基于自适应阈值方法实现迭代降噪鬼成像

为了有效降低传统鬼成像中相关噪声对成像质量的影响,本文提出一种基于最佳阈值的迭代降噪鬼成像.首先在迭代降噪鬼成像的基础上,采用自适应阈值迭代法,在不需要目标先验信息的前提下,找到一个逼近传统鬼成像中相关噪声的阈值,根据得到的阈值构造噪声干扰项.为了每次迭代初值更接近原始目标的透射系数,对其进行二值化,降低重构图像背景噪声对迭代性能的影响.仿真以及实验结果表明,本文提出的方法与传统方法相比,视觉效果以及峰值信噪比值有明显提高.     

Influence of the sparsity of random speckle illumination on ghost imaging in a noise environment

The influence of the sparsity of random speckle illumination on traditional ghost imaging(GI) and GI via sparsity constraint(GISC) in a noise environment is investigated. The experiments demonstrate that both GI and GISC obtain their best imaging quality when the sparsity of random speckle illumination is 0.5, which is also explained by some parameters such as detection of the signal to noise ratio and mutual coherence of the measurement matrix.     

Lens ghost imaging with thermal light: From the far field to the near field

For a fixed 2-f reference path, we demonstrate both theoretically and experimentally that based on the spatial correlation between two light fields, ghost imaging in spatial domain (GI) and Fourier-transform ghost interference (FRT) can be obtained by only increasing the transverse size of the thermal source D. Both explanation of the transformation from GI to FRT and their potential applications are also discussed.     

Correspondence normalized ghost imaging on compressive sensing

Ghost imaging(GI) offers great potential with respect to conventional imaging techniques. It is an open problem in GI systems that a long acquisition time is be required for reconstructing images with good visibility and signal-to-noise ratios(SNRs). In this paper, we propose a new scheme to get good performance with a shorter construction time. We call it correspondence normalized ghost imaging based on compressive sensing(CCNGI). In the scheme, we enhance the signal-to-noise performance by normalizing the reference beam intensity to eliminate the noise caused by laser power fluctuations, and reduce the reconstruction time by using both compressive sensing(CS) and time-correspondence imaging(CI) techniques. It is shown that the qualities of the images have been improved and the reconstruction time has been reduced using CCNGI scheme. For the two-grayscale "double-slit" image, the mean square error(MSE) by GI and the normalized GI(NGI) schemes with the measurement number of 5000 are 0.237 and 0.164, respectively, and that is 0.021by CCNGI scheme with 2500 measurements. For the eight-grayscale "lena" object, the peak signal-to-noise rates(PSNRs)are 10.506 and 13.098, respectively using GI and NGI schemes while the value turns to 16.198 using CCNGI scheme. The results also show that a high-fidelity GI reconstruction has been achieved using only 44% of the number of measurements corresponding to the Nyquist limit for the two-grayscale "double-slit" object. The qualities of the reconstructed images using CCNGI are almost the same as those from GI via sparsity constraints(GISC) with a shorter reconstruction time.     

Handwritten digit recognition based on ghost imaging with deep learning

We present a ghost handwritten digit recognition method for the unknown handwritten digits based on ghost imaging(GI) with deep neural network, where a few detection signals from the bucket detector, generated by the cosine transform speckle, are used as the characteristic information and the input of the designed deep neural network(DNN), and the output of the DNN is the classification. The results show that the proposed scheme has a higher recognition accuracy(as high as98% for the simulations, and 91% for the experiments) with a smaller sampling ratio(say 12.76%). With the increase of the sampling ratio, the recognition accuracy is enhanced. Compared with the traditional recognition scheme using the same DNN structure, the proposed scheme has slightly better performance with a lower complexity and non-locality property.The proposed scheme provides a promising way for remote sensing.     

Orthogonal-triangular decomposition ghost imaging

Ghost imaging (GI) offers great potential with respect to conventional imaging techniques. However, there are still some obstacles for reconstructing images with high quality, especially in the case that the orthogonal measurement matrix is impossible to construct. In this paper, we propose a new scheme based on the orthogonal-triangular (QR) decomposition, named QR decomposition ghost imaging (QRGI) to reconstruct a better image with good quality. In the scheme, we can change the randomly non-orthogonal measurement matrix into orthonormal matrix by performing QR decomposition in two cases. (1) When the random measurement matrix is square, it can be firstly decomposed into an orthogonal matrix $\bm Q$ and an upper triangular matrix $\bm R$. Then let the off-diagonal values of $\bm R$ equal to 0.0, the diagonal elements of $\bm R$ equal to a constant $k$, where $k$ is the average of all values of the main diagonal, so the resulting measurement matrix can be obtained. (2) When the random measurement matrix is with full rank, we firstly compute its transpose, and followed with above QR operation. Finally, the image of the object can be reconstructed by correlating the new measurement matrix and corresponding bucket values. Both experimental and simulation results verify the feasibility of the proposed QRGI scheme. Moreover, the results also show that the proposed QRGI scheme could improve the imaging quality comparing to traditional GI (TGI) and differential GI (DGI). Besides, in comparison with the singular value decomposition ghost imaging (SVDGI), the imaging quality and the reconstruction time by using QRGI are similar to those by using SVDGI, while the computing time (the time consuming on the light patterns computation) is substantially shortened.     

