  We calculate the quantum-limited shape of the comb lines from a mode-locked
Ti:sapphire laser using experimentally-derived parameters for the linear
response of the laser to perturbations. The free-running width of the comb
lines is found across the laser spectrum. By modeling the effect of a simple
feedback loop, we calculate the spectrum of the residual phase noise in terms
of the quantum noise and the feedback parameters. Finally, we calculate the
frequency uncertainty in an optical frequency measurement if the limiting
factor is quantum noise in the detection of the optical heterodyne beat.
