High-accurate unwrapped phases are demanded in various research fields such as optical imaging optical holography, optical diffraction tomography, and magnetic resonance imaging. However, the ground-truth phase is not accessible due to 2 ambiguity which arises from phase jumps in the wrapped phase. In this study, we propose to improve the accuracy of unwrapping process by increasing the sampling frequency to reconstruct the unwrapped phase with high accuracy for the application of optical imaging. The simulation results show increasing the optical magnification from 4X to 8X enables improvement of the phase estimation accuracy by 51% for a highly refractive object. Experimental results validate the sensitivity of phase estimation on the sampling size. Our approach demonstrates significant achievement in obtaining ground-truth phases for highly refractive objects.

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Microscopic imaging for studying plant-pathogen interactions is limited by its reliance on invasive histological techniques, like clearing and staining, or, for in vivo imaging, on complicated generation of transgenic pathogens. We present real-time 3D in vivo visualization of pathogen dynamics with label-free optical coherence tomography. Based on intrinsic signal fluctuations as tissue contrast we image filamentous pathogens and a nematode in vivo in 3D in plant tissue. We analyze 3D images of lettuce downy mildew infection (Bremia lactucae) to obtain hyphal volume and length in three different lettuce genotypes with different resistance levels showing the ability for precise (micro) phenotyping and quantification of the infection level. In addition, we demonstrate in vivo longitudinal imaging of the growth of individual pathogen (sub)structures with functional contrast on the pathogen micro-activity revealing pathogen vitality thereby opening a window on the underlying molecular processes.@en

We quantify the precision and bias of dynamic light scattering optical coherence tomography (DLS-OCT) measurements of the diffusion coefficient and flow speed for first and second-order normalized autocovariance functions. For both diffusion and flow, the measurement precision and accuracy are severely limited by correlations between the errors in the normalized autocovariance function. We demonstrate a method of mixing statistically independent normalized autocovariance functions at every time delay for removing these correlations. The mixing method reduces the uncertainty in the obtained parameters by a factor of two but has no effect on the standard error of the mean. We find that the precision in DLS-OCT is identical for different averaging techniques but that the lowest bias is obtained by averaging the measured correlation functions before fitting the model parameters. With our correlation mixing method, it is possible to quantify the precision in DLS-OCT and verify whether the Cramer-Rao bound is reached.

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We demonstrate wavenumber-dependent DLS-OCT measurements of collective and self-diffusion coefficients in concentrated silica suspensions across a broad q-range, utilizing a custom home-built OCT system. Depending on the sample polydispersity, either the collective or self-diffusion is measured. The measured collective-diffusion coefficient shows excellent agreement with hard-sphere theory and serves as an effective tool for accurately determining particle sizes. We employ the decoupling approximation for simultaneously measuring collective and self-diffusion coefficients, even in sufficiently monodisperse suspensions, using a high-speed Thorlabs OCT system. This enables particle size and volume fraction determination without the necessity of wavenumber-dependent measurements. We derive a relationship between the particle number-based polydispersity index and the ratio of self and collective mode amplitudes in the autocorrelation function and utilize it to measure the particle number-based polydispersity index. Notably, the polydispersity determined in this manner demonstrates improved sensitivity to smaller particle sizes compared to the standard intensity-based DLS cumulant analysis performed on dilute samples.

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We demonstrate lensless single-shot dual-wavelength digital holography for high-speed 3D imaging in industrial inspection. Single-shot measurement is realized by combining off-axis digital holography and spatial frequency multiplexing of the two wavelengths on the detector. The system has 9.1 µm lateral resolution and a 50 µm unambiguous depth range. We determine the theoretical accuracy of off-axis dual-wavelength phase reconstruction for the case of shot-noise-limited detection. Experimental results show good agreement with the proposed model. The system is applied to industrial metrology of calibrated test samples and chip manufacturing.@en

Optical coherence tomography (OCT) can image deep inside scattering plant tissue with micrometer resolution. However, conventional OCT lacks specificity to distinguish plant tissue from pathogen tissue. Here we show how dynamic OCT (dOCT) creates functional contrast of Bremia, a downy mildew in lettuce, based on sub-resolution dynamic activity inside the tissue. We demonstrate its applicability for disease resistance quantification and longitudinal study of pathogen growth.@en

