Currently, many research topics in the field of optical transmission systems are focused on increasing the total data transmission speed of an individual optical fiber. Most of them are grounded on novel modulation techniques. 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. The authors have developed an experimental and theoretical 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.