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Reconstructing complex field through opaque scattering layer with structured light illumination
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Abstract
The wavefront is scrambled when coherent light propagates through a random scattering medium and which makes direct use of the conventional optical methods ineffective. In this paper, we propose and demonstrate a structured light illumination for imaging through an opaque scattering layer. Proposed technique is reference free and capable to recover the complex field from intensities of the speckle patterns. This is realized by making use of the phase-shifting in the structured light illumination and applying spatial averaging of the speckle pattern in the intensity correlation measurement. An experimental design is presented and simulated results based on the experimental design are shown to demonstrate imaging of different complex-valued objects through scattering layer.
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Light propagating along a reversed path experiences the same transmission coefficient as in the forward direction, independent of the path complexity. This is called the optical reciprocity of light, which is valid for not too intense scattering media as well. Hence, by utilizing the reciprocity principle, the proposed novel technique can achieve axially and laterally tunable focus, non-invasively, through a scattering media without a priori knowledge or modeling of its scattering properties. Moreover, the uniqueness of the proposed technique lies in the fact that the illumination and detection are on the same side of the scattering media.
The coherence holography offers an unconventional way to reconstruct the hologram where an incoherent light illumination is used for reconstruction purposes, and object encoded into the hologram is reconstructed as the distribution of the complex coherence function. Measurement of the coherence function usually requires an interferometric setup and array detectors. This paper presents an entirely new idea of reconstruction of the complex coherence function in the coherence holography without an interferometric setup. This is realized by structured pattern projections on the incoherent source structure and implementing measurement of the cross-covariance of the intensities by a single-pixel detector. This technique, named structured transmittance illumination coherence holography (STICH), helps to reconstruct the complex coherence from the intensity measurement in a single-pixel detector without an interferometric setup and also keeps advantages of the intensity correlations. A simple experimental setup is presented as a first step to realize the technique, and results based on the computer modeling of the experimental setup are presented to show validation of the idea.
This research presents a coherent structured illumination single-pixel imaging scheme to image objects with complex amplitudes. By utilizing a phase-only spatial light modulator for phase modulation, we can efficiently generate the Hadamard basis structured light and the reference light that interfere with each other to form the coherent structured illumination. Using the 4-step phase-shifting, the spectrum of the object is acquired by detecting the zero-frequency component of the object light with a single-pixel photodetector. The desired complex-amplitude image can be further retrieved by applying an inverse Hadamard transform. The proposed scheme is experimentally demonstrated by imaging two etched glass objects, a dragonfly wing, and a resolution test chart. Benefiting from the phase modulation, this scheme has a high efficiency, a high imaging quality, a high spatial resolution, and a simple and stable configuration to obtain both the phase and amplitude information of the target object. The proposed scheme provides a promising complex-amplitude imaging modality with single-pixel detection. Thus it might find broad applications in optical metrology and biomedical science.
We present a spatial light modulator (SLM) assisted compact holographic method and illustrate its application by imaging through a random scattering medium. The merit of the proposed method is wavefront division multiplexing, i.e. the dual wavefront modulations over a single SLM. Two different wavefront shapes: a reference-light shape and a phase object, are combined over the SLM. One advantage of this scheme is the flexible modulation of the reference light. The experimental implementation of this method is demonstrated by quantitatively reconstructing different phase objects from the randomly scattered light. This new scheme greatly simplifies the experimental configuration and presents a better stability even in presence of external vibrations, opening avenues for the holography-based scattering imaging application.
This study presents a partially coherent illumination based (PCI-based) SIM apparatus for dual-modality (phase and fluorescent) microscopic imaging. The partially coherent illumination (PCI) is generated by placing a rotating diffuser on a monochromatic laser beam, which suppresses speckle noise in the dual-modality images and endows the apparatus with sound sectioning capability. With this system, label-free quantitative phase and super-resolved/sectioned fluorescent images can be obtained for the same sample. We have demonstrated the superiority of the system in phase imaging of transparent cells with high endogenous contrast and in a quantitative manner. In the meantime, we have also demonstrated fluorescent imaging of fluorescent beads, rat tail crosscut, wheat anther, and hibiscus pollen with super-resolution and optical sectioning. We envisage that the proposed method can be applied to many fields, including but not limited to biomedical, industrial, chemistry fields.
