Atomic Physics

Faculty of Engineering, LTH

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High-Power Laser Facility

The TW laser is a 10 Hz Ti:sapphire system based on chirped-pulse amplification (CPA). It has been operational since 1992 and upgraded in several steps. Up to 1 J, 35 fs long pulses can be delivered on target, focused by an f/3 off-axis parabolic mirror. The diameter of the focal spot is less than 10 µm, giving a light intensity in excess of 1019 W / cm2. An acousto-optic modulator (the Dazzler from Fastlite) allows precise control of the temporal pulse shape. The crystal in the final amplifier is cryogenically cooled to 70 K, making it possible to change the pulse energy by simply varying the energy of the pump lasers.

Key parameters:

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

Detailed description of the laser system

A number of steps are needed in order to produce 1 J, 35 fs pulses. First, the pulses are generated in the oscillator. It is a mode-locked Ti:sapphire laser, using the optical Kerr-effect for phase-locking the laser modes to one another. It emits an 80 MHz pulse train with an average power of 500 mW. The pulses are centered at a wavelength of around 800 nm and have a spectral width of some 60 nm.

Immediately after the oscillator is the Dazzler. It is an acousto-optical modulator that can shape the spectral (and thereby also the temporal as they are linked via a Fourier transform) structure of the pulses. It is used to compensate for higher order dispersion in the system and gain-narrowing in the regenerative amplifier. Next is the stretcher, which introduces a chirp in the pulses and stretches them to several hundred picoseconds.

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 ns (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 ns before the main pulse.

The second amplifier is a five-pass butterfly amplifier. It raises the pulse energy to 300 mJ. Then, the pulse is sent through a beamsplitter. One part is recompressed and used for experiments with up to a few TW of laser power. The other part is amplified a third time in another multi-pass amplifier up to 1.6 J. The crystal in this power amplifier is cryogenically cooled to eliminate thermal lensing from the five one-joule YAG lasers pumping it. That makes it possible to control the laser pulse energy simply by changing the amount of pump light. The grating compressor is housed in a vacuum chamber, otherwise the pulse would suffer from non-linear effects.

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