Correlation Of Speckle Patterns Research Articles

Highly sensitive refractometric sensing system based on the combination of lensing and laser speckle correlation

A simple optical sensing system based on laser speckle and lensing for monitoring refractive index variations of fluids is proposed and demonstrated. The fluid of interest is contained in a cylindrical glass vessel, which acts as an optical lens whose focal length depends on the refractive index of the fluid. Focal length variations imply optical wavefront alterations that are revealed through laser speckle pattern correlation. The device exhibits a refractive index resolution on the order of 10−6, which is comparable to those of more complex and costly refractometric sensors and demonstrates the benefits of combining these two optical techniques. The simplicity and high sensitivity of the presented configuration make it greatly suitable for a variety of applications ranging from chemical sensing to specimen detection.

OP-FCNN: an optronic fully convolutional neural network for imaging through scattering media.

Imaging through scattering media is a classical inverse issue in computational imaging. In recent years, deep learning(DL) methods have excelled in speckle reconstruction by extracting the correlation of speckle patterns. However, high-performance DL-based speckle reconstruction also costs huge hardware computation and energy consumption. Here, we develop an opto-electronic DL method with low computation complexity for imaging through scattering media. We design the "end-to-end" optronic structure for speckle reconstruction, namely optronic fully convolutional neural network (OP-FCNN). In OP-FCNN, we utilize lens groups and spatial light modulators to implement the convolution, down/up-sampling, and skip connection in optics, which significantly reduces the computational complexity by two orders of magnitude, compared with the digital CNN. Moreover, the reconfigurable and scalable structure supports the OP-FCNN to further improve imaging performance and accommodate object datasets of varying complexity. We utilize MNIST handwritten digits, EMNIST handwritten letters, fashion MNIST, and MIT-CBCL-face datasets to validate the OP-FCNN imaging performance through random diffusers. Our OP-FCNN reveals a good balance between computational complexity and imaging performance. The average imaging performance on four datasets achieves 0.84, 0.91, 0.79, and 16.3dB for JI, PCC, SSIM, and PSNR, respectively. The OP-FCNN paves the way for all-optical systems in imaging through scattering media.

One-to-all lightweight Fourier channel attention convolutional neural network for speckle reconstructions.

Speckle reconstruction is a classical inverse problem in computational imaging. Inspired by the memory effect of the scattering medium, deep learning methods reveal excellent performance in extracting the correlation of speckle patterns. Nowadays, advanced models generally include more than 10M parameters and mostly pay more attention to the spatial feature information. However, the frequency domain of images also contains precise hierarchical representations. Here we propose a one-to-all lightweight Fourier channel attention convolutional neural network (FCACNN) with Fourier channel attention and the res-connected bottleneck structure. Compared with the state-of-the-art model, i.e., self-attention armed convolutional neural network (SACNN), our architecture has better feature extraction and reconstruction ability. The Pearson correlation coefficient and Jaccard index scores of FCACNN increased by at least 5.2% and 13.6% compared with task-related models. And the parameter number of the lightweight FCACNN is only 1.15M. Furthermore, the validation results show that the one-to-all model, FCACNN, has excellent generalization capability on unseen speckle patterns such as handwritten letters and Quickdraws.

Real-time high sensibility vibration detection based on phase correlation of line speckle patterns

A vibration extraction system based on laser speckle with phase correlation method is presented. The speckle pattern sequences are recorded by a linear array CMOS, which allows the system to detect high frequency signals more than 20 kHz. The line speckle patterns are fast Fourier transformed and correlated between the adjacent rows in the frequency domain. The phase angles of correlation calculations are directly used as the displacements between the respective two rows in the object space. The recovered audio signals are evaluated by segmental SNR (SegSNR), normalized sub-band envelope correlation (NSEC), and log-likelihood ratio (LLR). Compared with the centroid and quadratic fitting method, the phase correlation method obtains advantages of high stability, high sensibility, and real-time.

Speckle pattern optimization for DIC technologies

This paper contains the relation between speckle pattern and Digital Image Correlation (DIC). The most important advance in experimental mechanics has been DIC since the strain gage. The deformation (strain) of an object can be visualized by DIC. Among all scientific fields, the DIC Technologies have seen a dynamic increase. The relationship between the paint and the sample - as the patterns mediate the deformation to the cameras - has been the most important technological issue. In this article the method developed for the detection of isolated particles in alloys is used to characterize the spots, which help the best speckle pattern has determined.

