The receiver optics comprises a 10cm aperture telescope with field of view (FOV) of 2°. It was placed 20cm from laser, with the optical axis of the emitter and receiver optics forming an angle of 1° in a horizontal plane. The two optical axes crossed at 10m away, while the distance between lidar and bubble cloud (labeled as L in Fig. 2) ranged from 7 to 12m. An interference filter (central wavelength of 532nm,bandwidth of 1nm) was placed in front of the PMT to achieve better contrast by blocking stray light. Due to the depolarizing effects of random particles in the water [11,12], a polarizer was used to select component of the return signal that is co-polarized with the laser [13,14].