Holographic Wavefront Research Articles

Author Correction: Image-guided computational holographic wavefront shaping

Author Correction: Image-guided computational holographic wavefront shaping

Comparing thin and volume regimes of analog holograms for wavefront sensing.

Two analog holographic wavefront sensors, for measurement of defocus, have been fabricated as both thin and volume phase transmission holograms in a self-developing photopolymer. This represents the first reported direct comparison of hologram regimes when designed for wavefront sensing. An analysis of the effect of crosstalk in the presence of one other aberration mode (astigmatism X (0/90°), coma X (horizontal), and primary spherical aberration) was carried out with each version of the sensor. The performance of thin and volume analog holographic wavefront sensors was characterized under emulated conditions associated with moderate atmospheric turbulence.

Analog holographic wavefront sensor for defocus and spherical aberration measurement recorded in a photopolymer.

An analog holographic wavefront sensor (AHWFS), for measurement of low and high order (defocus and spherical aberration) aberration modes has been developed as volume phase holograms in a photopolymer recording medium. This is the first time that high order aberrations such as spherical aberration can be sensed using a volume hologram in a photosensitive medium. Both defocus and spherical aberration were recorded in a multi-mode version of this AHWFS. Refractive elements were used to generate a maximum and minimum phase delay of each aberration which were multiplexed as a set of volume phase holograms in an acrylamide based-photopolymer layer. The single-mode sensors showed a high degree of accuracy in determining various magnitudes of defocus and spherical aberration generated refractively. The multi-mode sensor also exhibited promising measurement characteristics and similar trends to the single-mode sensors were observed. The method of quantifying defocus was improved upon and a brief study into material shrinkage and sensor linearity is presented.

Breaking of Wavelength-Dependence in Holographic Wavefront Sensors Using Spatial-Spectral Filtering.

Nowadays, wavefront sensors are widely used to control the shape of the wavefront and detect aberrations of the complex field amplitude in various fields of physics. However, almost all of the existing wavefront sensors work only with quasi-monochromatic radiation. Some of the methods and approaches applied to work with polychromatic radiation impose certain restrictions. However, the contemporary methods of computer and digital holography allow implementing a holographic wavefront sensor that operates with polychromatic radiation. This paper presents a study related to the analysis and evaluation of the error in the operation of holographic wavefront sensors with such radiation.

Пространственно-спектральная фильтрация светового поля с помощью фазового модулятора света

An improved approach to the synthesis of holographic filters, which are achromatic computer-generated Fourier holograms, is demonstrated, taking into account the spatial and spectral characteristics of the light field. An experimental testing of the obtained filters was carried out based on the use of a holographic wavefront sensor operating on the principles of correlation analysis, using several quasi-monochromatic radiation sources. Using the proposed filters, the longitudinal chromatic aberration was measured, the value of which was λ/3 with an error of λ/50.

Wavelength-Independent Correlation Detection of Aberrations Based on a Single Spatial Light Modulator

The cumulative achievements in the fields of science and technology have allowed us to substantially approach the solution of the phase problem in optics. Among all phasometric methods, single-beam methods are the most promising, since they are more variable and versatile. Single-beam methods are based either on the analysis of the intensity distribution, as is conducted by interferometers and wavefront sensors, or on the transformation of the phase into an intensity distribution due to spatial filtering, as is conducted by holographic methods. However, all these methods have the problem of working with polychromatic radiation and require spectral filters to process such radiation. This paper presents a new approach to the synthesis of Fourier holograms used in holographic wavefront sensors that make it possible to create achromatic elements and work with white light without the use of additional filters. The approach was numerically and experimentally verified.

