Comparison of infrared sources for a differential photoacoustic gas detection system

A prototype of the differential photoacoustic measurement system with an optical cantilever microphone has been developed. The system is based on gas filter correlation method. The proposed system allows real-time measurement of various IR-absorbing gases from the flowing sample or in the open air. Three setups with different kind of infrared sources were carried out to study selectivity and sensitivity of the prototype and applicability of the source types with differential method. The sources were a mechanically chopped blackbody radiator, electrically chopped blackbody radiator and mechanically chopped CO₂-laser. A detection limit for C₂H₄ was estimated with all three infrared sources. Cross sensitivity and detection limits of gases CH₄, C₂H₄ and CO₂ were measured with the mechanically chopped blackbody radiator. This crossinterference matrix was also modeled using HITRAN database and completed with CO and H₂O. The measurements indicate that at least ppb-level detection of ethylene using CO₂-laser, sub-ppm level with mechanically chopped blackbody and ppm-level with electrically modulated blackbody is possible with a proposed differential system.     

Resonance photoacoustic spectroscopy and gas analysis of gaseous flow at reduced pressure

The results of theoretical and experimental studies of sensitivity of a resonant photoacoustic Helmholtz resonator detector for gas flowing through a photoacoustic cell under reduced pressure are presented. The measurements of the sensitivity and ultimate sensitivity of the differential photoacoustic cell were performed with a near-IR room-temperature diode laser using the well-known H₂O absorption line (12496.1056 cm^-1) as a reference. The measured value of the sensitivity (6–17 Pa W m^-1) is in satisfactory agreement with the calculated one, which equals 6–35 Pa W m^-1. The obtained value of the ultimate sensitivity ((3–5)×10^-7 W m^-1 Hz^-1/2) provides measurements of the concentration of molecules at the ppb–ppm level. Received: 19 April 2001 / Revised version: 18 September 2001 / Published online: 7 November 2001     

Miniaturized resonant photoacoustic cell of inclined geometry for trace-gas detection

A photoacoustic cell intended for laser detection of trace gases is represented. The cell is adapted so as to enhance the gas-detection performance and, simultaneously, to reduce the cell size. The cell design provides an efficient cancellation of the window background (a parasite response due to absorption of laser beam in the cell windows) and acoustic isolation from the environment for an acoustic resonance of the cell. The useful photoacoustic response from a detected gas, window background and noise are analyzed in demonstration experiments as functions of the modulation frequency for a prototype photoacoustic cell with the internal volume ∼0.6 cm³. The minimal detectable absorption for the prototype is estimated to be ∼1.2×10⁻⁸ cm⁻¹ W Hz^−1/2.     

Cantilever-based photoacoustic detection of carbon dioxide using a fiber-amplified diode laser

A compact and sensitive photoacoustic setup has been developed based on a recently demonstrated cantilever technique. A micromechanical cantilever transducer is attached to a cylindrical photoacoustic cell and the cantilever’s deflection is monitored with a compact Michelson interferometer. A commercial 1-Watt optical fiber amplifier was used to enhance the performance of the system. A normalized sensitivity of 1.4×10^-10 cm^-1 W Hz^-1/2 was achieved in the detection of carbon dioxide at 1572 nm wavelength. Using 34 mW optical power from a DFB diode laser, the noise-equivalent detection limit for carbon dioxide at this wavelength is 4.0 ppm. Employing the fiber amplifier, we improved the sensitivity to yield measurement of sub-ppm concentrations. PACS 42.62.Fi; 42.55.Px; 82.80.Ch     

Photoacoustic detection of oxygen using cantilever enhanced technique

A tunable diode laser photoacoustic setup based on a recently demonstrated cantilever technique was used for sensitive detection of oxygen. As light sources, we used a distributed feedback (DFB) diode laser and a vertical-cavity surface-emitting (VCSEL) laser, both operating near 760 nm. With the 30 mW DFB laser a noise-equivalent detection limit of 20 ppm for oxygen was obtained, while a detection limit of 5 ‰ was achieved with the VCSEL having an output power of 0.5 mW. Our results yield a noise-equivalent sensitivity of 4.8×10^-9 cm^-1W Hz^-1/2 and demonstrate the potential of this technique for compact and sensitive measurement of oxygen. PACS 42.62.Fi; 42.55.Px; 82.80.Kq     

