15 Dec 2018

Chirped Pulse Amplification explained

Chirped Pulse Amplification was first described by Gérard Mourou and Donna Strickland back in 1985 in a three-page-long scientific paper, which strikingly was Strickland’s first scientific paper ever. The technique is now widely used to achieve extreme laser peek powers in the order of petawatts, which is a thousand times more than the power of all of the power stations in the world.

‘Since power is defined as energy divided by time, if you want to go to high powers, one way is to try to make ever shorter pulses,’ Mourou explained in an interview directly after hearing he would be awarded with the Nobel Prize in Physics.

Epiphany during ski trip
‘Back in the eighties, we could already produce very short pulses in the order of femtoseconds. However, it was very difficult to amplify them, because the high-intensity light would burn the amplifier,’ he explained. During a ski trip, Mourou came up with the winning idea: ‘What if we were to produce a laser pulse, stretch it over a longer period of time - thus lowering the power -, amplify the elongated pulse, and then compress it again?’ Strickland, his then PhD student, built the system, and demonstrated one milli-joule in 1 picosecond, which corresponds to 1 gigawatt.

Strickland and Mourou used an ingenious set of gratings to stretch and recompress their pulses, Strickland explained during her Nobel Lecture. ‘With the first pair of gratings we dispersed the light into different colors. Since red light travels a lot faster than blue light, this led to an effective stretching of the pulse in time. This elongated, low power pulse could subsequently be safely amplified with a laser rod. A second pair of gratings then recombined all of the colors and compressed the pulse, thereby effectively increasing the power.’