The simplified mode method explains the physical processes inside the grating in terms of the Mach- Zehnder interferometer model. There are three modes include mode 0, 1, 2 in grating under second Bragg angle incidence. We report a special case of fused silica grating when its period is close to 1.3 times of wavelength, where the effective refractive index of mode 1 might be equal to mode 2. In this special case, we found that the joint effect of mode 1 and 2 on the diffraction efficiency for -2 order is not accurate entirely, and the diffraction of the 0th order is approaching to the maximum, both with a slight oscillation of efficiency when the depth of grating is increased. This phenomenon by using simplified mode method could help to understand the physics of grating theory.
This paper proposed a transmission two-dimensional (2D) slanted grating based on a double-layer cylindrical structure. We used rigorous coupled-wave analysis (RCWA) and simulated annealing algorithm (SA) to optimize the grating parameters. Results show that the diffraction efficiency of the (-1,0) and (0,-1) order exceed 35% under normal incidence in the range of 429–468 nm wavelength for TE and TM polarization. Meanwhile, the total diffraction efficiency can reach up to 78%. We also discuss the tolerances for the grating parameters to ensure high quality manufacturing processes. The relatively large tolerances ensure fabrication of the two-dimensional slanted grating and provides the possibility for practical applications. The proposed 2D slanted grating can be applied to 2D exit pupil expansion, which is of great importance in AR/VR applications.
In view of the fact that the current research of detection technology for large-size grating surface defect was very little, this paper builds a set of grating defect detection device based on the principle of darkfield imaging. The scheme includes three subsystems: displacement system, monochrome light illumination system and image acquisition system. When the displacement system drives the motion stage to move, the monochromatic light illumination system and the image acquisition system jointly complete the acquisition of the grating surface images. In this study, the surface of the diffraction grating with 170×170mm2 was detected for defects, and the statistical data of different defects is given. Experimental results show that, the device meets the needs of defect detection and evaluation on large-size grating surfaces and the mechanical structure is simple and compact, which has the great potential for large-scale detection.
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