Feasibility study of digital holography for erosion measurements under extreme environmental conditions inside the International Thermonuclear Experimental Reactor tokamak [invited]

Convenient dual-wavelength digital holography based on orthogonal polarization strategy with a Wollaston prism

Dual-wavelength digital holography effectively expands the measurement range of digital holography, but it increases the complexity of optical system due to non-common-path of two wavelengths. Here, by using orthogonal polarization strategy, we present a dual-wavelength digital holography based on a Wollaston prism (DWDH-WP) to separate the reference beams of two wavelengths and realize the common-path of two wavelengths. A Wollaston prism is inset into the reference beam path of the off-axis digital holography system, so two orthogonal-polarized reference beams of two different wavelengths separated at different directions are generated. Then a dual-wavelength multiplexed interferogram with orthogonal interference fringes is captured by using a monochrome camera, in which both the polarization orientations and the interference fringe orientations of two wavelengths are orthogonal, so the spectral crosstalk of two wavelengths with arbitrary wavelength difference can be avoided. Compared with the existing DWDH method, the proposed DWDH-WP method can conveniently realize the common-path of the reference beams of two wavelengths, so it reveals obvious advantages in spectral separation, spectral crosstalk, system simplification, and adjustment flexibility. Both effectiveness and flexibility of the proposed DWDH-WP method are demonstrated by the phase measurement of the HeLa cell and vortex phase plate.

Advantages of holographic imaging through fog

In this paper, we demonstrate digital holographic imaging through a 27-m-long fog tube filled with ultrasonically generated fog. Its high sensitivity makes holography a powerful technology for imaging through scattering media. With our large-scale experiments, we investigate the potential of holographic imaging for road traffic applications, where autonomous driving vehicles require reliable environmental perception in all weather conditions. We compare single-shot off-axis digital holography to conventional imaging (with coherent illumination) and show that holographic imaging requires 30 times less illumination power for the same imaging range. Our work includes signal-to-noise ratio considerations, a simulation model, and quantitative statements on the influence of various physical parameters on the imaging range.

Single-shot wide-field topography measurement using spectrally multiplexed reflection intensity holography via space-domain Kramers-Kronig relations

Surface topology measurements of micro- or nanostructures are essential for both scientific and industrial applications. However, high-throughput measurements remain challenging in surface metrology. We present single-shot full-field surface topography measurement using Kramers-Kronig holographic imaging and spectral multiplexing. Three different intensity images at different incident angles were simultaneously measured with three different colors, from which a quantitative phase image was retrieved using spatial Kramers-Kronig relations. A high-resolution topographic image of the sample was then reconstructed using synthetic aperture holography. Various patterned structures at the nanometer scale were measured and cross-validated using atomic force microscopy.

Roadmap on digital holography [Invited]

This Roadmap article on digital holography provides an overview of a vast array of research activities in the field of digital holography. The paper consists of a series of 25 sections from the prominent experts in digital holography presenting various aspects of the field on sensing, 3D imaging and displays, virtual and augmented reality, microscopy, cell identification, tomography, label-free live cell imaging, and other applications. Each section represents the vision of its author to describe the significant progress, potential impact, important developments, and challenging issues in the field of digital holography.

Theoretical analysis of surface-shape-induced decorrelation noise in multi-wavelength digital holography

This paper presents analytical modelling for describing the speckle noise decorrelation in phase data from two- or multiple-wavelength digital holography. A novel expression for the modulus of the coherence factor is proposed for the case of two-wavelength speckle decorrelation from imaging roughness and surface shape through an optical system. The expression permits us to estimate the speckle decorrelation phase noise in surface shape measurements. The theoretical analysis is supported by realistic simulations including both the surface roughness and shape. The results demonstrate the very good agreement between the modulus of the coherence factor estimated with the simulation and the one calculated with theory.

Behavior of CAD/CAM ceramic veneers under stress: A 3D holographic study

OBJECTIVES

Ceramic veneers restorations may undergo damages, such as cracks, fractures, or debonding. Full-field measurements must be carried out in order to visualize and analyze the strain fields. This paper demonstrates that digital holography permits to investigate the mechanical behavior under stress of a natural incisor and a natural incisor reconstructed with CAD/CAM ceramic veneer.

