Showing papers on "Speckle imaging published in 2019"

Predicting Creep Failure from Cracks in a Heterogeneous Material using Acoustic Emission and Speckle Imaging

Abstract: Powered by TCPDF (www.tcpdf.org) This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Viitanen, Leevi; Ovaska, Markus; Ram, Sumit Kumar; Alava, Mikko J.; Karppinen, Pasi

Crosstalk-free volumetric in vivo imaging of a human retina with Fourier-domain full-field optical coherence tomography.

Abstract: Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is currently the fastest volumetric imaging technique that is able to generate a single 3-D volume of retina in less than 9 ms, corresponding to a voxel rate of 7.8 GHz. FD-FF-OCT is based on a fast camera, a rapidly tunable laser source, and Fourier-domain signal detection. However, crosstalk appearing due to multiply scattered light corrupts images with the speckle pattern, and therefore, lowers image quality. Here, for the first time, we report on a system that can acquire essentially crosstalk-free volumes of the retina by using a fast deformable membrane. It enables the visualization of choroids and a clear delineation of the retinal layers that is not possible with conventional FD-FF-OCT.

Hybrid photoacoustic-fluorescence microendoscopy through a multimode fiber using speckle illumination

Abstract: Advances in wavefront shaping technologies have enabled minimally invasive endoscopy using a single multimode fiber (MMF) and paved way to pushing the penetration depth limit inside scattering tissue. Photoacoustic imaging and fluorescence imaging are two key enabling imaging modalities that provide excellent anatomical and functional contrast, respectively. Combining their unique advantages is desirable; however, doing so in a minimally invasive endoscope has been a challenge. Here, we present an ultrathin hybrid imaging system with a 250 mm cross section based on an optical MMF and an optical fiber hydrophone and capable of combined photoacoustic and fluorescence microscopy. A reference-free calibration, using a set of prerecorded speckle intensity patterns combined with a reconstruction algorithm, enables compressive speckle imaging with optical resolution determined by the fiber numerical aperture.

Noninvasive three-dimensional imaging through scattering media by three-dimensional speckle correlation

Abstract: We present a method for noninvasive three-dimensional imaging through scattering media by using a three-dimensional memory effect in scattering phenomena. In the proposed method, an object in a scattering medium is reconstructed from a three-dimensional autocorrelation of speckle images captured by axially scanning an image sensor, based on a three-dimensional phase retrieval algorithm. We experimentally demonstrated our method with a lensless setup by using a three-dimensionally printed object between diffusers.

Application of laser speckle contrast imaging in laparoscopic surgery.

Abstract: Anastomotic leakage is a worldwide problem in gastrointestinal surgery which seems to be related to the state of microcirculation. Laser speckle contrast imaging (LSCI) could give surgeons insight in the state of microcirculation to attune the site of anastomosis. This work studies the feasibility of LSCI as a tool for this purpose. An experimental setup was developed using a commercially available laparoscopic video system. Laser speckle contrast imaging is capable of detecting ischemic areas on the large intestine. Further research and development are required before adaptation of this technique in the operating room.

Quantitative phase imaging camera with a weak diffuser

Abstract: We introduce the quantitative phase imaging camera with a weak diffuser (QPICWD) as a practical realization of quantitative phase imaging (QPI) on standard microscope platforms. The QPICWD is a compact stand-alone camera which measures sample induced phase delay under low-coherence quasi-monochromatic illumination by examining the deformation of the speckle intensity pattern. By interpreting the speckle deformation with an ensemble average of the geometric flow, we can obtain the high-resolution distortion field by solving the transport of intensity equation (TIE). Since the phase measured by TIE is the generalized phase of the partially coherent image, instead of the phase of the measured object, we analyze the effect of illumination coherence and imaging numerical aperture (NA) on the accuracy of phase retrieval, revealing that the phase of the object can be reliably retrieved when the coherence parameter (the ratio of illumination NA to objective NA) of the Kohler illumination is between 0.3 and 0.5. We present several applications for the new design including nondestructive optical testing of microlens array with nanometric thickness and imaging of fixed and live HeLa cells. Since the designed QPI camera does not require any modification of the widely available bright-field microscope or additional accessories for its use, it is expected to be adopted by the broader biology and medical community.

