The characteristics of frequency chirp of a two section distributed feedback (DFB) laser, which is under large signal modulation, is studied while the optical intensity and optical modulation depth are fixed. It is found that the measured time‐averaged power spectrum of the laser changes from one of a predominantly negative frequency chirp (red shift) to positive chirp (blue shift) as the injection current distribution between the two sections is changed. Our theoretical calculation of frequency chirp in DFB lasers based on the Green’s function method explains the shift of the frequency chirp in the two section DFB laser. ©1995 American Institute of Physics.