Multiwavelength digital holography with autocalibration of phase shifts and artificial wavelengths

Extended autofocusing in dual-wavelength digital holography

In single-wavelength digital holography (DH), the phase wrapping phenomenon limits the total object depth that can be measured due to the requirement for well-resolved phase fringes. To address this limitation, dual-wavelength DH is proposed, enabling measurement of much deeper objects. In single-wavelength DH, because the object depth is limited, the depth of focus (DOF) of DH's optical system at a reconstruction distance is sufficient to cover the object depth. To date, many autofocusing algorithms have been proposed to obtain a correct reconstruction distance. However, in dual-wavelength DH, because the object depth is extended, the DOF at a reconstruction distance cannot cover the extended object depth. The extended object depth can span multiple DOFs, causing partially out of focus object depth. Therefore, in dual-wavelength DH, relying solely on autofocusing algorithms for a single distance is insufficient. But extended autofocusing algorithms, which can autofocus objects through multiple DOFs, are demanded. However, there are no such extended autofocusing algorithms in dual-wavelength DH. Therefore, we propose an extended autofocusing algorithm for dual-wavelength DH based on a correlation coefficient. The proposed algorithm is able to focus the whole object depth when the depth spans multiple DOFs. Through theoretical analysis, simulations, and experiments, the necessity and effectiveness of the proposed algorithm are verified.

History and metrology applications of a game-changing technology: digital holography [Invited]

In digital holography (DH), information in the hologram is recorded and stored in digital format in discrete bits. Like its parent, holography, DH evolved over many years with periods of dormancy and revival. Almost abandoned, multiple times, unanticipated events or developments in separate industries revived it with explosive, quantum jumps, making it useful and popular to a wide audience. Although its history has been treated in many papers and books, the field is dynamic and constantly providing new opportunities. Having been born long before low-cost, fast, powerful digital computers and digital detectors were available, DH was confined to the academic world, where practical applications and commercial opportunities were few if any. Consumer demand that led to low-cost personal computers, high-resolution digital cameras, supporting software, and related products changed the situation drastically by providing every potential researcher affordable, powerful hardware and software needed to apply image processing algorithms and move DH to new practical application levels. In this paper, as part of the sixtieth anniversary of off-axis holography, we include a brief introduction to the fundamentals of DH and examine the history and evolution of DH during its periods of rise and fall. We summarize many new emerging techniques, applications, and potential future applications along with additional details for metrological examples from the authors' research.

Multiple angle digital holography for the shape measurement of the unpainted tympanic membrane

The shape of the tympanic membrane (TM) plays an important role in sound transmission through the ear for hearing. Previously we developed a high-speed holographic system employing a tunable wavelength laser for rapid TM shape measurement. However, the tunable laser illumination was not sufficient to measure the shape of the unpainted TM due to the semi-transparency of the TM and short exposure time of the camera. This paper presents a new multiple angle illumination technique that allows us to use a higher power single wavelength laser to perform shape measurements on the unpainted TM. Accuracy of the new method is demonstrated by a measure of a step gauge provided by the National Institute of Standards and Technology. We successfully applied the new shape measurement method on a fresh postmortem human TM without any paint.

Inline application of digital holography [Invited]

We describe the inline integration of the digital holographic sensor HoloTop in a precision turning plant. A fully automated part-handling system that fulfills the requirements for cycle time and stability was built and integrated into the production process. The inspection system has been running in multishift operation since 2015. For the first time, to the best of our knowledge, the results of one-year, long-term height measurements of 10 million parts under rough production conditions are presented to verify the suitability for industrial use.

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

In the International Thermonuclear Experimental Reactor under construction in southern France, there will be a need for continuous measuring of the erosion at the wall, after the reactor starts operating. A two-wavelength interferometric technique based on digital holography is proposed for the erosion measurement. This technique has the ability to tackle the challenging environmental conditions within the reactor by a long-distance measurement, where a relay optic will be used for imaging the investigated surface on the detector. We will show that the shape measurements of objects located at a distance of more than 20 m from the measuring head can be carried out in a short time (100 μs) by the two-wavelength interferometric technique. A depth accuracy of ±10  μm is achieved.

