[1] Univ Bath, Dept Phys, Bath BA2 7AY, Avon, England
[2] Bilkent Univ, Dept Phys, TR-06533 Bilkent, Turkey
[3] Univ Cambridge, Cavendish Lab, Cambridge CB3 OHE, England
[4] Univ Nottingham, Dept Phys, Nottingham NG7 2RD, England
[5] Nortel Networks Opt Components Inc, Ottawa, ON K2H 8E9, Canada