Fig. 1. Working principle of the microwave-resonator-enabled broadband EO comb generation process. (a), (b) Schematic comparison of EO comb generators with traditional lumped-capacitor design (a), where a circulator and an external 50  Ω load are needed to prevent electrical power reflection to the driving circuit, and the proposed λ/4 microwave resonator design (b). The microwave resonator serves both electrical field enhancement and prevention of electrical power reflection. (c) Schematic illustration of the electrical field distribution and phase-matching condition in the microwave-resonator-enhanced EO comb generator. Green pulses indicate the optical signal locations at different times t0, t0+τ/4, and t0+τ/2. Red arrows indicate electrical field distributions in the two gaps of the coplanar microwave resonator at t0, which would be reversed at t0+τ/2.

Fig. 2. Microwave resonator design. (a) Top view of the CPW resonator with shorted terminal at the left end and coupling interdigitated finger electrodes at the right-hand side. Insets: cross-section schematics of the CPW resonator at the shorted metallic bridge (blue dashed line) and in the middle of the resonator (green dashed line). (b) Equivalent circuit model of the short-circuited λ/4 CPW resonator (i), driven by an external circuit (ii) with a source impedance of RL=50  Ω. (c) Calculated (yellow) and measured (blue) microwave reflection coefficients of the CPW resonator, and that measured from a lumped-capacitor electrode (red).

Fig. 3. Broadband EO comb generation. (a) Micrographs of the fabricated on-chip EO comb generators with lumped-capacitor (top) and CPW resonator (bottom) electrodes. (b) SEM images of the metallic bridges at the shorted terminal (left) and the IDE coupler (right). The scale bars are 20 μm in both panels. (c) Measured EO comb spectra from the lumped-capacitor (i) and CPW resonator electrodes (ii) with the same input electrical power of 28.7 dBm. Insets show the device configurations (left) and measured optical transmission spectra (right) of the corresponding devices when microwave signal is off (red) and on (blue). The coupling rates 2Ω1=2×2π×0.264  GHz and 2Ω2=2×2π×0.449  GHz are measured under a moderate RF driven power of 5 dBm. (iii) Numerically simulated EO comb spectra of the current devices and setups shown in (i), (ii) and that of the current CPW resonator electrode design with potentially higher QL=1.5×106 and PMW=2.2  W.

Fig. 4. Tolerance to frequency mismatch between optical and microwave resonators. Blue dots show the measured comb span (top horizontal axis) for optical racetrack resonators with different FSRs (right vertical axis), driven with microwave frequencies (left vertical axis) matched with FSR. The corresponding comb spectra are shown in the left insets. Black solid curve shows the normalized electric field strength (bottom horizontal axis) derived from the measured S11 parameter of the CPW resonator (S11 in the full measured frequency range is shown in the bottom right inset).

Fig. 5. Microwave resonator for different target frequencies. (a) Micrograph of CPW resonators targeting resonance near 10 GHz. (b), (c) Measured S11 parameter of the devices with Ls=2990  μm (b) and Ls=3000  μm (c). The dashed lines indicate the calculated results, and gray solid lines indicate resonance frequency at 9.82 GHz and 9.67 GHz.