# RF Frequency Selective Switch by Multiple PMIM Conversions
## 2 Materials and Methods
In this way, when we control the power of each optical frequency tooth, we can then control the fading of  
Figure 1: The proposed RF FSS schematic diagram based on multiple PMIM conversions. The RF signal containing multiple channels are modulated on the optical frequency comb and acted on by the periodic notch filter. Because the comb spacing of the optical frequency comb is slightly different from the FSR of the periodic notch filter, the RF components of different frequency will be suppressed in different optical channels. After the optical WSS, different optical channels will be merged, and under the PMIM conversion, different RF channels will also appear in the designed port. corresponding RF channel. When we control the direction of each optical frequency tooth, we can then control the direction of corresponding RF channel, with the same principle. In order to achieve this goal, we input the light passing through the periodic notch filter into an optical WSS. Optical WSS is a multi-channel optical module with high reconfigurability. At present, the spectral resolution of commercial optical WSS can reach about 10 GHz, and with this resolution as the granularity, the power control and direction control of the optical spectral channel can be realized. For example, in a single output channel, arbitrary optical spectral control with a resolution of 10 GHz can be realized; In the application of multiple output channels, any optical channel can be output at given port or ports. However, due to the limited dispersion of optical elements, the resolution of optical WSS is difficult to further improve. That is, if it is directly used in RF FSS, the channel granularity can be as large as 10 GHz, so it cannot be directly applied to scenarios such as RF communication. However, in our design, since the RF channel is mapped to an optical channel, we can use the optical WSS with large spectral granularity to achieve high resolution processing of the RF spectrum. Obviously, according to Figure 1, the resolution is \(\Delta\). According to the level of existing optical components, the proposed RF FSS can be realized with a resolution better than 100 MHz. From optical WSS to RF FSS, we achieved resolution improvement of more than two orders of magnitude. The improvement comes from the Vernier caliper amplification effect between the optical frequency comb and the periodic notch filter. Considerable effort has been directed to the optical switch technologies in recent years [19; 20; 21]. An InGaAs/InP waveguide four-port switch by using free-carrier plasma-dispersion is demonstrated, where the switching speed may be greatly improved [19]. MEMS' inherent advantages such as small size and scalability have also been adopted into an optical switch [20]. Recently, an interferometric switching cell with a bandwidth larger than 150 nm was experimentally demonstrated [21]. In our proposal, the optical wavelength selective switch (WSS) is used to change the output ports of different spectral components, where the switching speed of our system totally depends on WSS. Higher switching speed is also demanded in future MIMO processing. Each channel will occupy 50 GHz on optical domain, while the operation bandwidth of WSS can reach \(\sim\)100 nm. From this perspective, the scalability of our system is hardly limited by WSS. In order to improve the resolution, the Vernier effect needs two periodic processes, namely, an optical frequency comb and periodic notch filter, which have strict periodicity. At present, there are many methods to generate optical frequency combs, such as time lens, Kerr frequency combs, and frequency combs based on ring modulators [22; 23; 24; 25; 26; 27]. The frequency response of the periodic notch filter can be realized by an optical resonator, including a ring resonator and Fabry Perot cavity. When the intracavity dispersion of these resonators is very small, their FSR is almost exactly the same in a certain optical bandwidth.