**6. Summary**

In this review, recent advancements in the ARHCF-based gas sensors were presented and discussed. It was shown that the properly designed ARHCFs could be successfully used as low volume, compact, and long absorption cells in sensing systems utilizing various laser spectroscopic techniques, i.e., TDLAS, WMS, PTS (in several dissimilar configurations), and PABS. The ARHCF absorption cells forming the gas molecules' interaction path length within the sensor can be efficiently and at a reasonably fast time filled with the target gas mixture using an overpressure induced gas flow, which results in an acceptable response time of such a sensor. A summary of the performance of the selected ARHCF-aided gas sensors is presented in Table 2.

The ARHCF-aided gas sensors utilizing the TDLAS technique indeed proved their capability to detect molecules of various gases in a simple and non-complex way, however, did not provide sufficient suppression of the background noise resulting from, i.e., intermodal interference. This drawback severely limits the sensitivity and long-term stability of such sensors, hence different spectroscopic techniques have to be combined with ARHCFs to fully benefit from the connection of both.


**Table 2.** Comparison of the performance of the ARHCF-based gas sensors.

The combination of ARHCFs with the WMS technique unquestionably provides a significant improvement in the noise level reduction in the ARHCF-assisted gas sensor configurations, which directly leads to a better sensor sensitivity. However, such sensors are still not immune to the uncontrolled fluctuations of the registered signal. Since especially short lengths of the ARHCFs are characterized by the presence of higher-order modes or longer pieces from the non-uniformity of the structure, the amplitude-based signal retrieval is sensitive to the parasitic changes of the analyzed light intensity, which significantly impacts the overall performance of this type of gas sensors. Hence, not only proper optimization of the fiber structure but also a change of the gas sensing method can provide the desired improvement in the operation of the ARHCF-based sensors.

The other approach for gas sensing aided from ARHCFs comes with the PTS technique, where the spectroscopic signal retrieval is based on the analysis of the *probe* light phase modulation induced by a local change in the RI due to heating of gas molecules. The phase modulation can be precisely investigated based on the interferometric signal measurement. The ARHCFs were implemented in the PTS sensors utilizing MZI, FPI, and MPD sensor setups. The main drawback of the MZI PTS gas sensors is the necessity of actively and precisely stabilizing the sensor's setup to maintain its operation at the quadrature point of the interferometer. This is not simple to realize, and the long-term stability of such sensors is very difficult to be obtained, which severely limits their ability to operate in real application conditions. The less complex sensor design is delivered by the application of an FPI. In an FPI PTS gas sensor utilizing ARHCFs, stabilizing the cavity at quadrature can be realized in a much more accessible and reliable way, by stabilizing the wavelength of the *probe* beam, e.g., with an aid of PID-based technique. With this technique, the sensitivity

of the ARHCF-assisted PTS sensors is improved in comparison to the sensors based on the MZI. Until now, the most sensitive configuration of the ARHCF-based gas sensors was based on the MPD PTS technique. The MPD PTS sensors were shown to provide significantly better long-term stability and immunity to the negative influence of external factors than, e.g., MZI PTS. However, it can be assumed that even simple WMS-based sensors aided with several tens of meters long ARHCFs could provide comparable or even better sensitivity benefiting from long interaction path lengths. The main requirements which have to be fulfilled are the pure single-mode guidance of the ARHCF and the uniformity of its structure, which both are mandatory to eliminate the fringe noise and the parasitic fluctuations of the signal amplitude. Furthermore, the application of heterodyne signal retrieval in the MZI PTS configurations instead of the homodyne technique will result in the possibility of analyzing the signal in the frequency, not amplitude domain which could provide the ultimate immunity of the sensor to the fringe noise (resulting from the intermodal interference in the ARHCFs) and the amplitude noise (e.g., residual amplitude modulation) [49].

A very interesting approach to gas sensing inside the ARHCFs was presented in [23], where the sensing system was based on the PABS technique. The interaction between the fiber-supported acoustics and optical modes enables signal retrieval in the MHz frequency range, which minimizes the 1/f noise. It was stated that with the further optimization of the fiber structure, hence the structure of the formed in it an acoustic resonator, ARHCF-based PABS sensor could deliver significantly greater sensitivity than reported by the authors.

The ARHCF-based gas sensors have already proved their excellent suitability for this application and opened a way to the new branch of sensitive, low-volume, and versatile detectors. It is expected that further development of the hollow-core fiber technology will result in the possibility of fabricating several tens of meters long fibers, maintaining uniform structure along their entire length, which will enable low-loss and single-mode transmission not only in the near- but also in the mid-IR spectral band. The utilization of different than fused silica materials, such as telluride or chalcogenide glasses should extend the operational wavelength range of these fibers to the spectral bands above 5.26 μm, hence allowing the detecting of various gases over a significantly broader than currently available range. The improvement in the transmission properties of the ARHCFs should also allow them to be successfully used in the broadband spectroscopy applications, i.e., in the frequency comb spectroscopy [62], which will enable an in-depth analysis of the complex mixture of gaseous substances, maintaining a low-volume of the sensing unit with high detection capability. Currently, the ARHCF-based gas sensors are in majority realized in laboratory conditions, which results in their still large size (despite the low-volume formed by the absorption cells) in comparison to field-deployable bulk-optics based sensors [63] and most commonly used in real-life applications non-optical gas detectors [64,65]. However, further minimization of the electronic components, accompanied with optics-free light coupling into the fiber and the possibility of bending the fiber with bend radius in the range of a few centimeters [29,66] should result in a significant reduction in the size of ARHCF-aided gas sensors, even beating the dimensions of the sensors utilizing multipass cells delivering several tens of meters long optical path lengths. Moreover, proper laserbased modification of the fiber structure, enabling loss-free access to the fiber core for gas filling purposes [67], should result in pure diffusion-based gas exchange in such sensors, significantly enhancing their versatility and usefulness together with further reduction in the overall size of the entire sensing unit. The combination of a low-volume, non-complex design, selectivity, excellent detection capability, and calibration-free operation should lead the ARHCF-aided gas sensing approach to the development of the sensors that could in future form a new branch of gas spectrometers with the parameters comparable or even beating the currently used devices.

**Funding:** This research was funded by Narodowe Centrum Nauki (NCN), grant number UMO−2018/ 30/Q/ST3/00809. The APC was funded by MDPI Sensors journal.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The author would like to thank Viktoria Hoppe for taking the SEM image of the borosilicate ARHCF. The author would like to acknowledge the financial support of MDPI Sensors journal for generously covering the publication fee.

**Conflicts of Interest:** There is no conflict of interest to be declared.
