An integrable optical-fiber source of polarization entangled   photon-pairs in the telecom band
  We demonstrate an optical-fiber based source of polarization entangled photon-pairs with improved quality and efficiency, which has been integrated with off-the-shelf telecom components and is, therefore, well suited for quantum communication applications in the 1550 nm telecom band. Polarization entanglement is produced by simultaneously pumping a loop of standard dispersion-shifted fiber with two orthogonally-polarized pump pulses, one propagating in the clockwise and the other in the counter-clockwise direction. We characterize this source by investigating two-photon interference between the generated signal-idler photon-pairs under various conditions. The experimental parameters are carefully optimized to maximize the generated photon-pair correlation and to minimize contamination of the entangled photon-pairs from extraneously scattered background photons that are produced by the pump pulses for two reasons: i) spontaneous Raman scattering causes uncorrelated photons to be emitted in the signal/idler bands and ii) broadening of the pump-pulse spectrum due to self-phase modulation causes pump photons to leak into the signal/idler bands. We obtain two-photon interference with visibility $>90$% without subtracting counts caused by the background photons (only dark counts of the detectors are subtracted), when the mean photon number in the signal (idler) channel is about 0.02/pulse, while no interference is observed in direct detection of either the signal or the idler photons. 
Xiaoying Li, Chuang Liang, Kim Fook Lee, Jun Chen, Paul L. Voss, and   Prem Kumar