Thick Holograms Research Articles

Investigation of -1st-order diffraction in conditions between thin and thick holograms

Investigation of -1st-order diffraction in conditions between thin and thick holograms

Information multiplexing from optical holography to multi-channel metaholography

Abstract Holography offers a vital platform for optical information storage and processing, which has a profound impact on many photonic applications, including 3D displays, LiDAR, optical encryption, and artificial intelligence. In this review, we provide a comprehensive overview of optical holography, moving from volume holography based on optically thick holograms to digital holography using ultrathin metasurface holograms in nanophotonics. We review the use of volume holograms for holographic multiplexing through the linear momentum selectivity and other approaches and highlight the emerging use of digital holograms that can be implemented by ultrathin metasurfaces. We will summarize the fabrication of different holographic recording media and digital holograms based on recent advances in flat meta-optics and nanotechnology. We highlight the rapidly developing field of metasurface holography, presenting the use of multi-functional metasurfaces for multiplexing holography in the use of polarization, wavelength, and incident angle of light. In the scope of holographic applications, we will focus on high bandwidth metasurface holograms that offer the strong sensitivity to the orbital angular momentum of light. At the end, we will provide a short summary of this review article and our perspectives on the future development of the vivid holography field.

Isotropic Two-Dimensional Differentiation Based on Dual Dynamic Volume Holograms

We study the use of two dynamic thick holograms to realize isotropic two-dimensional (2D) differentiation under Bragg diffraction. Acousto-optic modulators (AOMs) are used as dynamic volume holograms. Using a single volume hologram, we can accomplish a first-order derivative operation, corresponding to selective edge extraction of an image. Since the AOM is a 1D spatial light modulator, filtering of the image only occurs along the direction of the sound propagation. To achieve 2D image processing, two AOMs are used within a Mach–Zehnder interferometer (MZI). By aligning one AOM along the x-direction on the upper arm of the interferometer and another AOM along the y-direction on the lower arm, we accomplish the sum of two first-derivative operations, leading to isotropic edge extraction. We have performed both computer simulations and optical experiments to verify the proposed idea. The system provides additional operations in optical computing using AOMs as dynamic holograms.

Advanced volume holographic filter to improve the SNR of polychromatic sources in a noisy environment.

We present a new type of filter that improves the SNR of systems where polychromatic signal and noise are located at different distances within the same line of sight. The filter is based on holographic technology that allows for the discrimination of wavefronts by range. In using a combination of two holographic elements, a pre-disperser and a thick volume hologram, we were able to significantly increase the spectral bandwidth of the filter, from 9nm without the pre-disperser to 70nm with both holographic elements. Laboratory proof of concept demonstrated that such a filter is capable of an SNR improvement of 15 dB for a monochromatic source, and up to 7.6 dB for a polychromatic source. This filter can find applications in astronomic observation, satellite or space debris tracking, and free-space optical communication.

ACHROMATIZATION OF THE VOLUME HOLOGRAPHIC OPTICAL ELEMENS

The work considers a two-component holographic optical system having a base element in the form of a thick (volume) hologram optical element and intended for use in a given spectral range. The calculation of a two-component holographic system is carried out using formulas obtained from the mirror-lens model of the thick hologram element proposed by the author. It is indicated that according to the mirror model a thick hologram optical element is achromatic in a first approximation. For this the local period of the volume diffraction structure of the hologram element must be many times greater than the working wavelength, and the transverse dimensions of the element must be less than its thickness. Analytical expressions are given for the mutual correction of the chromatic aberration of the position of a thick hologram optical element and a relief kinoform element. The condition for achromatization of this two-component holographic system is formulated.

CALCULATION OF THE ACHROMATIC DIFFRACTION SYSTEM WITH CORRECTED SPHERICAL ABERRATION (part 2)

Achromatization of a three-component diffraction system consisting of one thick and two thin hologram optical elements is considered in the work. Analytical expressions are obtained for correcting the chromatic aberration of the position of a thick focusing hologram optical element by two scattering thin hologram optical elements in a given spectrum range. It is shown that achromatization is achieved for such a three-component system using two thin hologram elements located symmetrically on both sides of the thick element and having a value of the working diffraction order greater than the ratio of the focal length to the distance from the thin element to the image plane (at a given wavelength). The proposed three-component holographic system can be used to convert both an imaginary image into a real image and a real into an imaginary image in predetermined spectral regions of the visible, ultraviolet or infrared ranges of the spectrum.