Digital in-line holography (DIH) is broadly used to reconstruct 3D shapes of microscopic objects from their 2D holograms. One of the technical challenges in the reconstruction stage is eliminating the twin image originating from the phase-conjugate wavefront. The twin image removal is typically formulated as a non-linear inverse problem since the scattering process involved in generating the hologram is irreversible. Conventional phase recovery methods rely on multiple holographic imaging at different distances from the object plane along with iterative algorithms. Recently, end-to-end deep learning (DL) methods are utilized to reconstruct the object wavefront (as a surrogate for the 3D structure of the object) directly from the single-shot in-line digital hologram. However, massive data pairs are required to train the utilize DL model for an acceptable reconstruction precision. In contrast to typical image processing problems, well-curated datasets for in-line digital holography do not exist. The trained models are also highly influenced by the objects’ morphological properties, hence can vary from one application to another. Therefore, data collection can be prohibitively laborious and time-consuming, as a critical drawback of using DL methods for DH. In this article, we propose a novel DL method that takes advantages of the main characteristic of auto-encoders for blind single-shot hologram reconstruction solely based on the captured sample and without the need for a large dataset of samples with available ground truth to train the model. The simulation results demonstrate the superior performance of the proposed method compared to the state-of-the-art methods used for single-shot hologram reconstruction.
When it comes to “phase measurement” or “quantitative phase imaging”, many people will automatically connect them with “laser” and “interferometry”. Indeed, conventional quantitative phase imaging and phase measurement techniques generally rely on the superposition of two beams with a high degree of coherence: complex interferometric configurations, stringent requirements on the environmental stabilities, and associated laser speckle noise severely limit their applications in optical imaging and microscopy. On a different note, as one of the most well-known phase retrieval approaches, the transport of intensity equation (TIE) provides a new non-interferometric way to access quantitative phase information through intensity only measurement. Despite the insufficiency for interferometry, TIE is applicable under partially coherent illuminations (like the Köhler’s illumination in a conventional microscope), permitting optimum spatial resolution, higher signal-to-noise ratio, and better image quality. In this tutorial, we give an overview of the basic principle, research fields, and representative applications of TIE, focus particularly on optical imaging, metrology, and microscopy. The purpose of this tutorial is twofold. It should serve as a self-contained introduction to TIE for readers with little or no knowledge of TIE. On the other hand, it attempts to give an overview of recent developments in this field. These results highlight a new era in which strict coherence and interferometry are no longer prerequisites for quantitative phase imaging and diffraction tomography, paving the way toward new generation label-free three-dimensional microscopy, with applications in all branches of biomedicine.
Fundamental challenge of imaging through a scattering media has been resolved by various approaches in the past two decades. Optical wavefront shaping technique is one such method in which one shapes the wavefront of light entering a scattering media using a wavefront shaper such that it cancels the scattering effect. It has been the most effective technique in focusing light inside a scattering media. Unfortunately, most of these techniques require direct access to the scattering medium or need to know the scattering properties of the medium beforehand. Through the novel scheme presented on this paper, both the illumination module and the detection are on the same side of the inspected object and the imaging process is a real time fast converging operation. We model the scattering medium being a biological tissue as a matrix having mathematical properties matched to the physical and biological aspects of the sample. In our adaptive optics scheme, we aim to estimate the scattering function and thus to encode the intensity of the illuminating laser light source using DMD (Digital Micromirror Device) with an inverse scattering function of the scattering medium, such that after passing its scattering function a focused beam is obtained. We optimize the pattern to be displayed on the DMD using Particle Swarm Algorithm (PSO) which eventually help in retrieving a 1D object hidden behind the media.
This paper proposes a coded aperture structured illumination (CASI) technique in digital holographic microscopy (DHM). A CASI wave is generated using two binary phase codes (0° and 120°) for spatial phase shifting. The generated CASI wave then interferes with a reference wave to form a coded Fresnel hologram at a single exposure with compressive sensing (CS) to avoid the temporal phase-shifting process of the structured illumination (SI). The CS algorithm is applied to retrieve the missing data of decoded phase-shifted SI-modulated waves, which are used to separate overlapped spatial frequencies for obtaining a larger spatial frequency coverage to provide superresolution imaging. Two phase-only spatial light modulators are applied to generate a directional SI pattern for obtaining a coded aperture with a suitable size to perform one-shot acquisition in the DHM system.
