Digital Holograms Research Articles

21‐2: Invited Paper: 1µm Pixel Pitch Spatial Light Modulator Panel for Digital Holography

Digital hologram images reconstructed through interference of the modulated light waves requires ultra high resolution display panel. BCE oxide TFTs with channel width of 0.5µm and channel length of 2µm were stacked on data line for the integration of 1µm pitch pixel. 1µm pixel pitch 16K spatial light modulator on glass substrate was successfully fabricated with vertically stacked TFT structure.

Recording of a curved digital hologram for orthoscopic real image reconstruction.

Based on the concept of optical scanning holography, a holographic system for recording a curved digital hologram was proposed and demonstrated. In the system, an interference beam without the object information is first generated and then used to two-dimensionally scan a three-dimensional object along a cylindrical path. As a result, a complex curved hologram of a real object is digitally holographically recorded for the first time, to the best of our knowledge. The method of digital reconstruction and the properties of the curved digital hologram are then discussed.

Y4-Net: a deep learning solution to one-shot dual-wavelength digital holographic reconstruction.

In this Letter, a deep learning solution (Y4-Net, four output channels network) to one-shot dual-wavelength digital holography is proposed to simultaneously reconstruct the complex amplitude information of both wavelengths from a single digital hologram with high efficiency. In the meantime, by using single-wavelength results as network ground truth to train the Y4-Net, the challenging spectral overlapping problem in common-path situations is solved with high accuracy.

A Content Hiding Method for Digital Hologram Using Multiple Fresnel Diffraction

A digital hologram (DH) is so highly valued that it needs to be protected from exposure to an unpermitted person, which could be done by a content encryption. We propose an encryption scheme for digital holograms, whose goal is to hide their information with maximal visual distortion and minimal ration of the encrypted data. It uses the characteristics of the Fresnel transform and signal processing techniques. As the diffraction distance increases the region containing the object information relative to the whole diffraction plane becomes smaller. Therefore our scheme diffracts a given digital hologram twice: the first transform for reconstructing the image contained by the hologram and the second transform for concentrating the energy of the object into a small region. Then only the energy-concentrated region is encrypted to reduce the amount of data to be encrypted. Experimental results show that when the diffraction distance of the second transform is about 20 m, the encryption ratio is only 0.0058% of the hologram data, which is enough to hide the object information unrecognizably.

Providing a Visual Understanding of Holography Through Phase Space Representations

Digital holograms are a prime example for signals, which are best understood in phase space—the joint space of spatial coordinates and spatial frequencies. Many characteristics, as well as optical operations can be visualized therein with so called phase space representations (PSRs). However, literature relies often only on symbolic PSRs or on, in practice, visually insufficient PSRs like the Wigner–Ville representation. In this tutorial-style paper, we will showcase the S-method, which is both a PSR that can be calculated directly from any given signal, and that allows for a clear visual interpretation. We will highlight the power of space-frequency analysis in digital holography, explain why this specific PSR is recommended, discuss a broad range of basic operations, and briefly overview several interesting practical questions in digital holography.

A novel 4D digital holographic PIV/PTV (4D-DHPIV/PTV) methodology using iterative predictive inverse reconstruction

Digital imaging array technology and processing power are continuously improving and have reached a state where 4-dimensional (i.e. time-resolved 3C-3D) digital holographic PIV/PTV (4D-DHPIV/PTV) methods can be considered for macro fluid mechanics and turbulence investigations. This paper presents an in-line 4D-DHPIV/PTV methodology, which in addition to including the standard digital hologram reconstruction, incorporates advanced digital filtering to remove the virtual image effect, 3-dimensional volume deconvolution to reduce the depth-of-focus problem and the virtual image, followed by an efficient one-pass 3-dimensional clustering algorithm coupled with a predictive inverse reconstruction approach to increase the particle reconstruction dynamic range and 3-dimensional reconstruction domain, which is accelerated using particle position prediction. In addition to the presentation of the details of this novel 4D-DHPIV/PTV method, performance results pertaining to bias particle position error and the uncertainty associated with the particle position are presented as a function of (i) particle concentration and (ii) the shot noise present in the digitally recorded hologram. An experiment to measure laminar micro-channel flow has been performed to demonstrate the 4D-DHPIV/PTV methodology.

