Multi-wavelength Digital Holography Research Articles

Technology Selection for Inline Topography Measurement with Rover-Borne Laser Spectrometers.

This work studies enhancing the capabilities of compact laser spectroscopes integrated into space-exploration rovers by adding 3D topography measurement techniques. Laser spectroscopy enables the in situ analysis of sample composition, aiding in the understanding of the geological history of extraterrestrial bodies. To complement spectroscopic data, the inclusion of 3D imaging is proposed to provide unprecedented contextual information. The morphological information aids material characterization and hence the constraining of rock and mineral histories. Assigning height information to lateral pixels creates topographies, which offer a more complete spatial dataset than contextual 2D imaging. To aid the integration of 3D measurement into future proposals for rover-based laser spectrometers, the relevant scientific, rover, and sample constraints are outlined. The candidate 3D technologies are discussed, and estimates of performance, weight, and power consumptions guide the down-selection process in three application examples. Technology choice is discussed from different perspectives. Inline microscopic fringe-projection profilometry, incoherent digital holography, and multiwavelength digital holography are found to be promising candidates for further development.

Double acousto-optic spectral filtering in off-axis multi-wavelength digital holography

Multi-wavelength digital holography (MDH) is a label-free technique for mapping spectral and spatial properties of samples in scientific and industrial applications. Light sources for MDH often include acousto-optic tunable filters (AOTFs) for sequential or simultaneous selection of narrow spectral bands in broadband radiation. We theoretically and experimentally evaluated the influence of the spectral transmission function of single and double AOTFs on the quality of the acquired holograms and proposed ways to improve their characteristics. We analyzed the spatio-spectral properties of the acousto-optic interaction and changed the incident angle of light to reduce the spectral bandwidth and increase the width of the high-contrast area in the interference image. Double spectral filtering using two AOTFs significantly improves the visibility distribution of the interference pattern with peak spectral transmission close to 100% and provides a good compromise between an increase in the coherence length and a decrease in the peak intensity by shifting driving frequency of the second filter. The results of the study provide valuable information for the development of AOTF technology and can be used for both optimal design and adaptive control of AOTF-based tunable light sources in MDH.

Single-shot multiwavelength digital holography using Bragg diffraction of light by several ultrasound waves [Invited

We report on wavelength-multiplexed digital holographic imaging based on simultaneous Bragg diffraction of wideband light by several ultrasound waves of different frequencies in crystalline media. This technique is easy to implement, avoids spectral scanning, and is applicable in various digital holography schemes. It also enables single-shot acquisition of a few spectral fringe patterns by a single monochrome sensor and wavelength demultiplexing of the resulting interferogram. We have assembled a Mach-Zehnder interferometer with an acousto-optical tunable filter operating in the multifrequency mode and have validated the proposed technique by dual-, three-, and four-wavelength quantitative phase imaging of the test samples.

Phase microscopy and surface profilometry by digital holography

Quantitative phase microscopy by digital holography is a good candidate for high-speed, high precision profilometry. Multi-wavelength optical phase unwrapping avoids difficulties of numerical unwrapping methods, and can generate surface topographic images with large axial range and high axial resolution. But the large axial range is accompanied by proportionately large noise. An iterative process utilizing holograms acquired with a series of wavelengths is shown to be effective in reducing the noise to a few micrometers even over the axial range of several millimeters. An alternate approach with shifting of illumination angle, instead of using multiple laser sources, provides multiple effective wavelengths from a single laser, greatly simplifying the system complexity and providing great flexibility in the wavelength selection. Experiments are performed demonstrating the basic processes of multi-wavelength digital holography (MWDH) and multi-angle digital holography (MADH). Example images are presented for surface profiles of various types of surface structures. The methods have potential for versatile, high performance surface profilometry, with compact optical system and straightforward processing algorithms.

Melt pool monitoring in laser beam melting with two-wavelength holographic imaging

Over the past two decades, laser beam melting has emerged as the leading metal additive manufacturing process for producing small- and medium-size structures. However, a key obstacle for the application of this technique in industry is the lack of reliability and qualification mainly because of melt pool instabilities during the laser-powder interaction, which degrade the quality of the manufactured components. In this paper, we propose multi-wavelength digital holography as a proof of concept for in situ real-time investigation of the melt pool morphology. A two-wavelength digital holographic setup was co-axially implemented in a laser beam melting facility. The solidified aluminum tracks and melt pools during the manufacturing of 316L were obtained with full-field one-shot acquisitions at short exposure times and various scanning velocities. The evaluation of the complex coherence factor of digital holograms allowed the quality assessment of the phase reconstruction. The motion blur was analyzed by scanning the dynamic melt pool.

