1. Key Laboratory of In-Fiber Integrated Optics Ministry Education of China College of Physics and Optoelectronic Engineering Harbin Engineering University Harbin 150001 China
2. School of Electronic and Computer Engineering Peking University Shenzhen 518055 China
3. Tsinghua Shenzhen International Graduate School and Tsinghua-Berkeley Shenzhen Institute Tsinghua University Shenzhen 518055 China
4. School of Engineering Fraser Noble Building University of Aberdeen Aberdeen AB24 3UE UK