*2.2. Methodology*

The method of fabricating the microchannels in the outer cladding of the fiber is shown in Figure 2. The developed laser micromachining process allows for non-contact and damage-free processing of the fragile fiber structure with the required high accuracy and precision. At the point of impact of a high-energy fs laser beam, the material evaporates, leading to the formation of a crater, the depth of which depends on the selected process parameters, i.e., pulse duration, repetition rate, scan speed, pulse energy, and the number of

consecutive scans across the processed area. The key element of the method was the precise positioning of the fiber in such a way that the laser beam moved in the area between the inner capillaries, which in our case were 10–12 μm apart. For this reason, it was necessary to control the position of the fiber, which was realized using a CMOS (Complementary Metal-Oxide-Semiconductor) camera and precise motorized XYZ translation stages and theta rotation.

**Figure 2.** Methods used to process the lateral channels in the outer cladding of the ARHCF.

The critical point in the laser processing of such fibers is obtaining a contaminationfree microchannel in the outer cladding of the fiber. There is a risk of contamination of the air core by debris generated during the process, which is composed of the glass material that did not evaporate during the process. Any damage to the inner capillaries should also be avoided, as it directly leads to local changes in the resonance properties of this fiber. For this reason, the cutting process should be performed layer by layer so that debris does not penetrate the core and no damage to the capillaries is present.