All-dielectric bifocal isotropic metalens for a single-shot hologram generation device.

Planar metalenses are regarded as promising functional nanodevices because of their lightweight, nano-resolution properties, and, therefore, they can serve as versatile platforms for imaging and Fourier transforming. Here, we demonstrate a meta-device that functions as an isotropic bifocal all-dielectric Huygens' metalens to realize nanoscale real-time coaxial digital hologram generation. We design an isotropic bifocal metalens for micro/nano hologram recording, and the metalens utilizes the complete region compared to a previously reported interleaved multifocal metalens scheme. In addition, the hologram generation does not depend on complex polarization conversion, thereby improving the practical efficiency. For high-fidelity reconstruction, compressive reconstruction is utilized to remove twin-image and zero-order items and to suppress noise. Such concept would be extended to white-light achromatic meta-holography and three-dimensional micro/nano in vivo incoherent super-resolution imaging under subwavelength modulation.

Three-colour digital holographic microscopy based on a Mach – Zender interferometer and a solid-state RGB laser

We have developed a facility for recording colour digital holograms involving a Mach – Zender interferometer and a pulsed solid-state RGB laser, which radiates simultaneously at three wavelengths: 451, 532, and 634 nm. These wavelengths are obtained in the conversion of the radiation of a pulsed Nd3+ : YAG laser (1064 nm), which operates in the regime of intracavity parametric oscillation at a wavelength of 1570 nm, with the subsequent conversion of its frequencies to the red, green, and blue spectral regions in nonlinear KTP-crystal elements. The combination of this three-wavelength pulsed light source with the Mach – Zender interferometer provides the simultaneous recording of three spectral digital holograms with one matrix RGB radiation detector. Pulsed three-colour digital holography shows promise for the analysis of fast processes and rapidly varying colour phase objects.

A practical criterion for focusing of unstained cell samples using a digital holographic microscope.

Digital holographic microscopy (DHM) is an important technique that may be used for quantitative phase imaging of unstained biological cell samples. Since the DHM technology is not commonly used in clinics or bioscience research labs, at present there is no well-accepted focusing criterion for unstained samples that users can follow while recording image plane digital holograms of cells. The usual sharpness metrics that are useful for auto-focusing of stained cells do not work well for unstained cells as there is no amplitude contrast. In this work, we report a practical method for estimating the best focus plane for unstained cells in the digital hologram domain. The method is based on an interesting observation that for the best focus plane the fringe pattern associated with individual unstained cells predominantly shows phase modulation effect in the form of bending of fringes and minimal amplitude modulation. This criterion when applied to unstained red blood cells shows that the central dip in the doughnut-like phase profile of cells is maximal in this plane. The proposed methodology is helpful for standardizing the usage of DHM technology across different users and application developmentefforts. LAY DESCRIPTION: Digital holographic microscopy (DHM) is slowly but steadily becoming an important microscopy modality and gaining acceptability for basic bio-science research as well as clinical usage. One of the important features of DHM is that it allows users to perform quantitative imaging of unstained transparent cells. Instead of using dyes or fluorescent labelling, DHM systems use quantitative phase as a contrast mechanism which depends on the natural refractive index variation within the cell samples. Since minimal wet lab processing is required in order to image cell samples with a DHM, cells can be imaged in their natural state. While DHM is gaining popularity among users, the imaging protocols across the labs or users need to be standardized in order to make sure that the same quantitative phase parameters are used for tasks such as quantitative phased based cellclassification. One of the important operational tasks for any microscopy work is to focus the sample under study. While focusing comes naturally to users of brightfield microscopes based on image contrast, the focusing is not straightforward when samples are unstained so that they do not offer any amplitude contrast. When performing quantitative phase imaging, defocus can actually change the phase profile of the cell due to near-zone (Fresnel) diffraction effects. So unless a standardized focusing methodology is used, it will be difficult for multiple DHM users (potentially at different sites) to agree on quantitative results out of their phase images. DHM literature has prior works which perform numerical focusing of recovered complex wave-field in the hologram plane to find the best focus plane. However such methods are not user friendly and do not allow user the same focusing experience as in a brightfield microscope. The numerical focusing is therefore a reasonably good method for an optics researcher but not necessarily so for a microscopy technician looking at cell samples with a DHM system in a clinicalsetting. The present work provides a simple focusing criterion for unstained samples that works directly in the hologram domain. The technique is based on an interesting observation that the when an unstained cell sample is in the best-focus plane, its corresponding hologram (or fringe pattern) predominantly shows phase modulation manifested by bending of fringes at the location of the cell. This criterion can be converted into a simple numerical method as we have used to find the best-focus plane using a stack of through focus holograms. We believe that the technique can be used manually by visually observing the holograms or can be converted to an auto-focus algorithm for a motorized DHMsystem.

