Fig. 1. Femtosecond laser direct-writing optical waveguide system. (a) Schematic diagram of femtosecond laser direct-writing optical waveguide platform^[18]; (b) four types of optical waveguide structures prepared by using femtosecond lasers in transparent media, including Type-I, Type-II, ridge, and variable cross-section optical waveguide^[21]

Fig. 2. Structure and mode field images of Type-I optical waveguide^[21]. (a) Microscopic end face image of Type-I optical waveguide; (b) mode field of Type-I optical waveguide at a wavelength of 633 nm

Fig. 3. Schematic diagrams of various Type-II optical waveguide structures^{[21, 58-59]}. (a) Schematic diagram of Type-II optical waveguide directly-written by femtosecond laser; (b) double line Type-II optical waveguide and mode field generated by laser direct-writing in LiNbO₃ crystal; (c) Y-type Type-II optical waveguide directly-written by femtosecond laser in BiB₃O₆ crystal; (d) microscopic images with an 8-orbit diamond structure; (e) the induction of multifocal corresponding horizontal waveguides by femtosecond laser in LiTaO₃; (f) propagation modes of Type-II type Cr doped sapphire waveguides with single wire structure at 632 nm under different annealing temperatures, the inset shows the propagation mode of the waveguide at 632 nm at three different temperatures; (g) "cladding+dual-line" waveguide; (h) photonic lattice waveguide

Fig. 4. Preparation and schematic diagrams of different ridge-type optical waveguides^[3]. (a) Laser lithography assisted chemical mechanical etching technology; (b) micro-disk structure of erbium doped lithium niobate thin film; (c) optical micrograph of integrated micro-ring structure on chip; (d) cross section diagram of Z-cut Er³⁺∶TFLN lithium niobate cladding waveguide with Ta₂O₅ cladding deposited on top; (e) monolithic integrated erbium doped lithium niobate waveguide; (f) microimage of Er³⁺∶TFLN FP resonant cavity based on Sagnac ring reflector

Fig. 5. Femtosecond laser direct-writing of variable cross-section optical waveguides^[7,35]. (a) Schematic diagram of femtosecond laser direct-writing variable cross-section optical waveguide; (b)(c) schematic diagrams of the optical waveguide structure of LP₀₁ mode and LP₁₁ mode; (d) the optical waveguide mode field diagrams of LP₀₁ mode and LP₁₁ mode at wavelengths of 532 nm, 633 nm, 976 nm, and 1550 nm, respectively; (e)‒(h) end face and top view of fork grating optical waveguide at input and output terminals

Fig. 6. Amplifier based on Type-I optical waveguide, the net gain curve (red solid line) in the range of 1530 nm to 1565 nm of a 22 mm waveguide directly-written by a 885 kHz KYW laser, the blue dashed line represents the insertion loss of the waveguide^[18]

Fig. 7. Type-II optical waveguide amplifier^[26]. (a) Schematic diagram of the testing system for Type-II optical waveguide amplifier; (b) transmission loss; (c) the variation curve of output SH power (peak) and SHG conversion efficiency with fundamental wavelength input power

Fig. 8. Ridge type optical waveguide amplifier based on Er³⁺ doped lithium niobate thin film^[3]. (a) Schematic diagram of waveguide gain measurement system; (b) schematic diagram of a four-channel optical waveguide amplifier; (c) overall physical diagram of a four-channel amplifier; (d) the mode distribution and intensity distribution of 1550 nm signal light in a four-channel Er³⁺ doped waveguide (inset); (e) optical images of a four-channel array optical waveguide amplifier pumped by a 976 nm laser; (f) gain characteristic curves at 1550 nm; (g) gain characteristic curves at 1530 nm

Fig. 9. High-order mode optical waveguide amplifier^[35]. (a) An optical measurement system for high-order mode optical waveguide amplifiers,insets are designed LP₀₁ and LP₁₁ optical waveguide structures and corresponding mode field simulations; (b) the relationship between the gain of LP₁₁ high-order mode optical waveguide and pump power; (c) the peak intensity plots of LP₁₁ high-order mode optical waveguide with or without pumped laser; (d) net gain spectrum of LP₁₁ high-order mode optical waveguide in a window of 1500‒1600 nm

Fig. 10. Waveguide laser output using a bidirectional pumping scheme^[18]. (a) A measurement system for achieving waveguide laser output using a bidirectional pumping scheme; (b) waveguide laser input and output characteristic curves at 1534 nm, inset shows laser output spectra at pump power of 55 mW and 80 mW, respectively

Fig. 11. Lasers based on Type-II optical waveguide^[2]. (a)‒(e) Lasing mode distribution at 1 μm based on Type-II optical waveguide, including: (a) (b) y-type; (c) 1×4 type ; (d) circular shape; (e) photonic lattice; (f) laser direct-writing schematic diagram of Type-II S curved optical waveguide; (g) dual wavelength lasing spectra at 1064 and 1079 nm; (h) frequency spectrum of the output laser at 31.69 GHz

Fig. 12. A compact hybrid lithium niobate micro-ring waveguide laser pumped by a laser diode^[3]. (a) Schematic diagram of the integrated micro-ring waveguide laser structure, consisting of a CoS packaged semiconductor laser and a high-Q-value Er³⁺∶TFLN micro-ring laser; (b) optical image of a compact hybrid lithium niobate micro-ring laser; (c) optical micrographs of the interface between the semiconductor laser packaged with CoS and the Er³⁺∶TFLN microring waveguide; (d) lasing spectra of the micro-ring waveguide laser; (e) the relationship curve between the laser output power and pump power of a micro ring waveguide laser; (f) the relationship curve between laser output power and driving electrical power of micro ring waveguide laser

Fig. 13. A variable cross-section optical waveguide vortex laser^[7].(a) Schematic diagram of the experimental setup for generation and measurement of a variable cross-section optical waveguide vortex laser, inset shows the pump laser and the output vortex laser spectra; (b)‒(g) mode intensity distribution of far-field output laser, inset shows the corresponding two-dimensional intensity distribution, the numbers in parentheses represent the order of diffraction; (h)‒(m) the mode intensity distribution of the output laser obtained at the focal point of the cylindrical lens, l represents opological charge; (n)‒(s) relationship curve between output power and pump power of vortex laser with variable cross-section optical waveguide