# Photonics-Based Microwave Image-Reject Mixer
## 4 Photonics-Based Microwave IRM Applications  
The photonics-based microwave IRM can find potential applications in photonics-based RF channelizers, electronic warfare scanning receivers, and multi-band integrated RF front-ends for satellite and radar systems. RF channelization is a technique that can split a broadband signal into several frequency channels that have a bandwidth that can be processed by state-of-the-art electronics. The RF channelizer is usually applied in broadband RF receivers working in a high frequency regime. This high-frequency regime is required for modern RF systems, such as high-resolution radar, electronic warfare and multiband satellites [31]. By using the photonics-based microwave IRMs, it is possible to downconvert RF components at multiple frequencies over a wide bandwidth to the same IF band simultaneously, with great suppression of the in-channel interference. Previously, a typical coherent RF channelizer with large instantaneous bandwidth was proposed and demonstrated based on = photonics-based microwave IRMs [31; 32]. A pair of optical frequency combs (OPCs) provide multiple LOs to downconvert different parts of the RF signal to the same IF band, and the photonics-based microwave IRMs are employed to achieve large in-band interference suppression in each channel. In a proof-of- concept experiment [31], a Ku-band RF signal with a bandwidth of 5 GHz (13-18 GHz) is channelized into five channels with 1-GHz bandwidth simultaneously. The measured in-band interference suppression ratio was greater than 25 dB within the 1-GHz instantaneous processing bandwidth. Figure 8: (**a1,b1**) Electrical spectra and (**a2,b2**) time-frequency diagrams of the downconverted results for schemes in (**a1,a2**) [30] and (**a1,b1**) [25] when an RF signal with 2-GHz bandwidth and corresponding image are received simultaneously. The photonics-based microwave IRM can also play an important role in electronic surveillance, since the electronic warfare receiver needs to receive signals over a large instantaneous frequency range. Previously, a photonics-assisted electronic warfare scanning receiver was reported to incorporate the photonics-based microwave IRM [33; 34]. Figure 9 shows a schematic diagram. A tunable laser provides a master tone, which is then amplified and split into two arms. On the upper arm, the tunable laser acts as the optical carrier and is modulated by the RF input signal with CS-DSB modulation. On the lower arm, a coherent OFC is generated and serves as an LOs. By directing the OFC into a distributed-feedback slave laser, the laser will be injection locked, this leads to selection and amplification of the comb line at the required LO frequency. Then, down-conversion and precise anti-alias filtering are carried out through a photonics-based microwave mixer, incorporating a 90\({}^{\circ}\) optical hybrid. Scanning is realized by wavelength tuning of the master laser via its driving current. In addition, broadband flexible reconfigurable RF front-ends can be built based on photonics-based microwave IRMs [35]. This is of great importance for future RF systems, such as multi-functional radars, broadband electronic warfare and intelligent satellites, to achieve multi-band RF signals for receiving and emitting [36; 37; 38]. Previously, we have proposed and demonstrated a photonics-based RF front-end using photonics-based microwave IRMs [35], which can realize photonic multi-frequency LO generation, RF channelization, and multi-band up-conversion. Figure 10 shows a schematic diagram of a microwave photonic RF front-end. Dual OFCs with different free spectral ranges (FSRs) are generated and serve as photonic multi-frequency LOs. For signal receiving, the signal OFC is used as an optical carrier, which is modulated by the received RF signals with a CS-OSSB modulation format. The modulated OFC signal and the LO OFC are directed to a signal port and an LO port of a 90\({}^{\circ}\) optical hybrid, respectively.