Bandwidth Limit of Coherence Transfer in Kerr‐Synchronized Microcombs

Dissipative Kerr soliton microcombs are key coherent sources for precision metrology and optical communications, yet their linewidths suffer from mode‐dependent broadening that limits practical utility. Kerr‐induced synchronization (KIS) enables all‐optical linewidth control by locking a comb tooth to an external reference laser, but the finite locking bandwidth fundamentally limits noise transfer, a critical aspect that has remained quantitatively unexplored. Here we establish an analytical model that explicitly links the KIS locking bandwidth ( f c ) to the frequency‐noise power spectral density. This model reveals f c as a cutoff frequency: noise is fully suppressed far below f c , while the comb reverts to free‐running behavior above it. We validate this model systematically by tuning the injection power to achieve five distinct bandwidths from 1.3 to 8.8 MHz, confirming the predicted transition across all offset‐frequency regimes. Operating at the maximum bandwidth and using reference lasers with distinct noise levels ranging from 146 Hz to 13.8 kHz, we quantitatively verify the dual‐pinned elastic‐tape model across the comb spectrum, precisely mapping the optimized region and identifying the boundary beyond which KIS becomes detrimental. Our work uncovers a quantitative framework that transforms KIS from a qualitative noise‐reduction technique into an engineerable stabilization method for high‐performance microcomb applications.