Fig. 1. Different QPM structures and their reciprocal vectors: (a) QPM structure in the microdisk: (i) ideal and (ii) distorted; (b) numerical calculation of the reciprocal vector using FFT.

Fig. 2. Calculation of phase-matching condition with different reciprocal vectors. (a) Calculation of tuning curve at different temperatures, with pump wavelength at 1063.4 nm; the subplot illustrates the deviation of idler wavelength at different temperatures. (b) Calculation of other cascaded OPOs, all pumped at 1063.6 nm, deviation with different modes and M when the QPM structure is distorted; the subplot is the idler light corresponding to M₂ = 474.

Fig. 3. (a) QPM structure on the wafer under microscope. Domain walls appear as black lines with M₁ = 450. (b) Domain walls appear as black lines with M₂ = 410. (c) Fabrication process of the resonator with sandpaper and a high-speed rotating spindle.

Fig. 4. (a) Experimental setup. PBS, polarizing beam splitter; BS, beam splitter; L1 and L2, lenses; DM, dichroic mirror. (b) Transmission spectrum and measured Q-factor; (i) transmission spectrum of WGR with a sweeping range of 7.5 GHz above the threshold. The red line means the detected PD voltage of signal 1 light. (ii) Q-factor of the corresponding output light.

Fig. 5. Diverse cascade OPO in WGR. (a) Cascade OPO at 36°C: OPO₁ labeled green and cascade OPO light (OPO₂) labeled cyan. (1063.8 nm → 1566.6 nm + 3314.6 nm/1566.6 nm → 2970.4 nm + 3314.6 nm). (b) The other cascade OPO process at 28°C: OPO₁ labeled green and cascade OPO (OPO₂) labeled cyan (1063.7 nm → 1558.3 nm + 3351.3 nm/1558.3 nm → 2912.6 nm + 3351.3 nm).

Fig. 6. Cascade OPO with output light’s wavelength of 4802.9 nm. Pair of lights pumped by 1063.8 nm labeled green and signal light pumped by 1741.8 nm labeled purple (1063.8 nm → 1741.8 nm + 2732.9 nm/1741.8 nm → 2732.9 nm + 4802.9 nm).