Fig. 3. SEM images of each micro-ring resonator based on different material platforms. (a) SEM image of Ge ring resonator based on GeOI material platform^[21]; (b) SEM image of GaAs ring resonator on top of a Al_0.2Ga_0.8As buffer layer^[43]; (c)(d) SEM image of Ge ring resonator with air cladding and its quality factor value at operating wavelength^[2

Fig. 4. Chip layout and SEM image of mid-infrared Vernier cascaded micro-ring filter. (a) Chip of two racetrack resonators in a Vernier configuration^[25]; (b)(c) SEM and optical microscope images of heating tunable Vernier ring resonator^[48]

Fig. 5. Schematic of cascaded micro-ring resonator based on Vernier effect for sensing field^[47]

Fig. 6. Experimental results. (a) Transmission spectrum of filtering and sensing micro-ring; (b) wavelength shift of transmission spectrum of sensing micro-ring in the presence of detected substance; (c) total transmission spectrum of the cascaded micro-ring resonator based on Vernier effect; (d) wavelength shift of transmission spectrum of cascaded micro-ring resonator in the presence of detected substance

Fig. 7. Schematic of optical frequency comb generation based on micro-ring resonator^[66]

Fig. 8. Experimental results of mid-infrared Kerr optical frequency comb. (a) Kerr optical frequency comb based on Si₃N₄ ring resonator^[65]; (b) Kerr optical frequency comb based on non-etched silicon ring resonator^[15]; (c) coherent mid-infrared optical frequency comb based on four-wave mixing and Raman effect interaction in silicon ring resonator^{[Download full size}

Fig. 9. Property results of Ge strip waveguide. (a) Schematic of Ge-on-Si strip waveguide; (b) Schematic of Ge strip waveguide mode field(@3.5 μm); (c) dispersion of Ge strip waveguide

Fig. 10. Simulation results at 3.5 μm wavelength pump. (a) Input the pump light(@3.5 μm, pump power is 80 mW ); (b) output the Kerr optical frequency comb

Fig. 11. Simulation results at 4 μm wavelength pump. (a) Input the pump light(@4 μm, pump power is 112 mW ); (b) output the Kerr optical frequency comb

Table 1. Performance of ring resonators based on different material platforms