利用涡旋光场计算水中前向散射的频率响应

Frequency-response measurement of forward scattering clutters in water by using vortex beams

载波调制水下探测激光雷达利用散射光和信号光在频域特性上的差异来抑制杂散光,而不同水质中的散射光的截止频率是载波调制水下探测激光雷达设计中的重要参数。测量散射杂波的频率响应需要将目标回波与散射杂波分开,尤其是前向散射,与目标回波空间上重合,不容易被分开。文中提出了一种在载波强度调制激光雷达中分别测量水下目标和散射杂波的频率响应方法,利用散射光和目标反射的信号光在空间相干性上的差异,在回波光路上使用螺旋相位板将其分开并分别采集,再通过傅里叶变换得到它们的频谱。实验结果表明,前向散射杂波具有低通特性,而目标回波的响应是平坦的。将信号与散射在频谱中调制频率处的幅度之比定义为“信杂比”,并且用其来表示探测的有效性:当信杂比大于1时,认为探测到了目标。在较浑浊的水中,为了获得大于1的信杂比,载波的调制频率需要更高,基于蒙特卡洛方法对光子在海水中的传播进行了仿真模拟,结果与实验相符。测距实验也证明了,提高调制频率可以减小浑浊水中的测距误差。当信杂比小于1时,提高调制频率,测距误差降低明显。

Objective Lidar is widely used in underwater detection.However,laser is scattered during underwater propagation,and scattering is an important factor limiting the performance of radar systems.Lidar-radar techniques can suppress both forward and backward scattering,where the cut-off frequency of the scattered light is an important parameter.However,unlike backscattering which is easily separated from the target-reflected light in space,forward scattering is difficult to separate from the target-reflected light as they are intertwined in space.Therefore,we propose to separate the forward scattered light from the target-reflected light using vortex beams and analyze its frequency response to obtain the cut-off frequency of forward scattered light.Methods Vortex beams are used to distinguish between forward scattered light and target-reflected light in underwater target detection(Fig.3).The fast Fourier transform is done on the spatially filtered forward scattered light and the target-reflected light to obtain their frequency response(Fig.4).A Monte Carlo-based model for lidar-radar underwater target detection is also developed,and the fast Fourier transform of the forward scattered and reflected light in the echoes is also performed to analyze their frequency response(Fig.1).The simulation results are compared with the experimental results(Fig.2,Fig.6).A ranging experiment is designed to investigate the effect of modulation frequencies less than or greater than the cut-off frequency of forward scattering on the accuracy of ranging(Fig.7).Results and Discussions The experimental results show that scattering has an averaging effect on the modulation,and the high-frequency component is difficult to be maintained in the forward scattering.On the contrary,the high frequency modulation is well maintained in the signal light(Fig.4).The higher the modulation frequency is,the higher the signal-to-clutter ratio is.In addition,in order to achieve a signal-to-clutter ratio of 1,higher modulation frequencies are required in more turbid water(Fig.6).The experimental results are consistent with the results of Monte Carlo simulations(Fig.2).The results of the ranging experiments show that when the signal-to-clutter ratio is less than 1,increasing the modulation frequency reduces the ranging error significantly.While when the modulation frequency is high enough to make the signal-to-clutter ratio greater than 1,continuing to increase the modulation frequency makes the ranging error decrease roughly,but not change much(Fig.9).Conclusions A method for spatially distinguishing forward scattered light in underwater target detection echoes using vortex beams is proposed.Thereby,the forward scattered light and the target-reflected light are obtained separately in the experiment,and the frequency response of both is analyzed by fast Fourier transform to calculate the modulation frequency sufficient to suppress the forward scattered light in the lidar-radar technique.The frequency response of forward scattered light and target scattered light in underwater target detection is simulated based on Monte Carlo method and compared with the experimental results.Both simulation and experimental results show that a high modulation frequency can improve the signal-to-clutter ratio of the underwater lidar system.And for turbid water,a higher modulation frequency is required for more effective suppression of forward scattering.To achieve a signal-to-noise ratio greater than 1,modulation frequencies greater than 700 MHz are required for an attenuation length of 11 and greater than 900 MHz for an attenuation length of 12.Ranging experiments also demonstrate that when the modulation frequency is not high enough to make the signal-to-clutter ratio greater than 1,increasing the modulation frequency results in a significant reduction in the ranging error.While when the modulation frequency is high enough to make the signal-to-clutter ratio greater than 1,continuing to increase the modulation frequency results in a general reduction in the ranging error.When the modulation frequency is high enough to make the signal-to-clutter ratio greater than 1,the ranging error decreases roughly,but the change is not significant.The findings of this study can be used as a reference for the design of carrier modulated underwater lidar systems.