The Overlap of Two Photons

To measure ultrafast phenomena, researchers often use repetitive trains of very short laser pulses. For example, they can create two pulse trains from the same source and send them along different paths. To measure the length of the pulses within the trains, they shine both at a “nonlinear” crystal. The crystal produces extra light with double the original frequency when two pulses are present simultaneously. By changing the path followed by one train, perhaps making it a micron or so longer, researchers create a delay of a few femtoseconds. As they increase the extra path length, pulses from the two trains become out of sync, which reduces the crystal’s output and indicates the length of the pulses.