# Ultrafast and Wideband Microwave Photonic Frequency-Hopping Systems: A Review
## 2 Microwave Photonic Frequency-Hopping Systems
### MWP RF Switch for Frequency-Hopping Signal Generation
#### 2.4.2 Nonlinear Polarization Rotation Enabled Lyot Loop Filter as an RF Signal Switching  
To overcome the periodicity characteristic in the above MWP filter, finite impulse response (FIR)-based MWP bandpass filter can be used instead, such that distinct passbands at the desired frequencies can be obtained. Figure 9a shows the schematic of an optically controlled FIR-based reconfigurable MWP dual-band filter [27]. Unlike the last microwave photonic filter that uses the transmission characteristic of the optical comb to shape the RF frequency response, this MWP dual-band filter uses FIR (i.e., weighting and delaying of RF signal copies for summation) to generate the desired RF filter spectral shape. Moreover, nonlinear polarization rotation in semiconductor optical amplifier (SOA) is used instead as the switching mechanism, where the birefringence in a SOA changes according to the power of a pump light, resulting in polarization rotation of the signal light. A broadband light source is first shaped by an optical Gaussian shape filter and is then spectrally sliced by an optically controlled Lyot loop filter to generate the optical carriers needed for the FIR filter taps. The Lyot loop filter consists of a pair of polarizers (P1, P2), two optical circulators, a length of PMF, a SOA, an OC, and a distributed feedback laser (DFB). As shown in Figure 9b, the circulator-SOA loop enables the input light to transmit through a piece of PMF twice and allow rapid adjustment of the polarization state of the return light through optical injection at the SOA. Within the Lyot loop filter, the input optical signal is linearly polarized at 45\({}^{\circ}\) with respect to the principle axis of the PMF. After the first transmission through the PMF, the phase difference \(\Delta\varphi\) between fast and slow axis can be expressed as \(\Delta\varphi=2\pi BL/\lambda\), where \(B\) and \(L\) are the birefringence and length of the PMF, and \(\lambda\) is the light wavelength. Then, the light passes through the circulator-SOA loop and will travel back through the same PMF in the opposite propagation direction with a polarization rotation angle (\(\Delta\theta\)). By properly modify the optical pump power being injected to the SOA, the polarization rotation angle can be optically switched to 0, 45\({}^{\circ}\), and 90\({}^{\circ}\), resulting in a phase difference switching between 2\(\Delta\varphi\), \(\Delta\varphi\) and 0. Therefore, the resultant phase difference between the light aligned with the two axis changes accordingly to the equivalent length (\(L_{\epsilon}\)) of PMF being changed between 2\(L\), \(L\), and 0. Based on the phase difference, the Lyot loop filter will have the same reconfigurable periodic transmission function as the one described in above section, which is used to spectrally slice the broadband light source. The spectrally sliced optical carrier is then  
Figure 8: (**a**) Experimental setup of the PM-LMF-based MWP notch filter for RF signal switching. DFB: distributed feedback laser; DD-MZM: dual-drive Mach–Zehnder modulator; PM: phase modulator; PMF: polarization-maintaining fiber; PC: polarization controller; PD: photo detector; (**b**) Measured transmission optical spectra of the tunable PM-LMF at different control voltages. (**c**) Corresponding RF notch filter response at different control voltage; (**d**) Close-up of the frequency-hopping signal that switches between 6 GHz and 12 GHz. launched to a dispersive medium to obtain wavelength dependent delay, such that a FIR filter is formed at the photodetector, resulting in a microwave photonic filter with two reconfigurable passbands. It is important to note that the SOA combined with pump laser works as an optically controllable polarization controller. By applying different optical pump powers, birefringence of the SOA is changed, leading to nonlinear polarization rotation of the light inside the SOA.