## 2 High Diffraction Efficiency and Damage-Resistance Threshold Gratings
### Multilayer-Dielectric-Film Grating
#### 2.2.1 Multilayer-Dielectric-Film Reflection Grating
The grating line density was 960 lines/mm, and the diffraction efficiency in the 1040-1090 nm band at an incidence angle of 30.9\({}^{\circ}\) was greater than 95%, using the \(-1\) order Littrow structure. As illustrated in Figure 16, M\({}^{2}=1.9\), the beam quality was adjusted by using the double-grating-dispersion correction system, and it was demonstrated that the double-grating configuration is a promising high-brightness SBC approach. [PERSON], [PERSON], et al. of the Chinese Academy of Sciences Shanghai Institute of Optics and Mechanics [43] designed and built a polarization-independent broadband reflection grating with a high-refractive-index contrast double-layer isosceles trapezoidal grating ridge and a periodic multilayer-dielectric-film structure in 2017. The high-refractive-index contrast structure comprises several component combinations. This study demonstrated several grating design approaches. Figure 17 shows that the \(-1\)st order diffraction efficiencies in the 1020-1100 nm region were 99.57, 99.23, and 99.89%, respectively. The isosceles trapezoid design structure is easier to produce than the rectangular structure, and the dual-layer grating structure with high-refractive-index contrast enables efficient broadband polarization-independent diffraction gratings. The diffraction efficiency in the 1023-1080 nm region surpassed 98%, and Figure 18 shows the as-produced grating topography and reflection diffraction-efficiency spectrum. The broadband polarization-independent multilayer dielectric film grating may be employed as an excellent beam-combining device in a spectral beam-combining system, increasing the power of a system. Figure 5.5 Far-field pattern of the combined beam in (**a**) a low-power case and (**b**) a full-power (3 kW) case in the dual-grating SBC system. Figure 5.6 Different grating designs in the 1020–1100 nm band: (**a**) \(-1\)st diffraction efficiency = 99.57%, (**b**) \(-1\)st diffraction efficiency = 99.23%, and (**c**) \(-1\)st diffraction efficiency = 99.89%. The same year, [PERSON], [PERSON], et al. [44] devised and manufactured a revolutionary dual-layer-grating-ridge polarization-independent broadband reflection grating, which was primarily employed in an external-cavity SBC system. The grating structure was mostly made up of a multilayer-dielectric-reflective film and a double-layer-grating ridge structure on the surface. The structure of the multilayer-dielectric-reflective film stack is K9 (optical glass) 1(HL)\({}^{10}\)H\(\,\)I Air. The film was deposited by electron-beam thermal evaporation, where H represents 88.35 nm Ta\({}_{2}\)O\({}_{5}\) and L represents 248.60 nm SiO\({}_{2}\); the double-layer grating ridges were 320 nm HfO\({}_{2}\) and 380 nm SiO\({}_{2}\). It was fabricated by full-system exposure and reactive ion-beam etching. The grating period was 1041.67 nm, the duty cycle was 0.73, the Littrow angle was 30.25\({}^{\circ}\), and the bottom angle of the trapezoidal ridge was \(\theta_{\rm g}=80^{\circ}\). Figure 19 depicts a cross-sectional SEM image. The grating SiO\({}_{2}\) residual-layer thickness was 300 nm, the \(-\)1 st order diffraction efficiency in the 1010-1080 nm band region was 95%, and the output power after beam combining was 10.77 kW. Figure 5.7 SEM image of a grating cross-section. Figure 5.8 SEM image of a multilayer-dielectric-film reflection grating and a \(-\)1 order polarization-independent diffraction-efficiency spectrum. [PERSON] and [PERSON] of the Chinese Academy of Sciences Shanghai Institute of Optics and Mechanics devised and built an effective polarization-independent broadband multilayer-dielectric-film reflection grating in 2018 [45]. The top layer grating-groove structure was a combination trapezoid and groove structure that resembles a bullet. The reflection grating's exact technical specifications were as follows: the center wavelength was 1066 nm, the grating period was 900 nm, the first-order diffraction efficiency was 91% in a bandwidth of 50 nm, and the peak diffraction efficiency was 96%.