DWDM systems are a fundamental element of the current global networks and it is hard to imagine how it was before the systems were introduced, although their history is shorter than… 20 years.
In fibre optic transmission, WDM (Wavelength Division Multiplexing) is a technology which enables us to multiply the capacity of a single fibre. It is done through multiplexing a certain number of optical signals by using different wavelengths (colours). On the transmitter site, wavelength division multiplexing systems use appropriate multiplexers to couple several signals, while on the receiver site, they use demultiplexers to separate them. Since there are 3 different types of WDM systems, this technology is often referred to as xWDM.
- WDM – a system which uses 2 standard wavelengths: 1310 nm and 1550 nm in most of the cases,
- CWDM – coarse WDM system which uses 18 wavelengths from 1270 nm to 1610 nm at every 20 nm step, specified by ITU-T G.694.2 standard in 2002,
- DWDM – dense WDM system uses 48 wavelengths in C-Band and 100 GHz Grid, specified by ITU-T G.694.1 standard also in 2002. In later extensions to that standard, a 50 GHz grid was established (96 channels) and then other grids. On the day when this article is written the most popular are the systems based on one of these two grids.
DWDM systems are a fundamental element of the current global networks and it is hard to imagine how it was before the systems were introduced, although their history is shorter than… 20 years. It seems that after the wavelength generated by the laser was stabilized and the optical amplifier was invented, this has been the only right direction of development.
Dreams of wavelength multiplexing appeared in telecommunications a long time ago, and the invention of an optical amplifier at the end of the 1980s theoretically made it possible to amplify a great number of optical channels in one device, without the necessity of their demultiplexing. At that time optical transmission systems required the regeneration of individual streams independently every several dozens of kilometres, so a DWDM system with an optical amplifier was a promise of huge costs savings. The first WDM solutions with optical amplification were not sophisticated optical systems but primitive sets of devices connecting 2 or 4 wavelengths that required manual setting and tuning. It was not until 1996 when it changed and the first DWDM system consisting of 16 channels with a capacity of 2.5 Gbps each appeared. The present systems usually have 96 channels and each of them can work with the throughput of 100 Gbps and more.
This story shows that it is often necessary to combine two innovative methods to bring a revolution. Personal computers did not revolutionise office life until they were connected to printers. Similarly, DWDM networks have brought a number of advantages only when they were used together with an EDFA amplifier. Full regeneration was very expensive and EDFA amplifiers simply replicated the forthcoming photons and sent more photons of the same wavelength. Before that, each of the optical channels had to be regenerated independently, but thanks to DWDM it became possible to use only one EDFA amplifier for the entire optical fibre, instead of using dozens of regenerators. The revolution we benefit from even today has not eliminated the necessity of full regeneration of data streams, but has enabled us to build regeneration nodes not every 50-80 km, but every 1000-1500 km.
As we have already written in the article Understanding coherent transmission new technologies have enabled us to increase the capacity of the channels in a DWDM system. This is crucial because the EDFA area is limited to about 40 nm. Thus, the same fibre which in 1996 was able to carry 16 channels of 2.5 Gbps capacity, today can carry almost 20Tbps.