The time dependence of the kink power in weakly index guided high power stripe lasers in the AlGaAs system are investigated. A periodic dependence of the kink power on the length of the laser is observed in continuous and pulsed modes, but the pulsed mode kink curve is shifted with respect to the continuous one. This is observed to be proportional to1/&tgr;,where &tgr; is the pulse width. The kink is due to phase locking of the modes of the waveguide resulting from a shift of the effective indexes, which is driven by self-heating due to free-carrier absorption of the laser mode. ©1997 American Institute of Physics.