Single Speckle Research Articles

Interferometric speckle visibility spectroscopy (ISVS) for human cerebral blood flow monitoring

Infrared light scattering methods have been developed and employed to non-invasively monitor human cerebral blood flow (CBF). However, the number of reflected photons that interact with the brain is low when detecting blood flow in deep tissue. To tackle this photon-starved problem, we present and demonstrate the idea of interferometric speckle visibility spectroscopy (ISVS). In ISVS, an interferometric detection scheme is used to boost the weak signal light. The blood flow dynamics are inferred from the speckle statistics of a single frame speckle pattern. We experimentally demonstrated the improvement in the measurement of fidelity by introducing interferometric detection when the signal photon number is low. We apply the ISVS system to monitor the human CBF in situations where the light intensity is ∼100-fold less than that in common diffuse correlation spectroscopy (DCS) implementations. Due to the large number of pixels (∼2 × 105) used to capture light in the ISVS system, we are able to collect a similar number of photons within one exposure time as in normal DCS implementations. Our system operates at a sampling rate of 100 Hz. At the exposure time of 2 ms, the average signal photoelectron number is ∼0.95 count/pixel, yielding a single pixel interferometric measurement signal-to-noise ratio (SNR) of ∼0.97. The total ∼2 × 105 pixels provide an expected overall SNR of 436. We successfully demonstrate that the ISVS system is able to monitor the human brain pulsatile blood flow, as well as the blood flow change when a human subject is doing a breath-holding task.

Single speckle image analysis for monitoring the hardening kinetics of glass ionomer cements.

In this paper, we monitor the setting reaction of commercial glass ionomer cements using a laser speckle technique and adopting a spatial approach in the analysis of recorded speckle images. Experimental results showed that spatial contrast and speckle grain size increased as two studied cements underwent their setting reactions. After combining two geometrical configurations to measure the intensities of backscattered and transmitted light, we concluded that the increase in speckle grain size was caused by an increase in size of the scattering centers, since cement components aggregate and hence transition from a Rayleigh to a Mie scattering regime. Finally, two main phases were distinguished in the hardening process, as reported in the literature; however, the technique we propose has the advantage of easily identifying these two phases. The analysis of a single speckle image offers multiple advantages over the temporal analysis of a series of speckle images, in particular due to the low number of images recorded and a far shorter image processing time.

Single-shot color object reconstruction through scattering medium based on neural network

The reconstruction of the color object hidden behind the scattering medium is very important because the human eye is much more sensitive to color than grayscale. Traditional methods are still difficult to reconstruct an accurate image of the hidden target from one single speckle image. In this paper, a single-shot color object reconstruction technique is proposed by designing a Color Anti-scattering Convolutional Neural Network (CASNet), which is trained to output the color and structure of the hidden color target from the input of a single speckle image. The proposed technique enables us to reconstruct the target with accurate color and structure from a broadband speckle image, and the average PSNR of recovered targets with complex structure is higher than 24dB. Efficiency and accurateness are verified through experiments.

Implementing a microphysics model in hydrodynamic simulations to study the initial plasma formation in dielectric ablator materials for direct-drive implosions.

A microphysics model to describe the photoionization and impact ionization processes in dielectric ablator materials like plastic has been implemented into the one-dimensional hydrodynamic code LILACfor planar and spherical targets. At present, the initial plasma formation during the early stages of a laser drive are modeled in an ad hoc manner, until the formation of a critical surface. Implementation of the physics-based models predict higher values of electron temperature and pressure than the ad hoc model. Moreover, the numerical predictions are consistent with previous experimental observations of the shinethrough mechanism in plastic ablators. For planar targets, a decompression of the rear end of the target was observed that is similar to recent experiments. An application of this model is to understand the laser-imprint mechanism that is caused by nonuniform laser irradiation due to single beam speckle.

Speckle-based determination of the polarisation state of single and multiple laser beams

Laser speckle is generated by the multiple interference of light through a disordered medium. Here we study the premise that the speckle pattern retains information about the polarisation state of the incident field. We analytically verify that a linear relation exists between the Stokes vector of the light and the resulting speckle pattern. As a result, the polarisation state of a beam can be measured from the speckle pattern using a transmission matrix approach. We perform a quantitative analysis of the accuracy of the transmission matrix method to measure randomly time-varying polarisation states. In experiment, we find that the Stokes parameters of light from a diode laser can be retrieved with an uncertainty of 0.05 using speckle images of 150×150 pixels and 17 training states. We show both analytically and in experiment that this approach may be extended to the case of more than one laser field, demonstrating the measurement of the Stokes parameters of two laser beams simultaneously from a single speckle pattern and achieving the same uncertainty of 0.05.

