Fig. 1. Optical ring cavity with six mirrors

Fig. 2. The dependence of the beam radius ω0 at the center of 𝓁2 with the distance 𝓁1 between two concave mirrors

Fig. 3. The dependence of the gain G of the optical cavity with the transmittance T of the incident mirror

Fig. 4. Experimental setup for the four-wave mixing process in a cavity-enhanced cesium atomic ensemble,and double-Λ energy level transition of the Cs¹³³ D1 line. (a) Experimental setup; (b) energy level transition line

Fig. 5. The dependence of the four-wave mixing gain g and the squeezing degree with the input pump power for the cases without a resonant cavity and with a resonant cavity. (a) The temperature of the vapor cell is 112 °C; (b) the temperature of the vapor cell is 100 °C

Fig. 6. The dependence of four-wave mixing gain g and squeezing degree with temperature for the cases without a resonant cavity and with a resonant cavity. (a) The pump power is 600 mW; (b) the pump power is 340 mW

Fig. 7. Intensity-difference squeezing noise spectra generated by the four-wave mixing process in a cavity-enhanced cesium atomic ensemble