Depth-resolved speckle-correlations imaging through scattering layers via coherence gating

Scan-free time-of-flight-based three-dimensional imaging through a scattering layer

Reconstructing an object's three-dimensional shape behind a scattering layer with a single exposure is of great significance in real-life applications. However, due to the little information captured by a single exposure while strongly perturbed by the scattering layer and encoded by free-space propagation, existing methods cannot achieve scan-free three-dimensional reconstruction through the scattering layer in macroscopic scenarios using a short acquisition time of seconds. In this paper, we proposed a scan-free time-of-flight-based three-dimensional reconstruction method based on explicitly modeling and inverting the time-of-flight-based scattering light propagation in a non-confocal imaging system. The non-confocal time-of-flight-based scattering imaging model is developed to map the three-dimensional object shape information to the time-resolved measurements, by encoding the three-dimensional object shape into the free-space propagation result and then convolving with the scattering blur kernel derived from the diffusion equation. To solve the inverse problem, a three-dimensional shape reconstruction algorithm consisting of the deconvolution and diffractive wave propagation is developed to invert the effects caused by the scattering diffusion and the free-space propagation, which reshapes the temporal and spatial distribution of scattered signal photons and recovers the object shape information. Experiments on a real scattering imaging system are conducted to demonstrate the effectiveness of the proposed method. The single exposure used in the experiment only takes 3.5 s, which is more than 200 times faster than confocal scanning methods. Experimental results show that the proposed method outperforms existing methods in terms of three-dimensional reconstruction accuracy and imaging limit subjectively and objectively. Even though the signal photons captured by a single exposure are too highly scattered and attenuated to present any valid information in time gating, the proposed method can reconstruct three-dimensional objects located behind the scattering layer of 9.6 transport mean free paths (TMFPs), corresponding to the round-trip scattering length of 19.2 TMFPs.

Speckle-correlation imaging through a kaleidoscopic multimode fiber

Speckle-correlation imaging techniques are widely used for noninvasive imaging through complex scattering media. While light propagation through multimode fibers and scattering media share many analogies, reconstructing images through multimode fibers from speckle correlations remains an unsolved challenge. Here, we exploit a kaleidoscopic memory effect emerging in square-core multimode fibers and demonstrate fluorescence imaging with no prior knowledge on the fiber. Experimentally, our approach simply requires to translate random speckle patterns at the input of a square-core fiber and to measure the resulting fluorescence intensity with a bucket detector. The image of the fluorescent object is then reconstructed from the autocorrelation of the measured signal by solving an inverse problem. This strategy does not require the knowledge of the fragile deterministic relation between input and output fields, which makes it promising for the development of flexible minimally invasive endoscopes.

Imaging operator in indirect imaging correlography

Indirect imaging correlography (IIC) is a coherent imaging technique that provides access to the autocorrelation of the albedo of objects obscured from line-of-sight. This technique is used to recover sub-mm resolution images of obscured objects at large standoffs in non-line-of-sight (NLOS) imaging. However, predicting the exact resolving power of IIC in any given NLOS scene is complicated by the interplay between several factors, including object position and pose. This work puts forth a mathematical model for the imaging operator in IIC to accurately predict the images of objects in NLOS imaging scenes. Using the imaging operator, expressions for the spatial resolution as a function of scene parameters such as object position and pose are derived and validated experimentally. In addition, a self-supervised deep neural network framework to reconstruct images of objects from their autocorrelation is proposed. Using this framework, objects with ≈ 250 μ m features, located at 1 mt standoffs in an NLOS scene, are successfully reconstructed.

Using speckle correlations for single-shot 3D imaging

Recovery of a 3D object behind a scattering medium is an important problem in many fields, including biomedical and defense applications. Speckle correlation imaging can recover objects in a single shot but contains no depth information. To date, its extension to 3D recovery has relied on multiple measurements, multi-spectral light, or pre-calibration of the speckle with a reference object. Here, we show that the presence of a point source behind the scatterer enables single-shot reconstruction of multiple objects at multiple depths. The method relies on speckle scaling from the axial memory effect, in addition to the transverse one, and recovers objects directly, without the need for phase retrieval. We provide simulation and experimental results to show object reconstructions at different depths with a single-shot measurement. We also provide theoretical principles describing the region where speckle scales with axial distance and its effects on the depth of field. Our technique will be useful where a natural point source exists, such as fluorescence imaging or car headlights in fog.

