clinical trials.
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Highly sensitive, laser-based sensors often utilize extended gas interaction paths (e.g., multi-pass cells) that impact their size, weight, and susceptibility to misalignment, limiting their use to laboratory applications. This study reports the first demonstration of photothermal nitric oxide (NO) detection in the mid-infrared range at 5.26 µm utilizing a unique and miniature Solid-state Laser Intracavity Photothermal Sensor (SLIPS) with an ultra-short laser-gas interaction path length of 1 mm. The gas sample detection is realized inside the resonator of a monolithic diode-pumped solid-state laser (DPSSL) emitting at 1064 nm. Modulated mid-infrared radiation absorbed by NO inside the laser cavity causes local changes in photothermally-induced gas refractive index (RI). This effect results in the DPSSL frequency shifts proportional to gas concentration, which are being detected. A minimum detection limit of 50 ppbv (parts per billion of volume) and a noise equivalent absorption (NEA) coefficient of 7.9×10-7 cm-1 for 10 s integration time have been achieved in an ultra-compact sensing volume (4 µl) and ultra-short interaction length of the sensor as a result of its superb RI sensitivity of 1.8×10-11. Furthermore, the SLIPS is not limited in gas excitation laser wavelength because only a near-infrared detector is needed for spectroscopic signal readout.
本论文主要包括两个部分：一是基于室温连续波量子级联激光器光源，设 计并搭建了一套测量HONO 气体的系统，然后用该系统进行了HONO 气体的 探测和研究。二是利用室温操作的宽调谐差频产生中红外激光器系统进行痕量 气体(HCl, CH2O，HONO)的探测。OH 自由基是光化学循环的主要物种之一，并对臭氧的形成有重要影响，从而导致所谓的“光化学烟雾”污染。OH 自由基同时也影响着大气中烃类的氧化 能力。气态亚硝酸是清晨和白天OH 自由基的一个主要来源。因此，亚硝酸直 接影响大气的氧化能力，同时也间接推动了由于氧化过程而产生的二次污染物 的形成。大气中亚硝酸浓度的精确测量需要仪器具备高的灵敏度和稳定性，以 及好的时间和空间分辨率。本论文的主要工作之一就是设计一套以8 μm (~1255 cm-1)连续波量子级联激光器为基础对HONO 气体进行探测的装置。并利用由 H2SO4 和 NaNO2 发生化学反应而产生的HONO 气体估算和描述该装置的灵敏度和特性。产生的HONO 浓度由一个溶蚀器系统和一个NOX 分析仪来量化。在该量子级联激光器频率范围内(1254.6-1256.4 cm-1) ，只能从文献中查阅到 HONO 的5 条吸收线线强，因此测量了HONO 用于进行痕量探测的吸收线以及 其他吸收线的相对频率和线强，得到的十九条较强吸收线的线强范围大约在 (3-90)×10-21 cm。用直接吸收光谱技术结合125 m 的多次反射池对HONO 进行 了痕量探测。为了提高灵敏度，开展了波长调制技术的实验研究，得到HONO 的最低探测浓度(SNR=1) 在1 s 的积分时间内为400 ppt，适合进行外场测量。研究了由光学池壁表面而导致的HONO 的衰减效应并得到一个反应率常数，这个常数可能有助于以后HONO 的场测量，特别是用吸收池的测量。测量了实验 室空气中的HONO 和CH4，得到它们的室内浓度分别为116 ppb 和1.5 ppm, 相应的1 s 积分时间内1 σ 最小可以探测的浓度分别为396 ppt 和6 ppb。论文的另一部分工作是利用一个室温操作的，宽调谐窄线宽中红外差频产 生光源进行痕量气体的探测。该中红外差频产生系统是以准相位匹配为基础，利用两台近红外半导体激光器作为泵浦源，在PPLN 晶体中进行差频，得到的 相关中红外差频输出范围为3.2 μm 到 3.7 μm。用该系统进行了HCl 和 CH2O 气体探测，说明了该装置在工业和环境监测领域有一定的应用潜力。根据文献 得到的HONO 的另一个吸收带3590 cm-1，设计了另一套以钛宝石激光器和掺 Yb 光纤激光器作为泵浦源的差频光源系统，用CO2 气体进行了差频输出光频率 校准, 下一步的工作是进行HONO 气体测量实验。
Photothermal interferometry (PTI) with hollow core fibers (HCFs) have enabled highly sensitive spectroscopic gas sensors in an all-fiber format. Here we report remarkable improvement in the limit of detection of HCF-PTI, in terms of noise equivalent concentration (NEC), by exploiting the optical-phase-modulation amplifying (OPMA) effect of an HCF resonating cavity. By locking the wavelength of a 1550 nm probe laser to the resonance of a 10-cm-long HCF Fabry-Prot cavity with a finesse of ~700, OPMA of more than two orders of magnitude is achieved, which enables ultra-sensitive gas detection with large dynamic range. With 1654 nm, 1532 nm, and 761 nm pump lasers, we demonstrate detection of methane, acetylene, and oxygen with noise-equivalent-concentration of 15 parts-per-trillion (ppt), 2.7 ppt, and 0.56 parts-per-million (ppm), respectively. Further improvement in NEC is possible by use of a higher finesse cavity with a longer length of HCF. Extension of the technique to other gases, other types of phase or dispersion modulation-based sensors, and other optical resonating cavities is straightforward.
