Currently, many research topics in the field of optical transmission systems (mostly grounded on novel modulation techniques) are focused on increasing the total data transmission speed of an individual optical fiber. An alternative − but equally valid − approach to increasing the data transmission is to decrease the WDM channel spacing to high-dense dimensions while keeping the existing data transmission speed for an exact channel. We have developed an experimental HDWDM transmission system based on the quasi-rectangular optical filter technique. The results indicate that for 2.5 Gbit/s HDWDM transmission the suitable channel interval should be greater than 25 GHz, and for the 10 Gbit/s HDWDM solution − not less than 37.5 GHz between adjacent channels.