High-Order Adiabatic Elliptical-Microring Filter with an Ultra-Large Free-Spectral-Range

A silicon photonic filter with an ultra-large free spectral range (FSR) is proposed and demonstrated by introducing high-order adiabatic elliptical-microrings (AEMs) with bent directional couplers (DCs). For the AEMs, the waveguides in the non-coupling regions are wide and have a minimal bending radius, while the waveguides in the coupling regions are relatively narrow and have a maximal bending radius. Both the radius and width of AEMs have varied adiabatically. Furthermore, bent DCs are used to realize a sufficient coupling between the access waveguide and the microring waveguide and suppress the higher-order mode excitation. The fabricated filters based on the 2
^nd
-order, 3
^rd
-order, and 4
^th
-order AEMs have box-like spectral responses with an ultra-large FSR of ∼37 nm, which is a record for high-order microring filters to the best of our knowledge. Finally, an eight-channel silicon photonic filter with a channel-spacing of ∼3.2 nm and high extinction ratios of >30 dB is demonstrated.