Effect of external longitudinal magnetic field on the optical Free Induction Decay (FID) from a free radical was observed for the first time. The experiments were performed on the rotational transition of hydroxyl radical OH (²Π3/2(J=1) ← ²Π3/2(J=0) at 83.8 cm⁻¹) using the Terahertz Free Electron Laser. In contrast to the results of the experiments with a stable paramagnetic molecule NO, the observed effect of external magnetic field on the Free Induction Decay from hydroxyl radicals is more complicated. A longitudinal magnetic field leads to rotation of the polarization plane of the FID radiation, as well as to additional modulation of the signal intensity. The angle of rotation of the polarization plane is large, in agreement with the theoretical predictions. The observed FID kinetics in the time domain are in semi-quantitative agreement with the modeling. This observation opens an opportunity for selective detection by polarization discrimination of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species.
Croplands are important sources of atmospheric nitric oxide (NO). However, high-frequency measurements of NO fluxes over croplands using the eddy covariance (EC) technique are still scarce, mainly due to instrumental limitation. In this study, a closed-path NO analyzer based on a quantum cascade laser (QCL) absorption spectrometer was employed for EC flux measurements over a subtropical vegetable field during a two-month summer period with the lowest NO emission intensity of the year. The purpose was to investigate the detection limit of the EC system based on this NO analyzer and evaluate its applicability for measuring the turbulent fluxes of NO under field conditions. The performance of the analyzer was stable, showing an average precision (0.1 s) of 0.338 nmol mol⁻¹ and a corresponding flux detection limit of 5.6 μg N m⁻²h⁻¹ at the 95% confidence interval. The measured turbulent NO fluxes ranged from −7.1 to 61.4 μg N m⁻²h⁻¹ (median: 3.5 μg N m⁻²h⁻¹), with a relative random error of 386% before field ploughing and 76% thereafter. The systematic errors due to the high-frequency loss and the use of lag times of carbon dioxide for NO flux calculation were estimated at 12% and 3%, respectively. During the measurement period, 37% of the observed half-hourly fluxes were larger than the detection limit; the magnitude of these fluxes is comparable with that measured by the static chambers. Nevertheless, this EC system could be still qualified for measuring turbulent NO fluxes over common croplands if the flux averaged at daily or longer timescales are of interest, because either flux detection limit or random error would decrease by an order of n, wherein n is the number of half-hourly fluxes being taken for averages. This study shows that the closed-path dual-QCL analyzer could be an effective option for EC measurements of turbulent NO fluxes with the advantages of (i) stable performance, (ii) high precision and fast response, and (iii) feasible instrumental maintenance for long-term field measurements. However, the observed turbulent fluxes still underestimated the soil NO emissions likely due to chemical reaction loss of NO below the sensor height. Further studies are necessary to address this systematic error.
Laser spectroscopy outperforms electrochemical and semiconductor gas sensors in selectivity and environmental survivability. However, the performance of the state-of-the-art laser sensors is still insufficient for many high precision applications. Here, we report mode-phase-difference photothermal spectroscopy with a dual-mode anti-resonant hollow-core optical fiber and demonstrate all-fiber gas (acetylene) detection down to ppt (parts-per-trillion) and <1% instability over a period of 3 hours. An anti-resonant hollow-core fiber could be designed to transmit light signals over a broad wavelength range from visible to infrared, covering molecular absorption lines of many important gases. This would enable multi-component gas detection with a single sensing element and pave the way for ultra-precision gas sensing for medical, environmental and industrial applications.
