Abstract
Laser frequency combs are coherent light sources that simultaneously provide pristine frequency spacings for precision metrology and the fundamental basis for ultrafast and attosecond sciences. Recently, nonlinear parametric conversion in high-Q microresonators has been suggested as an alternative platform for optical frequency combs, though almost all in 100 GHz frequencies or more. Here we report a low-phase-noise on-chip Kerr frequency comb with mode spacing compatible with high-speed silicon optoelectronics. The waveguide cross-section of the silicon nitride spiral resonator is designed to possess small and flattened group velocity dispersion, so that the Kerr frequency comb contains a record-high number of 3,600 phase-locked comb lines. We study the single-sideband phase noise as well as the long-term frequency stability and report the lowest phase noise floor achieved to date with −130 dBc/Hz at 1 MHz offset for the 18 GHz Kerr comb oscillator, along with feedback stabilization to achieve frequency Allan deviations of 7 × 10−11 in 1 s. The reported system is a promising compact platform for achieving self-referenced Kerr frequency combs and also for high-capacity coherent communication architectures.
Highlights
We report a low-phase-noise Kerr frequency comb generated from a silicon nitride spiral resonator
A single bandwidth-limited RF beat note is observed and the single-sideband (SSB) phase noise analysis reveals the lowest phase noise floor achieved to date in free-running Kerr frequency combs, − 130 dBc/Hz at 1 MHz offset for the 18 GHz carrier
To obtain the Kerr frequency comb, the pump wavelength is first tuned into the resonance from the high frequency side at a step of 1 pm (~118 MHz) until a broadband comb is observed on the optical spectrum analyzer
Summary
First a 3 μ m thick SiO2 layer was deposited via plasma-enhanced chemical vapor deposition (PECVD) on p-type 8” silicon wafers to serve as the under-cladding oxide. Low-pressure chemical vapor deposition (LPCVD) was used to deposit a 750 nm silicon nitride for the spiral resonators, with a gas mixture of SiH2Cl2 and NH3. The resulting silicon nitride layer was patterned by optimized 248 nm deep-ultraviolet lithography and etched down to the buried SiO2 via optimized reactive ion dry etching. The silicon nitride spiral resonators were annealed at 1200 °C to reduce the N-H overtone absorption in the shorter wavelengths. The silicon nitride spiral resonators were over-cladded with a 3 μ m thick SiO2 layer, deposited initially with LPCVD (500 nm) and with PECVD (2500 nm)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
