Owing to the rapidly growing capacity requirements for long distance transmission, fiber optic telecommunications are advancing into high data rate and dense channel schemes enabled by wavelength division multiplexing (WDM). In order to maximize the system capacity and to minimize the performance degradation caused by transmission impairments the system investigation and optimization are very important [1]. In fiber optical transmission systems, the degradation effects can be categorized by the random noise and waveform distortion. For long-span high density WDM (HDWDM) systems, signal waveform distortion can be generated by linear chromatic dispersion, polarization mode dispersion, fiber nonlinearity, or their combination. In high-speed (more than 2.5 Gbit/s) time division multiplexing (TDM) optical systems, because of the short optical pulses and wide optical spectrum, the effect of complex dispersion dominates in the system performance degradation [2]. In multiwavelength WDM optical systems the inter-channel crosstalk originated by fiber nonlinearity, such as cross-phase modulation (XPM) and four-wave mixing (FWM), is a limiting factor. To maximize the WDM network capacity, the system’s design and optimization have to take into account all the contributing factors - such as the channel data rate, transmission distance, signal optical power, fiber linear and nonlinear effects and of course the channel interval. In a HDWDM system the latter factor the most important for high quality solution [3]. Currently, much research in optical communications is focused on increasing the total bit rate of individual optical fiber. Most of them are grounded on novel modulation schemes for the concrete wavelength. An alternative equally valid approach for increasing transmission capacity is to scale WDM channel spacing to high-dense dimensions, while keeping the existing bit rate. Our investigation of minimal allowed channel spacing in HDWDM systems will provide recommendations for future WDM solutions, since there are neither agreed recommendations nor specifications for less than 0.4 nm channel interval [4].