The femtosecond frontend is the source of short pulses for the PHELIX laser system. It delivers pulses at an energy level of 20 - 30 mJ and a 10-Hz repetition rate. The short pulses are generated by a commercial femtosecond laser oscillator (Coherent Mira) delivering pulses with a duration of 100 fs and energies around 4 nJ. These pulses are either pre-amplified by an ultrafast optical parametric amplifier (uOPA) which can be used for temporal contrast control or they just pass the uOPA without any amplification. In the second case the total gain is delivered by the following amplifiers.
An adjustable pulse stretcher is used as first element of the chirped pulse amplification scheme (CPA) where a pulse is stretched in time, amplified and recompressed again. A stretching ratio of 190 ps/nm keeps the peak powers below the laser damage threshold of the optical components in the following amplification chain.
The stretched pulse is then amplified by two Titanium-doped sapphire regenerative amplifiers with a repetition rate of 10 Hz. The first one is set up in a linear configuration while the second one is a ring amplifier. The typical output energy is 30 mJ. Using ultra fast Pockels cells the achievable intensity contrast ratio is better than 60 dB. In addition a Mach-Zehnder interferometer can be used for generating double pulses of variable delay, energy and different stretching ratio. This is of particular interest for the generation of X-ray laser pulses.
