FSR-Eliminated Vernier Racetrack Resonators Using Grating-Assisted Couplers

We present the design and experimental demonstration of a contra-directional grating-coupled racetrack resonator exhibiting the Vernier effect. The device consists of two racetrack resonators that are coupled together in a cascaded configuration. The input coupler in each racetrack resonator of the cascaded configuration consists of contra-directional gratings. The benefit of using contra-directional couplers is their small bandwidth, as compared to co-directional couplers that do not have gratings. The simulation results show that this device provides numerous performance advantages compared to cascaded racetrack resonators exhibiting the Vernier effect without contra-directional grating couplers. Specifically, we eliminate the free spectral range (FSR, i.e., in both the drop port and the through port) and show substantial improvement in the interstitial peak suppression and the through port insertion loss for the cascaded racetrack resonator with gratings, as compared to the case without gratings. In addition, experimental results are presented, which show an interstitial peak suppression of 29.3 dB, as well as the elimination of the FSR in the drop port and the through port.