Improved Analysis of Stimulated Raman Crosstalk in Wideband Optical WDM System

Abstract

Abstract: An improved model is proposed to approximate the actual Raman gain profile of silica, which in turn is used to calculate the power depletion of the shortest-wavelength channel due to all other channels in the system. In this improved model, the stimulated Raman scattering (SRS) effects from channels at the tail of Raman gain profile are taken into account. A statistical analysis is done for the SRS crosstalk in a wavelength division multiplexed system including modulation statistics. The bit error rate and power penalty are estimated, where the improved model for the Raman gain spectrum is used. Key Words: Bit error rate, power depletion, power penalty, Raman gain, stimulated Raman scattering, wavelength division multiplexing. 1. INTRODUCTION Optical Wavelength Division Multiplexed (WDM) system effectively utilizes the enormous bandwidth of an optical fiber and hence meets the demands for large transmission capacity of multimedia communication [1]. However, fiber nonlinear effects limit the performance of a high power optical WDM system. Among the nonlinear effects, the Stimulated Raman Scattering (SRS) is the ultimate power-limiting phenomenon [2]. The SRS causes power transfer from shorter-wavelength WDM channels to longer-wavelength WDM channels and thus induces interchannel modulation between each WDM channel [3]. The optical power limits are calculated by the worst-case assumption that all WDM channels are synchronized and transmitted at ONE level simultaneously [4]. In all the calculations of Raman crosstalk, the experimentally found Raman gain profile of silica reported in [5] is approximated by a triangular function, where the peak Raman gain occurs at 15 THz spacing between channels and no Raman gain is assumed at larger spacing [6]. The results so far reported in literature based on this approximation are sufficiently accurate as long as the bandwidth of the fiber amplifier available currently is concerned [1]. However, for future wideband WDM systems, this approximate model of Raman gain requires to be modified. The probability density function (PDF) of SRS induced crosstalk has been developed in Ref. [7]. But the complete performance evaluation based on the statistical analysis of the Raman crosstalk is yet to be reported. The objective of this paper is to develop an improved model for the Raman gain profile, and, accordingly, to calculate more accurately the power depletion of the shortest-wavelength channel due to the SRS effects from all possible channels in a wide-band WDM system. A statistical analysis is presented to estimate the bit error rate (BER) of a WDM system based on the improved model for the Raman gain profile. Finally the sensitivity penalty suffered by the system due to SRS crosstalk is evaluated.