A highly compact third-order silicon microring add-drop filter with a very large free spectral range, a flat passband and a low delay dispersion.

We demonstrate highly compact third-order silicon microring add-drop filters. The microring resonator has a small radius of 2.5 mum and a very large free spectral range of 32 nm at 1.55 mum. Experimental results show a low add-drop crosstalk of around -20 dB. Box-like channel dropping response is demonstrated, and it has a passband of ~ 1 nm (125 GHz), fast rolling-off (slope ~ 0.2 dB/GHz), high out-of-band signal rejection of around 40 dB and a low drop loss. Simulation agrees well with experiments in power transmission, and the group delay is also simulated and the variation is less than 1 ps within the passband. The propagation loss in microring resonators is optimized.