Fig. 1. Three types of waveguides directly written by femtosecond laser and their cross-sectional configurations (insets). (a) Type I; (b) type II; (c) depressed cladding waveguide^[18]

Fig. 2. Waveguide structures written by femtosecond laser in ZBLAN glass with different numbers of lower refractive index tracks. (a) 6; (b) 12; (c) 24^[41]

Fig. 3. End view of waveguides written by femtosecond laser in TWB-2 glass plate^[46]

Fig. 4. Microscopic images of depressed cladding waveguide end facets with different cross-sectional diameters written by femtosecond laser in PTR glass^[47]

Fig. 5. End view of depressed cladding waveguides with different cross sections inscribed in Nd∶YAG crystal. (a) Rectangular^[8]; (b) round^[48]

Fig. 6. Optical microscopic images of cross sections of 4 depressed cladding waveguides inscribed in LiNbO₃ and locations of waveguides indicated by red circles^[55]

Fig. 7. Depressed cladding waveguides with different diameters inscribed in ZnS crystal. (a) 50 μm; (b) 100 μm; (c) 150 μm^[56]

Fig. 8. Performance characterization of depressed cladding waveguide inscribed in sapphire. (a) Cross-section of depressed cladding waveguide inscribed in sapphire;(b) near-field intensity profile measured at waveguide's output at wavelength of 2850 nm; (c) radial refractive index profiles measured after successive annealing cycles and peak refractive index change of waveguides inscribed by lasers with different energies and under different annealing temperatures^[57<

Fig. 9. Depressed cladding waveguides inscribed in Ti₂O₃-doped sapphire by 795 nm femtosecond laser, and microscopic image of cross-section of waveguide shown in inset. (a) μ-PL intensity along cutting-line; (b) μ-PL spectra from different areas^[58]

Fig. 10. Depressed cladding waveguide directly written by femtosecond laser in polycrystalline diamond. (a) Schematic of fabrication; (b) microscopic image of cross-section of depressed cladding waveguide, superimposed by mode profiles corresponding to cross-sectional diameters of 20 μm and 25 μm^[61]

Fig. 11. Three types of techniques for femtosecond laser direct writing. (a) Linear translational scan; (b) transversal and helical scan; (c) longitudinal and helical scan^[63]

Fig. 12. Microscopic images of circular waveguides inscribed in Nd∶YAG ceramic. (a) DWG-1, ?= 100 μm, longitudinal and helical scan; (b) DWG-3, ?= 50 μm, longitudinal and helical scan; (c) DWG-4, ?=100 μm, linear transversal scan; top view of cladding layers of (d) DWG-1 and (e) DWG-4^[63]

Fig. 13. View of Nd∶YAG ceramic exit surface S2 under fiber-coupled diode pumping, and photos of waveguides without pumping shown in inset. (a) Bulk material; (b) DWG-1 (?=100 μm); (c) DWG-3 (?=50 μm) ; (d) DWG-4 (?=100 μm)^[63]

Fig. 14. Methods for generating hollow tubular beam to fabricate waveguide. (a) Schematic of experimental setup and spatial light intensity profile of high-order nondiffracting Bessel beam located between spatial light modulator and focusing lens shown in inset; (b) phase mask employed for producing Bessel beam and (c) transversal intensity distribution at focus of lens in air when n=10 and r₀=0.8 mm ^[64]

Fig. 15. Waveguide with tubular depressed-refractive-index cladding directly written by longitudinal scan based on focal field with circular intensity distribution. (a) Phase contrast microscope (PCM) image of waveguide with tubular depressed-refractive-index cladding (side view) and white part indicating negative index change; (b) optical transmission micrograph of the obtained tubular structure (end view); (c) near-field mode profile of tubular waveguide injected by 800 nm CW laser^[

Fig. 16. Schematic of transversal femtosecond laser direct writing of depressed cladding waveguide with phase masks used for focal field engineering shown in inset and experimental sample moving along X-axis. (a) Circular cladding composed of dozens of parallel filaments inscribed line by line using ellipsoidal focal spot; (b) square cladding with four sides inscribed one by one using slit shaped focus; (c) circular cladding formed in single scan using tilted longitudinal annular ring-shaped focal

Fig. 17. Schematic of experimental setup and phase masks for horizontal and vertical sides shown in inset^[36]

Fig. 18. Simulated peak laser intensity distributions in focal spots of slit-shaped beams. Intensity distributions of (a) horizontal and (b) vertical sides for fabrication of small-mode-area waveguides; (c) intensity distribution of 6 μm×6 μm square; intensity distributions of (d) horizontal and (e) vertical sides for fabrication of large-mode-area waveguides; (f) intensity distribution of 12 μm×12 μm square^[38]

Fig. 19. Optical micrograph and near-field mode profile of cross-section of waveguide. (a) Microscopic image of cross-section of tubular waveguide inscribed in ZBLAN glass with cross sectional size of 12 μm×12 μm; (b) near-field mode profile under bright field; (c) near-field mode profile under dark field^[38]; (d) microscopic image of cross section of tubular waveguide inscribed in LN crystal; (e) near-field mode profile of waveguide under s-polarization;