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.

多波长数字全息计量技术综述

多波长数字全息计量技术综述

Multiwavelength digital holography in the presence of vibrations: laterally resolved multistep phase-shift extraction.

With the application of multiwavelength digital holography in rough environments, such as where machine tools are used, we cannot rely on the complete absence of vibrations. The evaluation of temporal phase shifting in multiple interferograms regions allows one to determine and take into account random subwavelength tilt changes during image acquisition of the sensor with respect to a work piece. In this regard, experimental data inside a controlled laboratory setup, as well as data acquired within a five-axis machine tool suffering from random vibrations, are evaluated and affirmed by a simulation model.

Compressive holography with resolution improvement and lensless adjustable magnification

In lensless digital holography the resolving power is limited by factors such as the finite size of recorded area, pixel dimensions and sampling rate of digital acquisition system. These factors acts as a filter that discards high-frequency components with useful information about the analyzed object. In the present work, we propose a method to overcome these limitations to proving the resolution recovery in digital in-line holography by utilizing adjustable magnification in a compressive sensing framework. Here, we model the sensed field as low-pass filtered and projected on a Fresnel base with adjustable magnification and use sparse regularization in an inverse problem approach to image reconstruction. Our method enhances image reconstruction resolution from a single recorded hologram with magnification independent of parameters such as propagation distance and wavelength of light, which enables to be used in applications like multi-wavelength digital holography. In addition, this technique offers high noise immunity and eliminates typical problems of holographically reconstructed images such as twin image and speckle. Therefore, we demonstrate by simulations and experimentally results that our approach significantly improves the quality of reconstructed images when compared with conventional techniques.

Extending the Depth of Field beyond Geometrical Imaging Limitations Using Phase Noise as a Focus Measure in Multiwavelength Digital Holography

Digital holography is a well-established technology for optical quality control in industrial applications. Two common challenges in digital holographic measurement tasks are the ambiguity at phase steps and the limited depth of focus. With multiwavelength holography, multiple artificial wavelengths are used to extend the sensor’s measurement range up to several millimeters, allowing measurements on rough surfaces. To further extend the unambiguous range, additional highly stabilized and increasingly expensive laser sources can be used. Besides that, unwrapping algorithms can be used to overcome phase ambiguities—but these require continuous objects. With the unique feature of numerical refocusing, digital holography allows the numerical generation of an all-in-focus unambiguous image. We present a shape-from-focus algorithm that allows the extension of the depth of field beyond geometrical imaging limitations and yields unambiguous height information, even across discontinuities. Phase noise is used as a focus criterion and to generate a focus index map. The algorithm’s performance is demonstrated at a gear flank with steep slopes and a step sample with discontinuities far beyond the system’s geometrical limit. The benefit of this method on axially extended objects is discussed.

High-precision 3D surface topography measurement using high-stable multi-wavelength digital holography referenced by an optical frequency comb.

High-stable, two-wavelength, digital holography referenced by an optical frequency comb is presented. The technique is demonstrated by using optical waves from an external cavity laser diode phase-locked to a mode-locked Er-doped fiber comb laser light. The mixed beat signal is stabilized at 30MHz by a sensitive proportional integral derivative controller. The tunable continuous-waves stabilized with frequency uncertainty of 5×10-11 are employed to investigate large stepped structures.

Multiwavelength digital holography with wavelength-multiplexed holograms and arbitrary symmetric phase shifts

We propose multiwavelength in-line digital holography with wavelength-multiplexed phase-shifted holograms and arbitrary symmetric phase shifts. We use phase-shifting interferometry selectively extracting wavelength information to reconstruct multiwavelength object waves separately from wavelength-multiplexed monochromatic images. The proposed technique obtains systems of equations for real and imaginary parts of multiwavelength object waves from the holograms by introducing arbitrary symmetric phase shifts. Then, the technique derives each complex amplitude distribution of each object wave selectively and analytically by solving the two systems of equations. We formulate the algorithm in the case of an arbitrary number of wavelengths and confirm its validity numerically and experimentally in the cases where the number of wavelengths is two and three.

Multiwavelength off-axis digital holography with an angle of more than 40 degrees and no beam combiner to generate interference light.

