1. Institute of Optical Communication and Optoelectronics, Beijing University of Posts and Telecommunications, Beijing 100876, People’s Republic of China, Key Laboratory of Optical Communication & Lightwave Technologies, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, People’s Republic of China, and Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden