Endoscopic pulsed digital holography for 3D measurements

Ptycho-endoscopy on a lensless ultrathin fiber bundle tip

Synthetic aperture radar (SAR) utilizes an aircraft-carried antenna to emit electromagnetic pulses and detect the returning echoes. As the aircraft travels across a designated area, it synthesizes a large virtual aperture to improve image resolution. Inspired by SAR, we introduce synthetic aperture ptycho-endoscopy (SAPE) for micro-endoscopic imaging beyond the diffraction limit. SAPE operates by hand-holding a lensless fiber bundle tip to record coherent diffraction patterns from specimens. The fiber cores at the distal tip modulate the diffracted wavefield within a confined area, emulating the role of the 'airborne antenna' in SAR. The handheld operation introduces positional shifts to the tip, analogous to the aircraft's movement. These shifts facilitate the acquisition of a ptychogram and synthesize a large virtual aperture extending beyond the bundle's physical limit. We mitigate the influences of hand motion and fiber bending through a low-rank spatiotemporal decomposition of the bundle's modulation profile. Our tests demonstrate the ability to resolve a 548-nm linewidth on a resolution target. The achieved space-bandwidth product is ~1.1 million effective pixels, representing a 36-fold increase compared to that of the original fiber bundle. Furthermore, SAPE's refocusing capability enables imaging over an extended depth of field exceeding 2 cm. The aperture synthesizing process in SAPE surpasses the diffraction limit set by the probe's maximum collection angle, opening new opportunities for both fiber-based and distal-chip endoscopy in applications such as medical diagnostics and industrial inspection.

Quantitative phase imaging based on holography: trends and new perspectives

In 1948, Dennis Gabor proposed the concept of holography, providing a pioneering solution to a quantitative description of the optical wavefront. After 75 years of development, holographic imaging has become a powerful tool for optical wavefront measurement and quantitative phase imaging. The emergence of this technology has given fresh energy to physics, biology, and materials science. Digital holography (DH) possesses the quantitative advantages of wide-field, non-contact, precise, and dynamic measurement capability for complex-waves. DH has unique capabilities for the propagation of optical fields by measuring light scattering with phase information. It offers quantitative visualization of the refractive index and thickness distribution of weak absorption samples, which plays a vital role in the pathophysiology of various diseases and the characterization of various materials. It provides a possibility to bridge the gap between the imaging and scattering disciplines. The propagation of wavefront is described by the complex amplitude. The complex-value in the complex-domain is reconstructed from the intensity-value measurement by camera in the real-domain. Here, we regard the process of holographic recording and reconstruction as a transformation between complex-domain and real-domain, and discuss the mathematics and physical principles of reconstruction. We review the DH in underlying principles, technical approaches, and the breadth of applications. We conclude with emerging challenges and opportunities based on combining holographic imaging with other methodologies that expand the scope and utility of holographic imaging even further. The multidisciplinary nature brings technology and application experts together in label-free cell biology, analytical chemistry, clinical sciences, wavefront sensing, and semiconductor production.

Real-time holographic lensless micro-endoscopy through flexible fibers via fiber bundle distal holography

Fiber-based micro-endoscopes are a critically important tool for minimally-invasive deep-tissue imaging. However, current micro-endoscopes cannot perform three-dimensional imaging through dynamically-bent fibers without the use of bulky optical elements such as lenses and scanners at the distal end, increasing the footprint and tissue-damage. Great efforts have been invested in developing approaches that avoid distal bulky optical elements. However, the fundamental barrier of dynamic optical wavefront-distortions in propagation through flexible fibers limits current approaches to nearly-static or non-flexible fibers. Here, we present an approach that allows holographic, bend-insensitive, coherence-gated, micro-endoscopic imaging using commercially available multi-core fibers (MCFs). We achieve this by adding a partially-reflecting mirror to the distal fiber-tip, allowing to perform low-coherence full-field phase-shifting holography. We demonstrate widefield diffraction-limited reflection imaging of amplitude and phase targets through dynamically bent fibers at video-rate. Our approach holds potential for label-free investigations of dynamic samples.

