# High-Performance Microwave Photonic Transmission Enabled by an Adapter for Fundamental Mode in MMFs
## 3 Experimental Measurement and RF Transmission Performance  
Figure 3a,b demonstrate the performances of the MMF-based MPL that we discussed above, in comparison with a G.652D SMF MPL. In order to verify whether a larger mode field area can indeed lead to lower nonlinear effects and whether the length of fiber can influence nonlinearity, we design two sets of experiments for the SBS threshold measurement. In the first set, the lengths of SMF and MMF are 10 km and 8.8 km respectively, because the SBS fluctuations are generally not significant within 2 km, while in the second set, fiber lengths are increased to 16.5 km and 17.6 km, respectively. According to the existing fibers in our lab, we chose a 17.6-km MMF for the experiment, the length of which is already in the range of typical MPLs. Besides, the characteristics of the SMF and the MMF used in the experiments are presented in Table 1[15]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline \(\alpha_{\text{MMF\_LP01}}\) & 0.220 dB/km \\ \(\alpha_{\text{SMF}}\) & 0.187 dB/km \\ \(\text{A}_{\text{eff}}\) of MMF\_LP01 & 165 \(\upmu\text{m}^{2}\) \\ \(\text{A}_{\text{eff}}\) of SMF & 80 \(\upmu\text{m}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The performance characteristics of the SMF and OM3 MMF. Figure 3: The back-scattered optical power versus input optical power for (**a**) a 10-km SMF and an 8.8-km MMF and (**b**) a 16.2-km SMF and 17.6-km MMF with and without adapters. Figure 2: (**a**) The cross-sectional images of the MFAs obtained through six iterations of the reverse tapering technique, among which the 6 th sample’s mode field diameter can match that of the fundamental mode in the OM3 fiber. (**b**) The side view of the MFA, thickened slowly and smoothly from right to left. It can be seen that the SBS threshold of SMFs is only \(\sim\)10 dBm, while that of MMFs is \(\sim\)15 dBm. In addition, the SBS threshold without MFAs is 14 dBm, which is slightly smaller, indicating that the existence of the MFAs increases the SBS threshold by \(\sim\)1 dB. We also prove that the SBS threshold of longer fibers is almost unchanged, no matter SMFs or MMFs. However, the back-scattered light power of the longer SMF becomes exactly stronger, while that of the longer MMF increases less, proving that it is feasible to use MMFs to improve the nonlinearity. The crosstalk of the link without MFAs is very high because the HOM components of the MMF could be excited in a high proportion, which means that the quality of the baseband signal would be severely affected. Figure 4a demonstrates the relation of the output and input optical power of the link. When the input light power is 17 dBm, an increase of 3.8 dB can be obtained using MMFs compared to using SMFs. From Figure 4a, it can be seen that the transmitted power still increases after the incident optical power exceeds the SBS threshold, but not as fast as it is when below the SBS threshold, due to the presence of the Kerr effect in the fiber nonlinearity, where the power in the optical carrier is transferred to the sideband due to self-phase modulation and four-wave mixing (FWM). If the power in the sideband is below the SBS threshold, it continues to grow even though the optical carrier power is no longer growing due to exceeding the SBS threshold. The SFDR can be measured based on the signal output received by the ESA, which is shown in Figure 4b. The input dual-frequency RF signal can generate other frequency signals besides the fundamental frequency (FF)\(f_{1}\) and \(f_{2}\) due to link nonlinearity, among which the third-order intermodulation (IMD3) terms \(2f_{1}-f_{2}\) and \(2f_{2}-f_{1}\) are closest to the FF, which have a serious impact on the system. The FF and the IMD3 power values are measured a sufficient number of times at different input radio frequency (RF) signal powers and fitted to lines.