Researches On Optical Signal Quality Optimization For High-Speed Fiber-Optic Communication

The need of high capacity data has increased today and as a consequence the transportation capacity existing long-haul optical transmission systems has to be increased. This will lead to the design and realization of new equipment’s and the transmission link. The design of the transmission link and its realization will become complex. The existing optical transmission link data channel capacity is40Gb/s. This channel bit rate need to be increased to100Gb/s or even more to be deploy on the existing infrastructures. This will necessitate more sophisticated method to improve the signal quality.Chromatic dispersion in optical transmission system is managed by dispersion compensation methods with dispersion compensation fiber (DCF) in three different approaches:pre-compensation, post-compensation and the symmetrical compensation which is the combination of the two first methods. But in real world the pre-compensation and the post-compensation are adopted due to their flexibility compare to the symmetrical method which offers the best performance. Optical loss along the fiber is compensated for by the optical amplifiers such as erbium doped fiber amplifiers (EDFA) which in turn introduce noise in the transmission systems but minimize with the optical filter generally at the receiver end. To compensate for Kerr effects and even chromatic dispersion the mid-way optical phase conjugation (OPC) is prove to be a promising technology to increase the robustness of a transmission link with a simplify structure. But due to its position which is impose by the link and the signal, the mid-way OPC suffers the disadvantage of flexibility.In this thesis a new approach of improving received optical signal quality utilizing spectral inversion (SI) technique outside the transmission link is investigated in long-haul intensity and phase modulated transmission systems using channel data rate up to100Gb/s. The approach is implemented using a module called optical signal post-optimization (OSPO) composed with an OPC-unit followed by highly nonlinear fiber (HNLF) with high nonlinear coefficients. The OPC or Si-unit is realized with four wave mixing (FWM) media such as HNLF and semiconductor optical amplifier (SOA).The simulation results show an improvement of the transmission performance of40 Gb/s differential phase shift keying (DPSK) system of about2dB in Q factor compare the system without OSPO at transmission lengths of1500,1800and2100km.100Gb/s single channel was simulated with in differential quadrature phase shift keying (DQPSK) where an increased in Q factor of the system with OSPO of1.5dB is observed at transmission lengths of2000,2500and3000km. In100Gb/s5channels WDM the system Q factor improvement was about1dB at transmission lengths of2000and2500km. The nonlinear threshold (NLT) has also been improved to about1dB.To ascertain the effectiveness of the method an experimental investigation was carried out using on-off keying (OOK) systems and DPSK over300km transmission distance. Various results obtained showed an improvement of system Q factor of the order of1dB.Adopting this spectral inversion technique at the receiver not only prove effectiveness to improve received optical signal quality but also offers flexibility as digital back-propagation (DBP) and does not offer computation complexities observed in DBP and prior knowledge of fiber spans configuration is not required.