Description:

In fibre-optic communications, Wavelength-Division Multiplexing WDM is a technology which multiplexes a number of optical carrier signals onto a single optical fibre by using different wavelengths of laser light. This technique enables bidirectional communications over a single strand of fibre, also called wavelength-division duplexing, as well as multiplication of capacity. The term WDM 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. Specifically, frequency in Hertz, which is cycles per second multiplied by wavelength the physical length of one cycle, equals the velocity of the carrier wave. In a vacuum, this is the speed of light, usually denoted by the lowercase letter, c. In glass fibre, it is substantially slower, usually about 0.7 times c. The data rate in practical systems is a fraction of the carrier frequency. 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 system over a single fibre pair to over 16.

WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fibre. 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. The capacity of a given link can be expanded simply by upgrading the multiplexers and DE multiplexers at each end. This is often done by the 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 fibre optical cables which have a core diameter of 9 µm. 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 62.5 µm. Early WDM systems were expensive and complicated to run. However, recent standardization and a 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 DE multiplexer must provide the wavelength selectivity of the receiver in the WDM system. WDM systems are divided into three different wavelength patterns normal , coarse and dense . Normal WDM sometimes called BWDM uses the two normal wavelengths 1310 and 1550 nm on one fibre. Coarse WDM provides up to 16 channels across multiple transmission windows of silica fibres. Dense WDM 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 (1565–1625 nm), more or less doubling these numbers.

Thanks&regards                              

John Greesham

Journal coordinator

International Journal of innovative research in computer and communication engineering