*3.3. Bending Performance of the ARHCF with Manufactured Microchannels*

From an application point of view, an interesting aspect is determining to what extent the fabrication of lateral microchannels will affect the strength of the ARHCF and its transmission. For this purpose, an ARHCF with a length of 1 m was laser processed in its midsection to fabricate 5 microchannels with two different lengths of 100 and 250 μm and a width of 30 μm. Microchannels were produced with the use of an AL configuration and using the parameters defined at the beginning of Section 3. Figure 10a shows a schematic of the measurement system, where the fiber was bent at radii equal to 10, 15, 20 and 25 cm. Microchannels did not change their location relative to the plane during the shift of bending radius. The additional loss introduced by the modification of the fiber structure was defined by comparing the optical power transmitted through an unprocessed fiber and a fiber with the fabricated microchannels with different bending radiuses. The results are shown in Figure 10b.

The performed experiments indicated that for a 100 μm long microchannel (blue triangles), the bending-related loss is comparable to an unprocessed fiber for bending radius in the range between 15 and 25 cm. When the bending radius was reduced to 10 cm, a significant increase in loss (~9 dB) was observed, which was caused by a crack in the outer optical cladding within one of the microchannels. For 250 μm long microchannels (red dots), low-loss transmission was observed for a bending radius in the range of 20 to 25 cm. The lower bending radius resulted in permanent damage (fracture) to the fiber directly in one of the fabricated microchannels. This can be attributed to a significant weakening of the ARHCF structure in the processing region.

**Figure 10.** Bending loss of the ARHCF with microchannels: (**a**) schematic of the measurement setup and (**b**) results of measurements plotted as a function of the bending radius.

#### **4. Discussion**

In this work, we have demonstrated the method of fabricating microchannels in an ARHCF based on the laser ablation process. Conducting detailed research on the strategy and process parameters allowed for reproducible fabrication of microchannels in the ARHCF, which introduced relatively low losses (<0.01 dB for a single microchannel). So far, in the literature, the lowest loss for a single microchannel made with an fs laser on an HCF fiber was 0.35 dB [17]. In comparison, this result is over 35 times worse than the result obtained using the method developed within our research. In another article, a 20 μm microchannel made by laser ablation introduced 1 dB loss [29]. In both cited cases, the holes were made in the hollow-core photonic bandgap fiber (HC-PBG). Due to the complex structure and the light-guiding mechanism in HC-PBG, during the fabrication of microchannels interference in the structure of the fiber is inevitable through microdefects that locally disturb the periodicity and symmetry of the fiber, leading to a deformation of the PBG structure, and hence increase in the transmission loss of the fiber. Recently, there was a report about the creation of a 150 μm long channel using an fs laser in a hollow core negative curvature fiber (HC-NCF), which in its structure is similar to the fiber used by us, however, its cladding structure is formed by a set of non-gapless capillaries. The channel fabricated in the HC-NCF introduced a 0.45 dB loss, which mainly resulted from the microdestruction of the cladding capillaries, which are responsible for confining the light into the air core of the fiber. Table 2 summarizes the best results obtained for single microchannels made in the laser ablation process in HCFs.

The fabrication of a microchannel that showed a low loss (<0.01 dB) partly results from the relatively non-complex structure of the fiber used by us. The mechanism of light guidance in ARHCF takes place within the air core limited by internal capillaries, and interference with the outer cladding by the fabrication of microchannels of different lengths does not have a significant impact on the optical performance of the fiber, which has already been described in detail in [30]. In our case, the gap between the capillaries constituted a natural part of the channel, for which it was not necessary to introduce changes to the fiber structure. Therefore, the key element of the process was the precision of guiding the laser beam so that, in the final stage of the implementation of the microchannel in the outer cladding, the internal capillaries would not be accidentally damaged. The second important factor was the amount of debris formed during the laser ablation process, which, after the process was conducted, was found both outside and inside the ARHCF. It was possible to reduce the problem related to formed debris to some extent by means of an appropriate correlation of the applied process parameters: scanning speed, radiation power and pulse repetition rate, or spot size of the laser beam. An additional advantageous element was the use of air blown parallel to the fiber plane during the process. In this way, the debris generated during laser ablation was to some extent blown off the surface of the treated fiber, without causing excessive accumulation and priming of the laser beam. Obtaining low losses for single microchannels fabricated in the ARHCF allows us in future research to use them in a greater number to significantly accelerate the diffusion of gas into its interior.


**Table 2.** Comparison of the losses induced by microchannels in HCFs.

In most of the works, the fabricated microchannels were realized for relatively short sections of the fiber (up to several centimeters) and their analysis was limited only to the losses they introduced without addressing the issue of weakening the fiber structure and lowering mechanical resistance. In this work, it was shown that while maintaining precision and limiting the amount of generated debris, it is possible to fabricate microchannels from single μm to several mm. The height of the microchannels in this case was limited to the free space between the capillaries, which was 10–12 μm at the narrowest point. In addition, studies were carried out to determine to what extent the fabrication of microchannels affects the strength of the fiber and bending resistance, which for potential applications in the construction of long absorption cells or sensors is important in their miniaturization. It has been proven that when bending is required, it will be more advantageous to fabricate microchannels having a relatively short length, since they exhibit greater bending resistance due to the shorter length of the fiber subjected to modification.
