Possibility to break through limitation of measurement range in dual-wavelength digital holography

By using the beat frequency technique,the dual-wavelength digital holography(DWDH)can greatly increase the measurement range of the system.However,the beat frequency technique has a limitation in measurement range.The measurement range is not larger than a synthetic wavelength.Here,to break through this limitation,we propose a novel DWDH method based on the constrained underdetermined equations,which consists of three parts:(i)prove that the constrained underdetermined equation has a unique integer solution,(ii)design an algorithm to search for the unique integer solution,(iii)introduce a third wavelength into the DWDH system,and design a corresponding algorithm to enhance the anti-noise performance of DWDH.As far as we know,it is the first time that we have discovered that the problem of DWDH can belong in a problem of contained underdetermined equations,and it is also the first time that we have given the mathematical proof for breaking through the limitation of the measurement range.A series of results is shown to test the theory and the corresponding algorithms.More importantly,since the principle of proposed DWDH is based on basic mathematical principles,it can be further extended to various fields,such as dual-wavelength microwave imaging and dual-wavelength coherent diffraction imaging.

Quasi noise-free digital holography

One of the main drawbacks of Digital Holography(DH)is the coherent nature of the light source,which severely corrupts the quality of holographic reconstructions.Although numerous techniques to reduce noise in DH have provided good results,holographic noise suppression remains a challenging task.We propose a novel framework that combines the concepts of encoding multiple uncorrelated digital holograms,block grouping and collaborative filtering to achieve quasi noise-free DH reconstructions.The optimized joint action of these different image-denoising methods permits the removal of up to 98%of the noise while preserving the image contrast.The resulting quality of the hologram reconstructions is comparable to the quality achievable with non-coherent techniques and far beyond the current state of art in DH.Experimental validation is provided for both singlewavelength and multi-wavelength DH,and a comparison with the most used holographic denoising methods is performed.

Aperture synthesis based solely on phase images in digital holography

Aperture synthesis is an important approach to improve the lateral resolution of digital holography(DH) techniques.The limitation of the accuracy of registration positions between sub-holograms affects the quality of the synthesized image and even causes the failure of aperture synthesis.It is a major issue in aperture synthesis of DH.Currently intensity images are utilized to find the registration positions of sub-holograms in aperture synthesis.To improve the accuracy of registration positions, we proposed a method based on similarity calculations of the phase images between sub-holograms instead of intensity images.Furthermore, a quantitative indicator, degree of image distortion, was applied to evaluate the synthetic results.Experiments are performed and the results verify that the proposed phase-image-based method is better than the state-of-the-art intensity-image-based techniques in the estimation of registration positions and provides a better synthesized final three-dimensional shape image.

Crack Detection by Optical Voice Recorder Based on Digital Holography

The detection of crack on materials is an important issue in industry.On the contrary to conventional methods,such as manual inspection,sensor detection,and image processing techniques,a new simple method to detect the crack is proposed with optical voice recorder based on digital holography in this paper.Holograms obtained with sound waves passing through the materials are recorded by using the digital holography technique.Temporal behavior of the sound wave passing through the material,which is obtained from these holograms,gives image of crack.In this article,cracks in various materials are determined by the proposed new method,and crack images obtained with this new system are presented.

Digital holography as metrology tool at micronanoscale for soft matter

The appearance of the first laser approximately 12 years after the invention of holography by Gabor(1948)revolutionized the field of optical metrology.In fact,the invention of holographic interferometry enabled the exploitation of interferometry on non-mirror surfaces and full-scale objects.The holography-based measurement methods has been implemented to several industrial systems or in support of R&D with the aim of improving new products in many fields(automotive,aerospace,electronics,etc.).To date,holography has been considered an important measurement tool for non-destructive inspection(NDI),strain-stress measurement,and vibration analysis at various engineering sites.Recently,the new paradigm of Industry4.0 has seen the introduction of new technologies and methods of processing materials as well as the development of manufacturing approaches for the realization of innovative products.For example,direct printing,additive,and bottom-up manufacturing processes are expected to involve new ways of making products in future,and most innovative fabrication processes will be based on the manipulation of soft matter(e.g.,starting from the liquid phase)that will be shaped at the nanoscale.The inherent characteristics of digital holography(DH)make it a powerful and accurate tool for the visualization and testing of final products,as well as for in situ and real-time monitoring and quantitative characterization of the processes involved during the fabrication cycle.This review aims to report on the most useful applications of soft matter,where the capabilities offered by DH,such as three-dimensional(3D)imaging,extended focus,3D tracking,full-field analysis,high sensitivity,and a wide range of measurements from nanometers to centimeters,permit completely non-invasive characterizations on a full-scale.Several holographic experimental results of typical samples are reported and discussed where DH plays a primary role as a tool gauge for soft matter.

