1 Department of Physics, University of Science and Technology of China,He fei 230026, China
2 Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, Shanghai 201800, China
3 Wuhan Research Institute of P & T, Wuhan 430074, China
4 Optical Communication Center, Beijing University of Posts and Telecommunications, Beijing 100876, China
5 Institute of Semiconductor, The Chinese Academy of Sciences, Beijing 100083, China