High-speed Wavefront Research Articles

High-fidelity and high-speed wavefront shaping by leveraging complex media.

High-precision light manipulation is crucial for delivering information through complex media. However, existing spatial light modulation devices face a fundamental speed-fidelity tradeoff. Digital micromirror devices have emerged as a promising candidate for high-speed wavefront shaping but at the cost of compromised fidelity due to the limited control degrees of freedom. Here, we leverage the sparse-to-random transformation through complex media to overcome the dimensionality limitation of spatial light modulation devices. We demonstrate that pattern compression by sparsity-constrained wavefront optimization allows sparse and robust wavefront representations in complex media, improving the projection fidelity without sacrificing frame rate, hardware complexity, or optimization time. Our method is generalizable to different pattern types and complex media, supporting consistent performance with up to 89% and 126% improvements in projection accuracy and speckle suppression, respectively. The proposed optimization framework could enable high-fidelity high-speed wavefront shaping through different scattering media and platforms without changes to the existing holographic setups, facilitating a wide range of physics and real-world applications.

Focusing scattering light field with different states based on iterative algorithm

Transmitting optical information through scattering medium has broad application prospects in biomedical, aerospace and other fields. However, the light passing through the scattering medium will cause wavefront distortion and optical information blurring. Wavefront shaping technology uses a mathematical matrix to characterize the characteristics of scattering medium, which can achieve refocusing and imaging after light propagation through the scattering medium. It mainly includes optical phase conjugation, optical transmission matrix and wavefront shaping based on iterative optimization. However, the iterative wavefront shaping is considered to be a cost-effective method. Based on the wavefront amplitude modulation technology, the wavefront amplitude of the incident light is continuously adjusted by using the optimization algorithm to find the corresponding wavefront amplitude distribution that can maximize the light intensity in the target area. The system generates binary patterns implemented with digital-micromirror device (DMD) based on on-off state of micromirror, where “on” represents 1 and “off” refers to 0. The DMD has a high refresh rate and can achieve high speed wavefront amplitude modulation by using the iteration algorithm. In the experiment, the scattering medium is prepared with TiO<sub>2</sub>, water and gelatin, whose persistence times are controlled with the water-gelatin ratio (WGR). In addition, the Pearson correlation coefficient (Cor) curve obtained through 300-s-measurement under different WGR conditions, which shows that the greater WGR, the shorter the persistence time is. The experiment mainly studies the focusing of the spatial light through scattering media by wavefront amplitude modulation, and discusses the ability of point guard algorithm (PGA) and genetic algorithm (GA) to control the scattered light field with different persistence times in 64 × 64 segments. The experimental results show that the PGA can achieve higher enhancement factor and more uniform multi-point focusing than the GA after 1000 iterations in the scattering medium with the same persistence time. The relative standard deviation value is inversely proportional to the WGR value when multi-point focusing can be completed. We also demonstrate that GA can only achieve single-point focusing when WGR = 40, and it cannot accomplish multi-point focusing in self-made scattering medium. This study not only verifies a method to achieve focusing scattering light field, but also provides a new scheme for testing the performance of the iterative wavefront shaping.

A Review on High-Speed Wavefront Shaping System

A Review on High-Speed Wavefront Shaping System

High-speed feedback based wavefront shaping for spatiotemporal enhancement of incoherent light through dynamic scattering media.

Feedback-based wavefront shaping is a promising and versatile technique for enhancing the contrast of a target signal for both coherent and incoherent light through a highly scattering medium. However, this technique can fail for a dynamic sample with a short correlation time. So far, most proposed methods for high-speed wavefront shaping can only directly enhance the intensity of coherent light but not incoherent light. Here we try to fill this gap to directly enhance incoherent light with high speed, such as fluorescence, which is essential in extending wavefront shaping to biomedical applications. For this purpose, we develop a technique based on a single acousto-optic deflector (AOD) with field-programmable gate array (FPGA) acceleration for spatiotemporal focusing within milliseconds. With the digital time gating of the feedback signal, spatiotemporal focusing of laser light with high contrast can be formed behind dynamic scattering media in milliseconds resulting in fluorescence enhancement. Furthermore, FPGA-based wavefront shaping is shown to effectively enhance fluorescence directly behind dynamic samples with short correlation times.

