## 4 Experimental Results and Analysis
### Equalization Performance  
Based on the above analysis, the taps were configured as (85, 16) to balance the performance and computational complexity in our experiments. Due to the low time-varying rate of the channel system, \(\lambda=1\) was chosen to maintain the accuracy of the equilibrium convergence, and 1000 training sequences were used to ensure convergence stability. Figure 8a shows the BER versus ROP curves obtained using PA/MA-DFE (85, 16) and PA/MA-FFE (85) under BtB transmission. Note that they are both measured at their respective MZM optimal bias voltages. From the curves, it can be observed that the BER of the PA-DFE and MA-DFE are nearly identical. Using a BER of \(1\times 10^{-2}\) as the FEC threshold, both the PA-DFE and MA-DFE achieve a receiver sensitivity of \(-24\) dBm, which is a significant increase of 7 dB compared with the PA-FFE and MA-FFE. Figure 8b-e display the constellation diagrams after equalization by the four types of equalizers, where the circles representing the constellations of the PA-DFE and MA-DFE appear more convergent and distinct.  
Figure 7: BER performance and multiplications per symbol versus PA/MA-DFE with different taps.  
To increase the link budget of the system, we used an SOA to boost the launch power. However, it is important to note that when the SOA is in gain saturation state, it can introduce nonlinear distortions to the signal. For this reason, we investigated the performance of the PA-DFE and MA-DFE when the SOA was operating under the linear amplification and gain saturation states. We adjusted the input power to change the operating state of the SOA based on its input-output characteristics. When the input power is \(-12.8\) dBm, the SOA exhibits an output power of \(4\) dBm and operates within the linear amplification regime. Conversely, when the input power is \(-6.4\) dBm, the SOA outputs \(8\) dBm and transitions into the gain saturation state. The normalized power spectra of the signals in both states are shown in Figure 9a. The new frequency components generated by gain saturation can be clearly observed outside the band, and the in-band signal spectrum is also obviously distorted. These spectral impairments can lead to significant signal distortion. Figure 9b shows the BER performance of the PA-DFE and MA-DFE in two operating states after \(25\) km of SSMF transmission. Under linear amplification, the PA-DFE and MA-DFE achieve a sensitivity of \(-23\) dBm, and the system link budget is \(27\) dB. In the gain saturation region, the SOA experiences noise and nonlinear issues. The BER of the MA-DFE at the measured ROP fails to meet the FEC threshold of \(1\times 10^{-2}\). After increasing the number of taps to \((105,\,26)\), the MA-DFE meets FEC when the ROP is above \(-17\) dBm. However, a sensitivity of \(-20\) dBm is achieved when using the PA-DFE with tap numbers of \((85,\,16)\) and the sensitivity can be further improved by \(1\) dB when increasing the tap numbers to \((105,\,26)\). Considering the launch power of \(8\) dBm, a \(29\) dB link budget is obtained. Therefore, the PA-DFE demonstrates superior equalization performance compared to the MA-DFE in mitigating PAM4 signal impairments caused by SOA gain saturation.  
Finally, the data rate was limited to \(80\)Gb/s due to the bandwidth limitation of the \(25\) G ICR, and a higher data rate can be achieved by using higher bandwidth devices or applying polarization multiplexing techniques. Also, the link budget can be further improved by using an SOA with higher saturation power.  
Figure 8: (**a**) BER versus ROP curves obtained with different equalizers for BtB transmission. (**b**–**e**) Constellation diagrams obtained using different equalizers for BtB transmission at an ROP of \(-20\) dBm.