## 3 Experimental Setup and Results  
A proof of concept experiment of the proposed image-free microwave photonic subharmonic downconverter is performed. The experimental setting is shown in Figure 2. A light wave emitted from a laser source (Emcore1782) with a power of 11 dBm and a wavelength of 1549.6 nm is used as the optical carrier. The optical carrier is sent directly to the DP-QPSK (FTM7977 HQA), which has an insertion loss of about 12 dB, and the extinction ratio is about 35 dB. The half-wave voltage of the modulator is measured at about 3.5 V. The DP-QPSK modulator comprises two QPSK modulators (X-QPSK and Y-QPSK). The X-QPSK modulator is driven by an RF signal generated from a vector signal generator (Rohde & Schwarz, SWM200). The RF signal is split into two paths by a \(90^{\circ}\) electrical coupler (MARKI, QH-440) to drive the two X-QPSK RF ports. Thus, a 90\({}^{\circ}\) phase difference is introduced between the two signals driving the X-QPSK. The three bias points of X-QPSK are set at minimum, minimum, and quadrature transmission points, respectively, to achieve CS-SSB modulation of the RF signal. In the experiment, the direct current (DC) voltage is provided by two DC voltage sources (GWINSTEK, GPP-4323) with a regulation accuracy of 0.001 V. The LO signal generated from a microwave analog signal source (Agilent, 8630B) drives Y-QPSK and is also split into two paths via a tunable electric phase shifter (ARRAL9428B), introducing a 45-degree phase difference between the two paths. The three bias points of Y-QPSK are biased at maximum, maximum, and quadrature transmission points, respectively, to achieve SH-SSB modulation of the LO signal. We observe the optical spectra at the output of the DP-QPSK modulator on an optical spectrum analyzer (OSA; Advantest, Q8384), and the results are shown in Figure 3 (Blue line). Due to the limited resolution of our OSA (i.e., 0.01 nm), the modulated sidebands of the RF signal and LO signal cannot be distinguished. Afterward, we use a PBS followed by the DP-QPSK to separate orthogonal polarization-multiplexed optical signals and observe the optical spectra of the upper and lower arms, respectively. The results are shown in Figure 3 (the red and black dotted lines). It can be seen that one arm only includes the +1 st order sidebands and the other arm includes the +2 nd order sidebands and the optical carrier. Meanwhile, it can be seen that the sideband suppression ratio is larger than 30 dB. Figure 3: The output optical spectra of the DP-QPSK modulator. Figure 2: The experimental setup of the proposed image-free microwave photonic sub-harmonic downconverter. As the insertion loss of the DP-QPSK modulator is about 12 dB, we use an EDFA followed by the DP-QPSK modulator (KPS-STD-BT-C-18-PB) to improve the signal power, guaranteeing sufficient PD input power. The EDFA works in automatic power control (APC) mode, maintaining an output power of 16 dBm. The amplified signal is then split into two paths via a 50:50 optical coupler. In each path, a PC and a PBS are used to form a photonic microwave phase shifter. Here, the PBS is used as a polarizer. By properly adjusting PC1 and PC2, the angle between the principal axis of the modulator and one principal axis of the PBS can be set at 45\({}^{\circ}\). Thus, the polarization-multiplexed signals are combined into one polarization direction while ensuring maximum power. Meanwhile, to build the Hartley structure, a 90\({}^{\circ}\) phase shift is introduced in one path by adjusting the PC. After polarization combining, the optical signals are sent to two low-speed PDs to achieve 1/Q mixing. In our experiment, a variable optical attenuator (VOA) is used to compensate for the amplitude imbalance between the two paths. A tunable optical delay line (TODL) is used to match the delay between the two paths. We select the RF signal with a frequency of 11.7 GHz and a power of 10 dBm. The LO signal is fixed at 5.5 GHz with a power of 16 dBm.