1. Department of Physics, Technical University of Plovdiv, 25 Tzanko Djusstabanov Street, 4000 Plovdiv, Bulgaria
2. Department of Applied Mathematics and Modeling, Faculty of Mathematics and Informatics, Paisii Hilendarski University of Plovdiv, 24 Tsar Assen Street, 4000 Plovdiv, Bulgaria
3. Metal Vapour Lasers Department, Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tsarigradsko Shaussee Boulevard, 1784 Sofia, Bulgaria