  We have investigated the excitation-power dependence of
polarization-entangled photon-pairs generated from a CuCl single crystal using
biexciton resonant hyper-parametric scattering. The measured two-photon
polarization density matrix which corresponds to the two-photon polarization
state changes with increasing the excitation power. The evaluation of the
tangle and the linear entropy obtained from the measured density matrix
indicates that the two-photon polarization density matrix can be expressed as
the mixed state of an ideal state and a totally mixed state, and the mixture
ratio of the totally mixed state increases as the excitation density increases.
The variations of the density matrix and the mixture ratio with the excitation
power originate from the generation of the multiple photon pairs.
