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High-Power Laser Facility

The terawatt laser is a Ti:sapphire laser system based on chirped-pulse amplification (CPA). It has a repetition rate of 10 hertz, which means that it generates 10 laser pulses per second. It has been operational since 1992 and upgraded in several steps. Each laser pulse can have an energy up to 1 joule. The pulses are 35 femtoseconds long (1 femtosecond = 0.000000000000001 second). The pulses can be focused on target by an f/3 off-axis parabolic mirror. The diameter of the focal spot is less than 10 micrometer, giving a light intensity in excess of 1019 W / cm2. An acousto-optic modulator (called Dazzler, produced by Fastile) allows precise control of the temporal pulse shape. The crystal in the final amplifier is cryogenically cooled to 70 Kelvin, making it possible to change the pulse energy by simply varying the energy of the pump lasers.

Fastlite webpage

Key parameters:

Repetition rate 10 Hz
Pulse length 35 fs FWHM
Pulse energy 1 J on target
Peak intensity >1019 W/cm2
Peak power35 TW
Contrast1:108

Detailed description of the laser system

Generation

First, the pulses are generated in an oscillator. An oscillator consists of two mirrors and a gain medium. The mirrors make the light reflect back and forth in the oscillator and the gain medium is where the laser light is generated. In this oscillator the gain medium is a Ti:sapphire crystal. The pulses of light, in contrast to continuous light, are created by using the optical Kerr-effect. This makes the modes of the laser phase-locked, and is therefore called a mode-locked laser. Out from the oscillator comes a train of pulses with 80 million pulses per second, and with an average power of 500 milliwatt. The pulses are centered at a wavelength of around 800 nanometer and have a spectral width of around 60 nanometer.

Temporal shaping

Immediately after the oscillator is the Dazzler. It is an acousto-optical modulator that can shape the spectral structure of the pulses, and thereby also the temporal as they are linked via a Fourier transform. It is used to compensate for higher order dispersion in the system and gain-narrowing in the regenerative amplifier.

Stretching

Next is the stretcher, which introduces a chirp in the pulses and stretches them to several hundred picoseconds. This is the first step in the chirped-pulse amplification (CPA) process. The idea is to make the pulses longer in time so that the peak power is decreased. A lower peak power enables larger amplification without the risk of damaging optical components. 2018 Nobel Prize in physics was given to Donna Strickland and Gérard Mourou for inventing the chirped-pulse amplification. You can read more about the process at the Nobel Prize popular science description.

Popular science description of Chirped-Pulse Amplification (pdf, 613kB)

Amplification

Then it is time to amplify the pulses. The first amplifier is of a regenerative type, a closed cavity where the pulse to be amplified is switched in and out of the cavity with Pockels cells. After 15 passes, the energy in the pulse has gone from nanojoules to millijoules. Two extra Pockels cells clean the pulse of amplified spontaneous emission (ASE) and any prepulses up to 1.5 nanoseconds (limited by the rise time of the Pockels cells) before the main pulse. The Pockels cells can be timed to cut the pedestal between 1.5 and 4 nanoseconds before the main pulse.

The second amplifier is a five-pass butterfly amplifier. It raises the pulse energy to 300 millijoule.

Beam split, compression, third amplification

The pulse is sent through a beamsplitter. One part is compressed, which is the third and last step of the CPA. The pulses are compressed by a grating that is introducing the reverse dispersion that was given to the pulses at the stretcher. This makes the pulses short again. The grating is housed in a vacuum chamber, otherwise the pulse would suffer from non-linear effects. This part of the beam is used for experiments with up to a few terawatt of laser power.

The other part of the beam is amplified a third time in another multi-pass amplifier up to 2 joules. The crystal in this power amplifier is cryogenically cooled to eliminate thermal lensing. The amplification crystal is pumped by five one-joule YAG (Yttrium Aluminum Garnet) lasers. The cooling makes it possible to control the laser pulse energy simply by changing the amount of pump light.