Real-time monitoring of nitric oxide in diesel exhaust gas by mid-infrared cavity ring-down spectroscopy

We report the accurate and precise measurement of nitric oxide (NO) in automotive exhaust gas by cavity ring-down spectroscopy (CRDS) using a thermoelectrically cooled, pulsed quantum cascade laser (QCL) as a light source. A mid-infrared QCL with a 5.26 μm wavelength was used to detect fundamental vibrational transitions of NO. An effective optical path length of 2.1 km was achieved in a 50 cm long cell using high-reflectivity mirrors. In combination with a particle filter and a membrane gas dryer, stable and sensitive measurement of NO in exhaust gas was achieved for more than 30 minutes with a time resolution of 1 s. The results of this work indicate that a laser based NO sensor can be used to measure NO in exhaust gas over a dynamic range of three orders of magnitude.     

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.     

Relaxation effects and high sensitivity photoacoustic detection of NO<Subscript>2</Subscript> with a blue laser diode

A detection limit of 200 ppt of NO₂ in N₂ at atmospheric pressure was obtained with a photoacoustic detector and a high power blue laser diode. This corresponds to a normalized noise (1σ) equivalent absorption coefficient of 2×10^-9 cm^-1W/Hz^0.5. Measurements at different laser modulation frequencies showed no frequency dependence of the photoacoustic signal, indicating a relaxation time τ < 4 μs. Mixing O₂ into the NO₂ containing gas results in a decrease of the photoacoustic signal. A simple model shows that this effect can be attributed to an increased vibrational-vibrational relaxation of NO₂ to O₂. PACS 31.70.Hq; 34.50.Ez; 42.55.Px     

Mid-infrared laser spectroscopic determination of isotope ratios of N2O at trace levels using wavelength modulation and balanced path length detection

We present a new mid-infrared laser spectrometer for high-precision measurements of isotopic ratios of molecules at ppm concentrations. Results are discussed for nitrous oxide (N₂O), where a precision of 3‰ for a single measurement and a reproducibility of 6‰ have been achieved for a concentration of 825 ppm. The room-temperature laser source employed is based on difference-frequency generation delivering a continuous-wave power up to 23 μW at wavelengths between 4.3 μm and 4.7 μm and a line width of 1 MHz. Two different measurement methods are compared; wavelength modulation with first-harmonic detection and direct absorption spectroscopy by recording the spectrum with a data-acquisition card. Two different detection schemes were employed; either all isotopomers were measured using the long path (36 m) of the multipass cell or a balanced path length detection scheme was used, where the main isotope was measured with a beam along a shorter path (40 cm) in the multipass cell. A single-pass reference cell was designed, offering two different path lengths for balanced path length detection. All combinations of measurement methods and detection schemes were tested regarding precision of a single measurement and long-term stability. The advantages and disadvantages of various measurement approaches are discussed. PACS 42.62.Fi; 42.65.Ky; 32.10.Bi     

Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Pérot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10^-8 W cm^-1/Hz^1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself. PACS 43.60.Vx; 43.58.Wc; 43.58.Hp; 84.40.Xb     

Hidrogeochemical study in South Amazon region using Photoacoustic Spectroscopy

In this study we demonstrate the usefulness of the Photoacoustic Spectroscopy (PAS) in the investigation of water collected from a natural site located within the Amazon region, Brazil, during the wet to dry seasons transition (May/2006). The water samples were collected from different stages along a hydrologic pathway including precipitation water (Prec), groundwater (GW), through flow water (TF), overland flow water (OF), and stream flow water (SW). The observed photoacoustic spectral features, in the 0.3 to 1.0 μm wavelength region, fall within three distinct bands (C, S, and L). We found band-C, band-S and band-L occurring in the spectral range of 0.30 to 0.40 μm, 0.40 to 0.45 μm and 0.45 to 1.0 μm regions, respectively. The photoacoustic features shift peak positions and change intensities for all samples investigated, thus supporting the proposal of PAS as a useful technique to investigate water samples from natural environments.     

Photoacoustic detection of nitric oxide by use of a quantum-cascade laser

A photoacoustic trace-gas sensor for the measurement of nitric oxide with a detection limit of 500 parts in 10(9) has been demonstrated. The radiation source was a thermoelectrically cooled distributed-feedback quantum-cascade laser operating in pulsed mode near 5.3 microm with an average laser power of 8 mW. A resonant photoacoustic cell was excited in its first longitudinal mode by the modulated laser light. Preliminary measurements have been performed to test the performance of our photoacoustic sensor; possible improvements to reach lower detection limits are discussed.     

