Fig. 1. Light conduction mechanism of photonic band gap. (a) Schematic diagram of 2D photonic band gap structure with triangular micropore distribution^[24]; (b) Bragg scattering interference effect in photonic band gap light conduction mechanism^[8]

Fig. 2. Schematic diagrams of Bragg fiber structure and its refractive index profile^[5]

Fig. 3. Schematic diagrams of ARF light guiding mechanism. (a) Schematic diagrams of ARROW structure and its transmission spectrum (bottom)^[25]; (b) schematic diagram of anti-resonant reflection mechanism^[26]

Fig. 4. Hollow core fiber development history

Fig. 5. SEM images of HC-PBGF and loss trend diagrams of different bands. (a) SEM image of 7-core hole HC-PBGF^[40]; (b) SEM image of 19-core hole HC-PBGF^[42]; (c) low-loss band diagram of 7-core hole HC-PBGF^[40]; (d) loss trend chart of 19-core hole HC-PBGF in C+L band^[41]

Fig. 6. SEM images of Kagome hollow core fiber and its transmission spectra. (a) SEM image of Kagome hollow core fiber^[47]; (b) Kagome hollow core fiber with negative curvature wall of internal rotation wheel^[10]; (c) transmission spectra of Kagome hollow core fiber with negative curvature wall of internal rotation wheel^[49]

Fig. 7. Attenuation spectra and bending loss trend of Kagome hollow core fiber with negative curvature core. (a) Attenuation spectra^[50]; (b) trend chart of the measured bending loss^[51]

Fig. 8. Simplified structures of single-tube ARF, HC-PBGF and Kagome hollow core fiber. (a) Simplified ARF^[53]; (b) simplified HC-PBGF^[54]; (c) simplified Kagome hollow core fiber^[9]

Fig. 9. Comparison between single-tube ARF and NANF^[60]. (a) Structural comparison; (b) comparison of 3 dB contour limit principle

Fig. 10. Ultra-low-loss CTF structure,broadband transmission, and loss spectra^[67]. (a) SEM structure image; (b) transmission spectra; (c) measured loss (black) and simulated loss (grey) spectra

Fig. 11. DNANF structure and loss spectra^[14]. (a) SEM image of DNANF; (b) comparison of loss spectra of DNANF and SMF-28e measured by truncation method; (c) magnification diagram of C+L band loss spectra

Fig. 12. Schematic diagrams of the two-stage stack and draw process for ARF fabrication^[72]. (a) Schematic diagram of 6-tube ARF prefabricated rods; (b) schematic diagram of prefabricated rod drawn in the first stage of the drawing tower; (c) schematic diagram of the intermediate prefabricated rod in the second stage of the drawing tower

Fig. 13. Schematic diagrams of OTDR test with an SMF pigtail connected into the hollow core fiber^[84]. (a) Via SOP; (b) via EOP; (c) backward-reflecting signal curves measured on NANF sample

Fig. 14. Schematic diagrams of optical side-leakage radiation measurement^[85]. (a) Principle of leakage light collection by an IS(PD: photodetector); (b) schematic diagram of light leakage out of an ARF; (c) schematic diagrams of varied radial Poynting fluxes with different SOPs

Fig. 15. DNANF transmission losses measured by truncation (dark blue: short truncation, light blue:long truncation), insertion (red dots), and bidirectional OTDR (green dots)^[68]

Fig. 16. NANF transmission loss spectra in the range of 1250‒1650 nm, experimental test loss (blue), simulated total loss (red), limiting loss (black) and micro-bending loss (green)^[63]

Fig. 17. NANF transmission loss and dispersion trend with wavelength in 400 Gbit/s transmission over 1000 km^[90]. (a) Loss spectra; (b) dispersion curve

Fig. 18. Comparison of DGD simulation and measurement in ARF and SMF^[91]. (a) Comparison of measured and simulated DGD for the same NANF; (b) comparison of RMS DGD between symmetric 6-tube NANF and SMF

Fig. 19. Experimental structure of single-wavelength Tbit-level without nonlinear loop transmission in hollow core fiber^[94]

Fig. 20. Experimental structure for the first demonstration of quasi-continuous 154.5 Tbit/s transmission across the S+C+L band using ARF^[95]

Fig. 21. Experimental structure of the first penalty-free real-time full-compound optical fiber transmission with 5-tube NANF^[96]

Fig. 22. Structure diagrams of 502.6 Tbit/s transmission experiment system in S+C+L band with 5-tube DNANF^[97]

Fig. 23. Experimental structure diagram of hollow core fiber transmission wavelength division multiplexing system based on joint probabilistic shaping and preequalization^[101]

Fig. 24. Structure of hollow core fiber high-capacity long-distance transmission system based on the inverse design of Raman fiber amplifier^[102]. (a) Schematic diagram of input pump parameters of the Raman amplifier system obtained from the output yield curve through the neural network in the reverse design; (b) schematic diagram of hollow core fiber spectral transmission system with Raman fiber amplifier

Fig. 25. 1 kW continuous laser transmission over 1 km NANF^[87]. (a) Experimental structure diagram; (b) variation trend chart of NANF output power (P_out) versus NANF input power (P_in) and throughput efficiency

Fig. 26. Experimental system diagram of mid-infrared gas Raman ARF laser^[114]