*3.1. Influence of Laser Microchannel Processing on the ARHCF Transmission Characteristic*

To verify to what extent the fabricated microchannels affect the transmission properties of the ARHCF, an experiment was performed in which a set of microchannels were manufactured with selected processing parameters. In this stage, a 30 m long ARHCF was used and 5 sections with 5 microchannels each (250 × 30 μm) were manufactured. The distance between the microchannels was 1 mm and the separation between the sections was equal to 10 cm. A diagram of the experimental setup is presented in Figure 7a. A custom-built DFG COMB was used as a broadband light source operating in the 3.3–3.4 μm wavelength region, matching a part of the low-loss transmission band of the ARHCF. The loss introduced by the fiber modification was calculated by comparing the optical power delivered by the unprocessed fiber and the fiber with laser-machined microchannels. First, the fiber-delivered power was measured for the ARHCF with 5 sections of microchannels processed at its end. Subsequently, one section of the microchannels was cut off from the fiber using a ceramic blade, and the measurement of the delivered optical power was

performed again. This procedure was repeated for each section until the entire processed part of the fiber was removed. Note that during the measurements the input light coupling conditions were maintained the same for each step, so the average optical power of the DFG COMB radiation coupled into the ARHCF core was constant. The results obtained are presented in Figure 7b.

**Figure 7.** Influence of microchannel fabrication on transmission loss in the ARHCF for wavelengths 3.3–3.4 μm: (**a**) experimental setup and (**b**) loss measurement calculated as a function of the number of microchannels.

The microchannels produced with the F-theta lens introduced significantly greater transmission losses (black triangle) compared to the lateral cutting made with the aspherical lens (red square). For example, for the 25 microchannels made with the F-theta lens, the loss was 15.49 dB, while for the aspherical lens it was only 0.17 dB. Based on theoretical calculations presented in [21–23], the introduction of lateral cuts in the outer cladding of the nodeless ARHCF should not lead to significant transmission deterioration since the outer cladding does not participate directly in guiding the light inside the core. To investigate the cause of the increase in loss, a detailed analysis of the fabricated microchannels was carried out.

Figure 8 shows photographs of randomly selected microchannels made with the aid of the aspherical lens (Figure 8a,b) and the F-theta lens (Figure 8c,d). In both cases, the microchannels were made in a region exactly above the free space between the inner capillaries, without damaging their structure. The photographs taken with visible light illumination show the presence of impurities inside the fiber within the manufactured microchannels only in the case of using the F-theta lens approach. By analyzing the SEM images of the fiber cross-sections (Figure 8b,d), we can conclude that for the microchannels made with the use of an aspherical lens, only a low degree of impurities occurs in the space between the outer cladding and the capillaries. For microchannels made with the F-theta lens, the amount of impurities inside the fiber is much greater (Figure 8c,d). The formation of large particles, which settle on the walls of the capillaries forming the core, is visible not only in close proximity to the lateral cut, but they also displace into the inner section of the air core, for example due to the pressure difference, which is shown in Figure 8c. The presence of such large fractions leads to deterioration of transmission in the fiber, due to scattering effects, attenuation of the glass particles at wavelengths longer than 2.5 μm and as a result of influencing the ARHCF guiding mechanism, which is directly dependent on the thickness of the capillaries. The formation of debris is directly related to the laser ablation process, during which the material is removed by evaporation. The condensation of vapors induced by the collision with cooler gas molecules from the environment leads to the deposition of the debris in the vicinity of the laser beam or on the walls of the formed crater. This leads to a deterioration in the effectiveness of laser ablation in the next

cycle of the process [26–28]. Therefore, the increased amount of debris in the case of laser processing with the F-theta lens results mainly from the processed surface area (due to the relatively large focused spot size) from which the material is evaporated in a short time. With this in mind, further experiments were performed using an aspherical lens for which; due to almost an order of magnitude smaller focused beam diameter on the sample and the speed of the process, the amount of debris was significantly reduced.

**Figure 8.** Images of microchannels processed in the ARHCF: (**a**) using an aspherical lens, (**b**) SEM cross-sectional image with debris in the vicinity of the microchannel made using an aspherical lens, (**c**) using the F-theta lens, and (**d**) SEM cross-sectional images with debris in the vicinity of the microchannel made with the F-theta lens.
