Chirped pulse amplification

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.

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 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.

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).

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.

Each component in the whole chain from the seed laser to the output of the compressor contributes to the dispersion.

It turns out to be hard to tune the dispersions of the stretcher and compressor such that the resulting pulses are shorter than about 100 femtoseconds.

This setup is normally used as a compressor since it does not involve transmissive components that could lead to unwanted side-effects when dealing with high-intensity pulses.

Figure 2 shows the dispersion orders for a grating compressor with a groove density of

, as described in the original design by Donna Strickland and Gérard Mourou (1985),[2] and evaluated using Lah-Laguerre optical formalism - a generalized formulation of the high orders of dispersion.

[5][6] Figure 3 shows a more complicated grating configuration that involves focusing elements, here depicted as lenses.

As with the configuration in Figure 1, it is possible to use an additional mirror and use a single grating rather than two separate ones.

This setup requires that the beam diameter is very small compared to the length of the telescope; otherwise undesirable aberrations will be introduced.

For this reason, it is normally used as a stretcher before the amplification stage, since the low-intensity seed pulses can be collimated to a beam with a small diameter.

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.

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

, using Lah-Laguerre optical formalism - a generalized formulation of the high orders of dispersion.

[5][6] The compressor parameters at near Brewster incidence angle are: normal distance between the prisms of

The particular values depend on the prism material, the wavelength of interest as well as on the compressor parameters.

laser amplifiers may be phase locked via reflection from a phase-conjugating mirror[7] to increase the brightness as

For this purpose degenerate four-wave mixing Kerr Phase conjugation is relevant.

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.

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.

Figure 1. Schematic layout of a grating-based compressor with ne­gative dispersion, i.e., the short wavelengths (in blue) come out first.
Figure 2. Dispersion orders of a grating compressor. (p = 2 − GDD, p = 3 − TOD, p = 4 − FOD, p = 5 − FiOD, p = 6 − SiOD, p = 7 − SeOD, p = 8 − EOD, p = 9 − NOD, p = 10 − TeOD)
Figure 3. Schematic layout of a grating-based stretcher. In this case, , which leads to a positive dispersion, i.e. the long wavelengths (in red) come first.
Figure 4. Prism stretcher. This configuration has a positive dispersion. Although the different wavelengths appear to travel along very different paths, the effective path length differences are rather small, as indicated by the colors of the dispersed pulse.
Figure 5. Dispersion orders of a fused silica prism-pair compressor at 780 nm. (p = 2 − GDD, p = 3 − TOD, p = 4 − FOD, p = 5 − FiOD, p = 6 − SiOD, p = 7 − SeOD, p = 8 − EOD, p = 9 − NOD, p = 10 − TeOD)