A reflective sandwiched polarizing beam splitter (PBS) grating with a connecting layer in Littrow mounting is described and designed in this paper. By using two-beam-interference theory for a modal method. a duty cycle and a period of grating can be calculated. where the first order is well suppressed for transverse magnetic (TM) polarization. https://www.roneverhart.com/Doorking-2108-080-eVolve-Smart-Video-Intercom-System/