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Fiber nonlinearity impairments in a 40-Cb/s coherent optical orthogonal frequency division multiplexing (CO- OFDM) system are post-compensated for by a new method of fiber nonlinearity post-compensation (FNPC).The FNPC located before the CO-OFDM receiver includes an optical phase conjugation (OPC) unit and a subsequent 80-km-high nonlinear fiber (HNLF) as a fiber nonlinearity compensator.The OPC unit is based on a four wave mixing effect in a semiconductor optical amplifier.The fiber nonlinearity impairments in the transmission link are post-compensated for after OPC by transmission through the HNLF with a large nonlinearity coefficient.Simulation results show that the nonlinear threshold (NLT) (for Q > 10 dB) can be increased by about 2.5dB and the maximum Q factor is increased by about 1.2 dB for the single-channel 40-Gb/s CO-OFDM system with periodic dispersion maps.In the 50-GHz channel spacing wavelength-division-multiplexing system, the NLT increases by 1.1 dB, equating to a 0.7dB improvement for the maximum Q factor.