Compact hollow waveguide mid-infrared gas sensor for simultaneous measurements of ambient CO2 and water vapor

A compact, sensitive and stable hollow waveguide (HWG) mid-infrared gas sensor, based on gas absorption lines using wavelength modulation spectroscopy with a second harmonic (WMS-2f) detection scheme, was developed for simultaneous measurements of ambient CO 2 and water vapor. Optimization of the laser modulation parameters and pressure parameter in the HWG are performed to improve the strength of the WMS-2f signal and hence the detection limit, where 14.5-time for CO 2 and 8.5-time for water vapor improvement in system detection limit is achieved compared to those working at 1 atm. The stability of the sensor has been improved significantly by optimizing environmental disturbances, incoupling alignment of the HWG and laser scanning frequency. An Allan variance analysis shows detection limit of the developed sensor of ~3 ppmv for CO 2 and 0.018% for water vapor, which correspond to an absorbance of 2.4 × 10 -5 and 2.7 × 10 -5 , with a stability time of 160 s, respectively. Ambient CO 2 and water vapor measurement have been performed in two days in winter and spring separately. The measurement precision is further improved by applying a Kalman adaptive filter. The HWG gas sensor demonstrates the ability in environmental monitoring and the potential to be used in other areas, such as industrial production and biomedical diagnosis

Influence of light coupling configuration and alignment on the stability of HWG-based gas sensor system for real-time detection of exhaled carbon dioxide

A mid-infrared tunable diode laser absorption spectroscopy (TDLAS) gas sensor based on hollow waveguide (HWG) gas cell for real-time exhaled carbon dioxide (eCO2) detection is reported. A 2.73 μm distributed feedback (DFB) laser was used to target a strong CO2 absorption line, and wavelength modulation spectroscopy (WMS) with the second harmonic (WMS-2.) was used to retrieve the CO2 concentration with high sensitivity. The influence of different parameters, including coupling configuration of HWG, laser-to-HWG and HWG-to-detector coupling alignment on the stability of the HWG sensor is systematically studied. The HWG eCO2 sensor showed a fast response time of 2.7s, detection limit of 17 ppmv, and measurement precision of 20.9 ppmv with a 0.54 s temporal resolution. The eCO2 concentrations changed in breath cycles were measured in real time. The Allan variance indicated that the detection limit can reach 1.7 ppmv, corresponding to a detection sensitivity of 1.3(215)10-8 cm-1Hz-1/2, as the integration time increases to 26 s. This work demonstrates the performance characteristics and merits of HWG eCO2 sensor for exhaled breath analysis and potential detection for other exhaled gases

Laser Spectroscopy for Atmospheric and Environmental Sensing

Lasers and laser spectroscopic techniques have been extensively used in several applications since their advent, and the subject has been reviewed extensively in the last several decades. This review is focused on three areas of laser spectroscopic applications in atmospheric and environmental sensing; namely laser-induced fluorescence (LIF), cavity ring-down spectroscopy (CRDS), and photoluminescence (PL) techniques used in the detection of solids, liquids, aerosols, trace gases, and volatile organic compounds (VOCs)

Advanced laser based spectroscopic techniques for trace gas detection based on optical cavity enhancement and multipass absorption cells

In this thesis, three advanced experiments based on laser spectroscopy are introduced for the first time which address several experimental topics for trace gas analysis. The implementation of a diode laser based gas phase Raman detector is introduced, capable of parts per million (ppm) detection limits. The spectrometer features a low power laser diode (10 mW) which is enhanced by power build up in an optical cavity. This new technique is characterised by recording spectra of N2, O2, H2, CH4 and benzene. A second advanced laser based spectroscopy technique for trace gas detection, mid infrared cavity enhanced resonant photoacoustic spectroscopy (mid-IR CERPAS) is set up and characterised. This scheme uses optical cavity power build-up, optical feedback stabilisation and resonant photoacoustics. A single-mode continuous wave quantum cascade laser is coupled to a three mirror V-shape optical cavity. Gas phase species absorbing in the mid-IR are detected using the photoacoustic (PA) technique. Mid-IR CERPAS was characterised by measuring acetylene (limit of detection 50 ppt) and nitromethane (0.8 ppb). The mid-IR CERPAS equipment was also used to detect explosives’ vapours; TNT (1.2 and 5.5 ppb), 2, 4-DNT (7 ppb), TATP (4 ppb) and explosives’ taggants such as DMNB (11 ppb). Significant interferences from ambient water in lab air are observed and are overcome. Normalized noise-equivalent absorption coefficients are determined as » 6 x 10-10 cm-1 s1/2 (1 s integration time) and 6 x 10-11 cm-1 s1/2 W (1 s integration time and 1 W laser power). Finally, a near infrared Herriott cell enhanced resonant photoacoustic spectroscopy spectrometer is set up and characterised. This scheme uses enhancement from the absorption pathlength by a multipass Herriott cell and detection of the gas phase species by resonant photoacoustics, Herriott cell enhanced resonant photoacoustics, HERPAS. A single-mode continuous wave near infrared external cavity diode laser is coupled to a Herriott cell. Absorbing gas phase species are detected using the photoacoustic (PA) technique which was characterised by measuring acetylene (150 ppb detection limit at 100 ms integration time). HERPAS was extended to measure several toxic industrial gases including hydrogen sulfide, ammonia and carbon monoxide. Normalized noise-equivalent absorption coefficients are determined for H2S as » 5.3 x 10-9 cm-1 s1/2 (1 s integration time) and 1.6 x 10-10 cm-1 s1/2 W (1 s integration time and 1 W laser power). These three novel advanced spectroscopic techniques allow the detection of IR-inactive and IR- active gas phased species with great sensitivities and selectivity and improve significantly current capabilities for trace gas phase detection

