  Polarization-entangled photon pairs can be efficiently prepared into pure
Bell states with a high fidelity via type-II spontaneous parametric
down-conversion (SPDC) of narrow-band pump light. However, the use of
femtosecond pump pulses to generate multi-photon states with precise timing
often requires spectral filtering to maintain a high quality of polarization
entanglement. This typically reduces the efficiency of photon pair collection.
We experimentally map the polarization correlations of photon pairs from such a
source over a range of down-converted wavelengths with a high spectral
resolution and find strong polarization correlations everywhere. A spectrally
dependent imbalance between contributions from the two possible decay paths of
SPDC is identified as the reason for a reduction in entanglement quality
observed with femtosecond pump pulses. Our spectral measurements allow to
predict the polarization correlations for arbitrary filter profiles when the
frequency degree of freedom of the photon pairs is ignored.
