Digital holography of self-luminous objects by using a Mach-Zehnder setup

Deep learning-based incoherent holographic camera enabling acquisition of real-world holograms for holographic streaming system

While recent research has shown that holographic displays can represent photorealistic 3D holograms in real time, the difficulty in acquiring high-quality real-world holograms has limited the realization of holographic streaming systems. Incoherent holographic cameras, which record holograms under daylight conditions, are suitable candidates for real-world acquisition, as they prevent the safety issues associated with the use of lasers; however, these cameras are hindered by severe noise due to the optical imperfections of such systems. In this work, we develop a deep learning-based incoherent holographic camera system that can deliver visually enhanced holograms in real time. A neural network filters the noise in the captured holograms, maintaining a complex-valued hologram format throughout the whole process. Enabled by the computational efficiency of the proposed filtering strategy, we demonstrate a holographic streaming system integrating a holographic camera and holographic display, with the aim of developing the ultimate holographic ecosystem of the future.

Investigation of the effective aperture: towards high-resolution Fresnel incoherent correlation holography

Fresnel incoherent correlation holography (FINCH) shows great advantages of coherent-light-source-free, high lateral resolution, no scanning, and easy integration, and has exhibited great potential in recording three-dimensional information of objects. Despite the rapid advances in the resolution of the FINCH system, little attention has been paid to the influence of the effective aperture of the system. Here, the effective aperture of the point spread function (PSF) has been investigated both theoretically and experimentally. It is found that the effective aperture is mainly restricted by the aperture of the charge-coupled device (CCD), the pixel size of the CCD, and the actual aperture of the PSF at different recording distances. It is also found that the optimal spatial resolution exists only for a small range of recording distance, while this range would become smaller as the imaging wavelength gets longer, leading to the result that the optimal spatial resolution is solely determined by the actual aperture of the PSF. By further combining the FINCH system with a microscopy system and optimizing the recording distance, a spatial resolution as high as 0.78 μm at the wavelength of 633 nm has been obtained, enabling a much higher quality imaging of unstained living biological cells compared to the commercial optical microscope. The results of this work may provide some helpful insights into the design of high-resolution FINCH systems and pave the way for their application in biomedical imaging.

Digital Incoherent Compressive Holography Using a Geometric Phase Metalens

We propose a compressive self-interference incoherent digital holography (SIDH) with a geometric phase metalens for section-wise holographic object reconstruction. We specify the details of the SIDH with a geometric phase metalens design that covers the visible wavelength band, analyze a spatial distortion problem in the SIDH and address a process of a compressive holographic section-wise reconstruction with analytic spatial calibration. The metalens allows us to realize a compressive SIDH system in the visible wavelength band using an image sensor with relatively low bandwidth. The operation of the proposed compressive SIDH is verified through numerical simulations.

Single-shot phase calibration of a spatial light modulator using geometric phase interferometry

A vibration-insensitive, single-shot phase-calibration method for phase-only spatial light modulators (SLM) is reported. The proposed technique uses a geometric phase lens to form a phase-shifting radial shearing interferometer to enable common-path measurements. This configuration has several advantages: (a) unlike diffraction-based SLM calibration techniques, this technique is robust against intensity errors due to misalignment; (b) unlike two-beam interferometers, this technique offers a high environmental stability; and (c) unlike intensity-based methods, the phase-shifting capability provides a phase uncertainty routinely in the order of ${2}\pi /100$2π/100. The experimental results show a significantly higher accuracy when compared to the diffraction-based approaches.

Michelson-interferometric-configuration-based incoherent digital holography with a geometric phase shifter

The phase-shifting method is a simple and efficient approach to extract complex hologram information free of bias and twin-image noise. In this study, the geometric phase-shifting method is utilized for a self-interference incoherent digital holographic recording system based on the Michelson-type interferometer. The phase-shifting module consists of a horizontal polarizer, and two achromatic quarter-wave plates are employed inside the interferometer, replacing conventional phase-shifting devices, such as the piezo-actuated mirror. Since the phase-shifting amount of the introduced method herein is theoretical, regardless of the input wavelength, the simultaneous recording of step-wise phase-shifted interferograms for different color channels is available. Therefore, the multi-color hologram recording is achieved with fewer numbers of exposures. The demonstration of multi-color hologram recording and reconstruction are presented to validate the proposed idea.

Bimodal Incoherent Digital Holography for Both Three-Dimensional Imaging and Quasi-Infinite-Depth-of-Field Imaging

Although three-dimensional (3D) imaging and extended depth-of-field (DOF) imaging are completely opposite techniques, both provide much more information about 3D scenes and objects than does traditional two-dimensional imaging. Therefore, these imaging techniques strongly influence a wide variety of applications, such as broadcasting, entertainment, metrology, security and biology. In the present work, we derive a generalised theory involving incoherent digital holography to describe both 3D imaging and quasi-infinite-DOF (QIDOF) imaging, which allows us to comprehensively discuss the functions of each imaging technique. On the basis of this theory, we propose and develop a bimodal incoherent digital holography system that allows both 3D imaging and QIDOF imaging. The proposed system allows imaging objects using spatially incoherent light and reconstructing 3D images or QIDOF images solely by changing the phase pattern of a spatial light modulator and without requiring mechanical adjustments or any other modifications to the setup. As a proof-of-principle experiment, we evaluate the DOF and record holograms of a reflective object with the proposed system. The experimental results show that the generalised theory is effective; our demonstration platform provides the function of 3D and QIDOF imaging.

