Resonance frequency tuning of a double ring resonator in GaInAsP/InP. Experiment and simulation

A racetrack shaped double ring resonator (DRR) filter is demonstrated with radii of 200 pm. The double ring resonator contains two -3 dB muttimode interference (MMI) couplers for I/O coupling and a - 13 dB codirectional coupler in between the rings, A free spectral range of 50 GHz has been realized. A simulation model has been developed to describe the DRR. As fabrication tolerances do not allow the realization of two identical rings with required rim-circumference accuracy in the resonator, a frequency alignment of the resonator is indispensable. The resonance frequency tuning is performed thermally using platinum resistors which have been placed on top of the waveguides in both rings. An on-off ratio increase has been achieved of more than 3 dB, resulting in a total on-off ratio larger than 18 dB. The frequency alignment is inevitable in the case of multiple coupled micro ring resonators.