In this article the operation and thermal conductance of a series of quantum cascade lasers (QCLs) emitting at 4.6 μm grown with between 5 and 30 cascades in the active region is analyzed. A reduced number of cascades, but with fixed active region design, results in larger threshold current density but lower threshold power density. An analysis of the temperature gradient in the active region implies that for continuous-wave QCL operation, using fewer than 15 cascades in the active region is advantageous, especially for broad-area structures. We demonstrate a QCL with 10 cascades operating in continuous-wave mode at room temperature emitting 0.9 W from a 4 mm × 30 μm stripe.
Over the past few years mid infrared absorption spectroscopy (MIR-AS) over the region from 3 to 20 ?m has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of MIR-AS because most of these compounds and their decomposition products are infrared active. MIR-AS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a micro second, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from MIR-AS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of MIR-AS techniques to industrial requirements including the development of new diagnostic equipment. The aim of the present chapter is fourfold: (i) to briefly summarize the basic principles of infrared absorption spectroscopy and related instrumentation, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas using different types of MIR-AS techniques, (iii) to describe examples of industrial process monitoring in the mid infrared and (iv) to discuss the potential of advanced instrumentation based on quantum cascade lasers (QCLs) for plasma diagnostics.
We review our recent results in development of high-precision laser spectroscopic instrumentation using mid-infrared quantum cascade lasers, interband cascade lasers and antimonide diode lasers. These instruments are primarily for high-precision and high-sensitivity measurements of atmospheric trace gases, as required for atmospheric research. The instruments are based on direct absorption spectroscopy with rapid sweeps, integration and precision fitting, under the control of high-capability software. By operating in the mid-infrared with long absorption path lengths at reduced pressure, we achieve excellent sensitivity. Some instruments have demonstrated a fractional precision of 10−4 for atmospheric trace gases at ambient concentration, allowing real-time isotopologue measurements of CO2, CO, CH4, N2O and H2O. Trace gas detection in ambient air at the low part-per-trillion levels is feasible. We also describe signal processing methods to identify and reduce measurement noise. Analysis of spectral information is largely based on loading spectra into arrays and then applying block operations such as filters, Fourier analysis, multivariate fitting and principal component analysis. We present mathematical expressions for averaged spectra in arrays and note different ways frequency aliasing can occur. We present an extended example of analysis of instrument noise and find an electronic signal mixing with an interference fringe.
A spectroscopic trace-gas sensor employing a continuous wave, thermoelectrically cooled distributed feedback laser diode and a 100 m optical pathlength astigmatic Herriott cell for sensitive and selective detection of ethane near 3.3 ?m is reported.
We present a novel spectral method to measure atmospheric carbon dioxide
(CO2) with high precision and stability without resorting to
calibration tanks during long-term operation. This spectral null method
improves precision by reducing spectral proportional noise associated
with laser emission instabilities. We employ sealed quartz cells with
known CO2 column densities to serve as the permanent internal
references in the null method, which improve the instrument's stability
and accuracy. A prototype instrument - ABsolute Carbon dioxide (ABC) is
developed using this new approach. The instrument has one-second
precision of 0.02 ppm, which averages down to 0.007 ppm within one
minute. Long-term stability of within 0.1 ppm is achieved without any
calibrations for over a one-month period. These results have the
potential for eliminating the need for calibration cylinders for high
accuracy field measurements of carbon dioxide.
This document is part of Subvolume B 'Laser Systems', Part 3 of Volume 1
'Laser Physics and Applications' of Landolt-Börnstein Group VIII
'Advanced Materials and Technologies'.
Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics.
We review our recent results in development of high-precision laser spectroscopic instrumentation using midinfrared quantum cascade lasers (QCLs). Some of these instruments have been directed at measurements of atmospheric trace gases where a fractional precision of 10-3 or better of ambient concentration may be required. Such high precision is needed in measurements of fluxes of stable atmospheric gases and measurements of isotopic ratios. Instruments that are based on thermoelectrically cooled midinfrared QCLs and thermoelectrically cooled detectors have been demonstrated that meet the requirements of high-precision atmospheric measurements, without the need for cryogens. We also describe the design of and results from a new dual QCL instrument with a 200-m path-length absorption cell. This instrument has demonstrated 1-s noise of 32 ppt for formaldehyde (HCHO) and 9 ppt for carbonyl sulfide (OCS).