Flight photon counting electron multiplying charge coupled device development for the Roman Space Telescope coronagraph instrument

We describe the development of flight electron multiplying charge coupled devices (EMCCDs) for the photon-counting camera system of a coronagraph instrument (CGI) to be flown on the 2.4-m Nancy Grace Roman Space Telescope. Roman is a NASA flagship mission that will study dark energy and dark matter, and search for exoplanets with a planned launch in the mid-2020s. The CGI is intended to demonstrate technologies required for high-contrast imaging and spectroscopy of exoplanets, such as high-speed wavefront sensing and pointing control, adaptive optics with deformable mirrors, and ultralow noise signal detection with photon counting, visible-sensitive (350 to 950 nm) detectors. The camera system is at the heart of these demonstrations and is required to sense both faint and bright targets (10 − 4 − 107 counts-s − 1) adaptively at up to 1000 frames-s − 1 to provide the necessary feedback to the instrument control loops. The system includes two identical cameras, one to demonstrate faint light scientific capability, and the other to provide high-speed real-time sensing of instrument pointing disturbances. Our program at the Jet Propulsion Laboratory (Pasadena, California, United States) has evaluated the low-signal performance of radiation-damaged commercial EMCCD sensors and used those measurements as a basis for targeted radiation hardening modifications developed in partnership with the Open University (Milton Keynes, United Kingdom) and Teledyne-e2v (Chelmsford, United Kingdom). A pair of EMCCDs with test features was then developed and their low signal performance is reported here. The program has resulted in the development of a flight version of the EMCCD with low signal performance improved by more than a factor of three over the commercial one after exposure to 2.6 × 109 protons-cm − 2 (10 MeV equivalent). The flight EMCCD sensors are contributed by ESA through a contract with Teledyne-e2v (Chelmsford, United Kingdom). We will describe the program requirements, sensor design, test results and metrics used to evaluate photon counting performance.

High-gain and high-speed wavefront shaping through scattering media.

Wavefront shaping (WFS) is emerging as a promising tool for controlling and focusing light in complex scattering media. The shaping system's speed, the energy gain of the corrected wavefronts, and the control degrees of freedom (DOF) are the most important metrics for WFS, especially for highly scattering and dynamic samples. Despite recent advances, current methods suffer from trade-offs that limit satisfactory performance to only one or two of these metrics. Here, we report a WFS technique that simultaneously achieves high speed, high energy gain, and high control DOF. By combining photorefractive crystal-based analog optical phase conjugation (AOPC) and stimulated emission light amplification, our technique achieves an energy gain approaching unity, more than three orders of magnitude larger than conventional AOPC. The response time of ~10 μs with about 106 control modes corresponds to an average mode time of about 0.01 ns/mode, which is more than 50 times lower than some of the fastest WFS systems to date. We anticipate that this technique will be instrumental in overcoming the optical diffusion limit in photonics and translate WFS techniques to real-world applications.

Comparative analysis of speckle-based single-pixel imaging using uniform and non-redundant optical phased arrays.

An optical phased array (OPA) is a compact high-speed wavefront modulation device that is promising for next-generation optical sensing systems. In particular, speckle-based single-pixel imaging (SSPI) using OPA is an attractive scheme since precise tuning of optical phases is unnecessary. In this work, we present a comprehensive analysis of SSPI using an OPA with N phase shifters by comparing two classes of OPAs: uniformly spaced OPA (UOPA) and non-redundant OPA (NROPA). Through singular value decomposition analysis of the illumination patterns generated from the OPA, we clarify the theoretical limit of the imaging resolution for each case. As a result, the number of resolvable points can be as large as N 2-N+1 for the case of NROPA. This is in clear contrast to the case of UOPA, where the number of resolvable points can only be as large as 2N-1. Finally, imaging results of a test target are compared to study the impact of the array layout in OPA-based SSPI. Our work provides theoretical understanding of OPA-based SSPI and reveals the effectiveness of SSPI using NROPA.

