Fig. 1. Processing diagram of femtosecond laser-induced two-photon polymerization process. (a) Optical path diagram of femtosecond laser micromachining system^[35]; (b) methods of fixing micro/nano-fibers in the process of two-photon polymerization^[35]; (c) polymer Bragg grating diagram^[35]; (d) welding process^[36]; (e) ablation process^[36]; (f) polymerization process^[36]; (g) development process^[36]

Fig. 2. Various optical integrated waveguide devices fabricated by two-photon polymerization. (a) Linear polymer FBG integrated on microfiber surface ^[35]; (b) helical FBG integrated on microfiber surface^[39]; (c) polymer waveguide FBG integrated in hollow fiber^[40]; (d) polymer waveguide interferometer integrated in hollow fiber^[41]; (e) all-optical modulator based on a polymer FBG integrated in hollow fiber^[36]

Fig. 3. Hydrogen sensor based on fiber end polymer μ-cantilever^[42]. (a) Schematic diagram; (b) reflection spectra of μ-cantilever beams at different heights; (c) scanning electron microscopy image of the polymer μ-cantilever

Fig. 4. Fabrication process of the polymer μ-cantilever hydrogen sensor. (a) Modeling; (b) model slicing and scanning path design; (c) polymerization; (d) developing; (e) palladium film coating; (f) finished product

Fig. 5. SZP structure of optical fiber end face prepared by two-photon polymerization method^[55]

Fig. 6. Optical path diagram of the optical fiber end face written by the SZP microstructure^[57]

Fig. 7. Fiber end face SZP structure^[57]

Fig. 8. Kinoform SZP of fiber end face prepared by two-photon polymerization^[58]. (a) Structure design; (b) l=-1; (c) l=0; (d) l=1; (e) l=2