In order to satisfy the ever increasing demand for the bandwidth requirement in broadband services the optical orthogonal frequency division multiplexing (OOFDM) scheme is being considered as a promising technique for future high-capacity optical networks. The aim of this book is to investigate two kind of optical OFDM modem: IMDD optical OFDM modem and CO-OFDM modem. An investigation of coherent optical OFDM synchronization with Shmidl and COX method are presented, and then we introduce IMDD system analysis and we present the SVM equalizer as method for IMDD nonlinearities mitigation for 800 Km without dispersion compensation fiber. First, a study of Intensity modulation/Direct Detection (IM/DD) optical OFDM modem and the investigation of Intersymbol Interference (ISI) cancellation and cyclic prefix reduction is performed with different time equalizer Second, an investigation of the coherent optical OFDM signals nonlinearities with different nonlinear equalizers such as: Volterra, sparse Volterra, MLSE, Wiener-Hammerstein, SVM, ANN with experimental results.