Two-dimensional apodized grating coupler for polarization-independent and surface-normal optical coupling

A four-port grating coupler with etched square holes is proposed and demonstrated for surface-normal and polarization independent coupling. Benefiting from the perfectly vertical coupling and 1×4 power splitting/combing scheme, efficient polarization independent operation was achieved. The coupling efficiency was further improved by grating apodization. According to the simulations, the proposed two-dimensional (2D) apodized grating coupler can achieve a coupling efficiency (CE) of 64.5% (-1.9dB), a low upward back-reflection of 8.5% (-11dB) and polarization-dependent loss (PDL) lower than 0.04dB across the wavelength range of 1520-1620nm. A CE of 56.3% (-2.5dB) was experimentally measured, with PDL be-low 0.4dB within the C-band and lower than 0.7dB within the L-band. Waveguide delay lines were used to compensate the waveguide channel phase differences at the output port