US Patent 7661833 Light guide device and backlight module therewith

A light guide device (52) includes a light guide substrate (520) and a sub-wavelength grating (527). The light guide substrate has a light input surface (521), a light output surface (522) adjacent to the light input surface (521), a bottom surface (523) opposite to the light output surface (522). In the light guide plate, stress-induced birefringence is introduced to achieve the polarization state conversion. The SWG 527 located on the light output surface includes a top layer (525) and a bottom layer (526). The SWG 527 is configured to work as a reflective polarizing beam splitter, consistent with the principle of rigorous coupled-wave theory.