# Stable and Low-Spurs Optoelectronic Oscillators: A Review
## 3 Improvement of Optoelectronic Oscillators
### Frequency Stabilization
The phase-locked-loop (PLL) technique is the most practical and has been widely used in the frequency stabilization for microwave oscillators. The active PLL control circuit can lock the oscillation signal generated by OEO to an external highly-stable reference source. The final signal of PLL-based OEO has the same frequency stability with the external reference while maintaining phase noise performance at high-offset frequencies. Our research group proposed and demonstrated a novel broadband and wide-range feedback tuning scheme on the strength of a dual parallel Mach-Zehnder modulator (DPMZM) and optical band-pass filter for PLL stabilization of tunable OEO as shown in Figure 13[50]. The PLL-based stabilization for OEO was achieved at different oscillating frequencies and the long-term frequency stability was indeed improved with more than four orders of magnitude without any thermal control. However, the PLL-aided OEO in [50], which is realized by controlling the bias voltage of the modulator would affect the light intensity, thereby negatively affecting the OEO oscillating state. The analog voltage-controlled phase shifter is typical in adjusting the cavity length in the PLL-based OEO. In [51], [PERSON] et al. proposed a novel voltage-controlled oscillator  
Figure 12: The schematic diagram of the long-term frequency stabilization for OEO based on temperature control and PLL techniques. EOM: electro-optical modulator. DDF: dispersion deduced fiber. PD: photodiode. LNA: low noise amplifier. BPF: band-pass filter. EC: electrical coupler. PS: power splitter. LPF: low-pass filter. PID: proportional-integral-derivative. Reprinted with permission from Ref. [49], IEEE, 2018. (VCO) which is an OEO utilizing phase-locked loop architecture as shown in Figure 14. The OEO demonstrated excellent phase-noise performance at high-offset frequencies, while the PLL reduces the phase noise at low-offset frequencies. Nevertheless, the maximum phase shift range of the commercial microwave phase shifter decreases along with the operating frequency, and it will be too narrow to compensate the frequency drift for OEOs at a high operating frequency. Therefore, we proposed a frequency conversion pair to improve the frequency-drift compensation range for the PLL-stabilized OEO as shown in Figure 15[52]. The cavity length is adjusted by controlling the phase shift of oscillation signal at relatively low frequency via frequency division and frequency multiplication. Moreover, the stability of the locked optoelectronic oscillator was improved from 4.1 \(\times\) 10\({}^{-7}\) to 1.1 \(\times\) 10\({}^{-10}\) at 1000 s averaging time. In order to achieve the frequency stability of millimeter-wave (MMW) signal produced by OEO, we proposed a novel frequency-de-multiplication OEO (FD-OEO) scheme as shown in Figure 16[53]. The oscillation of the FD-OEO can be maintained without optical first-order sidebands, which would help to simplify the photonic-assisted frequency multiplication process while providing a wide frequency compensation range for the millimeter-wave signal. The overlapping Allan deviation of the generated 40-GHz MMW signal reaches 1.38 \(\times\) 10\({}^{-12}\) at the average time of 100 s as shown in Figure 17.  
Figure 14: The schematic diagram of the phase-locked loop architecture utilizing OEO as a voltage-controlled oscillator (VCO). MZM: Mach-Zehnder modulator. PC: polarization controller. PD: photodetector. EA: electrical amplifier. ESA: electrical spectrum analyzer. Reprinted with permission from Ref. [51], IEEE, 2018. Figure 13: The schematic diagram of the feedback tune scheme in PLL stabilization of OEO [50]. LD: laser diode. MZM: Mach-Zehnder modulator. DSF: dispersion-shifted fiber. PD: photodetector. EA: electrical amplifier. BFF: band-pass filter. EC: electronic coupler. ESA: electrical spectrum analyzer. PID: proportional-integral-derivative regulator module. Ref: microwave reference. Reprinted with permission from Ref. [50], OSA, 2018.