Simulation-based design optimization of the holographic wavefront sensor in closed-loop adaptive optics

Adaptive optics systems are used to compensate for wavefront distortions introduced by atmospheric turbulence. The distortions are corrected by an adaptable device, normally a deformable mirror. The control signal of the mirror is based on the measurement delivered by a wavefront sensor. Relevant characteristics of the wavefront sensor are the measurement accuracy, the achievable measurement speed and the robustness against scintillation. The modal holographic wavefront sensor can theoretically provide the highest bandwidth compared to other state of the art wavefront sensors and it is robust against scintillation effects. However, the measurement accuracy suffers from crosstalk effects between different aberration modes that are present in the wavefront. In this paper we evaluate whether the sensor can be used effectively in a closed-loop AO system under realistic turbulence conditions. We simulate realistic optical turbulence represented by more than 2500 aberration modes and take different signal-to-noise ratios into account. We determine the performance of a closed-loop AO system based on the holographic sensor. To counter the crosstalk effects, careful choice of the key design parameters of the sensor is necessary. Therefore, we apply an optimization method to find the best sensor design for maximizing the measurement accuracy. By modifying this method to take the changing effective turbulence conditions during closed-loop operation into account, we can improve the performance of the system, especially for demanding signal-to-noise-ratios, even more. Finally, we propose to implement multiple holographic wavefront sensors without the use of additional hardware, to perform multiple measurement at the same time. We show that the measurement accuracy of the sensor and with this the wavefront flatness can be increased significantly without reducing the bandwidth of the adaptive optics system.

Metasurface-Based Holographic Display With All-Dielectric Meta-Axilens

In a classical metasurface-based holographic display system, a clear holographic reconstruction image can be obtained only in the back-focal-plane of the lens. However, when the receiving plane deviates from the focal plane, the reconstructed holographic image suffers from degradations that limit the quality of the images. Hence, we propose a novel metasurface-based holographic display to realize clearly continuous imaging within the specified position range by superimposing the meta-axilens phase. It can effectively relieve the alignment requirements of the imaging system. Firstly, the parameters and properties of all-dielectric meta-atom is analyzed. Then we compare and demonstrate two sets of metasurface-based optical elements (meta-axicon, meta-lens, and meta-axilens), made by silicon meta-atoms working at 610 nm. Furthermore, we show that the metasurface hologram with the axilens phase can form a series of the relatively clear holographic images away from the focal plane. Our proposal suggests a method to reconstruct holographic wavefront in different planes simultaneously, and one can find their application domain, such as 3D biological imaging, spectroscopy, optical information storage, and encryption.

Optimization of the holographic wavefront sensor for open-loop adaptive optics under realistic turbulence. Part I: simulations.

The modal holographic wavefront sensor enables fast measurement of individual aberration modes without the need for time-consuming calculations. However, the measurement accuracy suffers greatly from intermodal crosstalk, caused when the wavefront contains more aberrations than the one to be measured. In this paper, we present sensor optimization to minimize this effect and show the improvement when using Karhunen-Lòeve instead of Zernike modes as the basis. Finally, we show in simulation that an open-loop adaptive optics system based on the optimized sensor can be used to correct the effect of realistic, dynamic atmospheric turbulence on a wavefront and increase its Strehl ratio significantly.

Observing distant objects with a multimode fiber-based holographic endoscope

Holographic wavefront manipulation enables converting hair-thin multimode optical fibers into minimally invasive lensless imaging instruments conveying much higher information densities than conventional endoscopes. Their most prominent applications focus on accessing delicate environments, including deep brain compartments, and recording micrometer-scale resolution images of structures in close proximity to the distal end of the instrument. Here, we introduce an alternative “far-field” endoscope capable of imaging macroscopic objects across a large depth of field. The endoscope shaft with dimensions of 0.2 × 0.4 mm2 consists of two parallel optical fibers: one for illumination and the other for signal collection. The system is optimized for speed, power efficiency, and signal quality, taking into account specific features of light transport through step-index multimode fibers. The characteristics of imaging quality are studied at distances between 20 mm and 400 mm. As a proof-of-concept, we provide imaging inside the cavities of a sweet pepper commonly used as a phantom for biomedically relevant conditions. Furthermore, we test the performance on a functioning mechanical clock, thus verifying its applicability in dynamically changing environments. With the performance reaching the standard definition of video endoscopes, this work paves the way toward the exploitation of minimally invasive holographic micro-endoscopes in clinical and diagnostics applications.

