Stretching the spectra of Kerr frequency combs with self adaptive boundary silicon waveguides

Dispersion engineering of optical waveguides is among the most important steps in enabling the realization of Kerr optical frequency combs. A recurring problem is the limited bandwidth in which the nonlinear phase matching condition is satisfied, due to the dispersion of the waveguide. This limitation is particularly stringent in high-index-contrast technologies such as silicon-on-insulator. We propose a general approach to stretch the bandwidth of Kerr frequency combs based on subwavelength engineering of single-mode waveguides with self-adaptive boundaries. The wideband flattened dispersion operation comes from the particular property of the waveguide optical mode that automatically self-adapts its spatial profile at different wavelengths to slightly different effective spatial spans determined by its effective index values. This flattened dispersion relies on the squeezing of small normal-dispersion regions between two anomalous spectral zones, which enables it to achieve two Cherenkov radiation points and substantially broaden the comb, achieving a bandwidth between 2.2 and 3.4  μm wavelength. This strategy opens up a design space for trimming the spectra of Kerr frequency combs using high-index-contrast platforms and can provide benefits to various nonlinear applications in which the manipulation of energy spacing and phase matching are pivotal.