# Photonics-Based Microwave Image-Reject Mixer
## 3 Photonic IRM Based on Phase-Cancellation Techniques
### Photonic IRM Based on 90\({}^{\circ}\) Optical Hybrid  
The third class of photonic IRMs based on phase cancellation applies a 90\({}^{\circ}\) optical hybrid to introduce a quadrature phase [25; 26; 27; 28; 29]. Benefiting from a simple structure and the small phase imbalance of the commercially available optical hybrid over a wide frequency range, a high image rejection ratio can be achieved in a large bandwidth. Figure 7 shows a schematic diagram of a typical IRM based on a 90\({}^{\circ}\) optical hybrid [25]. The optical carrier is split into two branches. One branch is modulated by an RF signal and the other is modulated by an LO signal, both with carrier-suppressed single-sideband (CS-SSB) modulation. The CS-SSB modulation can be realized by using either optical filters placed in each branch [25; 26; 27], or I/Q modulators operated at CS-SSB mode [28; 29]. Signals from the two branches are sent to the signal port and the LO port of the 90\({}^{\circ}\) optical hybrid, which typically has two in-phase output and two quadrature output ports. When one of the in-phase outputs and one of the quadrature outputs are sent to two PDs, an I/Q frequency mixer is obtained. By using a low-frequency electrical quadrature hybrid to combine the two electrical outputs of the I/Q mixer, a photonic IRM is achieved. It should be noted that if other outputs of the 90\({}^{\circ}\) optical hybrid are directed to PDs, single-ended and balanced mixers can also be realized. Figure 6: A photonic IRM using an electrical 90\({}^{\circ}\) hybrid to generate quadrature RFs. LD: laser diode; PD: photodetector [19]. Since parallel modulations, and/or parallel filterings, and parallel amplifications are required to obtain the independent RF and LO sidebands, the system performance will therefore be vulnerable to any path length difference between the two branches [26; 27; 28; 29]. Therefore, environmental vibration, which cause path length variations, would be a challenge for practical application of such IRMs. To overcome this problem, in [26] a reconfigurable microwave photonic mixer which can perform such functions, including the IRM, is achieved by using a DPol-MZM and an optical 90\({}^{\circ}\) hybrid. Parallel signal modulations are implemented in an integrated modulator and act as polarization multiplexors at the output. Thus, RF and LO sidebands can both be filtered and amplified by the same devices, guaranteeing links consistency for both signals. Separation lengths are minimized, which can be easily packaged to avoid environmental vibration. With the three aforementioned kinds of photonic IRMs, based on phase-cancellation techniques, images can be rejected over a wide frequency range. On the other hand, mixing spurs are generated by the nonlinearity of the mixer at frequencies of \(\left|kf_{\mathrm{RF}}+lf_{\mathrm{LO}}\right|\), where \(k\) and \(l\) are integers. Since the mixing spurs might be in the same band of the wanted signal, especially in a wideband operation scenario, they should be suppressed together with the image. To realize mixing-spur suppression, we can rely on advanced modulation formats. For example, the suppression of LO/RF leakages can be realized by introducing a carrier suppressed double sideband (CS-DSB) modulation format for both RF and LO signals [20]. In addition, a CS-SSB modulation format can be applied to eliminate more high-frequency spurs [21; 22; 23; 24; 25; 26; 27; 28; 29]. However, the implementation of advanced modulation formats always requires complex configuration, and typically needs optical filters or high-frequency electrical hybrids, which can limit working frequency range. In addition, the previously reported photonics-based microwave IRMs are tested and analyzed by respectively injecting an RF signal and image. In practical applications, RF signals and images are received simultaneously and will generate other unwanted mixing spurs.