Author Posting. (c) Taylor & Francis, 2007.
This is the author's version of the work. It is posted here by permission of Taylor & Francis for personal use, not for redistribution.
The definitive version was published in Journal of Modern Optics, Volume 54 Issue 6, April 2007.
doi:10.1080/09500340601101633 (http://dx.doi.org/10.1080/09500340601101633)