In this paper we demonstrate performance of three different optical filter transfer functions: Lorentzian, Raised Cosine and Supergaussian, and their influence on non return to zero (NRZ) coded optical signals in wavelength division multiplexing (WDM) transmission systems with different channel spacing and data transmission speed values. The effect of filter’s full width half maximum (FWHM) bandwidth’s influence on NRZ code format and spectral narrowing on signal quality has been studied extensively.