Info about WDM System Technology

Most WDM systems operate on single-mode fiber optical cables, which have a core diameter of 9 µm. Certain forms of WDM may also be used in multi-mode fiber cables (also referred to as premises cables) which have core diameters of fifty or 62.5 µm.

Early WDM systems were expensive and complicated to operate. However, recent standardization and better understanding of the dynamics of WDM systems make WDM less expensive to deploy.

Optical receivers, as opposed to laser sources, tend to be wideband devices. Therefore the demultiplexer must provide the wavelength selectivity of the receiver in the WDM system.

WDM systems are split into different wavelength patterns, conventional/coarse (CWDM) and dense (DWDM). Conventional WDM systems provide up to 8 channels within the 3rd transmission window (C-Band) of silica fibers around 1550 nm. Dense wavelength division multiplexing (DWDM) uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of 12.5 GHz spacing (sometimes called ultra-dense WDM).  Such spacing’s are today only achieved by free-space optics technology. New amplification options (Raman amplification) enable the extension of the usable wavelengths towards the L-band, pretty much doubling these numbers.

Coarse wavelength division multiplexing (CWDM) in contrast to conventional WDM and DWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. To supply 8 channels on one fiber CWDM uses the whole frequency band between second and third transmission window (1310/1550 nm respectively) including both windows (minimum dispersion window and minimum attenuation window) but the critical area where OH scattering may occur, recommending using OH-free silica fibers in case the wavelengths between second and third transmission window ought to be used. Avoiding this region, the channels 47, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used. Each WDM Optical MUX includes its optical insertion loss and isolation measures of every branch. WDMs are available in several fiber sizes and kinds (250µm fiber, loose tube, 900µm buffer, Ø 3mm cable, simplex fiber optic cable or duplex fiber cable).