Abstract
A small all-pass racetrack resonator with a large free spectral range (FSR) is proposed and designed with a 600-nm-thick LiNbO3-on-insulator (LNOI) optical waveguide. It is shown that intermode crosstalk happens in LiNbO3 waveguide bends because more than one modes are generated from the anisotropy of LiNbO3. A sharp LNOI waveguide arc-bend is realized theoretically by choosing the geometrical parameters optimally to achieve constructive interference between the excited modes. This is very useful to realize compact resonators on LNOI. In particular, a novel racetrack resonator with a straight coupling region as well as Euler-bends is proposed. Here the narrow straight waveguides are introduced in the coupling region so that sufficient coupling between the access waveguide and the resonator is achieved within a short length. Euler-bends are introduced to avoid mode-mismatching at the junction between the straight-bent sections. For the designed LNOI racetrack resonator with an equivalent bending radius of 18 μm, the free-spectral range (FSR) is as large as 5.8 nm, and the Q-factor is 6.5 ×105 when assuming that the LNOI waveguide has a propagation loss of 1 dB/cm.
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