Study of coupling loss on strongly-coupled, ultra compact microring resonators

Small-radius microring resonators with large free spectral range (FSR) are of great interest for optical communication and optical interconnect applications. The resonator loss of a waveguide-coupled ring resonator, if the gap width between the microring and the bus waveguide is extremely small, can be significantly influenced by the coupling loss which corresponds to the microring operated in a strong coupling regime. This effect is particularly prominent for small radius microrings. We have studied the coupling loss with respect to the gap width on a waveguide-coupled microring both experimentally and theoretically, using two-dimensional (2D) finite difference time domain (FDTD) and effective index method (EIM).The coupling loss was confirmed by measuring transmission spectra of Si microring filters fabricated on silicon-on-insulator (SOI) wafers. Our experimental data show that the ring loss increases rapidly as the coupling gap decreases to less than 200 nm. The measured results show that the ring loss of a silicon microring with a radius of 2.75 μm is around 0.01382 dB/circumference as the gap width is greater than 325 nm, referred to as the intrinsic ring loss. However, for a smaller gap of 150 nm, the loss of the microring increases to 0.07084dB/circumference. The added ring loss is attributed to the coupling loss at small coupling gap for small radius ring.