Investigations of OFDM-transmission for direct detection optical systems

Novel transmission techniques and modulation formats are always needed to increase the transmission capacity in optical communication system, which has large bandwidth and low loss compared to wireless communication system. Transmission techniques can be classified into two categories. The first is single carrier transmission, in which a single carrier is used for data transmission, and the second is multicarrier transmission technique, in which many closely spaced subcarriers are used for data transmission. Orthogonal frequency division multiplexing (OFDM) is a special case of multicarrier transmission technique, which is widely used in many standards in wireless communications due to its high spectral efficiency and robustness towards channel impairments, such as intersymbol interference (ISI). However, one of the main drawbacks in OFDM-transmission is the high peak-to-average power ratio (PAPR), which requires devices of high dynamic range to avoid the clipping distortion that degrades the system performance. OFDM has been proposed in recent years as a new candidate in optical communication systems. Optical transmission systems employing OFDM have gained considerable research because OFDM can combat linear fiber impairments, such as chromatic dispersion, and has the inherent capability to use higher-level modulation formats. Optical OFDM can be implemented by either using of coherent detection (CO-OFDM), which requires a local oscillator at the receiver, or using of direct detection (DD-OFDM), which is characterized by its simplicity. In this thesis, DD-OFDM has been mainly considered with some principles about CO-OFDM for comparison. Starting from the theory of OFDM until transmission over an uncompensated span of a single mode fiber with up to 32 WDM channels in DD-OFDM have been studied. From the results obtained in this dissertation, it seems that optical OFDM would be an intriguing candidate for optical high-speed transmission compared to other modulation formats like ON-OFF keying (OOK).