Silicon photonics packaging : characterization of a waveguide grating coupler and modeling of the fiber coupling ratio

The main issue for a fiber coupling structure with a nanophotonic circuit is of course to reach very low insertion losses. However, another major issue is the polarization sensitivity as optical networks usually use non polarization maintaining optical fibers. Therefore, to implement a nanophotonic circuit into optical networks, the fiber coupling structures must have a very low polarization sensitivity, especially concerning the coupling from the fiber towards the circuit. Surface grating couplers are very interesting fiber coupling structures as they allow for coupling anywhere on a circuit without the need for cleaving and polishing facets, which makes wafer-scale testing of photonic circuits possible. However, one-dimensional gratings couplers are well-known to be polarization sensitive. In this paper, we show that a one-dimensional grating may have very similar coupling performances for TE and TM polarizations. Only the optimal injection angle may still differ between TE and TM polarizations. Therefore, packaging consideration is discussed to design an optical coupling system between the fiber and the grating coupler which should allow the optimization of the incident angle for both fundamental polarized modes. This way a low polarization sensitive coupler using a simple one-dimensional grating coupler is proposed.