The Fractional Fourier Transform is a time–frequency distribution and an extension of the classical Fourier transform; it's used to make a reduction in the fiber nonlinear impairments by pre distorted signal before they launched into the fiber in WDM systems. Time domain FRFT can be implemented by using time lens according to the optical time–space duality theory. It consists of two phase modulators and a dispersive medium between them, the dispersion value of the dispersive medium has been varied from 10 ps/nm/km to 90 ps/nm/km (by 10 ps/nm/km for each step) and each value compared with the BER of the system, for a specific system configuration (4 channels, 300 Km, DPSK modulation format, 100 GHz channel spacing and 10Gb/s bitrate) results that in a dispersion value of 40 and 80 ps/nm/km the best system performance was found (improvement about 1.3 orders in BER at SNR of 10 dB).