Сдвиговый спекл-интерферометр с квадролинзой

The paper proposes a new optical element – Quad lens, which is used as part of a shearing speckle interferometer (sherograph) to provide measurements of stress-strain states of objects simultaneously in two mutually perpendicular directions. Quad lens consists of four identical sections cut from the original round lenses and spaced from each other to form a gap. Quad lens creates four images of an object that are offset relative to the optical axis by a distance that depends on the size of the gaps between the sectors. Phase recovery from a single speckle interferogram is performed using the spatial phase shift method based on the Fourier transform. To increase the contrast of interference bands, an aperture diaphragm with four holes is installed in front of quad lens sectors, and polarizing channel isolation can be used to separate channels and reduce the influence of cross interference. Experimental results of using a speckle interferometer with quad lens for the study of microdeformation of a round membrane are presented.

基于深度学习的散斑投影轮廓术

Traditional single speckle pattern matching algorithms always suffer from the low measurement accuracy and cannot be used to measure complex surface objects. A speckle projection profilometry with deep learning was proposed to realize the pixel-by-pixel matching. The siamese convolutional neural network structure was applied and extended where the main speckle pattern and the auxiliary speckle pattern were fed into the neural network patch by patch. It was expected that the feature from the speckle pattern patches could be extracted by the convolution operation. In this way, the features were fused and the matching coefficient between the two patches was obtained, which could be further used to formulate the disparity data and then the three-dimensional (3D) object was reconstructed. The experiment results demonstrate that with the proposed method 3D measurement with an accuracy of about 290 μm could be achieved through a single speckle pattern.

Photonic human identification based on deep learning of back scattered laser speckle patterns.

The analysis of the dynamics of speckle patterns that are generated when laser light is back scattered from a tissue has been recently shown as very applicable for remote sensing of various bio-medical parameters. In this work, we present how the analysis of a static single speckle pattern scattered from the forehead of a subject, together with advanced machine learning techniques based on multilayered neural networks, can offer novel approach to accurate identification within a small predefined number of classes (e.g., a 'smart home' setting which restricts its operations for family members only). Processing the static scattering speckle pattern by neural networks enables extraction of unique features with no previous expert knowledge being required. Using the right model allows for a very accurate differentiation between desirable categories, and that model can form a basis for using speckles patterns as a form of identity measure of 'forehead-print'.

Control of the temporal and polarization response of a multimode fiber

Control of the spatial and temporal properties of light propagating in disordered media have been demonstrated over the last decade using spatial light modulators. Most of the previous studies demonstrated spatial focusing to the speckle grain size, and manipulation of the temporal properties of the achieved focus. In this work, we demonstrate an approach to control the total temporal impulse response, not only at a single speckle grain but over all spatial degrees of freedom (spatial and polarization modes) at any arbitrary delay time through a multimode fiber. Global enhancement or suppression of the total light intensity exiting a multimode fibre is shown for arbitrary delays and polarization states. This work could benefit to applications that require pulse delivery in disordered media.

An effective compressive computational ghost imaging with hybrid speckle pattern

As an important factor to determine the quality of imaging, the sizes of the speckle pixel have gained increasing attention in ghost imaging. This paper proposes a new compressive computational ghost imaging which using hybrid speckle pattern to retrieve the target area. The hybrid speckle pattern is composed of different sizes of speckle pixel which depends on the resolution of target area. This method consists of two parts, the first part is the determination of interesting region and the second part is the imaging. The ghost images with hybrid speckle pattern are reconstructed by using CCGI. Simulation and experiment results have demonstrated the effectiveness and feasibility of the proposed method. The results indicate that the structural similarity index measure and signal-to-noise ratio are increased remarkably compared with single speckle pattern.

Efficient glare suppression with Hadamard-encoding-algorithm-based wavefront shaping.

In this Letter, we explore the effectiveness of a Hadamard encoding algorithm (HEA) for efficiently suppressing glare. Both numerical simulations and experimental data show that light intensity decays exponentially with respect to the number of HEA measurements. Specifically, we applied the HEA to reduce the intensity of a single speckle to 4.1% of its original value within only 16 measurements. In contrast, the commonly used genetic algorithm (GA) can reduce the speckle intensity only to 0.2 of its original value even after 1200 measurements. Therefore, the HEA greatly outperforms the previously adopted GA in terms of efficiency. We further show that the HEA is also applicable to suppress the integrated intensity of many speckles.

High-energy, high-resolution, fly-scan X-ray phase tomography

High energy X-ray phase contrast tomography is tremendously beneficial to the study of thick and dense materials with poor attenuation contrast. Recently, the X-ray speckle-based imaging technique has attracted widespread interest because multimodal contrast images can now be retrieved simultaneously using an inexpensive wavefront modulator and a less stringent experimental setup. However, it is time-consuming to perform high resolution phase tomography with the conventional step-scan mode because the accumulated time overhead severely limits the speed of data acquisition for each projection. Although phase information can be extracted from a single speckle image, the spatial resolution is deteriorated due to the use of a large correlation window to track the speckle displacement. Here we report a fast data acquisition strategy utilising a fly-scan mode for near field X-ray speckle-based phase tomography. Compared to the existing step-scan scheme, the data acquisition time can be significantly reduced by more than one order of magnitude without compromising spatial resolution. Furthermore, we have extended the proposed speckle-based fly-scan phase tomography into the previously challenging high X-ray energy region (120 keV). This development opens up opportunities for a wide range of applications where exposure time and radiation dose are critical.

Different methods for characterizing surface roughness using laser speckle technique

In this work, results from an optical technique (laser speckle technique) for measuring surface roughness was done by using statistical properties of speckle pattern from the point of view of computer image texture analysis. Four calibration relationships were used to cover wide range of measurement with the same laser speckle technique. The first one is based on intensity contrast of the speckle, the second is based on analysis of speckle binary image, the third is on size of speckle pattern spot, and the latest one is based on characterization of the energy feature of the gray level co-occurrence matrices for the speckle pattern. By these calibration relationships surface roughness of an object surface can be evaluated within these relations ranges from single speckle pattern image which was taken from the surface.

Processing of the output signal of the speckle interferometer on a single speckle at influence of a external noise

The influence of external noise on the processing of the output signal of a speckle interferometer on a single speckle is considered. The optical scheme of a speckle interferometer on a single speckle is shown. On the basis of theoretical studies, the main limitations imposed on the parameters of the optical circuit are determined. The proposed optical scheme allows simultaneously to determine both the vibration mode of the object under study and the amplitude of the vibrations from several nanometers to hundreds of microns and in a wide frequency range from zero Hz to tens of MHz. Theoretical and experimental studies of the influence of external noise on the output signal from the proposed speckle interferometer on a single speckle are presented. As a result of theoretical and experimental studies it was found that low-frequency noise does not affect the output signal from the speckle interferometer, especially when the amplitude of the information signal is greater than λ/8 (λ - is the wavelength of the laser radiation). Thus, the application of the proposed method of vibration registration of the investigated elements of mechanical structures makes it possible to use it in conditions close to industrial.

Single-shot large field of view imaging with scattering media by spatial demultiplexing.

Benefiting from the memory effect (ME) for speckle intensity correlations, only one single-shot speckle pattern can be used for the high-quality recovery of objects. However, ME gives a restriction to the field of view (FOV) for imaging with scattering media. Objects beyond the ME region cannot be recovered and produce unwanted speckle patterns, which cause reduction in the speckle contrast and recovery quality. Nevertheless, all the spatial information from a large object is embedded in a single speckle image. Here, we extract the spatial information from these unavoidable speckle patterns and enlarge the FOV of the imaging system with scattering media. Regional point spreading functions, which are fixed and only need to be recorded once for all-time use, are employed to recover corresponding spatial regions of an object by deconvolution. Then, an automatic weighted averaging in an iterative process is performed to obtain the object with significantly enlarged FOV. Our results present an important advancement of imaging techniques with strongly scattering media.

