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Raman fiber lasers (RFLs) are efficient light sources at frequencies where no other comparable all-solid-state sources are available. Especially if fiber Bragg gratings (FBGs) with narrow bandwidths are used, the bandwidth of the Stokes light is strongly broadened by Kerr nonlinearities like four-wave mixing (FWM), and self- and cross-phase modulation (SPM, XPM). In this letter, we discuss an exact numerical model to calculate the spectral behaviour of RFLs and show its application to determine the effective reflectivity of the FBGs. The model is based on a combination of the nonlinear Schroedinger equation including dispersion, FWM, SPM, and XPM with a shooting method to solve the power steady-state equations for RFLs. Numerical results are in good agreement with measurements.