  We report new limits on the phase coherence of the ultrafast mode-locking
process in an octave-spanning Ti:sapphire comb. We find that the mode-locking
mechanism correlates optical phase across a full optical octave with less than
2.5 micro Hz relative linewidth. This result is at least two orders of
magnitude below recent predictions for quantum-limited individual comb-mode
linewidths, verifying that the mode-locking mechanism strongly correlates
quantum noise across the comb spectrum.