Speckle Interferometry at SOAR in 2018

Abstract: The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research Telescope (SOAR) in 2018 are given, totaling 3097 measurements of 2427 resolved pairs with separations from 11 mas to 5.9" (median 0.15", magnitude difference up to 7 mag) and non-resolutions of 624 targets. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. Also, pre-main-sequence stars in the Orion OB1 association were surveyed, resolving 26 out of 118 targets. In addition, we report discovery of 35 new companions among field visual multiples (some of which are likely optical) and first-time resolutions of another 31 pairs. By combining the measurements given here with the published ones, we computed 76 orbits for the first time and updated orbital elements of 34 visual binaries. Their periods range from 0.65 to 1100 years, and their quality varies from first tentative solutions of grade 5 to accurate elements of grades 1 and 2. Finally, a list of 53 spurious pairs discovered by various techniques and unresolved at SOAR

Speckle denoising by variant nonlocal means methods.

Abstract: This study aims to demonstrate the performances of nonlocal means (NLM) and their variant denoising methods, mainly focusing on NLM-shaped adaptive patches and several NLM-reprojection schemes for speckle noise reduction in amplitude and phase images of the digital coherent imaging systems. In the digital coherent imaging systems such as digital speckle pattern interferometry, digital holographic interferometry, etc., the image quality is severely degraded by additive uncorrelated speckle noise, due to the coherent nature of the light source, and therefore limits the development of several applications of these imaging systems in many fields. NLM and its variant denoising methods are employed to denoise the intensity/phase images obtained from these imaging systems, and their effectiveness is evaluated by considering various parameters. The performance comparison of these methods with other existing speckle denoising methods is also presented. The performance of these methods for speckle noise reduction is quantified on the basis of two criteria matrices, namely, the peak-to-signal noise ratio and the image quality index. Based on these criteria matrices, it is observed that these denoising methods have the ability to improve the intensity and phase images favorably in comparison to other speckle denoising techniques, and these methods are more effective and feasible in speckle-noise reduction.

Multiple-image double-encryption via 2D rotations of a random phase mask with spatially incoherent illumination.

Abstract: Optical image encryption technique has become extremely important in these years. However, most of the proposed multiple-image encryption systems are illuminated with coherent light source. Here we present a multiple-image double-encryption method with spatially incoherent illumination. The first-encryption of multiple images is based on the speckle rotation decorrelation property, and the second-encryption of images’ order is based on the speckle shift decorrelation out of the angular memory-effect range. The double-encryption via two-dimensional rotations of the random phase mask enhances the security and keeps the simplicity of the cryptosystem. The capacity of the ciphertext is greatly increased by multiplexing, and further increased after crosstalk noise removal. The use of incoherent light source reduces the requirements for experimental conditions, and makes the cryptosystem easy to implement in various application scenarios.

Expansion of the FOV in speckle autocorrelation imaging by spatial filtering.

Abstract: Optical imaging through inhomogeneous media based on autocorrelations suffers from a limited field of view (FOV), since the optical memory effect (ME) of a scattering medium has its inherent angular extent. Here we successfully expand the angular ME range by exploiting a spatial filtering technique to select low-frequency components, mainly ballistic light and less scattered light, thereby increasing the FOV of the speckle autocorrelation imaging. Both a simulation and experimental verifications are presented. This technique, which is not limited to the discussed 4f structure, can provide a guideline for the design of an optical system to image through scattering media.

Improving reconstruction of speckle correlation imaging by using a modified phase retrieval algorithm with the number of nonzero-pixels constraint.

Abstract: Speckle correlation imaging (SCI) has been considered one of the most promising techniques for computational imaging through a scattering medium. However, the image quality is not always acceptable in conventional SCI, especially when a complex object is involved. In this work, a modified phase retrieval algorithm is introduced to significantly improve the imaging quality of SCI. In the proposed scheme, nonzero-pixel constraints, rather than the real and nonnegative constraints, are employed as the object domain constraints of the iterative algorithm in the image reconstruction process. Experimental results are presented to show the performance enhancement of this scheme, inclusive of less iterations, better image quality, and higher reliability, in comparison with the conventional SCI method.

Single-shot noninvasive three-dimensional imaging through scattering media

Abstract: We present a method for single-shot three-dimensional imaging through scattering media with a three-dimensional memory effect. In the proposed computational process, a captured speckle image is two-dimensionally correlated with different scales, and the object is three-dimensionally recovered with three-dimensional phase retrieval. Our method was experimentally demonstrated with a lensless setup and was compared with a multishot approach used in our previous work [Opt. Lett.44, 2526 (2019)OPLEDP0146-959210.1364/OL.44.002526].

Speckle interferometry at SOAR in 2018

Abstract: The results of speckle interferometric observations at the 4.1 m Southern Astrophysical Research Telescope (SOAR) in 2018 are given, totaling 3097 measurements of 2427 resolved pairs with separations from 11 mas to 5.9" (median 0.15", magnitude difference up to 7 mag) and non-resolutions of 624 targets. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. Also, pre-main-sequence stars in the Orion OB1 association were surveyed, resolving 26 out of 118 targets. In addition, we report discovery of 35 new companions among field visual multiples (some of which are likely optical) and first-time resolutions of another 31 pairs. By combining the measurements given here with the published ones, we computed 76 orbits for the first time and updated orbital elements of 34 visual binaries. Their periods range from 0.65 to 1100 years, and their quality varies from first tentative solutions of grade 5 to accurate elements of grades 1 and 2. Finally, a list of 53 spurious pairs discovered by various techniques and unresolved at SOAR

Simultaneous 3D measurement of deformation and its first derivative with speckle pattern interferometry and shearography

Abstract: Digital speckle pattern interferometry and digital shearography are widely used in nondestructive testing due to their advantages of high speed, full field, and high sensitivity. However, traditional speckle pattern interferometry and shearography can only measure a single variable of deformation or strain. This study presents a modified common measurement setup that can simultaneously measure deformation and its first derivative. In the optical setup, a reference beam is introduced behind the shearing device to interfere with the object beams, thereby simplifying the spectrum and improving the quality of the phase maps. Then the spatial carrier technology is used to extract phase and achieve dynamic measurements. The proposed system also expands the measurement range using a ${4}f$4f system. This study also presents a 3D optical setup based on the 1D system, which is more suitable for practical measurement applications in industrial areas. Theoretical derivation and experimental results are described and presented.

Robust quantitative single-exposure laser speckle imaging with true flow speckle contrast in the temporal and spatial domains

Abstract: A systematic and robust laser speckle contrast imaging (LSCI) method and procedure is presented covering the LSCI system calibration, static scattering removal, and measurement noise estimation and correction to obtain a true flow speckle contrast Kf 2and the flow speed from single-exposure LSCI measurements. We advocate the use of K2 as the speckle contrast instead of the conventional contrast K, as the former relates simply to the flow velocity and is with additive noise alone. We demonstrate the efficacy of the proposed true flow speckle contrast by imaging phantom flow at varying speeds, showing that (1) the proposed recipe greatly enhances the linear sensitivity of the flow index (inverse decorrelation time) and the linearity covers the full span of flow speeds from 0 to 40 mm/s; and (2) the true flow speed can be recovered regardless of the overlying static scattering layers and the type of speckle statistics (temporal or spatial). The fundamental difference between the apparent temporal and spatial speckle contrasts is further revealed. The flow index recovered in the spatial domain is much more susceptible to static scattering and exhibit a shorter linearity range than that obtained in the temporal domain. The proposed LSCI analysis framework paves the way to estimate the true flow speed in the wide array of laser speckle contrast imaging applications.

Ptychographic imaging of incoherently illuminated extended objects using speckle correlations.

Abstract: A scattering layer is usually considered an obstacle to imaging. However, using speckle correlation imaging techniques, the scattering layer effectively acts as a lens. To date, the speckle correlation imaging method has been limited to imaging sparse samples. Here, we demonstrate imaging of incoherently illuminated extended objects in transmission and around-the-corner geometries. We are able to image extended objects by constraining the illumination spot on the object and then scanning the object. A ptychography algorithm is used to reconstruct the extended object. This work demonstrates the applicability of ptychography to spatially incoherent light and enables a new method of imaging in spectral regions where there is limited choice in optics, such as the terahertz, extreme ultraviolet, and x-ray regions.

