A simple method for the full amplitude and phase characterization of the li
ght emitted by continuous-wave ultrahigh-repetition-rate lasers is experime
ntally demonstrated. The method is based on the measurement of the beat not
es obtained by filtering pairs of adjacent longitudinal laser modes in a hi
gh-resolution, zero-dispersion grating-lens spectroscope arrangement. The m
ethod only requires the use of a simple fast photodiode and does not involv
e any nonlinear optical process.