1. College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications , Nanjing 210023 , China
2. Shandong Provincial Engineering and Technical Center of Light Manipulations & Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University , Jinan 250014 , China
3. Jiangsu Province Engineering Research Center for Fabrication and Application of Special Optical Fiber Materials and Devices , Nanjing 210023 , China