Fig. 20. Depressed cladding waveguide with annular ring-shaped cross section based on focal fields with piecewise annular ring and continuous annular ring and experimental sample moving along X-axis. Focal field with piecewise annular ring: (a) side view, (b) phase mask, (c) 3D isosurface focal field obtained by simulation (isosurface given by intensity at 30% of peak value), (d) 2D intensity distributions in planes viewed in directions indicated by yellow arrows; focal field with continuous annul

Fig. 21. Optical micrograph and guided modes of waveguide written by continuous annular ring-shaped focal field. (a) Cross-section of waveguide and guided region indicated within dotted circle; 2D intensity distributions of guided modes at (b) 785 nm and (c) 1550 nm, respectively^[35]

Fig. 22. Design of focal field and calculation of phase mask with experimental sample moving along y axis. (a) 3D isosurface of focal field obtained by vector integration; (b) phase mask obtained by iterative calculation; (c) 3D isosurface of focal field simulated by phase mask^[14]

Fig. 23. Optical micrographs and guided modes of depressed cladding waveguide written by discrete ring-shaped focal field. (a) Cross-section of waveguide; (b) top view of waveguide; 2D intensity distributions of guided (c) H and (d) V polarization modes at 1550 nm^[14]

Fig. 24. Optical transmission microscopic images of depressed cladding waveguides with different cross-sectional shapes in Nd∶YAG ceramic. (a) Hexagon; (b) circle; (c) trapezoid^[27]

Fig. 25. Depressed cladding waveguides with different cross-sectional shapes. (a) Triangle; (b) square; (c) pentagon; (d) hexagon; (e) circle^[68]

Fig. 26. Cross-sectional microscopic images of waveguides. (a) Double-cladding waveguide; (b) single-cladding waveguide with diameter of 100 μm; (c) single-cladding waveguide with diameter of 30 μm^[69]

Fig. 27. Near-field intensity profiles of TE mode in waveguide at different wavelengths. (a)-(c) 632.8 nm; (d)-(f) 1064 nm^[69]

Fig. 28. Annual laser beams generated from tri-cladding and dual-cladding waveguides. Microscopic images of (a) tri-cladding and (b) dual-cladding waveguides in Nd∶YAG crystal; near-field modal profiles of (c) tri-cladding and (d) dual-cladding waveguides at 632.8 nm; annular laser modal profiles of (e) tri-cladding and (f) dual-cladding waveguides at 1064 nm under 808 nm optical pump with TE polarization^[29]

Fig. 29. Structure and modal profile of optical-lattice-like cladding waveguide. (a) Schematic of fabrication of typical photonic microstructure with guiding core surrounded by hexagonal track array; (b) cross-sectional sketch of photonic micro-structured cladding waveguide; (c) cross-sectional microscopic image of optical-lattice-like cladding waveguide Element-1 written by fs laser in Nd∶YAG crystal; (d) measured near-field modal profiles of TE and TM polarizations at 1064 nm^[

Fig. 30. Light confinement of optical-lattice-like cladding waveguides with different cladding layers^[71]

Fig. 31. Characterization of beam splitting for photonic structure in passive regime. (a) Schematic of waveguide elements 2 (top) and 3 (below); (b) microscopic images of waveguide elements 2 (top) and 3 (below); (c) prototype of beam splitter with connection of waveguide elements 1, 2 and 3; (d) simulated evolution of 1064 nm light propagating along optical-lattice-like cladding waveguide; (e) measured intensity distributions of beam splitter in both passive and active regimes^[

Fig. 32. Experimental setup for laser test of dual-wavelength waveguide and cross-sectional microscopic image of depressed cladding waveguide with diameter of 100 μm shown in inset^[30]

Fig. 33. Experimental results. Laser emission spectra at laser polarization angles of (a) 0°, (b) 45°, and (c) 90°, respectively; (d) relationship between output power and pump power at 812 nm pump and linear fit of experimental data indicated by solid line^[30]

Fig. 34. Cross-section of depressed cladding structure fabricated in nonlinear Nd∶YCOB crystal^[78]

Fig. 35. Experiments for waveguide lasing and self-frequency-doubling. (a) Schematic of end-face coupling system; (b) oscillation cavity for cladding waveguide lasing; (c) oscillation cavity for cladding waveguide self-frequency-doubling^[78]

Fig. 36. Lasing and self-frequency-doubling performances of depressed cladding waveguides in Nd∶YCOB crystal. (a) Near-field intensity distributions of waveguide lasers at 1062 nm and boundaries of the guiding areas indicated within circles; (b) relationship between near-infrared laser output power and absorbed pump power for WG1, WG2, and WG3; (c) photograph of FP cavity employed for waveguide laser self-frequency-doubling experiments; (d) modal distributions of green lasers generated from WG1, WG2 and

Fig. 37. Depressed cladding waveguides fabricated in MgO∶PPSLT sample for second harmonic generation. (a)Schematic of fan-out pattern on MgO∶PPSLT sample and FLW process; (b) microscopic images of WG1-WG4^[9]

