**4. Types of optical amplifiers**

The optical amplifiers which find widespread use in communication systems can be classified into three categories:-


The first two types, Fiber Raman Amplifier (FRA) and Erbium Doped Fiber Amplifier (EDFA) can be efficiently coupled to the transmission fiber by splicing with a minimum coupling loss. Of these two, EDFA requires lesser power for the pump source and the pump power requirements can be easily met by semiconductor laser diodes. Besides, the gain characteristics of EDFA are insensitive to polarization. Semiconductor Optical Amplifier (SOA) has the advantages of smaller size and lower power consumption. Its dimensional

Hybrid Fiber Amplifier 109

used as an amplifying medium. Er-doped fibers give an amplified output around 1550nm [Desurvire 2002; Becker, Olsson & Simpson 1999; Sun et.al.1997]. The EDFA is one of the key devices used for dense wavelength division multiplexed (DWDM) transmission systems. EDFAs are revolutionizing lightwave systems by reducing system costs and enhancing

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

network performance. Some of the advantages offered by EDFAs are:

Inherent compatibility to transmission fiber with low insertion loss

level diagram of Er3+ is shown in figure 4 ) [Keiser 2009;Mynbaev 2003].

High gain (~50dB)

 High output power (>100mW) Low noise figure (~4dB) Less gain variation

**5. Working principle of EDFA** 

Fig. 4. Energy Level Diagram of Er3+

Wide bandwidth of operating suiting DWDM

Cross talk immunity in multichannel systems

compatibility with the transmission fiber is obviously not as good as the fiber amplifier. However, SOA is suitable for optoelectronic integrated circuits.Table1 shows the basic difference between the three optical amplifiers and Table 2 shows the comparison of optical amplifiers.



Table 1. Difference of materials and operating bandwidth of three optical amplifiers

Table 2. Comparison of Optical Amplifiers

Although gain bandwidth of semiconductor laser amplifiers is ideally large, they have several drawbacks like polarization sensitivity, interchannel cross- talk and large coupling losses. Fiber amplifiers are preferable since the coupling loss due to fusion splice is negligible for them. Fiber amplifiers are also insensitive to polarization and have negligible noise for interchannel cross talk, which is one of the main noise sources in multichannel transmission or Dense Wavelength Division Multiplexing (DWDM). These reasons and available gain properties make the fiber amplifiers very suitable for modern optical transmission.

## **4.1 Advantages of EDFA**

It is clear that EDFAs are the best choice for optical amplification in present lightwave systems. Erbium (Er: 68) is used as dopant into glass host (fiber) and the 'doped fiber' is

compatibility with the transmission fiber is obviously not as good as the fiber amplifier. However, SOA is suitable for optoelectronic integrated circuits.Table1 shows the basic difference between the three optical amplifiers and Table 2 shows the comparison of optical

*Type of optical amplifier Material required Operating Working band* 

Semiconductor material from group III and V. *e.g.*phosphorous, gallium, indium and arsenic

> Lightly doping silica or tellurite with rare earth element *i.e.* erbium.

Fiber Raman Amplifier (FRA) Raman Lasers All Operating Bands Table 1. Difference of materials and operating bandwidth of three optical amplifiers

**1 Gain(dB)** >30 >40 >25 **2 Bandwidth (3dB)** 60 30-60 Pump

**4 Noise Figure (dB)** 8 5 5 **5 Pump Power** <400mA 25dBm >30dBm **6 Wavelength(nm)** 1260-1650 1530-1560 1260-1650 **7 Time Constant** 2x10-9 s 10-2s 10-15 s **8 Size** Compact Rack Mounted Bulk Module **9 Cost factor** Low Medium High

**(dBm)** 18 22 0.75 X pump

**Sensitivity** Yes No No

Although gain bandwidth of semiconductor laser amplifiers is ideally large, they have several drawbacks like polarization sensitivity, interchannel cross- talk and large coupling losses. Fiber amplifiers are preferable since the coupling loss due to fusion splice is negligible for them. Fiber amplifiers are also insensitive to polarization and have negligible noise for interchannel cross talk, which is one of the main noise sources in multichannel transmission or Dense Wavelength Division Multiplexing (DWDM). These reasons and available gain

It is clear that EDFAs are the best choice for optical amplification in present lightwave systems. Erbium (Er: 68) is used as dopant into glass host (fiber) and the 'doped fiber' is

properties make the fiber amplifiers very suitable for modern optical transmission.

**Semiconductor Optical Amplifier (SOA)** 

O-Band and C-Band

O-Band, S-Band, C-Band and L-Band

> **Fiber Raman Amplifier (FRA)**

dependent

**Erbium Doped Fiber Amplifier (EDFA)** 

amplifiers.

**S.** 

Semiconductor Optical Amplifier (SOA)

Erbium Doped Fiber Amplifier (EDFA)

**No. Parameter** 

**<sup>3</sup>Max. Saturation** 

**<sup>10</sup>Polarization** 

**4.1 Advantages of EDFA** 

Table 2. Comparison of Optical Amplifiers

used as an amplifying medium. Er-doped fibers give an amplified output around 1550nm [Desurvire 2002; Becker, Olsson & Simpson 1999; Sun et.al.1997]. The EDFA is one of the key devices used for dense wavelength division multiplexed (DWDM) transmission systems. EDFAs are revolutionizing lightwave systems by reducing system costs and enhancing network performance. Some of the advantages offered by EDFAs are:

