**1. Introduction**

The end of the previous century has brought a new type of optical fiber, the socalled hollow-core fiber (HCF), which due to its unique structure and ability to guide light in the air via the photonic bandgap effect, rather than via the conventional total internal reflection phenomenon, revolutionized the development and application areas of optical fiber technology [1]. Further development of the HCF structure and exploration of different guidance mechanisms of light in the air have enabled access to the HCFs, which deliver a superb ability to efficiently guide laser light, especially in the mid-infrared (mid-IR) spectral band, where conventional solid-core fibers suffer from high attenuation of the glass material [2,3]. Currently, three major types of HCFs have been proposed, fabricated, and successfully used in various applications [4–7], amongst which the laserbased spectroscopy of gases has attracted significant attention of researchers around the world [8–10]: the hollow-core photonic bandgap fiber (HC-PBGF) [1], the Kagome HCF [11] and the Antiresonant Hollow-Core Fiber (ARHCF) [8]. Benefiting from an empty core, which can be filled with the target gas, HCFs can be utilized as low-volume absorption gas cells, forming versatile light-gas molecules interaction paths with the desired length within a sensor setup [8]. Since the sensitivity of the majority of laser-based gas sensors can be relatively simply and significantly enhanced by increasing the interaction path length, access to non-complex and long optical paths is highly desired. Hence, the incorporation of HCF-based absorption cells into laser-based gas detectors could lead to high sensor detection capability and less complex design in comparison with commonly used bulk optics-based solutions, e.g., utilizing multipass cells [12,13]. Multipass cells delivering optical paths with several tens of meters length require advanced optical arrangements for coupling into them the laser beam in a way allowing for obtaining the proper number of light passes, hence the desired path length. Since optical and optomechanical components are sensitive to vibrations and temperature changes, which negatively affect their long-term stability, even a slight misalignment of the coupling optics disturbs the light propagation

inside the multipass cell. Unfortunately, optics-free coupling into a multipass cell is not possible. This leads to the increase in the noise level, the reduced amplitude of the measured signal, hence a significant drop in the detection capability of the sensor. On the other hand, light guidance in HCFs can be efficiently excited via an optics-free butt-coupling approach of the laser beam. Furthermore, the multipass cells based on the use of optical mirrors (e.g., Herriot- or White-type) mounted in e.g., metal optomechanical housings are sensitive to temperature changes due to thermal expansion of the material, which additionally affects the stability of the gas sensor. This can be minimized by using materials with lower thermal expansion coefficient, e.g., invar, however at cost of a significant increase in the sensor's price, especially when multipass cells delivering several tens of meters long paths lengths are used in the setup.

HC-PBGFs have been successfully used in various gas sensor configurations, however, they target transitions of different gases in the wavelength range not exceeding 3.4 μm [9,14,15]. It was established that the main issues connected with the use of this particular type of fiber that significantly limits the sensitivity and versatility of fiber-based gas sensors arise from the multimode nature of these fibers and their maximum operational wavelength range [9,16]. Multimode guidance leads to the intermodal interference between the fiber-supported fundamental mode and the higher-order modes, which negatively impacts the noise level in the sensor [16]. This can be minimized by combining HC-PBGFs with spectroscopic techniques that have a built-in capability of reducing the impact of the fringe noise on the measured signal, e.g., Chirped Laser Dispersion Spectroscopy (CLaDS) or Photothermal Spectroscopy (PTS) [15,16]. Moreover, despite guidance in air, this fiber is still characterized by a relatively high overlap between its glass structure and the guiding light, which limits the transmission bandwidth to approximately 3 μm spectral band [17]. Furthermore, due to the small core size (typically up to 20 μm), the gas filling time of HC-PBGFs can reach even several hours, which severely limits the response time of the sensing systems utilizing these fibers [16].

