Performance Analysis of Nonlinearities in Optical Fibers

Abstract

Non linearity effects arose as optical fiber data rates, transmission lengths, number of wavelengths and optical power levels increased. The only worries that plagued optical fiber in the early day were fiber attenuation and sometimes, fiber dispersion; however, these issues are easily dealt with using a variety of dispersion avoidance cancellation techniques. Fiber nonlinearities present a new realm of obstacles that must be overcome. The nonlinearities previously appeared in specialized applications such as undersea installations. However the new nonlinearities that need special attention when designing fiber optic system include stimulated brillouin scattering (SBS), stimulated Raman Scattering (SRS), four wave mixing (FWM), self phase modulation (SPM), cross phase modulation (XPM) and intermodulation. Fiber nonlinearities represent the fundamental limiting mechanism to the amount of data that can be transmitted on a single optic fiber. This thesis presents a study of these limitations and the steps that can be taken to minimize the effect of nonlinearities. The effect of nonlinearities at different data rates and effective length of fiber have been studied. The data rates, transmission lengths and the channel spacing have to be optimized in order to have an efficient transmission without any nonlinear effect. The non-linear effects tends to manifest themselves when optical power is very high, they become important in DWDM. FWM is one of the dominating degradation effects in WDM systems with dense channel spacing. If the channels are equally spaced, the new waves generated by FWM will fall at channel frequencies and will give rise to crosstalk. The effect of FWM at various channel spacing in WDM systems have been studied in this thesis, however the optimum channel spacing, effective length and cross-sectional area have to be found. Effect of nonlinearities and hence the choice of data format (NRZ, RZ, NRZ raised cosine, RZ raised cosine) have to be worked upon. Apart from FWM, SPM and XPM also degrade the system performance. All these nonlinearities along with chromatic dispersion have been studied. It is found that some amount of chromatic dispersion can minimize the nonlinearities. The simultaneous if compensation of dispersion and nonlinearities is presented.