Optimal coupling of entangled photons into single-mode optical fibers
  We present a consistent multimode theory that describes the coupling of single photons generated by collinear Type-I parametric down-conversion into single-mode optical fibers. We have calculated an analytic expression for the fiber diameter which maximizes the pair photon count rate. For a given focal length and wavelength, a lower limit of the fiber diameter for satisfactory coupling is obtained. 
R. Andrews, E. R. Pike and Sarben Sarkar