Fig. 1. Schematic diagrams for different types of MOPO principle. (a) Single-pass pumping; (b) dual-pass pumping; (c) pump-resonant

Fig. 2. Schematic diagrams of longitudinal modes for the signal and idler waves. (a) Near-degenerate type-Ⅰ PM dual-resonant MOPO; (b) non-degenerate type-Ⅰ CPM dual-resonant MOPO or type-Ⅱ CPM dual-resonant MOPO; (c) single-resonant MOPO

Fig. 3. Schematic diagrams of gain bandwidth and longitudinal mode change during tuning. (a) Optical comb by MOPO; (b)"mode hopping"of narrow band by MOPO; (c) traditional OPO

Fig. 4. Schematic diagrams of the oscillating light intensity change in MOPO based on CPM. (a) Dual-pass pumping MOPO; (b) single-pass pumping MOPO; (c) single-pass pumping MOPO based on 3^rd-order CPM

Fig. 5. Equivalent nonlinear coefficient of CPM versus L/Lc. (a) Single-pass pumping; (b) dual-pass pumping

Fig. 6. Evolution of the parametric light intensity versus L/Lc

Fig. 7. Theoretical analysis of MOPO^[15]. (a) Schematic diagram of the nonlinear propagation process in the microcavity; (b) modal overlap integral

Fig. 8. Single-pass pumping MOPO based on KTP crystal^[23]. (a) Experimental setup and longitudinal mode distribution; (b) output spectrum operating at crystal temperature of 113.5 ℃; (c) output wavelength versus temperature; (d) output wavelength versus pump wavelength

Fig. 9. Single-pass pumping MOPO based on MgO∶LN crystal^[31]. (a) Signal light wavelength versus temperature; (b) signal spectrum at crystal temperature of 171.5 ℃

Fig. 10. Dual-pass pumping MOPO based on KTP crystal^[32]. (a) Phase shift and transmittance curves of the output mirror; (b) output characteristics at crystal temperature of 43 ℃

Fig. 11. Triply resonant MOPO based on KTP crystal^[33]. (a) Schematic diagram of the triply resonant MOPO; (b) transmittance curves of the front and rear surfaces for crystal; (c) output wavelength versus temperature; (d) output spectrum at crystal temperature of 102 ℃

Fig. 12. Type-I CPM MOPO based on MgO∶‍LN crystal^[34]. (a) Schematic diagram of the broadband MOPO; (b) calculation results of coherence length and nonlinear coefficient; (c) output spectrum of the MOPO with the crystal thickness of 140 µm and the pump energy of 1.03 mJ; (d) output spectrum of MOPO with the crystal thickness of 485.25 µm and the pump energy of 830 µJ

Fig. 13. Microcavity terahertz parametric oscillator^[41]. (a) Schematic diagram of the microcavity terahertz parametric oscillator; (b) evolution process of pump, signal and terahertz light; (c) terahertz waveforms under different pump pulse widths; (d) terahertz waveforms under different seeding signal light powers; (e) terahertz pulse width versus injected seed signal power

Fig. 14. Low threshold terahertz parametric oscillator^[25]. (a) Schematic diagram of the structure; (b) equivalent nonlinear coefficient and mode distribution of the microcavity; (c) terahertz output characteristics; (d) time-domain evolution of each optical pulse

Fig. 15. OPA system based on MOPO^[48]. (a) Schematic diagram of the experimental setup; (b) signal spectra of the OPA and OPG; (c) total output energy of the OPA and OPG versus pump energy

Fig. 16. Schematic diagram of MOPO pumped by microchip subnanosecond laser