## 4 Experimental Setup Characterization  
In this experiment, real SOA-MZI and other devices are used in order to create the experimental setup that is the same as the simulation one (see Figure 2). Moreover, all the operating points used in the simulations are also used in the actual setup because of obtaining a fair differentiation between them. During the experimental work, the frequency range of the signal to be mixed only reached 59 GHz because of the restriction of the ESA bandwidth. Hence, the contrast of the signal to be mixed between the real and simulated results will be in the frequency range that corresponds to the frequencies of the signal to be mixed at the outer gate of the SOA-MZI from \(f_{s}+f_{IF}=20\) GHz to \(3f_{s}+f_{IF}=59\) GHz. The spectrum of the shifted signal in the electrical domain is also obtained at the outer port of the SOA-MZI for the real work. The first three harmonics of the sampling OPC signal, as well as the duplication of the shifted signal related to these harmonics, are shown in Figure 5.  
Hence, the degradation of the replicas is almost as similar to the ones calculated for simulations. In both works, the signals to be up-converted follow the retreating of \(H_{n}\) of the sampling OPC signal. The same results are achieved for simulations seen in Figure 3. However, the obvious enhancement of the electro-optical modulation of the differential mode is at the higher frequency range up to 195.5 GHz in simulations. The \(G_{c}\) of the up-converted signals is measured at \(f_{s}+f_{IF}\), \(2f_{s}+f_{IF}\), and \(3f_{s}+f_{IF}\), as shown in Figure 6. It is obtained at the same range as the simulations in order to confirm the results. The measured \(G_{c}\) is 1 dB below the simulated one as a consequence of the receiver noise and optical losses of the optical fiber. The experimental work confirms that the \(G_{c}\) has the same behavior in comparison with the simulations for the differential modulation. Figure 5: Experimental electrical spectrum of the signal to be up-converted at \(nf_{s}\pm f_{IF}\) at the outer port of the SOA–MZI. In order to gauge the quality of the electro-optical differential modulation, the EVM of the signal to be mixed that carries QPSK data is achieved and compared with the results obtained by using a VPI simulator at the data rate of 5 Gbit/s as exhibited in Figure 6. The real EVM values are larger than the ones in simulations due to the unexpected noise in the actual measurements. For the real experimentation, the EVM degrades 2% in comparison with the simulations. Moreover, all the EVM values of the mixed signals are below the EVM limits. This also confirms that the electro-optical differential modulation realizes good performances in terms of its quality through EVM as well as its efficiency through \(G_{c}\). It is worth noting that, for the real study, the real-time oscilloscope used in the experimental setup is called a digital sampling oscilloscope (DSO) that is used to digitalize the signal to be up-converted. Then a vector signal analyzer (VSA) software is directly exploited to compute the EVM values. Table 1, which is another view of Figure 6, shows the comparison between the real and simulated results of the electro-optical transmission system based on a sampling SOA-MZI at 59 GHz in terms of the efficiency through conversion gains and the quality performance through EVM values. It also displays the characteristics of the sampled signal at 195.5 GHz in simulation investigations. The simulated conversion gains and EVM values obtained through the VPI simulator are in very good agreement with those attained through the real schema. The results in both cases validate the principle of the electro-optical sampling SOA-MZI in differential modulation architectures.