1. Circuits and Systems Research Unit, University of Oulu, Oulu, Finland
2. Optoelectronics and Measurement Techniques Research Unit, University of Oulu, Oulu, Finland
3. Optoelectronics Research Centre, Physics Unit, Tampere University, Tampere, Finland
4. Fraunhofer Centre for Applied Photonics, Fraunhofer UK Research Ltd., Glasgow, U.K.
5. Department of Physics, Institute of Photonics, SUPA, University of Strathclyde, Glasgow, U.K.