Experimental demonstration of polarization-insensitive air-cladding grating couplers for silicon-on-insulator waveguides.

We present an air-cladding apodized focusing subwavelength grating that can effectively couple two polarizations into a single waveguide. For the transverse magnetic mode, -3.2  dB maximum coupling efficiency with ∼28  nm 1 dB bandwidth is achieved. With the same grating, -4.3  dB maximum coupling efficiency with ∼58  nm 1 dB bandwidth is achieved for the transverse electric mode. The minimum difference between two polarizations' coupling peaks is demonstrated to be ∼32  nm. At the 1525 nm wavelength range, the polarization-insensitive grating is demonstrated with -6.5  dB coupling efficiency. The polarization-insensitive coupling wavelength can be controlled experimentally.