First-order field autocorrelation measurements of high dynamic range and sensitivity have been performed on the pulse train of a synchronously pumped mode-locked dye laser with a two-plate birefringent filter as a tuning element. A double-phase-modulation detection technique developed by us for time-resolved stimulated Raman gain spectroscopy was employed. These measurements yield information about the mode structure of the dye-laser pulses. Our experimental results lend support to the domain model as a viable description of the mode structure of imperfect mode locking. Moreover, second-harmonic autocorrelation and cross-correlation widths and shapes are shown to be reconcilable with this model.