**5. Conclusions**

We have demonstrated an efficient method of producing microchannels in a nodeless ARHCF using an fs laser. The development of optimal process parameters and a laser cutting strategy allowed the fabrication of microchannels in the outer cladding of the ARHCF without damaging the internal capillaries forming the antiresonant structure. The key element of the proposed technology was to carry out the process in a way that allowed us to minimize the formation of the debris that occurred during laser ablation. For a single microchannel, there was a slight increase in transmission losses (<0.01 dB), which is more than 35 times lower than reported so far [13]. Moreover, the implementation of microchannels of different lengths did not cause a significant increase in losses, which proves that in the case of ARHCF it is possible to make microchannels of any length, provided that the fiber is precisely positioned in relation to the laser beam. Additionally, it has been shown that making lateral incisions in the ARHCF weakens its mechanical resistance to bending. Coiling of the fiber in a small diameter is possible only in the case of short microchannels <0.1 mm, since for longer microchannels the bending forces can fracture the ARHCF in the area subjected to laser modification. The presented work shows

that the proposed micromachining process can be used to manufacture microchannels in ARHCFs, thus allowing the construction of low-loss fiber-based absorption gas cells for applications in laser-based gas sensing.

**Author Contributions:** Conceptualization, P.K., P.J. and K.K.; methodology, P.K. and P.J.; formal analysis, P.K.; investigation, P.K., P.J., K.K. and G.D.; resources, F.Y. and D.W.; data curation, P.K.; writing—original draft preparation, P.K.; writing—review and editing, P.J. and K.K.; visualization, V.H.; supervision, M.L., J.K. and K.A.; project administration, M.L. and K.A.; funding acquisition, K.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science Centre (NCN), grant number UMO-2018/30/Q/ST3/00809, the Polish National Agency for Academic Exchange, grant number PPI/APM/2018/1/00031/U/001, the National Natural Science Foundation of China (61935002, 61961136003), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (ZDBS-LY- JSC020) and the CAS Pioneer Hundred Talents Program; the Open Fund of the Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques (South China University of Technology). The laser source for the MIR range was supported by the Foundation for Polish Science within the First TEAM program co-financed by the European Union under the European Regional Development Fund (contract no. POIR.04.04.00-00-434D/17-00).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The Authors would like to thank A. Anto ´nczak for providing a system for laser micromachining and Aleksander Budnicki from Trumpf Laser GmbH for lending a femtosecond laser source.

**Conflicts of Interest:** The authors declare no conflict of interest.
