Simultaneous measurement of NO and NO(2) by dual-wavelength quantum cascade laser spectroscopy

Non-uniform line-of-sight measurements of nitric oxide using mid-infrared Faraday rotation spectroscopy

Traditional absorption spectroscopy relies on detecting intensity variations along the line-of-sight to gauge average concentration and temperature. While methods like profile fitting and temperature binning offer insights into the non-uniformity of the path, they fall short of accurately capturing the precise spatial distribution with a single line-of-sight measurement. We propose a novel measurement scheme for non-uniformly distributed concentration of nitric oxide (NO) along the line-of-sight utilizing a single laser and path, by incorporating Faraday rotation spectroscopy with magnetic fields changing over time and space. We validate the proposed scheme by measuring a path of two regions in series with different NO concentrations, and comparing the measurement results with direct absorption spectroscopy of each respective region. In this work, the tuning range of the interband cascade laser used is from 1899.42 to 1900.97 cm-1, encompassing two sets of spectral lines corresponding to the 2Π1/2 and 2Π3/2 transitions of NO's R(6.5). The average relative uncertainty in the concentration measurement for each region is estimated to be within 1.5%, with the concentration for individual absorption cells ranging from 0.2% to 0.8%.

Fully Integrated Photoacoustic NO2 Sensor for Sub-ppb Level Measurement

A fully integrated photoacoustic nitrogen dioxide (NO₂) sensor is developed and demonstrated. In this sensor, an embedded photoacoustic cell was manufactured by using an up-to-date 3D printing technique. A blue laser diode was used as a light source for excitation of photoacoustic wave in the photoacoustic cell. The photoacoustic wave is detected by a sensitive microelectromechanical system (MEMS) microphone. Homemade circuits are integrated into the sensor for laser diode driving and signal processing. The sensor was calibrated by using a chemiluminescence NO–NO₂–NO_X gas analyzer. And the performance of this sensor was evaluated. The linear relationship between photoacoustic signals and NO₂ concentrations was verified in a range of below 202 ppb. The limit of detection was determined to 0.86 ppb with an integration time of 1 s. The corresponding normalized noise equivalent absorption was 2.0 × 10⁻⁸ cm⁻¹∙W∙Hz^−1/2. The stability and the optimal integration time were evaluated with an Allan deviation analysis, from which a detection limit of 0.25 ppb at the optimal integration time of 240 s was obtained. The sensor was used to measure outdoor air and the results agree with that obtained from the NO–NO₂–NO_X gas analyzer. The low-cost and portable photoacoustic NO₂ sensor has a potential application for atmospheric NO₂ monitoring.

Demonstration of mid-infrared gas sensing using an anti-resonant hollow core fiber and a quantum cascade laser

In this paper we report a mid-infrared sensor based on an anti-resonant hollow core fiber. A quantum cascade laser operating around 4.53 µm is used to target one of the strongest transition of nitrous oxide near 2203.7 cm^-1. The system provides 1-second minimum detection limit at single parts-per-billion level using 3.2-m-long fiber with the response time of less than 30 seconds. Presented sensing approach shows a good perspective for compact and sensitive mid-infrared fiber-based spectrometers.

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.

RES-Q-Trace: A Mobile CEAS-Based Demonstrator for Multi-Component Trace Gas Detection in the MIR

Sensitive trace gas detection plays an important role in current challenges occurring in areas such as industrial process control and environmental monitoring. In particular, for medical breath analysis and for the detection of illegal substances, e.g., drugs and explosives, a selective and sensitive detection of trace gases in real-time is required. We report on a compact and transportable multi-component system (RES-Q-Trace) for molecular trace gas detection based on cavity-enhanced techniques in the mid-infrared (MIR). The RES-Q-Trace system can operate four independent continuous wave quantum or interband cascade lasers each combined with an optical cavity. Twice the method of off-axis cavity-enhanced absorption spectroscopy (OA-CEAS) was used, twice the method of optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS), respectively. Multi-functional software has been implemented (i) for the general system control; (ii) to drive the four different laser sources and (iii) to analyze the detector signals for concentration determination of several molecular species. For the validation of the versatility and the performance of the RES-Q-Trace instrument the species NO, N₂O, CH₄, C₂H₄ and C₃H₆O, with relevance in the fields of breath gas analysis and the detection of explosives have been monitored in the MIR with detection limits at atmospheric pressure in the ppb and ppt range.