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We propose a novel dispersion monitoring method based on four-wave mixing (FWM) or parametric amplification effect in optical fiber. The signal pulse stream, which is used as a parametric pump, is mixed with a weak continuous wave light at a different wavelength in a fiber. Due to the FWM effect, a so-called idler light is created at a wavelength symmetric in respect to the pump. The output idler power can be measured and is dependent on the pulsewidth and, therefore, accumulated residual dispersion of the input signal. The concept was successfully demonstrated with 40-Gb/s return-to-zero signal. This approach potentially works for data rates up to terabits per second and can be applied to provide a feedback for automatic dispersion compensation.