Tracking objects outside the line of sight using laser Doppler coherent detection

Based on the laser Doppler coherent detection method, a laser Doppler Non-Line-of Sight imaging technique (LD-NLOS) is proposed to obtain a series of effective information about the detected objects outside the line of sight. According to the analysis of the frequency and light intensity characteristics of the scattered signal, the information of the detected object hidden in the intermediate scattering surface is decoded. Without relying on complicated back-end algorithm processing and expensive experimental detection cost, the proposed LD-NLOS technique can detect the target vibration velocity and stably reconstruct its 2D shape.

Single-shot imaging through scattering media under strong ambient light interference

Speckle correlation imaging (SCI) has found tremendous versatility compared with other scattering imaging approaches due to its single-shot data acquisition strategy, relatively simple optical setup, and high-fidelity reconstruction performance. However, this simplicity requires SCI experiments to be performed strictly in a darkroom condition. As background noise increases, the speckle contrast rapidly decreases, making precise interpretation of the data extremely difficult. Here, we demonstrate a method by refining the speckle in the autocorrelation domain to achieve high-performance single-shot imaging. Experiment results prove that our method is adapted to estimate objects in a low signal-to-background ratio (SBR) circumstance even if the SBR is about -23dB. Laboratory and outdoor SCI experiments are performed.

Coherence gate manipulation for enhanced imaging through scattering media by non-ballistic light in partially coherent interferometric systems

Coherence gating is typically exploited for imaging through disordered media by least-scattered (ballistic) light. Ballistic light-based approaches produce clear images only when the proportion of ballistic to multiply scattered (non-ballistic) light is relatively high. To overcome this limitation, we counterintuitively utilize the coherence gate to image by the non-ballistic light, enabling us to retrieve information missing in the ballistic image. We show that non-ballistic images acquired by transversal coherence gate shifting have image quality and spatial resolution comparable to the ballistic image. Combining images for different coherence gate positions, we synthesize an image of quality superior to ballistic light approaches. We experimentally demonstrate our concept on quantitative phase imaging through biological tissue.

Noninvasive imaging of two isolated objects through a thin scattering medium beyond the 3D optical memory effect

A speckle image formed by scattering lights can be decoded by recently invented techniques, owing to the optical memory effect, thereby enabling the observation of a hidden object behind a thin scattering medium. However, the range of three-dimensional OME is typically small; therefore, both the field of view and depth of field are limited. We propose a method that can significantly and simultaneously improve both values for a specific scenario, where one object moves around the other position-fixed object. The effectiveness of the proposed scheme is demonstrated through a set of experiments.

Locating and Imaging through Scattering Medium in a Large Depth

Scattering medium brings great difficulties to locate and reconstruct objects especially when the objects are distributed in different positions. In this paper, a novel physics and learning-heuristic method is presented to locate and image the object through a strong scattering medium. A novel physics-informed framework, named DINet, is constructed to predict the depth and the image of the hidden object from the captured speckle pattern. With the phase-space constraint and the efficient network structure, the proposed method enables to locate the object with a depth mean error less than 0.05 mm, and image the object with an average peak signal-to-noise ratio (PSNR) above 24 dB, ranging from 350 mm to 1150 mm. The constructed DINet firstly solves the problem of quantitative locating and imaging via a single speckle pattern in a large depth. Comparing with the traditional methods, it paves the way to the practical applications requiring multi-physics through scattering media.

Signal detection in turbid water using temporally encoded polarimetric integral imaging

To improve signal detection in a turbid medium, we propose temporally encoded single shot polarimetric integral imaging. An optical signal is temporally encoded using gold coded sequences and transmitted through a turbid medium. The encoded signals are captured as a sequence of elemental images by two orthogonal polarized image sensor arrays. Polarimetric and polarization difference imaging are used to suppress the partially polarized and unpolarized background noise such that only the polarized ballistic signal photons are captured at the sensor. Multidimensional integral imaging is used to obtain 4D reconstructed data, and multidimensional nonlinear correlation is performed on the reconstructed data to detect the optical signal. We compare the effectiveness of the proposed polarimetric underwater optical signal detection approach to conventional (non-polarimetric) integral imaging-based and 2D imaging-based signal detection systems. The underwater signal detection capabilities are measured through performance metrics such as receiver operating characteristic (ROC) curves, the area under the curve (AUC), and the number of detection errors. Furthermore, statistical measures, including the Kullback-Leibler divergence, signal-to-noise ratio (SNR), and peak-to-correlation energy (PCE), are also calculated to show the improved performance of the proposed system. Our experimental results show that the proposed polarimetric integral-imaging approach significantly outperforms the conventional imaging-based methods. To the best of our knowledge, this is the first report on temporally encoded single shot polarimetric integral imaging for signal detection in turbid water.

