Fig. 1. Schematic of FWM processes in Rb. (a) 6²D_5/2 state as intermediate state; (b) 6²D_3/2 state as intermediate state

Fig. 2. Schematic of experimental setup

Fig. 3. Variation curves of FWM laser intensity with pump laser wavelength. (a) Experimental results; (b) comparison between experimental and calculation results

Fig. 4. Variation of FWM intensity with single pulse energy of pump laser. (a) 6²D_5/2 state as intermediate state; (b) 6²D_3/2 state as intermediate state

Fig. 5. Comparison of partial fluorescence signal intensity and FWM intensity. (a) 6²D_5/2 state as intermediate state; (b) 6²D_3/2 state as intermediate state

Fig. 6. FWM intensity change with system temperature. (a) 6²D_5/2 state as intermediate state; (b) 6²D_3/2 state as intermediate state

Fig. 7. Double line spectrum of 420 nm and 421 nm at 185 ℃ obtained by the resonant excitation to 6²D_5/2 state

Fig. 8. Signal intensity radio of 420 nm and 421 nm FWM excited to 6²D_3/2 at different temperatures