Optical approaches to fluorescent, spectroscopic, and morphological imaging have made exceptional advances in the last decade. Super-resolution imaging and wide-field multiphoton imaging are now underpinning major advances across the biomedical sciences. While the advances have been startling, the key unmet challenge to date in all forms of optical imaging is to penetrate deeper. A number of schemes implement aberration correction or the use of complex photonics to address this need. In contrast, we approach this challenge by implementing a scheme that requires no a priori information about the medium nor its properties. Exploiting temporal focusing and single-pixel detection in our innovative scheme, we obtain wide-field two-photon images through various turbid media including a scattering phantom and tissue reaching a depth of up to seven scattering mean free path lengths. Our results show that it competes favorably with standard point-scanning two-photon imaging, with up to a fivefold improvement in signal-to-background ratio while showing significantly lower photobleaching.
A high-resolution Shack-Hartmann wavefront sensor has been used for coherent holographic imaging, by computer reconstruction and propagation of the complex field in a lensless imaging setup. The resolution of the images obtained with the experimental data is in a good agreement with the diffraction theory. Although a proper calibration with a reference beam improves the image quality, the method has a potential for reference-less holographic imaging with spatially coherent monochromatic and narrowband polychromatic sources in microscopy and imaging through turbulence.
The structured illumination (SI) technique has already been
well established as a resolution enhancer in many studies and
well demonstrated in many optical imaging systems during
the past decade. The ability to use the SI in incoherent imaging
systems was also introduced, especially in fluorescence
microscopy. In this Letter, we propose and demonstrate a
new approach to combine the SI technique with the recently
innovated motionless incoherent holographic system, called
Fresnel incoherent correlation holography (FINCH), in order
to enhance the resolution beyond the limits achieved in
regular imaging with SI. The results obtained by use of
SI-FINCH were compared against regular imaging, regular
FINCH and SI-imaging.
Single-pixel imaging techniques enable to capture a scene without a direct line of sight to the object, but high-quality imaging has been proven challenging especially in the presence of noisy environmental illumination. Here we present a single-pixel imaging technique that can achieve high-quality images by acquiring their Fourier spectrum. We use phase-shifting sinusoid structured illumination for the spectrum acquisition. Applying inverse Fourier transform to the obtained spectrum yields the desired image. The proposed technique is capable of capturing a scene without a direct view of it. Thus, it enables a feasible placement of detectors, only if the detectors can collect the light signals from the scene. The technique is also a compressive sampling like approach, so it can reconstruct an image from sub-Nyquist measurements. We experimentally obtain clear images by utilizing a detector not placed in direct view of the imaged scene even with noise introduced by environmental illuminations.
Classical statistical optics is revisited with the aim of introducing the concept of spatial statistical optics that places particular focus and special emphasis on spatial statistics of the optical field, rather than on their temporal statistics. The principles of emerging technology of statistical correlation holography based on spatial statistical optics are reviewed, and their unique capabilities for coherence and polarization shaping as well as synthesizing stochastic optical fields with desired statistical properties are introduced.
We report a new technique for the recovery of quantitative phase and amplitude information of an object hidden behind a scattering medium. Two-point intensity correlation measurement together with digital holography principles are utilized for this purpose. The hologram information of the object and a reference beam is scrambled by the presence of a scattering medium in its path. A direct digital holographic recording of this scattered light does not lead to the reconstruction of actual object information. We propose the idea of recovering this hologram information from the spatially fluctuating field of a laser speckle pattern using the intensity correlation, and subsequently apply digital reconstruction of the hologram for recovery of quantitative phase and amplitude information of objects hidden by a random diffuser.
We propose and experimentally demonstrate a technique for the recovery of the wavefront from spatially fluctuating fields using the two point intensity correlation, i.e., fourth order correlation. Assuming spatial ergodicity and Gaussian statistics for the speckle field, we connect the fourth order correlation to the modulus of the corresponding second order correlation. The idea is to retrieve the complex coherence function and consequently the wavefront using the off-axis holography. Application of this technique is demonstrated in the reconstruction of complex field of the object lying behind a weak scatterer. Experimental results of recovery of the complex field of phase objects “vortices” with three values of topological charges are presented.
