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This study presents a novel all-optical in-band optical signal-to-noise monitoring method based on degenerated four-wave mixing. The monitoring ability arises from the power transfer function (PTF) giving preferential gain to clean pulses when compared with noisy ones. By selecting appropriate phase-matching condition a much higher monitoring sensitivity can be achieved at a reduced input power compared to other methods based on PTF proposed before. An analytic expression to evaluate the sensitivity of the monitor is presented. Numerical simulations are then used to quantify the sensitivity enhancement for signals with different modulation formats.