Using Laser Spectroscopy Gas Sensing Technology
The use of laser spectroscopy gas sensing technology is crucial in several sectors, including biomedical diagnostics, ocean exploration, Mars exploration, and climate change. However, the ability to analyze multiple unknown components has been a highly technical challenge for developing optical gas sensors, considering system cost, power consumption, weight, and volume restrictions.
This is due to the ongoing deterioration of the global environmental ecology and energy problems. Due to their high price, mid-infrared (MIR) external cavity quantum cascade lasers (ECQCLs), which may be used to measure more than three gas components, are still not frequently employed in spectroscopic applications.
Near-infrared lasers have improved with the advancement of optical communication technology, particularly distributed feedback (DFB) diode lasers. These lasers have been shown to work with the majority of atmospheric molecules by combining them with various spectroscopy techniques, including cavity-enhanced absorption spectroscopy (CEAS), cavity ring-down spectroscopy (CRDS), off-axis integrated-cavity output spectroscopy (OA-ICOS), photoacoustic spectroscopy (PAS), wavelength modulation spectroscopy (WMS), and direct absorption spectroscopy (DAS).
Different Multi-Component Analysis Techniques
For multi-component analysis, a single broadband laser source or multilaser array is often used in conjunction with several photoelectric detectors or frequency-division multiplexing (FDM) temporal division multiplexing (TDM) techniques to cover the whole spectral range of interest.
For the simultaneous detection of CH4, CO2, and H2O, for instance, a small multi-gas sensor system based on several diode lasers around 1653, 1574, and1391 nm and a single quartz crystal tuning fork (QCTF) detector was developed.
It is also possible to identify air trace gases by analyzing the absorption spectroscopy in the NIR or MIR spectral regions. The NIR spectral gas sensors are favored because they can satisfy all field application criteria for cost, size, and power.
FDM Integrated with WMS
Due to the dependency of wavelength modulation spectroscopy (WMS) on variations in light intensity, which need regular adjustments to assure accuracy for real-time measurement applications, the FDM methodology is often integrated with the WMS detection method. However, one drawback of the TDM-based multilaser array technology is a reduction in temporal resolution, and the complexity of the gas sensor system is increased by the simultaneous use of many detectors or gas cells and other devices.
Novel Gas Sensing Method
A new gas detection approach based on fiber optical sensing and calibration-free DAS spectroscopy analysis is suggested in this study for the simultaneous detection of several gas component molecules to address the drawback described above.
Two fiber-coupled NIR DFB diode lasers that emit around 1653 nm and 1574 nm simultaneously measure the CH4 and CO2 absorption spectra to examine the proposed gas's properties detection method.
In-depth theoretical and practical research is carried out on the ideal sampling pressure, laser tuning characteristics, spectrum sampling spots, and probable optical interference. The constructed sensor system is then tested for measuring ambient air conditions.
Multi-Gas Sensor System's Configuration
The designed multi-gas sensor system's configuration comprises data collection, system control, and optical modules. Through a 2x1 fiber coupler and an optical fiber collimator, two fiber output DFB diode lasers with center wavelengths of approximately 1574 nm and 1653 nm, respectively, are directly linked into a single beam in the optical module.
The laser beam is then precisely adjusted to enter a multi-pass gas sample Herriott cell. It was focused on a photodetector after leaving the long-path gas cell via a gold-plated parabolic mirror.
All optical components were installed on an aluminum breadboard. A pressure meter, a few two- and three-way valves, two flow meters, an oil-type diaphragm pump and several standard gas cylinders for system calibration made up the bulk of the gas handling module.
System control and data collection were performed using a computer and a data acquisition (DAQ) I/O card. A LabVIEW-based digital lock-in amplifier handled the real-time signal processing and analysis.
Significant Findings of the Study
The findings showed that the designed laser absorption spectroscopy sensor is reliable and has been successfully tested for ambient CH4 and CO2 detection utilizing a single detector with no time lag. According to the Allan-Werle deviation analysis, the detection limits for CO2 and CH4 at a 1-second average time are 0.82 ppm and 447.68 ppm, respectively.
For CH4, the measurement sensitivity may be increased to 0.12 ppm at an ideal average time of 181 seconds, and for CO2, it can be increased to 35.97 ppm at an ideal averaging time of 166 seconds.
With an integration time of 181 seconds for CH4 and 166 seconds for CO2, detection limits of 0.12 ppm and 35.97 ppm may be reached. By combining a laser array, the suggested method may be extended to detect more molecules concurrently, opening up new possibilities for creating an ultra-compact, low-cost multi-gas laser spectroscopic sensing system.
Xu Wu, Yulong Du, Shijian Shi, Cong Jiang, Xueliang Deng, Song Zhu, Xiaolong Jin and Jingsong Li (2022) Simultaneous Detection of CO2 and CH4 Using a DFB Diode Laser-Based Absorption Spectrometer. Chemosensors. https://www.mdpi.com/2227-9040/10/10/390