## 3 Results and Discussion
### Constellation Diagrams  
In this section, we present retrieved constellation diagrams using the two mixers under electrical and optical pumping conditions. Figure 5 shows the resulting diagram for 40 and 80 Gbps of received data using electrical pumping. We observe that the GaAs-based mixer exhibited superior performance compared to the InGaAs-based mixer. The GaAs mixer demonstrated tighter and better-defined signal clusters indicative of reduced signal distortion and a better EVM. The BERs obtained in this case were \(1.5\cdot 10^{-4}\) and \(2.22\cdot 10^{-3}\) for the 40 and 80 Gbps of transmission, respectively. In contrast, the InGaAs mixer, while efficient, showed slightly wider and worse defined signal clusters, reflecting relatively higher levels of signal distortion and higher EVM values and showing a BER of \(5.04\cdot 10^{-3}\) and \(1.81\cdot 10^{-2}\) at 40 and 80 Gbps, respectively. Therefore, the GaAs mixer was able to recover the transmitted data, showing a BER below the hard-decision forward error correction (HD-FEC) limit, while the BER obtained with the InGaAs mixer stayed close to the soft-decision FEC (SD-FEC) limit. An interesting observation is that the remanent phase noise after the DSP routine was more pronounced in the InGaAs-based mixer case. This shows that the phase noise cancellation algorithm is indeed sensitive to the input signal quality, which is impaired when using the InGaAs-based mixer due to the higher conversion loss and overpumping conditions, as will be discussed later. Figure 5: Constellation diagram of the 40 and 80 Gbps transmission links using two different mixers electrically pumped. Left and right results correspond to the GaAs and InGaAs mixer, respectively. Notably, the results described above strongly corroborate the performance outcomes described in Table 1, which outlines the superior capabilities of GaAs mixers over InGaAs mixers under electrical pumping. This was already demonstrated in [43], where the first comparison between InGaAs and GaAs in terms of the CL was reported. In Figure 6, the captured IF spectrum of the received signal is shown. A clear representation of the carrier is observed at 25 GHz, being the 20 GHz of data bandwidth on either side, ensuring that the signal is within the available 33 GHz bandwidth of the oscilloscope. In addition to the carrier and data signals, the presence of an unwanted resonance is noticeable at 10.2 GHz. This effect arises due to non-linearities within the LO source, which can introduce unwanted frequencies resulting from interactions in the multiplication stages and potentially leading to undesired signal distortion and degradation in the received data. The presence of this resonance warrants careful consideration, as it can significantly impact the overall performance of the system. Unlike electrical pumping, the optically pumped mixer exhibited minimal to no presence of unwanted resonances in the received spectrum of the data, as seen in Figure 7. This crucial advantage not only enhances signal fidelity but also contributes to a substantial reduction in signal distortion and intermodulation effects. In the IF spectrum shown in Figure 7, the data transmission rate remained at 80 Gbps, and the bandwidth was again 20 GHz with a 1.5 GHz separ  
Figure 6: IF spectrum using electrical pumping; 20 GHz of data bandwidth captured, with the carrier centered at 25 GHz. An unwanted harmonic is noticeable at 10.2 GHz coming from the AMC. Figure 7: IF spectrum using optical pumping; 20 GHz of data bandwidth captured, with the carrier centered at 25 GHz. No unwanted harmonics appeared. Figure 8 illustrates the constellation diagrams obtained using an InGaAs narrow-band mixer, where well-defined clusters of constellation points can be observed. The measured BER reached a value of \(6.83\cdot 10^{-5}\) and \(2.73\cdot 10^{-3}\) at data rates of 40 and 80 Gbps, respectively. This indicates that a BER below the HD-FEC limit can be also achieved with the InGaAs-based mixer as a direct consequence of using optical pumping.