Chirped pulse amplification (CPA) is a technique that permits the amplification of ultrashort (20 - 1000 fs) laser pulses to high energy levels (0.1 - >100 J).
The key to CPA is to avoid amplifying the short pulses directly in the laser amplifier, where the intensity would become so high that nonlinear effects would lead to damage in the amplifier. Instead, low energy (nJ) short pulses produced by a ultrafast laser oscillator (1, below) are dramatically stretched in time (2). This is done by stretching out the various colors of the pulse using gratings.
These long (~ 1 ns) pulses are then amplified to the desired energy levels (3). The final step in CPA is to recompress the laser pulses by bringing the stretched colors back together (4). The result is a high energy laser pulse with an ultrashort duration near that of the initial low energy pulse from the oscillator.