  NOON states are path entangled states which can be exploited to enhance phase
resolution in interferometric measurements. In the present paper we analyze the
quantum states obtained by optical parametric amplification of polarization
NOON states. First we study, theoretically and experimentally, the
amplification of a 2-photon state by a collinear Quantum Injected Optical
Parametric Amplifier (QIOPA). We compared the stimulated emission regime with
the spontaneous one, studied by Sciarrino et al. (PRA 77, 012324), finding
comparable visibilities between the two cases but an enhancement of the signal
in the stimulated case. As a second step, we show that the collinear amplifier
cannot be successfully used for amplifying N-photon states with N>2 due to the
intrinsic \lambda/4 oscillation pattern of the crystal. To overcome this
limitation, we propose to adopt a scheme for the amplification of a generic
state based on a non-collinear QIOPA and we show that the state obtained by the
amplification process preserves \lambda/N feature and exhibits a high
resilience to losses. Furthermore, an asymptotic unity visibility can be
obtained when correlation functions with sufficiently high order M are
analyzed.