Optical Rectifying Linear Units for Back-Propagation Learning in a Deep Holographic Convolutional Neural Network

We present the design for a bidirectional coherent optical Rectifying Linear Unit (ReLU) device capable of phase thresholding and appropriately rectifying forward-propagating optical-neuron activity and gating optically back-propagating error that will enable the construction of an all-optical deep learning system. The ReLU device can be fabricated in large arrays using high-speed liquid-crystal-on-Silicon (LCoS) smart-pixel technology capable of implementing arrays of feature planes needed for convolutional neural networks. Interferometric detection of the phase of a split-off fraction of the forward-propagating neuron input is used to set the state of the bidirectional switch and gate both the rest of the forward-propagating neuron input as well as the back-propagating error signals. We show how the array of convolutional adaptive interconnections needed for deep learning can be physically implemented and learned in an all-optical multistage dynamic holographically-interconnected architecture using lenslet arrays addressing thick Fourier-plane dynamic holograms. This optical architecture is self-aligned, phase-calibrated, and aberration compensated by using phase-conjugate mirrors to record the dynamic-holographic interconnections in each layer. This system has the potential to achieve a computational throughput approaching that of supercomputer clusters at a much lower energy cost by synergistically combining the analog computational properties of coherent Fourier optics with the hardware fault-tolerance provided by error-driven deep-learning algorithms.

Design and fabrication of cascaded dichromate gelatin holographic filters for spectrum-splitting PV systems

The technique of designing, optimizing, and fabricating broadband volume transmission holograms using dichromate gelatin (DCG) is summarized for solar spectrum-splitting applications. The spectrum-splitting photovoltaic (PV) system uses a series of single-bandgap PV cells that have different spectral conversion efficiency properties to more fully utilize the solar spectrum. In such a system, one or more high-performance optical filters are usually required to split the solar spectrum and efficiently send them to the corresponding PV cells. An ideal spectral filter should have a rectangular shape with sharp transition wavelengths. A methodology of designing and modeling a transmission DCG hologram using coupled wave analysis for different PV bandgap combinations is described. To achieve a broad diffraction bandwidth and sharp cutoff wavelength, a cascaded structure of multiple thick holograms is described. A search algorithm is then developed to optimize both single- and two-layer cascaded holographic spectrum-splitting elements for the best bandgap combinations of two- and three-junction spectrum-splitting photovoltaic (SSPV) systems illuminated under the AM1.5 solar spectrum. The power conversion efficiencies of the optimized systems are found to be 42.56% and 48.41%, respectively, using the detailed balance method, and show an improvement compared with a tandem multijunction system. A fabrication method for cascaded DCG holographic filters is also described and used to prototype the optimized filter for the three-junction SSPV system.

全息打印技术研究进展

Holographic printing technique can achieve a true three-dimensional display of the scene well. Based on different sources and different recording methods of interference patterns, holographic printing techniques can be classified as synthetic holographic stereogram printing, computer-generated hologram printing, and wavefront printing. Synthetic holographic stereogram printing can't record the depth information of the three-dimensional scene accurately, so there occurs the vergence-accommodation conflicts during the reconstruction of the hologram. Computer-generated hologram printing can record and reconstruct the depth information of the scene accurately, and solve the vergence-accommodation conflicts. However, the hologram is only a thin transmission hologram which can't be reconstructed by white light. Wavefront printing can not only solve the vergence-accommodation conflicts, but also achieve a thick reflection hologram for white-light reconstruction with good observation effect. Principles of different holographic printing techniques were introduced, then research status of each technique was analyzed. Finally, the advantages and disadvantages of them were discussed to illustrate their own properties.

Highly concentrated phenanthrenequinone – polymethylmethacrylate composite for thick reflection holograms recording at 532 nm

The formation of stable phase reflection holograms in highly-concentrated phenanthrenequinone - polymethylmethacrylate (PQ-PMMA) layers with a thickness of about 100 µm at 532 nm recording wavelength has been investigated. In spite of the low absorbance of the photosensitive material at the long wave edge of the absorption spectrum, a refractive index modulation of 4.2·10−4 close to the practically reachable limit was achieved during the optical recording. Quantitative investigation of thermal and light treatment of the recorded holograms has demonstrated a tenfold increase of the diffraction efficiency (up to 60%) without disturbing the angular selectivity profile.

