Photonic-Chip Supercontinuum with Tailored Spectra for Counting Optical Frequencies

We explore a photonic-integrated-circuit platform that implements optical-frequency measurements and timekeeping with a perspective towards next-generation portable and spaceborne frequency references and optical-clock networks. The stoichiometric-silicon-nitride waveguides we create provide an efficient and low-noise medium for nonlinear spectral broadening and supercontinuum generation with fiber-based optical-frequency combs. In particular, we demonstrate detailed control over supercontinuum emission to target specific atomic-transition wavelengths and perform an optical-clock comparison using on-chip supercontinuum sources. We report a clock-limited relative frequency instability of 3.8 x 10(-15) at tau = 2 s between a 1550-nm cavity-stabilized reference laser and NIST's calcium atomic-clock laser at 657 nm using a two-octave waveguide-supercontinuum frequency comb.