Recent Advances of Binary Computed Holography in High-Speed Wavefront Modulation

Recent Advances of Binary Computed Holography in High-Speed Wavefront Modulation

Focusing light into scattering media with ultrasound-induced field perturbation

Focusing light into scattering media, although challenging, is highly desirable in many realms. With the invention of time-reversed ultrasonically encoded (TRUE) optical focusing, acousto-optic modulation was demonstrated as a promising guidestar mechanism for achieving noninvasive and addressable optical focusing into scattering media. Here, we report a new ultrasound-assisted technique, ultrasound-induced field perturbation optical focusing, abbreviated as UFP. Unlike in conventional TRUE optical focusing, where only the weak frequency-shifted first-order diffracted photons due to acousto-optic modulation are useful, here UFP leverages the brighter zeroth-order photons diffracted by an ultrasonic guidestar as information carriers to guide optical focusing. We find that the zeroth-order diffracted photons, although not frequency-shifted, do have a field perturbation caused by the existence of the ultrasonic guidestar. By detecting and time-reversing the differential field of the frequency-unshifted photons when the ultrasound is alternately ON and OFF, we can focus light to the position where the field perturbation occurs inside the scattering medium. We demonstrate here that UFP optical focusing has superior performance to conventional TRUE optical focusing, which benefits from the more intense zeroth-order photons. We further show that UFP optical focusing can be easily and flexibly developed into double-shot realization or even single-shot realization, which is desirable for high-speed wavefront shaping. This new method upsets conventional thinking on the utility of an ultrasonic guidestar and broadens the horizon of light control in scattering media. We hope that it provides a more efficient and flexible mechanism for implementing ultrasound-guided wavefront shaping.

High-speed wavefront determination method based on single in-and-out electric field analysis to focus light through highly scattering medium

We develop a new type of high-speed wavefront determination method with single feedback measurement to focus light through a 15.2 scattering mean free path in ∼113 ms. Our method is based on a heterodyne-detection phase sensitivity interferometer. First, the matrix which describes the light propagation process in the sample is measured by single input and output optical fields’ analysis. Then, by using a spatial light modulator to reshape the incident light with a matched wavefront, a focused beam is observed behind the sample. The proposed high-speed light focusing method will open new spot scanning mode toward deeper imaging through highly scattering biological tissues.

Single-shot time-reversed optical focusing into and through scattering media.

Optical time reversal can focus light through or into scattering media, which raises a new possibility for conquering optical diffusion. Because optical time reversal must be completed within the correlation time of speckles, enhancing the speed of time-reversed optical focusing is important for practical applications. Although employing faster digital devices for time-reversal helps, more efficient methodologies are also desired. Here, we report a single-shot time-reversed optical focusing method to minimize the wavefront measurement time. In our approach, all information requisite for optical time reversal is extracted from a single-shot hologram, and hence no other preconditions or measurements are required. In particular, we demonstrate the first realization of single-shot time-reversed ultrasonically encoded (TRUE) optical focusing into scattering media. By using the minimum amount of measurement, this work breaks the fundamental speed limit of digitally based time reversal for focusing into and through scattering media, and constitutes an important step toward high-speed wavefront shaping applications.

Wavefront shaping in complex media with a 350 kHz modulator via a 1D-to-2D transform

Controlling the propagation and interaction of light in complex media has sparked major interest in the last few years. Unfortunately, spatial light modulation devices suffer from limited speed that precludes real-time applications such as imaging in live tissue. To address this critical problem we introduce a phase-control technique to characterize complex media based on the use of fast 1D spatial light modulators and a 1D-to-2D transformation performed by the same medium being analyzed. We implement the concept using a micro-electro-mechanical grating light valve (GLV) with 1088 degrees of freedom modulated at 350 KHz, enabling unprecedented high-speed wavefront measurements. We continuously measure the transmission matrix, calculate the optimal wavefront and project a focus through various dynamic scattering samples in real-time, all within 2.4 ms per cycle. These results improve prior wavefront shaping modulation speed by more than an order of magnitude and open new opportunities for optical processing using 1D-to-2D transformations.

Large field of view correction by using conjugate adaptive optics with multiple guide stars.

Adaptive optics has been widely used in the optical microscopy to recover high-resolution images deep into the sample. However, the corrected field of view (FOV) with a single correction is generally limited, which seriously restricts the imaging speed. In this article, we demonstrate a high-speed wavefront correction method by using the conjugate adaptive optical correction with multiple guide stars (CAOMG) based on the coherent optical adaptive technique. The results show that the CAOMG method can greatly improve the corrected FOV. For 120-μm-thick mouse brain tissue, the corrected FOV can be improved up to ~243 times of the conventional pupil adaptive optics (PAO) without additional time consumption. Therefore, this study shows the potential of high-speed imaging through scattering medium in biological science.

直视合成孔径激光雷达自补偿高速空间波前调制器

直视合成孔径激光雷达自补偿高速空间波前调制器

High speed wavefront sensorless aberration correction in digital micromirror based confocal microscopy.

The quality of fluorescence microscopy images is often impaired by the presence of sample induced optical aberrations. Adaptive optical elements such as deformable mirrors or spatial light modulators can be used to correct aberrations. However, previously reported techniques either require special sample preparation, or time consuming optimization procedures for the correction of static aberrations. This paper reports a technique for optical sectioning fluorescence microscopy capable of correcting dynamic aberrations in any fluorescent sample during the acquisition. This is achieved by implementing adaptive optics in a non conventional confocal microscopy setup, with multiple programmable confocal apertures, in which out of focus light can be separately detected, and used to optimize the correction performance with a sampling frequency an order of magnitude faster than the imaging rate of the system. The paper reports results comparing the correction performances to traditional image optimization algorithms, and demonstrates how the system can compensate for dynamic changes in the aberrations, such as those introduced during a focal stack acquisition though a thick sample.

<italic>In Vivo</italic> Deep Tissue Imaging via Iterative Multiphoton Adaptive Compensation Technique

Multiphoton microscopy has been widely adopted in biological studies for its advantages in deep tissue imaging at subcellular resolutions. However, aberration and scattering in the sample often lead to a distorted laser focus with reduced focus intensity, limiting the achievable resolution and penetration depth. Adaptive optics has been adopted in microscopy to correct wavefront distortions and recover the diffraction-limited focus. Here, we review the Iterative MultiPhoton Adaptive Compensation Technique (IMPACT) that can perform in vivo high-speed wavefront measurement and compensation at large depth. We describe the working principle of IMPACT, demonstrate its applications for in vivo deep tissue imaging, and discuss its advantages and aspects to be improved.

Present research status of piezoelectric bimorph mirrors in synchrotron radiation sources

