###### Abstract  
A filter-free, image-reject, sub-harmonic downconverted RoF link is proposed based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. At the remote antenna unit, the receiving radio frequency signal is applied to the upper QPSK modulator to achieve carrier-suppressed single-sideband (CS-SSB) modulation. The local oscillator (LO) is applied to the lower QPSK modulator, achieving sub-harmonic single-sideband (SH-SSB) modulation. The I/Q mixing is realized by exploiting a two-channel photonic microwave phase shifter, which mainly consists of a modulator, two polarization controllers, and two polarizers. The image interference signal can be rejected when combing the I and Q IF signals through a 90\({}^{\circ}\) electrical hybrid. Because the scheme is simple and filter-free, it has a good image-reject capability over a large frequency tunable range. Moreover, due to the special SH-SSB modulation, the modulated signals are immune to the chromatic dispersion-introduced power fading effect. Last, the sub-harmonic downconverter can decrease the frequency requirement of the LO signal. Experimental results show that an image rejection ratio (IRR) greater than 50 dB can be achieved when transmitted through a 25 km single-mode fiber (SMF). Simultaneously, under different RF signals and IF signals, the IRR has no periodic power fading, only small fluctuations. Image rejection capability of the scheme for the 50-MBaud 16-QAM wideband vector signal is also verified and the demodulation of the desired IF signal with a good EVM of less than 5% is realized. microwave photonic mixer; sub-harmonic downconversion; image rejection; dispersion-introduced power fading 20241119  
translated to the same IF band. Without image rejection processing, the image-induced IF signal can hardly be filtered by electrical filters, resulting in the distortion of the expected IF signal. Therefore, the image-free downconverters are urgently required. Many methods have been proposed to realize image rejection. One popular method is based on the pre-filtering technique, where the image signal is removed directly via electrical or optical filters [10; 11; 12] before frequency downconversion. However, it is hard to realize broadband operation due to the electrical filter. In addition, optical filters with narrow bandwidths and sharp edge roll-offs are difficult to realize, limiting the working bandwidth and flexibility of the system. Another important approach to achieving image rejection is based on the phase-cancellation technique, implemented through Hartley architecture. Cascaded modulator structures, such as the phase modulator (PM) with the Mach-Zehnder modulator (MZM) structure [13; 14], a PM with a polarization modulator (PolM) [15], a polarization-division multiplexing-MZM (PDM-MZM) [16], or a dual-driven Mach-Zehnder modulator interferometer [17] are used to suppress the image-induced IF interference through the phase cancellation technique. In Ref. [18], a bidirectional PM is used to obtain two quadrature IF signals by controlling the polarization state. Other methods using a single modulator have utilized image rejection, which has a simple and compact structure [19; 20; 21; 22]. However, optical filters are generally required to generate pure single-sideband products from LO and RF signals. The system's working frequency range is limited and its environmental stability is compromised due to the non-compact structure. In Ref. [23], although no filter is used, the cascaded modulator structures have large insertion losses. In Refs. [24; 25], a balanced Hartley architecture with a 90\({}^{\circ}\) optical hybrid is proposed to suppress the image-induced IF signal. However, the use of two separate modulators introduces phase and amplitude imbalances in the two paths related to the LO and RF ports of the 90\({}^{\circ}\) optical hybrid. In addition, the structures mentioned above are not suitable for antenna remoting applications due to fiber-dispersion-induced power fading.