# L-Band Erbium-Doped Fiber Optimization and Transmission Investigation +
## 3 Transmission Investigation and Discussion
### L-Band Transmission Results  
Based on the demonstrated transmission system shown in Figure 9, the transmission performance was first evaluated at 1600 nm and 1610 nm. The input signal power was set to \(-20\) dBm, while the forward pump power was fixed at 400 mW. The backward pump power increased from 200 mW to 450 mW with an interval of 50 mW. The relationship between the BER and backward pump power was measured with a digital signal processing (DSP) free receiver, as shown in Figure 9. The results are demonstrated in Figure 11.  
As shown in Figure 11a, at the signal wavelength of 1600 nm, as backward pump power rises, the signal power of all mode channels grows synchronously, while the BER reduces correspondingly. At the backward pump power of 200 mW, only the BER of the LP\({}_{01}\) mode channel falls below the FEC threshold [22; 23]. As the backward pump power approaches 300 mW, the BERs of all three signal mode channels drop below the FEC threshold, with the BER of the LP\({}_{01}\) mode channel even falling below \(1\times 10^{-10}\). When the backward pump power reaches 400 mW, the BERs of all three mode channels are less than \(1\times 10^{-10}\).  
As shown in Figure 11b, at the signal wavelength of 1610 nm, the modal gain is relatively low compared to that at 1600 nm (see Figure 8). Consequently, when the backward pump power reaches 350 mW, the BERs of all three signal mode channels fall below the FEC threshold. Referring to Figure 7b, LP\({}_{11a}\) can obtain the highest modal gain at 1610 nm. Therefore, within the backward power range of 300 mW to 400 mW, the LP\({}_{11}\) mode channel outperforms the other two mode channels. When the backward pump power reaches 400 mW, the BERs of three mode channels are below \(1\times 10^{-10}\).  
The BERs were also tested at 1570 nm, 1580 nm, and 1590 nm with the same measurement method. The forward pump and backward pump powers were set to 400 mW and 450 mW, respectively. The relationship between BERs and signal wavelength is illustrated  
Figure 11: The relationship between BER and the backward pump power: (**a**) 1600 nm; (**b**) 1610 nm.  
Figure 10: The RI profile and the fiber profile of the transmission fiber under a microscope.  
in Figure 12a. Additionally, the ASE spectra within this wavelength range were measured and are shown in Figure 12b.  
As shown in Figure 12a, the BERs of the LP\({}_{01}\) signal channels among all the wavelengths are consistently below 1 \(\times\) 10\({}^{-10}\). In contrast, due to the similarity of LP\({}_{11\text{a}}\) and LP\({}_{11\text{b}}\), the BERs of the LP\({}_{11\text{a}}\) and LP\({}_{11\text{b}}\) mode channels among different wavelengths exhibit a similar trend of variation: As the signal wavelength grows from 1570 nm to 1590 nm, the BERs of both the LP\({}_{11\text{a}}\) and LP\({}_{11\text{b}}\) mode channels continuously decrease; in the wavelength range of 1590 nm to 1610 nm, the BERs of these two mode channels remain below 1 \(\times\) 10\({}^{-10}\).  
The DSP-free receiver indicates that the BERs directly reflect the performance of the FM-EDFA. The variation in the BER is analyzed as follows: The wavelength range of 1570 nm to 1610 nm is divided into two sub-ranges at 1590 nm. In the wavelength range of 1570 nm to 1590 nm, the modal gains of the three signal modes are relatively high. However, as the wavelength decreases, the DMG (see Figure 8) and the ASE power (see Figure 12b) increase, which leads to a degradation in the signal quality. Consequently, the BERs of the LP\({}_{11}\) mode group rise as the wavelength decreases. Meanwhile, as the wavelength increases from 1590 nm to 1610 nm, although the modal gains reduce, the ASE power drops rapidly, and the DMGs remain well controlled. As a result, the BERs among all channels remain below 1 \(\times\) 10\({}^{-10}\). This indicates the good performance of the L-band EDFA.