**6. Limitations of EDFA**

The main practical limitation of an EDFA stems from the spectral non-uniformity of the amplifier gain. As a result, different channels of a DWDM system are amplified by different amounts. These problems become quite severe in long-haul systems, employing cascaded chain of EDFAs. Secondly, for many EDFA deployments, automatic gain control (AGC) is used to ensure that the output signal power is proportional to the input power. However, there are times when a constant optical signal output, independent of input power, is more desirable, e.g., in an optical preamplifier at an optical receiver [Qiao & Vella 2007]. The figure 6) [Keiser 2009;Mynbaev 2003].Figure 6 shows the gain spectrum of EDFA, from which it is clear that EDFA has peak gain at 1530nm, beyond which the gain reduces slightly and remains flat almost until 1550nm. After that, the gain reduces sharply. Several gain flattening techniques of EDFA are available [Lee et,al 1996; Ono et.al.1997; Kim et.al.1998; Park et.al.1998; Kawai et.al.1999; Yun et al. 1999; Lu & Chu 2000; Pasquale & Federighi 1995; Kemtchou et. al.1996; Hwang et al.2000; .Bakshi et.al.2001; Sohn et.al.2002; Arbore et.al.2002; Kaur & Gupta 2009 and Lobo et.al. 2003]

So, EDFAs are widely used in the C-band (1530-1560nm) for optical communication networks. So, there is a necessity to improve the amplification bandwidth of EDFA (i.e. broadening as well as flattening of gain spectrum). This would help to cater the needs of present day communication systems. In order to overcome this limitation of EDFA, different doping elements are coming into existence. One of such doping material is thulium and the doped fiber amplifier is known as Thulium Doped Fiber Amplifier (TDFA). TDFAs are highly viable alternative to meet out the limitations of EDFAs and have bright future prospects to be used in optical communication systems.

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

The main practical limitation of an EDFA stems from the spectral non-uniformity of the amplifier gain. As a result, different channels of a DWDM system are amplified by different amounts. These problems become quite severe in long-haul systems, employing cascaded chain of EDFAs. Secondly, for many EDFA deployments, automatic gain control (AGC) is used to ensure that the output signal power is proportional to the input power. However, there are times when a constant optical signal output, independent of input power, is more desirable, e.g., in an optical preamplifier at an optical receiver [Qiao & Vella 2007]. The figure 6) [Keiser 2009;Mynbaev 2003].Figure 6 shows the gain spectrum of EDFA, from which it is clear that EDFA has peak gain at 1530nm, beyond which the gain reduces slightly and remains flat almost until 1550nm. After that, the gain reduces sharply. Several gain flattening techniques of EDFA are available [Lee et,al 1996; Ono et.al.1997; Kim et.al.1998; Park et.al.1998; Kawai et.al.1999; Yun et al. 1999; Lu & Chu 2000; Pasquale & Federighi 1995; Kemtchou et. al.1996; Hwang et al.2000; .Bakshi et.al.2001; Sohn et.al.2002; Arbore et.al.2002;

So, EDFAs are widely used in the C-band (1530-1560nm) for optical communication networks. So, there is a necessity to improve the amplification bandwidth of EDFA (i.e. broadening as well as flattening of gain spectrum). This would help to cater the needs of present day communication systems. In order to overcome this limitation of EDFA, different doping elements are coming into existence. One of such doping material is thulium and the doped fiber amplifier is known as Thulium Doped Fiber Amplifier (TDFA). TDFAs are highly viable alternative to meet out the limitations of EDFAs and have bright future

Fig. 5. Absorption and Emission Spectra of EDFA

diodes are available and therefore used as pump sources.

**6. Limitations of EDFA** 

Kaur & Gupta 2009 and Lobo et.al. 2003]

prospects to be used in optical communication systems.

Fig. 6. Gain Spectrum of EDFA

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

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. 7. Energy Level Diagram of Tm3+

Hybrid Fiber Amplifier 113

So, to utilize the S-band, TDFA is proposed to be used by various authors. The S-band has attracted attentions because it has low fiber loss, low dispersion and also high gain and efficiency. The summary of the work done on EDFA, TDFA and TDFA-EDFA amplifiers is

> **TDFA [Aozasa et.al.2008]**

**TDFA-EDFA [Sakamoto et.al.2006]** 

Considered Not considered

given in Table 3.

