# RF Frequency Selective Switch by Multiple PMIM Conversions
## 1 Introduction  
At present and in the near future, radio frequency (RF) resources are being fully developed. In radar, communication, electronic countermeasures and many other applications, the RF signal has occupied or will occupy the bandwidth above several GHz [1,2]. Simultaneously, the phenomenon of a multi-channel is increasingly being presented in various applications. The frequency and channel have become important features of signals. Frequency- or channel-related processing is often one of the most important functions of RF signal processing. In this case, RF signal processing is to be the broadband and multichannel, which poses a great challenge to traditional electronic or digital technology. For example, traditional microwave and RF technology often has its limited functions (such as simple filtering), poor tunable or reconfigurable capability, and there are large electromagnetic compatibility (EMC) problems. Digital technology, when the bandwidth is very large, will face the problems of requiring deep storage space, large computing delay, and large volume, weight, and power consumption. Microwave photonics technology provides a feasible method for large-bandwidth frequency or channel-related signal processing [3,4]. Due to the high carrier frequency of light, it can carry a large bandwidth of the RF signal. At the same time, with the development of optical fiber communication technology, more and more optical devices and optoelectronic devices can directly face signals with bandwidth above tens of GHz. In many microwave photonic devices and systems, the microwave photonic filter is a very important frequency or channel-dependent signal processor. The finite impulse response (FIR) structure is a typical scheme to obtain a microwave photonic filter [5,6]. Here, the signal is divided into many taps, and then these taps experience different delays, weights,and are finally superimposed to obtain the output. By adjusting the amplitude and phase of the tap, a reconfigurable microwave photonic filter can be obtained. In order to avoid the instability caused by superposition, the FIR filter usually adopts incoherent combination, so it is difficult to realize a complex tap. Photonic field programmable gate array (FPGA) is a new method which can be used to realize a full-function reconfigurable microwave photonic filter [7; 8; 9; 10; 11; 12; 13; 14]. The tap phase control is obtained by coherently detecting the optical phase shift. However, we believe that the existing coherent or incoherent combining schemes face several problems. Firstly, the optical FPGA needs coherent synthesis, and the fluctuation of the external environment will cause obvious interference to the synthesis, so the filter needs very precise control. Secondly, according to the theory of FIR structure, the resolution of a signal processor or filter is inversely proportional to the longest time interval among many taps, that is to say, the maximum delay difference required for a processing resolution of 1 GHz is about 1 ns. This delay corresponds to a waveguide with a length of 10 centimeters. A waveguide of this length will face large loss or optical phase jitters, but this resolution is too wide for RF communications. If one wants to achieve a resolution below 100 MHz, the corresponding optical waveguide length must also be increased 10 times. Finally, the future microwave photonic filter must have the ability of multiple-input and multiple-output (MIMO) processing, that is, it must be able to process multiple radio frequency signals in parallel at the same time. At present, many related demonstrations are aimed at single inputs and outputs, especially various incoherent structures. In this paper, we propose a multi-channel reconfigurable microwave photonic processer based on incoherent synthesis. Although we use incoherent synthesis, it is different from the traditional FIR structure. In our scheme, each tap corresponds to a narrow-band channel, which is realized by phase modulation to intensity modulation (PMIM) conversion [15].