Fig. 1. Optical parametric oscillation (a) and optical parametric amplification (b) experimental schematic diagrams

Fig. 2. The oscilloscope displays the piezoelectric transducer voltage and intracavity seed light intensity in the optical parametric oscillator cavity. (a) Linear voltage loading; (b) scan-hold-trigger voltage loading

Fig. 3. Schematic diagram of the lidar structure employing a frequency doubling and sum frequency laser emission system

Fig. 4. Original echo signal of the Ca layer and Ca⁺ layer at Yanqing station on November 3, 2020

Fig. 5. Detection threshold of the mohe calcium ion lidar at Mohe station

Fig. 6. Nighttime density evolution of the Ca layer and the Ca⁺ layer at Yanqing station, Beijing. (a) Ca layer, September 1, 2021; (b) Ca⁺ layer, September 1, 2021; (c) Ca layer, October 9, 2022; (d) Ca⁺ layer, October 9, 2022

Fig. 7. Ca⁺ density at 80–300 km detected by lidar at Yanqing station, Beijing on July 4, 2021 (a) and Mohe station on January 31, 2024 (b)