We propose single-shot multiwavelength digital holography with an extremely large incident angle and show the digital recording of multiple object waves at multiple wavelengths with an angle of more than 40 degrees and no beam combiner to generate interference light. Both the avoidance of the crosstalk between the object waves at different wavelengths and the space-bandwidth extension are simultaneously achieved with a single-shot exposure of a monochromatic image sensor and a reference beam even when the wavelength difference between the object waves is small. An extremely large angle can be set by utilizing the signal theory. An angle of up to 40.6 degrees was introduced, interference fringes with an 818 nm period at the wavelength of 532 nm were generated, and an image sensor recorded a two-wavelength-multiplexed hologram. Resolution improvement was experimentally demonstrated using two-wavelength digital holography with the wavelengths of 640 and 532 nm.

High-resolution topograms of fingerprints using multiwavelength digital holography

Fingerprint analysis is a popular identification technique due to the uniqueness of fingerprints and the convenience of recording them. The quality of a latent fingerprint on a surface can depend on various conditions, such as the time of the day, temperature, and the composition of sweat. We first developed latent fingerprints on transparent and blackened glass slides by depositing 1000-nm-thick columnar thin films (CTFs) of chalcogenide glass of nominal composition Ge28Sb12Se60. Then, we used transmission-/reflection-mode multiwavelength digital holography to construct the topograms of CTF-developed fingerprints on transparent/blackened glass slides. The two wavelengths chosen were 514.5 and 457.9 nm, yielding a synthetic wavelength of 4.1624 μm, which is sufficient to resolve pores of depths 1 to 2 μm. Thus, our method can be used to measure the level-3 details that are usually difficult to observe with most other techniques applied to latent fingerprints.

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 single-wavelength and multi-wavelength DH, and a comparison with the most used holographic denoising methods is performed.

Single-Shot Multiwavelength Digital Holography Using Angular Multiplexing and Spatial Bandwidth Enhancement for Extending the Field of View

Single-shot multiwavelength digital holography using angular multiplexing and spatial bandwidth enhancement is proposed to extend the field of view without the superimposition of undesired images. Angular multiplexing is used to record multiple wavelength information with a single-shot exposure of a monochromatic image sensor. Enlargement of the field of view without the superimposition of undesired images is achieved by designing both the object-wave spectra and angle conditions to obtain wide spatial bandwidths in the respective wavelengths with a high signal-to-noise ratio. Theoretical analyses clarified that the field of view is 1.33 times extended in square by the proposed technique in the case of dual wavelengths. Single-shot, multiple wavelengths, and 3D image recording with both the extended field of view and no superimposition of the zeroth-order diffraction wave is experimentally demonstrated while the zeroth-order wave remains, and the color-synthesized image is degraded in the conventional technique. Experimental results agree well to the theoretical analysis.

Single-shot multiwavelength phase unwrapping using a single reference beam and a monochromatic image sensor

We propose single-shot multiwavelength phase unwrapping using a single reference beam and a monochromatic image sensor. A single beam containing multiple wavelengths passes through a reference arm and acts as reference waves. Multiple-wavelength information is multiplexed in the space domain on the monochromatic image sensor and is separated in the spatial frequency domain by utilizing the spatial frequency difference of interference fringes at multiple wavelengths. From a single hologram, multiwavelength phase unwrapping is conducted and an unwrapped phase image is obtained. Both transmission and reflective single-shot multiwavelength digital holography systems using a monochromatic image sensor can be constructed with a simple optical setup employing the proposed technique. Single-shot multiwavelength phase unwrapping by the proposed technique is numerically simulated and experimentally demonstrated.

Holographic volume displacement calculations via multiwavelength digital holography

In this work multiwavelength digital holography is applied to calculate the volume displacement of various topographic surface features. To accurately measure the volume displacement of macroscopic features, long synthetic wavelengths up to several millimeters are generated using tunable IR laser sources. Practical methods of implementation are considered, including geometric effects of both Michelson and Mach-Zehnder recording configurations and error due to wavelength selection.

Applications of digital and analog holography in three-dimensional imaging

Digital and analog holography, along with its many variations, viz., holographic interferometry, holographic microscopy, holographic tomography, multiwavelength digital holography, phase-shifting holography, compressive holography, coherence holography, etc., have become the methods of choice for various metrological applications in three-dimensional (3D) imaging. In this review, we discuss the basic principles of analog and digital holography and the various topics mentioned above, with selected applications to real-world problems. We also discuss other related topics such as dynamic holography, non-Bragg orders, and compressive holographic tomography, nonlinear holography, holographic TV, as well as a nonholographic technique for 3D visualization, viz., transport of intensity imaging. Finally, we expose interested readers to contemporary topics in the area, viz., nonlinear holography and real-time holographic TV.

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.