The Pre-Amplifier employs two flashlamp-pumped Nd:glass amplifiers with free apertures of 19 mm and 45 mm diameter respectively, which are operated in double-pass mode. The input beam delivered by either the femtosecond or nanosecond frontend is stepwise magnified by Keppler telescopes up to a maximum exit beam diameter of 70 mm in order to keep the fluence within safe limits, below the damage threshold. Furthermore, a deformable mirror can correct wavefront aberrations. This mirror is actively driven in a closed-loop control cycle with a Shack-Hartmann sensor. Since the sensor is located at the end of the laser chain, the deformable mirror also precompensates wavefront aberrations introduced in the main amplifier (precompensation).
In addition, the Pre-Amplifier uses a serratured aperture in combination with a spatial filter (pinhole) for generating round and elliptic beam shapes with an almost top-hat intensity profile. The apertures are produced as lithographic aluminum masks on glass substrates.