2 Systems with Wavelength Division Multiplexing - CWDM and DWDM grid
2.1 General idea of multiplexing

The general idea how to increase the bit rate and information capacity of a system is to transmit information at many channels (wavelengths) concurrently. Long distance networks with the capacity of tens of Tbps must operate at hundreds channels, each running at the rate of tens of Gbps.

DWDM - Dense Wavelength Division Multiplexing requires using narrow-spectrum source of radiations, which are Distributed Feedback Lasers (DFB). In addition, the central wavelength must be stable (it is generally known that laser wavelength is temperature tunable); for this purpose it is recommended to use cooled DFB where operating temperature is fixed between 20 and 30°C.

Metropolitan networks can operate at much lower bit rates. CWDM - Coarse Wavelength Division Multiplexing uses 4 to 16 channels, with huge spacing of 20 nm, which allows using lasers with broader spectrum of emitted wavelengths, such as Fabry-Perot lasers, or non-cooled DFB. Such arrangement is less expensive.