RESUMEN
A high response and sensitive dual-gas sensor based on calibration-free wavelength modulation spectroscopy (CF-WMS) has been developed for the simultaneous detection of carbon monoxide (CO) and nitrous oxide (N2O) to eliminate the detection errors caused by light intensity variations. A multi-pass cell (MPC) was employed to lengthen the optical path to improve the precision of the sensing system by combining with a 4.56 µm mid-infrared quantum cascade laser (MIR-QCL). Meanwhile, a LabVIEW-based bi-molecular iterative fitting algorithm was used to infer the respective abundances of each species. The performance of the completed system was accurately evaluated with precisions of 3.4 ppb for CO and 3.8 ppb for N2O at a 1 s averaging time, which could be improved to 0.48 ppb for CO and 0.53 ppb for N2O at an averaging time of 154 s and 278 s, respectively. The grassland soil respiration analysis of CO and N2O was performed under different moisture conditions, which indicated that dried soil samples appeared to be a significant source of CO, while the sinks of CO and the sources of N2O occurred in the moist soil samples. The maximum exchange rates of the two gases were exhibited in moderate moisture soil samples rather than in the over-wet or arid soil samples. Moreover, a possible positive relationship between the sinks of CO and sources of N2O was established to illustrate the correlation of the two species in soil respiration.
RESUMEN
A compact multigas sensor system based on a single quartz crystal tuning fork (QCTF) and multifrequency synchronous modulation strategy is proposed for trace gas detection. To demonstrate the novel detection technique, three near-infrared continuous-wave (CW) distributed feedback (DFB) diode lasers with center wavelengths of near 1391, 1574, and 1653 nm and a standard 32 kHz QCTF were integrated for simultaneous detection of H2O, CO2, and CH4, respectively. Wavelength modulation spectroscopy with second harmonic detection (WMS-2f) was selected for enhancing sensitivity. Design of the sensor configuration and primary performance between the traditional single-frequency modulation and the proposed tri-frequency modulation were experimentally investigated and compared in detail. The results indicate that the proposed sensing technique has significant advantages of cost effectiveness, portability, and ease-of-use, and detection limits of 1.4, 353, and 3.1 ppm for simultaneously measuring H2O, CO2, and CH4, respectively, are obtained, corresponding to the normalized noise equivalent absorption (NNEA) coefficients of 2.65 × 10-10, 8.09 × 10-10, and 8.28 × 10-10 cm-1 W/âHz, respectively. Moreover, the use of an erbium-doped fiber amplifier (EDFA) has been demonstrated as an effective method for sensitivity enhancement.
RESUMEN
An innovative trace gas-sensing technique utilizing a single quartz crystal tuning fork (QCTF) based on a photoelectric detector and dual-frequency modulation technique was demonstrated for the first time for simultaneous multi-species detection. Instead of traditional semiconductor detectors and lock-in amplifier, we utilized the piezoelectric effect and resonant effect of the QCTF to measure the light intensity. A fast signal analysis method based on fast Fourier transform (FFT) algorithm is proposed for overlapping signal extraction. To explore the capabilities of this technique, a gas-sensing system based on two lasers having center emission wavelength of 1.653 µm (a DFB laser diode in the near-IR) and 7.66 µm (an EC QCL in the mid-IR) is successfully demonstrated for simultaneous CH4 spectroscopy measurements. The results indicate a normalized noise equivalent absorption (NNEA) coefficients of 1.33×10-9 cm-1W·Hz-1/2 at 1.653 µm and 2.20×10-10 cm-1W·Hz-1/2 at 7.66 µm, were achieved. This proposed sensor architecture has the advantages of easier optical alignment, lower cost, and a compactness compared to the design of a conventional TDLAS sensor using multiple semiconductor detectors for laser signal collection. The proposed technique can also be expanded to common QEPAS technique with multi-frequency modulation for multiple species detection simultaneously.
RESUMEN
A dual-optimized adaptive Kalman filtering (DO-AKF) algorithm based on back propagation (BP) neural network and variance compensation was developed for high-sensitivity trace gas detection in laser spectroscopy. The BP neural network was used to optimize the Kalman filter (KF) parameters. Variance compensation was introduced to track the state of the system and to eliminate the variations in the parameters of dynamic systems. The proposed DO-AKF algorithm showed the best performance compared with the traditional multi-signal average, extended KF, unscented KF, KF optimized by BP neural network (BP-KF) and KF optimized by variance compensation (VC-KF). The optimized DO-AKF algorithm was applied to a QCL-based gas sensor system for an exhaled CO analysis. The experimental results revealed a sensitivity enhancement factor of 23. The proposed algorithm can be widely used in the fields of environmental pollutant monitoring, industrial process control, and breath gas diagnosis.
RESUMEN
A compact gas sensing system based on the quartz crystal tuning fork (QCTF) and 2f/1f wavelength modulation spectroscopy (2f/1f-WMS) technique was reported for the first time. An ingenious laser modulation strategy and frequency division multiplexing demodulation algorithm were developed for realizing a single QCTF to detect the first harmonic and second harmonic signals simultaneously. The influence of laser power change and excitation position the QCTF based 2f/1f-WMS technique was first investigated in detail. The results show that, compared with the traditional QCTF-2f method, the reported QCTF based 2f/1f technique has better immunity. To further evaluate this sensing technique, real-time monitoring of ambient water vapor (H2O) was made, the results show that the developed QCTF based 2f/1f-WMS technique can effectively improve the long-term stability and has super anti-interference ability to various environmental disturbance factors, such as light beam jitter, airflow fluctuation, and mechanical vibration, which proves it has great potential in practical field applications, especially for harsh environment.
