1. Photonics Laboratory, Physics Unit, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland
2. Iridian Spectral Technologies Inc., 2700 Swansea Crescent, Ottawa, ON K1G 6R8, Canada
3. Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States