The experimental observation of blue‐chirped pulses for passive mode‐locking and red‐chirped pulses for active mode‐locking is theoretically explained as a consequence of different &agr; parameters in the gain and absorber section. The influence of nonlinear gain due to carrier heating is also discussed. The knowledge of the chirp is crucial for the optimization of passively mode‐locked lasers for spectrally narrow emission or for the emission of linearly chirped pulses. ©1995 American Institute of Physics.