Computationally Efficient Digital Backward Propagation For Fiber Nonlinearity Compensation

The next generation fiber transmission system is limited by fiber nonlinearity. A distributed nonlinearity compensation method, known as Digital Backward Propagation (DBP), is necessary for effective compensation of the joint effect of dispersion and nonlinearity. However, in order for DBP to be accurate, a large number of steps are usually required for long-haul transmission, resulting in a heavy computational load. In real time DBP implementation, the FIR filters can be used for dispersion compensation and account for most of the computation per step. A method of designing a complementary filter pair is proposed. The individual errors in the frequency response of the two filters in a complementary filter pair cancel each other. As a result, larger individual filter error can be tolerated and the required filter length is significantly reduced. Unequal step size can be used in DBP to minimize the number of steps. For unrepeatered transmission with distributed Raman amplification, the Raman gain as a function of the distance and the effective fiber length of each DBP step need to be calculated by solving the differential equations of Raman amplification. The split-step DBP is performed only for transmission where the signal power is high. In comparison with solving the nonlinear Schrodinger equation (NLSE) for the total field of the WDM signal, solving the coupled NLSE requires a smaller step number and a lower sampling rate. In addition, the phase-locking between the local IV oscillators is not necessary for solving the coupled NLSE. The XPM compensation of WDM long-haul transmission by solving the coupled NLSE is experimentally demonstrated. At the optimum power level of fiber transmission, the total nonlinear phase shift is on the order of 1 radian. Therefore, for transoceanic fiber transmission systems which consist of many ( \u3e 100) amplified fiber spans, the nonlinear effects in each span are weak. As a result, the optical waveform evolution is dominated by the dispersion. Taking advantage of the periodic waveform evolution in periodically dispersion managed fiber link, the DBP of K fiber spans can be folded into one span with K times the nonlinearity. This method can be called distance-folded DBP . Under the weakly nonlinear assumption, the optical waveform repeats at locations where accumulated dispersions are identical. Consequently, the nonlinear behavior of the optical signal also repeats at locations of identical accumulative dispersion. Hence for a fiber link with arbitrary dispersion map, the DBP steps can be folded according to the accumulated dispersion. Experimental results show considerable savings in computation using this dispersion-folded DBP method. Simulation results show that the dramatically reduced computational load makes the nonlinearity-compensated dispersion-managed fiber link a competitive candidate for the next-generation transmission systems