Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61875257) and the Natural Science Foundation of Shaanxi Province, China (Grant No. 2017JM6035).
Received Date:22 August 2018
Accepted Date:17 November 2018
Available Online:01 March 2019
Published Online:05 March 2019
Abstract:Noise is an important factor affecting the image quality of laser coherent field high resolution imaging system. And there exists not only background light additive noise but also laser multiplicative speckle noise in a laser coherent field imaging system. Both of the above noise affect the imaging quality of laser coherent field system. In order to improve the imaging quality from the perspective of noise suppression and settle the imaging quality degradation problem of laser multiplicative speckle noise and background additive noise in the laser coherent field imaging system, the model for the influence of multiplicative speckle noise and background additive noise on laser echo field demodulated signal is established in atmospheric downlink. Then, based on the model, a novel homomorphic filter and sparse matrix trace cascade compound de-noising algorithm is put forward. Firstly, based on the homomorphic filtering theory, the laser multiplicative speckle noise in the laser echo demodulated signal is converted into the additive noise by logarithmic transformation. Then the low-frequency laser multiplicative speckle noise is filtered by the high-pass filter, and the high-frequency demodulated signal is retained. The logarithmic inverse transform is used to obtain the laser echo demodulation signal after the multiplicative speckle noise has been filtered out. Next, the phase random disturbance of atmosphere in laser echo demodulated signal is suppressed by phase closure technology and the imaging spectrum component is reconstructed by the spectrum iterative reconstruction method. Then the high resolution image is obtained by spectrum component inverse Fourier transform. Finally, the effect of background additive noise on the image quality is suppressed by the sparse base tracking theory. The simulated and outdoor experiment result are used to verify the denoising effect and image quality enhancement effect of the composite de-noising method. Compared with the existing single denoising method, the composite denoising method is shown to be able to effectively eliminate laser multiplicative speckle noise and background additive noise at one time. The proposed method can improve image contrast and promote the Strehl ratio of imaging quality in a coherent imaging system. It provides a theoretical basis for improving imaging quality and denosing laser multiplicative speckle noise and background additive noise in coherent field imaging system. Keywords:laser speckle noise/ background light noise/ homomorphic filter/ sparse denoising
激光回波场信号可看作激光光强函数I(x, y, t)与目标强度反射率函数O(x, y)的卷积, 因此叠加了激光散斑噪声和背景光噪声的相干场回波信号$S'(t) $可表示为:
$\begin{split}&S'\left( t \right) = \iint {I\left( {x,y,t} \right)O\left( {x,y} \right){\rm{d}}x{\rm{d}}y} \\&\quad \quad=\iint {\left( {{I_{\rm{s}}} + {I_n} + {I_{{\rm{s}}n}}} \right)O\left( {x,y} \right){\rm{d}}x{\rm{d}}y} ,\\&\quad S'\left( t \right) = S\left( t \right) + \Delta n\left( t \right) + \Delta sn\left( t \right),\end{split}$
简写为 $S'\left( t \right) = S\left( t \right) + {\Delta }n\left( t \right) + {\Delta }sn\left( t \right),$ $S'(t) $表示受散斑噪声干扰的激光回波信号; S(t)表示不受散斑和背景噪声干扰的激光回波信号; $\Delta n\left( t \right)$表示背景光和探测器等加性随机噪声; $\Delta sn\left( t \right)$为激光乘性散斑噪声. 激光回波能量信号经大面积探测器汇聚处理后, 还会受到探测器噪声$\Delta c\left( t \right)$的影响, 本研究假设激光散斑噪声、探测器噪声、背景噪声两两相互独立, 则叠加探测器噪声$\Delta c\left( t \right)$后的激光回波信号可表示为
$S''\left( t \right) = S\left( t \right) + {\Delta }n\left( t \right) + {\Delta }sn\left( t \right) + {\Delta }c\left( t \right).$