We present a novel perturbation method for the nonlinear Schrödinger equation (NLSE) that governs the propagation of light in optical fibers. We apply this method to study signal-noise interactions in amplified multispan fiber-optic systems. Being based on a combination of the regular perturbation (RP) and logarithmic perturbation, the method is especially suitable for modeling the simultaneous presence of nonlinear and dispersive effects. Even after linearization, it retains the contribution of the quadratic perturbation terms of the NLSE, thereby achieving higher accuracy than an RP with comparable complexity. We revise parametric gain and nonlinear phase-noise effects under the new theory. We finally consider several examples and evaluate the probability density function of the optical or postdetection signal and the bit-error rate of an NRZ–OOK system. All of the results are compared with other models and with multicanonical Monte Carlo simulations.
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