Review of Holographic Printers for Computer-Generated Holograms

Two different types of holographic printers have been developed that can output holograms on photosensitive materials from digital data calculated as computer-generated holograms to display three-dimensional (3-D) images. The first one is the holographic fringe printer that can write a thin transmission hologram. The other one is the volume hologram printer to write a thick reflection hologram. These printers can output holograms that can provide all physiological depth cues the same as real objects. Therefore, reconstructed images are very natural and do not provide the accommodation-convergence conflict like other 3-D display systems, such as stereoscopic displays. This paper introduces basics of printers, improvements, and reconstructed images.

Photothermal Conversion of Color Centers in $$\hbox {CaF}_{2}$$ CaF 2 Crystals: A Process Underlying the Use of Crystals as a Holographic Medium

Photothermal effects in a holographic medium of calcium fluoride crystals with color centers \((\hbox {CaF}_{2})\) are discussed. It is shown that photochromism of this crystal makes it possible to record volume holograms within its volume and to change their physical properties by photothermal treatments. The diffusion–drift mechanism of a hologram recording in \(\hbox {CaF}_{2}\) includes the conversion of centers and their redistribution over the crystal bulk. Depending on the readout wavelength, it is possible to read out amplitude, amplitude phase, or phase holograms. The opportunity to record thick holograms in \(\hbox {CaF}_{2}\) and their high resistance with respect to non-coherent illumination at an elevated temperature allows forming narrow-band angular and spectral holographic filters in \(\hbox {CaF}_{2}\).

Tailoring 3-Component Photoinitiating Systems for Use as Efficient Photopolymerizable Holographic Material

To enhance the efficiency of photopolymerizable systems as holographic recording materials, the use of 3-component photoinitiating systems (PIS) was explored. In order to get more insight into the hologram formation, gratings' recording curves were compared to those of monomer conversion obtained by RTFTIR spectroscopy. This work outlines the differences between the photoinitiating systems. A holographic 3-component recording material giving rise to thick phase holograms with both high diffraction yield and high rate of formation is highlighted.

Explaining the success of Kogelnik's coupled-wave theory by means of perturbation analysis: discussion.

The problem of diffraction of an electromagnetic wave by a thick hologram grating can be solved by the famous Kogelnik's coupled-wave theory (CWT) to a very high degree of accuracy. We confirm this finding by comparing the CWT and the exact result for a typical example and propose an explanation in terms of perturbation theory. To this end we formulate the problem of diffraction as a matrix problem following similar well-known approaches, especially rigorous coupled-wave theory (RCWT). We allow for a complex permittivity modulation and a possible phase shift between refractive index and absorption grating and explicitly incorporate appropriate boundary conditions. The problem is solved numerically exact for the specific case of a planar unslanted grating and a set of realistic values of the material's parameters and experimental conditions. Analogously, the same problem is solved for a two-dimensional truncation of the underlying matrix that would correspond to a CWT approximation but without the usual further approximations. We verify a close coincidence of both results even in the off-Bragg region and explain this result by means of a perturbation analysis of the underlying matrix problem. Moreover, the CWT is found not only to coincide with the perturbational approximation in the in-Bragg and the extreme off-Bragg cases, but also to interpolate between these extremal regimes.

Dual-grating confocal-rainbow volume holographic imaging system designs for high depth resolution

Confocal microscopy rejects out-of-focus light from the object by scanning a pinhole through the image and reconstructing the image point by point. Volume holographic imaging systems with bright-field illumination have been proposed as an alternative to conventional confocal-type microscopes that does not require scanning of a pinhole or a slit. However, due to wavelength-position degeneracy of the hologram, the high Bragg selectivity of the volume hologram is not utilized and system performance is not optimized. Confocal-rainbow illumination has been proposed as a means to remove the degeneracy and improve optical sectioning in these systems. In prior work, two versions of this system were illustrated: the first version had a separate illumination and imaging grating and the second used a single grating to disperse the incident light and to separate wavelengths in the imaging path. The initial illustration of the dual-grating system has limited depth resolution due to the low selectivity of the illumination grating. The initial illustration of the single-grating system has high depth resolution but does not allow optimization of the illumination path and requires high optical quality of the holographic filters. In this paper we consider the design and tolerance requirements of the dual-grating system for high depth resolution and demonstrate the results with an experimental system. An experimental system with two 1.8 mm thick planar holograms achieved a depth resolution of 7 μm with a field of view of 1.9 mm and a hologram dispersion matching tolerance of ±0.008°.

