Chirped pulse amplification
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Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort laser pulse up to the terawatt level and above with the laser pulse being stretched out temporally prior to amplification.
CPA was invented by Gérard Mourou at the University of Rochester in the late 1980s. Before then, the peak power of laser pulses was limited due to the fact that a laser pulse at intensities of gigawatts per square centimeter causes serious damage to the gain medium through nonlinear processes such as self-focusing. In order to keep the intensity of laser pulses below the threshold of the nonlinear effects, the laser systems had to be large and expensive, and the peak power of laser pulses was limited to the gigawatt level.
In CPA, on the other hand, an ultrashort laser pulse is stretched out in time prior to introducing it to the gain medium using a pair of gratings that are arranged so that the low-frequency component of the laser pulse travels a shorter path than the high-frequency component does. After going through the grating pair, the laser pulse becomes positively chirped, that is, the high-frequency component lags behind the low-frequency component, and has longer pulse duration than the original by a factor of 103 to 105. Then the stretched pulse, whose intensity is sufficiently low compared with the intensity limit of gigawatts per square centimeter, is safely introduced to the gain medium and amplified by a factor of approximately 1010. Finally, the amplified laser pulse is recompressed back to the original pulse width through the reversal process of stretching, achieving orders of magnitude higher peak power than laser systems could generate before the invention of CPA.
In addition to the higher peak power, CPA makes it possible to miniaturize laser systems. A compact high-power laser, known as a tabletop terawatt laser (T3 laser), is created based on the CPA technique.
