Digital holographic content manipulation for wide-angle holographic near-eye displays.

In recent years, the development of holographic near-eye displays (HNED) has surpassed the progress of digital hologram recording systems, especially in terms of wide-angle viewing capabilities. Thus, there is capture-display parameters incompatibility, which makes it impossible to reconstruct recorded objects in wide-angle display. This paper presents a complete imaging chain extending the available content for wide-angle HNED of pupil and non-pupil configuration with narrow-angle digital holograms of real objects. To this end, a new framework based on the phase-space approach is proposed that includes a set of affine transformations required to account for all differences in capture-display cases. The developed method allows free manipulation of the geometry of reconstructed objects, including axial and lateral positioning and size scaling. At the same time, it has a low computational effort. The presented work is supported with non-paraxial formulas developed using the phase-space approach, enabling accurate tracing of the holographic signal, its reconstruction, and measuring appearing deformations. The applicability of the proposed hologram manipulation method is proven with experimental results of digital hologram reconstruction in wide-angle HNED.

Spherical wave illumination scanning digital holographic profilometry.

In this work, we proposed what we believe to be a novel scanning solution for the assessment of high-NA samples, referred to as spherical-wave illumination scanning digital holographic profilometry (SWS-DHP). This approach introduces a 2F optimization methodology, based on the measurement of the focal length of the object to determine the spherical component of the scanning. Furthermore, re-optimization of 2F, whether it needs to be operated depends on the measured object's NA to inspect more information. Meanwhile, utilizing phase space analysis shows SWS superiority in information transfer for high-NA samples compared to plane-wave illumination scanning. In addition, this method introduces a shape reconstruction algorithm with volumetric aberration compensation based on the propagation of the aberrated object and illumination waves to obtain high-quality measurements. Finally, the imaging merits of SWS-DHP were proved through simulations and were experimentally verified for the object of NA up to 0.87.

Eyebox expansion with accurate hologram generation for wide-angle holographic near-eye display.

Small eyebox in wide-angle holographic near-eye display is a severe limitation for 3D visual immersion of the device. In this paper, an opto-numerical solution for extending the eyebox size in these types of devices is presented. The hardware part of our solution expands the eyebox by inserting a grating of frequency fg within a non-pupil forming display configuration. The grating multiplies eyebox, increasing the possible eye motion. The numerical part of our solution is an algorithm that enables proper coding of wide-angle holographic information for projecting correct object reconstruction at arbitrary eye position within the extended eyebox. The algorithm is developed through the employment of the phase-space representation, which facilitates the analysis of the holographic information and the impact of the diffraction grating in the wide-angle display system. It is shown that accurate encoding of the wavefront information components for the eyebox replicas is possible. In this way, the problem of missing or incorrect views in wide angle near-eye display with multiplied eyeboxes is efficiently solved. Moreover, this study investigates the space-frequency relation between the object and the eyebox and how the hologram information is shared between eyebox replicas. The functionality of our solution is tested experimentally in an augmented reality holographic near-eye display that has maximum field of view of 25.89°. Obtained optical reconstructions demonstrate that correct object view is obtained for arbitrary eye position within extended eyebox.

LED near-eye holographic display with a large non-paraxial hologram generation.

In this paper, two solutions are proposed to improve the quality of a large image that is reconstructed in front of the observer in a near-eye holographic display. One of the proposed techniques, to the best of our knowledge, is the first wide-angle solution that successfully uses a non-coherent LED source. It is shown that the resulting image when employing these types of sources has less speckle noise but a resolution comparable to that obtained with coherent light. These results are explained by the developed theory, which also shows that the coherence effect is angle varying. Furthermore, for the used pupil forming display architecture, it is necessary to compute a large virtual nonparaxial hologram. We demonstrate that for this hologram there exists a small support region that has a frequency range capable of encoding information generated by a single point of the object. This small support region is beneficial since it enables to propose a wide-angle rigorous CGH computational method, which allows processing very dense cloud of points that represents three-dimensional objects. This is our second proposed key development. To determine the corresponding support region, the concept of local wavefront spatial curvature is introduced, which is proportional to the tangent line to the local spatial frequency of the spherical wavefront. The proposed analytical solution shows that the size of this area strongly depends on the transverse and longitudinal coordinate of the corresponding object point.

Single-shot digital multiplexed holography for the measurement of deep shapes.

This work develops a single-shot holographic profilometer that enables shape characterization of discontinuous deep surfaces. This is achieved by combining hologram frequency multiplexing and an illumination technique of complex amplitude in multi-incidence angle profilometer. Object illumination is carried out from seven directions simultaneously, where the radial angular coordinates of illumination plane waves obey the geometric series. It is shown that: (i) the illumination pattern provides the required frequency separation of all object wavefronts in transverse frequency space, which is necessary for hologram demultiplexing, and (ii) numerical generation of longitudinal scanning function (LSF) is possible, which has large measurement range, high axial resolution, and small side lobes. Low side lobes of LSF and the developed multiplexed field dependent aberration compensation method are essential to minimize the negative influence of speckle noise of single-shot capture on the measurement result. The utility of the proposed method is demonstrated with experimental measurement of heights of two step-like objects.

Multi-Incidence Holographic Profilometry for Large Gradient Surfaces with Sub-Micron Focusing Accuracy.

Surface reconstruction for micro-samples with large discontinuities using digital holography is a challenge. To overcome this problem, multi-incidence digital holographic profilometry (MIDHP) has been proposed. MIDHP relies on the numerical generation of the longitudinal scanning function (LSF) for reconstructing the topography of the sample with large depth and high axial resolution. Nevertheless, the method is unable to reconstruct surfaces with large gradients due to the need of: (i) high precision focusing that manual adjustment cannot fulfill and (ii) preserving the functionality of the LSF that requires capturing and processing many digital holograms. In this work, we propose a novel MIDHP method to solve these limitations. First, an autofocusing algorithm based on the comparison of shapes obtained by the LSF and the thin tilted element approximation is proposed. It is proven that this autofocusing algorithm is capable to deliver in-focus plane localization with submicron resolution. Second, we propose that wavefield summation for the generation of the LSF is carried out in Fourier space. It is shown that this scheme enables a significant reduction of arithmetic operations and can minimize the number of Fourier transforms needed. Hence, a fast generation of the LSF is possible without compromising its accuracy. The functionality of MIDHP for measuring surfaces with large gradients is supported by numerical and experimental results.