Parallax limitations in digital holography: a phase space approach

The viewing direction in Digital Holography can be varied if different parts of a hologram are reconstructed.In this article parallax limitations are discussed using the phase space formalism.An equation for the parallax angle is derived with this formalism from simple geometric quantities.The result is discussed in terms of pixel size and pixel number of the image sensor.Change of perspective is demonstrated experimentally by two numerical hologram reconstructions from different parts of one single digital hologram.

The application of digital holography for accurate three-dimensional localisation of mosquito-bednet interaction

Understanding mosquito interaction with long-lasting insecticidal bednets is crucial in the development of more effective intervention methods to protect humans from malaria transmission.As such,a 240×240×1000 mm laboratory setup for the in-line recording of digital holograms and subsequent in-focus reconstruction and 3D localisation of mosquitoes is presented.Simple bednet background removal methods are used to accurately localise a mosquito obscured by a bednet in 3D coordinates.Simulations and physical data demonstrate that this method is suitable for mosquitoes positioned 3−1000 mm behind a bednet.A novel post-processing technique,involving a cascade-correlation of a Tamura of Intensity focus metric extracted from digitally reconstructed scenes,accurately localises mosquitoes positioned 35−100 mm behind a bednet from a single digital hologram.The result of this study is a scalable digital holographic methodology to examine mosquito-bednet interaction in 3D at a level of accuracy previously only seen in 2D imaging of mosquitoes in a much smaller volume.

Incoherent digital holographic spectral imaging with high accuracy of image pixel registration

Fresnel incoherent correlation holography(FINCH) is a unique three-dimensional(3D) imaging technique which has the advantages of scanning-free,high resolution,and easy matching with existing mature optical systems.In this article,an incoherent digital holographic spectral imaging method with high accuracy of spectral reconstruction based on liquid crystal tunable filter(LCTF) and FINCH is proposed.Using the programmable characteristics of spatial light modulator(SLM),a series of phase masks,none of whose focal lengths changes with wavelength,is designed and made.For each wavelength of LCTF output,SLM calls three phase masks with different phase constants at the corresponding wavelength,and CCD records three holograms.The spectral images obtained by this method have a constant magnification,which can achieve pixel-level image registration,restrain image registration errors,and improve spectral reconstruction accuracy.The results show that this method can not only obtain the 3D spatial information and spectral information of the object simultaneously,but also have high accuracy of spectral reconstruction and excellent color reproducibility.

Vibration parameter measurement using the temporal digital hologram sequence and windowed Fourier transform

Real time digital recording and numerical reconstruction of a temporal digital hologram sequence have become feasible in recent years.They provide a new measurement method which enjoys the valuable advantages of being full-field,noncontact and high precision.In this paper,a combined method of temporal digital hologram sequence and windowed Fourier transform is proposed to measure the kinematic parameters of random vibration.A series of holograms are recorded by CCD camera and the original phase can be reconstructed by Fresnel reconstruction algorithm.The three-dimensional windowed Fourier transform is used to filter noise in phase and extract the instantaneous kinematic parameters of the specimen,such as the displacement,velocity and acceleration.An experiment is conducted on a chloroprene rubber latex membrane.Results demonstrate that the proposed method determines the vibration parameters precisely and enjoys many merits.

Deep learning assisted variational Hilbert quantitative phase imaging

We propose a high-accuracy artifacts-free single-frame digital holographic phase demodulation scheme for relatively lowcarrier frequency holograms-deep learning assisted variational Hilbert quantitative phase imaging(DL-VHQPI).The method,incorporating a conventional deep neural network into a complete physical model utilizing the idea of residual compensation,reliably and robustly recovers the quantitative phase information of the test objects.It can significantly alleviate spectrum-overlapping-caused phase artifacts under the slightly off-axis digital holographic system.Compared to the conventional end-to-end networks(without a physical model),the proposed method can reduce the dataset size dramatically while maintaining the imaging quality and model generalization.The DL-VHQPI is quantitatively studied by numerical simulation.The live-cell experiment is designed to demonstrate the method's practicality in biological research.The proposed idea of the deep learning-assisted physical model might be extended to diverse computational imaging techniques.

