**5. Working principle of EDFA**

The invention of the Erbium Doped Fiber Amplifier (EDFA) in the late eighties was one of the major events in the history of optical communication systems. It provided new life to the research of technologies that allow high bit rate transmission over long distances. EDFA has a narrow high gain peak at 1532nm and a broad peak with a lower centered at 1550nm. The use of an increasing number of channels in the present day DWDM optical networks requires a flat gain spectrum across the whole usable bandwidth. Owing to their versatility, useful gain bandwidth, high pumping efficiency and low intrinsic noise, EDFAs are the amplifier of choice for most of the network applications. They are based on single mode optical fibers with cores that have been doped, typically to a few hundred part per million, with the trivalent erbium ion, Er3+. The gain is provided through stimulated emission, as in laser. The Er3+ ion acts mostly as a three level system, in which the main participants are the 4I15/2 ground state, the 4I13/2 first excited level and the 4I11/2second excited level. The energy level diagram of Er3+ is shown in figure 4 ) [Keiser 2009;Mynbaev 2003].

Fig. 4. Energy Level Diagram of Er3+

Hybrid Fiber Amplifier 111

The optical fiber can be doped with any of the rare earth element, such as Erbium (Er), Ytterbium (Yb), Neodymium (Nd) or Praseodymium (Pr), Thulium (Tm). The host fiber material can be either standard silica, a fluoride based glass or a multicomponent glass. The operating regions of these devices depend on the host material and the doping elements. Fluorozirconate glasses doped with Pr or Nd are used for operation in the 1300nm window, since neither of the ions can amplify 1300nm signals when embedded in silica glass. The next popular material for long haul telecommunication applications is a silica fiber doped with Thulium, which is known as Thulium Doped Fiber Amplifier (TDFA). In some cases as Yb is added to increase the pumping efficiency and the amplifier gain. The TDFA are used in S-band (1460-1530nm). The energy state diagram of Tm3+ is shown in figure 7 [Aozasa et.al.2008].

Fig. 6. Gain Spectrum of EDFA

Fig. 7. Energy Level Diagram of Tm3+

**7. Role of TDFA in communication systems** 

Fig. 5. Absorption and Emission Spectra of EDFA

EDFAs are of particular interest in telecommunications, because their emission spectrum shows a gain of more than 20dB over the range of 1530-1560nm. This is also the third window used in optical communication. The absorption spectrum reveals that good absorption takes place around 380nm, 520nm, 800nm, 980nm, and 1480nm. The absorption bands at shorter wavelengths are not of interest owing to the non- availability of semiconductor laser diodes at these wavelengths. At 980nm and 1480nm, efficient laser diodes are available and therefore used as pump sources.
