This work presents a novel fiber-optic sensing system to measure the local air-volume (void) fraction of bubbly flows caused by breaking waves near a submerged breakwater. Initially, a sensing system with a single fiber-optic probe is developed to detect the local void fractions of bubbly flows. The rating curve between the time fraction ratio of the bubble signal and the void fraction is then established by performing calibration experiments in a cylindrical tank. The high correlation coefficient between the time fraction ratio and the void fraction suggests that the proposed fiber-optic sensing system can measure local void fractions in the range of 1 % to 16 %. Additionally, based on the proposed system the bubbly flows near a submerged breakwater caused by breaking waves are investigated. Effects of different variables on the distribution of void fraction at the lee side of breakwater are systematically studied. These variables include the incident wave height, period and width of the breakwater.