Optical Waveguides for Infrared Spectroscopic Detection of Molecular Gases

Fields like medical diagnostics, urban and industrial environmental monitoring or basic microbiological research greatly benefit from advances in chemical and biological sensing. These applications require rapid sample analysis, reduced needs for sample handling, or good sensor network. Such demands can be met with miniaturised sensors utilising methods which secure sufficient sensitivity and selectivity such laser absorption spectroscopy. However, such instruments are nowadays bulky, expensive, and require large sample volumes. Optical waveguides, a cornerstone of integrated opto-chemical sensors, are aiming at replacing current bulky and costly instrumentation based on free-space optics. They can realize large optical interaction pathlengths as well as provide simple functions of beam splitting or combining on a compact photonic chip, thus substituting e.g., multi-pass cells, Fabry-Perot cavities, or free-space interferometers such as those in FTIR instruments. To achieve competitive sensitivities, however, the waveguide device needs to meet two criteria: Low loss to allow long interaction paths, and large light–analyte interaction per unit length. This thesis presents the analysis, fabrication methods, and characterisation of optical waveguides for infrared tuneable diode laser absorption spectroscopy. A free standing waveguide for use in the mid-infrared spectral domain was developed to tackle the challenges above. Moreover, the waveguide features negligible etalon fringes in transmission, which otherwise interfere with measured spectral signatures. Compared to a free space beam, an outstanding 7 % stronger light-analyte interaction strength was measured with the waveguide

Kinetic Studies of the Gas Phase CH2OO Criegee Intermediate Relevant to Atmospheric Chemistry Using Time-Resolved UV and IR Absorption Spectroscopy

The chemistry of Criegee intermediates, produced in the atmosphere via oxidation of unsaturated volatile organic compounds by ozone, has potentially important impacts on atmospheric composition and thus on air quality and climate. In recent years, there have been significant advances in our understanding of the properties and chemistry of Criegee intermediates following the advent of photolytic sources for use in laboratory experiments. Since the discovery of this photolytic production method of Criegee intermediates, various methods have been employed for their detection, which have yielded vast information on Criegee spectra and the kinetics of Criegee reactions with other atmospheric species. However, discrepancies persist in Criegee intermediate spectra, rate coefficients of Criegee intermediate reactions and also in product yields. In this work, the UV absorption cross sections of the simplest Criegee intermediate CH2OO, and kinetics of the CH2OO self reaction and the reaction of CH2OO with I are reported as a function of pressure. Measurements were made at 298 K using 248 nm pulsed laser flash photolysis of CH2I2/O2/N2 gas mixtures coupled with time resolved broadband UV absorption spectroscopy at pressures between 6 and 300 Torr. Results give a peak absorption cross¬ section of (1.37 ± 0.29) × 10-17 cm2 at ~340 nm and a rate coefficient for the CH2OO self reaction of (8.0 ± 1.1) × 10-11 cm3 s-1, with no significant pressure dependence of the absorption cross sections or the self reaction kinetics over the range investigated. On the contrary, the rate coefficient for the reaction between CH2OO and I demonstrates pressure dependence over the range investigated, with a Lindemann fit giving k0 = (4.4 ± 1.0) × 10-29 cm6 s-1 and k∞ = (6.7 ± 0.6) × 10-11 cm3 s-1. The origins of IO in the system have been investigated, the implications of which are discussed. Additionally, the CH2OO + SO2 reaction at room temperature was selected to develop and characterise a robust and economical instrument that can be applied to a wide range of problems in atmospheric chemistry and beyond. The development, characterisation and initial results from the experiment using 266 nm pulsed laser flash photolysis coupled with time resolved mid IR quantum cascade laser (QCL) absorption spectroscopy are reported. The IR absorption spectrum of CH2OO and rate coefficient of the CH2OO + SO2 reaction with respect CH2OO loss and SO3 production are reported at 298 K and pressures in the range 20-100 Torr. Results indicate the CH2OO spectrum to be in good agreement with that of a previously reported measurement in terms of relative peak heights and positions in the wavenumber region 1285.5917-1286.0605 cm-1, and a rate coefficient for the CH2OO + SO2 reaction of (3.8 ± 0.5) × 10-11 cm3 s-1 from both CH2OO and SO3 measurements, with no significant pressure dependence over the pressure range investigated. The product yield of SO3 from the reaction of CH2OO + SO2 is determined to be independent of pressure over the range investigated, with an SO3 absorption cross section of (5.5 ± 2.3) × 10-19 cm2 at ~1388.7 cm-1

Energy: A continuing bibliography with indexes

This bibliography lists 1920 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System from July 1, 1980 through September 30, 1980