Asunto(s)
Rayos Láser , Cuarzo , Luz , Análisis Espectral , VibraciónRESUMEN
A compact multi-gas sensor has been developed for simultaneous detection of atmospheric carbon monoxide (CO), nitrous oxide (N2O), and methane (CH4). Instead of the traditional time-division multiplexing detection technique, two lasers having center emission wavelengths of 1.653 µm (near-infrared (NIR) diode feedback (DFB) laser diode) and 4.56 µm (mid-infrared (MIR) quantum cascade laser) were simultaneously coupled to a multipass cell using a dichroic mirror, which significantly decreased the complexity of the measurement and increased the temporal resolution of the spectrometer. Wavelength modulation spectroscopy (WMS) with the second-harmonic detection technique (WMS-2f) was used to improve the detection sensitivity. A LabVIEW-based digital lock-in amplifier (DLIA) algorithm and system control unit was developed to make the system more compact and flexible. Allan deviation analysis indicates that detection limits of 6.36 ppb by volume for CO, 4.9 ppb by volume for N2O, and 23.6 ppb by volume for CH4 are obtained at 1 s averaging time, and the sensitivity can be improved to 0.44 ppb for CO, 0.41 ppb for N2O, and 2 ppb for CH4 at an optimal averaging time of 900 s. Two-day real-time measurement in ambient air was performed to demonstrate the long-term stability of the sensor system.
Asunto(s)
Metano , Óxido Nitroso , Monóxido de Carbono , Rayos Láser , Análisis EspectralRESUMEN
A compact high-resolution spectroscopic sensor using a thermoelectrically (TE) cooled continuous-wave (CW) room temperature (RT) quantum cascade laser (QCL) was demonstrated for simultaneous measurements of exhaled carbon monoxide (CO) and nitrous oxide (N2O). The sampling pressure was optimized to improve the sensitivity, the optimal pressure was determined to be 150â¯mbar based on an optical density analysis of simulated and measured absorption spectra. An adaptive Kalman filtering algorithm based on back-propagation (BP) neural network was developed and proposed for real-time exhaled breath analysis in order to perform fast and high precision on-line measurements. The detection limits (1σ) of 1.14â¯ppb and 1.12â¯ppb were experimentally achieved for CO and N2O detection, respectively. Typical concentrations of exhaled CO and N2O from smokers and non-smokers were analyzed. The experimental results indicated that the state-of-the-art CW-QCL based sensor has a great potential for non-invasive, on-line identification and quantification of biomarkers in human breath.
Asunto(s)
Algoritmos , Monóxido de Carbono/análisis , Espiración , Redes Neurales de la Computación , Óxido Nitroso/análisis , Contaminación del Aire Interior/análisis , Humanos , Presión , Fumar , Análisis Espectral , AguaRESUMEN
A compact high-resolution spectroscopic sensor using a thermoelectrically (TE) cooled continuous-wave (CW) room temperature (RT) quantum cascade laser (QCL) operating at 4.6⯵m, is employed for simultaneous detection of three main isotopic species (14N15N16O, 15N14N16O and 14N14N16O). To enable a high-precision analysis of N2O isotopic species at ambient mixing ratios, a liquid nitrogen-free preconcentration unit is built to trap and load atmospheric N2O. The absorption spectra of 14N15N16O, 15N14N16O, and 14N14N16O between 2188.6â¯cm-1 and 2189â¯cm-1 are measured, and the respective ratios of the rare to the abundant isotopologues abundances are demonstrated. Moreover, spectroscopic parameters of pressure-broadening coefficient for selected absorption lines have been determined, and a good agreement is obtained by comparing with HITRAN database.
RESUMEN
A laser spectroscopy system based on a broadband tunable external cavity quantum cascade laser (ECQCL) and a mini quartz crystal tuning fork (QCTF) detector was developed for standoff detection of volatile organic compounds (VOCs). The self-established spectral analysis model based on multiple algorithms for quantitative and qualitative analysis of VOC components (i.e. ethanol and acetone) was detailedly investigated in both closed cell and open path configurations. A good agreement was obtained between the experimentally observed spectra and the standard reference spectra. For open path detection of VOCs, the sensor system was demonstrated at a distance of 30m. The preliminary laboratory results show that standoff detection of VOCs at a distance of over 100m is very promising.
RESUMEN
A new investigation of acetylene absorption lines between 6526.5 and 6531.5cm-1 spectral region was performed by using a long-path absorption cell based tunable diode laser absorption spectrometer. The multi-spectrum fitting procedure has been applied to these intensive absorption lines of acetylene within the spectral range of particular interest to recover the line parameters. Line intensities and line positions of total 42 lines of acetylene were reported, including 22 new lines precisely identified for the first time. The reported results will be valuable to complete the spectroscopic databases of acetylene, and also be useful for upgrading our newly developed TDLAS sensor system for industrial C2H2 gas detection.
RESUMEN
We developed a laser absorption sensor based on a pulsed, broadband tunable external cavity quantum cascade laser (ECQCL) centered at 1285 cm-1. Unlike traditional infrared spectroscopy system, a quartz crystal tuning fork (QCTF) as a light detector was used for laser signal detection. Fast Fourier transform was applied to extract vibration intensity information of QCTF. The sensor system is successfully tested on nitrous oxide (N2O) spectroscopy measurements and compared with a standard infrared detector. The wide wavelength tunability of ECQCL will allow us to access the fundamental vibrational bands of many chemical agents, which are well-suited for trace explosive, chemical warfare agent, and toxic industrial chemical detection and spectroscopic analysis.