In this paper we demonstrate performance of three different optical filter transfer functions: Lorentzian, Raised Cosine and Supergaussian, and their influence on 10 Gbit/s non-return-to-zero (NRZ) coded optical signals in HDWDM transmission systems. 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. We foresee that for 10 Gbit/s data transmission speed NRZ coded optical signal optimal filter’s FWHM bandwidth level must be greater than 0.1 nm or 12.5 GHz.