WDM is a method of separating or combining multiple wavelengths out of or into a single fiber strand with each wavelength carrying a different signal. Using optical filters lets a certain range of wavelengths pass through, while another range is allowed. Thin-film filter technology (TFF) is often used to achieve this effect. Multiple thin layers are stacked and interference effects are created by sequential reflections on the interface between the layers. This lets light reflect for certain wavelengths and pass through for others.
The capacity of a network can be increased cost effectively by using WDM. Two types of WDM are commonly used:
- Dense Wave Division Multiplexing (DWDM) devices are mainly used when more wavelengths are required between sites and when the network extends over a very long distance. Forty wavelength channels from 1530 nm to 1570 nm are distributed in the C-band. To increase capacity, DWDM can be overlaid on a CWDM infrastructure.
- Coarse Wave Division Multiplexing (CWDM) has 18 different wavelength channels standard, spaced 20 nanometers (nm) apart between 1270 nm and 1610 nm. Most systems only use the top eight channels between from 1470 nm and 1610 nm. CWDM systems have the advantage that they can always be upgraded at a later stage. This limits the initial installation costs. The requirements on the lasers is not severe due to the wide channel spacing, allowing less expensive lasers without any temperature control to be used.
The insertion loss of DWDM and CWDM is typically lower than that of optical splitters. This increases the reach of a network from a centralized office substantially. As every customer has wavelength(s) assigned to them, this provides better security and makes eavesdropping virtually impossible.
WDMs can be utilized in different ways:
Add/drop vs mux/demux.
A multiplexer, also known as a mux, combines several wavelength channels on one fiber, while a de-multiplexer (demux) separates them at the other side. A mux/demux configuration is very useful to increase a fiber’s end-to-end capacity. A mux is normally located at a central office, while demuxes are placed in either a splice closure or cabinet. From there the fibers are routed in a star-shaped topology to their ultimate destination.
An alternative to separating the wavelengths at one side, individual wavelengths can be added or dropped at various points across the line. This process does not affect other wavelengths. This is often preferable when the distance between sites is long or they are grouped in a circular structure.
One or two fibers?
An alternative to sending signals at different wavelengths through the same fiber is to use two different fibers. Many CWDM systems use two fibers where one is used for upstream signals and the other for downstream. In this configuration, each customer uses two fibers and one wavelength. Each customer will have two wavelengths if they use a single fiber.
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