1-GHz VIS-to-MIR frequency combs enabled by CMOS-compatible nanophotonic waveguides

Abstract

A fully stabilized frequency comb is essential for precision metrology and coherent optical synthesis. However, fully-stabilized frequency combs generally require separate stages for supercontinuum generation (SCG) and self-referencing, largely limiting their compactness. Here, enabled by the low-threshold multi-octave supercontinuum generation and concurrent third-harmonic generation in low-loss silicon nitride waveguides, we present a novel approach to a self-referenced frequency comb source at 1 GHz repetition rate spanning from the full visible (VIS) to the mid-infrared (MIR). Our coherent comb is seeded by an all-polarization-maintaining ultrafast fiber laser at 1556 nm, with a pulse duration of 73 fs at 1 GHz repetition rate. With an injected energy of merely 110 pJ, the pulses propagate through dispersion-engineered Si3N4 waveguides, generating supercontinuum spanning over three octaves from 350-3280 nm i.e. 0.76 PHz of coherent bandwidth. Moreover, the on-chip third harmonic generation provides a carrier envelope offset beat note via f-3f with a signal-to-noise ratio of 43 dB. Fueled by the evolving photonic integration providing possibilities of on-chip filtering and photo-detectors, this approach for single-chip self-referencing of high-repetition-rate frequency combs paves the way for ultrabroadband comb sources with unprecedented compactness and field-readiness.