High-sensitivity detection of NO2 using a 740 nm semiconductor diode laser

 An AlGaAs diode laser was used to detect NO₂ absorption lines belonging to the (0 0 0)–(2 13 1) vibrational band, within the X˜² A ₁ electronic ground state, at 739 nm. A simple absorption spectrometer based on wavelength-modulation spectroscopy with second-harmonic detection was developed. The minimum detectable pressure of pure NO₂ was 0.1 μbar with 2 m absorption path-length, corresponding to an absorbance of 10^-6. High-sensitivity detection of NO₂ was also performed in the presence of N₂ and air at different total pressures: The effects on the detection limit of our apparatus were accurately investigated. The minimum NO₂ concentration at 500 mbar of air was measured to be 2 ppm. Received: 11 June 1996 / Revised version: 11 October 1996     

Absolute, high resolution water transpiration rate measurements on single plant leaves via tunable diode laser absorption spectroscopy (TDLAS) at 1.37 μm

A new sampling-free and calibration-free multi-channel hygrometer using near infrared (NIR) tunable diode laser absorption spectroscopy (TDLAS) at 1.37 μm was developed and used to determine absolute transpiration rates of single plant leafs. Four 8×6× 4 cm³, fiber-coupled absorption cells are used to simultaneously measure absolute water vapor concentrations with an absolute accuracy of about 5% and a temporal resolution of about 2 s. Two chambers (BOTTOM, TOP) are directly attached to the leaf surface, while two chambers (IN, OUT) analyze the purge gas supplied to the plant leaf and the total outflow of the leaf chambers. The BOTTOM–TOP comparison provided a direct, leaf-side resolved ratio of stomatal conductance and–by taking into account the purge gas flow and the leaf area exposed–leaf side resolved water transpiration rates. The OUT–IN-difference yielded the total leaf transpiration rate with 2 μmol/m²/s resolution. The new multi-point hygrometer was validated by monitoring of the transpiration dynamics of a plant of the species Epipremnum pinnatum (L.) Engl. during diurnal variation of the leaf irradiation. During these experiments the differential H₂O concentration resolution between two chambers was determined to be better than 3 ppm at Δt= 2 s (i.e. better than 711 ppb m Hz^1/2). This performance was verified by an Allan analysis over a 30 min time period using CH₄ as a surrogate absorber and yielded an average optimum optical resolution of 4.9×10^-6 for 83 s measurement time, i.e. a CH₄ resolution of 892 ppb, which corresponds to the optical resolution needed for a water sensitivity of 454 ppb m Hz^1/2. PACS  07.57.Ty; 42.62.Fi; 42.62.Be; 42.55.Px; 82.80.Gk     

Thermal performance analysis of vacuum variable-temperature blackbody system

In this paper, the design and structure of a vacuum variable-temperature blackbody system were described, and the steady-state thermal analysis of a 3-D blackbody model was presented. Also, the thermal performance of the blackbody was evaluated using an infrared camera system. The blackbody system was constructed to operate under vacuum conditions (2.67 × 10⁻² Pa) to reduce its temperature uncertainty, which can be caused by vapor condensation at low temperatures usually below 273.15 K. A heat sink and heat shield including a cold shield were embedded around the radiator to maintain the heat balance of the blackbody. A simplified 3-D model of the blackbody including a radiator, heat sink, heat shield, cold shield, and heat source was thermophysically evaluated by performing finite elements analysis using the extended Stefan–Boltzmann’s rule, and the infrared radiating performance of the developed system was analyzed using an infrared camera system. On the basis of the results of measurements and simulations, we expect that the suggested blackbody system can serve as a highly stable reference source for the calibration and measurement of infrared optical systems within operational temperature ranges.     

