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, then amplified, and then compressed again.[1] The stretching and compression uses devices that ensure that the different color components of the pulse travel different distances.
CPA for lasers was introduced by Donna Strickland and Gérard Mourou at the University of Rochester in the mid-1980s,[2] work for which they received the Nobel Prize in Physics in 2018.[3]
CPA is the technique used by most high-powered lasers in the world.
Before the introduction of CPA in the mid-1980s, 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 GW/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 1000 to .
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 a million or more. Finally, the amplified laser pulse is recompressed back to the original pulse width through reversal of the 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, typically delivering of energy within), can be created based on the CPA technique.[4]
There are several ways to construct compressors and stretchers. However, a typical Ti:sapphire-based chirped-pulse amplifier requires that the pulses are stretched to several hundred picoseconds, which means that the different wavelength components must experience about 10 cm difference in path length. The most practical way to achieve this is with grating-based stretchers and compressors. Stretchers and compressors are characterized by their dispersion. With negative dispersion, light with higher frequencies (shorter wavelengths) takes less time to travel through the device than light with lower frequencies (longer wavelengths). With positive dispersion, it is the other way around. In a CPA, the dispersions of the stretcher and compressor must cancel out. Because of practical considerations, the (high-power) compressor is usually designed with negative dispersion, and the (low-power) stretcher is therefore designed with positive dispersion.
In principle, the dispersion of an optical device is a function
\tau(\omega)
\tau
\omega
\phi(\omega)=2\pi\tau(\omega)c/λ(\omega)
λ
Figure 1 shows the simplest grating configuration, where long-wavelength components travel a larger distance than the short-wavelength components (negative dispersion). Often, only a single grating is used, with extra mirrors such that the beam hits the grating four times rather than two times as shown in the picture.
\sigma=1700 lines/mm
\thetain=65\circ
L=25 cm
Figure 3 shows a more complicated grating configuration that involves focusing elements, here depicted as lenses. The lenses are placed at a distance
2f
L
L<f
L>f
L=f
See main article: Prism compressor.
It is possible to use prisms rather than gratings as dispersive elements, as in Figure 4. Despite such a simple change, the set-up behaves quite differently, as to first order no group delay dispersion is introduced. Such a stretcher/compressor can have both a positive or negative dispersion, depending on the geometry and the material properties of the prisms. With lenses, the sign of the dispersion can be reversed, similar to Figure 3. For a given distance between the dispersive elements, prisms generate much less dispersion than gratings. Prisms and gratings are sometimes combined to correct higher order dispersion ("grisms"), in which case the distance between the prisms is on the order of 10 meters rather than 50 cm as with a grating compressor. Gratings lose power into the other orders, while prisms lose power due to Rayleigh scattering.As an example, the dispersion orders of a fused silica prism-pair compressor are illustrated in Figure 5 for variable insertion depth of the first prism
\ell1
780nm
\Deltaλ=30nm
L=30cm
\ell2=1mm
λmin
\alpha=69.06\circ
Chirped pulses from
N
N2
Some other techniques can be used for stretching and compressing pulses, but these are not suitable as the main stretcher/compressor in CPA due to their limited amount of dispersion and due to their inability to handle high-intensity pulses.
CPA is used in all of the highest-power lasers (greater than about 100 terawatts) in the world, with the exception of the ≈500 TW National Ignition Facility. Some examples of these lasers are the Vulcan laser at the Rutherford Appleton Laboratory's Central Laser Facility, the Diocles laser at the University of Nebraska–Lincoln, the GEKKO XII laser at the GEKKO XII facility in the Institute for Laser Engineering at Osaka University, the OMEGA and OMEGA EP lasers 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. Gérard Mourou has proposed using CPA to generate high-energy and low-duration laser pulses to transmute highly radioactive material (contained in a target) to significantly reduce its half-life, from thousands of years to only a few minutes.[9] [10]
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.