Spatio-temporal filtering in laser Doppler holography for retinal blood flow imaging.

Laser Doppler holography (LDH) is a full-field interferometric imaging technique recently applied in ophthalmology to measure blood flow, a parameter of high clinical interest. From the temporal fluctuations of digital holograms acquired at ultrafast frame rates, LDH reveals retinal and choroidal blood flow with a few milliseconds of temporal resolution. However, LDH experiences difficulties to detect slower blood flow as it requires to work with low Doppler frequency shifts which are corrupted by eye motion. We here demonstrate the use of a spatio-temporal decomposition adapted from Doppler ultrasound that provides a basis appropriate to the discrimination of blood flow from eye motion. A singular value decomposition (SVD) can be used as a simple, robust, and efficient way to separate the Doppler fluctuations of blood flow from those of strong spatial coherence such as eye motion. We show that the SVD outperforms the conventional Fourier based filter to reveal slower blood flow, and dramatically improves the ability of LDH to reveal vessels of smaller size or with a pathologically reduced blood flow.

Digital laser for on-demand intracavity selective excitation of second harmonic higher-order modes.

In this article, we demonstrate selective excitation of second harmonic higher-order modes inside a diode end-pumped solid-state laser resonator that comprises of a nonlinear potassium titanyl phosphate (KTP) crystal and a digitally addressed holographic end-mirror in a form of a reflective phase-only spatial light modulator (SLM). The emitted second harmonic higher-order modes at 532 nm are generated by an intracavity nonlinear KTP crystal that is pumped by high-order fundamental modes operating at 1064 nm. The fundamental modes are digitally controlled by displaying a computer-generated hologram in the form of a grey-scale image to the SLM screen for on-demand high-order modes. The phase matching of the fundamental mode to the generated frequency-doubled mode is achieved by controlling the phase of the digital hologram to either achieve a high or quasi-degree of orbital angular momentum conservation. We show that we can intracavity generate frequency-doubled high-order Laguerre-Gaussian modes and Hermit-Gaussian modes that are either quasi or fully reproducible in the far-field. To the best of our knowledge, this is the first laser to generate frequency-doubled on-demand higher-order modes inside the cavity at the visible (green) wavelength of 532 nm.

Three-Dimensional Reconstruction of Mechanical Parts Based on Digital Holography

In reconstruction of the off-axis digital hologram of diffuse reflection objects, the position of the positive first-order image cannot be accurately obtained because of the low quality of the reconstruction image. This paper focuses on the above problem and proposes a method for marking the first-order image of the 1-FFT surface based on the fast Fourier transform (1-FFT). The parameters of angle of illumination light were investigated, and the maximum relative measurement error is 5.6% by standard objects. The multiaperture stitching technique in cylindrical coordinates is applied to digital holography technology, and the particle swarm optimization algorithm is used to transform the nonlinear equations into optimization problems to solve the splicing parameters. Finally, the 3D display of a typical rotary three-dimensional mechanical part is successfully realized with holography stitching using the above method.

