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We present a theoretical and experimental investigation of the emissioncharacteristics and the flux of photon pairs generated by spontaneousparametric downconversion in quasi-phase matched bulk crystals for the use inquantum communication sources. We show that, by careful design, one can attainwell defined modes close to the fundamental mode of optical fibers and obtainhigh coupling efficiencies also for bulk crystals, these being more easilyaligned than crystal waveguides. We distinguish between singles coupling,conditional coincidence, and pair coupling, and show how each of theseparameters can be maximized by varying the focusing of the pump mode and thefiber-matched modes using standard optical elements. Specifically we analyze aperiodically poled KTP-crystal pumped by a 532 nm laser creating photon pairsat 810 nm and 1550 nm. Numerical calculations lead to coupling efficienciesabove 94% at optimal focusing, which is found by the geometrical relation L/z_Rto be ~ 1 to 2 for the pump mode and ~ 2 to 3 for the fiber-modes, where L isthe crystal length and z_R is the Rayleigh-range of the mode-profile. Theseresults are independent on L. By showing that the single-mode bandwidthdecreases as 1/L, we can therefore design the source to produce and couplenarrow bandwidth photon pairs well into the fibers. Smaller bandwidth meansboth less chromatic dispersion for long propagation distances in fibers, andthat telecom Bragg gratings can be utilized to compensate for broadened photonpackets--a vital problem for time-multiplexed qubits. Longer crystals alsoyield an increase in fiber photon flux proportional to sqrt{L}, and so,assuming correct focusing, we can only see advantages using long crystals.Comment: 19 pages, 15 figures, ReVTeX4, minor revisio

Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers