Incident Wavefront Research Articles

Research on Nanometer Precision Measurement Method of High Order Even Aspheres

Optical aspheres are demanded with extremely high precision to meet functional requirements in space telescopes, extreme ultraviolet lithography, and other modern large optical systems. The nano-precision fabrication of optical aspheres requires high-precision measurements to guide deterministic optical processing. Null test is the preferred method for high-precision measurements. Null optics are required to compensate for the incident wavefront in the null test of optical aspheres. However, wavefront aberrations caused by the transmission flat or transmission sphere of interferometer and null optics can limit measurement accuracy and need to be separated. A nano-precision measurement method is proposed for the even optical aspheres of high order in this paper. A computer-generated hologram is used as a null optic to realize a null test on optical aspheres. Mapping distortion correction is performed on the measurement results to ensure that the transverse coordinates of the measurement results correspond correctly to those of the test surface. Absolute testing is applied to separate the wavefront aberrations caused by the computer-generated hologram and interferometer optics. Finally, the results obtained by this method were used to guide deterministic optical processing, enabling the nano-precision fabrication of optical aspheres.

Metagratings for underwater acoustic wavefront manipulation

Metagratings are flat periodic assemblies of small scatterers, engineered to achieve effects such as steering, beamforming, or absorption. They have recently received attention in both electromagnetism and acoustic fields. In this work, we report the design, modeling, and experimental characterization of metagratings to steer underwater acoustic wavefronts towards anomalous directions (i.e. not classically allowed by Snell-Decartes relationships) with high efficiency (close to 100%). They are build from simple assemblies of small brass cylinders, hold by 3D-printed plastic supports, and can redirect ultrasonic wavefronts either in reflection mode (when placed in front of a reflective surface like e.g. the water / air interface) or in transmission mode. Furthermore, we demonstrate how metagratings build from an asymmetrical pattern of multiple basic elements can achieve near perfect asymmetrical transmission: a wavefront incident from side of the structure is fully transmitted, while a similar wavefront incoming from the other side is fully reflected, achieving an acoustic one-way mirror effect. This work combine theoretical analysis, finite element modeling, and experimentation in water tanks to explore and demonstrate the potential of such metagratings for various applications in underwater acoustics, like communication, noise mitigation, and stealth.

Full transmission of vectorial waves through 3D multiple-scattering media.

A striking prediction from the random matrix theory (RMT) in mesoscopic physics is the existence of "open channels": waves that use multipath interference to achieve perfect transmission across an opaque disordered medium even in the multiple-scattering regime. Realization of such open channels requires a coherent control of the complete incident wavefront and has only been achieved for scalar waves in two dimensions (2D) so far. Here, we utilize a recently proposed "augmented partial factorization" full-wave simulation method to compute the polarization-resolved scattering matrix from 3D vectorial Maxwell's equations and demonstrate the existence of open channels in 3D disordered media. We examine the spatial profile of such open channels, demonstrate the existence of a bimodal transmission eigenvalue distribution, and study the effects of incomplete polarization control and finite-area illumination. The simulations provide full access to all spatiotemporal properties of the complex wave transport in 3D disordered systems, filling the gap left by experimental capabilities.

Extending the operational limit of a cooled spatial light modulator exposed to 200 W average power for holographic picosecond laser materials processing

The phase range of a Spatial Light Modulator (SLM) requires full 2π response for accurate phase modulation of the incident laser wavefront to create the desired structured intensity distribution. While cooled SLM’s allow increased power exposures, degradation in performance occurs with average powers P>100 W in a Gaussian beam due to residual absorption and consequent heating of the liquid crystal (LC) layer which seriously limits the phase stroke and diffraction efficiency. By introducing a refractive flat top intensity generator ahead of a cooled SLM, we have extended the phase range to Δφ = 2π at incident power P=210 W with only a small light field depolarisation. The operational limit has therefore been extended significantly, allowing efficient, parallel laser materials processing with a high-power picosecond laser. The calculation and implementation of binary Damman gratings for multi-beam laser processing is also shown to be useful at high power exposure.

Extreme wave impacts on a suspended box above free surface

A systematic approach incorporating precision-controlled experiments and high-fidelity numerical simulations has been employed to delve deeper into the physics of extreme wave impact pressures. Using a suite of high-speed imaging and high-resolution pressure sensing techniques that are accurately synchronized with the wave generation, a consistent and well-correlated set of data providing details of the focusing incident wave front, flow velocities, and impact pressure time histories has been successfully obtained. The numerical simulations using a validated incompressible smoothed particle hydrodynamics (ISPH) provided further details that were not tractable in the experiments. Based on the results obtained, there were five broad impact scenarios identified, ranging from focusing crest impacts to plunging jet impacts with varying entrapped air. Among these impact types, the highest impact pressures were those associated with a focusing wave crest. It has been found that high impact pressures were correlated with a convergence of the horizontally impinging overturning crest and the vertical surge of the wall boundary water mass, but with the overturning crest impingement just ahead of the arrival of the wall boundary water mass, which in essence amounted to the impingement of a focusing wave front. Depending on the relative arrival times of the impinging crest and the wall boundary upsurge, the impact scenarios could vary from an up-slosh to a jet impingement with a clear entrapped air pocket, followed by a flip-through of the incident crest. For the scenario with the highest impact pressures, broadly classified as type II impact in this paper, the peak pressures could reach as high as 85ρC2, an order of magnitude higher than impact scenarios without the focusing effect, such as the scenario with a plunging jet impact with entrapped air (classified as types III and IV impact in this study). As the volume of entrapped air for this scenario was relatively small, the one-phase ISPH model used in this study was able to capture the peak pressure characteristics, including the peak pressure amplitudes. However, in scenarios with significantly highly entrapped air during impact, a two-phase model would be necessary.

Direct imaging of magnetohydrodynamic wave mode conversion near a 3D null point on the sun

Mutual conversion of various kinds of magnetohydrodynamic (MHD) waves can have profound impacts on wave propagation, energy transfer, and heating of the solar chromosphere and corona. Mode conversion occurs when an MHD wave travels through a region where the Alfvén and sound speeds are equal (e.g., a 3D magnetic null point). Here we report the direct extreme ultraviolet (EUV) imaging of mode conversion from a fast-mode to a slow-mode MHD wave near a 3D null point using Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. An incident fast EUV wavefront associated with an adjacent eruptive flare propagates laterally through a neighboring pseudostreamer. Shortly after the passage of the fast EUV wave through the null point, a slow-mode wave appears near the null that propagates upward along the open structures and simultaneously downward along the separatrix encompassing the fan loops of the pseudostreamer base. These observations suggest the existence of mode conversion near 3D nulls in the solar corona, as predicted by theory and MHD simulations. Moreover, we observe decaying transverse oscillations in both the open and closed structures of the pseudostreamer, along with quasiperiodic type III radio bursts indicative of repetitive episodes of electron acceleration.

Principle and application of holographic modal wavefront sensor for extended objects and general aberration modes

By employing a computer-generated hologram (CGH) to modulate the incident wavefront, the holographic modal wavefront sensor (HMWFS) can efficiently measure multiple aberration modes in a single shot, providing significant advantages such as computational simplicity, rapid measurement speed, and customizable assessment of aberration modes. However, the current HMWFS is confined to point-like objects and aberrations with specific orthogonality, thereby limiting its application scope. In this paper, we introduce a novel HMWFS that, to the best of our knowledge, is compatible with extended objects and general aberration modes. We establish the theoretical relationship between the second moment of the image intensity and the coefficients of general aberration modes for extended objects. Simulation and experimental results confirm the feasibility and performance of the proposed HMWFS. Additionally, we demonstrated its practical use in measuring misalignments of the secondary mirror of a space telescope. Simulations show that the misalignments can be effectively corrected within 2∼3 iterations.

О распространении микроволнового излучения в слоистых биотканях грудной клетки человека

The features of microwave propagation have been studied in a model of layered biotissue simulating a section of the human chest in a GHz frequency range. The model contained four layers (skin, subcutaneous fat, muscle, lung), and is bounded by a rectangular parallelepiped. The selection of layer thicknesses was carried out basing on the biological model of the human body. The following frequencies were selected for numerical simulation: 1, 2, 5, 10, 20, 50 and 100 GHz. The transverse size of the model and the incident wave front were chosen to be equal to 5λ, excluding an influence of wave diffraction on the lateral surface of the model on wave propagation along the axis of the model. The choice of the thickness of the last layer in the model ensured minimization of interference effects with the wave penetrating into the model from its back surface. As a result of modeling, the features of microwave propagation in layered biological tissue were revealed and the dependence of the depth of penetration of microwave radiation into the biostructure on the frequency was calculated.

Multifocal meta-fiber based on the fractional Talbot effect.

Multi-focusing of light is a crucial capability for photonic devices that can be effectively achieved by precisely modulating the phase delay on the incident wavefront. However, integrating functional structures into optical fibers for remote light focusing remains challenging due to the complex device design and limited fabrication approaches. Here, we present the design and fabrication of metalens array on the end-face of a tailored single-mode step-index fiber for focusing light field into closely packed focal spot array. The metalenses are configured based on the fractional Talbot effect and benefit a modular design capability. Light passing through the optical fiber can be focused into different focal planes. With a synergistic 3D laser nanoprinting technique based on two-photon polymerization, high-quality meta-fibers are demonstrated for focusing light parallelly with a uniform numerical aperture (NA) as high as approximately 0.77. This may facilitate various applications such as optical trapping, generation of sophisticated beam profiles, and boosting light coupling efficiencies.

Optical wavefront phase-tilt measurement using Si-photonic waveguide grating couplers.

Silicon photonic wavefront phase-tilt sensors for wavefront monitoring using surface coupling grating arrays are demonstrated. The first design employs the intrinsic angle dependence of the grating coupling efficiency to determine local wavefront tilt, with a measured sensitivity of 7 dB/°. The second design connects four gratings in an interferometric waveguide circuit to determine incident wavefront phase variation across the sensor area. In this device, one fringe spacing corresponds to 2° wavefront tilt change. These sensor elements sample a wavefront incident on the chip surface without the use of bulk optic elements, fiber arrays, or imaging arrays. Both sensor elements are less than 60 µm across and are potential unit cell sensor elements for large arrays that monitor wavefront shape across an image or pupil plane in adaptive optics systems for free-space optical communications, astronomy, and beam pointing applications.

Prediction on X-ray output of free electron laser based on artificial neural networks

Knowledge of x-ray free electron lasers’ (XFELs) pulse characteristics delivered to a sample is crucial for ensuring high-quality x-rays for scientific experiments. XFELs’ self-amplified spontaneous emission process causes spatial and spectral variations in x-ray pulses entering a sample, which leads to measurement uncertainties for experiments relying on multiple XFEL pulses. Accurate in-situ measurements of x-ray wavefront and energy spectrum incident upon a sample poses challenges. Here we address this by developing a virtual diagnostics framework using an artificial neural network (ANN) to predict x-ray photon beam properties from electron beam properties. We recorded XFEL electron parameters while adjusting the accelerator’s configurations and measured the resulting x-ray wavefront and energy spectrum shot-to-shot. Training the ANN with this data enables effective prediction of single-shot or average x-ray beam output based on XFEL undulator and electron parameters. This demonstrates the potential of utilizing ANNs for virtual diagnostics linking XFEL electron and photon beam properties.

Construction and use of an adaptive optics two-photon microscope with direct wavefront sensing.

Two-photon microscopy, combined with the appropriate optical labelling, enables the measurement and tracking of submicrometer structures within brain cells, as well as the spatiotemporal mapping of spikes in individual neurons and of neurotransmitter release in individual synapses. Yet, the spatial resolution of two-photon microscopy rapidly degrades as imaging is attempted at depths of more than a few scattering lengths into tissue, i.e., below the superficial layers that constitute the top 300-400 µm of the neocortex. To obviate this limitation, we shape the focal volume, generated by the excitation beam, by modulating the incident wavefront via guidestar-assisted adaptive optics. Here, we describe the construction, calibration and operation of a two-photon microscope that incorporates adaptive optics to restore diffraction-limited resolution at depths close to 900 µm in the mouse cortex. Our setup detects a guidestar formed by the excitation of a red-shifted dye in blood serum, used to directly measure the wavefront. We incorporate predominantly commercially available optical, optomechanical, mechanical and electronic components, and supply computer-aided design models of other customized components. The resulting adaptive optics two-photon microscope is modular and allows for expanded imaging and optical excitation capabilities. We demonstrate our methodology in the mouse neocortex by imaging the morphology of somatostatin-expressing neurons that lie 700 µm beneath the pia, calcium dynamics of layer 5b projection neurons and thalamocortical glutamate transmission to L4 neurons. The protocol requires ~30 d to complete and is suitable for users with graduate-level expertise in optics.

Single-shot, phase-diversity phase retrieval for high-energy beam focal-spot diagnostics

A single-shot, phase-diversity phase-retrieval method is demonstrated for directly characterizing the on-target focal spot at kilojoule energy in the OMEGA 60-beam laser. A 2D holographic grating that introduces diversity focal spots is installed in an existing on-target focal-spot diagnostic to obtain a single-shot diversity focal-spot image. A nonlinear optimization phase-retrieval algorithm with four diversity focal spots is employed to determine modal coefficients for the incident wavefront. The agreement between the measured and calculated focal spots is excellent. This approach can be easily adapted for other high-energy focal-spot diagnostics.

Focusing Coherent Light through Volume Scattering Phantoms via Wavefront Shaping.

Manipulating the wavefront of coherent light incident on scattering media to enhance the imaging depth, sensitivity, and resolution is a common technique in biomedical applications. Local phase variations cause changes in the interference and can be used to create a focus inside or behind a scattering medium. We use wavefront shaping (WFS) to force constructive interference at an arbitrary location. The amount of light transmitted into a given region strongly depends on the scattering and absorption characteristics. These are described by their respective coefficients μs and μa and the scattering phase function. Controlling the scattering and absorption coefficients, we study the behavior of wavefront shaping and the achievable intensity enhancement behind volume scattering media with well-defined optical properties. The phantoms designed in this publication are made of epoxy resin. Into these epoxy matrices, specific amounts of scattering and absorbing particles, such as titanium dioxide pigments and molecular dyes, are mixed. The mixture obtained is filled into 3D-printed frames of various thicknesses. After a precise fabrication procedure, an integrating sphere-based setup characterizes the phantoms experimentally. It detects the total hemispherical transmission and reflection. Further theoretical characterization is performed with a newly developed hybrid PN method. This method senses the flux of light into a particular angular range at the lower boundary of a slab. The calculations are performed without suffering from ringing and fulfill the exact boundary conditions there. A decoupled two-path detection system allows for fast optimization as well as sensitive detection. The measurements yield results that agree well with the theoretically expected behavior.

Full-Vectorial Light Propagation Simulation of Optimized Beams in Scattering Media

Volumetric scattering prevents imaging modalities in biomedical optics from imaging deep inside tissue. The optimization of the incident wavefront has the potential to improve these imaging modalities. To investigate the optimization and light propagation of such beams inside scattering media rigorously, full-vectorial simulations based on solutions of Maxwell’s equations are necessary. In this publication, we present a versatile two-step beam synthesis method to efficiently simulate the scanning and phase optimization of a focused beam inside a static scattering medium. We present four different approaches to the phase optimization of the energy density and the absolute value of the Poynting vector. We find that these quantities have two regions with different, almost exponential decays over depth for a non-optimized beam. Optimization by conjugating the phase of the projected electric field in various directions at the focus shows an improvement below a certain penetration depth. Seeking global solutions to the optimization problems reveals an even better enhancement in the energy density and the absolute value of the Poynting vector in the focus. For Poynting vector optimization, the differences between the presented optimization approaches are more significant than for the energy density. With the presented method, it is possible to efficiently simulate different imaging methods improved by wavefront shaping to investigate their possible penetration depths.

High robustness single-shot wavefront sensing method using a near-field profile image and fully-connected retrieval neural network for a high power laser facility.

This paper proposes a single-shot high robustness wavefront sensing method based on deep-learning for wavefront distortion measurement in high power lasers. This method could achieve fast and robust wavefront retrieval by using a single-shot near-field profile image and trained network. The deep-learning network uses fully-skip cross connections to extract and integrate multi-scale feature maps from various layers and stages, which improves the wavefront retrieval speed and enhances the robustness of the method. The numerical simulation proves that the method could directly predict the wavefront distortion of high power lasers with high accuracy. The experiment demonstrates the residual RMS between the method and a Shack-Hartmann wavefront sensor is less than 0.01 µm. The simulational and experimental results show that the method could accurately predict the incident wavefront distortion in high power lasers, exhibiting high speed and good robustness in wavefront retrieval.