Optical transmission through complex media such as biological tissue is fundamentally limited by multiple light scattering. Precise control of the optical wavefield potentially holds the key to advancing a broad range of light-based techniques and applications for imaging or optical delivery. We present a simple and robust digital optical phase conjugation (DOPC) implementation for suppressing multiple light scattering. Utilizing wavefront shaping via a spatial light modulator (SLM), we demonstrate its turbidity-suppression capability by reconstructing the image of a complex two-dimensional wide-field target through a highly scattering medium. Employing an interferometer with a Sagnac-like ring design, we successfully overcome the challenging alignment and wavefront-matching constraints in DOPC, reflecting the requirement that the forward- and reverse-propagation paths through the turbid medium be identical. By measuring the output response to digital distortion of the SLM write pattern, we validate the sub-wavelength sensitivity of the system.
The transmission of light through a disordered medium is described in microscopic detail by a high-dimensional matrix. Researchers have now measured this transmission matrix directly, providing a new approach to control light propagation.
Speckle intensity measurements utilized for phase retrieval (PR) are sequentially taken with a digital camera, which introduces quantization error that diminishes the signal quality. Influences of quantization on the speckle intensity distribution and PR are investigated numerically and experimentally in the static wavefront sensing setup. Results show that 3 to 4 bits are adequate to represent the speckle intensities and yield acceptable reconstructions at relatively fast convergence rates. Computer memory requirements may be eased down by 2.4 times if a 4 bit instead of an 8 bit camera is used. This may facilitate rapid speckle data acquisition for dynamic wavefront sensing.
The imaging depth of two-photon excitation fluorescence microscopy is partly limited by the inhomogeneity of the refractive index in biological specimens. This inhomogeneity results in a distortion of the wavefront of the excitation light. This wavefront distortion results in image resolution degradation and lower signal level. Using an adaptive optics system consisting of a Shack-Hartmann wavefront sensor and a deformable mirror, wavefront distortion can be measured and corrected. With adaptive optics compensation, we demonstrate that the resolution and signal level can be better preserved at greater imaging depth in a variety of ex-vivo tissue specimens including mouse tongue muscle, heart muscle, and brain. However, for these highly scattering tissues, we find signal degradation due to scattering to be a more dominant factor than aberration.
We present a modified Gabor-like setup able to recover the complex amplitude distribution of the object wavefront from a set of inline recorded holograms. The proposed configuration is characterized by the insertion of a condenser lens and a spatial light modulator (SLM) into the classical Gabor configuration. The phase shift is introduced by the SLM that modulates the central spot (dc term) in an intermediate plane, without an additional reference beam. Experimental results validate the proposed method and produce superior results to the Gabor method.
In this paper, a novel approach using deep learning-assisted wavefront correction in beam rotation holographic tomography to acquire three-dimensional images of native biological cell samples is described. With digitally recorded holograms, the wavefront aberration is contained in the reconstructed phases. However, there are large computation costs for compensating the phase aberration during the reconstruction. With the aid of a deep convolution network, we present an effective algorithm on the reconstructed phases with sparse data for active wavefront correction. To accomplish this, we developed a Res-Unet scheme to segment the cell region from its background aberration and a deep regression network for the representation of the aberration on Zernike orthonormal basis. Moreover, a sparse data fitting algorithm was used to predict the Zernike coefficients of whole scanning angles from the collected sparse data. As a result, the proposed algorithm is capable of accurately correcting the background aberration and much faster than the original plain algorithm.
The ability to overcome the adverse effect induced by the obstacles within the transmission link is a central challenge in long-distance optical image transmission and is significantly crucial in free-space
optical communication. In this work, we introduce an efficient protocol to realize the robust far-field optical image-signal transmission by modulating the spatial-coherence structure of a partially coherent
random light source. The image information encoded in the spatial-coherence structure can be stably transmitted to the far field and can resist the influence of obstructions within the communication link.
This is due to the self-reconstruction property of the spatial-coherence structure embedded with the cross phase in the far field. We demonstrate experimentally that the image information can be recovered well by measuring the second-order spatial-coherence structure of the obstructed random light in the far field. Our findings open a door for robust optical signal transmission through the complex environment and may find application in optical communication through a turbulent atmosphere.