A partial solution to the issues that are present in HC-PBGFs comes with the aid of the Kagome type HCFs, which guide light via the inhibited coupling mechanism [11]. As a result of a modified fiber structure and different light guidance principles, these fibers can efficiently transmit light in the near-infrared (near-IR) and mid-IR [11,18]. Furthermore, the core size of the Kagome fibers is a few times greater (116 μm) in comparison with the HC-PBGFs guiding light within the same spectral band, which results in the reduction of the gas filling time down to several seconds [18]. Nevertheless, it was indicated in [10,18] that the problem connected with the multimode guidance is also present in the Kagome HCFs, which was identified as the main limiting factor in the performance of the gas sensors utilizing these fibers.

ARHCFs, in which light transmission is realized by the Antiresonant Reflecting Optical Waveguiding (ARROW) principle [19], can deliver low loss in both near- and mid-IR spectral bands, fast gas exchange time, and single-transversal mode operation if a proper fiber structure is designed [8,20]. Currently, ARHCFs have been successfully used in gas sensors utilizing a variety of laser-based sensing techniques, i.e., Tunable Diode Laser Absorption Spectroscopy (TDLAS), Wavelength Modulation Spectroscopy (WMS), PTS, and Photoacoustic Spectroscopy (PAS) [8,21–23]. Examples of the ARHCFs used in gas sensing applications are depicted in Figure 1. Researchers have shown that the ARHCFaided gas sensors can target molecules with transitions in the wavelength range up to 5.26 μm, which is unreachable with the use of other types of HCFs [20,24]. Furthermore, benefiting from their ability to simultaneously guide laser radiation within two dissimilar spectral bands, the ARHCF-based detectors can be used to analyze gas mixtures that contain molecules having transitions in both near- and mid-IR [8]. Similar to the Kagome HCFs, ARROW-guiding fibers are characterized by the core size in the range of several tens of micrometers, which in combination with a proper gas delivery system allows obtaining gas exchange times in the range of several seconds [20]. The combination of ARHCF-based gas absorption cells with, e.g., the PTS technique enables obtaining superb

long-term stability of the sensor, giving a promising perspective for their future application in out-of-lab conditions [25]. The sensors utilizing such fibers have been demonstrated to provide detection capability even at a level comparable to the bulk optics-based setups, indicating that the fiber-based configuration of the sensors can form a new branch of sensitive, selective, and non-complex gas sensing platforms.

**Figure 1.** Examples of ARHCFs used in gas sensing systems. (**a**) Silica-based ARHCF is designed to operate in the 2 μm wavelength range with a core size of 70 μm. Reprinted with permission from [21] © The Optical Society. (**b**) Six-capillary cladding silica-based ARHCF with a core diameter of 65 μm for guidance at ~1.55 μm and 3.34 μm. Reprinted with permission from [26] © The Optical Society. (**c**) Seven-capillary cladding silica-based ARHCF with a core size of 84 μm providing low-loss transmission in the near- and mid-IR. Reprinted with permission by MDPI from [8]. (**d**) Silica-based ARHCF with nested capillary cladding and a core diameter of 65 μm for guidance at ~4.54 μm. Reprinted with permission from [27] © The Optical Society. (**e**) Tellurite ARHCF enabling light guidance ~5 μm inside a hollow core with a 139 μm diameter. Reprinted with permission from [28] © The Optical Society. (**f**) 5.26 μm-guiding borosilicate glass-based ARHCF with a core size of 122 μm.

In this review, the recent progress in ARHCF-based gas sensors utilizing the aforementioned gas sensing techniques will be discussed. Several different sensor configurations are presented and their advantages along with main limiting factors are reviewed. Section 2 of this review aims at explaining the light guidance properties of the ARHCFs. Section 3 is devoted to the implementation of the ARHCFs into TDLAS-based gas sensors. Section 4 is focused on the WMS gas sensors aided with different types of ARHCFs. Section 5 presents the PTS technique supported by the ARHCFs and explains how the few-moded guidance of the fiber can be transferred to the high sensor stability and sensitivity together with an introduction to the new gas sensing method in ARHCFs, the so-called Photoacoustic Brillouin Spectroscopy (PABS) [23]. Section 6 summarizes the performance of the reported up-to-date ARHCF-aided gas sensor configurations.