Observation of ultrafast phenomena using chirped pulse digital holography

We experimentally demonstrate chirped pulse digital holography for measuring the sequence of ultrafast optical wavefronts in the temporal resolution of 35 fs and the time separation of 3.7 ps. In this method, multiple ultrafast object pulses and a chirped reference pulse are interfered to record a digital hologram and optical wavefronts at different times can be reconstructed separately. The phase images of pressured air injected from a nozzle are obtained using multiple optical pulses. Also the phase images of spark discharge in a spark plug are obtained using a single-shot optical pulse. The phase distortion effect in chirped pulse digital holography and the method of its correction are shown.

Design and development of volume phase holographic grating based digital holographic interferometer for label-free quantitative cell imaging.

In this paper, a volume phase holographic optical element based digital holographic interferometer is designed and used for quantitative phase imaging of biological cells [white blood cells, red blood cells, platelets, and Staphylococcus aureus (S. aureus) bacteria cells]. The experimental results reveal that sharp images of the S. aureus bacteria cells of the order of ${\sim}{1}\;{\unicode{x00B5}{\rm m}}$∼1µm can be clearly seen. The volume phase holographic grating will remove the stray light from the system reaching toward the grating and will minimize the coherent noise (speckle noise). This will improve the sharpness in the image reconstructed from the recorded digital hologram.

Enhancing the Quality of Sampled Phase-Only Hologram (SPOH) Based on Time-Division Comb Filtering

Generation of digital phase-only Fresnel holograms is an important research area in digital holography, as it leads to a substantial simplification of a holographic display system. However, the quality of the reconstructed image of a hologram without the magnitude component is heavily degraded. The problem can be reduced by down-sampling the intensity of an image prior to generating the hologram. The method, referred to as “sampled phase-only hologram” (SPOH) generation, results in reconstructed images that are masked with the pattern of the down-sampling lattice. This paper reports a novel, low complexity method to alleviate this problem through the concept of comb filtering. Results reveal prominent enhancement on the reconstructed image of a SPOH.

Object-based digital hologram segmentation and motion compensation.

Digital video holography faces two main problems: 1) computer-generation of holograms is computationally very costly, even more when dynamic content is considered; 2) the transmission of many high-resolution holograms requires large bandwidths. Motion compensation algorithms leverage temporal redundancies and can be used to address both issues by predicting future frames from preceding ones. Unfortunately, existing holographic motion compensation methods can only model uniform motions of entire 3D scenes. We address this limitation by proposing both a segmentation scheme for multi-object holograms based on Gabor masks and derive a Gabor mask-based multi-object motion compensation (GMMC) method for the compensation of independently moving objects within a single hologram. The utilized Gabor masks are defined in 4D space-frequency domain (also known as time-frequency domain or optical phase-space). GMMC can segment holograms containing an arbitrary number of mutually occluding objects by means of a coarse triangulation of the scene as side information. We demonstrate high segmentation quality (down to ≤ 0.01% normalized mean-squared error) with Gabor masks for scenes with spatial occlusions. The support of holographic motion compensation for arbitrary multi-object scenes can enable faster generation or improved video compression rates for dynamic digital holography.

A rotating holographic imager for stationary cloud droplet and ice crystal measurements

