On-chip FSR manipulation of optical frequency combs with a four-wave mixing time lens

Precise control of the free spectral range (FSR) in high-repetition-rate (e.g., 100 GHz) optical frequency combs is critical for advanced applications but remains technically challenging. The frequency domain Talbot effect has emerged as a predominant mechanism for such spectral reconfiguration. Conventional Talbot-based methods face distinct limitations: electro-optic modulation is constrained by bandwidth, while cross-phase modulation demands complex pulse shaping. Furthermore, traditional four-wave mixing (FWM) schemes suffer from bulky footprints and instability due to their reliance on long fibers. Here, we demonstrate on-chip spectral densification by integrating a silicon nitride chirped waveguide Bragg grating (CWBG) with an FWM time lens. Replacing the kilometer-scale fibers used in standard setups, our CWBG generates the requisite large group delay within a centimeter-scale footprint. This compact chip-based dispersion module significantly reduces the long-fiber footprint and mitigates environmental instability and higher-order dispersion distortions. We experimentally compressed the FSR of a 100-GHz comb by integer factors (N = 2, 3, 4), thereby validating a robust, scalable platform for miniature spectral reconstruction in microwave photonics.