# Stable and Low-Spurs Optoelectronic Oscillators: A Review
## 3 Improvement of Optoelectronic Oscillators
### Frequency Stabilization  
The superior phase noise performance of OEO can be attributed to the employment of low-loss, long optical fiber with high Q factor. However, the effective refractive index and length of fiber are sensitive to ambient temperature variations. Meanwhile, the oscillating cavity is easily affected by mechanical vibrations, stress, and device aging, which make the cavity length change and the oscillating frequency drift. Besides, in view of the low Q-value microwave band-pass filter, there would be multiple spurious modes in the gain bandwidth of OEO loop. The change of the cavity length will cause some spurious modes to obtain sufficient oscillation gain and replace the original frequency, leading to stochastic frequency hopping. Those characters have a non-negligible contribution to oscillation frequency drift, and they would greatly deteriorate the long-term frequency stability of optoelectronic oscillators [45]. In terms of improving the long-term frequency stability of OEOs, it is  
Figure 11: The schematic diagram of the millimeter-wave OEO employing an optoelectronic hybrid band-pass filter with ultra-high Q value. MZM: Mach-Zehnder modulator. PC: polarization controller. OC: optical coupler. DSF: dispersion shifted fiber. PD: photodetector. EA: electrical amplifier. ESA: electrical spectrum analyzer. Reprinted with permission from Ref, [44], IEEE, 2018. Figure 10: The schematic diagram of the OEO employing a Fabry–Perot etalon and Pound–Drever–Hall frequency stabilization. PC: polarization controller. IM: intensity modulator. VOD: variable optical delay. EDFA: erbium-doped fiber amplifier. AOM: acousto-optic modulator. PM: phase modulator. CIR: circulator. FFE: Fabry–Perot etalon. PD: photodetector. LPF: low-pass filter. PID: proportional–integral–derivative controller. VCO: voltage-controlled oscillator. Reprinted with permission from Ref. [43], IEEE, 2018. Figure 9: The schematic diagram of the OEO employing a WGM resonator. WGM: whispering gallery mode. PC: polarization controller. PD: photodetector. LNA: low noise amplifier. Reprinted with permission from Ref. [26], OSA, 2018.  
essentially necessary to control the effective cavity length of the OEO and reduce the impact of the ambient environment. Several methods of improving the frequency stability of OEOs have been proposed, including temperature-insensitive fiber [46], thermal stabilization [47; 48] and phase-locked loop [49]. Employing temperature-insensitive fiber in the OEOs can diminish the impact of ambient environment variation. In [46], [PERSON] et al. replaced the common single-mode fiber with solid-core photonic crystal fiber (SC-PCF) serving as energy storage element for OEO and the frequency stability was improved to some extent. Nevertheless, the phase noise of OEOs with SC-PCF is less-than-ideal due to the sacrifice of Q value resulting from larger transmission loss. Thermal stabilization, including temperature control and system isolation for OEOs, is also an effective solution to relieve the negative influence of ambient environment variation. For instance, [PERSON] et al. placed the OEO system in a bubble-filled box to isolate the effects of sound and vibration [47]. In [48], [PERSON] et al. made use of resistive heaters and temperature controllers for the high-Q devices of OEO, such as the optical fiber and the narrow band-pass microwave filter. The free-running OEO under thermal stabilization featured a short-term frequency stability of 0.02 ppm. The slope of frequency versus. temperature improved from \(-8.3\) ppm/\({}^{\circ}\)C for non-thermally stabilized OEO to \(-0.1\) ppm/\({}^{\circ}\)C. However, it is worth mentioning that the means of thermal stabilization inevitably increased bulk and power consumption, which brings new issues. Meanwhile, the performance for frequency stability of OEOs could not meet higher requirements in special applications, which means that further stabilization methods would still be necessary, as shown in Figure 12[49].