The advent of continuous wave quantum cascade lasers operating at near room temperature has greatly expanded the capability of spectroscopic detection of atmospheric trace gases using infrared absorption at wavelengths from 4 to 12 mum. The high optical power, narrow line width, and high degree of single mode purity result in minimal fractional absorptions of 5x10-6 Hz-1/2 detectable in direct absorption with path lengths up to 210 meters. The Allan plot minima correspond to a fractional absorbance of 1x10-6 or a minimum absorption per unit path length 5x10-11 cm-1 in 50s. This allows trace gas mixing ratio detection limits in the low part-per-trillion (1 ppt = 10-12) range for many trace gases of atmospheric interest. We present ambient measurements of NO2 with detection precision of 10 ppt Hz-1/2. The detection precision for the methane isotopologue 13CH4 is 25 ppt Hz-1/2 which allows direct measurements of ambient ratios of 13CH4/12CH4 with a precision of 0.5 in 100 s without pre-concentration. Projections are given for detection limits for other gases including COS, HONO and HCHO as CWRT lasers become available at appropriate wavelengths.
We describe the performance of two mid-infrared laser spectrometers for carbon monoxide, nitrous oxide and nitric oxide detection.
The first spectrometer for CO and N2O detection around 2203 cm-1 is based upon all-diode laser difference frequency generation (DFG) in a quasi-phase matched periodically-poled lithium niobate
(PPLN) crystal using two continuous-wave room-temperature distributed feedback diode lasers at 859 and 1059 nm. We also report
on the performance of a mid-infrared spectrometer for NO detection at ∼ 1900 cm-1 based upon a thermoelectrically-cooled continuous-wave distributed feedback quantum cascade laser (QCL). Both spectrometers
had a single-pass optical cell and a thermoelectrically cooled HgCdZnTe photovoltaic detector. Typical minimum detection limits
of 2.8 ppmv for CO, 0.6 ppmv for N2O and 2.7 ppmv for NO have been demonstrated for a 100 averaged spectra acquired within 1.25 s and a cell base length of 21 cm
at ∼ 100 Torr. Noise-equivalent absorptions of 10-5 and 10-4 Hz-1/2 are typically demonstrated for the QCL and the DFG based spectrometers, respectively.
This talk will focus on recent advances in the development of sensors based on the use of both diode lasers as well as quantum cascade (QC) and interband cascade (IC) lasers for the detection, quantification and monitoring of trace gas species and their application to environmental monitoring, industrial process control, national security and medical diagnostics. Ultra-sensitive, selective and fast response chemical analysis of gases based on molecular absorption laser spectroscopy is a well-established technology. The architecture and performance of several sensitive, selective and real-time gas sensors based on near and mid-infrared semiconductor lasers will be described. To date we have detected 16 gases (CH4, H2S, N2O, CO2, CO, NO, H2O, SO2, NH3, C2H2, OCS, C2H4, H2CO, C2H5OH, C2HF5 and CH3COCH3) at the ppm to ppt level (1,2). Isotopic signatures of carbon and oxygen have also been observed. High sensitivity requires sensitivity enhancement schemes such as a multipass gas absorption cell, cavity absorption enhancement, or photoacoustic spectroscopy. These methods can achieve minimum detectable absorbances in the range from 10-4 to 10-5 for field deployable gas sensors. A novel technique called Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) (3,4) will be emphasized. Our progress in QEPAS optimization has now resulted in a 60 fold increase in detection sensitivity as a result of incremental improvements in optical coupling, acoustic design and electronics. QEPAS allows a breakthrough in size, weight, robustness and cost as well as wireless sensor network nodes (5) for laser-based chemical trace gas sensors. Several recent examples of real- world applications including environmental monitoring will be reported.
We demonstrate an asymmetric multipass cavity with meter-scale optical path lengths in a volume of 68 cm3 and diameter of 5.24 cm. The cavity is machined from copper and coated with gold. For the characterization of the optical path length in the cavity, we perform a pulse-delay experiment by measuring the time delay between a laser pulse coupled into and out of the cavity. A midinfrared quantum cascade laser is employed as the source. By rotating the cavity, a variety of reentrant patterns and path lengths ranging from 2.0 to 6.0 m can be obtained with straightforward optical alignment.
