DUAL DIFFRACTION GRATING IN-COUPLER FOR REDUCED WAVEGUIDE THICKNESS

In example embodiments, an apparatus includes a waveguide having an in-coupler an out-coupler. The waveguide has a first surface and an opposite second surface, and the waveguide provides at least one optical path from the in-coupler to the out-coupler. The in-coupler comprises a first diffraction grating with a first grating vector on the first surface and a second diffraction grating with a second grating vector on the second surface. At least one of the first and second grating vectors is not oriented along any of the optical paths from the in-coupler to the out-coupler. The second grating may alter the polarization state of in-coupled light and/or change the direction of the in-coupled light to aid in preventing the light from being out-coupled by the first diffraction grating.