In this dissertation, a simulation study of coherent-optical OFDM (CO-OFDM) systems has been conducted. Fiber nonlinearities are the main limitation for long-haul CO-OFDM transmission. A second limitation is the high impact of laser phase noise (LPN) imposed by the large linewidth of low-cost distributed feedback (DFB) lasers. Furthermore, high-order constellations (16-QAM) combined with large OFDM symbols (1024-point inverse fast Fourier transform (IFFT)/fast Fourier transform (FFT)) increase the penalty of such impairments. The larger OFDM symbols reduce the overhead of the cyclic prefix, and high-order constellations provide high spectral efficiency. Therefore, several combination techniques have been proposed and studied to alleviate the effect of fiber nonlinearities for long-haul CO-OFDM systems and of LPN for CO-OFDM systems that use DFB lasers. The experimental demonstration of CO-OFDM systems has been achieved in this dissertation using off-line processing, where the received data are stored by a sampling oscilloscope and then evaluated using MATLAB. First, the experimental setups of a homodyne CO-OFDM system and a self-homodyne CO-OFDM system have been carried out with DFB lasers. Two-stage LPN mitigation is then proposed, for which the experimental results exhibit an improvement in the bit error ratio (BER). Second, some experiments have been carried out to investigate the impact of LPN on DFT-spread CO-OFDM systems. Furthermore, a new spectral shaping technique for DFT-spread OFDM has been experimentally studied over a distance of approximately 347 km.