State of the art optical systems become more complex. There are more lenses required in the optical design
and optical coatings have more layers. These complex designs are prone to induce more thermal stress into the optical
system which causes birefringence. In addition, there is a certain degree of freedom required to meet optical specifications
during the assembly process. The mechanical fixation of these degrees of freedom can also lead to mechanical stress in
the optical system and therefore to birefringence. To be able to distinguish those two types of stress a method to image
the birefringence in the optical system is required. In the proposed setup light is polarized by a circular polarization filter
and then is transmitted through a rotatable linear retarder and the tested optical system. The light then is reflected on the
same path by a mirror. After the light passes the circular polarization filter on the way back, the intensity is recorded.
When the rotatable retarder is rotated, the recorded intensity is modulated depending on the birefringence of the tested
optical system. This modulation can be analyzed in Fourier domain and the linear retardance angle between the slow and
the fast axis as well as the angle of the fast axis can be calculated. The retardance distribution over the pupil of the optical
system then can be analyzed using Zernike decomposition. From the Zernike decomposition, the origin of the birefringence
can be identified. Since it is required to quantify small amounts of retardance well below 10nm, the birefringence of the
measurement system must be characterized before the measurement and considered in the calculation of the resulting
birefringence. Temperature change of the measurement system still can produce measurement artifacts in the calculated
result, which must also be compensated for.
Hans-Martin Heuck, Ulrich Wittrock, Jérôme Fils, Stefan Borneis, Klaus Witte, Udo Eisenbart, Dasa Javorkova, Vincent Bagnoud, Stefan Götte, Andreas Tauschwitz, Eckehard Onkels
GSI Darmstadt currently builds a high-energy petawatt Nd:glass laser system, called PHELIX (Petawatt High-Energy Laser for Heavy-Ion Experiments). PHELIX will offer the world-wide unique combination of a high current, high-energy heavy-ion beam with an intense laser beam. Aberrations due to the beam transport and
due to the amplification process limit the focusability and the intensity at the target. We have investigated the
aberrations of the different amplification stages. The pre-amplifier stage consists of three rod-amplifiers which
cause mainly defocus, but also a small part of coma and astigmatism. The main amplifier consists of five disk
amplifiers with a clear aperture of 315 mm. These large
disk-amplifiers cause pump-shot aberrations which occur
instantly. After a shot, the disk amplifiers need a cooling time of several hours to relax to their initial state.
This limits the repetition rate and causes long-term aberrations. We will present first measurements of the
pump-shot and long-term aberrations caused by the pre- and the main amplifier in a single-pass configuration.
In this context, we will present the adaptive optics system which is implemented in the PHELIX beam line and
discuss its capability to compensate for the pump-shot and long-term aberrations.
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