LLG (Laser-Laboratorium-Göttingen)

       We have a long-standing cooperation with the Laser-Laboratorium-Göttingen (LLG) in Germany in the field of the research and the development of short-pulse excimer lasers.

       The worldwide first compact high-brightness UV laser system was developed and built in the frame of this cooperation. In contrast to other common high-power lasers operating in the infrared, the system developed by the LLG group produces deep ultraviolet femtosecond laser pulses by amplification of frequency doubled short pulses.

       In short pulse UV lasers the key components of the system are excimer modules used as amplifiers in dual-wavelength schemes, where high quality short pulses are generated in the long-wavelength region and then shifted into the ultraviolet through frequency doubling or tripling. For physical and technical reasons, among excimers, the KrF mixture suits best for short pulse amplification.

       In a row of experimental studies, the operation of the entire system was carefully characterized and optimized.

       We have studied the spatial evolution of the chirp and the pulse duration of an originally positively chirped pulse when it passes through a dispersive element and a subsequent image system. We demonstrated, that spatial evolution of the pulse duration allows us to simulate the conventional chirped-pulse amplification technique in a much simpler way, only by inserting an amplifier into the beam where the pulse duration is stretched. In this way the amplifier and the target can be separated only by a lens or a mirror without the need of separate pulse stretchers and compressors. This arrangement is ideally suited for travelling wave excitation (TWE) of targets.

       We also studied intensity-dependent loss mechanisms occurring in window materials at 248 nm. It was found that the loss is mainly due to light scattering and absorption. Absorption in fused silica is primarily caused by two-photon absorption, while in CaF2, LiF, and MgF2 the combined effect of color-center formation and three-photon absorption must be considered. We have demonstrated that taking proper care of the directional properties of the beam, the high-power table-top laser system can generate focused intensities of as high as 1019 W/cm2. In excimer amplifiers – due to the short storage time of the active medium – successive replenishment of the momentarily stored energy is the only way to have access to the whole stored energy. This can be done by optical multiplexing. We have developed a new multiplexing technique which allows automatic (phase-locked) synchronization of the partial beams, therefore ideally suited for multiplexing of femtosecond pulses.