Effect of the average number of reference speckles in speckle imaging using off-axis speckle holography

Abstract: The propagation of a coherent beam of light through a random scattering medium results in the generation of a speckle pattern. Although the intensity distribution of the speckle pattern is random in nature, the information of the object hidden behind a scattering medium is scrambled into it. The scrambled object information can be retrieved using the off-axis speckle holographic technique, where the object retrieval is made from a recorded interferogram, formed by the superposition of the object speckles and a tilted reference speckle pattern. In the present paper, the effect of the average number of reference speckles on the signal-to-noise ratio of the retrieved object is investigated in two different random domains of the reference speckles, which are defined from the study of the Shannon entropy of the reference speckle patterns. The observed results can be useful in tuning the visibility and sharpness of the object, retrieved by employing the off-axis speckle holographic technique.

Speckle-interferometry and speckle-correlometry of GB-speckles.

Abstract: A new method of coding of genetic information using laser speckles has been developed. Specific technique of transforming the nucleotide of gene into a speckle pattern (gene-based speckles or GB-speckles) is suggested. Reference speckle patterns of omp1 gene of typical wild strains of Chlamydia trachomatis of genovars D, E, F, G, and J are generated. This is the first report in which perspectives of the proposed technique in the bacterial gene identification and detection of natural genetic mutations in bacteria as a single nucleotide polymorphism (SNP) are demonstrated. The usage of GB-speckles can be viewed as the next step on the way to the era of digital biology.

Phase-sensitive interferometry of decorrelated speckle patterns.

Abstract: Phase-sensitive coherent imaging exploits changes in the phases of backscattered light to observe tiny alterations of scattering structures or variations of the refractive index. But moving scatterers or a fluctuating refractive index decorrelate the phases and speckle patterns in the images. It is generally believed that once the speckle pattern has changed, the phases are scrambled and any meaningful phase difference to the original pattern is removed. As a consequence, diffusion and tissue motion that cannot be resolved, prevent phase-sensitive imaging of biological specimens. Here, we show that a phase comparison between decorrelated speckle patterns is still possible by utilizing a series of images acquired during decorrelation. The resulting evaluation scheme is mathematically equivalent to methods for astronomic imaging through the turbulent sky by speckle interferometry. We thus adopt the idea of speckle interferometry to phase-sensitive imaging in biological tissues and demonstrate its efficacy for simulated data and imaging of photoreceptor activity with phase-sensitive optical coherence tomography. We believe the described methods can be applied to many imaging modalities that use phase values for interferometry.

Multiple speckle exposure imaging for the study of blood flow changes induced by functional activation of barrel cortex and olfactory bulb in mice

Abstract: Speckle contrast imaging allows in vivo imaging of relative blood flow changes. Multiple exposure speckle imaging (MESI) is more accurate than the standard single-exposure method since it allows separating the contribution of the static and moving scatters of the recorded speckle patterns. MESI requires experimental validation on phantoms prior to in vivo experiments to ensure the proper calibration of the system and the robustness of the model. The data analysis relies on the calculation of the speckle contrast for each exposure and a subsequent nonlinear fit to the MESI model to extract the scatterers correlation time and the relative contribution of moving scatters. We have designed two multichannel polydimethylsiloxane chips to study the influence of multiple and static scattering on the accuracy of MESI quantitation. We also propose a method based on standard C++ libraries to implement a computationally efficient analysis of the MESI data. Finally, the system was used to obtain in vivo hemodynamic data on two distinct sensory areas of the mice brain: the barrel cortex and the olfactory bulb.

Precision roll angle measurement based on digital speckle pattern interferometry

Abstract: An approach to precise measurement of small roll angle based on digital speckle pattern interferometry (DSPI) is introduced. A simple relationship between the roll angle and interferometric phase distribution is established by analyzing the mechanism of fringe formation in the process of roll detection using a dual-beam DSPI, and reasonable approximate processing. The presented method enjoys some important advantages, such as being free of a cooperative target and a very high measurement resolution. In our experiments, a measurement resolution of 1 µrad, which can compete with the best resolution of the existing methods, is achieved. Theoretically, the measurement resolution can even be improved to tens of nano-radians at the cost of increasing the size of the measuring device. The performance of the method is also demonstrated by a comparison experiment.

