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We propose to use a backward Raman pump to suppress cross-gain modulation (XGM) during multiwavelength amplification in a fiber optical parametric amplifier (FOPA). The performances of two-wavelength amplification have been experimentally compared between FOPAs with and without the Raman pump under the condition of identical unsaturated gains. With Raman-assisted amplification, a larger range of input signal power is supported without causing significant depletion of the parametric pump. Consequently, the XGM crosstalk is effectively suppressed. The improvement is explained by compensation of the parametric pump power through the Raman gain. In our experiment, a 6.3-dB improvement has been obtained in the receiver sensitivity of a 10-Gb/s return-to-zero ON/OFF keying signal at the 10-8 bit-error rate level under simultaneous amplification of two data streams. The error floor has also been removed.