Chip‐Based f ‐2 f Interferometry in Periodically Tapered Lithium Niobate Nanophotonic Waveguides

Nanophotonic supercontinuum generation offers a practical route to chip‐based ‐2 interferometry by leveraging coexisting and nonlinearities. In conventional uniform waveguides, the phase‐matching bandwidth for second‐harmonic generation (SHG) is intrinsically narrow, restricting the spectral overlap factor for heterodyne beating. To address this limitation, we introduce a periodically tapered nanophotonic waveguide made from MgO‐doped, z‐cut thin‐film lithium niobate for energy‐efficient and fabrication‐robust ‐2 operation. By adiabatically varying the waveguide width within a dual phase‐matching window that supports concurrent dispersive wave (DW) emission and SHG, we routinely achieved a broad spectral overlap between the SHG and DW components. This capability enables robust detection of the carrier–envelope offset frequency () at substantially lower pulse energies than that in uniform‐waveguide approaches. We further developed a compact waveguide module that operates reliably under temperature fluctuations and is capable of interfacing with high‐repetition‐rate (500 MHz) mode‐locked lasers, enabling detection and phase locking of with a signal‐to‐noise ratio of 48 dB. These results highlight the potential of nanophotonic chips for developing compact, field‐deployable frequency comb systems.