1. State Key Laboratory for Information Photonics and Optical CommunicationsSchool of Electronic EngineeringBeijing Key Laboratory of Work Safety Intelligent MonitoringBeijing University of Posts and Telecommunications Beijing 100876 P. R. China
2. Institute of Laser EngineeringBeijing University of Technology Beijing 100124 P. R. China
3. School of InformationBeijing University of Technology Beijing 100124 P. R. China