Soil respiration analysis using a mid-infrared quantum cascade laser and calibration-free WMS-based dual-gas sensor

Re-injection off-axis integrated cavity output spectroscopy for the simultaneous detection of N2O, H2O and CO with a mid-infrared QCL laser

Off-axis integrated cavity output spectroscopy (OA-ICOS) has attracted much interest because it potentially allows highly sensitive field measurements with robust optical alignment. In this paper, a novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of multi-component gases N2O, H2O and CO in the atmosphere based on a mid-infrared quantum cascade laser (QCL) and OA-ICOS. In order to improve the energy utilization and increase the signal-to-noise ratio (SNR) of the signal, a new type of optical path structure of the laser re-injection method is adopted. Furthermore, the system performance can be effectively improved by using a new intervention method of injecting radio frequency (RF) white noise into a laser driver to suppress cavity mode noise and combining the wavelength modulation method (WMS). We compared the sensitivity of the second harmonic signal demodulation between the re-injection method and the standard OA-ICOS, and the SNR increased by 2.68 times compared to the latter. Analysis of the spectral measurements with Allan variance indicates that within an integration time of 1 s, the measurement accuracy of N2O, H2O, and CO is 6.71 ppb, 13.945 ppm, and 1.81 ppb, respectively, and within an integration time of 820 s, the measurement accuracy of N2O, H2O, and CO can be further improved to 1.26 ppb, 2.089 ppm, and 172 ppt, respectively. Our approach represents an underlying analytical method that provides guidelines for monitoring of representative gases in the atmosphere, industrial processes, emergency safety, etc.

Frequency division multiplexing and wavelength stabilized 2f/1f wavelength modulation spectroscopy for simultaneous trace CH4 and CO2 detection

A near-infrared (NIR) dual-gas sensor has been developed for simultaneous detection of atmospheric trace methane (CH4) and carbon dioxide (CO2). To realize high sensitivity and high precision, wavelength modulation spectroscopy with 2f/1f (WMS-2f/1f) detection method was adopted for eliminating laser light intensity fluctuation, and laser wavelength locking strategy based on a self-developed proportion integration differentiation (PID) algorithm was used for suppressing laser wavelength shifting effect. Two fiber-coupled DFB diode lasers with central wavelengths near 1653.7 nm and 1579.6 nm are applied for simultaneously measuring CH4 and CO2 spectra, respectively, and frequency division multiplexing (FDM) technique is employed to resolve the potential crosstalk effect. Real-time measurement of ambient atmospheric trace CH4 and CO2 was performed to demonstrate the long-term stability of the sensor system. Allan deviation analysis indicates that detection sensitivity of 0.1 ppm for CH4 and 2.27 ppm for CO2 was achieved with a 1 s average time, which can be further improved to 18 ppb and 0.3 ppm with the optimal integration time of 462 s and 392 s, respectively.

Development of a mid-infrared sensor system for early fire identification in cotton harvesting operations

To realize early fire identification in cotton harvesting operations, a mid-infrared carbon monoxide (CO) sensor system was developed. To match the broadband light source with a 15° divergence angle, a multipass gas cell (MPGC) with an effective path length of 180 cm was designed to improve sensor sensitivity, leading to a limit of detection (LoD) of 0.83 parts-per-million by volume (ppmv). A damping module with springs at the bottom and front/back sides was fabricated, which can effectively reduce the vibration intensity by >80%. The sensor system can operate normally from -40 °C to 85 °C by stabilizing the temperature of the optical module through heating or cooling as well as using automotive electronic components. An adaptive early fire identification algorithm based on a dual-parameter threshold alarming method was proposed to avoid false and missing alarms. Field deployments on a harvester verified the good practicability of the sensor system.

Recovery integral absorbance method in the full concentration range to eliminate the interference of background gas

At present, gas sensors are extremely susceptible to interference from background gases in the field environment, which leads to greatly reduced accuracy. For this reason, we propose an improved method of recovering integral absorbance (IA) using Y component of first harmonic to achieve accurate prediction of the full range of concentration (not reaching absorption saturation). This approach can eliminate the interference of background gas at a low modulation depth (m < 0.25). When the background gas is pure nitrogen and a mixture of nitrogen and carbon dioxide, the recovery effect of this method on methane is both close to the theoretical value when the background gas is air. The linear fitting coefficients for the methane concentration range of 2000-7000 ppm are all greater than 0.999. The prediction effect is satisfactory regardless of the background gas, with a relative error of less than 1%. In summary, this method has considerable application prospects.

A trace CH4 detection system based on DAS calibrated WMS technique

We report the development of a compact near-infrared (NIR) laser-based trace methane (CH₄) detection system. This detection system relied on a 2334 nm distributed feedback (DFB) fiber laser as the light source. A parallel dense light-spot pattern multipass gas cell (MGC) with 41.5 m effective absorption path length was utilized to improve the system sensitivity. A self-calibration approach based on direct absorption spectroscopy (DAS) calibrated wavelength modulation spectroscopy (WMS) technique was employed to solve the problem of extra concentration calibration requirement in traditional WMS technique, and to improve the accuracy and stability of the system. According to the Allan deviation analysis, 1-s measurement precision of 0.61 ppmv for DAS and 0.16 ppmv for WMS was obtained, which could be further reduced to 0.11 ppmv for DAS and 0.03 ppmv for WMS by averaging up to 80 s and 50 s, respectively. A week-long continuous atmospheric CH₄ concentration measurement was also carried out to demonstrate the long-term performance of our CH₄ detection system. With a fast dynamic response characteristics, high-accuracy and high-sensitivity, the proposed detection system is suitable for CH₄ measurement in many fields such as atmospheric chemistry analyzation, industrial safety monitoring, agricultural information acquisition, etc.