# Photonics-Based Microwave Image-Reject Mixer
## 2 Photonic IRMs Based on Pre-Filtering  
A typical way to achieve image rejection is to directly remove the image from the input RF signal by a filter, either in the electrical [12] or optical domain [13], as shown in Figure 2. As can be seen from Figure 2c, the performance of the image rejection depends on the response of the filter. For a scenario using an electrical bandpass filter (EBPF), to select the required RF signal, as shown in Figure 2a, electrical filters with high center frequency will be required to realize high frequency operation capabilities of IRMs. In addition, the slope of the filter must be sharp enough to suppress the image without affecting the desired signal, especially when the IF frequency is very low (i.e., the frequency difference between \(f_{\mathrm{RF}}\) and \(f_{\mathrm{IM}}\) is small). For example, the EBPF used in [12] has a passband of 8-9 GHz, thus, instantaneous working bandwidth is limited to 1 GHz. For a required IF working frequency (10 GHz [12]), the RF/LO working frequency will also be limited by the EBPF (20 GHz in [12]). When using an optical bandpass filter (OBPF) to select the required modulated RF signal in the optical domain, as shown in Figure 2b, optical filters with narrow bandwidth and sharp edge roll-offs are desirable. However, both the electrical filters with high center frequency, and optical filters with narrow bandwidth and sharp edge roll-offs, are difficult to realize, limiting working bandwidth and system performance. For example, the optical filter described by [PERSON] and [PERSON], [13] is a Fabry-Perot filter with a 3-dB bandwidth of 0.6 GHz, which is too wide for many applications. In addition, the filter bandwidths and center frequencies are usually fixed and wide tunability is hard to realize, limiting the flexibility of the system. Figure 1: Schematic diagram illustrating image interference. Figure 2: Schematic diagram of photonic image-reject mixers (IRMs) based on direct pre-filtering in an (**a**) electrical and (**b**) optical domain; (**c**) pre-filtering principle of IRMs. EBPF: electrical bandpass filter; OBPF: optical bandpass filter; EO: electrical to optical conversion; OE: optical to electrical conversion. To solve these problems, another method using pre-filtering was realized, based on multiple-stage frequency conversion [14; 15; 16; 17]. Figure 3 shows a schematic diagram of typical IRMs based on multiple-stage frequency conversion, and Figure 4 illustrates its principle. The original RF signal \(f_{\text{Received}}\) (containing the image \(f_{\text{IM}}\)) shown in Figure 4a is first downconverted using LO1 at a photonics-based microwave mixer. The converted result is shown in Figure 4b. Then a bandpass filter is used to select the desired RF signal and to remove the undesired image, as shown in Figure 4b. The selected RF signal enters the second photonics-based microwave mixer and is downconverted using LO2 to the desired IF band. The final downconverted result is shown in Figure 4c. As compared with the IRM based on direct pre-filtering, shown in Figure 2c, frequency requirement of the EBPF is lowered by increasing the number of frequency-conversion stages. Thus, when filtering is performed in the electrical domain after optical-to-electrical conversion, a good image rejection can be realized. It should be noted that in the first frequency-conversion step, one can carefully select LO1 frequency to downconvert received RF signal into a proper band, where EBPF with narrow bandwidth and sharp edge roll-offs is easy to achieve [14; 15]. For example, an image-rejection ratio of larger than 150 dB has been achieved by [PERSON] and [PERSON] [14] using two cascaded EBPFs with large out-of-band suppression, and in another paper [15] the image-rejection ratio reached 60 dB when the EBPF out-of-band suppression was 60 dB. Filtering after the first-stage frequency conversion can also be realized in the optical domain [16; 17]. When using optical filters, achieving adequate edge sharpness and frequency-/bandwidth-tuning is still challenging.