Measurement of the composition of gas mixtures is essential for many purposes. Laser spectroscopy has been used as a measurement technique for the composition of gases for many years. It has evolved throughout time by introducing new methods and enhancing those currently used because of the pressing need to limit the presence of dangerous gases.
Traditional sensors employ single-pass configurations limiting their sensitivity. Sensors of miniature size achieve high sensitivity, but these are very complex. Extending the optical channel is one of the traditional methods for lowering the detection limits of an absorption-based laser gas sensor.
Multi-Pass Cell (MPC)
The detection limit of a device is improved when the optical path is extended due to more absorption of the gas analyte.
Extending the optical path is employed using a multi-pass cell (MPC). However, there are several drawbacks associated with MPC, including the introduction of optical noise, the requirement of tested gas samples in large volumes, and the heaviness and unwieldiness of the MPC. These disadvantages have forced scientists to explore alternative solutions.
Hollow-Core Fibers (HCFs) and Anti-Resonant Hollow-Core Fibers (ARHCFs)
Hollow-core fibers (HCFs) are one method that performs measurement and guides the laser beam simultaneously and can be filled with test gas sample. Laser spectroscopy has been introduced with unique sensors, and photonic bandgap fibers (PBG) are one of them. However, two issues are associated with PBG: limited transmittance capability in the mid-infrared spectrum and the multimode nature of light transmission altering results.
These issues are solved by introducing anti-resonant hollow-core fibers (ARHCFs) technology based on ARROW (anti-resonant reflecting optical waveguide). This unique hollow-core fiber technology simplifies the gas exchange procedure since a large transmission spectrum is achievable with the quasi-single-mode transmission and has wide air cores.
These properties of ARHCFs technology have gained the attention of researchers worldwide who experimentally demonstrated the utility of ARHCFs in developing several high-sensitivity gas sensors based on several spectroscopic methods. The results of these experiments show that these sensors have outstanding sensitivity. However, these sensors need a low-noise detector to convert the spectroscopic signal to electrical signals, usually done via semiconductor-based photodetectors.
The limitation of using semiconductor-based photodetectors is that the photodiode material determines their operating wavelength range. Furthermore, low operating temperatures are required for maintaining low-noise characteristics of mid-IR detectors, and the manufacturing material used is also hazardous. A quartz tuning fork (QTF) as a traditional photodetector is an alternative solution for this problem.
Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS)
Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a method that uses a quartz tuning fork for detecting sound waves generated between its prongs as a result of laser beam absorption in the gas sample.
QEPAS-based sensors are highly sensitive and selective in detecting molecules of different gases near and mid-IR spectral bands. Quartz-enhanced photothermal spectroscopy (QEPTS) is a variation of this method in which a quartz tuning fork is used as a high Q-factor detector.
A laser beam conveyed through the test gas sample causes the quartz tuning fork resonator to excite by locally heating the surface of QTF, which faces temporary deformation in the luminescent region due to thermoelastic expansion of the quartz.
Key Takeaways From This Study
The researchers utilized the aforementioned ARHCF as an absorption cell, QTF detector and QEPTS detection method to make a unique laser-based gas sensor. This sensor helped researchers eliminate the system's narrowband IR detectors and bulky MPC. This technique is proven to be very cost-effective compared to a broadband detector since QTF resonators are of very low price and allow measurements in near and mid-IR regions.
The study also demonstrates methane gas detection as proof of the proposed method. The results of this experimentation show that this system allowed normalized noise-equivalent absorption (NNEA) of 2.04 x 10-11 W cm-1 Hz -1/2 level. Even though ARHCF was not enhanced for the required wavelength, it did not affect the detectability of the system.
The findings point to further opportunities for system improvement by employing both a fiber with improved transmission properties and a more isolated and precise detecting module. This system can be expanded in the future to simultaneously detect multiple gases to target the absorption lines in the mid-and near-IR spectral bands. This will further demonstrate the potential capabilities of this system.
Piotr Bojęś , Piotr Pokryszka, Piotr Jaworski, Fei Yu, Dakun Wu, and Karol Krzempek (2022) Quartz-Enhanced Photothermal Spectroscopy-Based Methane Detection in an Anti-Resonant Hollow-Core Fiber. Sensors. https://www.mdpi.com/1424-8220/22/15/5504/htm