  Supercontinuum generation using chip-integrated photonic waveguides is a
powerful approach for spectrally broadening pulsed laser sources with very low
pulse energies and compact form factors. When pumped with a mode-locked laser
frequency comb, these waveguides can coherently expand the comb spectrum to
more than an octave in bandwidth to enable self-referenced stabilization.
However, for applications in frequency metrology and precision spectroscopy, it
is desirable to not only support self-referencing, but also to generate
low-noise combs with customizable broadband spectra. In this work, we
demonstrate dispersion-engineered waveguides based on silicon nitride that are
designed to meet these goals and enable precision optical metrology experiments
across large wavelength spans. We perform a clock comparison measurement and
report a clock-limited relative frequency instability of $3.8\times10^{-15}$ at
$\tau = 2$ seconds between a 1550 nm cavity-stabilized reference laser and
NIST's calcium atomic clock laser at 657 nm using a two-octave
waveguide-supercontinuum comb.