Coupling-mediated ghost resonance in mutually injected lasers

We experimentally and numerically study the phenomenon of ghost resonance in coupled nonlinear systems. Two mutually injected semiconductor lasers are externally perturbed in their pump currents by two respective periodic signals of different frequencies f(1) and f(2). For small amplitudes of the external modulations, the two laser intensities display synchronized optical pulses, in the form of dropout events occurring at irregular times. By adjusting the amplitude and frequencies of the driving signals, the system exhibits a ghost resonance in the dropout appearance at a frequency f(r) not present in the distributed inputs.     

Optimization of speckle patterns in ghost imaging via sparse constraints by mutual coherence minimization

In this Letter the problem of optimization of speckle patterns in a ghost imaging(GI) system is addressed. The mutual coherence between the measuring matrix and the sparsifying dictionary matrix is minimized to obtain the required speckle patterns. Simulation and experimental results are presented, both showing that the quality of the reconstructed results obtained with the optimized speckle patterns is much improved in comparison with that obtained with the general unoptimized ones. We expect this method can be used to design GI systems with high performance.     

Robust Ghost Imaging Based on Degenerate Spontaneous Parametric Down-Conversion

In traditional ghost imaging, the entangled photon pairs produced from the spontaneous parametric down conversion(SPDC) process are used. There is an intrinsic disadvantage that the utilization efficiency of the photon pairs is very low. Inasmuch as all the correlated photon pairs produced by the degenerate SPDC process can be used to record the image of an object, the ghost imaging scheme we present here has a higher utilization efficiency of the photon pairs. We also investigate the robustness of our experimental scheme. The experimental results show that, no matter whether the photon-pair source is two light cones or two beam-like spots, the clear image of the object can be obtained. The slight rotation of the nonlinear crystal has no influence on the imaging quality.Our experimental results also demonstrate that when the part of the photon-pair source in the signal path or the idler path is blocked by unwanted things, the clear ghost image of the object can still be recorded.     

Resolution of ghost imaging with entangled photons for different types of momentum correlation

We present an analytical analysis of the spatial resolution of quantum ghost imaging implemented by entangled photons from a general, spontaneously parametric, down-conversion process. We find that the resolution is affected by both the pump beam waist and the nonlinear crystal length. Hence, we determined a method to improve the resolution for a certain imaging setup. It should be noted that the resolution is not uniquely related to the degree of entanglement of the photon pair since the resolution can be optimized for a certain degree of entanglement. For certain types of Einstein-Podolsky-Rosen(EPR) states——namely the momentum-correlated or momentum-positively correlated states——the resolution exhibits a simpler relationship with the pump beam waist and crystal length. Further, a vivid numerical simulation of ghost imaging is presented for different types of EPR states,which supports our analysis. This work discusses applicable references to the applications of quantum ghost imaging.     

Experimental investigation of the quality of lensless super-resolution ghost imaging via sparsity constraints

Ghost imaging via sparsity constraints (GISC) can nonlocally realize super-resolution imaging. Factors influencing the quality of lensless super-resolution GISC are investigated and the experimental results show that, the quality of GISC is enhanced as the object?s sparse ratio in the representation basis or the spatial transverse coherence lengths on the object plane are decreased. The differences between ghost imaging (GI) and GISC are also discussed.     

Effects of light intensity on reflective ghost imaging

In this letter, we analyze the effects of light intensity find that the brightness of reflective ghost image can on reflective ghost imaging with thermal source. We be changed by modulating the light intensity of the source and the splitting ratio of the beam splitter. The signal-to-noise ratio will be improved by increa.sing the light intensity of the source. More important, we can obtain the reflective ghost image with high image quality by adopting a low light intensity signal beam and a high light intensity reference beam, which is better than the classical optical imaging, because it can reduce the effects of light on the object.     

Probability theory of intensity correlation in ghost imaging with thermal light

To well reveal the imaging mechanism of ghost imaging (GI) with thermal light, we propose a probability theory using more general assumptions than existing theoretical explanations that the reference patterns obey an arbitrary identical distribution and the objects are of grayscale. It is not limited to a specific correlation function, even multi-functional variants of reference patterns can be used in correlation calculation. We have proven in both theory and experiments that the reconstruction means are linear transformations of original object's gray values and the probabilities of recovered pixel values in the pixel region of the same original gray value follow a Gaussian distribution, whose variance explains the appearance of reconstruction noise. We also derive the relationship between pattern variance and contrast-to-noise ratio under noisy conditions, which will be instructive for better pattern construction. This work will deepen the understanding of GI and enrich intensity correlation forms.     

