Abstract

Silicon-based integrated optic technology has been the subject of a growing interest over the last ten years. As with other waveguide technologies, and in spite of its numerous intrinsic merits, not much can practically be made before the problem of the access to/from the waveguide has received a viable solution. Grating coupling is conceptually the most attractive solution since it fully planarizes the access function. One of the limitations of grating coupling is the weakness of the coupling efficiency unless smart but difficult solutions are used such as groove blazing¹ or corrugation phase shifting.² An inherent and remarkable advantage of Si-based technology is the very existence of the highly reflective silicon substrate which may allow a significant enhancement of the coupling efficiency without resorting to any additional technological step. It is easier to understand this by considering a waveguide grating coupler in its outcoupling regime. Let us imagine, as illustrated in Fig. 1, a step index slab waveguide of thickness h, index n₂ on top of a silica buffer of thickness b, index n₃. The substrate has a complex index n₄ with large real part. The cover is generally air but can have the index n₁. The zigzagging guided beam undergoes diffraction in both the cover and substrate at the grating of sinusoidal amplitude σ. The beam diffracted downwards experiences a strong partial reflection at the high index silicon surface; it will thus superpose with the wave diffracted upwards. Depending on the relative phase shift between these two waves, the net diffracted power will flow preferably into the air (a situation which is desired in an optical antenna application) or into the substrate (which is desired for waveguide power detection purpose).

© 1994 IEEE