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We describe high-density deep-etched fused-silica transmission polarizing beam splitter (PBS) gratings and their simple physical explanation. Optimized numerical results of the grating depth and period are given using the rigorous coupled-wave analysis in order to achieve high extinction ratio and efficiency for the usual laser wavelengths 351, 441.6, 532, 632.8, 800, and 1053 nm. The physical mechanism of such a PBS grating can be well explained with the effective indices of the modes for TE/TM polarization, which is a useful extension of the modal method for different structures reported in this paper. Compared with the work of Clausnitzer et al., fused-silica gratings can work as PBSs for not only the special duty cycle of 0.51 but also the usual duty cycle of 0.50. These numerical results and simple physical explanation provide a useful guideline for the design of a PBS grating.