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Components for optical transmission for special applications

Wavelength Division Multiplexing
Since the light of the different wavelengths is not mixed in the fiber, it is possible to simultaneously transmit signals at different wavelengths through a single fiber. Fiber is economical, but installing new cables can be expensive, so using already installed fibers to transmit more signals can be very cost-effective.
Wavelength division multiplexing (WDM) was first used with multimode fiber at the beginning of the optical fiber, using both 850 and 1310 nm in multimode fiber. Currently, single-mode fiber networks can carry signals at 10Gb / s in 64 wavelengths or more, which is known as dense wavelength division multiplexing (DWDM). Multimode fiber systems that use wavelength division multiplexing (WDM) have been less popular; Nevertheless,

Repeaters and optical amplifiers
Although the optical fiber has low losses, which allows the signal to travel hundreds of kilometers, for extremely long distances and even on submarine cables, regenerators or repeaters are needed to periodically amplify the signal. At first, the repeaters consisted basically of a transmitter followed by a receiver. This receiver converted the optical input signal into an electrical signal, cleaned it to eliminate all possible noise and then another laser transmitter retransmitted it. These repeaters added noise to the signal, consumed a lot of energy and were complex, which means they were a cause of failures. Further,

The solution to the optical repeaters were the optical amplifiers. A standard fiber amplifier operates in the band 1480-1650 nm. It consists of fiber doped with erbium pumped with a laser at 980 or 1480 nm. The pumping laser supplies the power to the amplifier, while the input signal stimulates the emission as the pulse passes through the doped fiber. This stimulated emission, in turn, stimulates greater emission so that rapid and exponential growth of energy is generated in the doped fiber. It is possible to obtain gains of up to 40 dB (10000X) with available powers of up to +26 dBm (400 mW).
In addition to being used as repeaters, optical amplifiers are used to increase the signal level in cable television systems, which require high energy levels in the receiver to maintain adequate signal-to-noise performance, which allows extended cable runs or the use of splitters to "emit" a single signal through a coupler to many fibers and thus save the cost of additional transmitters. In telephony, fiber amplifiers are combined with DWDM (dense wavelength division multiplexing) to overcome the inefficiencies of DWDM couplers for long distance transmissions.

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