Fig. 1. (a) Schematic diagram of laser direct writing technology process^[18]; (b) Main factors affecting the quality of DOE preparation

Fig. 2. (a) Schematic diagram of the autofocus subsystem (left) and magnification of the in-focus and oblique beams (right)^[31]; (b) SEM photo of chromium grating with period of 2 µm^[31]; (c) Schematic diagram of laser direct writing system for preparing curved diffraction structures^[32]; (d) Schematic diagram of autofocus system^[32]; (e) Spherical lens with binary Fresnel zone plate structure^[32]

Fig. 3. (a) Exposure energy density distribution at different depths in the photoresist layer^[18]; (b) Schematic diagram of the relationship between scanning speed and scanning line width^[36]; (c) Schematic diagram of the relationship between laser power and scan line width^[36]

Fig. 4. (a) Schematic diagram of the femtosecond laser direct writing micro-nano processing system^[45]; (b) VPG microscopic magnification images obtained under different pulse energies^[46]; (c) SEM images of HOVML with topological charge numbers of 1, 3, and 16, respectively^[47]; (d) Focusing properties of HOVML under different topological numbers^[47]

Fig. 5. (a) Schematic diagram of femtosecond direct writing system with scanning galvanometer^[54]; (b) SEM image of a Fresnel zone plate^[54]; (c) SEM image of multilayer diffractive optical element^[45]

Fig. 6. (a) Schematic diagram of polar femtosecond laser direct writing system^[56]; (b) Image of the circular grating on curved surface of the lens and LSCM image of the circular grating^[56]; (c) System configuration for patterning of diffractive micro-optics^[57]; (d) Patterning procedure for ultra-thin diffractive optics array^[57]

Fig. 7. (a) Schematic diagram of the double-beam laser direct writing interference device^[59]; (b) Schematic diagram of the proximity effect of 5×5 spot DOE fabricated under different laser powers^[62]; (c) Light path diagram of femtosecond laser processing system^[63]; (d) Phase hologram of 1×11 grating array and CCD observation image^[63]

Fig. 8. (a) Schupmann compensation diffractive telescope system; (b) Compact Schupmann diffractive telescope system by inserting a relay lens^[86]

Fig. 9. Schematic diagram of ablating square holes and beam scanning on a glass substrate using FBS type diffractive optics to shape a single pulse laser^[95]