**6.1 Equalization of gain with optimization of EDFA parameters**

The figures bellows show the power and noise spectrum as a function of wavelength at the output of the EDFA for different concentrations of erbium ions (from 1.10 +24/m<sup>3</sup> to 1.10+25/m<sup>3</sup> ) and different lengths of the doped fiber (from 2.5 m to 12.5 m), eye diagrams of the simulated system are also shown. According to the graphs obtained, the gray wave represents the noise that decreases as the length and concentration of the doped fiber decreases, while the red symbol in the graphs indicates the sample wavelength (eight wavelengths).

The results obtained show that the concentration of de 5,5.10+24/m<sup>3</sup> and the length of the doped fiber of 7.5 m give the best results in terms of maximum gain (around 26.63 dB) and equalization of the amplified optical spectrum.

It is noticeable that the eye aperture is well open which the quality factor is between 9 and 10 which are higher than 6 that mean that the system works correctly. The BER is higher than 10<sup>12</sup> which expresses that the transmission is errorfree (**Figures 14**–**18**).

The table below summarizes the simulation results at the output of the WDM analyzer for different values of the erbium ion concentration and the length of the doped fiber, where the term RG represents the difference between the maximum and minimum value of the EDFA gain (maximum ratio), while RNF indicates the variation of the noise figure.

#### **Figure 14.**

*(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 5.10+24/m<sup>3</sup> and L = 7, 5 m).*

**Figure 15.**

*(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (L = 5 m and C = 3,5.10+24/m*�*<sup>3</sup> ) .*

**Figure 16.**

*(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 1.10+24/m*�*<sup>3</sup> and L = 2.5 m).*

Comparison of the five graphs leads to the conclusion that the gains are flattened with a RG ¼ 0*:*7 in the band 1537 nm to 1545 nm wavelength around a gain of 26.3 dB with a noise Figure (NF) of less than 4 dB for 8 transmission channels for a concentration of 5, 5.1024/m�<sup>3</sup> and a doped fiber length of 7.5 m the worst case (gain less equalized with a RG ¼ 1*:*54) is obtained with a fiber length of 12.5 m (**Table 3**).

*Waveguide Amplifier for Extended Reach of WDM/FSO DOI: http://dx.doi.org/10.5772/intechopen.104790*

**Figure 17.**

*(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 7,5.10+24/m<sup>3</sup> and L = 10 m).*

#### **Figure 18.**

*(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 1.10+25/m<sup>3</sup> and L = 12, 5 m) .*

#### **6.2 Gain equalization through the use of a GFF filter**

Gain Equalizing Filters, also known as GFF (Gain Flattening Filters), are integrated into optical systems and are usually combined with optical amplifiers in the


**Table 3.**

*Optimization results obtained by the WDM analyzer.*

transmission chain to ensure good gain flattening. This process provides a solution to the problem of equalizing the amplifier output power of a WDM multiplexed system. This part refers to a process of combining the EDFA amplifier with a GFF filter and visualizing the power and noise spectrum.

The tables below show the results of the WDM analyzer at the output of the EDFA "**Table 4**"and the output of the gain equalizing filter "**Table 5**" in terms of wavelength, gain, noise figure and the deference between the maximum and minimum values of these figures. Comparison between the tables below leads to the conclusion that the gains are flattened with a RG ¼ 0*:*77in the band 1537 nm to 1545 nm wavelength around a gain of 26.97 dB with a noise Figure (NF) of less than 2.15 dB for 8 transmission channels.

The figures below show a schematic representation of the spectrum emitted at the output of the EDFA and output of the gain equalizing filter GFF. In **Figure 19**, the different wavelengths of the multiplex at the output of the EDFA (at the input of the filter) can be recognized, represented by red symbols from 1 to 8. It can be seen that the spectrum received at the output of the EDFA has a different total


**Table 4.**

*Results of the WDM analyzer at the EDFA output.*