Quantitative phase imaging (QPI) plays an essential role in exploring properties of objects exemplified by transparent and absorption-free samples. Within the realm of QPI, the holography that records both the amplitude and the phase of optical fields is widely used. However, when the object is obscured by a scattering medium, conventional holographic methods can no longer be used to extract complex-value field information of the object from the directly recorded intensity pattern. To address this challenge, we demonstrate a lensless QPI using holographic methods derived from the coherence theory. Based on the Hanbury Brown-Twiss (HBT) approach, these methods include off-axis holography, phase-shifting holography, and polarization-based phase-shifting holography. We make quantitative comparisons among these three HBT-based holography techniques for the phase imaging through a diffuser in a common experimental setup. In comparison with first two, the third one, which utilizes an orthogonally-polarized light that transmits coaxially with object light as the reference light, upgrades the imaging capability by suppressing noises and augmenting the field of view (FOV). Finally, its outperformance over other two methods is illustrated by imaging a complicated phase sample.
In this Letter, we present the modeling, design, and characterization of a light sheet-based structured light illumination (SLI) light field microscopy (LFM) system for fast 3D imaging, where a digital micromirror device is employed to rapidly generate designed sinusoidal patterns in the imaging field. Specifically, we sequentially obtain uniformly illuminated and structured light field images, followed by post-processing with a new, to the best of our knowledge, algorithm that combines the deconvolution and HiLo algorithms. This enables fast volumetric imaging with improved optical cross-sectioning capability at a speed of 50 volumes per second over an imaging field of 250×250×80µm3 in the x, y, and z axis, respectively. Mathematical models have been derived to explain the performance enhancement due to suppressed background noises. To verify the results, imaging experiments on fluorescence beads, fern spore, and Drosophila brain samples, have been performed. The results indicate that the light sheet-based SLI-LFM presents a fast 3D imaging solution with substantially improved optical cross-sectioning capability in comparison with a standard light sheet-based LFM. The new light field imaging method may find important applications in the field of biophotonics.
The imaging quality of quantitative phase imaging (QPI) based on the transport of intensity equation (TIE) can be improved using a higher-order approximation for defocused intensity distributions. However, this requires mechanically scanning an image sensor or object along the optical axis, which in turn requires a precisely aligned optical setup. To overcome this problem, a computer-generated hologram (CGH) technique is introduced to TIE-based QPI. A CGH generating defocused point spread function is inserted in the Fourier plane of an object. The CGH acts as a lens and grating with various focal lengths and orientations, allowing multiple defocused intensity distributions to be simultaneously detected on an image sensor plane. The results of a numerical simulation and optical experiment demonstrated the feasibility of the proposed method.
Time multiplexing is a super-resolution technique that sacrifices time to overcome the resolution reduction obtained because of diffraction. There are many super resolution methods based on time multiplexing, but all of them require a priori knowledge of the time changing encoding mask, which is projected on the object and used to encode and decode the high-resolution information. In this paper, we present a time multiplexing technique that does not require the a priori knowledge on the projected encoding mask. First, the theoretical concept of the technique is demonstrated; then, numerical simulations and experimental results are presented.
In low light conditions, such as in astronomy and non-invasive bio-imaging applications, the imaging performance is mostly degraded due to shot noise. In this paper, we demonstrate a transport of intensity equation-based technique that uses photon-counting phase imaging. To achieve the phase imaging in photon starved condition, a method proposed by Paganin et al. [J. Micros. 214 (2004) 51] has been used. The method uses the fact that the magnitude of the wavefront curvature determines the quality of the recovered phase image for a given noise level and defocus distance. The effectiveness of the proposed method has been illustrated through simulations and experimental results using inexpensive partially incoherent illumination. The study can find applications in non-invasive phase imaging.
We report a phase shifting holography for coherence waves in the Hanbury Brown and Twiss approach. This technique relies on the wave nature and interference of the coherence waves in the two-point intensity correlation. Experimentation is carried out by recovery of the complex coherence using phase shifting in the intensity correlation.
As an application, imaging of the phase target obscured by a random scattering medium is demonstrated, and the results are presented for three different cases. These results
are also compared with imaging of the targets in absence of the scattering medium by a conventional phase shifting digital holography.
