1. Department of Applied Physics, KTH, 10691 Stockholm, Sweden
2. Department of Fiber Optics, RISE Acreo, Box 1070, SE-164 25 Kista, Sweden
3. Department of Physics, Pontifícia Universidade Católica do Rio de Janeiro (PUC-RIO), Rio de Janeiro 22451-900, Brazil
4. Naval Research Laboratory, 4555 Overlook Ave., Washington, DC 20375, USA