Suppression of High Order Diffraction in Holographic Wave-Front Sensor

Suppression of High Order Diffraction in Holographic Wave-Front Sensor

Holographic wavefront sensors

A brief historical review of the first designs of holographic wavefront sensors (WFS’s) and the concepts lying in their basis is presented. The main directions in the development of these sensors are highlighted and considered. One of these directions implies a two-stage transition from the use of several separate measuring channels with holograms filtering only one Zernike mode: first to the use of one channel with a multiplexed hologram filtering several first Zernike modes, and then to filtering the entire set of Zernike modes with the aid of one combined multiplexed hologram. Another line of research in this field (related to the first one) is the optimisation of the filter hologram structure in order to reduce cross-modulation interferences, increase multiplexing level, etc. One more line of research implements principles of dynamic holography by introducing spatial light modulators into the WFS composition. Hence, the advantages of time multiplexing of holograms can be used. The approach developed by G. Andersen’s team, aimed at adapting the holographic WFS design for operation as an element of adaptive optical system with a zonal corrector, as well as an approach implying development of hybrid holographic sensors, are considered separately. The results of the authors’ studies of holographic WFS’s with application of the methods of Fourier holography (i.e., holography of focused beams, in particular, using diffuse scatterers in a hologram recording channel) are also reported.

Complete Complex Amplitude Modulation with Electronically Tunable Graphene Plasmonic Metamolecules

Dynamic high-resolution wavefront modulation of light is a long-standing quest in photonics. Metasurfaces have shown potential for realizing light manipulation with subwavelength resolution through nanoscale optical elements, or metaatoms, to overcome the limitations of conventional spatial light modulators. State-of-the-art active metasurfaces operate via phase modulation of the metaatoms, and their inability to also independently control the scattered amplitude leads to an inferior reconstruction of the desired wavefronts. This fundamental problem posed severe performance limitations particularly for applications relying on subwavelength spatiotemporal complex field modulation, which includes dynamic holography, high-resolution imaging, optical tweezing, and optical information processing. Here, we present the "metamolecule" strategy, which incorporates two independent subwavelength scatterers composed of noble metal antennas coupled to gate-tunable graphene plasmonic nanoresonators. The two-parametric control of the metamolecule secures the complete control of both amplitude and phase of light, enabling 2π phase shift as well as large amplitude modulation including perfect absorption. We further develop a generalized graphical model to examine the underlying requirements for complete complex amplitude modulation, offering intuitive design guidelines to maximize the tunability in metasurfaces. To illustrate the reconfigurable capability of our designs, we demonstrate dynamic beam steering and holographic wavefront reconstruction in periodically arranged metamolecules.

Angle-compensated holographic wave front printing for the fabrication of holographic optical elements operating in the infrared

We introduce a technique for recording volume holographic optical elements (vHOEs) designed for operation at a wavelength different from the recording wavelength. This technique relies on our holographic wave front printer, which employs two spatial light modulators for recording wave front generation. We present the procedure for computing the required recording wave fronts accurately and noniteratively from the desired vHOE optical transformation function. We exploit this technique for the fabrication of a converging lens vHOE with an operation wavelength of 915 nm at a recording wavelength of 515 nm. Finally, we demonstrate the holographic lens functionality directly in the infrared spectral region.

Hardware/Software Support for Correlation Detection in Holographic Wavefront Sensors

An algorithm that automatically calculates aberrations in the holographic wavefront sensor scheme previously proposed by the authors is described, which enables measuring aberrations by means of iterative generation of holograms on a phase spatial light modulator (SLM). The practical implementation of this algorithm on the basis of the feedback of the camera and SLM is given. Using the proposed algorithm, defocusing was measured with an accuracy of λ/160.

