The emission transients of optically excited high‐speed multiple quantum well laser diodes have been measured by up‐converting the emission in a nonlinear crystal. From the gain‐switched oscillation the intrinsic resonance frequency and damping factor are determined. The high temporal resolution allows us to observe additional short period oscillations with a frequency of about 170 GHz. Numerical solution of the traveling wave rate equations indicates that these high frequency oscillations arise due to inhomogeneous carrier injection. ©1995 American Institute of Physics.