Quantum-cascade lasers operating at λ ≈ 3.9 μm at room temperature with narrow w ≈ 5 μm ridge widths are described. The lateral confinement due to the narrow ridge is similar to the vertical confinement and the resulting beam is circular in cross section with a single TM00 spatial mode. The beam divergence is 46° both parallel and perpendicular to the surface. The beam quality factor along the slow axis is about M2 = 1.6. The narrow ridges also increase the relative lateral heat dissipation from the active region, resulting in a thermal conductance per unit area of about Gth = 380 W K−1 cm−2 for a 3 mm long laser. Maximum average power is obtained with duty cycles between 10% and 30%; in spite of the very narrow ridge, the total average power with thermoelectric cooling exceeds 60 mW with a peak power of 460 mW. The circularly symmetric beam with very good beam quality suggests essentially zero astigmatism and indicates that these narrow-ridge quantum-cascade lasers are well suited for applications in midinfrared spectroscopy and imaging.
Nanoparticle doped hybrid organogels were prepared using the pseudopeptidic macrocycle 1 and CdSe/ZnS quantum dots (QDs). The new semi-solid materials show the same thermal stability and excellent optical transparency as compared to the parent organogel in the absence of the nanoparticles, but they are fluorescent due to the presence of embedded semiconductor nanocrystals. The fluorescence lifetime of a QD-organogel composite is reported for the first time, and it was found to be similar to that of the QDs in solution in a related solvent, independent of the concentration of gelator. The chemical sensing ability of the hybrid organogels towards gaseous nitric oxide was investigated by steady-state fluorescence spectroscopy. The gels show fluorescence sensitivity towards NO ranging from 0.05 to 0.5 (vol%). The results reported herein constitute a proof of principle of the potential of the hybrid supramolecular soft materials to develop nitric oxide sensor devices of practical application, especially because the semi-solid state of the organogel is preserved after interaction with the analyte. This is remarkable since, for the vast majority of organogels with analytical capabilities reported so far, the signaling mechanism relies upon the disassembly of the supramolecular structure.
Mid infrared (MIR) absorption spectroscopy between 3 and 20 µm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas and for trace gas analysis. The increasing interest in molecular processing plasmas has lead to further applications of IRLAS. IRLAS provides a means of determining the absolute concentrations and temperatures of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition and semiconductor processing this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes and for trace gas analysis. The aim of the present contribution is threefold: (i) to report on selected studies of the spectroscopic properties and kinetic behaviour of the methyl radical, (ii) to review recent achievements in our understanding of molecular phenomena in plasmas and the influence of surfaces, and (iii) to describe the current status of advanced instrumentation for quantum cascade laser absorption spectroscopy (QCLAS).
Recent advances in the development of sensors based on infrared diode and quantum cascade lasers for the detection of trace
gas species is reported. Several examples of applications in environmental and industrial process monitoring as well as in
medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be described.
Keywords: Trace gas detection, near infrared diode lasers, mid infrared quantum and interband cascade lasers, quartz enhanced
photoacoustic spectroscopy, laser absorption spectroscopy.
We have developed a compact instrument for sensitive, rapid and continuous measurement of trace gases in air, with results
presented here for methane (CH4), nitric oxide (NO), nitrous oxide (N2O) and ammonia (NH3). This instrument takes advantage of recent technology in quantum cascade (QC) lasers and infrared detectors, which allows
high sensitivity without cryogenic liquids, e.g., 0.2ppb (0.2×10-9) of NH3 in air in 1s. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument operates
continuously, requiring little operator attention, and web-based remote access is provided for instrument control, calibration
and data retrieval. The instrument design includes a thermoelectrically (TE) cooled pulsed distributed feedback (DFB) QC laser,
a low volume (0.5l) multipass cell offering 76m absorption path length and a TE cooled detector. Integrated software for
laser control and data analysis using direct absorption provides quantitative trace gas measurements without calibration gases.
The instrument may be applied to field measurements of gases of environmental concern.
Within the last decade, mid-infrared absorption spectroscopy between 3 and 20μm – known as infrared laser absorption spectroscopy (IRLAS) and based on tunable semiconductor lasers, namely lead salt diode lasers, often called tunable diode lasers (TDLs), and quantum cascade lasers (QCLs) – has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, and organosilicon compounds has led to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Information about gas temperature and population densities can also be derived from IRLAS measurements. A variety of free radicals and molecular ions have been detected, especially using TDLs. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes.