Fig. 38. Waveguide-integrated light-induced quasi-phase matching structures. (a) Design of LiQPM depressed cladding waveguide; (b) modulation scheme of QPM grating structure inside waveguide core^[80]

Fig. 39. Experimental results of temperature tuning for SHG of two kinds of LiQPM grating devices. (a) Dual-wavelength SHG; (b) multi-wavelength SHG^[80]

Fig. 40. Optical microscopic images of superficial depressed cladding waveguides written by femtosecond laser in Nd∶YAG crystal. (a) Sectional view of semicircular waveguide; (b) sectional view of rectangular waveguide; (c) top view of rectangular waveguide; speckle patterns collected from output faces of (d) semicircular and (e) rectangular waveguides at 460 nm; (f) experimental setup for waveguide specklegram temperature sensor based on end-face coupling system in which thermo electric cooler used to c

Fig. 41. Effects of laser intensity and wavelength on sensitivity of different temperature sensors. (a)(c) Semicircular waveguide; (b)(d) rectangular waveguide^[33]

Fig. 42. Waveguide Bragg gratings (WBGs) in ZBLAN glass. (a) Hexagonal lattice of points within core of waveguide and three-dimensional rendering of structure shown in inset (not to scale); (b) top-down differential interference contrast images at three different positions showing grating phase relation across core and reduction of grating contrast indicated apparently in close proximity to waveguide cladding (top profile)^[42]

Fig. 43. Electro-optical tunable lithium niobate waveguide embedded with Bragg gratings. (a) Schematic of direct integration and characterization; (b) e light of circular waveguide structure with diameter of 15 μm; (c) multiscan Bragg grating with period of Λ=704 nm and upper and lower waveguide lines omitted for clear imaging; (d) cross section of polished WBG with integrated electrodes; (e) closed-circular waveguide; its modal profiles and insertion losses of (f) o light and (g) e light at λ<

Fig. 44. Bragg grating performance of depressed cladding waveguide. Reflection spectra for (a) o light and (b) e light as input light; (c) maximum spectral tuning of more than a peak width achieved with applied voltage of ±840 V for p light; (d) relative shift of central Bragg reflection maxima for s light (o light) and p light (e light) and high bandwidth operation for frequency test shown in inset^[13]

Fig. 45. Mode field converter. (a) Schematic of fs-laser inscription process; (b) schematic of tapered waveguide; (c) model for implementation of tapered cladding with reduction factor of d_max∶d_min by decreasing track separation; (d) model for implementation of tapered cladding with reduction factor of d_max∶(d_min/2) by decreasing track separation and reducing number of tracks^[81]

Fig. 46. Morphology and mode field characterization of mode field converter. (a) Microscopic image taken in transmission mode of 4∶1 tapered test structure fabricated with input radius of 24 μm and output radius of 6 μm (taper length L shortened to well appreciate details); (b) modal profiles at output of 2∶1 and 4∶1 tapered waveguides at 633 nm (left) and 850 nm (right), respectively^[81]

Fig. 47. Photonic devices based on Y-junctions. (a) Schematic of fabricated structures: straight waveguide, Y-junctions and Mach-Zehnder interferometer; (b) optical microscopic images of cross sections at different planes of Y-junctions and image at bottom indicating longitudinal view of splitting region and approximate positions of above images^[82]

Fig. 48. Y-branch splitter based on superficial depressed-cladding waveguides. (a)-(e) Cross-sectional images and (f) top view of surface cladding waveguides; simulated refractive index distributions at cross sections of (g) input port, (h) joint port, and (i) output port, as well as (j) mode propagation process^[34]

Fig. 49. 2×2 directional coupler based on depressed cladding waveguides. (a) Layout of interaction region with several cladding tracks skipped; (b) geometry of 2×2 waveguide coupler; (c) microscopic image of fabricated directional coupler obtained after stitching; (d) image of output facet of directional coupler; (e) intensity distribution at output facet of directional coupler with splitting ratio of 48∶52^[15]

Fig. 50. 3×3 directional coupler based on depressed cladding waveguides. (a) Geometry of 3×3 directional coupler; (b) cross-sectional geometry of interaction region; (c) top view image of 3×3 coupler obtained after stitching; (d) microscopic image of end facet of 3×3 directional coupler; (e) intensity distribution at output facet of directional coupler with coupling distance of 14 μm^[15]

Fig. 51. Schematic of depressed cladding waveguide directional coupler and transmission differential interference contrast microscopic images taken at different positions with dashed circles marking defects created by hard on/off switching of laser and output facet of directional coupler shown on right-hand side^[39]

Fig. 52. Reconfigurable 2×2 directional coupler based on depressed cladding waveguides with ring cladding composed of 16 parallel damage tracks and circles tilted 10° with respect to X axis^[14]

Fig. 53. Electro-optically tunable directional coupler. (a) Top views of straight interaction region (middle) and two curved segments (left and right) of directional coupler; (b) partial top views of electrodes and directional coupler with interaction length of about 1.8 mm; (c) equipotential contour of electric field around electrodes;(d) splitting ratio versus voltage applied to electrode; (e) output modes at 1550 nm of coupler under three different voltages^[14