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We propose a simple route to obtain an ultrawide free spectral range (FSR) using a composite racetrack microring resonator (RMRR) with Fabry–Perot cavities. The presence of two Fabry–Perot cavities in the straight arm of the RMRR suppresses other peaks while yielding a single resonance dip with a high extinction ratio. A comprehensive, detailed mathematical description and finite difference time domain (FDTD) simulations investigate the proposed device performances. The modified RMRR yields an FSR value greater than 150 nm while retaining the unmodified ring resonator’s Q -factor. Other than its application in communications, the device also will be highly useful for sensing. FDTD simulations show a sensitivity of 185 nm/RIU, which is 2.6 times larger than the unmodified RMRR. The proposed device also enjoys compactness and a wide tunability range and has less fabrication complexity. The proposed sensor could be a strong candidate in optical communications, optical switching, and many other applications.

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