Shape of the spatial mode function of photons generated in noncollinear   spontaneous parametric downconversion
  We show experimentally how noncollinear geometries in spontaneous parametric downconversion induce ellipticity of the shape of the spatial mode function. The degree of ellipticity depends on the pump beam width, especially for highly focused beams. We also discuss the ellipticity induced by the spectrum of the pump beam. 
Gabriel Molina-Terriza, Stefano Minardi, Yana Deyanova, Clara I.   Osorio, Martin Hendrych, and Juan P. Torres