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...infrastructure multiplying its capacity by up to 160 times.
DWDM is complex, however, and if designers are to optimize their long-haul DWDM designs in terms of power, space, cost and reliability, clear understanding is needed of current design techniques, as well as available technologies and components. Only then can DWDM's full potential be realized.
DWDM overview
In conventional long-haul fiber-transmission systems, data is transmitted with a certain bit rate, using (for low signal dispersion) a single wavelength from the second optical window (1,300-nanometer range) or (for low signal attenuation) a single wavelength from the third optical window (1,500- or 1,600-nm range). Higher transmission capacity can be achieved by raising the bit rate based on time-division multiplexing (TDM) or by adding more fiber cabling in parallel to existing cabling. Both will help. Since the latter requires an expensive buildout, increasing the bit rate becomes the logical path to higher bandwidth within an existing fiber-optic network.
The absence of a mature and cost-effective process for high-speed IC development, combined with the limitations of physical fiber media, means that it is not yet possible to realize practical commercial transmission systems beyond 40 Gbits/second. While upgrading a single-wavelength fiber link from 2.5 Gbits/s, for example, to 10 Gbits/s quadruples the bandwidth, DWDM can multiply the capacity by up to 160 times. It does this by simultaneously transmitting multiple high-bit-rate signals, each on a separate wavelength. Another big advantage of WDM transmission over TDM long-haul trunks with electronic regenerators is "bit-rate transparency," as conferred by the purely optical functions that are mandatory in such systems. These functions include optical multiplexers and demultipexers, optical-line amplifiers (OLAs) and (in the future) optical 3R regenerators for ultralong link distances. In principle, therefore, the link includes no elements that would require a change of optical-line components to achieve a higher bit rate.
The basic elements of a DWDM transmission system are the optical multiplexer, the optical demultiplexer and the optical-line amplifier (OLA) or so-called erbium-doped fiber amplifier (EDFA). These amplify optical signals to compensate for fiber attenuation caused by material impurities and the filter losses of the optical demultiplexer (see Fig. 1).
An optical multiplexer combines all the received wavelengths of the L band (1,530 to 1,565 nm) and C band (1,570 to 1,620 nm) into one wavelength-multiplexed light signal. Today's systems achieve wavelength separations of 0.4 nm or less, which allows about 160 potentially usable wavelengths. The L- and C-band ranges are determined by the OLA, which is...
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