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Photon modes have an important role in characterizing the quantum sources oflight. Proper coupling of various photon modes obtained in spontaneousparametric down conversion (SPDC) process in optical fibers is essential togenerate an effective source of entangled photons. The two main pre-detectionfactors affecting the biphoton mode coupling in SPDC are the pump beam focusingparameter and the crystal thickness. We present the numerical and experimentalresults on the effect of pump focusing on conditional down-converted photonmodes for a Type-I BBO crystal. We experimentally verify that biphoton couplingefficiency decreases asymptotically with pump beam focusing parameter. Weattribute this behaviour to (a) the asymmetry in the spatial distribution ofdown-converted photons with the pump beam focusing parameter and (b) theellipticity of biphoton modes introduced due to the focusing of the pump beam.We also show experimentally the ellipticity as well as quantify the asymmetrywith the pump focusing parameter. These results may be useful in selectingoptimum conditions in the down-conversion process for generating efficientsources of entangled photons for quantum information applications.

Selecting the pre-detection characteristics for effective fiber coupling of entangled photon sources