1. State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China; Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, South China University of Technology, Guangzhou 510640, China; and Guangdong Provincial Key Laboratory of Fiber Laser Material and Applied Techniques, South China University of Technology, Guangzhou 510640, China