1. Optoelectronic Technology Center, Institute of Microelectronics of the Chinese Academy of Sciences, Beijing 100029, China
2. Division of Time and Frequency Metrology, National Institute of Metrology, Beijing 100029, China
3. Key Laboratory of State Administration for Market Regulation (Time Frequency and Gravity Primary Standard), Beijing 100029, China
4. University of Chinese Academy of Sciences, Beijing 100049, China