The third-generation synchrotron radiation sources are widely used in physics, chemistry, material science, etc. due to their light beams with high brilliance and low emittance. In order to efficiently utilize such light beams for scientific research, reflective mirrors with excellent figure quality are required. The reflective mirrors on the beamlines of synchrotron radiation sources consist of fixed polished shape mirrors and bendable mirrors. Bendable mirrors have been attracting the attention of the synchrotron radiation community because their curvatures can be varied to realize different focusing properties. Classical bendable mirrors are realized by applying mechanical moment at the ends of the mirror substrates. In this paper, we introduce a new concept of bendable mirrors, X-ray adaptive mirrors which are based on the adaptive optics technology and the properties of piezoelectric bimorph systems. X-ray adaptive mirrors exhibit many advantages over the classical bendable mirrors, such as mechanics-free, figure local corrections, and good focusing properties. The piezoelectric bimorph mirrors have been used in astronomy to correct the wavefront distortions introduced by atmospheric turbulence in real time. The piezoelectric bimorph mirror was first introduced into the field of synchrotron radiation by European Synchrotron Radiation Facility (ESRF) in the 1990s for making an X-ray reflective mirror. Compared with astronomy community, synchrotron radiation community is not interested in high-speed wavefront correction, but looking for the ultimate precision of the surface shape of piezoelectric bimorph mirror. In the second part of this paper, the usual structure and working principle are briefly described. Piezoelectric bimorph mirrors are laminated structures consisting of two strips of an active material such as zirconate lead titanate (PZT) and two faceplates of a reflecting material such as silicon. A discrete or continuous control electrode is located between the interfaces of PZT-PZT, while two continuous ground electrodes are located between the interfaces of Si-PZT. The PZTs that are polarized normally to their surface, any voltage applied across the bimorph results in a different change of the lateral dimensions of two PZTs, thereby leading to a bending of the whole structure. The relationship between the curvature of the bending mirror and voltage is given. In the third part of this paper, the technical issues as well as the design concepts are discussed in detail. Several Si-PZT-PZT-Si bimorph mirrors are first fabricated and tested by ESRF. The dimensions of each of them are 150 mm in length, 4045 mm in width, and 1518 mm in thickness. PZT is selected as an active material because of its high coupling factor, high piezoelectric coefficient, and high Curie temperature. The faceplates need to be easy to polish such as silicon and silica. Owing to the symmetrical layered structure Si-PZT-PZT-Si, the mirror is less sensitive to temperature variations from the process of bonding and polishing. The bimorph mirrors are confirmed to be promising by experimental tests. As the state-of-art polishing technique, elastic emission machining (EEM) becomes available commercially, and diamond light source brings EEM into the bimorph mirror to achieve a novel adaptive X-ray mirror coupling adaptive zonal control with a super-smooth surface. This super-polished adaptive mirror becomes the first optics with a bendable ellipse with sub-nanometer figure error. Spring-8 fabricates an adaptive mirror with different structures, and two strips of PZTs are glued to the side faces of the mirror. This mirror shows a diffraction-limited performance. Finally, the wavefront measuring methods and control algorithm are introduced. Wavefront measuring devices used in the metrology cleanroom include long trace profiler, nanometer optics component measuring machine, and interferometer. At-wavelength measuring methods used on the beamline include pencil-beam method, phase retrieval method, X-ray speckle tracking technique, and Hartmann test. The wavefront control algorithm is aimed at obtaining the voltages applied according to the inverse of the interaction matrix.

Implementing transmission eigenchannels of disordered media by a binary-control digital micromirror device

We report a method for measuring the transmission matrix of a disordered medium using a binary-control of a digital micromirror device (DMD). With knowledge of the measured transmission matrix, we identified the transmission eigenchannels of the medium. We then used binary control of the DMD to shape the wavefront of incident waves and to experimentally couple light to individual eigenchannels. When the wave was coupled to the eigenchannel with the largest eigenvalue, in particular, we were able to achieve about two times more energy transmission than the mean transmittance of the medium. Our study provides an elaborated use of the DMD as a high-speed wavefront shaping device for controlling the multiple scattering of waves in highly scattering media.

Adaptive optical two-photon microscopy using autofluorescent guide stars

We demonstrate a fast, direct wavefront-sensing method for dynamic in vivo adaptive optical two-photon microscopy. By using a Shack-Hartmann wavefront sensor and open-loop control, the system provides high-speed wavefront measurement and correction. To measure the wavefront in the middle of a Drosophila embryo at early stages, autofluorescence from endogenous fluorophores in the yolk were used as reference guide stars. The method was tested through live imaging of a Drosophila embryo. The aberration in the middle of the embryo was measured directly for the first time. After correction, the contrast and signal intensity of the structure in the middle of the embryo was improved.

Forward to special issue on blast and impact of engineering structures

The blast and impact behaviour of structures are dependent on many factors, such as the type of loading and the behaviour of constituent materials at high strain rates. Blast can be defined as a large scale, rapid and sudden release of energy. For example, in physical explosions, energy may be released from the catastrophic failure of a pressure vessel, or from the chemical interaction of two or more liquids. Structural impact deals with the behaviour of structures and components subjected to large dynamic loads involving contact, which could be the outcome, for example, of a high velocity projectile or a high speed crash. The prediction of the effect of a blast load on a structure is an important step in designing advanced blast protective structures. Blast energy is released within a very short period of time, which can be less than 1ms, making it challenging for protection material to absorb a majority of the blast energy. For blast impact analysis, the complexity of the problem can be attributed to the high speed wave front propagation, the flow of materials under these conditions, and the large structural deformation. Researchers are actively pursuing all these issues related to blast and impact analysis and means of developing protection against blast and impact forces. Since an explosion is a hypervelocity impact, numerical simulation is an attractive method to