Fig. 1. Spot distributions of different modes. (a) LP₀₂ mode; (b) LP₃₁ mode

Fig. 2. Quasi-single-mode emitting spot and energy distribution curve. (a) Emitting spot; (b) energy distribution curve

Fig. 3. Loss curve of fundamental mode in fiber

Fig. 4. Diagram of quasi single mode realization system of multimode fiber MZI

Fig. 5. Output light spots after light coupling into optical fiber and propagating with different incident angles. (a) Incident angle is 0.81°; (b) incident angle is 1.43°; (c) incident angle is 2.02°

Fig. 6. Spot energy distribution of emitting light at different incident angles, and energy proportion in full width at half-maximum at different incident angles. (a) Spot energy distribution of emitting light at different incident angles; (b) energy proportion in full width at half-maximum at different incident angles

Fig. 7. Output faculae at different number of disturbance mode circles when diameter of disturbance mode is 6.6 cm. (a) 0 circle; (b) 1 circle; (c) 2 circles; (d) 3 circles

Fig. 8. Normalized energy distribution curves under different number of disturbance mode circles when diameter of disturbance mode is 6.6 cm, and variation of normalized energy with number of disturbance mode circles. (a) Normalized energy distribution curves under different number of disturbance mode circles; (b) variation of normalized energy with number of disturbance mode circles

Fig. 9. Output faculae under different number of disturbance mode circles when diameter of disturbance mode is 4.8 cm. (a) 1 circle; (b) 2 circles; (c) 3 circles

Fig. 10. Normalized energy distribution curves under different number of disturbance mode circles when diameter of disturbance mode is 4.8 cm, and variation of normalized energy with number of disturbance mode circles. (a) Normalized energy distribution curves under different number of disturbance mode circles; (b) variation of normalized energy with number of disturbance mode circles

Fig. 11. Output faculae at different number of disturbance mode circles when diameter of disturbance mode is 3.0 cm. (a) 1 circle; (b) 2 circles; (c) 3 circles

Fig. 12. Normalized energy distribution curves under different number of disturbance mode circles when diameter of disturbance mode is 3.0 cm, and variation of normalized energy with number of disturbance mode circles. (a) Normalized energy distribution curves under different number of disturbance mode circles; (b) variation of normalized energy with number of disturbance mode circles

Fig. 13. Output faculae at different number of disturbance mode circles when diameter of disturbance mode is 2.2 cm. (a) 1 circle; (b) 2 circles; (c) 3 circles; (d) 4 circles

Fig. 14. Normalized energy distribution curves under different number of disturbance mode circles when diameter of disturbance mode is 2.2 cm, and variation of normalized energy with number of disturbance mode circles. (a) Normalized energy distribution curves under different number of disturbance mode circles; (b) variation of normalized energy with number of disturbance mode circles

Fig. 15. Comparison of energy curves and normalized output energy under different disturbance mode diameters when number of disturbance mode circle is 1. (a) Energy curve comparison; (b) normalized output energy

Fig. 16. Comparison of energy curves and normalized output energy under different disturbance mode diameters when number of disturbance mode circles is 2, and normalized output energy. (a) Energy curve comparison; (b) normalized output energy

Fig. 17. Comparison of energy curves and normalized output energy under different disturbance mode diameters when number of disturbance mode circles is 3, and normalized output energy. (a) Energy curve comparison; (b) normalized output energy

Fig. 18. Energy proportion in full width at half-maximum under different disturbance mode diameters varies with number of disturbance mode circles

Fig. 19. Detection SNR of Doppler lidar system at night versus detection height

Table 1. Parameters of 2×2 step refractive index multimode fiber coupler (FG200UEA)

Table 2. Parameters of multimode fiber (ultraviolet quartz double-cladding fiber)