FLY project

The usual technique used to measure the temporal width of ultra short light pulses is the autocorrelation based on a Michelson interferometer. The others commercial instruments use the following techniques : FROG (Frequency Resolved Optical Gating), GRENOUILLE (technique FROG améliorée) ou SPIDER (Spectral Phase Interferometry for Direct Electric-Field Reconstruction). However, these instruments give the amplitude and the phase of the light pulse, there are complex and expensive (ten times the cost of an autocorrelation instrument). Moreover, these instruments use interferometric techniques or CCD for the measurement.

Along my thesis work, I devoted some time to the study of two photons photodiodes. These photodiodes have a large band-gap. The electron hole pair creation needs the simultaneous absorption of up to two photons. This study, in complement of a theoritical statistic point of view on the spatial and temporal variables of the laser pulse allows my research group to imagine a clever instrument able to measure the temporal width of a laser pulse. This non interferometric instrument is based on the laser beam focusing on two photons photodiodes. The instrument compares the statiscal temporal variables before and after propagation through a dispersive medium. With this technique, the instrument gives the absolute value of the temporal width of a femtosecond laser pulse, and possibly the value of the chirp (chirp of the frequencies in the pulse) with the add of a third two photons photodiode.