We present a new approach to the measurement of stable isotopic ratios of carbon dioxide using a near-room-temperature pulsed
quantum cascade laser and a spectral ratio method based upon dual multiple pass absorption cells. The spectral ratio method
improves precision and accuracy by reducing sensitivity to variations in the laser tuning rate, power and line width. The
laser is scanned across three spectral lines (near 2310cm-1) quantifying three CO2 isotopologues: 12C16O2, 13C16O2 and 12C16O18O. Isotopic ratios are determined simultaneously with a precision of 0.2δ for each ratio with a one-second measurement. Signal
averaging for 400s improves the precision to better than 0.03δ for both isotopic ratios (13
R and 18
R). Long-term accuracy of 0.2 to 0.3δ is demonstrated with replicate measurements of the same sample over a one-month period.
The fast time response of this instrument is suitable for eddy flux measurements.
Mid-infrared laser absorption sensors based on quantum cascade laser (QCL) technology offer the potential for high-sensitivity,
selective, and high-speed measurements of temperature and concentration for species of interest in high-temperature environments,
such as those found in combustion devices. Anew mid-infrared QCL absorption sensor for carbon monoxide and temperature measurements
has been developed near the intensity peak of the CO fundamental band at 4.6μm, providing orders-of-magnitude greater sensitivity
than the overtone bands accessible with telecommunications lasers. The sensor is capable of probing the R(9), R(10), R(17),
and R(18) transitions of the CO fundamental ro-vibrational band which are located at frequencies where H2O and CO2 spectral interference is minimal. Temperature measurements are made via scanned-wavelength two-line ratio techniques using
either the R(9) and R(17) or the R(10) and R(18) line pairs. The high-speed (1–2kHz) scanned-wavelength sensor is demonstrated
in room-temperature gas cell measurements of CO and, to demonstrate the potential of the sensor for high-temperature thermometry,
in shock-heated gases containing CO for a very wide range of temperature (950–3500K) near 1atm. To our knowledge, these
measurements represent the first use of QCL-based absorption sensor for thermometry at elevated combustion-like temperatures.
The high-temperature measurements of CO mole fraction and temperature agree with the post-reflected-shock conditions within
±1.5% and ±1.2% (1σ deviation), respectively.
A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out
of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering.
Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum
wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A
strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2
micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely
determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure
material.
We demonstrate room-temperature, single-mode, continuous-wave operation of a λ ≃ 5.4 μm quantum-cascade laser up to the temperature of 30 °C. Processing is done using standard lithography in a ridge waveguide mounted junction-up. The active region is based on a bound-to-continuum transition. The high performances were achieved with a low active region doping and a thick electroplated gold deposition, resulting in a characteristic temperature of T0 = 155 K in continuous-wave with a threshold current density of jth = 2.05 kA/cm2 at 300 K.
A nitric oxide (NO) gas sensor based on a thermoelectrically cooled, continuous-wave, distributed feedback quantum cascade
laser operating at 5.45μm (1835cm-1) and off-axis integrated cavity output spectroscopy combined with a wavelength-modulation technique was developed to determine
NO concentrations at the sub-ppbv levels that are essential for a number of applications, such as medical diagnostics, environmental
monitoring, and industrial process control. The sensor employs a 50-cm-long high-finesse optical cavity that provides an effective
path length of ∼700m. A noise equivalent (SNR=1) minimum detection limit of 0.7ppbv with a 1-s observation time was achieved.
This paper describes the status of the 2004 edition of the HITRAN molecular spectroscopic database. The HITRAN compilation consists of several components that serve as input for radiative transfer calculation codes: individual line parameters for the microwave through visible spectra of molecules in the gas phase; absorption cross-sections for molecules having dense spectral features, i.e., spectra in which the individual lines are unresolvable; individual line parameters and absorption cross-sections for bands in the ultra-violet; refractive indices of aerosols; tables and files of general properties associated with the database; and database management software. The line-by-line portion of the database contains spectroscopic parameters for 39 molecules including many of their isotopologues.The format of the section of the database on individual line parameters of HITRAN has undergone the most extensive enhancement in almost two decades. It now lists the Einstein A-coefficients, statistical weights of the upper and lower levels of the transitions, a better system for the representation of quantum identifications, and enhanced referencing and uncertainty codes. In addition, there is a provision for making corrections to the broadening of line transitions due to line mixing.
Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 micrometers. The results demonstrate the potential of quantum cascade lasers as continuous wave mid-infrared light sources for high-resolution spectroscopy, chemical sensing applications, and free-space optical communication systems.
