**1. Introduction**

The main goal of laser-based gas spectroscopy research is the development of sensors with high sensitivity and selectivity, preferably maintaining a non-complex design of their setup [1–3]. According to the Beer–Lambert law, the detection ability of such sensors depends mainly on the length of the interaction path between the light beam and the measured gas molecules [4]. Hence, long optical paths implemented within the sensors setup are highly desired, especially in the case of low-concentration (typically single parts-per-million (ppm) or below—parts-per-billion (ppb) levels) gas molecules detection. Usually, long optical paths are realized by bulk-optics-based multipass cells (MPCs) that indeed fulfill this crucial requirement, however, at the expense of reduced stability of the sensor due to possible opto-mechanical drifts and optical fringes contributing to the overall noise level. Recent reports demonstrate the use of hollow-core fibers (HCFs), especially

Krzempek, K.; Hoppe, V.; Dudzik, G.; Yu, F.; Wu, D.; Liao, M.; Knight, J.; Abramski, K. Fabrication of Microchannels in a Nodeless Antiresonant Hollow-Core Fiber Using Femtosecond Laser Pulses. *Sensors* **2021**, *21*, 7591. https:// doi.org/10.3390/s21227591

**Citation:** Kozioł, P.; Jaworski, P.;

Academic Editor: Vittorio M.N. Passaro

Received: 27 October 2021 Accepted: 14 November 2021 Published: 16 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the relatively new type of such fibers, ARHCFs, as an alternative to sensors based on MPCs [5–8]. A gas-filled ARHCF can form a versatile absorption cell characterized by a small volume and can be tailored to a desired length with respect to the specific application. The filling process of the ARHCF core with the target gas mixture can be realized by either gas diffusion through both fiber ends or via specially designed gas cells located at both fiber ends, through which the gas is delivered to the hollow core using overpressure. The gas filling time can vary from several hours per meter for free gas diffusion [9,10] to a few seconds for pressure-supported methods [11–13]. A purely diffusion-based gas exchange approach is less complex; however, it significantly increases the response time of an ARHCF-based gas sensor. On the other hand, the use of overpressure filling via additional gas cells increases the sensor complexity and can negatively affect the long-term stability of the sensor. Hence, an alternative solution that will enable noninvasive, simple, and efficient gas filling of the ARHCF core is highly desired.

An alternative method of filling the ARHCF core with the target gas can be realized via side incisions that allow direct access to the fiber air core along the entire length of the fiber. This solution allows non-complex gas exchange without interfering with the optical coupling at the fiber end facets [14]. Due to the fragility of the material, noncontact processing methods are most often used. The most popular method is based on the usage of a femtosecond (fs) laser. For example, a V-shaped hole was made in an HCF [15,16] using the layer-by-layer scanning method. In [17], the researchers presented fabrication of a side hole in an HCF using an fs laser, where the process was carried out in a fluid with a matching refractive index to reduce the debris that is generated during the laser ablation process. The fabrication of microchannels with the use of femtosecond laser pulses introduces certain transmission losses, which mostly result from local damage of the fiber structure or the presence of debris generated during laser ablation.

Another noncontact method used for the production of microchannels is based on the use of a focused ion beam (FIB) as presented in [18,19] for which, due to lack of destructive factors, no losses resulting from the fabrication of the microchannel were recorded. The disadvantage of this method is the time required to process a single hole (~30 min for a single hole with a diameter of 10 μm [19]). Furthermore, because of the limited space in the FIB-based setup, it is impossible to process long (i.e., several meters) fibers in multiple places, which is required for providing optimum gas filling conditions within the long optical path provided by the fiber. For this reason, an attempt was made to find a way to improve the quality of microchannels produced using laser micromachining.

The methods of microchannel realization using an fs laser presented in the literature so far have introduced relatively high losses at the level of 0.45 dB per hole [20], and more. The lowest loss of 0.35 dB per hole was recorded for a single microchannel made in an HCF assisted by liquid [17]. These results are not satisfactory from the potential applications viewpoint since in the case where a higher number of holes is required, the total losses will effectively limit light propagation in such fiber. The most common cause of losses are local modifications of the fiber structure due to laser ablation, leading to an increased attenuation of the propagating light. An additional factor that increases losses is the debris that is formed during the process and is deposited inside the fiber. Both of these factors are the result of the non-optimized laser micromachining process used for modifying the HCF structure.

In this work, we propose a new approach based on the use of the fs laser micromachining process, enabling damage-free and efficient processing of the outer cladding of nodeless ARHCFs. To the best of our knowledge, this is the first attempt to utilize fs laser pulses to access the air core of nodeless ARHCFs via side-drilled microchannels. The developed process does not affect the cladding capillaries that define the low-loss guidance properties of this particular fiber. Such a modification in the outer structure of the ARHFC fiber, as shown in the literature [21–23], should not introduce additional transmission losses since the outer cladding has a negligible impact on the light confinement in the air core. With a properly optimized laser-micromachining process, the gap between the capillaries

can be used to directly access the hollow region of the microstructured fiber. The key element of the proposed method is the precision of laser processing and proper selection of process parameters in order to significantly reduce the formation of the debris that can accumulate in the air core of the fiber. Minimization of the debris is crucial as it significantly influences the guidance properties of ARHCFs. Here, we will compare two approaches of fabricating lateral channels in ARHCFs in terms of amount of debris, channel surface quality, and processing time. The influence of the microchannel dimensions and bending of the processed fiber will be experimentally verified.
