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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. [ABSTRACT FROM AUTHOR]

Multimode silicon resonators with ultralow propagation losses for ultrahigh quality (Q) factors have been attracting attention recently. However, conventional multimode silicon resonators only have high Q factors at certain wavelengths because the Q factors are reduced at wavelengths where fundamental modes and higher-order modes are both near resonances. Here, by implementing a broadband pulley directional coupler and concentric racetracks, we present a broadband high-Q multimode silicon resonator with average loaded Q factors of 1.4 × 106 over a wavelength range of 440 nm (1240–1680 nm). The mutual coupling between the two multimode racetracks can lead to two supermodes that mitigate the reduction in Q factors caused by the mode coupling of the higher-order modes. Based on the broadband high-Q multimode resonator, we experimentally demonstrated a broadly tunable Raman silicon laser with over 516 nm wavelength tuning range (1325–1841 nm), a threshold power of (0.4 ± 0.1) mW and a slope efficiency of (8.5 ± 1.5) % at 25 V reverse bias. The authors demonstrate a new approach for multimode resonators to maintain high-quality-factor resonances across a broad wavelength range using the detuning between the symmetric and anti-symmetric supermodes in concentric racetrack resonators. [ABSTRACT FROM AUTHOR]