# Ultrafast and Wideband Microwave Photonic Frequency-Hopping Systems: A Review
## 2 Microwave Photonic Frequency-Hopping Systems
### Integrated Photonic Based MWP Bandpass Filter for Frequency-Hopping Signal Generation  
Currently, most photonics-based frequency-hopping systems have been demonstrated using discrete fiber-optic components, which are bulky, with larger loss, and are sensitive to peripheral environment such as temperature changes and vibration, resulting in a significant degradation in the frequency-hopping system performance. Therefore, photonic integration is a necessity to minimize the frequency-hopping system footprint, increase system stability, and make it more comparable with RF circuits [51; 52]. Integrated microwave photonics have been used for demonstrating photonic filters [34],  
Figure 9: (**a**) Experimental setup of the optically controlled fast reconfigurable microwave photonic dual-band filter for RF signal switching based on nonlinear polarization rotation. BBS: broadband light source. EDFA: erbium-doped fiber amplifier. DFB: distributed feedback laser. SOA: semiconductor optical amplifier. P1–P2: polarizers. PM: phase modulator. PMF: polarization-maintaining fiber. PD: photodetector. DCF: dispersion compensating fiber. (**b**) Operation principle of the optically reconfigurable Lyot loop filter; (**c**) Measured response speed of the nonlinear polarization rotation effect. Purple waveform: 1 Gbps hopping code sequence with a “10101100” pattern. Blue waveform: Corresponding output signal of the SOA that experienced nonlinear polarization rotation. (**d**) Switching speed measurement of the optically controlled RF signal switching with rise/fall time less than 200 ps.  
on-chip stimulated Brillouin scattering-based RF signal processor [53], and photonic oscillator based on Kerr comb [54; 55]. Regarding the generation of microwave signal on an optical chip, a Kerr optical frequency comb excited in a high-Q magnesium fluoride whispering-gallery-mode resonator was reported to have a phase noise as low as \(-120\) dB/Hz at a 1-kHz offset frequency [54]. Figure 10 shows the diagram of a fast-switching integrated optical spectral shaper that forms part of an MWP filter (i.e., essentially replacing the Lyot loop filter in the previous section by the optical spectral shaper). The optical spectral shaper chip has a size of \(9.6\) cm \(\times\) 2.4 cm, which consists of six delay elements made by silica [34]. The polarization independent \(2\times 2\) thermal optical switches are used as interconnectors between delay lines, which can be switched between bar-state and cross-state by change the driving voltage. Since six stages of time delay are used in this chip, \(2^{6}-1=63\) different delay combinations can be obtained. Therefore, the resultant passband frequency can be adjusted through the control of the time delay configuration via the optical switches, as shown in Figure 11. The fast-switching frequency-hopping signal waveform between 1.782 GHz and 3.635 GHz is shown in Figure 11b, which has a hopping speed of 0.7 ms. So far, the limitation on tuning speed of most integrated photonics circuits and silicon photonic signal processors are governed by the large coefficient of thermal-optic effect [56].