Spectral Analysis of Nonlinear Phase Noise Arising in Coherent Fiber-Optic Transmission Systems of OFDM Signals

A coherent fiber-optic system for transmitting OFDM signals is considered. The spectrum of nonlinear phase noise arising in an optical fiber under nonlinear operation conditions of the optical path is investigated. It is shown that the spectrum of nonlinear phase noise consists of frequency components both at the fundamental frequency of the channel and combination frequencies falling within the band of OFDM channel signals. Estimation formulas are given for determining the noise immunity of channel signals by calculating the Q-factor depending on the parameters of the transmission system, in particular, on the number, modulation format (QPSK, M-QAM) and power of channel signals. An example of calculation of the Q-factor for channel signals for the QPSK formal is given. It is shown that it is necessary to limit the optical power level in the fiber to achieve the required signal quality factor. For example, when transmitting a multichannel signal over a 100-km fiber, it is necessary to limit the power level to 6.5, 7.4, and 8.8 dBm when using error correction methods FEC, HD-FEC, and SD-FEC, respectively.