Description of the femtosecond laser system

Femtosecond system general description:

Recently (March 2006) the femtosecond laser system has been upgraded. The new system should deliver 2.5 mJ per (100 fs) pulse with a maximum repetition rate of 3 kHz. The manifacturer is KMlabs.

The femtosecond laser is the main instrument for ultra fast initiation of photo-chemical reactions. It is phase locked to the x-rays and used for single-bunch Laue diffraction, stroboscopic experiments up to 1 kHz (diffraction and wide-angle scattering) and to trigger a jitter-free femtosecond x-ray streak camera.
The laser system consists of three stages.

The first stage is a Kerr-effect mode-locked Ti: sapphire laser that is phase locked to the synchrotron RF/4 and designed to produce weak 100 fs pulses near 800 nm with a repetition frequency of 88 MHz.
The second stage, a Ti:sapphire multipass chirped pulse amplifier (CPA) operating at a repetition frequency up to 3 kHz (so far extensively tested only up 3 kHz). It boosts the energy of a single fs pulse to about 1.5-2.5 mJ. Before amplification, the pulse is stretched from 100 fs to 200 ps. The stretching is necessary because the high peak power of a 1 mJ, 1 mm2, 100 fs pulse (1 TW cm-2) would cause non-linear absorption and damage to several optical elements in the amplifier, including the Ti: sapphire crystal and the pockels cell. After amplification, the stretched pulse is ejected from the amplifier, and compressed back down to 100 fs. Click on the picture for more pictures.
The third stage employs non-linear methods to generate a variety of wavelengths. The methods include frequency doubling (400 nm), third harmonic generation (267 nm), and optical parametric generation/amplification (OPG/OPA). The OPA/OPG can generate tuneable visible pulses (460-760 nm) with up to 200 uJ per pulse. The entire laser system is supported on a 4.25 x 1.5 m2 optical table.

A laser pulse contains more than 3 x 10¹³ photons, which matches the number of binding sites in a 100 x 100 x 100 um³ MbCO crystal or the number of absorbers in a few mM solution (100x100x300 um³. This pulse energy makes it possible, at least in principle, to produce a high degree of excitation. In practice one would like to have more photons to compensate for losses in the beamline between the laser and the sample. The laser is focused by a motorised telescope, which makes it possible to scan the laser focus onto the sample.

Performance: