Bessel beam | EPFL Graph Search

A Bessel beam is a wave whose amplitude is described by a Bessel function of the first kind. Electromagnetic, acoustic, gravitational, and matter waves can all be in the form of Bessel beams. A true Bessel beam is non-diffractive. This means that as it propagates, it does not diffract and spread out; this is in contrast to the usual behavior of light (or sound), which spreads out after being focused down to a small spot. Bessel beams are also self-healing, meaning that the beam can be partially obstructed at one point, but will re-form at a point further down the beam axis. As with a plane wave, a true Bessel beam cannot be created, as it is unbounded and would require an infinite amount of energy. Reasonably good approximations can be made, however, and these are important in many optical applications because they exhibit little or no diffraction over a limited distance. Approximations to Bessel beams are made in practice either by focusing a Gaussian beam with

Coherent imaging from bacteria to multicellular organisms

Séverine Coquoz

Structural and functional imaging of cells, tissues and organisms is crucial for understanding biomedical processes. Fluorescence microscopy is an established tool and has contributed to many discoveries in the life sciences. This technique provides molecular contrast but has limitations inherent to the fluorescent probes, namely phototoxicity and photobleaching. Phase microscopy provides a label-free alternative. Contrast is achieved by translating the phase variations induced by the sample into measurable intensity variations. While phase contrast and differential interference contrast microscopy offer a means to generate contrast without exogenous contrast agents, quantitative phase imaging (QPI) techniques have emerged, enabling the measurement of the phase delays. This measurement yields information on the thickness, refractive index and dry mass of the sample. QPI is thus a powerful tool and has proven its use in various biomedical applications. In the first part of this thesis, new concepts for extending the capabilities of QPI and applications exploiting them are presented. We implement piezo-based Fourier phase microscopy, exhibiting high spatial and temporal sensitivities and resolutions for label-free imaging of cell dynamics at various time scales. The phase is retrieved via phase-shifting intereferometry using a piezo-actuated module allowing very fast tunable phase modulation. Several examples of applications are described. Furthermore, we exploit the phase-to-mass equivalence for investigating the growth of uropathogenic E. coli and the effect of different antibiotics, thereby providing new insights into fundamental mechanisms of bacterial growth. Because QPI methods are limited to optically thin and weakly scattering objects, the second part of this thesis focuses on optical coherence microscopy (OCM). OCM is an interferometric technique providing 3D images of highly scattering biological samples with micrometric resolution and penetration depth of up to several hundreds of micrometers. Compared to optical coherence tomography, OCM uses high numerical aperture (NA) optics to achieve higher transverse resolution, leading to a decrease of the depth of field (DOF). Several methods have been developed to overcome this trade-off between resolution and DOF. Extended-focus OCM (xfOCM) provides a particularly interesting solution by illuminating the sample with a Bessel beam. Moreover, interferometric synthetic aperture microscopy (ISAM) achieves depth-independent resolution through an approximate solution to the inverse scattering problem. We develop extended ISAM (xISAM) to combine the benefits of both approaches, and demonstrate its potential on simulated and experimental data. We also propose a new application of visible OCM (visOCM), a xfOCM system using visible light and a high-NA objective to achieve 3D imaging with sub-micrometer spatial resolution. The capabilities of visOCM are well suited for 3D in vivo imaging of C. elegans, an extensively used model organism in biomedical research. We demonstrate that the anatomy of C. elegans can be visualized with high speed and high contrast down to the sub-cellular level. We further show that visOCM can be employed for imaging age-related morphological changes. Altogether, the methods developed in this thesis pave the way to many applications in the life sciences by featuring label-free imaging at various temporal and spatial scales. Several promising applications are pursued.

EPFL2017

Crossed fiber optic Bessel beams for curvilinear optofluidic transport of dielectric particles

Fabrice Merenda, Kyunghwan Oh, René Salathé

Due to its unique non-diffracting and self-reconstructing nature, Bessel beams have been successfully adopted to trap multiple particles along the beam's axial direction. However, prior bulk-optic based Bessel beams have a fundamental form-factor limitation for in situ, in-vitro, and in-vivo applications. Here we present a novel implementation of Fourier optics along a single strand of hybrid optical fiber in a monolithic manner that can generate pseudo Bessel beam arrays in two-dimensional space. We successfully demonstrate unique optofluidic transport of the trapped dielectric particles along a curvilinear optical route by multiplexing the fiber optic pseudo Bessel beams. The proposed technique can form a new building block to realize reconfigurable optofluidic transportation of particulates that can break the limitations of both prior bulk-optic Bessel beam generation techniques and conventional microfluidic channels. (C) 2013 Optical Society of America

Optical Soc Amer2013

Structural and functional brain imaging using extended-focus optical coherence tomography and microscopy

Paul James Marchand

Neuroimaging techniques aim at revealing the anatomy and functional organisation of cerebral structures. Over the past decades, functional magnetic resonance imaging (fMRI) has revolutionized our understanding of human cerebral physiology through its ability to probe neural activity throughout the entire brain in a non-invasive fashion. Nevertheless, despite recent technological improvements, the spatial resolution of fMRI remains limited to a few hundreds of microns, restricting its use to macroscopic studies. Microscopic imaging solutions have been proposed to circumvent this limitation and have enabled revealing the existence of various cerebral structures, such as neuronal and vascular networks and their contribution to information processing and blood flow regulation within the brain. Optical imaging has proven, through its increased resolution and available contrast mechanisms, to be a valuable complement to fMRI for cellular-scale imaging. In this context, we demonstrate here the capabilities of an extension of optical coherence tomography, termed extended-focus optical coherence tomography (xf-OCT), in imaging cerebral structure and function at high resolution and very high acquisitions rates. Optical coherence tomography is an interferometric imaging technique using a low-coherence illumination source to provide fast, three-dimensional imaging of the back-scattering of tissue and cells. By multiplexing the interferometric ranging over several spectral channels, Fourier-domain OCT performs depth-resolved imaging at very high acquisition rates and high sensitivity. Increasing the lateral resolution of optical systems typically reduces the available depth-of-field and thus hampers this depth multiplexing advantage of OCT. Extended-focus systems aim at alleviating this trade-off between imaging depth and lateral resolution through the use of diffraction-less beams such as Bessel beams, providing high resolution imaging over large depths. The xf-OCT system therefore combines fast acquisition rates and high resolution, both characteristics required to image and study the structure and function of microscopic constituents of cerebral tissue. In this work, we performed functional brain imaging using the ability of xf-OCT to obtain quantita- tive measurements of blood flow in the brain. Changes in blood velocity evoked by neuronal activation were monitored and maps of hemodynamic activity were generated by adapting tools routinely used in fMRI to xf-OCT imaging. Additionally, three novel xf-OCT instruments are presented, wherein the advantages of different spectral ranges are exploited to reach specific imaging parameters. The increased contrast and resolution afforded by an illumination in the visible spectral range was used in two extended-focus optical coherence microscopy (xf-OCM) implementations for subcellular imaging of ex-vivo brain slices and cellular imaging of neurons, capillaries and myelinated axons in the superficial cortex in-vivo. Subsequently, an xf-OCT system is presented, operating in the infrared spectral range, wherein the reduced scattering enabled imaging the smallest capillaries deep in the murine cortex in-vivo.

EPFL2018