Compensation of anisotropy effects in the generation of two-photon light
  We analyse a method to compensate for anisotropy effects in the spatial distribution of parametric down-conversion (PDC) radiation in bulk crystals. In this method, a single nonlinear crystal is replaced by two consecutive crystals with opposite transverse walk-off directions. We implement a simple numerical model to calculate the spatial distribution of intensity and correlations, as well as the Schmidt mode structure, with an account for the anisotropy. Experimental results are presented which prove the validity of both the model and the method. 
Andrea Cavanna, Angela M. P\'erez, Felix Just, Maria V. Chekhova, Gerd   Leuchs
quant-ph physics.optics