Theoretical And Experimental Investigations Of Phase Noise In Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM)

A potential way to reduce the necessity of optical dispersion compensation for next generation high speed transmission is orthogonal frequency division multiplexing (OFDM). There are two major types of optical OFDM; Direct detection (DD) OFDM and coherent optical (CO) OFDM that have been suggested for fibre-optic networks. CO-OFDM has many benefits over DD-OFDM which are presented within this thesis. The goal of this thesis is to study, theoretically and experimentally new phase noise compensation methods for CO-OOFDM technique in long haul transmission networks. Employing the effective digital signal processing (DSP), coherent optical OFDM receivers enable the compensation of physical layer impairments such as optical chromatic dispersion (CD), polarization mode dispersion (PMD), Local Oscillator (LO) phase noise (PN) and sampling clock jitter (SCJ). The two last impairments are each a source of phase noise (PN). While SCJ is a term describing the timing errors in sampling operation due to clock noise in analog to digital converter (ADC), local oscillator phase noise (LO PN) means noise is due to mismatch of crystal oscillator between the transmitter and receiver lasers or the random timing change of the oscillator.Theoretical analysis has verified that local oscillator phase noise and sampling clock error might be converted from one to another one in order to manage easily the design tradeoffs and to realize the optimal system and data-converter efficiency. A phase noise compensation process, based on new pilots design and feed forward maximum likelihood (FFML) estimator, is proposed. Simulations have established that LO laser linewidth is responsible for the quality of signal reception and4-QAM outperforms16-QAM,64-QAM and256-QAM.To improve the receiver performance, minimum mean square error (MMSE) is included for mitigation issue. Optical signal to noise ratio (OSNR) of pilot-data-aided feed forward maximum likelihood method (PA-FF-ML) using MMSE has performed better than simple PA-FF-ML method. The dependency of RMSE with pilots symbols numbers and filter taps numbers has been established. A16-QAM/128OFDM experiment has confirmed the simulation results. At a nominal bit rate of lOGb/s, we have successfully reconstructed the16-QAM constellations after25-km transmission distance over standard single mode fiber (SSMF) without optical dispersion compensation.