ABSTRACT
Grating couplers are dielectric gratings put over planar optical waveguides or slabs. They are used to couple an incident laser beam into a guided mode, or vice-versa. The gratings may be photo resists holographic gratings deposited and recorded on the slab, gratings etched in the guiding layer, or resin gratings replicated using standard techniques. Sometimes they are designed with curved or chirped grooves to focus light outside the device or to couple in a focused laser beam. Grating couplers can be used to couple different modes of the waveguide, or to change the direction of propagation of the same mode in the slab, such are the distributed Bragg mirrors used to reflect the mode. Whatever the application, the curvature radius of the grooves can be considered much larger than the groove period, so that the light diffraction by the coupler can be considered locally as if the grooves were straight and equidistant.
