[1] Instituto de Telecomunica co es, Department of Electrical and Computer Engineering,Instituto Superior T′ecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal
[2] Nanophotonics Technology Centre, Universitat Polit`ecnica de Val`encia, 46022 Valencia, Spain
[3] THALES Communications, Gennevilliers, France
4. Science and Technology Department, Corning Incorporated, One Riverfront Plaza, Corning, NY 14831, USA