  We demonstrate a new technique for the experimental measurement of the
spectral coherence of microresonator optical frequency combs. Specifically, we
use a spectral interference method, typically used in the context of
supercontinuum generation, to explore the variation of the complex degree of
first order coherence across the full comb bandwidth. We measure the coherence
of two different frequency combs, and observe wholly different coherence
characteristics. In particular, we find that the observed dynamical regimes are
similar to the stable and unstable modulation instability regimes reported in
previous theoretical studies. Results from numerical simulations are found to
be in good agreement with experimental observations. In addition to
demonstrating a new technique to assess comb stability, our results provide
strong experimental support for previous theoretical analyses.
