1. a LMCP, Univ. Paris VI , case 115, 4 pl. Jussieu, 75252 , Paris cedex 05
2. b LURE, bǎt. 209, Univ. Paris-Sud , 91405 , Orsay cedex , France
3. c Dept. of Elect. Eng., Univ. of Pennsylvania , Philadelphia , PA , 19104-6390 , USA
4. d Complex Systems Theory Branch, NRL , Washington , DC , 20375 , USA
5. e LCSM, Univ. Nancy I , BP 239, 54506 , Vandoeuvre-Cedex , France
6. f ESRF , BP 220, 38043 , Grenoble-Cedex , France