Experimental study on the desulfurization and evaporation characteristics of Ca(OH)_(2) droplets

The experiments were conducted to focus on the desulfurization and evaporation characteristics of lime slurry droplets at 298-383 K. We designed an evaporation-reaction chamber with quartz glass windows.The monodisperse slurry droplet stream was injected into the evaporation reaction chamber, and the inlet gas components(air, air + SO_(2)) were introduced into the chamber. We applied the magnified digital in-line holography to measure the droplet parameters and calculated the evaporation rate. The effects of temperature, droplet concentration, and SO_(2) concentration on the evaporation rate of Ca(OH)_(2) droplets were discussed. Moreover, the Ca(OH)_(2) droplets under different experimental conditions were sampled,and the droplets were observed and analyzed using an off-line microscope. The evaporation rate of the Ca(OH)_(2) droplet increased at first, and then decreased during the falling process, and remained constant at last. The average evaporation rate of the Ca(OH)_(2) droplets increased significantly with the temperature increasing.

Iterative projection meets sparsity regularization:towards practical single-shot quantitative phase imaging with in-line holography

Holography provides access to the optical phase.The emerging compressive phase retrieval approach can achieve in-line holographic imaging beyond the information-theoretic limit or even from a single shot by exploring the signal priors.However,iterative projection methods based on physical knowledge of the wavefield suffer from poor imaging quality,whereas the regularization techniques sacrifice robustness for fidelity.In this work,we present a unified compressive phase retrieval framework for in-line holography that encapsulates the unique advantages of both physical constraints and sparsity priors.In particular,a constrained complex total variation(CCTV)regularizer is introduced that explores the well-known absorption and support constraints together with sparsity in the gradient domain,enabling practical high-quality in-line holographic imaging from a single intensity image.We developed efficient solvers based on the proximal gradient method for the non-smooth regularized inverse problem and the corresponding denoising subproblem.Theoretical analyses further guarantee the convergence of the algorithms with prespecified parameters,obviating the need for manual parameter tuning.As both simulated and optical experiments demonstrate,the proposed CCTV model can characterize complex natural scenes while utilizing physically tractable constraints for quality enhancement.This new compressive phase retrieval approach can be extended,with minor adjustments,to various imaging configurations,sparsifying operators,and physical knowledge.It may cast new light on both theoretical and empirical studies.

Three dimensional flow motions in the viscous sublayer

We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale(onthe order of viscous wall units)spanwise meandering motions.The probability density functions(PDFs)ofwall shear stresses show a clear discrepancy in high stress range with those from direct numerical simu-lation(DNS),which is attributed to the unresolved streamwise and spanwise motions by DNS.Moreover,the PDF of Lagrangian particle accelerations yields a stretched exponential shape like that in homogenousisotropic turbulence,indicating strong intermittency in the sublayer.We find a significant fraction of highaccelerations is associated with the small-scale meandering motions.Our study helps explain the effectof sublayer-scale roughness on reducing drag and flow separation reported in the literature.

Suppressing defocus noise with U-net in optical scanning holography

Optical scanning holography(OSH)records both the amplitude and phase information of a 3D object by a 2D scan.To reconstruct a 3D volumetric image from an OSH hologram is difficult,as it suffers from the defocus noise from the other sections.The use of a random phase pupil can convert defocus noise into speckle-like noise,which may require further processing in sectional image reconstruction.In this paper,we propose a U-shaped neural network to reduce this speckle haze.Simulation results show that the proposed method works effectively and efficiently both in simple and complex graphics.

Portable lensless wide-field microscopy imaging platform based on digital inline holography and multi-frame pixel super-resolution

In this paper,an irregular displacement-based lensless wide-field microscopy imaging platform is presented by combining digital in-line holography and computational pixel super-resolution using multi-frame processing.The samples are illuminated by a nearly coherent illumination system,where the hologram shadows are projected into a complementary metal-oxide semiconductor-based imaging sensor.To increase the resolution,a multi-frame pixel resolution approach is employed to produce a single holographic image from multiple frame observations of the scene,with small planar displacements.Displacements are resolved by a hybrid approach:(i)alignment of the LR images by a fast feature-based registration method,and(ii)fine adjustment of the sub-pixel information using a continuous optimization approach designed to find the global optimum solution.Numerical method for phase-retrieval is applied to decode the signal and reconstruct the morphological details of the analyzed sample.The presented approach was evaluated with various biological samples including sperm and platelets,whose dimensions are in the order of a few microns.The obtained results demonstrate a spatial resolution of 1.55 μm on a field-of-view of<30 mm^(2).