Continuous-wave operation of an external cavity quantum-cascade laser on a thermoelectric cooler is reported. The active region of the gain element was based on a bound-to-continuum design emitting near 5.15 microm. The external cavity setup was arranged in a Littrow configuration. The front facet of the gain chip was antireflection coated. The laser could be tuned over more than 170 cm(-1) from 4.94 to 5.4 microm and was single mode over more than 140 cm(-1). The output power was in excess of 10 mW over approximately 100 cm(-1) and in excess of 5 mW over approximately 130 cm(-1) at -30 degrees C.
We used a thermoelectrically cooled, continuous-wave, quantum cascade laser operating between 1847 and 1854 cm(-1) in combination with wavelength modulation spectroscopy for the detection of nitric oxide (NO) at the sub-part-per-billion by volume (ppbv) level. The laser emission overlaps the P7.5 doublet of NO centered around 1850.18 cm(-1). Using an astigmatic multiple-pass absorption cell with an optical path length of 76 m, we achieved a detection limit of 0.2 ppbv at 10 kPa, with a total acquisition time of 30 s. The corresponding minimal detectable absorption is 8.8 x 10(-9) cm(-1) Hz(-1/2).
Recent advances in quantum-cascade (QC) laser active-region design
are reviewed. Based on a rate equation model of the active region, we
show why new gain regions. based on a two-phonon resonance or a
bound-to-continuum transition exhibit significantly better performance
than the traditional design based on a three-quantum-well active region.
Threshold current densities as low as 3 kA/cm<sup>2</sup> at T=300 K,
operation with a peak power of 90 mW at 425 K, single-mode high-power
operation up to temperatures above 330 K at λ≈16 μm and
continuous wave operation up to T=311 K are demonstrated. QC lasers able
to operate at high duty cycles (50%) on a Peltier cooler were used in a
demonstration of a 300-MHz free-space optical link between two buildings
separated by 350 m
Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases
in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1nmole/mole (<1ppbv) using
a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26μm and coupled to a 210-m path length multiple-pass
absorption cell at reduced pressure (50Torr). The sensitivity of the system is enhanced by operating under pulsing conditions
which reduce the laser line width to 0.010cm-1 (300MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector.
The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode
laser spectrometer sampling from a common inlet. A detection precision of 0.12ppb Hz-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10-5Hz-1/2 or an absorbance precision per unit path length of 5×10-10cm-1 Hz-1/2. A precision of 0.3ppb Hz-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods
with a totally cryogen-free instrument.
A high-power continuous-wave (CW) operation of distributed-feedback quantum-cascade lasers using a buried grating with epitaxial regrowth up to temperatures of above 60 ° C is demonstrated. For a high-reflectivity-coated 13 μ m -wide and 3 mm -long cavity, CW output powers of 135 mW at 25 ° C and still 37 mW at 60 ° C are obtained. The device exhibits a CW threshold current density of 1.1 kA / cm <sup>2</sup> , a maximum CW wall-plug efficiency of 1.48% at 25 ° C , and a characteristic temperature of 177 K in pulsed mode. Single-mode emission near 4.8 μ m with a side-mode suppression ratio of ≫30 dB and a tuning range of ∼8.1 cm <sup>-1</sup> (i.e., tunability of 0.18 cm <sup>-1</sup>/ K ) in the temperature range from 15 to 60 ° C is observed.
A multipass absorption cell, based on an astigmatic variant of the off-axis resonator (Herriott) configuration, has been designed to obtain long path lengths in small volumes. Rotation of the mirror axes is used to obtain an effective adjustability in the two mirror radii. This allows one to compensate for errors in mirror radii that are encountered in manufacture, thereby generating the desired reentrant patterns with less-precise mirrors. Acombination of mirror rotation and separation changes can be used to reach a variety of reentrant patterns and path lengths with a fixed set of astigmatic mirrors. The accessible patterns can be determined from trajectories, as a function of rotation and separation, through a general map of reentrant solutions. Desirable patterns for long-path spectroscopy can be chosen on the basis of path length, distance of the closest beam spot from the coupling hole, and tilt insensitivity. We describe the mathematics and analysis methods for the astigmatic cell with mirror rotation and then describe the design and test of prototype cells with this concept. Two cell designs are presented, a cell with 100-m path length in a volume of 3 L and a cell with 36-m path length in a volume of 0.3 L. Tests of low-volume absorption cells that use mirror rotation, designed for fast-flow atmospheric sampling, show the validity and the usefulness of the techniques that we have developed.
We report a heterodyne beat with a linewidth of 5.6+/-0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 microm . We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop.