Estimation of light penetration depth in turbid media using laser speckles

We present novel experimental method for estimation of the light penetration depth (LPD) in turbid media based on the analysis of the speckle pattern structure. Under the certain illumination conditions this structure is strongly dependent on the penetration depth. Presented theoretical model based on the Bragg diffraction from the thick holograms allows LPD estimation if only one parameter of the material, namely refractive index, of the material is known. Feasibility of the method was checked experimentally. Experimental results obtained for variety of the materials are in good agreement with the theoretical assumptions. It was shown that qualitative LPD comparison does not require knowledge of the material properties.

Optical vortex discrimination with a transmission volume hologram

Transmissive volume holograms are considered as mode-selective optical elements for the de-multiplexing and detecting of optical vortex modes according to the topological charge or mode number. Diffraction of vortex modes by a fundamental mode hologram is modeled using a physical optics model that treats the volume hologram as an angle-dependent transfer function. Diffracted irradiance profiles and diffraction efficiencies are calculated numerically as a function of the incident mode number. The results of the model are compared with experimental results obtained with volume holograms of fundamental and higher-order vortex modes. When considered as a function of detuning between the incident and recorded mode numbers, the measured diffraction efficiencies are found to be invariant with respect to the recorded mode number, provided that the order difference remains unchanged, and in close agreement with the predictions of the model. Measurements are made with a 1.3 mm thick permanent photo-thermo-refractive glass hologram and a 9 mm thick re-writable photorefractive lithium niobate hologram. A liquid-crystal spatial light modulator generates the vortex modes used to record and read the holograms. The results indicate that a simple volume hologram can discriminate between vortex modes; however, adjacent mode discrimination with low crosstalk would require a very thick hologram. Furthermore, broadening of the vortex angular spectrum, due to diffraction at a finite aperture, can adversely affect diffraction efficiencies.

Efficient photon sorter in a high-dimensional state space

An increase in the dimension of state space for quantum key distribution (QKD) can decrease its fidelity requirements while also increasing its bandwidth. A significant obstacle for QKD with qu$d$its ($d\geq 3$) has been an efficient and practical quantum state sorter for photons whose complex fields are modulated in both amplitude and phase. We propose such a sorter based on a multiplexed thick hologram, constructed e.g. from photo-thermal refractive (PTR) glass. We validate this approach using coupled-mode theory with parameters consistent with PTR glass to simulate a holographic sorter. The model assumes a three-dimensional state space spanned by three tilted planewaves. The utility of such a sorter for broader quantum information processing applications can be substantial.

Coupled-wave analysis of vector holograms: effects of modulation depth of anisotropic phase retardation

The diffraction properties of thick vector holograms were analyzed with the use of a simple coupled-wave theory. Two eigenpolarizations in the holograms were determined based on the dielectric perturbation, and diffraction efficiencies for the polarizations were calculated by applying the Kogelnik method. The results were compared with those simulated by the finite-difference time-domain method. As a result, it was demonstrated that the diffraction efficiencies calculated by the two methods are in good agreement for any incident polarization when the modulation depth of the anisotropic phase retardation is substantially smaller than the mean retardation. In addition, we confirmed that coupled-wave analysis provides reasonable accuracy for relatively large modulation in the case of Bragg incidence with eigenpolarization.

Analysis of Diffraction Characteristics of Holographic Grating in Photopolymer Films by Beam Propagation Method

Photopolymers are photopolymerized organic material, which are suitable for recording thick phase holograms. In addition, photopolymers are stable media with high diffraction efficiency; therefore, they are widely used as holographic recording media. A photopolymer must satisfy certain criteria to qualify as a practical recording medium. However, the reaction processes of such photopolymers are highly complicated. In this study, we have simulated the formation of diffraction gratings in photopolymers using the diffusion model based on elementary reactions, and analyzed their diffraction characteristics in detail by the beam propagation method. By this method, we analyzed and obtained diffraction characteristics that are otherwise difficult to analyze by conventional methods such as coupled-wave and rigorous coupled-wave analyses. Moreover, we also performed experimental interference fringe recording and evaluated the recording characteristics of photopolymer media. We consider that the results of our analysis will provide useful information for the development of holographic data storage.