1 Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK
2 Current address: Institute of Advanced Photonics Technology, School of Information Engineering, Guangdong University of Technology, Guangzhou 510006, China
3 Current address: Guangdong Provincial Key Laboratory of Photonics Information Technology, Guangdong University of Technology, Guangzhou 510006, China
4 School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
5 Fujikura Ltd., Chiba 285-8550, Japan