强度调制532 nm激光水下测距

激光水下探测在水下目标搜寻、资源勘探等领域具有重要的应用,而散射是激光水下探测面临的主要挑战.载波调制激光雷达具有抗散射、抗干扰的优点,本文利用自行研制的532 nm强度调制激光源,在3 m长的水箱中搭建激光水下探测系统. 532 nm激光源最大输出功率为2.56 W,强度调制范围为10.0 MHz—2.1 GHz,光束发散角约0.5 mrad.通过在水箱中添加氢氧化镁(Mg(OH)_2)粉末,测量了不同浑浊度下水的衰减系数.采用相位测距的方法,目标反射光的调制信号为探测信号,对激光源进行调制的电信号作为参考信号,利用相关运算获得激光的延时时间,进而可以获得水下目标的距离.最大调制频率为500 MHz时,实现了距离为4.3个衰减长度目标的探测,测距误差约12 cm.探测距离越远,测距误差越大,调制频率越高,测距精度越高.

Underwater ranging with intensity modulated 532 nm laser source

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,highprecision 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 homemade 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.