Efficient coupling of down-converted photon pairs into single mode fiber

We present the analysis of the down-conversion emission profile from a critically phase-matched type-I nonlinear crystal. The emission profile of the down-conversion modes shows asymmetry in cylindrical coordinate system due to the birefringent effects of the nonlinear crystal, i.e. pump walk-off. This effect leads to reduced collection of the pairs from the source to the single mode fiber, which limits the brightness as a function of the crystal length. We propose a method to improve the symmetry of the down-conversion modes by inserting a lens off-centered with respect to the optical axis. Our theoretical and experimental results show that the asymmetry of the system increases with the crystal length by L0.5, and the brightness is proportional to crystal length by L0.72 and L0.91 for uncorrected and corrected down conversion mode profiles, respectively. The proposed method allows more efficient use of photon pair sources in terms of brightness which is to be used in quantum optics applications.