  A semi-classical 7-wave model is developed to investigate the noise
performances of a degenerate dual-pump phase sensitive amplifier. This approach
takes into account the transfer to the signal, through multiple four-wave
mixing processes, of the vacuum fluctuations injected in the high-order waves.
This effect leads to a degradation of the noise figure of the amplifier with
respect to the 0 dB value predicted by the usual 3-wave model. However, it is
proved that a careful choice of the fiber dispersion allows to use the
high-order waves to enhance the signal gain without degrading the noise figure
above 1 dB.