Single shot laser speckle based 3D acquisition system for medical applications

The state of the art techniques used by medical practitioners to extract the three-dimensional (3D) geometry of different body parts requires a series of images/frames such as laser line profiling or structured light scanning. Movement of the patients during scanning process often leads to inaccurate measurements due to sequential image acquisition. Single shot structured techniques are robust to motion but the prevalent challenges in single shot structured light methods are the low density and algorithm complexity. In this research, a single shot 3D measurement system is presented that extracts the 3D point cloud of human skin by projecting a laser speckle pattern using a single pair of images captured by two synchronized cameras. In contrast to conventional laser speckle 3D measurement systems that realize stereo correspondence by digital correlation of projected speckle patterns, the proposed system employs KLT tracking method to locate the corresponding points. The 3D point cloud contains no outliers and sufficient quality of 3D reconstruction is achieved. The 3D shape acquisition of human body parts validates the potential application of the proposed system in the medical industry.

Measuring vibrational motion from a moving platform using speckle field detection.

We investigate the ability of a holographic laser vibrometer to mitigate noise introduced when operating on a moving platform or when measuring a moving target. This motion introduces a fundamental limitation on the measurement sensitivity due to the time-varying speckle pattern produced as the illumination beam scans across the target surface. In addition, since existing systems record the phase of only a single speckle grain, speckle fading imposes a limit on the coherent processing interval and thus the frequency resolution of these measurements. In this work, we show that by measuring N speckle grains in parallel using holographic detection, we are able to provide a N1/2 improvement in the system sensitivity while simultaneously overcoming the limitations on the coherent processing interval imposed by speckle fading. The ability to perform sensitive vibrational measurements of a moving target or from a moving platform could dramatically increase the applications available to laser vibrometry.

Near-field speckle imaging of light localization in disordered photonic systems

Optical localization in strongly disordered photonic media is an attractive topic for proposing novel cavity-like structures. Light interference can produce random modes confined within small volumes, whose spatial distribution in the near-field is predicted to show hot spots at the nanoscale. However, these near-field speckles have not yet been experimentally investigated due to the lack of a high spatial resolution imaging techniques. Here, we study a system where the disorder is induced by random drilling air holes in a GaAs suspended membrane with internal InAs quantum dots. We perform deep-subwavelength near-field experiments in the telecom window to directly image the spatial distribution of the electric field intensity of disordered-induced localized optical modes. We retrieve the near-field speckle patterns that extend over few micrometers and show several single speckles of the order of λ/10 size. The results are compared with the numerical calculations and with the recent findings in the literature of disordered media. Notably, the hot spots of random modes are found in proximity of the air holes of the disordered system.

Single-shot diffraction-limited imaging through scattering layers via bispectrum analysis.

Recently introduced speckle correlations-based techniques enable noninvasive imaging of objects hidden behind scattering layers. In these techniques, the hidden object Fourier amplitude is retrieved from the scattered light autocorrelation, and the lost Fourier phase is recovered via iterative phase-retrieval algorithms, which suffer from convergence to wrong local minimums solutions and cannot solve ambiguities in object orientation. Here, inspired by notions used in astronomy, we experimentally demonstrate that in addition to Fourier amplitude, the object-phase information is naturally and inherently encoded in the scattered light bispectrum (the Fourier transform of triple correlation) and can also be extracted from a single high-resolution speckle pattern, based on which we present a single-shot imaging scheme to deterministically and unambiguously retrieve diffraction-limited images of objects hidden behind scattering layers.

Controlling the intensity of light in large areas at the interfaces of a scattering medium

The recent advent of wave-shaping methods has demonstrated the focusing of light through and inside even the most strongly scattering materials. Typically in wavefront shaping, light is focused in an area with the size of one speckle spot. It has been shown that the intensity is not only increased in the target speckle spot, but also in an area outside the optimized speckle spot. Consequently, the total transmission is enhanced, even though only the intensity in a single speckle spot is controlled. Here, we experimentally study how the intensity enhancement on both interfaces of a scattering medium depends on the optimization area on the transmission side. We observe that as the optimization radius increases, the enhancement of the total transmitted intensity increases. We find a concomitant decrease of the total reflected intensity, which implies an energy redistribution between transmission and reflection channels. In addition, we find qualitative evidence of a long-range reflection-transmission correlation. Our result is useful for efficient light harvesting in solar cells, multichannel quantum secure communications, imaging, and complex beam delivery through a scattering medium.