  Recent investigations of microcavity frequency combs based on cascaded
four-wave mixing have revealed a link between the evolution of the optical
spectrum and the observed temporal coherence. Here we study a silicon nitride
microresonator for which the initial four-wave mixing sidebands are spaced by
multiple free spectral ranges (FSRs) from the pump, then fill in to yield a
comb with single FSR spacing, resulting in partial coherence. By using a pulse
shaper to select and manipulate the phase of various subsets of spectral lines,
we are able to probe the structure of the coherence within the partially
coherent comb. Our data demonstrate strong variation in the degree of mutual
coherence between different groups of lines and provide support for a simple
model of partially coherent comb formation.
