A wavelength division multiplexing optical transmission system, an optical loading-channel transmitters, a WDM optical transmitter and a method of optically loading an optical transmission line with multiple optical loading channels. The multiple optical loading channels are produced in the form of an optical comb by using a periodic optical filtering apparatus incorporated within a resonator constructed as an optical loop. A plurality of optical loading channels of the generated optical comb are then output without data being modulated thereon.