Compact self-interference incoherent digital holographic camera system with real-time operation

The video recording-capable compact incoherent digital holographic camera system is proposed. The system consists of the linear polarizer, convex lens, geometric phase lens, and the polarized image sensor. The Fresnel hologram is recorded by this simple configuration in real time. The system parameters are analyzed and evaluated to record a better-quality hologram in a compact form-factor. The real-time holographic recording and its digitally reconstructed video playback are demonstrated with the proposed system.

Achromatic phase shifting self-interference incoherent digital holography using linear polarizer and geometric phase lens

A simple Fresnel-type self-interference incoherent digital holographic recording system is proposed. The main part of the system consists of the two linear polarizers and geometric phase lens. The geometric phase lens is employed as a polarization selective common-path interferometer. One of the polarizers is rotated by the motor and serves as a phase-shifter with the geometric phase lens, to eliminate the bias and twin image noise. A topological phase is obtained by the relative angle between the polarizer and geometric phase lens. Since this phase shifting method does not depend on the change of the optical path length, the phase shifting performance is almost constant in the broad spectral range. Using the proposed achromatic phase shifting method, a simultaneous three-color phase shifting digital hologram recording under the incoherent light source is demonstrated.

Single-shot phase-shifting incoherent digital holography with multiplexed checkerboard phase gratings

Single-shot phase-shifting incoherent digital holography with multiplexed checkerboard phase gratings is proposed for acquiring holograms of moving objects. The gratings presented here play the following three roles: dividing the beams, modulating the curvature of spherical beams, and introducing different phase shifts. With the gratings of our proposed method, four individual holograms of a spatially incoherent light are formed on an image sensor. Therefore, it is possible to simultaneously capture four holograms and implement a phase-shifting technique. A proof-of-principle experiment was conducted to show the feasibility of the proposed method.

Optical configuration with fixed transverse magnification for self-interference incoherent digital holography

The optical configuration proposed in this paper consists of a 4-f optical setup with the wavefront modulation device on the Fourier plane, such as a concave mirror and a spatial light modulator. The transverse magnification of reconstructed images with the proposed configuration is independent of locations of an object and an image sensor; therefore, reconstructed images of object(s) at different distances can be scaled with a fixed transverse magnification. It is yielded based on Fourier optics and mathematically verified with the optical matrix method. Numerical simulation results and experimental results are also given to confirm the fixity of the reconstructed images.

Self-interference digital holography with a geometric-phase hologram lens

Self-interference digital holography (SIDH) is actively studied because the hologram acquisition under the incoherent illumination condition is available. The key component in this system is wavefront modulating optics, which modulates an incoming object wave into two different wavefront curvatures. In this Letter, the geometric-phase hologram lens is introduced in the SIDH system to perform as a polarization-sensitive wavefront modulator and a single-path beam splitter. This special optics has several features, such as high transparency, a modulation efficiency up to 99%, a thinness of a few millimeters, and a flat structure. The demonstration system is devised, and the numerical reconstruction results from an acquired complex hologram are presented.

Three-dimensional mapping of fluorescent nanoparticles using incoherent digital holography

Three-dimensional mapping of fluorescent nanoparticles was performed by using incoherent digital holography. The positions of the nanoparticles were quantitatively determined by using Gaussian fitting of the axial- and lateral-diffraction distributions through position calibration from the observation space to the sample space. It was found that the axial magnification was constant whereas the lateral magnification linearly depended on the axial position of the fluorescent nanoparticles. The mapping of multiple fluorescent nanoparticles fixed in gelatin and a single fluorescent nanoparticle manipulated with optical tweezers in water were demonstrated.

Modified Lagrange invariants and their role in determining transverse and axial imaging resolutions of self-interference incoherent holographic systems

The Lagrange invariant is a well-known law for optical imaging systems formulated in the frame of ray optics. In this study, we reformulate this law in terms of wave optics and relate it to the resolution limits of various imaging systems. Furthermore, this modified Lagrange invariant is generalized for imaging along the z axis, resulting with the axial Lagrange invariant which can be used to analyze the axial resolution of various imaging systems. To demonstrate the effectiveness of the theory, analysis of the lateral and the axial imaging resolutions is provided for Fresnel incoherent correlation holography (FINCH) systems.

Recent advances in digital holography [invited]

This article presents an overview of recent advances in the field of digital holography, ranging from holographic techniques designed to increase the resolution of microscopic images, holographic imaging using incoherent illumination, phase retrieval with incoherent illumination, imaging of occluded objects, and the holographic recording of depth-extended objects using a frequency-comb laser, to the design of an infrastructure for remote laboratories for digital-holographic microscopy and metrology. The paper refers to current trends in digital holography and explains them using new results that were recently achieved at the Institute for Applied Optics of the University Stuttgart.

