1. School of Engineering, University of Glasgow, Glasgow G12 8LT, United Kingdom
2. School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, United Kingdom
3. Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 4DE, United Kingdom
4. QD Laser Inc, Keihin Bldg. 1F, 1-1 Minamiwataridacho, Kawasaki-ku, Kawasaki, Kanagawa 210-0855, Japan