The complex refractive index is analyzed by measuring its scattering attenuation µ_s, group index n_g, and group velocity dispersion (GVD) for 100 nm diameter silica nanoparticles dispersed in water. The experiments were performed for wavelengths between 410 nm and 930 nm. The experimental results were compared with different mixing models for the complex refractive index of colloidal suspensions. The group index linearly scaled with the volume fraction both in experiment and for all tested models. It was found that the GVD has a nonlinear dependence on volume fraction in agreement with the coupled dipole model of Parola et al. [J. Chem. Phys. 141, 124902 (2014)] The scattering attenuation is in good agreement with both the coupled dipole model and the low frequency quasi-crystalline approximation [J. Electromagn. Waves Appl. 2, 757 (1988)] that take particle correlations into account. With an iterative fitting procedure of all the data based on both the coupled dipole model and the quasi-crystalline approximation, the refractive index, porosity, and size of the nanoparticles were determined. We determined that the coupled dipole model is in best agreement with the data.

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Phase-preserving spectral estimation optical coherence tomography (SE-OCT) enables combining axial resolution improvement with computational depth of focus (DOF) extension. We combine SE-OCT with interferometric synthetic aperture microscopy (ISAM) to obtain a high 3D resolution over a large depth range with a narrow bandwidth visible light super-luminescent diode (SLD). SE-OCT gives a five times axial resolution improvement to 1.5 micrometer. The combination with ISAM gives a sub-micron lateral resolution over a 300 micrometer axial range, 12 times the conventional DOF. The results show that phase-preserving SE-OCT is sufficiently accurate for coherent post-processing, enabling the use of cost-effective SLDs in the visible light range for high spatial resolution OCT.

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The group index, n_g , group velocity dispersion (GVD), and scattering attenuation coefficient, µ_s, were measured for dilutions of glycerol, ethanol, and Intralipid 20% with water. Experiments were performed with a supercontinuum laser based Mach–Zehnder spectroscopic interferometry setup for wavelengths between 400 and 930 nm. All optical properties could be retrieved from a single calibrated measurement of the interference spectrum. Scattering attenuation was determined from the envelope of the interference. The group index and GVD were retrieved from the unwrapped spectral phase. It was found that the group indices of glycerol and ethanol dilutions are in accordance with the Lorentz–Lorenz mixing formula. The scattering attenuation matches well to a semi-empirical model based on the Twerksy effective packing fraction.

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We have implemented number fluctuation dynamic light scattering optical coherence tomography (OCT) for measuring extremely slow, sub-diffusion flows of dilute particle suspensions using the second-order autocovariance function. Our method has a lower minimum measurable velocity than conventional correlation-based OCT or phase-resolved Doppler OCT, as the velocity estimation is not affected by the particle diffusion. Similar to non-dilute correlation-based OCT, our technique works for any Doppler angle. With our analysis we can quantitatively determine the concentration of particles under flow. Finally, we demonstrate 2D sub-diffusion flow imaging with a scanning OCT system at high rate by performing number fluctuation correlation analysis on subsequent B-scans@en

Phase-preserving spectral estimation optical coherence tomography (SE-OCT) enables combining axial resolution improvement with computational depth of field (DOF) extension. We show that the combination of SE-OCT with interferometric synthetic aperture microscopy (ISAM) and computational adaptive optics (CAO) results in high 3D resolution over a large depth range for an OCT system with a narrow bandwidth visible light super-luminescent diode (SLD). SE-OCT results in up to five times axial resolution improvement from 8 µm to 1.5 µm. The combination with ISAM gives a sub-micron lateral resolution over a 400 µm axial range, which is at least 16 times the conventional depth of field. CAO can be successfully applied after SE and ISAM and removes residual aberrations, resulting in high quality images. The results show that phase-preserving SE-OCT is sufficiently accurate for coherent post-processing, enabling the use of cost-effective SLDs in the visible light range for high spatial resolution OCT.

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We show number fluctuations dynamic light scattering optical coherence tomography (OCT) for measuring extremely slow, sub-diffusion flows of dilute particle suspensions. Our method removes the minimum measurable velocity limitation of conventional correlation-based or phase-resolved Doppler OCT, set by flowing particles’ Brownian motion. Our technique works for any Doppler angle, is applicable to 2D flow imaging with scanning OCT systems and can be used to determine concentration of particles under flow.

