[1] Univ Strathclyde, Dept Elect & Elect Engn, Glasgow G1 1XW, Lanark, Scotland
[2] Univ Cantabria, Photon Engn Grp TEISA, Santander 39005, Cantabria, Spain
[3] Univ Publ Navarra, Dept Elect & Elect Engn, Navarra 31006, Spain
[4] Heriot Watt Univ, Dept Phys, Edinburgh EH14 4AS, Midlothian, Scotland
[5] INESCO Porto, Unidade Optelect & Sistemas Elect, P-4485 Oporto, Portugal