Microresonator frequency combs, or microcombs, provide a new opportunity for ultra-compact and chip-integrated optical and microwave frequency synthesis. A high-Q dielectric microresonator pumped by a continuous wave laser generates a frequency comb (typically 10-1000 GHz mode spacing) via parametric four‑wave mixing. Due to the tight waveguide confinement and high nonlinearity of the integrated-photonics platform, comb generation occurs with only milliwatts of input power. Additionally, precise geometric control of the waveguide dispersion has led to ultra‑broad bandwidth microcombs suitable for f-2f self-referencing. We have leveraged these advances to demonstrate a dual microcomb optical frequency synthesizer that employs inter-locked silicon nitride and silica microcombs. A silicon nitride resonator with 1 THz mode spacing provides an octave-spanning spectra for self-referencing and connection of the 200 THz pump laser to the silicon nitride repetition rate. The mode-spacing of this comb is subsequently phase-locked with a 22 GHz silica comb that provides the countable repetition rate and final link between microwave and optical domains. This dual microcomb system thus forms a backbone in the telecom C-band against which a tunable chip-integrated laser is phase-locked with user-defined frequency at 1 Hz resolution and absolute accuracy. The core elements of this system are fully reversible, and can also be used to generate low-noise microwaves on both short and long timescales.
Scott Diddams and Scott Papp, "Kerr-microresonator combs for low-noise frequency synthesis (Conference Presentation)," Proc. SPIE 10516, Nonlinear Frequency Generation and Conversion: Materials and Devices XVII, 105160D (Presented at SPIE LASE: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2295517.5751419908001.
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