Chinese Optics Letters(《中国光学快报》)2009年“Nanophotonics”和“Digital and Computer-Generated Holography”专题征稿启事

(Chinese Optics Letters(《中国光学快报》)计划于2009年10月在正刊上推出'Nanophotonics'('纳米光子学')专题,特向国内外广大读者以及作者征集该方面原创性的研究论文。征稿范围包括:·Metamaterials:negative index,cloaking and nonlinear

End-to-end deep learning framework for digital holographic reconstruction

Digital holography records the entire wavefront of an object,including amplitude and phase.To reconstruct the object numerically,we can backpropagate the hologram with Fresnel–Kirchhoff integralbased algorithms such as the angular spectrum method and the convolution method.Although effective,these techniques require prior knowledge,such as the object distance,the incident angle between the two beams,and the source wavelength.Undesirable zero-order and twin images have to be removed by an additional filtering operation,which is usually manual and consumes more time in off-axis configuration.In addition,for phase imaging,the phase aberration has to be compensated,and subsequently an unwrapping step is needed to recover the true object thickness.The former either requires additional hardware or strong assumptions,whereas the phase unwrapping algorithms are often sensitive to noise and distortion.Furthermore,for a multisectional object,an all-in-focus image and depth map are desired for many applications,but current approaches tend to be computationally demanding.We propose an end-to-end deep learning framework,called a holographic reconstruction network,to tackle these holographic reconstruction problems.Through this data-driven approach,we show that it is possible to reconstruct a noise-free image that does not require any prior knowledge and can handle phase imaging as well as depth map generation.

Correlation Holography with A Single-Pixel Detector:A Review

Correlation holography uses incoherent light to reconstruct holograms.This technique reconstructs objects as distributions of two-point coherence function rather than using optical fields,as in conventional holography.The basic principle of correlation holography is derived from the van Cittert--Zernike theorem and relies on the similarity between the optical field and the coherence functions.Experimental implementation of the correlation holography techniques requires a field or intensity interferometer,and fringe analysis and crosscovariance measurement in these interferometers require a conventional camera with array detectors.With the availability of digitally controlled diffractive elements,it is possible to replace the incoherent light source,such as a rotating ground glass,with a digital source loaded with the random patterns in sequence.Such strategies ease the burden on the detector and allow for correlation holography with a single-pixel detector(SPD)to be used.This review paper discusses a close connection between digital holography and correlation holography.The principles of correlation holography with the SPD are reviewed in detail,and the advantages of using digital sources to mimic incoherent illumination in the correlation holography are examined in the context of three-dimensional and complex field imaging.

Holography in the invisible. From the thermal infrared to the terahertz waves: outstanding applications and fundamental limits

Since its invention,holography has been mostly applied at visible wavelengths in a variety of applications.Specifically,non-destructive testing of manufactured objects was a driver for developing holographic methods and all related ones based on the speckle pattern recording.One substantial limitation of holographic non-destructive testing is the setup stability requirements directly related to the laser wavelength.This observation has driven some works for 15 years:developing holography at wavelengths much longer than visible ones.In this paper,we will first review researches carried out in the infrared,mostly digital holography at thermal infrared wavelengths around 10 micrometers.We will discuss the advantages of using such wavelengths and show different examples of applications.In nondestructive testing,large wavelengths allow using digital holography in perturbed environments on large objects and measure large deformations,typical of the aerospace domain.Other astonishing applications such as reconstructing scenes through smoke and flames were proposed.When moving further in the spectrum,digital holography with so-called Terahertz waves(up to 3 millimeters wavelength)has also been studied.The main advantage here is that these waves easily penetrate some materials.Therefore,one can envisage Terahertz digital holography to reconstruct the amplitude and phase of visually opaque objects.We review some cases in which Terahertz digital holography has shown potential in biomedical and industrial applications.We will also address some fundamental bottlenecks that prevent fully benefiting from the advantages of digital holography when increasing the wavelength.

Deep holography

With the explosive growth of mathematical optimization and computing hardware,deep neural networks(DNN)have become tremendously powerful tools to solve many challenging problems in various fields,ranging from decision making to computational imaging and holography.In this manuscript,I focus on the prosperous interactions between DNN and holography.On the one hand,DNN has been demonstrated to be in particular proficient for holographic reconstruction and computer-generated holography almost in every aspect.On the other hand,holography is an enabling tool for the optical implementation of DNN the other way around owing to the capability of interconnection and light speed processing in parallel.The purpose of this article is to give a comprehensive literature review on the recent progress of deep holography,an emerging interdisciplinary research field that is mutually inspired by holography and DNN.I first give a brief overview of the basic theory and architectures of DNN,and then discuss some of the most important progresses of deep holography.I hope that the present unified exposition will stimulate further development in this promising and exciting field of research.