Incoherent off-axis Fourier triangular color holography

We present a new method for recording off-axis digital Fourier holograms of three-dimensional objects under spatially incoherent illumination. The method is implemented by modifying the optical configuration of triangular interferometer. The recording properties and 3D reconstruction ability of the proposed method are investigated theoretically and experimentally. Multicolor holographic recording and reconstruction of spatially incoherent illuminated object are achieved by using the proposed off-axis Fourier triangular interferometer and monochromatic digital camera. Only three holograms are sufficient to rebuild a color image without zero-order and twin image disturbing effect. Combining with some image fusion skills during reconstruction, the reconstructed color images with satisfied quality are demonstrated.

Spectrally resolved incoherent holography: 3D spatial and spectral imaging using a Mach-Zehnder radial-shearing interferometer

Spatial and spectral information holds the key for characterizing incoherently illuminated or self-luminous objects, as well as for imaging fluorescence. We propose spectrally resolved incoherent holography using a multifunctional Mach-Zehnder interferometer that can introduce both a radial shear and a variable time delay between the interfering optical fields and permits the measurement of both spatial and temporal coherence functions, from which a 3D spatial and spectral image of the object is reconstructed. We propose and demonstrate the accurate 3D imaging of the object spectra by in situ calibration.

Overview of techniques applicable to self-interference incoherent digital holography

Self-interference incoherent digital holography (SIDH) retrieves the complex hologram from the object illuminated by the incoherent light. Supported by the adaptive optic feature, SIDH is readily applicable to the ocular imaging to investigate the human retinal cells. Considering the practical issues, issues related to resolution, phase-shifting, and contrast should be addressed to implement the viable SIDH system which is capable of recording the holographic information of human retinal cells under the incoherent illumination. Super resolution image reconstruction technique can be directly applied to SIDH to enhance the resolution of the system without any change of configuration. We present the improved way to incorporate the phase-shifting itself into the lateral shift required by the super resolution technique. To deal with the phase-shifting issue, we present an arbitrary phase shift retrieval algorithm which can reduce the number of phase-shift and accept the blind phase-shift. The single-shot imaging is also possible by adopting the off-axis configuration of SIDH. We will provide the detailed procedures to retrieve the complex hologram using the proposed arbitrary phase shifting algorithm and the off-axis configuration.

Single-shot self-interference incoherent digital holography using off-axis configuration

We propose a single-shot incoherent holographic imaging technique that adopts self-interference incoherent digital holography (SIDH) with slight tilt of the plane mirror in the optical configuration. The limited temporal coherence length of the illumination leads the guide-star hologram of the proposed system to have a Gaussian envelope of elliptical ring shape. The observation shows that the reconstruction by cross correlation with the guide-star hologram achieves better quality than the usual propagation methods. Experimentally, we verify that the hologram and 3D reconstruction can be implemented incoherently with the proposed single-shot off-axis SIDH.

Off-axis digital holographic microscopy with LED illumination based on polarization filtering

A reflection mode digital holographic microscope with light emitting diode (LED) illumination and off-axis interferometry is proposed. The setup is comprised of a Linnik interferometer and a grating-based 4f imaging unit. Both object and reference waves travel coaxially and are split into multiple diffraction orders in the Fourier plane by the grating. The zeroth and first orders are filtered by a polarizing array to select orthogonally polarized object waves and reference waves. Subsequently, the object and reference waves are combined again in the output plane of the 4f system, and then the hologram with uniform contrast over the entire field of view can be acquired with the aid of a polarizer. The one-shot nature in the off-axis configuration enables an interferometric recording time on a millisecond scale. The validity of the proposed setup is illustrated by imaging nanostructured substrates, and the experimental results demonstrate that the phase noise is reduced drastically by an order of 68% when compared to a He-Ne laser-based result.

Enhanced resolution in Fourier incoherent single channel holography (FISCH) with reduced optical path difference

Fourier incoherent single channel holography (FISCH) is a method for recording spatially incoherent digital Fourier holograms. We present a general design of enhanced FISCH with a smaller optical path difference between interfering beams, when compared to our initial design [Opt. Lett. 37, 3723 (2012)]. This reduction enables a proper system operation with a wider bandwidth. Potential resolution enhancement of the images reconstructed from the FISCH holograms consequentially follows.

Recording of incoherent-object hologram as complex spatial coherence function using Sagnac radial shearing interferometer and a Pockels cell

The ideas of incoherent holography were conceived after the invention of coherent-light holography and their concepts seems indirectly related to it. In this work, we adopt an approach based on statistical optics to describe the process of recording of an incoherent-object hologram as a complex spatial coherence function. A Sagnac radial shearing interferometer is used for the correlation of optical fields and a Pockels cell is used to phase shift the interfering fields with the objective to quantify and to retrieve the spatial coherence function.