EDFA

Module 15 : ERBIUM DOPED FIBER AMPLIFIERS (EDFA)

Lecture : ERBIUM DOPED FIBER AMPLIFIERS (EDFA)

	ERBIUM DOPED FIBER AMPLIFIERS (EDFA)
1.	Introduction :
	In optical communication network, signals travel through fibers for very large distances without significant attenuation. However, when distances become hundreds of kilometers, it becomes necessary to amplify the signal during transit. Optical fiber amplifiers provide in-line amplification of optical signals by effecting stimulated emission of photons by rare earth ions implanted in the core of the optical fiber. Erbium is the preferred rare earth for this purpose though amplifiers using Praseodymium are also in use. EDFAs are used to provide amplification in long distance optical communication with fiber loss less than0.2 dB/km by providing amplification in the long wavelength window near 1550 nm. The principle of rare earth doped fiber amplifier is the same as that of lasers excepting that such amplifiers do not require a cavity whereas a cavity is required for laser oscillation. Advantages of EDFA are as follows:
	It provides in-line amplification of signal without requiring electronics i.e., the signal does not need to be converted to
	electrical signal before amplification. The amplification is entirely optical.
	It provides high power transfer efficiency from pump to signal power.
	The amplification is independent of data rate.
	The gain is relatively flat so that they can be cascaded for long distance use.
	On the debit side, the devices are large, there is gain saturation and there is also presence of amplified spontaneous emission (ASE).

2.	Composition of Rare Earth Doped Fiber :
	The fiber core consists of glassy material such as SiO $_2$ and GeO $_2$ . Rare earth ions like Er, Pr are doped into the core. The cladding material is mainly SiO $_2$ .
	Glass is, in reality, a liquid with such high viscosity that it mimics a solid, though with disordered arrangement of atoms as in the case of a liquid. As a material for lasr host, glass has several advantages over other solid state material, e.g., transparency, high optical damage threshold, optical quality etc. Oxides, such as SiO $_2$ , GeO $_2$ , Sb $_2$ O $_3$ etc. which can form glassy structures either by themselves or in combination with other oxides having similar properties are called glass network formers .