1. Hochschule Emden/Leer, Institute for Laser and Optics, University of Applied Sciences, Constantiaplatz 4, 26723 Emden, Germany
2. Institute of Physics, Carl von Ossietzky University of Oldenburg, 26111 Oldenburg, Germany
3. Université d’Aix Marseille, Polytech Marseille-IUSTI/UMR 7153, 5 Rue Enrico Fermi, 13453 Marseille Cedex 13, France