The optical outputs of single-section quantum-dash and
quantum-dot mode-locked lasers (MLL) are well-known
to exhibit strong group velocity dispersion. Based on
careful measurements of the spectral phase of the pulses
from these MLLs, we confirm that the difference in group
delay between the modes at either end of the MLL
spectrum equals the cavity roundtrip time. This
observation allows us to deduce an empirical formula
relating the accumulated dispersion of the output pulse
to the spectral extent and free-spectral-range of the MLL.
We find excellent agreement with previously reported
dispersion measurements of both quantum-dash and
quantum-dot MLLs over a wide range of operating
conditions.