Quantitative phase microscopy (QPM), a technique combining phase imaging and microscopy, enables visualization of the 3D topography in reflective samples, as well as the inner structure or refractive index distribution of transparent and translucent samples. Similar to other imaging modalities, QPM is constrained by the conflict between numerical aperture (NA) and field of view (FOV): an imaging system with a low NA has to be employed to maintain a large FOV. This fact severely limits the resolution in QPM up to 0.82λ/NA, λ being the illumination wavelength. Consequently, finer structures of samples cannot be resolved by using modest NA objectives in QPM. Aimed to that, many approaches, such as oblique illumination, structured illumination, and speckle illumination (just to cite a few), have been proposed to improve the spatial resolution (or the space–bandwidth product) in phase microscopy by restricting other degrees of freedom (mostly time). This paper aims to provide an up-to-date review on the resolution enhancement approaches in QPM, discussing the pros and cons of each technique as well as the confusion on resolution definition claims on QPM and other coherent microscopy methods. Through this survey, we will review the most appealing and useful techniques for superresolution in coherent microscopy, working with and without lenses and with special attention to QPM. Note that, throughout this review, with the term “superresolution” we denote enhancing the resolution to surpass the limit imposed by diffraction and proportional to λ/NA, rather than the physics limit λ/(2n med ), with n med being the refractive index value of the immersion medium.
Quantitative phase imaging (QPI) has emerged as a valuable method for investigating cells and tissues. QPI operates on unlabelled specimens and, as such, is complementary to established fluorescence microscopy, exhibiting lower phototoxicity and no photobleaching. As the images represent quantitative maps of optical path length delays introduced by the specimen, QPI provides an objective measure of morphology and dynamics, free of variability due to contrast agents. Owing to the tremendous progress witnessed especially in the past 10–15 years, a number of technologies have become sufficiently reliable and translated to biomedical laboratories. Commercialization efforts are under way and, as a result, the QPI field is now transitioning from a technology-development-driven to an application-focused field. In this Review, we aim to provide a critical and objective overview of this dynamic research field by presenting the scientific context, main principles of operation and current biomedical applications.
Optical imaging through complex scattering media is one of the major technical challenges with important applications
in many research fields, ranging from biomedical imaging to astronomical telescopy to spatially multiplexed
optical communications. Various approaches for imaging through a turbid layer have been recently
proposed that exploit the advantage of object information encoded in correlations of the random optical fields.
Here we propose and experimentally demonstrate an alternative approach for single-shot imaging of objects
hidden behind an opaque scattering layer. The proposed technique relies on retrieving the interference fringes
projected behind the scattering medium, which leads to complex field reconstruction, from far-field laser speckle
interferometry with two-point intensity correlation measurement. We demonstrate that under suitable conditions,
it is possible to perform imaging to reconstruct the complex amplitude of objects situated at different
depths.
Correlation holography reconstructs three-dimensional
(3D) objects as a distribution of two-point correlations
of the random field detected by two-dimensional detector
arrays. Here, we describe a hybrid method, a combination
of optical and computational channels, to reconstruct the
objects from only a single pixel detector. An experimental
arrangement is proposed as a first step to realizing the technique;
we have simulated the experimental model for both
scalar and vectorial objects. The proposed technique provides
depth focusing and 3D reconstruction with digital
suppression of unwanted frequency spectrum.
We propose and experimentally demonstrate lensless complex amplitude image retrieval through a visually
opaque scattering medium from spatially fluctuating fields using intensity measurement and a phase-retrieval
algorithm. The complex amplitude information of the hidden object is encoded in the form of a real and nonnegative amplitude function represented as an interference pattern. A single charge coupled device (CCD) image of the scattered light collected through a visually opaque optical diffuser contains enough information to digitally regenerate the interference pattern. Furthermore, a lensless configuration is implemented which eliminates any possible aberration effects associated with optical components, and this further has promising applications where the use of imaging optics is not feasible. Experimental results for the recovery of complex fields corresponding to optical vortices of two different topological charges are presented.
We demonstrate imaging of complex amplitude objects through digital holography with phase-structured illumination and bucket detection. The object is sampled with a set of micro-structured phase patterns implemented onto a liquid-crystal spatial light modulator while a bucket detector sequentially records the irradiance fluctuations corresponding to the interference between object and reference beams. Our reconstruction algorithm retrieves the unknown phase information from the full set of photocurrent measurements. Interestingly, the sampling functions can be codified onto the reference beam, so they can be nonlocal with respect to the object. Finally, we show that the system is well-fitted for transmission of the object information through scattering media.
A wavefront sensing method based on a spatial light modulator (SLM) and an incremental binary random sampling (IBRS) algorithm is proposed. In this method, the recording setup is built just by a transmittance SLM and an image sensor. The tested wavefront incident to the SLM plane can be quantitatively retrieved from the diffraction intensities of the wavefront passed through the SLM displaying a IBRS pattern. Because only two modulation states (opaque and transparent) of the SLM are used, the method does not need to know the concrete modulation function of the SLM in advance. In addition by introducing the concept of the incremental random sampling into wavefront sensing, the adaptability of phase retrieving based on the diffraction intensities is significantly improved. To the best of our knowledge, no previous study has used this concept for the same purpose. Some experimental results are given for demonstrating the feasibility of our method.
