Once a pulse has been measured, it can be used as a reference pulse to measure other pulses. Then other techniques can be used. One such technique is Spectral Interferometry.

SI was first introduced by Claude Froehly and coworkers in the 1970s.

Because it involves no optical nonlinearity, SI is very sensitive. We used it to measure a train of pulses, each of which contained less than a photon! See our 1996 paper.