1. Key Laboratory of In-Fiber Integrated Optics of Ministry of Education; College of Science; Harbin Engineering University; Harbin 150001 China
2. School of Physics and Astronomy; University of Birmingham; Birmingham B15 2TT UK
3. Department of Physics; Harbin Institute of Technology; Harbin 150001 China
4. College of Advanced Interdisciplinary Studies; National University of Defense Technology; Changsha 410073 China