Nonlinear effects inherently limit the amount of optical amplification possible in a gain medium. Effects such as Brillouin scattering reduce gain, and effects such as self-focusing can cause optical damage. These effects are proportional to the peak power in the medium, putting an upper limit on the gain possible.
Chirped pulse amplification circumvents this limit by spreading the energy in the pulse over a longer period of time, thus reducing the peak power throughout the longer pulse. This is done by sending the input pulse through a medium with a high wavelength dispersion, such as a pair of gratings or prisms, or a length of dispersive optical fiber. The pulse that emerges from the dispersive medium is chromatically dispersed, with the short wavelengths at one end and the long wavelengths at the other. The degree of dispersion depends both on the medium and the spectral width of the pulse. In practice, chirped pulse amplification works best with pulses lasting tens to hundreds of femtoseconds, which are inherently broadband.
The longer dispersed pulse is amplified in a broadband gain medium, then passed through a medium with dispersion of the opposite sign, so the wavelengths that passed first through the amplifier are delayed and those that passed through the amplifier later in the pulse can catch up. The pre-amplification and post-amplification dispersion do not have to cancel each other out, although the minimum pulse duration still depends on the spectral bandwidth.
Chirped pulse amplification also can be used in optical parametric amplifiers, which have broader bandwidth than laser oscillators and thus can be chirped more strongly to generate higher peak powers. Optical parametric chirped-pulse amplification (OPCPA) will be used in Europe's Extreme Light Infrastructure.
How a CPA works. A pair of gratings that delay the blue end of the spectrum stretches input pulses about a factor of 1000 in duration. Those pulses then pass through a broadband amplifier, and the higher-power output is compressed by a second pair of gratings that delay red wavelengths to produce a high-energy ultrashort pulse.