*2.3. Experimental Setup and Procedure*

The experiments were carried out using a TruMicro 2030 laser (Trumpf Laser GmbH, Schramberg, Germany), generating fs pulses with adjustable duration from 270 fs to 20 ps, at a wavelength of 1030 nm, an average power of 20 W, and adjustable repetition rate from 200 kHz (100 μJ pulse energy) to 2 MHz (10 μJ pulse energy). Furthermore, the laser has a built-in "pulse picker" that allows operation at selected frequencies (>1 kHz). In order to develop the most effective way of making the lateral cuts in terms of quality and time of implementation, two configurations of delivering the laser beam to the material surface were investigated. In the first approach, shown in Figure 3a, the laser beam, after passing through the beam expander, was directed to the biaxial galvanometer scanners (model IntelliSCAN 14, ScanLab, Munich, Germany) controlled by the TruTops PFO software (Trumpf Laser GmbH, Schramberg, Germany), which was used for fast and precise positioning of the mirrors to deflect the laser beam. The beam (1/e<sup>2</sup> diameter of 10 mm) leaving the scanner was focused on the sample using an F-theta lens with a focal length of f = 56 mm, producing a focused spot with 1/e2 diameter of ~12 μm. Galvanometer scanners in combination with the F-theta lens allowed for rapid scanning of the surface of the processed material (with speeds of m/s or greater), which significantly reduced the time of the process. To ensure precise positioning of the ARHCF with respect to the incident beam, the fiber was placed in a rotating holder (model CRM1, Thorlabs GmbH, Bergkirchen, Germany) mounted on biaxial motorized stages (model 8MTF-102, Standa Inc., Vilnius, Lithuania). Visual control of the process was achieved using a CMOS camera (ALPHA1080 Series, Hangzhou, China).

**Figure 3.** Schematic of the experimental setup for: (**a**) a system with galvanometer scanners and an F-theta lens and (**b**) a moving field and an aspherical lens.

The second variant of laser micromachining is a system with a moving field of work as depicted in Figure 3b, where the beam, after passing through the beam expander, is focused on the fiber using an aspherical lens (model AL1210-B, Thorlabs GmbH, Bergkirchen, Germany) with a focal length of 10 mm and NA = 0.55, providing a focused spot diameter. ~2 μm (1/e2). The fiber placed in a rotating holder (model CRM1, Thorlabs GmbH, Bergkirchen, Germany) embedded in the platform was moved in relation to the incident laser beam using precise motorized XYZ stages at a speed of ~1 mm/s. All three stages were driven by linear motors on the X-Y (model 8MTF-102, Standa Inc., Vilnius, Lithuania), Z axis (model 8MT175-50, Standa Inc., Vilnius, Lithuania), and the positioning resolution for each axis was 0.31 μm. The advantage of this system is the possibility of using the CMOS camera on the same axis as the laser beam, which enables real-time monitoring of the laser ablation processes.

To effectively reduce the occurrence of debris in the laser treated area for both configurations, experiments were conducted in a natural environment without the use of shielding gases except for the transverse airflow used to remove ablation residues.

The research work began with the precise alignment of the fiber in relation to the incident laser beam. For this purpose, a CMOS camera with a source of light transmitted through the fiber, situated perpendicularly to the top layer of the fiber, and a rotating holder were used. By focusing the camera on the surface of the inner capillaries and rotating the fiber with the rotating holder, the tangent points of the inner capillaries to the outer cladding were controlled so that their positions were on the same level. Thus, the space between the capillary pair was centrally located just below the focus area of the laser beam, as shown in Figure 2. In the next step, the location of the focus of the laser beam in relation to the fiber surface was determined. This was achieved by intentionally positioning the fiber far below the focal length of the lens and subsequently moving the fiber mounting platform on the Z axis upward in 2.5 μm steps. During this process a single line scan across the ARHCF was performed at low pulse energy, until the effects of laser ablation were observed (polymer coating was locally mechanically removed using a scalpel before the laser processing). After the location of the focal point was defined with respect to the position of the fiber, subsequent experiments were conducted. The laser cutting is a complex process dependent on several variable parameters related to both the laser source (i.e., pulse energy, repetition rate, average power) and the process itself (e.g., scanning speed, distance between lines—hatching, multiplicity of the process, or a change of the lens focus position in relation to the material). We chose manufactured microchannels with dimensions 250 × 30 μm, which we found to be optimal for the ARHCF used in the experiments (due to the distance between the capillaries of ~12 μm as shown in Figure 1).

In order to define the optimal parameters for the process of cutting a microchannel and the pattern of scanning the surface, a series of systematic experiments were carried out for each setup configuration. A summary of the individual parameters used during the experiments is presented in Table 1. Additionally, we investigated the possibility to remove the protective polymer layer from the fiber with the use of fs laser pulses for both techniques.


**Table 1.** Laser and process parameters used for optimization of laser cutting of the microchannel.

After laser treatment, the macroscale analysis of the quality of the microchannels was performed using a digital microscope (model VHX 5000 Keyence, Osaka, Japan) with transmitted light illumination of the sample. Detailed analysis was carried out using the scanning electron microscope (SEM, model EVO MA 25, Zeiss, Oberkochen, Germany). To minimize the charging effect, the fiber was coated with a ~5 nm gold layer using a sputter coater (model Quorum Q150R ES, Lewes, UK). Moreover, the fiber was placed on top of a conductive carbon strip attached to a grounded sample holder.

To investigate the influence of the fabricated microchannels on transmission losses we used a custom-built difference frequency generation-based optical frequency comb source (DFG COMB) operating within the fundamental transmission band of the fiber (3.3–3.4 μm). The broadband comb source was coupled into the ARHCF, and the losses were estimated by measuring the difference in the exiting optical power before and after laser processing of the microchannels. Measurements were taken with the aid of a digital optical power meter (PM100D with high-resolution thermal power sensors S401C, Thorlabs GmbH, Bergkirchen, Germany). This setup was also used to measure the ARHCF bending losses (see Section 3.3 for further details).
