This paper presents a novel all-optical in-band optical
signal-to-noise (OSNR) monitor. The monitoring ability arises
from the power transfer function (PTF) provided by un-phase-matched four-wave mixing (FWM). A much higher output
contrast compared to formal methods can be obtained at a much lower required power. Numerical simulations are then
used to quantify the sensitivity enhancement for signals with different modulation formats which show that the
monitoring sensitivity can be greatly improved by more than 10 dB for NRZ, CSRZ, RZ modulation formats while the
required power can be reduced by about 3 dB.