*3.2. Influence of Microchannel Length on the ARHCF Transmission*

To determine the usefulness of the proposed method for modifying the ARHCF structure with the aid of the ultrafast laser micromachining process, we have investigated the possibility of cutting long microchannels with a length of up to ~2 mm. Here, 35 microchannels were made at the output end of a 15 m-long ARHCF (the same fiber as in the previous experiment) in 7 sections, each containing 5 microchannels with different lengths and fixed widths equal to 30 μm. The distance between the sections was 10 cm and the microchannels were separated by 1 mm. The polymer coating was removed locally, mechanically using a scalpel prior to laser processing. All microchannels were made using an aspherical lens for optimal process parameters, as defined at the beginning of this section. Figure 9a,b shows fabricated microchannels with lengths of 0.1, 0.25, 0.5, 0.75, 1, 1.5 and 2 mm. The quality of the fabricated microchannels was at the same level regardless of their length. In-depth analysis of the fabricated structures has shown that no damage occurred to the internal capillaries that form the actual fiber cladding. The impact of a microchannel length on the transmission properties of the ARHCF was determined using a similar approach to that previously described in Section 3.1. The optical power at the fiber output was measured after individual sections of the microchannels were cut off. The results are plotted in Figure 9c. It can be seen that microchannels with a length below

0.5 mm introduce very low transmission loss, not exceeding 0.1 dB. For microchannels with lengths greater than 0.5 mm, an increase in light attenuation was observed. The higher loss level observed in the case of longer microchannels was caused by the accumulation of debris in the vicinity of the processing area, which was pushed into the core by the air flow used to clean the processed region.

**Figure 9.** Additional loss of the ARHFC plotted as a function of the length of the microchannel: (**a**) view of a single section containing 5 microchannels with a length of 0.25 mm, (**b**) microchannels with lengths equal to 0.1, 0.25, 0.5, 0.75, 1, 1.5, 2 mm, respectively, and (**c**) loss vs. microchannel length.
