Fig. 1. SEM images of toroid cavities: (a) cavity with principle diameter of 514 μm, (b) cavity with principle diameter of 800 μm, and (c) cavity with principle diameter of 1.88 mm.

Fig. 2. (a) Transmission spectrum of toroid cavity with a diameter of 514 μm and the corresponding intrinsic Q factor of 2.7×108; (b) transmission spectrum of a cavity with diameter of 800 μm and the corresponding intrinsic Q factor of 3.0×108; (c) transmission spectrum of cavity with a diameter of 1.88 mm and the corresponding intrinsic Q factor of 3.3×108; (d) experimental setup. EDFA, erbium doped fiber amplifier; FPC, fiber polarization controller; TBF, tunable bandpass filter; VOA, variable optical attenuator; PM, powermeter; PD photodiode; and OSA, optical spectrum analyzer.

Fig. 3. (a) Calculated dispersions represented as ΔFSR and (b) calculated dispersions represented as D (in ps/nm/km).

Fig. 4. (a) Measured threshold power of toroid with diameter of 514 μm, loaded Q factor of 8.5×107, and corresponding coupled pump power of 330 μW. (b) Measured threshold power of toroid with diameter of 800 μm, loaded Q factor of 2.4×108, and corresponding coupled pump power of 508 μW. (c) Measured threshold power of toroid with diameter of 1.88 mm, loaded Q factor of 1.7×108, and corresponding coupled pump power of 980 μW.

Fig. 5. First comb lines excited for (a) 514 μm toroid, first comb line excited 10 FSR away from the central mode; (b) 800 μm toroid, first comb line excited 10 FSR away from the central mode; and (c) 1.88 mm toroid, first comb line excited 18 FSR away from the central mode.

Fig. 6. Spectra of optical combs: (a-1) optical frequency comb in toroid with diameter of 514 μm, and corresponding repetition rate of 126 GHz; (a-2) closeup of nine comb lines around the pump mode; (b-1) optical frequency comb in toroid with diameter of 800 μm, and corresponding repetition rate of 82 GHz; (b-2) closeup of nine comb lines around the pump mode; (c-1) optical frequency comb in toroid with diameter of 1.88 mm, and corresponding repetition rate of 36 GHz; (c-2) closeup of nine comb lines around the pump mode.