Theoretical and Simulation Studies of Fiber Optical Communication Systems with Higher Order Dispersion, Fiber Type and Self Phase Modulation

Abstract

Focus on development of broadband optical communication systems is incredible since it offers combination of wide bandwidth and low losses unmatched by any other transmission medium but group velocity dispersion and fiber nonlinearities remain inherent limitations of such systems thereby degrading the performance. The main motivation of this work was to study theoretical and simulation studies of broadband optical communication systems due to dispersion and fiber nonlinearities. The GVD effect is the major factor that degrades the performance of high bit-rate long-distance optical communications systems. The studies on dispersion are very limited as far as the significance of higher order dispersion terms are concerned. Fiber nonlinearities have become one of most significant limiting factors of system performance since the advent of erbium-doped fiber amplifiers (EDFAs) because input power is increasing and the effects of fiber nonlinearities are accumulating with the use of EDFAs. In wavelength-division-multiplexing (WDM) systems, inter-channel interference due to fiber nonlinearities may limit the system performance significantly. First, the FM-AM conversion with respect to binary intensity modulated PCM systems including higher order dispersion term are discussed using large signal analysis for dispersive optical fiber. The modified expression for power penalty has been derived and its impact on laser linewidth and bit rate has been investigated. For power penalty less than 0.5 dB, the plots between bit rate and transmission distance are plotted. It is seen that the transmission distance increases with decrease in linewidth over significant bit rates. The transmission distance with first order dispersion term for 150 KHz linewidth is approximately 900 km for 40Gb/s bit rate and 10-12 bit error rate. With proper first order dispersion compensation i.e. with second order dispersion only, the transmission distance can be enhanced to km for this linewidth for the same bit rate. It is also seen that the linewidth requirement is narrow for larger bit rates and large transmission distances. For achieving transmission distance of 200 km, the linewidth requirement is 3 KHz, 60 KHz and 5 MHz for bit rates 100 Gb/s, 40 Gb/s and 10 Gb/s respectively with bit error rate of 10-9. For WDM systems, with acceptable bit error rate of 10-12, the linewidth requirement reduces to 2KHz, 40 KHz and 2 MHz for bit rates 100 Gb/s, 40 Gb/s and 10 Gb/s respectively. Secondly, it has been shown that the bit error rate becomes deciding factor to select the fiber over long distance. From the results & discussion, it can be concluded that Dispersion shifted fibers anomalous & normal are performing better for long distances. Finally, we investigate power effects on simulation of 10 Gb/s NRZ optical communication systems with self phase modulation (SPM). In this, the behavior of SPM versus the optical power for a two spans amplified system has been investigated. A 10 Gb/s NRZ signal is launched over two DS fiber spans (D=0.4 ps/nm/km) of 50 km, each. The power at the input to each span is varied from 10 to 17.5 dBm by using the parametric run feature in OptSim. EDFA noise has been turned off in order to simplify the analysis of SPM. By increasing the power, SPM grows and depletes the signal, and the measured power (in a bandwidth equal to twice bit rate) actually decreases with the increasing of the transmitted power. Moreover, the channel has been demodulated. The eye diagram highlights the PM-AM conversion due to the SPM. Specifically the eye opening decreases with increasing transmitted power. Since there is no noise, estimation of the Q values is irrelevant.