[1] Univ Sannio, Optoelect Div Engn, I-82100 Benevento, Italy
[2] Univ Cantabria, Photon Engn Grp, E-39005 Santander, Cantabria, Spain
[3] Univ Publ Navarra, Dept Elect & Elect Engn, Pamplona 31006, Spain
[4] Univ Strathclyde, Dept Elect & Elect Engn, Glasgow G1 1XW, Lanark, Scotland