Optical orthogonal frequency division multiplexing(O-OFDM) has been demonstrated to combat fiber chromatic dispersion(CD) and polarization-mode dispersion(PMD),which is being considered as a promising technology for future long-haul optical transmission.Because of computation efficiency and lower cost,the direct detection optical OFDM(DDO-OFDM) system is one of its schemes.However,the amplified-spontaneous noise induced by direct-detection seriously affects the system performance.The impact of n...