Measurement of Whole Blood Coagulation Time by Laser Speckle Pattern Correlation

We present here the results obtained by correlation analysis of digital speckle patterns to assess coagulation time in citrated whole blood. The spread of coagulation time values in experiments was no more than 25% for an essentially healthy cohort of laboratory animals. Studies of normal and pathological plasma showed that the method is able to detect pathological hemostasis. The ability to create a new universal optical coagulometer analyzing citrated and native blood, as well as whole blood, is discussed. Estimation of citrated blood clotting time is an intermediate stage in developing a test for native blood.

Direct en-face, speckle-reduced images using angular-compounded Master–Slave optical coherence tomography

In this paper, an angular compounding method to achieve speckle contrast reduction in optical coherence tomography (OCT) imaging is explored in detail. The angular compounding approach involves collecting multiple images at different angles of incidence, registering the images to correct for induced distortions, and then incoherently summing the images to reduce speckle. The method was experimentally demonstrated with a spectrometer-based Master–Slave enhanced OCT instrument capable of directly generating en-face images. We have investigated the impact of the angular range and number of averaged frames on the degree of speckle artefact reduction, as well as the effect on image resolution and sharpness. The minimum angular step necessary to secure a sufficiently small speckle pattern correlation between the images has also been determined, and the method has subsequently been validated on a biological sample (potato cells).

Utilizing multiple scattering effect for highly sensitive optical refractive index sensing

The sensitivity of random optical diffraction to refractive index inhomogeneity is usually considered as a challenging obstacle for imaging or controlling light through scattering media. Here, on the opposite side, we utilize the natural randomness of strongly scattering media and their sensitivity to refractive index inhomogeneity to develop a novel optical sensor. Unlike various sensing technologies with trade-off between complexity and sensitivity, we demonstrate a very simple sensing technique, which uses just ground glass optical diffusers to achieve very high sensitivity. Light propagating through a scattering medium is scattered multiple times at multiple interfaces between ground glass and the surrounding environment creating a speckle pattern, which is sensitive to the environment’s refractive index. The correlation of speckle patterns indicates the change of refractive index around the scattering medium. Simply placing the rough surface of ground glasses in contact with sensing solution, we are able to detect the solution’s refractive index change at resolution of 3.87 × 10−6 RIU, which is equivalent to the glucose concentration change of 12 ppm. More interestingly, the sensitivity of the proposed approach could be improved simply by adding more scattering surfaces in contact with the target medium. Our simple technique could be very useful for prominent applications in refractive index sensing such as measuring concentration of solution or gas.

Camera calibration using synthetic random speckle pattern and digital image correlation

Accurate camera calibration is of fundamental importance to various vision-based 3D metrological techniques. Despite camera calibration methods using regular planar calibration targets (e.g., checkerboard or circular pattern) have been widely adopted, their accuracy is less-than-desirable due to the limited number and the low registration accuracy of control points. This work presents an alternative calibration method that uses a calibration target based on a synthetic random speckle pattern. Specifically, a set of regularly distributed control points is first specified on the synthetic speckle pattern displayed on a monitor. These control points are then precisely matched to their counterparts on the captured calibration images using the state-of-the-art digital image correlation (DIC) algorithm. Compared with the existing camera calibration methods, the proposed method possesses the advantages of much more effective control points, higher control point match accuracy, which lead to more accurate and precise estimation of camera parameters. To evaluate the performance of the proposed calibration method, simulated calibration tests using images with varied noise levels and real calibration tests were performed on the planar calibration targets using speckle, circular and checkerboard patterns. The influences of various DIC calculation parameters (i.e. the number of control points and the subset size) on calibration results are also studied using noiseless simulated images. The reprojection errors on synthetic noiseless images and real images are computed as 0.004 and 0.038 pixels, respectively, confirming the high calibration accuracy delivered by the proposed method.

