# Ultrafast and Wideband Microwave Photonic Frequency-Hopping Systems: A Review
## 2 Microwave Photonic Frequency-Hopping Systems
### Microwave Photonic Opto-Electronic Oscillator Enabled Frequency-Hopping System  
Opto-electronic oscillator (OEO) has attracted great interest for decades [35; 36; 37] to replace the conventional RF signal generator for applications that have precise timing requirement due to the OEO's high stability and low phase noise performance over a wide frequency bandwidth. OEO usually consists of a laser source, an electro-optic modulator, a section of optical fiber, a photodetector, an electrical amplifier, a microwave bandpass filter, and an electrical power coupler, such that the RF signal is repeatedly selected by an electrical bandpass filter and amplified to ensure gain is larger than the loss in the OEO cavity to achieve a high-Q factor. Once the accumulated gain and phase of the oscillation condition are satisfied, high quality microwave signal is resultant, where the oscillating frequency is governed by the center frequency of the microwave bandpass filter. To generate \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Techniques** & **Speed** &
\begin{tabular}{c} **Potential for** \\ **Multi-level** \\ \end{tabular} & **Methods** & **Tuning Mechanism** \\ \hline
[23] & Four wave mixing & 50 ps & No & switch & Polarization tuning \\
[24] & SBS filter & 100 ps & Yes & switch & SBS tuning \\
[25] & EO-based comb filter & 100 ps & Yes & switch & EO Pockels effect \\
[26] & Notch filter & 190 ps & No & switch & EO Pockels effect \\
[27] & Lyot filter & 200 ps & Yes & switch & Nonlinear \\
[28] & DD-MZM bias control & 1 ns & No & switch & EO Pockels effect \\
[29] & Optical injection & 10 ns & Yes & synthesizer & Nonlinear dynamics \\
[30] & Delay line-based comb filter & 40 ns & Yes & switch & Optical delay line \\
[31] & PM-PSFBG OEO & 100 ns & Yes & synthesizer & Polarization tuning \\
[32] & Microring resonator & 500 us & Yes & synthesizer & Thermal tuning \\
[33] & RF-MEMS & 300 us & Yes & switch & MEMS capacitor \\
[34] & Integrated spectrum shaper & 7 ms & Yes & switch & Thermal tuning \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the State-of-the-Art Frequency-Hopping Signal-Generation Schemes. Figure 2: Schematic of microwave photonic frequency-hopping transmission systems. (**a**) Input—encoded data at baseband; (**b**) Ultrafast MWP-based frequency synthesizer controlled by an applied hopping pattern. (**c**) Frequency-hopping carrier generation using MWP RF switch (**d**) Output—frequency-hopping signal. frequency-hopping signal using OEO, the center frequency of the microwave bandpass filter must be changed rapidly according to the applied hopping code sequence. Unfortunately, microwave bandpass filters have narrow frequency tuning range and slow tuning speed due to the inherent limitation in tuning mechanism, making it not suitable for high-speed and large dynamic range frequency-hopping generation [38; 39]. To solve this problem, microwave photonic filter is used in place of the microwave bandpass filter that is incorporated in the OEO. Figure 3a shows the use of a polarization-maintaining phase-shifted fiber Bragg grating (PM-PSFBG)-based OEO for the generation of wideband frequency-hopping signal [31]. The polarization modulator (PolM), optical circulator, PM-PSFBG, and photodetector form an MWP filter. When the PM-PSFBG is used with a PolM, the MWP filter exhibits a polarization-dependent RF response with narrow 3-dB bandwidth, as illustrated on the right of Figure 3b. It is important to note that there are two notches in the PM-PSFBG reflection spectrum for each principal polarization axis of the PM-PSFBG to achieve phase to intensity conversion during photodetection. Therefore, two different frequency responses, each with two transmission peaks are observed through a vector network analyzer (VNA), as shown in Figure 3b. To achieve frequency-hopping signal generation, polarization of a continuous wave light wave is aligned at 45\({}^{\circ}\) with respect to the principal axis of the PolM for polarization modulation.