Multiple phase extraction using graphics processing unit assisted unitary transformation method in digital holographic interferometry

The article presents a method to estimate multiple phase maps from a moiré fringe signal obtained using digital holographic interferometry. The proposed method uses a unitary transformation based signal subspace approach, and shows high robustness against noise. In addition, the method facilitates the estimation of multiple phase maps in a single shot operation without the need for spectral filtering or multiple images. The computational efficiency of the method was improved by a high performance implementation using a graphics processing unit. The practical utility of the proposed method is demonstrated using simulation and experiment results.

Pulsed laser source digital holography efficiency measurements

In this paper, a 1064 nm pulsed laser source and a short-wave IR (SWIR) camera are used to measure the total system efficiency associated with a digital holography system in the off-axis image plane recording geometry. At a zero path-length difference between the signal and reference pulses, the measured total system efficiency (15.9%) is consistent with that previously obtained with a 532 nm continuous-wave laser source and a visible camera [Appl. Opt.58, G19 (2019)APOPAI0003-693510.1364/AO.58.000G19]. In addition, as a function of the temporal delay between the signal and reference pulses, the total system efficiency is accurately characterized by a component efficiency, which is formulated from the ambiguity function. Even with multimode behavior from the pulsed laser source and substantial dark current noise from the SWIR camera, the system performance is accurately characterized by the resulting ambiguity efficiency.

Simultaneous 3D measurement of deformation and its first derivative with speckle pattern interferometry and shearography

Digital speckle pattern interferometry and digital shearography are widely used in nondestructive testing due to their advantages of high speed, full field, and high sensitivity. However, traditional speckle pattern interferometry and shearography can only measure a single variable of deformation or strain. This study presents a modified common measurement setup that can simultaneously measure deformation and its first derivative. In the optical setup, a reference beam is introduced behind the shearing device to interfere with the object beams, thereby simplifying the spectrum and improving the quality of the phase maps. Then the spatial carrier technology is used to extract phase and achieve dynamic measurements. The proposed system also expands the measurement range using a ${4}f$4f system. This study also presents a 3D optical setup based on the 1D system, which is more suitable for practical measurement applications in industrial areas. Theoretical derivation and experimental results are described and presented.

Speckle illumination holographic non-scanning fluorescence endoscopy

Optical sectioning endoscopy such as confocal endoscopy offers capabilities to obtain three-dimensional (3D) information from various biological samples by discriminating between the desired in-focus signals and out-of-focus background. However, in general confocal images are formed through point-by-point scanning and the scanning time is proportional to the 3D space-bandwidth product. Recently, structured illumination endoscopy has been utilized for optically sectioned wide-field imaging, but it still needs axial scanning to acquire images from different depths of focal plane. Here, we report wide-field, multiplane, optical sectioning endoscopic imaging, incorporating 3D active speckle-based illumination and multiplexed volume holographic gratings, to simultaneously obtain images of fluorescently labeled tissue structures from different depths, without the need of scanning. We present the design, and implementation, as well as experimental data, demonstrating this endoscopic system's ability to obtain optically sectioned multiplane fluorescent images of tissue samples, with cellular level resolution in wide-field fashion, and no need for mechanical or optical axial scanning.(A) Schematic drawing of the SIHN endoscopy to simultaneously acquire multiplane images from different depths. (B) Uniform, and (C) SIHN illuminated images of standard fluorescence beads (25 μm in diameter) for the two axial planes. (D) Intensity profile on fluorescently labeled signal (ie, in-focus) and background (ie, out-of-focus) of microspheres.

