Fig. 1. PT-symmetric OEO. (a) Structure and schematic diagram of the PT-symmetric OEO^[30]; (b) dual-loop PT-symmetric OEO^[30]; (c) wavelength-division multiplexed PT-symmetric OEO^[32]

Fig. 2. Fast frequency-scanning Fourier domain mode-locked OEO^[39]. (a) Principle of the Fourier domain mode-locked OEO; (b) schematic diagram of the passband of the frequency-scanning loop filter; (c) time domain waveform of the produced frequency-scanning microwave signal; (d) instantaneous frequency of the produced frequency-scanning microwave signal

Fig. 3. OEPO^[58]. (a) Structure diagram of the OEPO; (b) principle of the parametric frequency conversion process; (c) spectrum of the stable multi-mode oscillation; (d) instantaneous frequency when the multi-mode oscillation is established

Fig. 4. Broadband random OEO^[59]. (a) Structure and schematic diagram; (b) spectrum of the output signal; (c) instantaneous frequency of the output signal

Fig. 5. Soliton frequency-hopping OEO^[63]. (a) Structure diagram; (b) spontaneous frequency-hopping microwave signal; (c) soliton sequence; (d) detail of the soliton pulse

Fig. 6. Actively mode-locked OEO^[64]. (a) Schematic diagram of actively mode-locked OEO using additional intensity modulator as the active mode locking module; (b) principle; (c)(d) output spectra under different harmonic mode locking orders

Fig. 7. Integrated OEO. (a) InP integrated OEO with fully integrated photonic parts^[79]; (b) sillicon integrated OEO^[80]; (c) sillicon integrated PT-symmetric OEO^[85]; (d) hybrid integrated OEO^[86]

Fig. 8. Examples of applications of the novel OEOs. (a) Microwave photonic radar based on the Fourier domain mode-locked OEO^[87]; (b) microwave photonic temperature sensor based on the Fourier domain mode-locked OEO^[88]; (c) random number generation based on the broadband random OEO^[60]; (d) microwave photonic coherent Ising machine based on the OEPO^[91]