1.School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China 2.Beijing Key Laboratory for Precision Optoelectronics Measurement Instrument and Technology, Beijing 100081, China 3.Kunming Institute of Physics, Kunming 650223, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61835001, 61875011)
Received Date:29 September 2020
Accepted Date:10 November 2020
Available Online:02 April 2021
Published Online:20 April 2021
Abstract: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. Keywords:laser/ intensity modulation/ underwater ranging/ correlation calculation
采用示波器采集调制信号和回波信号的波形, 同时对两个通道进行数据采集. 把水箱入射窗口的内表面作为距离零点, 水的衰减系数为0.99 m–1, 激光的调制频率为50 MHz时, 回波信号和参考信号的波形如图2(a)所示, 将回波信号和参考信号做相关运算, 结果如图2(b)所示, 峰值对应时间为–0.48 ns. 把目标移动到水中0.5 m的位置, 回波信号和参考信号的波形如图2(c)所示, 将回波信号和参考信号做相关运算, 结果如图2(d)所示, 峰值对应时间为4.158 ns. 水的折射率为1.333@532 nm, 调制频率为50 MHz, 在水中的测尺长度为2.251 m. 0.5 m的距离在一个测尺之内, 则两个位置之间的延时时间为4.638 ns, 距离为0.522 m, 测距误差为2.2 cm. 目标距离的标定为固定在水箱上精度为1 mm卷尺的测量结果, 因此测量误差为激光测距和卷尺测距之间的差值. 图 2 回波信号和参考信号的波形及相关运算结果 (a), (c) 0和0.5 m处的波形; (b), (d) 0和0.5 m处的相关结果 Figure2. Waveform of echo signal and reference signal, results of correlation calculation: (a), (c) Waveform at 0 and 0.5 m; (b), (d) results of correlation calculation at 0 and 0.5 m.
水的衰减系数为0.99 m–1, 分别把目标放置在0.5, 1.0, 1.5, 2.0和2.5 m的位置, 采用相位法测距, 每个位置采集五组数据, 不同调制频率下的测距结果如图3所示. 调制频率分别为50, 100, 200, 300, 400和500 MHz, 在水中对应的测尺长度约为2.251, 1.125, 0.563, 0.375, 0.281和0.225 m. 调制频率为50 MHz的测尺基本可以覆盖水箱中的测量距离, 2.5 m位置处的测量结果需要补全一个测尺长度. 调制频率增大, 测尺减小, 测尺长度小于测量长度时可以根据调制频率为50 MHz的测距结果补全相应倍数的测尺长度. 图3(f)为不同距离不同调制频率的测距误差, 随着测量距离的增大, 测量误差增大, 最高达到了7.45 cm. 同一距离, 调制频率越高, 测距误差越小. 图 3 不同距离的测距结果及误差(c = 0.99 m–1) (a) 0.5 m; (b) 1.0 m; (c) 1.5 m; (d) 2.0 m; (e) 2.5 m; (f) 测距误差 Figure3. Ranging results and errors at different distances (c = 0.99 m–1): (a) 0.5 m; (b) 1.0 m; (c) 1.5 m; (d) 2.0 m; (e) 2.5 m; (f) ranging error.
水的衰减系数为1.72 m–1, 调制频率分别为50, 100, 200, 300, 400和500 MHz, 采用相位法测量不同位置的距离, 测距结果如图4(a)—(e)所示. 当测量距离较近时同一调制频率同一目标的多次测量结果一致性较好. 如图4(e)所示, 多次测量结果比较分散, 与图3(e)相比, 水的浑浊度提高, 测距结果波动增大; 与图4(a)—(d)相比, 测量距离增大, 测距结果波动增大. 图4(f)表明了不同距离不同调制频率的测距结果, 随着测量距离的增大, 测量误差也增大, 最高达到了约12 cm. 由于水的浑浊度增加, 测距误差整体增大. 目标距离为2.5 m时, 调制频率为500 MHz的测距误差明显小于低频调制时的测距误差, 调制频率越高, 测距精度越高. 图 4 不同距离的测距结果及误差 (c = 1.72 m–1) (a) 0.5 m; (b) 1.0 m; (c) 1.5 m; (d) 2.0 m; (e) 2.5 m; (f) 测距误差 Figure4. Ranging results and errors at different distances (c = 1.72 m–1): (a) 0.5 m; (b) 1.0 m; (c) 1.5 m; (d) 2.0 m; (e) 2.5 m; (f) ranging error.