Introduction

In order to investigate laser-matter interaction high power laser systems are needed to provide laser pulses that can be focused to reach high intensities (I>10¹⁴ W/cm²). Thus laser pulses generated by femtosecond oscillators with typical energies of a few nanojoules have to be amplified. This amplification of the ultrashort pulses becomes complicated by the fact that even low pulse energies result in intensities that can cause damage of the optical laser components.

Lowering the intensities by larger beam diameters and larger amplifying crystals would be a very expensive solution to this problem. A more economic method is the chirped pulse amplification (CPA) technique, which was developed in the 1980s [1].

The CPA technique is shown in Fig.1 and consists of four parts:

1. A low power femtosecond laser pulse is generated in an oscillator. The involved process is called modelocking because the pulse is a superposition of different longitudinal laser modes (wavelengths) that have a fixed (locked) phase.

2. The femtosecond pulse is temporally stretched in order to reduce the intensity and to allow smaller beam diameters without reaching the damage threshold of the optics. The stretching is done by gratings which delay the shorter wavelengths resulting in a longer pulse that consits of light with successive decreasing wavelength. Because of the decreasing wavelength inside the pulse it is called a positive chirped pulse (hence the name: chirped pulse amplification).

3. Due to the strongly reduced intensity of the laser pulse it can be amplified without reaching the damage threshold of the optics. To obtain high energies multiple amplification stages can be installed.

4. In this last step the pulse is compressed back nearly to the length it had before passing the stretcher. This raises the intensity dramatically and makes a magnification of the beam neccessary in order not to destroy the optics. The compression reverses the stretching with gratings that delay the longer wavelengths.
    

The laser systems

Our two commercial CPA-systems contain Kerr-lens modelocked titanium-sapphire oscillators that are pumped by diode pumped frequency doubled Nd:YVO₄ lasers. The generated femtosecond pulse is stretched to about 200 picoseconds before entering the amplification stage. In the Thales Alpha system this stage consists of one regenerative amplifier and two multipass amplifier. The KMLabs Dragon system uses a 12-pass amplifier to reach its output power. The specifications of the laser systems are shown below.