A laser interferometric method is proposed to detect the audible frequencies of underwater acoustic signals. A laser beam is used to irradiate the air-water interface when an underwater acoustic source causes the fluctuation of the water surface

the scattered beam with acoustic wave information will interfere with a reference beam according to the principle of the Michelson interferometer

and the frequencies of water surface waves can be concluded in the interference results.A precise optical detection device is used to detect the change of optical path difference caused by the water surface fluctuation and to achieve the real-time information concerning audible frequency of the underwater acoustic source by demodulating the data in processing system. It can be seen from the real-time frequency-domain graphs that the central frequency of the frequency band is the audible frequency of underwater acoustic signals

while the low-frequency band results from the Doppler shift caused by the natural fluctuation of the water surface.Experiment results prove that the method can be used to detect real-time underwater acoustic signals with the frequency from 4 kHz to 15 kHz and the test error is less than 7 Hz.These results can meet the requirement of the underwater acoustic signal detection for the real time and precision.