We propose a method for resolution enhancement of a diffraction limited optical system based on the capture of a set of low resolution images. These images are obtained after projection of an ensemble of unknown speckle patterns on top of the high resolution object that is to be imaged. Each speckle pattern is generated by the same thin (and unknown) diffuser, but illuminated with a slightly different wavelength. From the ensemble of low resolution images, we obtain a system of equations that can be solved in an iterative manner that enables reconstruction of the high resolution object. As a result, we also achieve the projected high resolution speckle patterns used for the encoding.
We introduce a simple but practical method to measure the optical
transmission matrix (TM) of complex media. The optical TM of a complex medium
is obtained by modulating the wavefront of a beam impinging on the complex
medium and imaging the transmitted full-field speckle intensity patterns. Using
the retrieved TM, we demonstrate the generation and linear combination of
multiple foci on demand through the complex medium. This method will be used as
a versatile tool for coherence control of waves through turbid media.
Imaging with optical resolution through and inside complex samples is a
difficult challenge with important applications in many fields. The fundamental
problem is that inhomogeneous samples, such as biological tissues, randomly
scatter and diffuse light, impeding conventional image formation. Despite many
advancements, no current method enables to noninvasively image in real-time
using diffused light. Here, we show that owing to the memory-effect for speckle
correlations, a single image of the scattered light, captured with a standard
high-resolution camera, encodes all the information that is required to image
through the medium or around a corner. We experimentally demonstrate
single-shot imaging through scattering media and around corners using
incoherent light and various samples, from white paint to dynamic biological
samples. Our lensless technique is simple, does not require laser sources,
wavefront-shaping, nor time-gated detection, and is realized here using a
camera-phone. It has the potential to enable imaging in currently inaccessible
scenarios.
The state of the art in the study of laser speckle patterns is reviewed,
with speckle phenomena treated as useful tools or as noise. Statistical
properties of speckle patterns (autocorrelations, spectra), blurred or
integrated, and covered along with speckle patterns in partially
coherent light (polychromatic or quasimonochromatic). Techniques for
minimizing speckle noise in holography are examined in detail. The use
of speckle patterns in information processing and storage and in
interferometry are discussed separately. Speckle photography, electronic
speckle pattern interferometry (ESPI), speckle holography, stellar
speckle interferometry, and speckle interferometry using photographic
recording are among the topics discussed.
Imaging with optical resolution through turbid media is a long
sought-after goal with important applications in deep tissue imaging.
Although extensively studied, this goal was considered impractical until
recently. Adaptive-optics techniques, which can correct weak
aberrations, are inadequate for turbid samples, where light is scattered
to complex speckle patterns with a number of modes greatly exceeding the
number of degrees of control. This conception changed after the
demonstration of coherent focusing through turbid media by
wavefront-shaping, using spatial light modulators. Here, we show that
wavefront-shaping enables wide-field imaging through turbid layers with
incoherent illumination, and imaging of occluded objects using light
scattered from diffuse walls. In contrast to the recently introduced
schemes for imaging through turbid media, our technique does not require
coherent sources, interferometric detection, raster-scanning or off-line
reconstruction. Our results bring wavefront-shaping closer to practical
applications and realize the vision of looking through `walls' and
around corners.
Wavefronts incident on a random phase plate are reconstructed via phase retrieval utilizing axially displaced speckle intensity measurements and the wave propagation equation. Retrieved phases and phase subtraction facilitate the investigations of wavefronts from test objects before and after undergoing a small rotation or deformation without sign ambiguity. Angular displacement (Deltatheta) between incident planar wavefronts is determined from the light source vacuum wavelength (lambda) divided by the fringe spacing (Lambda). Fourier analysis of the wavefront phase difference yields a peak frequency that is inversely proportional to Lambda, and the sign gives the direction of rotation. Numerical simulations confirm the experimental results. In the experiments, the smallest Deltatheta measured is 0.031 degrees . The technique also permits deformation analysis of a reflecting test object under thermal loading. The technique offers simple, high resolution, noncontact, and whole field evaluation of three-dimensional objects before and after undergoing rotation or deformation.
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