Chirped Pulse Amplification

Chirped pulse amplification (CPA) (dt .: amplifying chirped pulses) is a method in laser physics , which allows laser pulses with very high intensity to produce. CPA is also used synonymously for an optical assembly that uses the method.

With this method, maximum pulse peak powers in the petawatt range, i.e. more than 10 ¹⁵ W, can be achieved. Such high peak powers cannot be generated directly by laser beam sources, since the amplification media of the laser would usually be destroyed by non-linear optical effects . As a result, laser pulses are stretched in time outside the resonator in an amplifier, reducing their energy density, and then pass through an amplifier medium. After amplification, they are compressed and then used in experiments or industrial applications with the higher power density.

With CPA lasers with intensities of up to 10 ²² W / cm² with a pulse length of a few femtoseconds were built up.

CPA was introduced in 1985 by Gérard Mourou and Donna Strickland , who received the Nobel Prize in Physics for it in 2018 .

principle

Schematic structure of a CPA

The shorter a light pulse, the wider the spectrum of frequencies it contains. This results from the description of the light pulse as a wave packet (see the same article). For a Gaussian wave packet, the following relationship applies between the temporal length Δt and the spectral width Δω :

${\displaystyle \Delta t\propto {\frac {1}{\Delta \omega }}.}$

Light of different frequencies can be refracted or delayed differently by optical components . By arranging optical components, mainly grids and prisms, the different frequency components of the (short) laser pulse can be delayed differently and the pulse can be spatially separated and then compressed again. Figuratively speaking, the red (low-frequency) components hurry ahead of the pulse, while the blue (high-frequency) components are delayed more (or vice versa, depending on the sign of the dispersion). The total pulse is thereby lengthened and the pulse peak power is reduced accordingly. The intensity can then be increased again by amplification to below the influence of non-linear optical effects in the amplifier medium.

The expansion or compression is shown schematically in the figure. A first grating fans out the light depending on the frequency. A second grid is set up in such a way that it parallelizes the light, but also causes it to expand or compress it. A mirror reflects the laser light in such a way that the beam passes the grating again and is further expanded or compressed in the process. With grids parallel to each other, you get a positive dispersion ( the blue end is faster ). With a simple lens system between the grids, these can be tilted towards each other and you get a negative dispersion ( the blue end is slower ).

Individual evidence

1. ↑ D. Strickland, G. Mourou: Compression of amplified chirped optical pulses. In: Optics communications, Vol. 55, No. 6 . 1985, p. 447-449 , doi : 10.1016 / 0030-4018 (85) 90151-8 .