1. Institute of Optics and Atomic Physics; Technical University Berlin; Berlin 10623 Germany
2. Optics and Optoelectronics Laboratory; Ocean University of China; Qingdao 266100 China
3. Science and Information College; Qingdao Agriculture University; Qingdao 266109 China
4. Institute of Lasers; Academy of Sciences; Unjong District Pyongyang DPR Korea