自由电子激光相干强太赫兹源装置鬼脉冲初步研究

研究中国工程物理研究院自由电子激光相干强太赫兹源(FEL-THz)装置上鬼脉冲产生的原因及其对装置调试测量的影响,通过束流动力学模拟、热分析、蒙特卡罗模拟以及初步实验结果分析,证明了鬼脉冲将会明显破坏阻拦型测量,并会在束线管道上造成温度升高和辐射强度增大。研究表明：鬼脉冲成为限制FEL-THz装置正常工作的重要原因,为保证FEL-THz顺利出光,鬼脉冲必须予以消除,并简要讨论了鬼脉冲的消除方法。     

Improvement of the optical image reconstruction based on multiplexed quantum ghost images

Ghost imaging allows one to obtain information on an object from the spatial correlation function between photons propagating through or reflected from the object and photons of the reference arm. In this case, detection in the object arm is performed over the entire aperture of the beam and, therefore, it does not give information on the object. The reference beam does not interact with the object, but is recorded with a scanning point detector or a CCD array permitting the measurement of the spatial correlation function of photons in two arms. The use of multimode entangled quantum light beams by illuminating the object by one beam and orienting other beams to reference arms makes it possible to obtain simultaneously several ghost images (GIs). Cross correlations of multiplexed GIs (MGIs) are determined by eighth-order field correlation functions. A special algorithm is developed for calculating higher-order correlations of Bose operators. The presence of GI cross correlations is used for improving the quality of the reconstructed object’s image by their processing using the measurement reduction method. An example of the computer simulation of the image reconstruction by MGIs formed in the field of four-frequency entangled quantum states is considered. It is found that in this case the reduced GI has a signal-to-noise ratio several times higher than that of GIs.     

自由电子激光相干强太赫兹源装置鬼脉冲初步研究

研究中国工程物理研究院自由电子激光相干强太赫兹源(FEL-THz)装置上鬼脉冲产生的原因及其对装置调试测量的影响,通过束流动力学模拟、热分析、蒙特卡罗模拟以及初步实验结果分析,证明了鬼脉冲将会明显破坏阻拦型测量,并会在束线管道上造成温度升高和辐射强度增大。研究表明:鬼脉冲成为限制FEL-THz装置正常工作的重要原因,为保证FEL-THz顺利出光,鬼脉冲必须予以消除,并简要讨论了鬼脉冲的消除方法。     

A method to improve the visibility of ghost images obtained by thermal light

A method, which is based on the correlation measurement of intensity fluctuations in time-space-average domain, is invented to improve the visibility of ghost images obtained by thermal light. We demonstrate that the visibility of ghost images can be significantly enhanced using this method. A corresponding evaluation standard of the visibility of images and effects influencing the visibility of ghost images, such as the weight factor γ and speckle frame numbers, are also discussed.     

Simulation study of ghost image phenomenon induced by contrast enhancement film in plasma display panels

We have performed simulations of the ghost image phenomenon induced by a contrast enhancement film used for plasma display panels (PDPs). An optical imaging system was designed in order to investigate the ghost image qualitatively and quantitatively. We report on and discuss the effects of the material parameters and film orientation on the relative illuminance of the ghost image for a conventional louver-type contrast enhancement film adopting a trapezoidal black-stripe structure. The ghost image phenomenon is strongly affected by the film orientation relative to the PDP module and the difference between the refractive index of the ultraviolet curable urethane acrylate layer and that of the black stripes. We show that when the top side of the black stripes faces the PDP module and the refractive index difference is small, the ghost image phenomenon is weakened.     

The factors of spontaneous parametric down conversion process in BiB3O6 crystal with a broadband pump

We investigate the spontaneous parametric down-conversion (SPDC) process in biaxial nonlinear crystal BiB₃O₆ (BIBO) with a broadband pump. Under the type I and type II phase-matching conditions, we numerically calculate the influencing factors on SPDC process in BIBO crystal, such as temporal and spatial walk-off, the acceptance angles, and spectral acceptance bandwidth. Comparing the two types of phase-matching, we could conclude that the type II phase-matching in BIBO crystal is better for the SPDC because of zero dispersion and bigger acceptance parameters. These results can be used to construct a system to generate ultra-fast entangled photon pairs.     

Three-dimensional ghost imaging based on periodic diffraction correlation imaging

We experimentally demonstrate a three-dimensional （3D） ghost imaging method based on period diffraction correlation imaging. Compared with conventional ghost imaging, our method can easily retrieve the images of different focal planes. Due to the correlation between the disturbed object beam and the reference beams which do not pass through any scattering, the clear images can be periodically obtained in the uncovered zones even through a scattering medium. The analysis of the 3D imaging resolution reveals that the proper resolution for actual demand can be achieved by designing our devices. The implementation of this experiment is quite simple and low-cost. It facilitates the practical applications of ghost imaging.