METHODS

The facial surface of a maxillary central incisor is prepared to receive a monolithic ceramic reconstruction manufactured using a chairside computer-aided design and computer aided manufacturing (CAD/CAM) system (Cerec AC® system, Sirona Dental System®, Bensheim, Germany). One incisor is kept intact for comparison. The samples are sectioned longitudinally to obtain a planar observation of the region of interest. A mechanical indentation head and digital holographic set-ups permit a full-field, contact-less and single-shot measurement of the three-dimensional displacement fields at the surface of the tooth sample when subjected to load. Stain fields are then estimated and comparison of the results between two samples can be carried out.

RESULTS

3D displacement, fields and strain fields are measured and highlight the behavior of the region of interest in three directions of space for the ceramic veneer and the natural incisor. The strain maps reveal the local behavior, especially the concentration or the sudden change in strain. The transition zones are clearly observed, particularly for the veneered sample.

CONCLUSION

Digital holography highlights the localization of stress concentration zones in regions of interest and yields comparative analysis between samples with different tooth preparations.

SIGNIFICANCE

holography permits to visualize and compare the mechanical response of the ceramic veneer and natural tooth. This helps choosing the mechanical properties of the bonding interface.

A digital holography ex situ measurement characterization of plasma-exposed surface erosion from an electrothermal arc source

Digital holography has been proposed to fulfill a need for an imaging diagnostic capable of in situ monitoring of surface erosion caused by plasma-material interaction in nuclear fusion devices. A digital holography diagnostic for 3D surface erosion measurement has been developed at Oak Ridge National Laboratory with the goal of deployment on a plasma device. A proof-of-concept in situ demonstration is planned which would involve measurement of plasma erosion on targets exposed to an electrothermal arc source. This work presents the results of an ex situ characterization of the capability and limitations of holographic imaging of targets exposed to the arc source. Targets were designed to provide a fiducial for comparison of deformed and unaffected areas. The results indicated that the average net erosion was ∼150 nm/plasma exposure, which is expected to be within the diagnostic's measurement capacity. Surface roughness averages determined by holographic image analysis showed good agreement with measurements taken with a profilometer. The limit of the holography diagnostic's x-y spatial resolution was characterized by comparison with scanning electron microscope imaging.

Measurements of dynamic surface changes by digital holography for in situ plasma erosion applications

There are currently few viable diagnostic techniques for in situ measurement of plasma facing component erosion. Digital holography is intended to fill this gap. Progress on the development of single and dual CO₂ laser digital holography diagnostics for in situ plasma facing component erosion is discussed. The dual laser mode's synthetic wavelength allows the measurable range to be expanded by a factor of ∼400 compared to single laser digital holography. This allows the diagnostic to measure surface height changes of up to 4.5 μm in single laser mode and up to 2 mm in dual laser mode. Results include ex situ measurements of plasma eroded targets and also dynamic measurements of nm and μm scale motion of a target mounted on a precision translation stage. Dynamic measurements have successfully been made with the system operating in both single and dual laser modes, from ∼50 nm to ∼4 μm in single laser mode and up to ∼400 μm in dual laser mode (limited only by the stage speed and camera acquisition duration). These results demonstrate the feasibility of using digital holography to characterize plasma facing component erosion dynamically, i.e., during plasma exposure. Results of proof-of-principle in situ digital holographic measurements of targets exposed to an electrothermal arc plasma source are presented.

Precise building deformation measurement using holographic multipoint replication

For the active control of large-scale structures, especially high-rise buildings and bridges, fast and accurate measurement of local deformations is required. We present a highly accurate and fast vision-based measurement technique and, to the best of our knowledge, first experimental results for the control of an adaptive-structures prototype frame, equipped with hydraulic actuators. Deformations are detected at multiple discrete points, based on a photogrammetric approach with additional holographic spot replication. The replication leads to effective averaging of most error contributions, especially discretization and photon noise. Measurements over a distance of 11.4 m result in a measurement uncertainty of 0.0077 pixel (corresponding to 0.055 mm in object space).