M^2-factor of coherent and partially coherent dark hollow beams propagating in turbulent atmosphere Yuan, Yangsheng ; Cai, Yangjian ; Qu, Jun ; Eyyuboğlu, Halil T. ; Baykal, Yahya ; Korotkova, Olga Abstract Publication: Optics Express Pub Date: September 2009 DOI: 10.1364/OE.17.017344 Bibcode: 2009OExpr..1717344Y