## 3 Experiment Results and Discussion
The two optical signals are simultaneously input to the BPD for photoelectric conversion, and the output signals are differentially combined. Figure 4c shows the combined output electrical spectrum. It can be seen that IMD3 components are suppressed greatly with the SIR of 79.3 dB, which is increased by 25 dB. During the up-conversion test of the two-tone signal and the subsequent SFDR test, the bandwidth (BW) of the spectrum analyzer is set to 50 MHz, and the resolution bandwidth (RBW) and video bandwidth (VBW) are both set to 10 kHz. Figure 3: Measured up-conversion electrical spectra of the upper path when the DD-MZM working at the minimum point optical spectra of the upper path before EDFA (RBW = 500 kHz). Figure 2: Measured optical spectra of the upper path before EDFA. Blue solid line stands for the DD-MZM working at the minimum point. Red dashed line stands for the DD-MZM working at the quadrature point. Then, the SFDR of the system is measured with the IF signal power increase step of 1 dB. The diagram of the measured result is shown in Figure 5a with the SFDR of 112.3 dB-\(\mathrm{HZ^{4/5}}\). The slope of the fitting line of the intermodulation distortion component is 5, which indicates that the IMD3 component in the system is almost suppressed, and the fifth-order intermodulation distortion becomes the main nonlinear component. Due to the reduction in common-mode noise by BPD, the noise floor of the proposed up-conversion scheme can still maintain at a relatively low value when two EDFAs are included. The measured noise floor of the proposed scheme is \(-147.8\) dBm/Hz. At the same time, the SFDR of the traditional frequency up-conversion link based on CS-DSB modulation is also measured. By disconnecting the optical path to the lower input port of BPD and adjusting the VOC, all the optical carrier is sent into the upper path. Directly input the IF signal and LO signal to the DD-MZM in the upper path. The measured results are shown in Figure 5b with the SFDR of 97.5 dB\(\cdot\)Hz\({}^{2/3}\) and the measured noise floor of \(-146.9\) dBm/Hz. Under the condition of the optical power into BPD being unchanged, the increase in the optical power reduces the gain of the EDFA, thereby reducing the ASE noise. By comparing Figure 5a,b, the SFDR of the proposed frequency up-conversion system is improved by 14.8 dB, demonstrating the effectiveness of the optimization scheme. Finally, the frequency up-conversion performance for the broadband signal with the established system is tested. The IF signal with a center frequency of 2 GHz and bandwidth of 50-Mbaud of 16-QAM is applied. The output up-conversion electrical spectra with the input IF signal power of 12 dBm, 15 dBm and 18 dBm are measured and shown in Figure 6. Figure 6a,c,e are the measured electrical spectra of the frequency up-conversion system based on traditional CS-DSB modulation. It can be seen that the intermodulation distortion  
Figure 4: The frequency up-conversion electrical spectrum of (**a**) upper path, (**b**) lower path and (**c**) combination output from BPD (RBW = 10 kHz). Figure 5: Measured fundamental and IMD3 output powers as a function of the input IF signal power: (**a**) the proposed frequency up-conversion system and (**b**) the up-conversion link based on carrier suppression double-sideband modulation. components grow greatly, as illustrated by the spurious out-of-band components with the increase in the input IF power. By using the proposed scheme, the IMD3 components can be suppressed effectively, as shown in Figure 6b,d,f. Certainly, there are some residual spurious components when the IF signal power is 18 dBm, which are mainly the fifth-order intermodulation distortion. The error vector magnitude (EVM) of the demodulated signal in Figure 6f is 5.12% rms, satisfying the international 3 GPP guidelines with the maximum EVM value of 12.5% rms for 16-QAM modulation [22]. For comparison, Table 1 lists the recent work in developing microwave photonics mixing links.