Fig. 1. Several typical structures of cladding waveguides. (a) Photonic crystal waveguide fabricated by discrete inscription approach^[21]; (b) circular cladding waveguide fabricated by discrete inscription approach^[45]; (c)(d) waveguides fabricated by helical inscription approach in different directions^[45]

Fig. 2. Influence of experimental parameters on etching selectivity of fused silica etched by different etching agents^[58]

Fig. 3. Study on selective etching of YAG^[61]. (a) Influence of different etching agents on the selective etching of YAG; (b) influence of pulse energy and scanning speed on the selective etching of YAG; (c) influence of scanning speed and etching temperature on the selective etching of YAG

Fig. 4. Research on the principle of selective etching. (a) Raman spectra of fused silica modified by femtosecond laser with single and double pulses^[65]; (b) confocal microfluorescence images of Nd∶YAG modified by femtosecond laser^[60]; (c) phonon Raman mode images of Nd∶YAG modified by femtosecond laser^[60]; (d) Raman spectra of the modified and unmodified area of sapphire^[66]; (e) XRD spectrum of the unmodified area of sapphire^[66]; (f) XRD spectrum of the modified area of sapphire^[66]

Fig. 5. Fabrication of hollow channel based waveguides in fused silica by femtosecond laser enhanced wet etching. (a) Schematic diagram of hollow channel waveguide fabrication^[23]; (b) schematic diagram of liquid-filled hollow channel waveguide^[67]; (c) controlled sealing of liquid inlet by CO₂ laser^[68]

Fig. 6. Hollow channel YAG photonic crystal waveguides fabricated by femtosecond laser enhanced wet etching^[14]. (a) Variation curves of hollow channel size with laser power and polarization after etching, and the influence of polarization on the aspect ratio of hollow holes; (b) schematic diagrams of fabricating vertical etched pores; (c) mode profile and dispersion control ability of photonic crystal waveguides fabricated by femtosecond laser enhanced wet etching

Fig. 7. Fabrication of YAG hollow channel waveguides by femtosecond laser enhanced wet etching. (a) Schematic diagrams of photonic crystals hollow channel waveguides with different etching time^[21]; (b) comparison of the end-faces of photonic crystal hollow channel waveguides before and after polishing^[21]; (c) transmission mode and polarization characteristics of photonic crystal hollow channel waveguides^[21]; (d) schematic diagrams of the etching influenced by crystal orientation in YAG^[22]; (e) section diagrams and top views of waveguides fabricated by facetted etching^[22]; (f) waveguide transmission mode and the loss tested by FP method of the waveguide^[22]

Fig. 8. Fabrication of helical hollow channel waveguide by femtosecond laser enhanced wet etching^[69]