Due to the limitation of the electrical OFDM signal and electrical Fast Fourier Transform (FFT), all-optical OFDMs have recently received much attention. Accordingly, this research study was conducted to investigate the effect of phase noise in the performance of an all-optical OFDM transmission system with 4-point FFT single mode fiber (SMF) links by considering the effects of fiber length, input laser power and a number of channels. In all optical systems, the transmitter side consists of a comb power generator, wavelength selected switch and an optical QAM generator. A comb power generator generates channels with a frequency separation of ∆f=25 GHz. Subsequently, a Wavelength Selected Switch (WSS) was used to split subcarriers and then the subcarriers were modulated individually with Optical QAM modulators. As the results show, a higher number of channels led more phase noise in terms of XPM and FWM nonlinearities, and signal power was the main factor in nonlinear fiber optics. As a consequence, there is more phase noise distortion at a higher signal power for a higher number of channels rather than the lower number of channels.