  We report on a systematic experimental study of heralding efficiency and
generation rate of telecom-band infrared photon pairs generated by spontaneous
parametric down-conversion and coupled to single mode optical fibers. We define
the correlated-mode coupling efficiency--an inherent source efficiency--and
explain its relation to heralding efficiency. For our experiment, we developed
a reconfigurable computer controlled pump-beam and collection-mode optical
apparatus which we used to measure the generation rate and correlated-mode
coupling efficiency. The use of low-noise, high-efficiency superconducting
nanowire single-photon detectors in this setup allowed us to explore focus
configurations with low overall photon flux. The measured data agree well with
theory and we demonstrated a correlated-mode coupling efficiency of $97 \pm
2\%$, which is the highest efficiency yet achieved for this type of system.
These results confirm theoretical treatments and demonstrate that very high
overall heralding efficiencies can, in principle, be achieved in quantum
optical systems. It is expected that these results and techniques will be
widely incorporated into future systems that require, or benefit from, a high
heralding efficiency.
