**3. Results**

Figure 4 presents the selected examples of laser cutting of microchannels in the solid outer cladding of the ARHCF. Photographs of the fabricated microchannels were compared in five sections according to different process parameters used during the laser ablation process. The main criterion for assessing the laser cutting process was the quality of the cut and the efficiency of the process. Qualitative analysis was carried out on the basis of the number and size of microchannels, the occurrence of microcracks in the processing region of the material and the amount of debris generated during the process. Comparison of the quality of the laser-processed microchannels has indicated that the structures made with the use of the F-theta lens are characterized by greater debris in the area of laser treatment, both on the surface and in the air core of the ARHCF. On the other hand, the aspherical lens-aided process introduced significantly less debris and provided a higher quality of the machined features in the fiber structure. In addition, incisions made with the F-theta lens exhibited microcracks that resulted from the generation of high stresses due to the

interaction of the high-energy laser beam over a much larger area than in the process where an aspherical lens was used. When analyzing the execution time for the microchannels fabricated by using the F-theta lens, significantly shorter time was achieved (less than one minute) as compared to the process completed using an aspherical lens, where the average time of a laser cutting was in the range between 4 and 15 min per microchannel (depending on the parameters used). Such large differences in the processing time resulted from different diameters of the laser beam and the speed of the processes used.

**Figure 4.** Microscope photographs of microchannels fabricated with the aid of a laser using a galvanometer scanner with an F-theta lens—F-theta and a setup based on a moving field and an aspherical lens, AL. Results are presented for varying parameters: (**a**) pulse energy, (**b**) scanning speed, (**c**) pulse repetition frequency, (**d**) different hatching lines, and (**e**) the Z-axis step of the beam focus position.

On the basis of the microscope photographs analysis, the process parameters giving the best results are indicated in green. For the AL-based configuration the optimal parameters were: pulse energy 2 μJ, speed 1 mm/s, repetition frequency 5 kHz, distance between lines 1.25 μm (hatching) and increment along the Z axis with a 2.5 μm step. With these parameters, a single microchannel was fabricated with dimensions equal to 250 × 30 μm in 7 min. For the F-theta configuration, the following parameters were found to be optimal: pulse energy 10 μJ, speed 200 mm/s, repetition ratio 50 kHz, distance between lines 5 μm (hatching) and increment in the Z axis with a step of 10 μm. The optimal parameters result in a total processing time equal to 30 s. After optimization of the processing parameters, both methods of fabricating the microchannels resulted in acceptable quality. The air core of the ARHFC was exposed with the desired microchannel cross-section, no capillaries were damaged, no microcracks in the outer cladding were present, and low accumulation of debris was observed inside the fiber.

Figure 5 shows the results of the fabrication of the microchannels in the ARHCF using both the AL and F-theta setup for the optimized parameters. In both cases, unobstructed microchannels were created along their entire length. The SEM images shown in Figure 5b clearly show that the AL approach delivers a superior overall quality of the fabricated microchannels compared to the F-theta lens-based approach. It can be seen that the amount of debris accumulated in the laser cutting area is negligible, and the edge of each microchannel is very sharp with no visible chipping of the glass material. This is due to the fact that the processing of materials using a small diameter laser beam (~2 μm 1/e2) allows the material to be removed in a more controlled manner—layer by layer with a small increment in the Z axis, however, at a cost of increased process duration. A microscope photography of the fabricated microchannel taken with visible light illumination clearly shows the differences in the quality of the fabricated features.

**Figure 5.** SEM images and microscope photographs of fabricated microchannels using fs laser pulses: (**a**) for the configuration with the F-theta lens and (**b**) for the AL configuration.

> Figure 6 shows the results of the experiments during which the microchannels were fabricated without prior removal of the outer polymer layer. This approach eliminates the requirement of stripping the polymer coating of the fiber using, e.g., a scalpel blade, which is not straightforward and can easily damage the fiber, especially in the case when several tens of meters long fiber need to be laser-processed in the middle of its length. For both processing setups, the polymer layer was successfully laser-removed during the process before ablating the glass-based outer clad of the fiber. This method of selective laser removal of the polymer layer is superior when compared to preprocessing mechanical removal from a larger, neighboring area. The laser-based method is ablating the polymer

directly above the microchannel area, which does not weaken the overall structure of the fiber as much as pre-processing removal of the fiber. The parameters used for polymer removal with the fs laser were identical as those for the microchannel cutting, except that for the AL method the number of steps on the Z-axis was increased to fully remove the polymer layer. Analyzing the results obtained for the F-theta lens, a much more efficient removal of the polymer was observed (Figure 6a) than for the AL system. This is a direct result of using a larger beam, which resulted in evaporation of the material from a larger surface for a single laser pulse. Moreover, it was observed that the exposure of the outer optical cladding in a small area causes disturbance of the air flow used to remove the free fractions of the debris from the site of interaction of the laser beam with the fiber (polymer layer thickness ~120 μm), which is visible in the case of the AL technique (Figure 6b). As a result, the debris that arose during the polymer coating removal was deposited in the crater formed during laser cutting, effectively blocking the laser ablation process and directly leading to the partial free-flow fabricated microchannel. The use of vertical air flow in this case would mitigate the problem, but there is a risk of blowing the generated debris into the ARHCF core, which would have a negative impact on the transmission properties of the fiber, in the worst case causing severe losses. Therefore, the proposed solution to this problem is based on material removal from a larger area to ensure improved external air flow at the bottom of the microchannel being processed.

**Figure 6.** SEM images and microscope photographs of the polymer coating removal process and the laser microcutting of the microchannel realized in one process: (**a**) with the F-theta configuration and (**b**) with the AL configuration.