Multitarget imaging through scattering media beyond the 3D optical memory effect

A robust method for efficient spatial separation optical imaging through scattering media regardless of the three-dimensional (3D) optical memory effect is proposed. In this method, the problems of imaging dealiasing, decomposition, and separation of speckle patterns are solved by employing independent component analysis. Multitarget imaging behind a scattering layer with diverse spatial positions is observed experimentally, for the first time, to the best of our knowledge. In this work, we demonstrate that, by knowing the number of targets and keeping each subtargets' size in the optical memory effect range while isolating them beyond this range without overlap in the axial direction, speckle dealiasing and multitarget imaging are solved effectively. The strategy provides a potentially useful means for incoherent imaging through scattering media in a wide class of fields such as optical microscopy, biomedical imaging, and astronomical imaging.

Passive optical time-of-flight for non line-of-sight localization

Optical imaging through diffusive, visually-opaque barriers and around corners is an important challenge in many fields, ranging from defense to medical applications. Recently, novel techniques that combine time-of-flight (TOF) measurements with computational reconstruction have allowed breakthrough imaging and tracking of objects hidden from view. These light detection and ranging (LiDAR)-based approaches require active short-pulsed illumination and ultrafast time-resolved detection. Here, bringing notions from passive radio detection and ranging (RADAR) and passive geophysical mapping approaches, we present an optical TOF technique that allows passive localization of light sources and reflective objects through diffusive barriers and around corners. Our approach retrieves TOF information from temporal cross-correlations of scattered light, via interferometry, providing temporal resolution that surpasses state-of-the-art ultrafast detectors by three orders of magnitude. While our passive approach is limited by signal-to-noise to relatively sparse scenes, we demonstrate passive localization of multiple white-light sources and reflective objects hidden from view using a simple setup.

Depth-resolved and auto-focus imaging through scattering layer with wavelength compensation

Imaging techniques through turbid materials have been extensively studied in recent years. The challenge now is to recover objects in a large field of view with depth-resolving ability. We present a method to image through a thin scattering layer automatically with the depth of the object detectable. By revealing the wavelength-depth-matching relation based on the axial memory effect, this method can automatically search the optimal wavelength of the reference light and compute the depth of the object. The no-reference image quality assessment function and rule-based searching algorithm are used in the searching process. The proposed method is promising for dynamic object tracking.

Speckle rotation decorrelation based single-shot video through scattering media

Optical imaging and tracking moving objects through scattering media is a challenge with important applications. However, previous works suffer from time-consuming recovery process, object complexity limit, or object information lost. Here we present a method based on the speckle rotation decorrelation property. The rotational speckles detected at short intervals are uncorrelated and multiplexed in a single-shot camera image. Object frames of the video are recovered by cross-correlation deconvolution of the camera image with a computationally rotated point spread function. The near real-time recovery provides sharp object image frames with accurate object relative positions, exact movement velocity, and continuous motion trails. This multiplexing technique has important implications for a wide range of real-world imaging scenarios.

Extending the depth-of-field of imaging systems with a scattering diffuser

Large depth of field (DOF) is a longstanding goal in optical imaging field. In this paper we presented a simple but efficient method to extend the DOF of a diffraction-limited imaging system using a thin scattering diffuser. The DOF characteristic of the imaging system with random phase modulation was analyzed based on the analytical model of ambiguity function as a polar display of the optical transfer function (OTF). The results of numerical simulation showed that more high-frequency components existed in the defocused OTF curve when the exit pupil of the imaging system exhibited a random phase modulation. It proved the important role of the scattering diffuser in extending the DOF of imaging systems. For the reconstruction, a stack of point spread functions (PSFs) corresponding to different axial locations within a measurement range were superimposed to construct the stacked PSF. Then the large DOF image was recovered from a speckle pattern by deconvolution. In this proof-of-concept, we experimentally demonstrated the single-shot imaging with larger DOF using a thin glass scattering diffuser in both a single-lens imaging system and a microscopic imaging system.