Wavelength division multiplexing

In telecommunications wavelength division multiplexing (WDM) is a technology which multiplexes several optical carrier signals on a single optical fibre by using different wavelengths (colours) of laser light to carry different signals.

Note that this term applies 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). However, since wavelength and frequency are inversely proportional, and since radio and light are both forms of electromagnetic radiation, the distinction is somewhat arbitrary.

The device that joins the signals together is known as a multiplexer, and the one that splits them apart is a demultiplexer. With the right type of fibre you can have a device that does both at once, and can function as an optical add-drop multiplexer.

The first WDM systems combined two signals and appeared around 1985. Modern systems can handle up to 160 signals and can expand a basic 10 Gbit/s fibre system to a theoretical total capacity of over 1.6 Tbit/s over a single fiber pair.

WDM systems are popular with telecommunications companies because they allow them to expand the capacity. 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. All they have to do is to upgrade the multiplexers and demultiplexers at each end.

This is often done by using optical-to-electrical-to-optical translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.

Dense and coarse WDM

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

Early WDM systems were expensive and complicated to run. However, recent standardization and better understanding of the dynamics of WDM systems have made WDM much cheaper to deploy. The market has segmented into two parts, "dense" and "coarse" WDM.

Dense WDM (DWDM) is generally held to be WDM with more than 8 active wavelengths per fibre, with systems with fewer active wavelengths being classed as coarse WDM (CWDM).

As of 2003, CWDM devices have dropped in price to the point where they are similar in price to end-user equipment such as Ethernet switches.

The introduction of the ITU-T G.694.1 frequency raster in 2002 has made it easier to integrate WDM with older but more standard SONET systems. (I don't have the details to hand, but I believe it specifies a 200 GHz frequency raster, with 100 GHz channel spacing as a refinement). Today's DWDM systems use 50 GHz channel spacing for up to 80 channel operation.

Recently the ITU has standardized a 20 nanometre channel spacing grid for use with CWDM, using the wavelengths between 1310nm and 1610nm. Many CWDM wavelengths below 1470nm are considered "unuseable" on older G.652 spec fibres, due to the increased attentuation in the 1310-1470nm bands. Newer fibres which conform to the G.652.C and G.652.D standards, such as Corning SMF-28e and Samsung Widepass nearly eliminate the "water peak" attenuation peak and allow for full operation of all twenty ITU CWDM channels in metropolitan networks. For more information on G.652.C and .D compliant fibres please see:

DWDM systems are significantly more expensive than CWDM because the laser transmitters need to be significantly more stable than those needed for CWDM. Precision temperature control of transmitter lasers is required in DWDM systems to prevent "drift" off a very narrow centre wavelength. In addition, DWDM tends to be used at a higher level in the communications hierarchy, and is therefore associated with higher modulation rates, thus creating a smaller market for DWDM devices with very high performance levels, and corresponding high prices.

Optical receivers, on the other hand, tend to be wideband devices, with the wavelength selectivity at the receive end provided as part of the optical demultiplexer.