Fig. 1. Two optimal photonic bandgap hollow-core fibers. (a) Photonic bandgap hollow-core fiber with transmission loss as low as 1.7 dB/km@1565 nm^[10]; (b) non-surface-mode bandgap fiber with transmission loss of 3.5 dB/km over 160 nm bandwidth^[15]; (c) loss plots

Fig. 2. SEM images of various types of HC-ARFs. (a) Traditional hexagonal Kagome HC-ARF^[22]; (b) hypocycloid Kagome HC-ARF^[25]; (c) UV transmission HC-ARF^[46]; (d) UV transmission HC-ARF with improved performance^[47]; (e) negative curvature HC-ARF with cladding consisting of nodes^[

Fig. 3. Ultra-low loss HC-ARFs. (a) Nested HC-ARF^[51]; (b) nested elliptical element HC-ARF^[52];(c) single ring-elliptical-tube HC-ARF^[53]; (d) nested three-adjacent-tube HC-ARF^[54]

Fig. 4. Ultralow-loss broadband conjoined-tube HC-ARF. (a) SEM image of conjoined-tube HC-PCF; (b) measured loss (black) and simulated loss (grey)^[55]

Fig. 5. Images (top) and spectra (below) of high-order stimulated Raman scattering through 1-m-long hydrogen-filled Kagome fiber. (a) Linearly polarized pump; (b) circularly polarized pump^[2]

Fig. 6. Raman frequency comb with stable phase. (a) Schematic of setup; (b) spectrum of Raman frequency comb and total wavevector mismatch Δβ^[60]

Fig. 7. Raman spectra of air. (a) Vibrational Raman spectrum of ambient air plotted in log scale with upper photo showing vibrational Raman lines at output of fiber using prism; (b) rotational Raman spectrum of nitrogen molecules in ambient air plotted in linear scale with upper photo showing rotational Raman lines at output of fiber using prism and insert being rotational Raman spectrum on blue side of 473 nm^[61]

Fig. 8. Supercontinuum spectra. (a) Supercontinuum spectra obtained using three different spectrometers, and inset is supercontinuum spectra in near infrared for different pulse energies; (b) supercontinuum spectrum after re-calibration ^[63]

Fig. 9. UV-Visible Raman optical frequency comb. (a) Measured spectrum of Raman optical frequency comb with near-field optical images of modal patterns at fiber end face shown above (as₄ is too weak to be directly imaged); (b) photograph of output spectrum dispersed at CaF₂ prism and cast onto fluorescent screen, with as₄ signal highlighted by box exhibiting complex far-field profile^[64]

Fig. 10. Absorption lines of different gas molecules in mid-infrared

Fig. 11. H₂ Raman laser output of 4.4 μm in mid-infrared. (a) Output spectrum; (b) average (peak) output power versus average launched (peak) pump power, with curves 1, 2, and 3 being average (peak) powers of all Raman lasers, 1.906 μm Raman laser, and 4.4 μm Raman laser, respectively^[70]

Fig. 12. High peak power 2.812 μm mid-infrared Raman laser. (a) Setup of mid-infrared gas Raman laser; (b) Raman spectrum at pump power of 381 mW and CH₄ pressure of 1.5 MPa, inset: near-field mode profiles at 1.064 μm, 1.544 μm, and 2.812 μm, respectively; (c) output Raman power versus coupled pump power^[44]