## 4 Results and Discussions  
Adopting the experimental setup shown in Figure 1, the experimental results at the 2 \(\upmu\)m/1.55 \(\upmu\)m/1.31 \(\upmu\)m wavelengths were acquired based on the same indium-rich InGaAs photodetector (cut-off wavelength covering 1.31 \(\upmu\)m to 2 \(\upmu\)m wavelengths), similar experimental setups consisting of the same 18 dBm sinusoidal-type RF signal of the MZMs, and similar optical spectrums output from the MZMs with \(\sim\)\(-\)7.7 dBm optical peak power. At the spectral resolution (\(\sim\)0.05 nm), the optical spectrums output by the 2 \(\upmu\)m/1.55 \(\upmu\)m/1.31 \(\upmu\)m wavelength MZMs working in the DSB-CS modulation method were obtained in order to generate the 12 GHz microwave carriers, as shown in Figure 2a,b. Obviously, the optical spectrums demonstrated similar optical signals using the same DSB-CS modulation method and practically the same optical peak power (\(\sim\)\(-\)7.7 dBm) at those three different wavelengths. An Optical Side Mode Suppression Ratio (OSMSR) up to 49 dB (46.3 dB at 1.55 \(\upmu\)m; 49.3 dB at 2 \(\upmu\)m) and suppression of the higher mode components in the Bessel expansion (\(J_{2}(0.29)=0.01,\ J_{3}(0.29)=0.0005\) when the MZM modulation index \(m=\left(\pi V_{RF}\right)/\left(2V_{\pi}\right)=0.29\)) suppresses the effect of the higher-order sidebands away from the target bands and kinks on the shoulders of the modulated peaks. Figure 3a-c have shown the electric spectrums of the 12 GHz frequency-doubling photo-generated microwave carriers using the MZMs at the DSB-CS modulation method at the 2 \(\upmu\)m/1.55 \(\upmu\)m/1.31 \(\upmu\)m wavelengths. It is shown that the high-quality microwave carriers with low phase noise were generated and that the Side Mode Suppression Ratios (SMSRs) were 23.2 dB at 2 \(\upmu\)m wavelengths, 23.6 dB at 1.55 \(\upmu\)m wavelengths, and 22.1 dB at 1.31 \(\upmu\)m wavelengths. Therefore, only the peak power generated from the first-order side band was considered during the following measurements or discussion due to the high SMSR value. Figure 1: Experimental setup of the photonics-generated microwave carrier system at 2 \(\upmu\)m/1.55 \(\upmu\)m/1.31 \(\upmu\)m wavelengths using the DSB-CS modulation method. Figure 4a-c give the tunable frequency-doubling microwave carriers obtained at the same setting parameters as the 12 GHz high-quality microwave carriers generated and shown in Figure 3a-c. The 7-12 GHz, stable, frequency-doubling, and tunable microwave carriers were generated at those three different wavelengths because of the 12 GHz bandwidth limitation for the MZMs and the diode, as shown in Figure 4a-c. As shown in Table 1, the phase noise of about \(-100\) dBc/Hz at 10 MHz and linear dynamic range up to 20 dB were obtained at the tested frequency of 7-12 GHz for the 2 \(\upmu\)m wavelengths. In addition, the phase noise of our setup obtained at 2 \(\upmu\)m wavelengths is still much higher than the performance of the available commercial electronic microwave synthesizers at the same frequency of 10 GHz (see Table 1), which suffers from noise issues such as ASE noise. To highlight the influence of the wavelength \(\lambda\) on the photo-to-electric power conversion efficiency, the generated microwave carrier power is normalized to the square of the corresponding external quantum efficiency \((\eta)\) under the same optical signal power used as the input. Then, based on Equation (13), the theoretical ideal power conversion efficiency variation between 2 \(\upmu\)m and 1. As shown, the \(r_{\text{nm(2~{}\upmu m/1.55~{}\upmu m)}}\) values measured are close to the theoretical estimation (see Figure 5a). Correspondingly, an opto-electric power conversion efficiency increesment of \(10 log_{10}\Big{[}(P_{F2~{}\upmu m}/\eta_{\text{2~{}\upmu m}}^{2})/(P_{F1.55~{} \upmu m}/\eta_{\text{1.55~{}\upmu m}}^{2})\Big{]}\) can be obtained, as seen in Figure 5b, with the average value of \(\sim\)1.16 dB.