Laser underwater detection has important applications in underwater target search, resource exploration, and other fields. The absorption and scattering of light by water are a big challenge to underwater detection. Absorption causes the laser signal to attenuate, thus limiting the detection distance. Scattering causes not only attenuation but also noise, the strong scattering noise can even submerge the target information. To reduce the absorption, the blue-green light band in the transmission window of water is chosen for lidar. Optically carried microwave radar (OCMR) has the advantages of resistance to turbulence and scattering. The intensity of the detection beam is modulated at radio frequency. The photons reflected by the target retain the intensity modulation information, while interference phase-out is generated between photons scattered by particles suspending in turbid water at different distances, resulting in the average of high-frequency modulation signals. The signal-to-noise ratio is improved when the received signal by the detector is correlated with the modulation signal.

High-power broadband intensity modulated light source is the key to achieving the long-distance, high-precision underwater ranging with the carrier modulation method. However, the carrier modulation technology for underwater detection is limited by the development of light source. The maximum power of intensity modulation green light used in underwater detection is on the order of hundreds of milliwatts, the receiver needs to adopt a photomultiplier tube (PMT).

In this paper, a laser underwater detection system is built with a 3-m-long water tank by using a home-made 532 nm light source. The maximum output power of the intensity-modulated 532 nm laser is 2.56 W. The modulation frequency is turned from 10 MHz to 2.1 GHz. Water with different attenuation coefficients is obtained by adding Mg(OH)₂ into the water tank. When the maximum modulation frequency is 500 MHz by phase ranging, 4.3 attenuation lengths(a.l.) are measured. The ranging error is about 12 cm. In the future study, a PMT will be used as the detector to increase the range resolution. We will also increase the bandwidth of the signal processing unit in order to take full advantage of the broadband intensity to modulate light source.