A highly sensitive tunnel diode trigger useful in temporal intensity buildup measurements of mode‐locked lasers has been developed; the device reduces notably the time walk due to the lack of repeatability in intensity of the laser output. The performance of the trigger has been established by means of a 5‐GHz wideband, 0.1 V/cm sensitive real‐time oscilloscope and of an image converter camera of picosecond resolution. The experimental results show that a variation of the amplitude of the laser pulse train of a factor of 5 leads to a time jitter of less than 30 psec.