Surface topography measurement by frequency sweeping digital holography

High-precision measurements of mechanical parts' surface topography represent an essential task in many industry sectors. Examples of such tasks are, e.g., precise alignments of opto-mechanical systems, large object deformation measurements, evaluation of object shape, and many others. Today, the standard method used for such measurements is based on use of coordinate measuring machines (CMMs). Unfortunately, CMMs have severe shortcomings: low measurement point density, long measurement time, risk of surface damage, etc. Indeed, the measurement time rapidly increases with the object complexity and with the density of measurement points. In this paper, we have developed a method for surface topography measurements called "frequency sweeping digital holography" (FSDH). Our developed FSDH method is based on the principles of wavelength scanning interferometry. It allows surface topography measurements of objects with a diameter of several hundred of mms and a high axial accuracy reaching 10 μm. The greatest advantage of the presented FSDH is the fact that the surface topology data are captured in a motionless manner by means of a relatively simple setup. This makes the FSDH method a suitable technique for topography measurements of objects with complex geometries made of common materials (such as metals, plastics, etc.), as well as for the characterization of complex composite structures such as acoustic metamaterials, active acoustic metasurfaces, etc. Measurement method principles, setup details, lateral resolution, and axial accuracy are discussed.

Digital holography on moving objects: interference contrast as a function of velocity and aperture width

Digital holographic measurements on planar moving objects are investigated. We discuss the dependence of the interference contrast on velocity and aperture width for both diffusely and specularly reflecting objects. Using spatial phase shifting, the experimental results for motion in parallel and perpendicular to the optical axis are in good agreement with the theoretical considerations. Measurements with object velocities of up to 100 mm/s are conducted using only less than 1 mW of continuous-wave laser light. These considerations are used to determine the optimal angle between the direction of motion and the illuminating beam, resulting in the lowest decrease in contrast with increasing velocity.

Strong forward-backward asymmetry of stimulated Raman scattering in lithium-niobate-based whispering gallery resonators

We show experimentally and prove theoretically that the pump-power thresholds of stimulated Raman scattering (SRS) in lithium-niobate-based whispering gallery resonators (WGRs) are strongly different for the signal waves propagating in the backward and forward directions with respect to the pump wave. This feature is due to a strong polaritonic effect. It leads to a cascade of alternating forward-backward Raman lines with increasing pump power. The measured polarization and spectral properties of SRS are in good agreement with theory. Similar properties have to be inherent in other WGRs made of polar crystals.

Synthetic-wavelength interferometry improved with frequency calibration and unambiguity range extension

We improve the accuracy of distance measurements with synthetic-wavelength interferometry by referencing the spectral spacing of the free-running light sources to a high-precision radio-frequency oscillator. In addition, we increase the unambiguity range with a time-of-flight technique. Distances to scattering technical surfaces can be measured with micrometer accuracy and an unambiguity range of 1.17 m. The measurement rate amounts to 300 Hz.

Multiple-wavelength-scanning-based phase unwrapping method for digital holographic microscopy

A phase unwrapping approach based on multiple-wavelength scanning is presented for digital holographic microscopy. It unwrapped the ambiguous phase image layer by layer by synthesizing the extracted continuous components from a set of multiple phase images obtained by varying the optical wavelength, where the discontinuities occur at different places and the phase speckle noise presents various distributions in state. The total time for data acquisition is approximately 22 min for 10 wavelengths. The simulation and experimental results demonstrate that the proposed method has a more accurate calculation and better counteraction of phase noise compared with those of previously reported approaches. In addition, the wrapped phase image of the object containing the steps has also been unwrapped successfully.

High-precision three-dimensional shape reconstruction via digital refocusing in multi-wavelength digital holography

Three-dimensional (3D) shape reconstructions and metrology measurements are often limited by depth-of-field constraints. Current focus-detection-based techniques are insufficient to profile out-of-focus 3D objects with high axial accuracy. Extended-focus imaging (EFI) techniques can improve the range and precision of such measurements. By incorporating digital refocusing with multiwavelength interferometry, a holographic imaging solution is presented in this paper to accurately measure 3D objects over a large depth range. Accuracy and repeatability of the proposed EFI technique are validated by digital simulations and refocusing experiments. A reconstruction example demonstrates the feasibility of high-precision 3D measurements of objects deeper than the system's classical depth of field.

Estimation of wavelength difference using scale adjustment in two-wavelength digital holographic interferometry

We propose a method for an estimation of wavelength difference using scale adjustment in two-wavelength digital holographic interferometry. To estimate wavelength difference, two holograms recorded with different wavelengths are reconstructed on the basis of the Fresnel diffraction integral, and pixel sizes in the reconstruction plane, which depend on the wavelength in recording hologram, are analyzed. In the analysis, a zero-padding method and an intensity correlation function are used to adjust pixel sizes in the reconstruction plane and then obtain a wavelength difference given by a difference between the pixel sizes. Theoretical predictions and experimental results are shown to indicate the usefulness of the proposed method in this paper.

Efficient phase unwrapping architecture for digital holographic microscopy

This paper presents a novel phase unwrapping architecture for accelerating the computational speed of digital holographic microscopy (DHM). A fast Fourier transform (FFT) based phase unwrapping algorithm providing a minimum squared error solution is adopted for hardware implementation because of its simplicity and robustness to noise. The proposed architecture is realized in a pipeline fashion to maximize throughput of the computation. Moreover, the number of hardware multipliers and dividers are minimized to reduce the hardware costs. The proposed architecture is used as a custom user logic in a system on programmable chip (SOPC) for physical performance measurement. Experimental results reveal that the proposed architecture is effective for expediting the computational speed while consuming low hardware resources for designing an embedded DHM system.

Focus detection criterion for refocusing in multi-wavelength digital holography

The majority of focus detection criteria reported is based on amplitude contrast. Due to phase wrapping, phase contrast was previously reported unsuitable for focus finding tasks. By taking the advantage of multi-wavelength digital holography, we propose a new focus detection criterion based on phase contrast. Experimental results are presented to prove the feasibility of the developed criterion. Possible applications of the developed technology include inspecting machined surfaces in the auto industry.

Positioning and localization of two-wavelength interferograms for wavefront reconstruction with volume holographic media

This work studies both theoretically and experimentally the formation of the contour interference patterns generated by a two-wavelength real-time holographic interferometer. The resulting contour interference fringes are due to the intersection of the measured surface with parallel, equally spaced planes of constant elevation. The theoretical analysis describes how the spatial frequency of the elevation planes, their angular position, and the localization of the fringes depend on parameters of the optical setup. A theoretical model for fringe localization is developed and confirmed by the experiments, showing a strong dependence of the interferogram position on the slope of the studied surface. Due to the thick Bi(12)TiO(20) crystal employed as the storage medium the Bragg selectivity of the holographic readout is also considered.

Full phase and amplitude control in computer-generated holography

We report what we believe to be the first realization of a computer-generated complex-valued hologram recorded in a single film of photoactive polymer. Complex-valued holograms give rise to a diffracted optical field with control over its amplitude and phase. The holograms are generated by a one-step direct laser writing process in which a spatial light modulator (SLM) is imaged onto a polymer film. Temporal modulation of the SLM during exposure controls both the strength of the induced birefringence and the orientation of the fast axis. We demonstrate that complex holograms can be used to impart arbitrary amplitude and phase profiles onto a beam and thereby open new possibilities in the control of optical beams.