1. National Institute of Telecommunications, Szachowa 1, 04-894 Warsaw, Poland
2. LINKS Foundation, Via Piercarlo Boggio 61, 10138 Torino, Italy
3. Instituto de Óptica, IO-CSIC, Serrano 121, 28006 Madrid, Spain
4. Aston Institute of Photonics Technologies, Aston University, Birmingham B4 7ET, UK