An optical cloud droplet and ice crystal measurement system ICEMET (icing condition evaluation method), designed for present icing condition monitoring in field conditions, is presented. The aim in this work has been to develop a simple but precise imaging technique to measure the two often missing parameters needed in icing rate calculations caused by icing clouds—the droplet size distribution (DSD) and the liquid water content (LWC) of the air. The measurement principle of the sensor is based on lens-less digital in-line holographic imaging. Cloud droplets and ice crystals are illuminated by a short laser light pulse and the resulting hologram is digitally sampled by a digital image sensor and the digital hologram is then numerically analyzed to calculate the present DSD and LWC values. The sensor has anti-icing heating power up to 500 W and it is freely rotating by the wind for an optimal sampling direction and aerodynamics. A volume of 0.5 cm3 is sampled in each hologram and the maximum sampling rate is 3 cm3/s. Laboratory tests and simulations were made to ensure the adequate operation of the measurement sensor. Computational flow dynamics simulations showed good agreement with droplet concentration distributions measured from an icing wind tunnel. The anti-icing heating of the sensor kept the sensor operational even in severe icing conditions; the most severe test conditions were the temperature − 15 °C, wind speed 20 m/s and the LWC 0.185 g/m3. The verification measurements made using NIST traceable monodisperse particle standard glass spheres showed that the ICEMET sensor measurement median diameter 25.54 µm matched well with 25.60 µm ± 0.70 µm diameter confidence level given by the manufacturer.

Analysis on angular field of view of holographic image dependent on hologram numerical aperture in holographic display

We investigate the angular field of view (AFOV) of a holographic image reconstructed from the digital Fresnel hologram in holographic display. The theoretical analysis clarifies that the AFOV of a holographic image is fundamentally determined by the hologram numerical aperture (HNA) other than a diffraction angle of pixel pitch of a pixelated modulator. This property is proved for various types of the digital holograms using a numerical simulation and optical experiments. The high-HNA hologram reconstructs the image with a high viewing angle, whereas the image contraction is inevitable due to the Nyquist sampling criterion in the hologram synthesis. We demonstrate the method for reconstructing the holographic image with a high viewing angle without image contraction.

Bicubic interpolation and extrapolation iteration method for high resolution digital holographic reconstruction

Digital holography enables the digitization processing of holograms. The resolution of the reconstructed image is greatly limited by the pixelated imaging detectors. To improve the resolution, a bicubic interpolation and extrapolation iteration (BIPEPI) method is proposed to compensate the limited size of the digital holograms. Based on the prior knowledge of the model of Fresnel zone plate with circular diffraction gratings, the pixel density is increased by the bicubic interpolation (BIP) method inside the hologram to enhance the low-frequency terms of the object. With extrapolation iterations (EPI) method, more high order fringes outside the hologram are generated to enhance the high-frequency terms of the object. The resolution of the reconstructed image can be effectively improved by combining this predicted low- and high-frequency terms. Numerical simulations and optical experiments are presented and the feasibility of the proposed method is quantitatively evaluated.

Coherent imager module with a large field of view for synthetic aperture interferometry applications.

Optical areal profilometry of large precision-engineered surfaces require high-resolution measurements over large fields of view. Synthetic Aperture Interferometry (SAI) offers an alternative to the conventional approach of stitching small fields of view (FOV) obtained with Coherent Scanning Interferometry (CSI) using high-NA objectives. In SAI, low-resolution digital holograms are recorded for different illumination and observation directions and they are added coherently to produce a high-resolution reconstruction over a large FOV. This paper describes the design, fabrication and characterization of a large FOV, compact and low-cost coherent imager (CI) as a building block of a coherent sensor array for a SAI system. The CI consists of a CMOS photodetector array with 1.12 µm pixel pitch, a square entrance pupil and a highly divergent reference beam that emerges from a pinhole milled with a focused ion beam on the cylindrical cladding at the tip of an optical fibre. In order to accurately reconstruct the digital holograms, the wavefront of the reference beam is estimated by localizing the reference source relative to the photodetector array. This is done using an optimization approach that simultaneously reconstructs plane waves that reach the aperture from 121 different illumination directions and guarantees a phase root-mean-squared (RMS) error of less than a fifth of the wavelength across the CI entrance pupil at a boundary of the FOV. The CI performance is demonstrated with a holographic reconstruction of a 0.110 m wide object placed at a distance of 0.085 m, i.e. a FOV = ±0.57 rad, the highest reported to date with a holographic camera.