# Review of Recent Advances in QEPAS-Based Trace Gas Sensing
## 7 Conclusions and Future Outlook  
In this review, recent developments of the QEPAS-based trace gas sensing technique in the past five years are summarized and discussed. Compared to other absorption spectroscopy such as tunable diode laser absorption spectroscopy (TDLAS), a distinct advantage of the QEPAS sensor is that its performance can be improved when the output power of the excitation laser source is increased. Based on this feature, the near-infrared light source of diode laser was power amplified by an erbium-doped fiber amplifier in the communications bands to improve the QEPAS sensor performance. Due to the output coupling loss, the laser power intensity in the intra-cavity is much higher than outside. Therefore, placing a QTF into the intra-cavity can enhance the signal level of QEPAS. When excitation sources such as LEDs, VCSELs, and terahertz (Thz) QCLs are used in QEPAS sensors, the bad beam quality of large beam diameter and divergence angle make the light beam unable to pass through the micro-resonators and the 300 \(\upmu\)m gap between the two prongs of standard QTFs,  
Figure 12: Schematic of the 3D-printed ADM (**a**) Designed 3D model; (**b**) 3D-printed ADM with optical and photoacoustic detection components; (**c**) the flow field within the ADM.  
resulting in obvious optical noise. A custom QTF with a large prong gap of \(\sim\)1 mm can solve this issue well. If the fundamental resonance frequency of the custom QTF is small (a few kHz), the resonance frequency of the first overtone flexural mode should not be too high (<20 kHz). Therefore, the first overtone flexural mode of QTF can be used in the QEPAS sensor. The several-fold increase in the Q factor of the first overtone mode improved the QEPAS sensor detection sensitivity significantly. Unlike the custom QTF with a large prong gap, a custom QTF with a small gap of 200 \(\upmu\)m with the merit of lower energy loss of propagation allowed the acoustic wave to push the fork prongs more efficiently. A decrease of QTF mass compared to a standard QTF would result in bigger displacement. Therefore, employing a QTF with a small gap of 200 \(\upmu\)m in QEPAS is another way to improve the detection limit. Fiber evanescent wave (FEW) has the advantages of being flexible in operation and on-line measurement. By combining FEW and QEPAS techniques together, a long distance of 3 km and distributed gas sensing were achieved. The 3D printing technique has the merits of \"free manufacturing\", reduction in processing time and procedure, high integration, and stability. A portable, small QEPAS sensor was obtained using a 3D-printed QEPAS acoustic detection module with a weight of \(\sim\)5 g. Further improvement of the QEPAS sensor detection sensitivity can be obtained by using a THz laser source with high output power in order to achieve the minimum detectable concentration at the ppb or sub-ppb level. Furthermore, the intra-cavity QEPAS sensor can be realized in the novel mid-infrared solid-state laser oscillations with the merits of high laser power intensity and excellent beam quality [81; 82]. Unlike optical detectors used in other laser spectroscopy such as TDLAS, QTF is immune to the laser wavelength. This feature is especially attractive for QEPAS gas sensing in the \(>\)10 \(\upmu\)m region, where the availability of optical detectors is limited and the strong absorption band of gas molecules exist. The overall size of a typical QEPAS sensor platform can be significantly reduced to a size that makes this device suitable for applications requiring lightweight and compact sensors such as balloon- or miniature unmanned aerial vehicle (UAV)-based atmospheric measurements or portable low-cost sensors for first responders, such as in mining accidents or forest fires near urban centers. The QEPAS-based sensor has a wide range of applications in environmental monitoring, atmospheric chemistry, industrial chemical analysis, and medical and biomedical diagnostics, as well as in law enforcement.