Fig. 1. Schematic of the heptagonal core ARF. D_c is the diameter of the inscribed circle. The tube wall thickness is t. The inner and outer curvature radii of the cladding nodes are R_b and R_s, respectively.

Fig. 2. Simulated results of the proposed ARF with t = 0.7 µm, D_c = 30 µm, R_b = 1.2 µm, and R_s = 0.3 µm. (a) Simulation of the loss spectra for the fundamental mode (red curve) and the first-order higher-order mode (blue curve). The inset figures are mode-field distributions. (b) Simulated confinement loss of a proposed ARF as a function of the radius of the node’s curvatures R_b and R_s.

Fig. 3. (a) SEM image of the fabricated ARF. (b) Zoomed-in view of the cladding structure.

Fig. 4. Transmission characteristics of the fabricated ARF. (a) Transmission spectra for the long fiber lengths (blue curve) and the short fiber lengths (red curve), as observed from experimental data. (b) The measured transmission spectra (green curve) are compared with the simulated loss spectra (orange curve). The shaded area in light green represents the measurement uncertainty. The inset figure displays the loss spectra within the wavelength range of 800–1300 nm.

Fig. 5. Schematic diagram of the laser beam mode shaper. L1, L2, and L3 are the plano-convex lenses with focal lengths of f1 = 50 mm, f2 = 50 mm, and f3 = 100 mm, respectively. M1, M2, and M3 are the dielectric mirrors with a reflectance exceeding 99.5%. Pellicle beam splitters (PBSs, 92:8) are used to split the partial power from the input and output ends of the FUT. Power meters (PMs) calibrate the incident power from the laser source and monitor the power coupled into the ARF and the output after M3. The M² measurement system (M² system) assesses beam quality, mode-field distributions, and the beam width as a function of the longitudinal distance. The inset figure depicts the near-field intensity pattern at the output of the ARF.

Fig. 6. (a) Light source beam quality. (b) Gaussian fitting of the beam mode-field distribution. (c) ARF output beam quality. (d) Gaussian fitting of the beam mode-field distribution. The inset figures depict mode-field distributions of the laser source and the output of the 1-m ARF.

Fig. 7. Measured output power and coupling efficiency as a function of the incident laser power.

Fig. 8. (a) Output power and beam quality versus time when the transmission power is 600 mW. (b), (c) Fiber output beam quality at 0 and 60 min. Inset figures are the corresponding measured beam profiles.