**EDFA** 

1546nm

**in [Qiao & Vella2007]** 

Considered with signal wavelength

**Stages** Two Single Four

**/Range of Gain** 0-37dB 22.6dB 20dB **Gain Excursion** 0.35dB 0.35dB 2dB

**Noise Figure** Not considered < 6.5dB <7dB

Hybrid configurations can be made by combination of the following:

et.al.2007, Kaur &Gupta 2008].

**Power** -5 to 5 dBm -32 to -2 dBm -20 to -10dBm

**Band** 1525-1565nm 1479-1507nm 1460-1537nm

Table 3. Summary of work done in [Qiao & Vella2007], [Aozasa et.al.2008], [Sakamoto

8 8 4

There is one more method of utilizing fiber amplifiers for optimum utilization of available fiber bandwidth i.e. by way of using various combinations of optical amplifiers in different wavelength ranges. The amplifiers can be connected either in parallel or in series. This configuration is termed as Hybrid Amplifier which is highly viable for the above discussed cause. In parallel configuration, the DWDM signals are first demultiplexed into several wavelength-band groups with a coupler, then they are amplified by amplifiers that have gains in the corresponding wavelength band and then they are multiplexed again with a coupler. The parallel configuration is very simple and applicable to all amplifiers. However, it has disadvantages also e.g. an unusable wavelength region exists between each gain band originated from the guard band of the coupler. Also, the noise figure degrades due to the loss of the coupler located in front of each amplifier. On the contrary, the amplifiers connected in series have relatively wide gain band, because they do not require couplers.

 **EDFAs and FRAs:** It has been observed that the gain spectrum of FRAs can be tailored by adjusting the pump powers and pump wavelengths. So this property is used to increase the amplification bandwidth of EDFA [Thyagarajan& Kakkar 2004; Oliveira

**Type of Amplifier/ Parameters** 

**Number of** 

**ASE and its Correction Function** 

**Peak Gain** 

**Range of Input** 

**No. of DWDM Channels Considered** 

**8. Hybrid amplifiers** 

et.al.2006**]** 

**Signal Gain** 

Tm+3 has three energy levels that are considered with respect to the Tm+3 populations. TDFA uses upconversion pumping method. The upconversion pumping consists of the twostep excitation of 3H6 to 3F4 and 3F4 to 3H4 with the same pump wavelength and this makes it possible to form a population inversion state between 3F4and 3H4. The gain and loss of TDFA in the 1460-1530nm wavelength region are determined not only by the excited state absorption (ESA) ( 3F4 to 3H4 ) and stimulated emission(SE) (3H4 to 3F4) but also by the ground state absorption (GSA) (3H6 to 3F4 ) The absorption, emission and ground state cross section emission of Tm3+ is shown in figure 8 [Aozasa et.al.2008,2002].

Fig. 8. Cross Sections of Tm3+

The gain spectrum of TDFA is shown in figure 9 [Aozasa et.al.2008, 2002]. A gain of 22dB (approximate) is obtained from 1460-1485nm wavelength range. After this wavelength range the gain reduces sharply.

Fig. 9. Gain Spectrum of TDFA

Tm+3 has three energy levels that are considered with respect to the Tm+3 populations. TDFA uses upconversion pumping method. The upconversion pumping consists of the twostep excitation of 3H6 to 3F4 and 3F4 to 3H4 with the same pump wavelength and this makes it possible to form a population inversion state between 3F4and 3H4. The gain and loss of TDFA in the 1460-1530nm wavelength region are determined not only by the excited state absorption (ESA) ( 3F4 to 3H4 ) and stimulated emission(SE) (3H4 to 3F4) but also by the ground state absorption (GSA) (3H6 to 3F4 ) The absorption, emission and ground state cross

The gain spectrum of TDFA is shown in figure 9 [Aozasa et.al.2008, 2002]. A gain of 22dB (approximate) is obtained from 1460-1485nm wavelength range. After this wavelength

section emission of Tm3+ is shown in figure 8 [Aozasa et.al.2008,2002].

Fig. 8. Cross Sections of Tm3+

range the gain reduces sharply.

Fig. 9. Gain Spectrum of TDFA

So, to utilize the S-band, TDFA is proposed to be used by various authors. The S-band has attracted attentions because it has low fiber loss, low dispersion and also high gain and efficiency. The summary of the work done on EDFA, TDFA and TDFA-EDFA amplifiers is given in Table 3.


Table 3. Summary of work done in [Qiao & Vella2007], [Aozasa et.al.2008], [Sakamoto et.al.2006**]** 
