Wavelength division multiplexing (WDM) is a technology or technique modulating numerous data streams, i.e. optical carrier signals of varying wavelengths (colors) of laser light, onto a single optical fiber. WDM enables bi-directional communication as well as multiplication of signal capacity.Many customers may don’t know the meaning of wave division multiplexing, here is the datails.
What is the development of WDM?
With the exponential growth in communications, caused mainly by the wide acceptance of the Internet, many carriers are finding that their estimates of fiber needs have been highly underestimated. Although most cables included many spare fibers when installed, this growth has used many of them and new capacity is needed.
Three methods exist for expanding capacity:
1) installing more cables.
2) increasing system bit rate to multiplex more signals.
3) wavelength division multiplexing.
Installing more cables will be the preferred method in many cases, especially in metropolitan areas, since fiber has become incredibly inexpensive and installation methods more efficient (like mass fusion splicing.) But if conduit space is not available or major construction is necessary, this may not be the most cost effective.Increasing system bit rate may not prove cost effective either.
Many systems are already running at SONET OC-48 rates (2.5 GB/s) and upgrading to OC-192 (10 GB/s) is expensive, requires changing out all the electronics in a network, and adds 4 times the capacity, more than may be necessary.The third alternative, wavelength division multiplexing (WDM), has proven more cost effective in many instances. It allows using current electronics and current fibers, but simply shares fibers by transmitting different channels at different wavelengths (colors) of light. Systems that already use fiber optic amplifiers as repeaters also do not require upgrading for most WDM systems.
Two different versions of WDM
Two different versions of WDM, defined by standards of the International Telecommunication Union (ITU), are distinguished:
(1) Coarse wavelength division multiplexing (CWDM, ITU standard G.694.2 ) uses a relatively small number of channels, e.g. four or eight, and a large channel spacing of 20 nm. The nominal wavelengths range from 1310 nm to 1610 nm. The wavelength tolerance for the transmitters is fairly large, e.g. ±3 nm, so that unstabilized DFB lasers can be used. The single-channel bit rate is usually between 1 and 3.125 Gbit/s. The resulting total data rates are useful e.g. within metropolitan areas, as long as broadband technologies are not widespread in households
(2) Dense wavelength division multiplexing (DWDM, ITU standard G.694.1) is the extended method for very large data capacities, as required e.g. in the Internet backbone. It uses a large number of channels (e.g. 40, 80, or 160), and a correspondingly small channel spacing of 12.5, 25, 50 or 100 GHz. All optical channel frequencies refer to a reference frequency which has been fixed at 193.10 THz (1552.5 nm). The transmitters have to meet tight wavelength tolerances. Typically, they are temperature-stabilized DFB lasers. The single-channel bit rate can be between 1 and 100 Gbit/s, and in the future even higher.
How does WDM work?
WDM with couplers and filters
It is easy to understand WDM. Consider the fact that you can see many different colors of light - red, green, yellow, blue, etc. all at once. The colors are transmitted through the air together and may mix, but they can be easily separated using a simple device like a prism, just like we separate the "white" light from the sun into a spectrum of colors with the prism.
Advantages of WDM
A WDM system has some features that make them very usable. Each wavelength can be from a normal link, for example a OC-48 link, so you do not obsolete most of your current equipment. You merely need laser transmitters chosen for wavelengths that match the WDM demultiplexer to make sure each channel is properly decoded at the receiving end.
If you use an OC-48 SONET input, you can have 4X2.5 GB/s = 10 GB/s up to 32 X 2.5 GB/s = 80 GB/s. While 32 channels are the maximum today, future enhancements are expected to offer 80-128 channels!And you are not limited to SONET, you can use Gigabit Ethernet for example, or you can mix and match SONET and Gigabit Ethernet or any other digital signals! You can even mix in analog channels like CATV, as is done with BPON FTTH systems.
Two obvious applications are already in use, submarine cables and extending the lifetime of cables where all fibers are being used. For submarine cables, DWDM enhances the capacity without adding fibers, which create larger cables and bulkier and more complicated repeaters. Adding service in areas where cables are now full is another good application.But this technology may also reduce the cost on all land-based long distance communications links and new technology may lead to totally new network architectures.
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