Three-Dimensional Shape Measurement Beyond Diffraction Limit for Measurement of Dynamic Events

Abstract: Speckle interferometry is one of the important measurement methods of deformation on an object with rough surfaces. In this paper, a method which can be applied to a three-dimensional (3-D) shape measurement for dynamic events is proposed. In the method, the differential coefficient distribution of the shape of such an object is detected by giving a known lateral shift in the computer memory in order to analyze using one-shot speckle pattern. The 3-D shape can be reconstructed by integrating the differential coefficient distribution. The method is also applied to the 3-D shape measurement of superfine structure beyond the diffraction limit. Furthermore, the influence of magnitude of lateral shift on shape is discussed.

Consistency of the Infrared Variability of Sgr A* over 22 years.

Abstract: We report new infrared measurements of the supermassive black hole at the Galactic Center, Sgr A*, over a decade that was previously inaccessible at these wavelengths. This enables a variability study that addresses variability timescales that are ten times longer than earlier published studies. Sgr A* was initially detected in the near-infrared with adaptive optics observations in 2002. While earlier data exists in form of speckle imaging (1995 - 2005), Sgr A* was not detected in the initial analysis. Here, we improved our speckle holography analysis techniques. This has improved the sensitivity of the resulting speckle images by up to a factor of three. Sgr A* is now detectable in the majority of epochs covering 7 years. The brightness of Sgr A* in the speckle data has an average observed K magnitude of 16.0, which corresponds to a dereddened flux density of $3.4$ mJy. Furthermore, the flat power spectral density (PSD) of Sgr A* between $\sim$80 days and 7 years shows its uncorrelation in time beyond the proposed single power-law break of $\sim$245 minutes. We report that the brightness and its variability is consistent over 22 years. This analysis is based on simulations using Witzel et al. (2018) model to characterize infrared variability from 2006 to 2016. Finally, we note that the 2001 periapse of the extended, dusty object G1 had no apparent effect on the near-infrared emission from accretion flow onto Sgr A*. The result is consistent with G1 being a self-gravitating object rather than a disrupting gas cloud.

Digital holographic interferometry and speckle interferometry applied on objects with heterogeneous reflecting properties

Abstract: In digital holographic and speckle interferometry devoted to solid object displacement measurement, the reflecting properties of the object under study are of importance in designing the observation and laser illumination systems. In practical cases, the objects can show separate zones in which the surface property can simultaneously cause either scattering or specular reflectivity. We present strategies for dealing with both reflectivity types at a time in digital holographic and speckle interferometers. The scattered surface is illuminated with a point source whereas the specular one is illuminated by a diffuser. Both types of surfaces visible across the field-of-view give rise to a specific interferogram with gaps in between, which in turn are interpreted separately related to the sensitivity vector, the latter being defined differently for scattering and specular areas.

Non-contact photoacoustic imaging using laser speckle contrast analysis

Abstract: A novel method for non-contact and continuous detection of photoacoustic signals is presented and experimentally demonstrated. The approach is based on analysis of the contrast of time-varying speckle patterns, and suggests a more robust alternative in respect to interferometric and refractometric available solutions.

Fast diffraction-limited image recovery through turbulence via subsampled bispectrum analysis

Abstract: Imaging through temporally changing aberrations is a common challenge that can be found in many different fields such as astronomy, long-range surveillance, and deep tissue bioimaging. Based on the notions originally developed in speckle interferometry, time-varying aberrations can be used to our advantage to obtain diffraction-limited resolution images through turbulence via bispectrum analysis. However, due to the heavy computational load brought on by the triple correlation and the phase extraction process, widespread use has been limited. Here, we demonstrate a Fourier domain subsampling scheme that can accelerate the speed of bispectrum analysis by more than 2 orders of magnitude. In contrast to other approaches for parallelization such as those based on Radon transform or image segmentation, our proposed method enables diffraction-limited imaging without suffering from resolution loss or image artifacts.