1. Aston Institute of Photonic Technologies, Aston University, Birmingham, U.K.
2. Instituto de Telecomunicacoes and Universidade de Aveiro, Campus Universitario de Santiago, Aveiro, Portugal
3. University of Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers, Atomes et Molecules, Lille, France