2.15   .Principle of operation of Wavelength Division Multiplexing

The basic configuration of an SCM–WDM optical system is shown in Fig. 1. In this example, n independent high-speed digital signals are mixed by N different microwave carrier frequencies fi. These are combined and optically modulated onto an optical carrier. M wavelengths are then multiplexed together in an optical WDM configuration. At the receiver, an optical de-multiplexer separates the wavelengths for individual optical detectors. Then, RF coherent detection is used at the SCM level to separate the digital signal channels. Channel add–drop is also possible at both the wavelength and SCM levels. Although this SCM–WDM is, in fact, an ultra dense WDM system, sophisticated microwave and RF technology enables the channel spacing to be comparable to the spectral width of the baseband, which is otherwise not feasible by using optical technology. Compared to conventional high-speed TDM systems, SCM is less sensitive to fiber dispersion because the dispersion penalty is determined by the width of the baseband of each individual signal channel. Compared to conventional WDM systems, on the other hand, it has better optical spectral efficiency because much narrower channel spacing is allowed. Conventional SCM generally occupies a wide modulation bandwidth because of its double-sideband spectrum structure and, therefore, is susceptible to chromatic dispersion. In order to reduce dispersion penalty and increase optical bandwidth efficiency, optical SSB modulation is essential for long-haul SCM–WDM optical systems.