Rapid wide-field imaging through scattering media by digital holographic wavefront correction.

Imaging through scattering media has been a long standing challenge in many disciplines. One of the promising solutions to address the challenge is the wavefront shaping technique, in which the phase distortion due to a scattering medium is corrected by a phase modulation device such as a spatial light modulator (SLM). However, the wide-field imaging speed is limited either by the feedback-based optimization to search the correction phase or by the update rate of SLMs. In this report, we introduce a new method called digital holographic wavefront correction, in which the correction phase is determined by a single-shot off-axis holography. The correction phase establishes the so-called "scattering lens", which allows any objects to be imaged through scattering media; in our case, the "scattering lens" is a digital one established through computational methods. As no SLM is involved in the imaging process, the imaging speed is significantly improved. We have demonstrated that moving objects behind scattering media can be recorded at the speed of 2.8fps with each frame corrected by the updated correction phase while the image contrast is maintained as high as 0.9. The image speed can potentially reach the video rate if the computing power is sufficiently high. We have also demonstrated that the digital wavefront correction method also works when the light intensity is low, which implicates its potential usefulness in imaging dynamic processes in biological tissues.

Measurement of wavefront curvature using computer-generated holograms.

The present article is dedicated to the problem of computer-generated holograms application for measurement of optical wavefront curvature with high precision. A holographic wavefront sensor scheme based on a phase-only spatial light modulator, which is used for CGH displaying, is proposed. The presented optical scheme and processing algorithm are validated with numerical simulations and experimental modelling.

新型功能超颖表面波前调制技术的发展与应用(特邀)

As a kind of smart surface, metasurfaces are usually composed of sub-wavelength nano-antenna arrays which are specially designed and processed with characteristic sizes close to or less than their working wavelengths. Metasurfaces can arbitrarily modulate the amplitude, phase and polarization of the light field. It has the advantages of ultra-thinness, ultra-small pixels, broadband, low loss, easy processing, flexible design and great functionality. This paper reviews the research progress of metasurfaces in holographic display, wavefront modulation and polarization conversion, active tunable, nonlinear wavefront modulation, and looks forward to the future development trend of metasurfaces. As an ultra-thin and miniaturized wavefront modulation device, metasurfaces have great information capacity and are more suitable for the future development of highly integrated micro optoelectronic systems. Metasurfaces provide potential feasibility and new perspectives for a plethora of applications such as holographic display, beam shaping, vortex beam generation, data storage, encryption and anti-counterfeiting, metalens and dispersion control, color printing, asymmetric transmission, nonlinear optics, the spin-Hall effect of light, optical communication, integrated optoelectronics. They are expected to replace the traditional optoelectronic devices and show broad prospects for future development.

Аппаратно-алгоритмическое обеспечение корреляционного детектирования в голографических датчиках волнового фронта

AbstractAn algorithm that automatically calculates aberrations in the holographic wavefront sensor scheme previously proposed by the authors is described, which enables measuring aberrations by means of iterative generation of holograms on a phase spatial light modulator (SLM). The practical implementation of this algorithm on the basis of the feedback of the camera and SLM is given. Using the proposed algorithm, defocusing was measured with an accuracy of λ/160.

Wavefront imaging by using an inline holographic microscopy system based on a double-sideband filter

In this letter, we propose an inline holographic microscopy (ILHM) system, based on the double-sideband technique (DST), for wavefront imaging. The presented optical system shows all the benefits of the previously reported DST (instantaneous removing of ghost images in an inline scheme) but adapted to a microscopic system. The double-sideband filter is implemented by using a transparent liquid crystal (LC) bi-pixel device. Specifically, by addressing the proper phase values to each half of the LC bi-panel, which is located at the Fourier plane of the holographic system, the conjugate image is removed. What is more, by using a high numerical aperture microscope objective, we achieved the microscopic wavefront holography imaging. Finally, the feasibility of the proposed system is testified by obtaining holographic wavefront images of different objects.