Compact multimode silicon racetrack resonators for high-efficiency tunable Raman lasers

Multimode integrated waveguides have large fabrication tolerances and enable low propagation losses. Multimode waveguide bends have, therefore, been used for high-quality (Q) factor multimode resonators. Conventional multimode circular bends typically require large bend radii to avoid the excitation of the higher-order modes. In this paper, we make use of multimode adiabatic bends in a multimode silicon racetrack resonator with a compact footprint of 0.16 mm2. The adiabatic bends help suppress the higher-order modes. The Q factor of the racetrack resonator has an average value of 2.2 × 106 in the wavelength range of 1260–1480 nm. Benefiting from the broadband high-Q multimode racetrack resonator, we experimentally demonstrated a continuous-wave Raman laser with a widely tunable wavelength range of 157 nm. The Raman laser has a threshold power of 0.2 mW and one of the highest slope efficiencies of 27.5%. This work shows how a high-efficiency integrated Raman laser can be achieved with a wide tunable wavelength range, compact footprint, and low threshold power for a tunable source that can extend the output wavelength beyond the direct output range of the pump laser.