  We present experimental demonstration and modeling of the optimization of a
phase-sensitive optical parametric amplifier by tuning the relative position
between the pump- and signal-beam waists along the propagation direction. At
the optimum position, the pump beam focuses after the signal beam, and this
departure from co-located waists increases with increasing pump power. Such
optimization leads to more than 3\,dB improvement in the measured
de-amplification response of the amplifier.
