Fig. 1. Three-dimensional optical waveguide devices fabricated by FsLDW technique^[5-9]

Fig. 2. Schematics of the free electron plasma formation. (a) Multiphoton ionization; (b) tunneling ionization; (c) avalanche ionization

Fig. 3. Schematics of the cross-section images of waveguides fabricated by FsLDW (the areas within the black dash lines represent the location of waveguide core). (a) Direct-written refractive index elevated waveguide; (b) stress-extruded dual line waveguide; (c) depressed cladding waveguide; (d) ablated ridge waveguide

Fig. 4. FIFO devices fabricated by FsLDW technique. (a) Sketch of the first FsLDW FIFO device for 4-core fiber^[63]; (b) schematic of an FIFO device for 121-core fiber based on a photonic lantern^[64]; (c) a fan-out device in optical quantum chips for the observation of N00N state Bloch oscillations^[65]; (d) FsLDW prepared FIFO device with board-bandwidth and low insertion loss for 19-core fiber^[66]

Fig. 5. Mode controlling devices fabricated by FsLDW technique. (a) Schematic of the first FsLDW OAM beam emitter^[75]; (b) schematic of tapered couplers^[76]; (c) concept and principle of the trench-based OAM mode multiplexer^[5]

Fig. 6. 1×N splitters fabricated by FsLDW technique. (a) Schematic of the SLM-assisted single-sweep laser direct-written 1×4 splitter^[83]; (b) schematic of three-dimensional LiNbO₃ splitter based on depressed cladding waveguide^[87]; (c)(d) schematic of splitter with the optical-lattice-like cladding and its output modal profile^[88]

Fig. 7. Three-dimensional quantum photonic chips fabricated by FsLDW technique. (a) Schematic of the first three qubit quantum Toffoli gate^[96]; (b) three-dimensional waveguide array for the two-mode braiding^[6]; (c) the array with 2401 waveguides for two-dimensional quantum walks^[98]; (d) three-waveguide coupled system introducing the birefringence^[99]; (e) schematics for four-mode and eight-mode integrated interferometers for the implementation of qFFT^[100]; (f) schematic of a glued hexagonal tree^[101]

Fig. 8. Topological phase in optical waveguide arrays. (a) Spiral induced Floquet topological insulator^[7]; (b) bulk band structure of Floquet topological insulators^[7]; (c)(d) optical field distributions on the output surface of the waveguide array at different propagation distances^[7]; (e) one-dimensional non-Hermitian SSH model^[104]; (f) experimental and simulation results of mean displacement〈Δm〉for different distance differences Δd between waveguids^[104]; (g) nonlinear induced photon anomalous Floquet topological insulator^[105]; (h) bulk band structure of anomalous Floquet topological insulators^[105]; (i) light diffraction in the anomalous Floquet arrays under linear and nonlinear edge excitation regimes^[105]

Fig. 9. Three-dimensional optical waveguide devices for astrophotonics. (a) Schematic of the PL with Bragg gratings^[110]; (b) schematic of the photonic dicer^[8]; (c) schematic of the pupil remapper^[111]; (d) schematic of three-dimensional beam combiner^[112]

Fig. 10. Three-dimensional waveguide sensors. (a) Schematic of the femtosecond-laser-fabricated microfluidic channel and integrated MZI^[117]; (b)(c) schematic of the RI sensor in silver containing glasses and transmission curves of the sensor under different refractive index oils^[118]; (d) schematic of the cascaded FPI and MZI sensing structure^[121]; (e) schematic of an HBGW created in coreless fiber^[122]

Fig. 11. Spherical aberration and its compensation techniques. (a) Introduction of spherical aberration^[138]; (b) schematic of the slit shaping optical path^[145]; (c) optical microscopy photo of the circular cross-section optical waveguide written by the cylindrical lens and slit combination optical path and the scale bar is 10 μm^[146]; (d) deep structures direct writing with the aid of SLM compensation algorithm^[148]; (e) schematic of the multi-foci-shaped femtosecond laser direct writing optical path^[150]

Fig. 12. Direct writing of large-depth waveguides by compensating the spherical aberration by SLM. (a) Symmetric cross-sections of waveguides written under different deptths^[152]; (b) large-scale uniform waveguide array for achieving structural invisibility^[153]

Fig. 13. Three-dimensional reconfigurable optical waveguides. (a) Depth-varying overpass waveguides^[157]; (b) waveguide in a cantilever beam^[158]

Table 1. Low-loss waveguides in various substrates fabricated by FsLDW technology