# Stable and Low-Spurs Optoelectronic Oscillators: A Review
## 3 Improvement of Optoelectronic Oscillators
### Spurious Suppression
The spurs can be suppressed as much as 40 dB in a 10-GHz COEO. Recently, we proposed and experimentally demonstrated a simple COEO scheme embedding a free-running microwave oscillator as shown in Figure 6[39]. The COEO and the embedded microwave oscillator share the same electronic feedback and then inject each other to achieve mutual injection locking effect. Due to the single-mode oscillation of the free-running oscillator, the multi-mode oscillation of COEO is eased and the spurious noise can be significantly suppressed to below \(-120\) dBc. Figure 3: The schematic diagram of dual-loop optoelectronic oscillator based on open-loop gain control in [37]. OC: optical coupler. Reprinted with permission from Ref. [37], IEEE, 2018. Figure 2: The schematic diagram of dual-loop optoelectronic oscillator. EOM: electro-optical modulator. PD: photodetector. EA: electronic amplifier. PS: phase shifter. PC: power coupler. According to the frequency pulling effect, the spurious suppression of OEO can be achieved by injecting another signal, which has close frequency with some oscillating mode of OEO, commonly referred to as injection locking. In [40], [PERSON] et al. employed an electronic oscillator to  
Figure 4: The schematic diagram of coupled optoelectronic oscillator. EDF: erbium doped fiber. WDM: wavelength division multiplex. Figure 5: The schematic diagram of the COEO based on optical power feedforward technique. EDF: erbium doped fiber. WDM: wavelength division multiplex. OBPF: optical band-pass filter. MZM: Mach-Zehnder modulator. PD: photodetector. LNA: low noise amplifier. EBPF: electric band-pass filter. ELPF: electric low pass filter. Reprinted with permission from Ref. [38], OSA, 2018. Figure 6: The schematic diagram of the mutually injection-locked COEO. EDF: erbium doped fiber. WDM: wavelength division multiplex. Reprinted with permission from Ref. [39], OSA, 2018.  
injection-lock the OEO without a narrow-band filter as shown in Figure 7, and the spurs levels closest to the carrier frequency were about 55 dB smaller than those obtained by a self-sustained OEO employing electronic filter having the Q-factor of 720. However, the phase noise of the final oscillating signal would be limited to some extent due to the injection signal. Therefore, a dual injection-locked OEO (DIL-OEO) scheme [41] proposed by [PERSON] et al. has been employed to achieve, not only low phase noise, but also low spurs via two OEO systems with different length oscillating cavities. The two OEOs inject and affect each other to achieve mutual injection-locking as shown in Figure 8. The steady-state power fluctuation in either loop will alter both the phase noise and spur levels of the DIL-OEO. In addition, both oscillation modes of the slave and master loops need accurate matches to ensure the phase locking effect. High-Q microwave photonic filters such as whispering gallery mode (WGM, Figure 9) resonator [26], Fabry-Perot etalon [35], and atomic cells [42] can simultaneously implement energy storage and frequency selection for OEOs, replacing the conventional long fiber and microwave filter. In [43], [PERSON] et al. utilized a Fabry-Perot etalon with a finesse of 100,000 as a photonic filter in a conventional single loop OEO as shown in \[\Delta f=\frac{1}{\tau}=\frac{c}{nL}. \tag{2}\]0 The etalon provided a high Q-value microwave band-pass filtering at harmonics of 1.5 GHz free spectral range for oscillation frequency in the range of 6 to 60 GHz. There were no visible spurious modes above the noise floor, despite employing as long as a 2 km fiber delay. In [44], we proposed an ultra-high Q optoelectronic hybrid band-pass filter to suppress a spurious mode of OEO as shown in \[\Delta f=\frac{1}{\tau}=\frac{c}{nL}. \tag{2}\]1 The 3-dB bandwidth of the proposed optoelectronic hybrid band-pass filter was about 1 MHz at 29.99-GHz center frequency, and the spurious suppression ratio can reach more than 83 dB. \[\Delta f=\frac{1}{\tau}=\frac{c}{nL}. \tag{2}\]2 The schematic diagram of the dual-injection-locked OEO.