1. State Key Laboratory of Information Photonics and Optical Communications, School of Science, Beijing University of Posts and Telecommunications, P.O. Box 122, Beijing 100876, People’s Republic of China.
2. Corresponding author. State Key Laboratory of Information Photonics and Optical Communications, School of Science, Beijing University of Posts and Telecommunications, P.O. Box 122, Beijing 100876, People’s Republic of China, and Hebei Key Laboratory of Physics and Energy Technology, North China Electric Power University, Baoding 071000, People’s Republic of China.