Online 3D Displacement Measurement Using Speckle Interferometer with a Single Illumination-Detection Path

Measurement systems for online nondestructive full-field three-dimensional (3D) displacement based on the single-shot and multiplexing techniques attract more and more interest, especially throughout the manufacturing industries. This paper proposes an accurate and easy-to-implement method based on an electronic speckle pattern interferometer (ESPI) with single illumination-detection path to realize the online nondestructive full-field 3D displacement measurement. The simple and compact optical system generates three different sensitivity vectors to enable the evaluation of the three orthogonal displacement components. By applying the spatial carrier phase-shifting technique, the desired information can be obtained in real time. The theoretical analysis and the measurement results have proven the feasibility of this ESPI system and quantified its relative measurement error.

Talbot multi-focal holographic fluorescence endoscopy for optically sectioned imaging

A wide-field multi-plane endoscopic system incorporating multiplexed volume holographic gratings and Talbot illumination to simultaneously acquire optically sectioned fluorescence images of tissue structures from different depths is presented. The proposed endoscopic system is configured such that multiple Talbot-illumination planes occur inside a volumetric sample and serve as the input focal planes for the subsequent multiplexed volume holographic imaging gratings. We describe the design, implementation, and experimental data demonstrating this endoscopic system's ability to obtain optically sectioned multi-plane fluorescent images of tissue samples in wide-field fashion without scanning in lateral and axial directions.

Single-shot dual-wavelength phase unwrapping in parallel phase-shifting digital holography

We propose a single-shot phase-unwrapping method using two wavelengths in parallel phase-shifting digital holography (PPSDH). The proposed method enables one to solve the phase ambiguity problem in PPSDH. We conducted an experiment of the proposed method using two lasers whose wavelengths are 473 and 532 nm. An object having about 1.9 μm step, which is 7.1 times larger than the half wavelength of one of the lasers (266 nm), was fabricated by using vapor deposition of aluminum. Single-shot measurement of the height of the object was successfully demonstrated, and the validity of the proposed method was verified.

Multiwavelength parallel phase-shifting digital holography using angular multiplexing

We propose a single-shot digital holography for recording multiwavelength and complex amplitude information by using a single monochromatic image sensor. The zeroth-order wave and conjugate image in each wavelength are removed from a recorded single hologram by applying parallel phase-shifting interferometry. Angular multiplexing is utilized to record the complex amplitude of an object wave in each wavelength separately, and no color filter is required. The effectiveness of the proposed technique was experimentally verified.

Contouring of diffused objects using lensless Fourier transform digital moiré holography

A method is proposed for contouring of diffused objects using digital holographic moiré interferometry in lensless Fourier transform configuration. Fringe projection moiré technique combined with digital double-exposure holography produces the contours in this method. Two digital holograms of a 10 mm aluminum alloy cube are recorded by tilting the illumination angle slightly between exposures, and a third one is recorded by translating the detector a little laterally with the final illumination angle unchanged. Upon numerical processing of the first two holograms, a plane parallel fringe system seems to be projected onto the object. This fringe system can be referred to as the modified grid. Processing of the second and the third hologram results in another grid, the reference grid. In effect, processing of the first and the third hologram combines the modified and the reference grids to produce the moiré contour fringes. The range of contour intervals obtained remains between 2.73 and 0.38 mm with seven different contours in between. The present method can measure details of a great variety of sizes on objects of large dimensional range. Deviations in the measured contour intervals from the theoretically calculated values are found to be within 12%-18%. This seems to be because of the deviation in the present experimental geometry from the ideal theoretical configuration, the hologram digitization, and the particular reconstruction algorithm used in the present experimental arrangement.

