Chirped Pulse Amplification

Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification. CPA is the current state of the art technique which all of the highest power lasers (greater than about 100 terawatts, with the exception of the ~500 TW National Ignition Facility) in the world currently utilize. Some examples of these lasers are the Vulcan Petawatt Upgrade at the Rutherford Appleton Laboratory's central laser facility, the Diocles Laser at the University of Nebraska-Lincoln, the Gekko Petawatt laser at the Gekko XII facility in the Institute of Laser Engineering at Osaka University, the OMEGA EP laser at the University of Rochester's Lab for Laser Energetics and the now dismantled petawatt line on the former Nova laser at the Lawrence Livermore National Laboratory. Apart from these state-of-the-art research systems, a number of commercial manufacturers sell Ti:sapphire-based CPAs with peak powers of 10 to 100 gigawatts.

Chirped-pulse amplification was originally introduced as a technique to increase the available power in radar in 1960. CPA for lasers was invented by Gérard Mourou and Donna Strickland at the University of Rochester in the mid 1980s. Before then, the peak power of laser pulses was limited because a laser pulse at intensities of gigawatts per square centimeter causes serious damage to the gain medium through nonlinear processes such as self-focusing. For example, some of the most powerful compressed CPA laser beams, even in an unfocused large aperture (after exiting the compression grating) can exceed intensities of 700 gigawatts/cm2, which if allowed to propagate in air or the laser gain medium would instantly self focus and form a plasma or cause filament propagation, both of which would ruin the original beam's desirable qualities and could even cause back-reflection potentially damaging the laser's components. 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 high gigawatt level or terawatt level for very large multi beam facilities.

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 106 or more. 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 (the compressor being the biggest part). A compact high-power laser, known as a tabletop terawatt laser (T3 laser), can be created based on the CPA technique.