What about the dispersion pre-compensation, what do I need it for?

The ultrashort pulses are made up of a coherent superposition of many different wavelengths, this is especially true for few-cycle pulses. When propagating through material, the wavelength dependent refractive index results in different propagation speeds for the different individual wavelengths (dispersion), which in turn results in a temporal broadening of the pulse (chirping of the pulse due to group velocity dispersion). To be able to measure the duration of few-cycle pulses, all dispersion in the beam path has to be compensated for, this is also true for the optics inside the measuring instrument. At these large bandwidths, the effect of the dispersion by propagating through a piece of 1mm fused silica is not negligible. As an example, the dispersion of the beam splitter of a typical autocorrelator has to be compensated for to be able to measure 5fs pulses.

Related to: gecco / venteon / taccor