Influence of Image Processing Method on Wavefront Reconstruction Accuracy of Large-Aperture Laser

In order to improve the wavefront reconstruction accuracy of a large-aperture laser, this paper proposed an adaptive window preprocessing algorithm based on the threshold center of gravity method (AW-TCoG). The effects of median filtering and mean filtering on spot image processing and wavefront reconstruction accuracy are simulated and analyzed. The results show that the mean filtering method has a better effect on noise elimination and can further improve the accuracy of wavefront reconstruction. In addition, the centroid detection errors of large-aperture laser wavefront reconstruction through the center of gravity (CoG), the threshold center of gravity (T-CoG), and the Windowing method were studied. The analysis shows that, due to the influence of noise, the wavefront reconstruction accuracy is poor when the CoG and Windowing methods are used to calculate centroid parameters, while the wavefront reconstruction accuracy of the threshold centroid method is better and can reach 0.2λ. When using the AW-TCoG proposed in this paper, the wavefront reconstruction accuracy can be maintained within 0.1λ for different incident wavefront RMS values and spot images with different signal-to-noise ratio (SNR) levels. Compared with the traditional threshold centroid method, the wavefront reconstruction accuracy of this method is significantly improved.

Enhanced secrecy in optical communication using speckle from multiple scattering layers.

We study the secrecy of an optical communication system with two scattering layers, to hide both the sender and receiver, by measuring the correlation of the intermediate speckle generated between the two layers. The binary message is modulated as spatially shaped wavefronts, and the high number of transmission modes of the scattering layers allows for many uncorrelated incident wavefronts to send the same message, making it difficult for an attacker to intercept or decode the message and thus increasing secrecy. We collect 50,000 intermediate speckle patterns and analyze their correlation distribution using the Kolmogorov-Smirnov (K-S) test. We search for further correlations using the K-Means and Hierarchical unsupervised classification algorithms. We find no correlation between the intermediate speckle and the message, suggesting a person-in-the-middle attack is not possible. This method is compatible with any digital encryption method and is applicable for codifications in optical wireless communication (OWC).

Controlling light propagation in multimode fibers for imaging, spectroscopy, and beyond

Light transport in a highly multimode fiber exhibits complex behavior in space, time, frequency, and polarization, especially in the presence of mode coupling. The newly developed techniques of spatial wavefront shaping turn out to be highly suitable to harness such enormous complexity: a spatial light modulator enables precise characterization of field propagation through a multimode fiber, and by adjusting the incident wavefront it can accurately tailor the transmitted spatial pattern, temporal profile, and polarization state. This unprecedented control leads to multimode fiber applications in imaging, endoscopy, optical trapping, and microfabrication. Furthermore, the output speckle pattern from a multimode fiber encodes spatial, temporal, spectral, and polarization properties of the input light, allowing such information to be retrieved from spatial measurements only. This article provides an overview of recent advances and breakthroughs in controlling light propagation in multimode fibers, and discusses newly emerging applications.

Analytical modelling of adaptive optics systems: Role of the influence function

Context. Adaptive optics (AO) is now a tool commonly deployed in astronomy. The real time correction of the atmospheric turbulence that AO enables allows telescopes to perform close to the diffraction limit at the core of their point spread function (PSF). Among other factors, AO-corrected PSFs depend on the ability of the wavefront corrector (WFC), generally a deformable mirror, to fit the incident wavefront corrugations. Aims. In this work, we focus on this error introduced by the WFC, the so-called fitting error. To date, analytical models only depend on the WFC cut-off frequency, and Monte Carlo simulations are the only solution for studying the impact of the WFC influence function shape on the AO-corrected PSF. We aim to develop an analytical model accounting for the influence function shape. Methods. We first obtain a general analytical model of the fitting error structure function. With additional hypotheses, we then derive an analytical model of the AO-corrected power spectral density. These two analytical solutions are compared with Monte Carlo simulations on different ideal profiles (piston, pyramid, Gaussian) as well as with real hardware (DM192 from ALPAO). Results. Our analytical predictions show a very good agreement with the Monte Carlo simulations. We show that in the image plane, the depth of the correction as well as the transition profile between the AO-corrected area and the remaining turbulent halo depend on the influence functions of the WFC. We also show that the generally assumed hypothesis of stationarity of the AO correction is actually not met. Conclusions. As the fitting error is the intrinsic optimal limit of an AO system, our analytical model allows for the assessment of the theoretical limits of extreme AO systems limited by the WFC in high-contrast imaging through a context where other errors become comparable.