1. Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan 430074 P. R. China
2. State Key Laboratory of Optical Communication Technologies and Networks Wuhan Research Institute of Posts Telecommunications Wuhan 430074 P. R. China
3. State Key Laboratory on Integrated Optoelectronics Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P. R. China