Fig. 1. Schematic of double collision scattering of a single photon

Fig. 2. Program diagram of photon simulated motion

Fig. 3. Bubble echo signal under different water qualities

Fig. 4. Peak photon number received by receiving detector under different water qualities

Fig. 5. Echo signal of bubbles. (a) Pure seawater; (b) clean seawater; (c) coastal seawater; (d) port seawater

Fig. 6. Echo signal of bubbles. (a) Pure seawater; (b) clean seawater; (c) coastal seawater; (d) port seawater

Fig. 7. Experimental system design

Fig. 8. APD bias power supply before design

Fig. 9. APD bias power supply before design

Fig. 10. Laser scattering echo signal of ship wake bubbles with 10 dB receiving gain. (a) Mean value; (b) peak value

Fig. 11. Laser scattering echo signal of ship wake bubbles with 15 dB receiving gain. (a) Mean value; (b) peak value

Fig. 12. Laser scattering echo signal of ship wake bubbles with 15 dB receiving gain. (a) Mean value; (b) peak value

Fig. 13. Amplitude ratio of target signal to water background signal. (a) Far field; (b) near field

Fig. 14. Comparison between water body laser scattering background signal and large-scale bubble laser scattering echo signal. (a) Original signal; (b) smoothed signal

Fig. 15. Comparison between water body laser scattering background signal and small-scale bubble laser scattering echo signal. (a) Original signal; (b) smoothed signal

Fig. 16. Comparison between water body background laser scattering signal and ship wake laser scattering echo signal. (a) Original signal; (b) smoothed signal