The authors have investigated the maximum permissible transmission distance for the high density wavelength division multiplexed (WDM) fiber-optic transmission systems (FOTSs) that complies with the following criteria: 1) optical signals are transmitted with two different per channel bitrates – 10 and 40 Gbit/s; 2) not only in FOTS traditionally used modulation formats, such as no-return to zero encoded on-off keying (NRZ-OOK) or NRZ encoded differential phase shift keying (NRZ-DPSK) but also the advanced modulation formats (e.g., 2-POLSK – orthogonal polarization shift keying) are used in system’s channels; 3) the minimum equal and unequal “non ITU-T” Recommendation G.694.2 defined channel spacing values are used for the obtaining of the maximum system’s spectral efficiency.