Motion-based coherent optical imaging in heavily scattering random media

Coherent optical imaging inside scattering media at substantial depths is key for biomedical applications, as well as various environmental and materials sensing situations, and has remained elusive. We present a simple imaging method involving the correlation of speckle patterns as a function of object position that provides access to the object autocorrelation function and, hence, allows the formation of an image using phase retrieval. With multiple measured speckle patterns, our method uses the ensemble-averaged spatial correlations to enable the extraction of an image of the moving object. The approach is effective for thick and heavily scattering media. Subject to adequate detector signal level, the method is simplified by heavy scatter. Reconstructions of both patch and aperture objects placed between heavily scattering acrylic slabs and between thick chicken breast slices demonstrate the feasibility of the approach.

Introducing non-local correlations into laser speckles.

Laser speckles have become a fundamental component of the modern optics-research toolbox. Not only are speckle patterns the basis of numerous imaging techniques, but also, they are employed to generate optical potentials for cold atoms and colloidal particles. The ability to manipulate a speckle pattern's spatial intensity correlations, particularly long-range (non-local) ones, is essential in numerous applications. A typical fully-developed speckle pattern, however, only possesses short-ranged (local) intensity correlations which are determined by the spatial field correlations. Here we experimentally demonstrate and theoretically develop a general method for creating fully-developed speckles with strong non-local intensity correlations. The functional form of the spatial intensity correlations can be arbitrarily tailored without altering the field correlations. Our approach provides a versatile and utilitarian framework for enhancing and controlling non-local correlations in speckle patterns.

Single-shot multispectral imaging with a monochromatic camera

Multispectral imaging plays an important role in many applications from astronomical imaging, earth observation to biomedical imaging. However, the current technologies are complex with multiple alignment-sensitive components, predetermined spatial and spectral parameters by manufactures. Here, we demonstrate a single-shot multispectral imaging technique that gives flexibility to end-users with a very simple optical setup, thank to spatial correlation and spectral decorrelation of speckle patterns. These seemingly random speckle patterns are point spreading functions (PSFs) generated by light from point sources propagating through a strongly scattering medium. The spatial correlation of PSFs allows image recovery with deconvolution techniques, while the spectral decorrelation allows them to play the role of tune-able spectral filters in the deconvolution process. Our demonstrations utilizing optical physics of strongly scattering media and computational imaging present the most cost-effective approach for multispectral imaging with great advantages.

Stable Speckle Patterns for Nano-scale Strain Mapping up to 700\xa0\xb0C

The digital image correlation (DIC) of speckle patterns obtained by vapour-assisted gold remodelling at 200 – 350 °C has already been used to map plastic strains with submicron resolution. However, it has not so far proved possible to use such patterns for testing at high temperatures. Here we demonstrate how a gold speckle pattern can be made that is stable at 700 °C, to study deformation in a commercial TiAl alloy (Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB2). The pattern is made up of a uniformly sized random array of Au islands as small as 15 nm in diameter, depending on reconstruction parameters, with a sufficiently small spacing to be suitable for nano-scale, nDIC, strain mapping at a subset size of 60 × 60 nm2. It can be used at temperatures up to 700 °C for many hours, for high cycle fatigue testing for instance. There is good particle attachment to the substrate. It can withstand ultra-sound cleaning, is thermally stable and has a high atomic number contrast for topography-free backscatter electron imaging.

Studying of Optical Fiber Strain Using Speckle Pattern Correlation

The present work discusses the optical fiber strain induced by the thermal and the mechanical effects. Two different optical fibers of different core diameters were used in the study. The thermal effect was induced by temperature control system in order to raise optical fibers temperatures to controlled values. The mechanical stress was induced on the optical fibers by a micro mechanical stage. The sensitivity of the optical fibers to both effects was studied by speckle pattern correlation in which the speckles are recorded at each disturbance and analyzed. By tracing the recorded speckles , x and y displacements could be measured and the strain could be determined in two dimensions. Power attenuation in each fiber was studied in each case as well. Both speckle correlation and power attenuation techniques used in the study showed similar indication on the sensitivity of the optical fibers to the external disturbers.

Syneresis and delayed detachment in agar plates.