We report continuous-wave (CW) operation of a 4.3-μm quantum-cascade laser from 80 K to 313 K. For a high-reflectivity-coated 11-μm-wide and 4-mm-long laser, CW output powers of 1.34 W at 80 K and 26 mW at 313 K are achieved. At 298 K, the CW threshold current density of 1.5 kA/cm<sup>2</sup> is observed with a CW output power of 166 mW and maximum wall-plug efficiency of 1.47%. The CW emission wavelength varies from 4.15 μm at 80 K to 4.34 μm at 298 K, corresponding to a temperature-tuning rate of 0.87 nm/K. The beam full-width at half-maximum values for the parallel and the perpendicular far-field patterns are 26° and 49° in CW mode, respectively.
A theoretical development is presented which results in a relationship between the expectation value of the standard deviation of the frequency fluctuations for any finite number of data samples and the infinite time average value of the standard deviation, which provides an invariant measure of an important quality factor of a frequency standard. A practical and straightforward method of determining the power spectral density of the frequency fluctuations from the variance of the frequency fluctuations, the sampling time, the number of samples taken, and the dependence on system bandwidth is also developed. Additional insight is also given into some of the problems that arise from the presence of "flicker noise" (spectrum proportional to |ω|<sup>-1</sup>) modulation of the frequency of an oscillator. The theory is applied in classifying the types of noise on the signals of frequency standards made available at NBS, Boulder Laboratories, such as: masers (both H and N<sup>15</sup>H 3 ), the cesium beam frequency standard employed as the U. S. Frequency Standard, and rubidium gas cells. "Flicker noise" frequency modulation was not observed on the signals of masers for sampling times ranging from 0.1 second to 4 hours. In a comparison between the NBS hydrogen maser and the NBS III cesium beam, uncorrelated random noise was observed on the frequency fluctuations for sampling times extending to 4 hours; the fractional standard deviations of the frequency fluctuations were as low as 5 parts in 10<sup>14</sup>.
- M Beck
- D Hofstetter
- T Aellen
- J Faist
- U Oesterle
- M Ilegems
- E Gini
- H Melchior
M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle,
M. Ilegems, E. Gini, and H. Melchior, Science 295, 301
(2002).
- J S Yu
- A Evans
- S Slivken
- S R Darvish
- M Razeghi
J. S. Yu, A. Evans, S. Slivken, S. R. Darvish, and M.
Razeghi, IEEE Photon. Technol. Lett. 17, 1154 (2005).
- Y A Bakhirkin
- A A Kosterev
- R F Curl
- F K Tittel
- D A Yarekha
- L Hvozdara
- M Giovannini
- J Faist
Y. A. Bakhirkin, A. A. Kosterev, R. F. Curl, F. K. Tittel,
D. A. Yarekha, L. Hvozdara, M. Giovannini, and J.
Faist, Appl. Phys. B 82, 149 (2006).
- J Faist
- D Hofstetter
- M Beck
- T Aellen
- M Rochat
- S Blaser
J. Faist, D. Hofstetter, M. Beck, T. Aellen, M. Rochat,
and S. Blaser, IEEE J. Quantum Electron. 38, 533
(2002).
- D D Nelson
- J S Shorter
- J B Mcmanus
- M S Zahniser
D. D. Nelson, J. S. Shorter, J. B. McManus, and M. S.
Zahniser, Appl. Phys. B 75, 343 (2002).
- J Massie
- A Orphal
- C P Perrin
- M A H Rinsland
- J Smith
- R N Tennyson
- R A Tolchenov
- J Toth
- P Vander Auwera
- G Varanasi
- Wagner
Massie, J. Orphal, A. Perrin, C. P. Rinsland, M. A. H.
Smith, J. Tennyson, R. N. Tolchenov, R. A. Toth, J.
Vander Auwera, P. Varanasi, and G. Wagner, J. Quant.
Spectrosc. Radiat. Transf. 96, 139 (2005).
- J B Mcmanus
- P L Kebabian
- M S Zahniser
J. B. McManus, P. L. Kebabian, and M. S. Zahniser,
Appl. Opt. 34, 3336 (1995).
- M Taubmann
- T Myers
- B Cannon
- R Williams
- F Capasso
- C Gmachl
- D L Sivco
- A Y Cho
M. Taubmann, T. Myers, B. Cannon, R. Williams, F.
Capasso, C. Gmachl, D. L. Sivco, and A. Y. Cho, Opt.
Lett. 27, 2164 (2002).