An optical performance monitor particularly well-suited for use in dense wavelength-division multiplexed (DWDM) systems includes both a nonlinear optical detector and a conventional linear detector. The nonlinear optical detector, which may comprise a quadratic detector, is used to provide information, on a channel-by-channel basis, regarding chromatic dispersion, polarization mode dispersion and accumulated amplified spontaneous emission (ASE) noise in each signal wavelength.