43 GHz repetition rate Fourier transform‐limited light pulses of a width of 1.5 ps are achieved by passively mode‐locking an ion‐implanted semiconductor laser in a very short external cavity. A higher repetition rate is limited by strongly mixed self‐pulsation. A numerical investigation of passive model‐locking is carried out based on the solution of traveling wave rate equations of an external cavity semiconductor laser. We find that the gain nonlinearity factor can significantly influence the present range of the mixed self‐pulsation. With the increase of the nonlinearity factor, the operation tends to pure passive mode‐locking at a low injection current. In addition, the proper adjustment of the ratio of differential absorption to differential gain and the carrier lifetime in the absorber can reduce the necessary current for pure passive mode‐locking. ©1995 American Institute of Physics.