Design of a compact high-performance InP ring resonator

This work reports on the design of a ring resonator with a high quality factor compatible with InP based generic foundry processes. The resonator consists of a spiral delay line, coupled to a waveguide via a Multimode Interference Coupler with unequal splitting ratio. The spiral loss is greatly reduced by using low-doped shallowly etched waveguides with millimeter bending radii. Based on a measured propagation loss of 0.3 dB/cm, a quality factor of 10⁶ and an extinction ratio of about 10 dB are expected for a ring length of 80 mm. By folding the ring into a spiral the chip size is reduced significantly, resulting in a footprint of 2.5 mm × 3.6 mm.