1. Physics Department, Yury Gagarin State Technical University of Saratov, 77 Polytechnicheskaya St., 410054 Saratov, Russia
2. Precision Mechanics and Control Institute of Russian Academy of Sciences, 24 Rabochaya St., 410024 Saratov, Russia
3. Institute of Physics, Saratov State University, 83 Astrakhanskaya St., 410012 Saratov, Russia