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We show number fluctuations dynamic light scattering optical coherence tomography (OCT) for measuring extremely slow, sub-diffusion flows of dilute particle suspensions. Our method removes the minimum measurable velocity limitation of conventional correlation-based or phase-resolved Doppler OCT, set by flowing particles’ Brownian motion. Our technique works for any Doppler angle, is applicable to 2D flow imaging with scanning OCT systems and can be used to determine concentration of particles under flow.

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Phase sensitive optical coherence tomography (OCT) is able to measure small axial motion at the level of ten nanometer. However, when interfaces are located close to each other, the phase of one interface may influence the phase of the other interface. This spectral leakage hampers the ability to see relative motion between structures within the sample, especially when the separation is below the axial resolution. Spectral estimation OCT (SE-OCT) based on IAA can not only improve the axial resolution beyond the conventional bandwidth limitation, but also reduce this spectral leakage. Here we show accurate reconstruction of the vibration of an interface at sub-resolution distance from a high-intensity interface with a different vibration frequency. Phase preserved IAA successfully reduces spectral leakage and outperforms conventional DFT-based reconstruction methods.

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Organ-on-chip (OoC) systems are novel microfluidic microsystems that combine the advantages of well-characterised human cells with the benefits of engineered, physiological-like microenvironments manufactured in the system. The extracellular matrix (ECM) is the natural microenvironment of cells in the human body responsible for providing the appropriate stimuli to cells to control cell processes such as proliferation, migration, and apoptosis. OoCs can mimic the ECM, via channels and porous membranes, by providing the cells with physiological-like mechanical stimuli governed by the fluid dynamics in the system [1]. Understanding the fluid dynamics in OOC can aid in fine-tuning the stimuli sensed by the cultured cells, understanding cell behavior and cell fate. The current state of the art methods for characterizing fluid dynamics in the OoC systems are simulations, theoretical calculations, and empirical observations, therefore a quantitative characterization technique is lacking. Optical coherence tomography (OCT) has been used in previous studies to measure omnidirectional flow velocities in flow systems [2]. In this study, we measured the flow in a cuvette using a Thorlabs GANYMEDE II HR series (high axial resolution of 3 mm in air) spectral domain OCT system. We made quantitative 2D flow measurements using the phase-resolved Doppler method. This work was then extended to extract flow dynamics, in the Bi/ond inCHIPit using titania scattering nanoparticles, which would be a novel way of flow characterization in the field of OOC. The results are compared to the theoretical Hagen-Poiseuille equations and COMSOL simulations and found to be in good agreement. The results of the study were further extended to determine the shear stress experienced by the cells in the culture well of the OoC. @en

We show scanning dynamic light scattering optical coherence tomography (OCT) omnidirectional flow measurements. Our method improves the velocity measurement limit over conventional correlation-based or phase-resolved Doppler OCT by more than a factor of 2. Our technique is applicable without a-priori knowledge of the flow geometry as our method works both for non-zero Doppler angle and non-ideal scan alignment. In addition, the method improves the particle diffusion coefficient estimation for particles under flow.

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We report on omnidirectional flow measurements using B-scan correlationbased OCT. Our method extends velocity measurement limits of OCT and provides improvements over the conventional correlation-based or phase-resolved Doppler flow measurement techniques.

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Properties of powders produced from drying solute-containing droplets arise from the dynamic redistribution of solute during drying. While insights on the dynamic redistribution are instrumental for the rational design of powders and for the optimized operation of equipment such as spray dryers, experimental techniques that allow measuring the spatio-temporal concentration of solute in drying droplets are scarce. In this work, we explore and demonstrate the use of optical coherence tomography (OCT) to measure the spatio-temporal concentration of solute in drying droplets and the development of a solidifying shell at the liquid-air interface, using aqueous droplets of maltodextrin as a model system. This work provides a solid foundation for the use of OCT to quantify the dynamic redistribution of solute and link it to the development of the morphology of the produced particles and agglomerates.

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We show scanning dynamic light scattering optical coherence tomography (OCT) omnidirectional flow measurements. Our method improves the velocity measurement limit over conventional correlation-based or phase-resolved Doppler OCT by more than a factor of 2. Our technique is applicable without a-priori knowledge of the flow geometry as our method works both for non-zero Doppler angle and non-ideal scan alignment. In addition, the method improves the particle diffusion coefficient estimation for particles under flow.

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Spectral estimation can improve axial resolution in optical coherence tomography over its traditional limit. Contrary to sparsity-based methods, the iterative adaptive approach combines resolution enhancement with good tissue contrast reconstruction.

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