WDM (Wavelength Division Multiplexing)

In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.

The term wavelength-division multiplexing is commonly applied to an optical carrier, which is typically described by its wavelength, whereas frequency-division multiplexing typically applies to a radio carrier which is more often described by frequency. This is purely conventional because wavelength and frequency communicate the same information.

WDM Systems

A WDM system uses a multiplexer at the transmitter to join the several signals together and a demultiplexer at the receiver to split them apart. With the right type of fiber, it is possible to have a device that does both simultaneously and can function as an optical add-drop multiplexer. The optical filtering devices used have conventionally been etalons (stable solid-state single-frequency Fabry–Pérot interferometers in the form of thin-film-coated optical glass). As there are three different WDM types, whereof one is called “WDM”, the notation “xWDM” is normally used when discussing the technology as such.

The concept was first published in 1978, and by 1980 WDM systems were being realized in the laboratory. The first WDM systems combined only two signals. Modern systems can handle 160 signals and can thus expand a basic 100 Gbit/s system over a single fiber pair to over 16 Tbit/s. A system of 320 channels is also present (12.5 GHz channel spacing, see below.)

WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.

This is often done by use of optical-to-electrical-to-optical (O/E/O) translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.

Most WDM systems operate on single-mode fiber optical cables which have a core diameter of 9 µm. Certain forms of WDM can also be used in multi-mode fiber cables (also known as premises cables) which have core diameters of 50 or 62.5 µm.

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

Optical receivers, in contrast 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 divided into three different wavelength patterns: normal (WDM), coarse (CWDM) and dense (DWDM). Normal WDM (sometimes called BWDM) uses the two normal wavelengths 1310 and 1550 on one fiber. Coarse WDM provides up to 16 channels across multiple transmission windows of silica fibers. Dense WDM (DWDM) uses the C-Band (1530 nm-1565 nm) transmission window but with denser channel spacing. Channel plans vary, but a typical DWDM 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). New amplification options (Raman amplification) enable the extension of the usable wavelengths to the L-band (1565 nm-1625 nm), more or less doubling these numbers.

Coarse wavelength division multiplexing (CWDM), in contrast to DWDM, uses increased channel spacing to allow less-sophisticated and thus cheaper transceiver designs. To provide 16 channels on a single fiber, CWDM uses the entire frequency band spanning the second and third transmission windows (1310/1550 nm respectively) including the critical frequencies where OH scattering may occur. OH-free silica fibers are recommended if the wavelengths between second and third transmission windows is to be used[citation needed]. Avoiding this region, the channels 47, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used. With OS2 fibers the water peak problem is overcome, and all possible 18 channels can be used.

WDM, CWDM and DWDM are based on the same concept of using multiple wavelengths of light on a single fiber but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. EDFA provide an efficient wideband amplification for the C-band, Raman amplification adds a mechanism for amplification in the L-band. For CWDM, wideband optical amplification is not available, limiting the optical spans to several tens of kilometres.

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