Intracavity Cr4+ :YAG laser absorption analyzed by time-resolved Fourier transform spectroscopy

A high-resolution time-resolved Fourier transform interferometer is combined with a multimode Cr⁴⁺:YAG laser for intracavity laser absorption spectroscopy (ICLAS) experiments. Atmospheric absorption spectra are recorded in the 1.5 μm region with a minimum detectable absorption coefficient equal to 8×10^-11 cm^-1 Hz^-1/2. The broad gain bandwidth of the crystal allows a simultaneous spectral coverage at most equal to 38 nm. The laser tunability covers the 1360–1577 nm range. Water vapor detection domain extends from the 100 ppmv down to the 0.1 ppbv level. PACS 42.62.Fi; 39.30.+w; 07.60.Ly; 33.20.Ea     

Space-borne remote sensing of CO2, CH4, and N2O by integrated path differential absorption lidar: a sensitivity analysis

CO₂, CH₄, and N₂O are recognised as the most important greenhouse gases, the concentrations of which increase rapidly through human activities. Space-borne integrated path differential absorption lidar allows global observations at day and night over land and water surfaces in all climates. In this study we investigate potential sources of measurement errors and compare them with the scientific requirements. Our simulations reveal that moderate-size instruments in terms of telescope aperture (0.5–1.5 m) and laser average power (0.4–4 W) potentially have a low random error of the greenhouse gas column which is 0.2% for CO₂ and 0.4% for CH₄ for soundings at 1.6 μm, 0.4% for CO₂ at 2.1 μm, 0.6% for CH₄ at 2.3 μm, and 0.3% for N₂O at 3.9 μm. Coherent detection instruments are generally limited by speckle noise, while direct detection instruments suffer from high detector noise using current technology. The wavelength selection in the vicinity of the absorption line is critical as it controls the height region of highest sensitivity, the temperature cross-sensitivity, and the demands on frequency stability. For CO₂, an error budget of 0.08% is derived from our analysis of the sources of systematic errors. Among them, the frequency stability of ± 0.3 MHz for the laser transmitter and spectral purity of 99.9% in conjunction with a narrow-band spectral filter of 1 GHz (FWHM) are identified to be challenging instrument requirements for a direct detection CO₂ system operating at 1.6 μm. PACS 42.68.Wt; 95.75.Qr     

Optimization of a compact photoacoustic quantum cascade laser spectrometer for atmospheric flux measurements: application to the detection of methane and nitrous oxide

Room temperature (RT) quantum cascade lasers (QCL) are now available even in continuous wave (cw) mode, which is very promising for in situ gas detectors. Ambient air monitoring requires high sensitivity with robust and simple apparatus. For that purpose, a compact photoacoustic setup was combined with two cw QCLs to measure ambient methane and nitrous oxide in the 8 μm range. The first laser had already been used to calibrate the sensitivity of the photoacoustic cell and a detection limit of 3 ppb of CH₄ with a 1s integration time per point was demonstrated. In situ monitoring with this laser was difficult because of liquid nitrogen cooling. The second laser is a new RT cw QCL with lower power, which enabled one to reach a detection limit of 34 ppb of methane in flow. The loss in sensitivity is mainly due to the weaker power as photoacoustic signal is proportional to light power. The calibration for methane detection leads to an estimated detection limit of 14 ppb for N₂O flux measurements. Various ways of modulation have been tested. The possibility to monitor ambient air CH₄ and N₂O at ground level with this PA spectrometer was demonstrated in flux with these QCLs. PACS 07.88; 92.60.Sz     

The potential of CO2 laser photoacoustic spectrometry for?detection of methanol in alcoholic beverage

The first use of CO₂ laser photoacoustic measurements for detecting the methanol contents in alcohol-like solutions is presented. With an intracavity cell configuration, the minimum detectable concentration was ∼200 ppm for methanol and the linear range of the calibration curve for methanol was from 200 to 70000 ppm. For demonstrating the reliability of analysis in alcoholic beverages, a series of different concentrations of two-component samples was prepared and measured by the same procedures. The results showed the feasibility on determining methanol and ethanol contents accurately within a specific tolerance, limited mainly by background signal and laser stability. This potential method with no pre-treatment of samples takes only ∼10 min to finish one single measurement. It suggests that the PA detection is suitable for routine diagnosis of adulterated wines in commercial products.     

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

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 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm^-1 (300 MHz) 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 0.12 ppb Hz^-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10^-5 Hz^-1/2 or an absorbance precision per unit path length of 5×10^-10 cm^-1 Hz^-1/2. A precision of 0.3 ppb 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. Received: 1 May 2002 / Revised version: 6 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/663-4918, E-mail: ddn@aerodyne.com