# A Review of Photothermal Detection Techniques for Gas Sensing Applications
## 6 Conclusions  
In this review, progresses in PT gas spectroscopy have been presented and discussed based on several sensor layouts and signal retrieval techniques. As presented in this review, PT gas detection can be accomplished in numerous dissimilar configurations, thus encouraging researchers to search for novel, yet unpublished, methods of utilizing the PT effect for precise and selective trace gas analysis. An appropriately designed PT gas sensor can reach sensitivities comparable with WMS or TDLAS techniques, without the necessity of using bulky and fragile multipass cells. Basic parameters of the sensors described in this review are summarized in Table 1.  
Based on the summary provided in Table 1, the most promising PT gas detection techniques are the FP interferometer-based technique and the ones based on a hollow-core fiber. FP interferometric readout of the PT-induced RI modulation has proven to be very sensitive and foremost miniature. Moreover, with proper design, this configuration is capable of reaching strong absorption lines of molecules located in the mid-infrared wavelength region. Similar results, in terms of sensitivity, were achieved in HC-PCF-based PT sensors. This configuration is particularly interesting due to the fact that the sensors were constructed all in fiber, thus having limited susceptibility to environmental noise. The only limitation is the wavelength of operation. Currently available HC fibers have tremendous attenuation in the mid-IR wavelength region, therefore restricting the implementation of such PT sensors to gases having absorption lines in the near infrared. This configuration will surely be further investigated and improved based on novel antiresonant hollow-core fibers with limited losses in the 3-7 \(\upmu\)m wavelength  
Figure 14: Balanced-detection Fabry-Perot interferometer PT gas sensor. Reprinted with permission from [60] C The Optical Society. region. An interesting approach to PTS as proposed in Reference [16], where the induced RI modulation was directly translated to frequency variation of an ML laser repetition rate. This ensured non-complex signal processing and versatility of the constructed sensor. The most pronounced drawback of the sensor was the necessity of combining fiber-based components with bulk optics-based gas absorption cell. This issue could be easily mitigated by incorporating a section of a hollow-core fiber-based gas cell into the ML laser resonator. This would limit the influence of environmental noise and considerably improve the robustness and compactness of the sensor. Significant improvement in terms of the registered signal would also be possible in the least complicated, yet very sensitive FP-based PTS gas sensor. This would require designing the FP resonator to utilize an additional acoustic resonance inside the gas cell, similar to configurations used in quartz-enhanced photoacoustic spectroscopy technique (QEPAS) [17]. The acoustic resonance should increase the registered signal several times, making the sensor configuration sensitive, selective and miniaturized. Nevertheless, similar to PAS, in PTS, the sensor can be designed to target strong particle transitions in the mid-IR, but at the same time have signal readout based on inexpensive fiber components. Considering the fact that the PT-induced RI modulation can be probed in numerous different ways (e.g., as variations in laser intensity, frequency or in a interferometric configuration), there is there is still plenty of space for further development of these techniques, which will surely be perused by researchers working on laser-based gas sensing. **Funding:** This work was financially supported by the statutory funds of the Chair of EM Field Theory, Electronic Circuits and Optoelectronics, Wroclaw University of Science and Technology (0401/0030/18). **Conflicts of Interest:** The author declares no conflict of interest.