A non-linear method for the study of the coherence of femtosecond optical pulses is proposed. The information about coherence is obtained from the observation of the second harmonic of the signal transmitted by a Michelson-type interferometer. Fringe visibility is enhanced by the non-linear process since the detected signal includes a term with the 4th power of the combination of the fields, instead of the second power in conventional linear detection. The envelope of the autocorrelation function has been used to characterize the phase distribution of the pulses. The authors describe two types of phase modulation-non-linear and linear-that they have observed in the ultrashort pulses generated by a passively mode-locked ring dye laser. These modulations decrease the coherence of the pulses. Anomalous dispersion compensation through normal dispersion introduced in the cavity gives rise to coherent pulses.