Fig. 1. The current world's lowest phase noise single-loop OEO scheme^[37]. (a) Schematic diagram of the OEO; (b) the phase noise of this OEO at oscillation frequency of 10 GHz

Fig. 2. Schematic diagrams of the dual-loop OEOs. (a) Schematic diagram and the phase noise of dual-loop OEO based on vernier effect^[38]; (b) schematic diagram and the phase noise of COEO^[39]

Fig. 3. OEO with phase locked loop^[40]. (a) Schematic diagram of the phase-locked OEO; (b) the phase noise of the phase-locked OEO at oscillation frequency of 10 GHz

Fig. 4. OEO based on MPF. (a) Schematic diagram of the OEO based on PM and MRR and the phase noise of the OEO at oscillation frequency of 25.65 GHz^[41]; (b) schematic diagram of the OEO based on PM and phase shifted Bragg grating and the phase noise of the OEO at oscillation frequency of 10 GHz^[42]; (c) schematic diagram of the OEO based on PM and SBS effect and the phase noise of the OEO at oscillation frequency of 8.18 GHz, 13.25 GHz and 32.02 GHz^[43]

Fig. 5. PT symmetric OEO schemes. (a) Schematic diagram of the dual-fiber loop PT symmetric OEO and the phase noise^[19]; (b) schematic diagram of the PT symmetric OEO based on Sagnac loop and the phase noise^[45]; (c) schematic diagram of the single-fiber loop PT symmetric OEO and the simulated phase noise^[46]

Fig. 6. FDML OEO scheme^[20]. (a) Schematic diagram of the FDML OEO; (b) the frequency spectra of the output signal of the FDML OEO with different frequency sweeping range; (c) the phase noise of the FDML OEO

Fig. 7. Broadband random OEO^[47]. (a) Schematic diagram of the broadband random OEO; (b) the phase noise of the broadband random OEO with RF filter centered at 5 GHz with a 3 dB bandwidth of 60 MHz

Fig. 8. Compact OEO based on WGMR^[49]. (a) Image of the OEO based on WGMR; (b) the phase noise of the OEO at oscillation frequency of 34.7 GHz

Fig. 9. Silicon photonic integrated OEO schemes. (a) Schematic diagram of the silicon photonic OEO based on phase modulator and micro-disk resonator and the phase noise of the OEO at oscillation frequency of 4.74 GHz ^[23]; (b) schematic diagram of the silicon photonic integrated PT symmetric OEO and the phase noise of the OEO at oscillation frequency of 4.97 GHz and 13.67 GHz^[50]; (c) schematic diagram of the silicon photonic integrated OEO based on RF filter and the phase noise of the OEO at oscillation frequency of 10 GHz^[51]

Fig. 10. Indium phosphide photonic integrated PT symmetric OEO^[24]. (a) Image of the InP photonic chip of the integrated OEO; (b) the phase noise of the OEO at oscillation frequency of 8.87 GHz

Fig. 11. Integrated parity-time symmetric OEO based on InP integrated mode-locked laser^[52]. (a) Schematic diagram of the PT symmetric OEO based on integrated mode-locked laser; (b) the phase noise of the OEO at oscillation frequency of 24.02 GHz, 24.50 GHz, and 24.96 GHz

Fig. 12. Chalcogenide photonic integrated OEO^[25]. (a) Conceptual diagram of the fully photonic integrated OEO based on SBS; (b) the phase noise of the OEO at oscillation frequency of 10.9 GHz and 40 GHz

Fig. 13. The fixed frequency TFLN OEO scheme^[53]. (a) Schematic diagram of the fixed frequency TFLN OEO; (b) the phase noise of the fixed frequency TFLN OEO scheme and commercial Keysight microwave source

Fig. 14. The frequency tunable TFLN OEO scheme^[53]. (a) Schematic diagram of the frequency tunable TFLN OEO; (b) operation principle of the frequency tunable TFLN OEO; (c) the RF spectrum of the frequency tunable OEO from 20 GHz to 35 GHz; (d) the phase noise of the frequency tunable OEO scheme and commercial Keysight microwave source