1. Key Laboratory of In-fiber Integrated Optics of the Ministry of Education, College of Science, Harbin Engineering University, Harbin 150001, China
2. Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K.
3. Academy of Marine Information Technology, Guilin University of Electronic Technology, Beihai 536000, China
4. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China