Advancements in Quartz-Enhanced Laser Spectroscopy (QEPAS and LITES)

In a recent review article published in the journal Light: Science & Applications, researchers provided a comprehensive analysis of quartz-enhanced laser spectroscopy sensing, a technology widely utilized in fields such as environmental monitoring, medical diagnostics, industrial control, and safety warnings.

Image Credit: luchschenF/Shutterstock.com

Background

The continuous acceleration of industrialization and environmental challenges requires highly sensitive and selective detection methods for trace gases. Traditional gas sensors, like electrochemical and semiconductor sensors, are cost-effective but lack the sensitivity and selectivity offered by spectral gas sensors. Spectroscopic sensing relies on the interaction of light with matter through absorption, emission, or scattering to determine gas type and concentration.

While most absorption spectroscopy techniques use photodetectors, these have limitations, such as a restricted response wavelength range (unable to detect lasers longer than m) and the high cost and need for cooling for mid-infrared detectors. This review focuses on two specialized spectroscopic techniques that overcome these limitations by using a quartz tuning fork (QTF) as the detection element: Quartz-enhanced photoacoustic spectroscopy (QEPAS) and light-induced thermoelastic spectroscopy (LITES). The core of both QEPAS and LITES is the quartz tuning fork (QTF), which offers exceptional advantages for spectral sensing, including a high-quality factor (-factor), strong noise immunity, compact size, and low cost. Specifically, the QTF's resonant properties significantly boost system signal strength. QEPAS, introduced in 2002, replaced the traditional microphone in photoacoustic spectroscopy (PAS) with a QTF. PAS is based on the photoacoustic effect, where absorbed light energy is converted into heat, causing periodic pressure variations (acoustic waves) proportional to gas concentration

Studies Highlighted in this Review

The review details pivotal innovations aimed at enhancing the performance of QEPAS and LITES sensors.

For QEPAS, innovations encompass using high-power excitation methods and novel excitation sources to maximize gas absorption, and employing resonant cavities or multi-pass structures to amplify the acoustic waves. Additionally, low-frequency QTFs are utilized to increase energy accumulation time, further enhancing signal strength.

For LITES, a major focus is on leveraging optical designs to boost absorbance. One area of research involves multi-pass cells (MPCs) to significantly enhance light absorption. For instance, a Herriott MPC-based LITES sensor achieved an effective optical path length (OPL) of 10.1 m. Furthermore, dense-spot MPCs have been proposed to improve mirror utilization efficiency. One such dense-spot MPC achieved an OPL of 37.7 m, corresponding to an approximately eightfold improvement over traditional Herriott cells. Another innovation involves using optical cavities, such as off-axis integral cavities, to increase the effective OPL. A reported off-axis cavity LITES sensor achieved an effective OPL of 15 m, about 150 times the intracavity propagation distance, thereby improving sensitivity and reducing optical noise. To address the size and alignment complexity of MPCs, optical waveguide-based LITES sensors are being developed. One example used a 75 cm hollow-core anti-resonant fiber (HC-ARF) as both the optical medium and the gas cell, providing a compact structure, simplified alignment, and low transmission loss while increasing the effective interaction length between light and gas.

A highly effective strategy is the combination of QEPAS and LITES into dual-spectroscopy systems. This combined technology, such as single-quartz-enhanced dual spectroscopy (S-QEDS), measures both photoacoustic and light-induced thermoelastic signals synergistically. S-QEDS configurations have shown a signal amplitude nearly equal to the sum of the individual QEPAS and LITES signals. Further enhancements include integrating advanced optical structures, such as combining an acoustic micro-resonator (AmR) to enhance QEPAS signals with an MPC to amplify LITES signals, optimizing laser energy utilization and simplifying optical alignment.

Discussion

The advancements in quartz-enhanced laser spectroscopy have enabled detection limits from ppb to ppt levels for trace gases. The core advantage remains the QTF, which, unlike traditional photodetectors, is insensitive to laser wavelength and can respond to signals generated by excitation from lasers of any wavelength. QEPAS and LITES offer highly complementary performance characteristics. Future development of this technology is moving toward ubiquitous, real-time, and intelligent gas sensing. This includes the integration of machine learning algorithms for optimized signal processing and the creation of portable devices that facilitate on-site rapid detection. Real-time multi-species detection requires high-speed spectral analysis and adaptive algorithms.

Despite the clear commercial value demonstrated in high-demand fields like industrial process monitoring, commercialization faces practical challenges. Primary obstacles include cost control due to complex fabrication processes for high-performance QTFs and stable laser sources, and environmental adaptability challenges where fluctuations in temperature, humidity, and vibrational noise can impact QTF stability. Additionally, a lack of standardization across different application needs hinders industry adoption.

Conclusion

Future breakthroughs require focusing on miniaturized optical integration to reduce costs, developing adaptive compensation algorithms for complex environments, and establishing unified technical standard systems. Ultimately, as cross-disciplinary convergence deepens, these QTF-based spectroscopic technologies are poised to evolve towards higher sensitivity, higher integration (including on-chip integration), and greater ease of practical application.

Download the PDF of this article