Optical wavelength division multiplexing (WDM) communication systems are forging ahead towards higher data transmission rate and lower channel spacing to utilize available bandwidth more effectively [1]. Increasing spectral efficiency is important for building efficient WDM communication systems, since this allows the optical infrastructure to be shared among many WDM channels, and thus reduces the cost per transmitted information bit in a fully loaded system [2]. In this paper we demonstrate performance of three different apodization profiles and their influence on high speed dense WDM systems main parameters: channel spacing and data transmission rate. We foresee that lower channel spacing and higher data transmission rate WDM systems will need FBG optical filters with inscribed more complex apodization profiles.