## 3 Experimental Setup and Results
Thus, two IF signals at a frequency of 700 MHz with a 90\({}^{\circ}\) phase difference can be obtained at the output of the two PDs (PIN, TRPD-P-3-A-85-FCAPC). The temporal waveforms of the generated IF signals are recorded on a digital sampling oscilloscope (Keysight, DSOV334A), as shown in Figure 4a. It can be seen that two IF signals with the same amplitude and quadrature phase are produced. When the RF frequency is changed to image frequency (i.e., 10.3 GHz), the temporal waveforms of I and Q channels are shown in Figure 4b. The two signals remain in quadrature, but the image-induced IF signal on the Q path shifts by 180\({}^{\circ}\) compared to the IF signal downconverted from the target signal of the Q path. When it comes to signal processing, a low-frequency 90\({}^{\circ}\) EHC (Hirose HDH00803) with an operating frequency of 0.5-1 GHz is used to combine the two IF signals. The electrical spectra of the corresponding IF signals measured on an electrical spectrum analyzer (ESA, Rohde & Schwarz, FSW50) are shown in Figure 5a. When an RF signal at 11.7 GHz is applied, a 700 MHz IF signal with a power of \(-\)25 dBm is generated. When the RF frequency is changed to the image frequency (i.e., 10.3 GHz), the IF signal with a power of \(-\)81 dBm is generated, which is 56 dB lower than that of the IF signal converted from the target RF. The results demonstrate the good ability of image rejection. The corresponding waveforms of the IF signals generated from the target RF signal and IM signal, respectively, are shown in Figure 5b. It can be seen that after image rejection, the amplitude of the image-induced IF signal is nearly zero while the useful IF signal has good amplitude. Figure 4: Measured temporal waveforms of downconverted 700-MHz IF signals of I and Q paths from (**a**) the target signal and (**b**) the image signal. Spurious-free dynamic range (SFDR) as an import performance parameter is also measured during the experiment. A two-tone signal with frequencies of 11.3 GHz and 11.35 GHz is used to measure the SFDR. The frequency of the LO signal is fixed at 5.5 GHz. As shown in Figure 6, by recording the powers of the output IF signal, IMD3, and noise under different RF power inputs, the SFDR is calculated to be 100.2 dB-HZ\({}^{2/3}\). The conversion gain of this system is about \(-\)35.7 dB. The relatively low conversion efficiency of our method is due to the lack of suppression of the optical carrier. In fact, an optical filter could be used to filter out the optical carrier and thereby increase the conversion efficiency. However, to accommodate the lower frequency range, we opted not to use optical filters in our method. On the other hand, within the power limits of the modulator, the conversion efficiency can also be enhanced by increasing the power of the LO signal. We also measure the IRR under different RF and IF frequencies to investigate the frequency tunability of this image rejection downconverter. Figure 7a shows the IRR versus different RF frequencies when the IF signal is fixed at 700 MHz. The frequency of the RF signal ranges from 1 to 37 GHz. Moreover, if the 90\({}^{\circ}\) hybrid coupler with a frequency range from 8 to 67-GHz (Marki Microwave, QH-0867) is used, the operating frequency can be extended to the U waveband. As shown in Figure 7a, the black line is the back-to-back transmission, that is, the signal is not transmitted through the long-distance fiber. The IRRs are around 55 dB with a fluctuation of below 3 dB. Meanwhile, to demonstrate the performance of the anti-chromatic dispersion transmission, a single-mode fiber (SMF) with a length of 25 km and dispersion coefficient of 16 ps/nm/km is connected behind the EDFA. It can be seen from Figure 7a that the IRRs drop after transmitting through 25-km SMF, but  
Figure 5: Measured (**a**) electrical spectrum and (**b**) temporal waveform of the downconverted useful signal and image signal. Figure 6: Measured SFDR of the proposed image-free sub-harmonic downconverter.