Biogels made of crosslinked polymers such as proteins or polysaccharides behave as porous soft solids and store large amounts of solvent. These gels undergo spontaneous aging, called syneresis, which consists of the shrinkage of the gel matrix and the progressive expulsion of solvent. As a result, a biogel originally casted in a container often loses contact with the container sidewalls, and the detachment time is difficult to anticipate a priori, since it may occur over variable time spans (from hours to days). Here we report on syneresis phenomena in agar plates, which consist of Petri dishes filled with a gel mainly composed of agar. Direct observations and speckle pattern correlation analysis allow us to rationalize the delayed detachment of the gel from the sidewall of the Petri dish. The detachment time t* is surprisingly not controlled by the mass loss as one would intuitively expect. Instead, t* is strongly correlated to the gel minimum thickness emin measured along the sidewall of the plate, and increases as a robust function of emin, independently of the prior mass-loss history. Time-resolved correlation spectroscopy atypically applied to such weakly diffusive media gives access to the local thinning rate of the gel. This technique also allows us to detect the gel micro-displacements that are triggered by water evaporation prior to the detachment, and even to anticipate the latter from a few hours. Our work provides observables to predict the detachment time of agar gels in dishes, and highlights the relevance of speckle pattern correlation analysis for the quantitative investigation of the syneresis dynamics in biopolymer gels.

Fabrication of a DIC sensor for in-plane deformation measurement

A thin epoxy film with a micro speckle pattern was developed as a deformation sensor, and replicated on the surface of a sample. In order to improve the toughness of the thin epoxy film, a flexibilizer was added into the epoxy compound and the mechanical property of the thin epoxy film was analyzed. The measurement results show that the toughness of the thin film has been improved greatly, which enlarges the range of application of deformation measurement. Finally, the feasibility of application of the thin film digital image correlation (DIC) sensor was verified using a standard tensile experiment. And the interface deformation of the signal lap joint was measured by using fabrication and replication of the micro speckle pattern and micro digital image correlation. The failure mode of a single lap joint at micro-scale lengths was determined by analyzing the captured images of the micro speckle pattern.

Radiative transfer and diffusion limits for wave field correlations in locally shifted random media

The aim of this paper is to develop a mathematical framework for opto-elastography. In opto-elastography, a mechanical perturbation of the medium produces a decorrelation of optical speckle patterns due to the displacements of optical scatterers. To model this, we consider two optically random media, with the second medium obtained by shifting the first medium in some local region. We derive the radiative transfer equation for the cross-correlation of the wave fields in the media. Then we derive its diffusion approximation. In both the radiative transfer and the diffusion regimes, we relate the correlation of speckle patterns to the solutions of the radiative transfer and the diffusion equations. We present numerical simulations based on our model which are in agreement with recent experimental measurements.

Compensation Based Displacement Measurement Using Objective Laser Speckles

Objective laser speckles can be observed as intensity pattern from free space propagation of coherent light that is diffracted from a technically rough surface. Utilizing this pattern allows for very localized non-contacting measurement of surface displacement as well as strain, and carries a high potential for miniaturization. Existing objective speckle techniques suffer from decorrelation effects, dramatically decreasing the performance of such measurement techniques. In this paper we present a compensation based displacement measurement scheme, where we keep the measured speckle pattern displacement close to zero by actively following the measuring objects movement in a feedback operation. This converts the image sensor into a zero-detector, what considerably improves the speckle pattern correlation over a significantly extended measurement range.

Distance measurements by speckle correlation of objective speckle patterns, structured by the illumination

Objective speckles produced by two beams overlapping and interfering on a rough object surface contain information about the angle of incidence of the two beams, and how well they overlap. We obtain the autocovariance function for such a speckle pattern, and demonstrate how the information carried by the objective speckles can be used to probe the distance between the object and the observation plane. From a distance of 75 mm to a distance of 150 mm, and using an angle of 0.3 deg between the two incident beams, we can measure the actual distance with an uncertainty of better than ±0.1% of the full range. As long as the beams overlap at the object surface, the proposed method can measure distance with an uncertainty inversely proportional to the spot size at the object.

Coherent incident field information through thick random scattering media from speckle correlations over source position

With two nonoverlapping beams incident at different angles on a heavily scattering medium, the spatial correlation of speckle patterns over source position has a beat that is related to the incident angle difference. A model presented explains the measurement. The spatial correlation is shown to decorrelate faster than the beam intensity correlation function and to be sensitive to the incident field profile. Increased scatter results in more rapid decorrelation. This work suggests new opportunities for imaging through scattering media.