Fig. 1. An envisioned example of 3D optical waveguide devices integrated in one flexible PDMS substrate^[27]

Fig. 2. Femtosecond laser direct-writing optical waveguides with positive and negative refractive index changes. (a) Type-I^[36]; (b) Type-II^[37]

Fig. 3. AFM of the waveguide cross-section of a quartz glass^[40]

Fig. 4. Relative concentration distribution of six ions in crown glass after laser irradiation^[66]

Fig. 5. Two types of waveguides in x-cut LiNbO₃ written by laser at a pulse energy of 0.2 µJ and different pulse widths^[62]. (a) (b) Extraordinary refractive index profile and guided optical mode at a wavelength of 633 nm for a pulse duration of 220 fs, Type-I; (c) (d) extraordinary refractive index profile and guided optical mode at a wavelength of 633 nm for a pulse duration of 1.1 ps, Type-II

Fig. 6. Schematics of two-photon polymerization processing. (a) Two-photon absorption processing^[90]; (b) scheme for fabrication of optical waveguide based on two-photon polymerization ^[4]

Fig. 7. Femtosecond laser direct-writing Type-I optical waveguides in PDMS immersed with photosensitized monomer^[27]. (a) PDMS curing; (b) immersing the cured PDMS into photosensitized monomer solution; (c) femtosecond laser direct-writing Type-I optical waveguides; (d) heat treatment to remove residual photosensitized monomer; (e) absorption spectrum of photosensitized monomer phenylacetylene in acetonitrile solvent, purple arrows indicate two-photon and three-photon absorption wavelengths, inset shows the multiphoton polymerization reaction equation of phenylacetylene; (f)(g) top view and cross-sectional view of the Type-I optical waveguide directly written in PDMS by femtosecond laser; (h) plots of optical waveguide width and height as a function of direct-writing depth

Fig. 8. Femtosecond laser direct-writing compound optical waveguide via multiple scanning^[28-29]. (a) Schematic of direct-writing optical waveguide by multi-scan stacking method;(b1)-(b3) cross-sectional view (upper) and top view (lower) of the optical waveguide written by single-, double-, and triple-scan stacks; (c) large array of 12 × 12 optical waveguides written directly based on the triple-scan stacking method

Fig. 9. Femtosecond laser direct-writing Type-I optical waveguide in cured PDMS doped with photosensitizer^[30-31]. (a) Schematic of 515 nm femtosecond laser direct-writing optical waveguide in flexible PDMS; (b) transmission spectra of PDMS doped with different photosensitizers, inset, cured PDMS bulk after doping with photosensitizer; (c) microscopic near-field images of optical waveguides direct-written under different laser power densities, left, burnt, right, not burnt; (d) two photosensitive mechanisms, namely, cleavage and hydrogen-abstraction; (e) measurement of optical waveguide coupling, transmission loss, and mode field

Fig. 10. Electrode array devices inserted inside the human cochlea^[29]. (a) Schematic of a cochlear implant electrode array module inserted inside the human cochlea; (b) cochlear implant module with the PDMS waveguide bundle endoscope

Fig. 11. Simulation and calculation for the input and output imaging signals based on optical waveguide arrays^[29]. (a)(b) Input from the MNIST database and refractive index distribution of a 12×12 PDMS waveguide bundle; (c)(d) x-z and y-z propagation profiles along 2 cm propagation length at 600 nm wavelength; (e)(f) output intensity profile after 2 cm and 4 cm propagation length