Endoscopic low-coherence topography measurement for upper airways and hollow samples

To evaluate the severity of airway pathologies, quantitative dimensioning of airways is of utmost importance. Endoscopic vision gives a projective image and thus no true scaling information can be directly deduced from it. In this article, an approach based on an interferometric setup, a low-coherence laser source and a standard rigid endoscope is presented, and applied to hollow samples measurements. More generally, the use of the low-coherence interferometric setup detailed here could be extended to any other endoscopy-related field of interest, e.g., gastroscopy, arthroscopy and other medical or industrial applications where tri-dimensional topology is required. The setup design with a multiple fibers illumination system is presented. Demonstration of the method ability to operate on biological samples is assessed through measurements on ex vivo pig bronchi.

Digital holographic interferometer using simultaneously three lasers and a single monochrome sensor for 3D displacement measurements

The use of digital holographic interferometry for 3D measurements using simultaneously three illumination directions was demonstrated by Saucedo et al. (Optics Express 14(4) 2006). The technique records two consecutive images where each one contains three holograms in it, e.g., one before the deformation and one after the deformation. A short coherence length laser must be used to obtain the simultaneous 3D information from the same laser source. In this manuscript we present an extension of this technique now illuminating simultaneously with three different lasers at 458, 532 and 633 nm, and using only one high resolution monochrome CMOS sensor. This new configuration gives the opportunity to use long coherence length lasers allowing the measurement of large object areas. A series of digital holographic interferograms are recorded and the information corresponding to each laser is isolated in the Fourier spectral domain where the corresponding phase difference is calculated. Experimental results render the orthogonal displacement components u, v and w during a simple load deformation.

3D shape measurement of macroscopic objects in digital off-axis holography using structured illumination

We propose what we believe to be a novel approach to measure the 3D shape of arbitrary diffuse-reflecting macroscopic objects in holographic setups. Using a standard holographic setup, a second CCD and a liquid-crystal-on-silicon spatial light modulator to modulate the object wave, the method yields a dense 3D point cloud of an object or a scene. The calibration process is presented, and first quantitative results of a shape measurement are shown and discussed. Furthermore, a shape measurement of a complex object is displayed to demonstrate its universal use.

Parallel optical-path-length-shifting digital holography

The authors propose an optical-path-length-shifting digital holography as a technique capable of single-shot recording of three-dimensional information of objects. With a single image sensor, the proposed technique can simultaneously record all of the holograms required for the in-line digital holography that reconstruct the image of an object from two intensity measurements at different planes. The technique can be optically implemented by using an optical-path-length-shifting array device located in the common path of the reference and object waves. The array device has periodic structure of two-step optical-path difference. The configuration of the array device of the proposed technique is simpler than the phase-shifting array device required for parallel phase-shifting digital holographies. Therefore, the optical system of the proposed technique is more suitable for the realization of a single-shot in-line digital holography system that removes the conjugate image from the reconstructed image. The authors conducted both a numerical simulation and a preliminary experiment of the proposed technique. The reconstructed images were quantitatively evaluated by using root mean squared error. In comparison to single-shot digital holography using the Fresnel transform alone, with the proposed technique the root mean squared errors of the technique were reduced to less than 1/6 in amplitude and 1/3 in phase. Also the results of the simulation and experiment agreed well with the images of an object. Thus the effectiveness of the proposed technique is verified.

Parallel two-step phase-shifting digital holography

We propose a parallel two-step phase-shifting digital holography technique capable of instantaneous measurement of three-dimensional objects, with a view toward measurement of dynamically moving objects. The technique is based on phase-shifting interferometry. The proposed technique carries out the two-step phase-shifting method at one time and can be optically implemented by using a phase-shifting array device located in the reference beam. The array device has a periodic two-step phase distribution, and its configuration is simplified compared with that required for three-step and four-step parallel phase-shifting digital holographies. Therefore the optical system of the proposed technique is more suitable for the realization of a parallel phase-shifting digital holography system. We conduct both a numerical simulation and a preliminary experiment in the proposed technique. The results of the simulation and the experiment agree well with those of sequential phase-shifting digital holography, and results are superior to those obtained by conventional digital holography using the Fresnel transform alone. Thus the effectiveness of the proposed technique is verified.