A Source of Entangled Photon-Pairs: Optimizing Emission in Two Quasi-Phasematched Crystals

We extend our earlier results of a bright source of non-degenerate polarization entangled photon-pairs by making a thorough theoretical and experimental investigation of its single-mode emission and state-decoherence. Using two perpendicular bulk crystals of periodically poled KTP, the source directly creates the polarization-entangled state (|HH > align=middle border=0 > + |VV > align=middle border=0 > )/ with signal and idler wavelengths of 810 nm and 1550 nm, respectively. In order to optimize the coupling into single-mode fiber we have developed a numerical algorithm which determines the emission mode and the fiber coupling efficiency for a given crystal length and focussing of the pump. For a 4.5 mm long crystal the optimal focussing of the pump is to a beam waist radius of 14 mum. With these focussing conditions the computer calculations give the lowest M2 value for the emission mode, M2 3, and the best possible fiber coupling efficiency, 75%. Using the theoretical results to optimize our experimental setup we obtained photon count rates of 1.4 MHz singles and 12 kHz coincidences, and visibilities H, V: 99.6% and 45 deg : 94.2%.