Optical Wavefront Research Articles

Reinforcement Learning Environment for Wavefront Sensorless Adaptive Optics in Single-Mode Fiber Coupled Optical Satellite Communications Downlinks

Optical satellite communications (OSC) downlinks can support much higher bandwidths than radio-frequency channels. However, atmospheric turbulence degrades the optical beam wavefront, leading to reduced data transfer rates. In this study, we propose using reinforcement learning (RL) as a lower-cost alternative to standard wavefront sensor-based solutions. We estimate that RL has the potential to reduce system latency, while lowering system costs by omitting the wavefront sensor and low-latency wavefront processing electronics. This is achieved by adopting a control policy learned through interactions with a cost-effective and ultra-fast readout of a low-dimensional photodetector array, rather than relying on a wavefront phase profiling camera. However, RL-based wavefront sensorless adaptive optics (AO) for OSC downlinks faces challenges relating to prediction latency, sample efficiency, and adaptability. To gain a deeper insight into these challenges, we have developed and shared the first OSC downlink RL environment and evaluated a diverse set of deep RL algorithms in the environment. Our results indicate that the Proximal Policy Optimization (PPO) algorithm outperforms the Soft Actor–Critic (SAC) and Deep Deterministic Policy Gradient (DDPG) algorithms. Moreover, PPO converges to within 86% of the maximum performance achievable by the predominant Shack–Hartmann wavefront sensor-based AO system. Our findings indicate the potential of RL in replacing wavefront sensor-based AO while reducing the cost of OSC downlinks.

Dispersion engineering of metalenses

Optical dispersion, the variation of the speed of light with frequency in a material, presents significant challenges in modern optical systems, including chromatic aberration and pulse signal distortion. Traditional approaches to dispersion engineering of an optical lens system require the use of a set of sub-lenses of opposite dispersion properties, largely increasing the overall lens thickness. Ultrathin metasurfaces offer unprecedented control over optical wavefronts with advanced functionalities. Developing achromatic metalenses has thereby emerged as a timely research topic for metasurface research. This Perspective article provides a comprehensive overview of dispersion engineering methods in metalenses, including the use of 2D and 3D meta-atoms fabricated from planar lithography and 3D laser lithography methods, respectively. We compare key figures of merit of achromatic metalenses developed for different wavelength ranges and discuss recent inverse design of large-scale achromatic metalenses. We believe advanced machine learning methods as well as hybrid nanofabrication of diffractive metalenses, refractive lenses, and metamaterials-like spaceplates could offer promising avenues for overcoming current challenges and eventually push ultrathin achromatic optics to practical applications in optics-related fields.

Demonstrating more than 2π phase modulation in non-Hermitian asymmetric multilayers

Phase modulation has come to be recognized as a fundamental paradigm for optical device design in applications involving the spatiotemporal control of optical wavefronts. Here, the cavities use a combination of absorptive and lossless layers with ideal conductive metal to control reflection and phase shift, similar to the Fabry–Pérot system, which has zero reflection and perfect transmission. Multilayers with different sequences and composed of several stack layers were examined for reflectivity when activated in a prism-coupled layout. The layer’s different arrangements, resulting in open and closed asymmetric multilayers, display reflectance spectra with distinct features as well as phase and shifts. In the open configuration, there is a strong coupling between the hybridized resonance mode and the exciton from the red dye. As a result, energy-based topological considerations identify a correlation between exciton/polariton strong coupling, topological darkness, and phase tuning, and they motivate the phase shift range.

On the exploration of structured light transmission through a multimode fiber in a reference-less system

Recent advancements in optical wavefront shaping have brought multimode fibers (MMFs) into the spotlight as potential contenders for long-haul communication, positioning them as promising substitutes to single-mode fibers. MMFs offer greater data rates, countering the impending congestion of fiber-based networks. Additionally, their suitability for single fiber endoscope procedures presents them as compelling alternatives for minimally invasive endoscopy, providing information comparable to, if not surpassing, current cutting-edge technology. However, the complex modal behavior of light in MMFs hinders the implementation of these promising applications. Hence, precise modal excitation and control are crucial for improving the transmission of structured light in MMFs. This study introduces a groundbreaking approach that achieves the retrieval of the transmission matrix in a single step, thereby facilitating coherent light propagation through highly dispersive MMFs. By combining iterative phase retrieval algorithms with the measurement of phase shifts between experimentally established focal points, potential arbitrary interference control is enabled, leading to effective phase correction. The efficacy of our method is validated through the successful transmission of diverse structured light beams, including Laguerre–Gauss and Hermite–Gaussian types, as well as handwritten characters via MMF. The examination of structured light is simplified using an off-axis holographic technique that accurately captures both intensity and phase information. These results hold significant potential, paving the way for major advancements in long-distance communication and minimally invasive medical procedures, thereby transforming the telecommunications and healthcare sectors.

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.

Spatially resolved readout of a Fabry-Perot ultrasound sensor interrogated through a multimode optical fiber using wavefront shaping.

The spatially resolved interrogation of a Fabry-Perot ultrasound sensor using a laser beam focused through a multimode fiber is demonstrated. To scan the beam across the sensor as required to read it out, optical wavefront shaping was employed to compensate for the scrambling of light in the fiber. By providing a means to map ultrasound through inexpensive, lightweight fibers, this could lead to new ultrasonic and photoacoustic imaging systems, such as endoscopes and flexible handheld probes.

Open fringes interferograms generation through the phase contrast technique

Phase contrast imaging is a proven technique for sensing optical wavefronts and obtaining qualitative and quantitative insights into phase disturbances. Traditionally, this method involves introducing a phase mask at the Fourier transform plane of the phase object to create a synthetic reference wave that interferes with the analyzed phase disturbance. In this study, we incorporate an intensity-dependent photorefractive material, facilitating the self-induction of a Zernike-type phase filter. We introduce an innovative modification by inducing an angular offset between the synthetic reference wave and the phase distribution of the object. Effective implementation necessitates an illumination area larger than the phase object's support. This method stands out due to its resilience to environmental factors, shared by all common-path interferometers. Furthermore, it leverages open fringe interferograms, consolidating all essential phase information into a single image. This innovation streamlines phase retrieval, eliminating the need for multiple images, and extends applicability beyond small-phase object perturbations. We present results demonstrating the recovered phase distribution using the well-established Takeda technique for open fringe interferograms. This novel approach promises to enhance phase distortion analysis, offering versatility and simplicity in a variety of applications.

Measuring the transmitted wavefront of large-aperture flat optics

This paper proposes a scanning distance measuring interferometer (SDMI) method to measure the transmitted wavefront of large-aperture flat optics accurately. The biaxial positioning mechanism moves the test component through the interferometric path of a differential distance-measuring interferometer; both the optical path difference and distribution are obtained; thus, the wavefront can be reconstructed. A Ø360 mm flat optical substrate is measured, and the result is compared with a phase measuring interferometer (PMI). The SDMI result is 68 nm in PV and 13 nm in RMS. The PMI result is 90.8 nm in PV and 16.4 nm in RMS. The forms of the two results show a good correlation, which gives confidence in the viability of the SDMI method for large-aperture optical transmitted wavefront measurements out of range of the commercial PMI.

Fast modal reconstruction of large plane wavefronts from sparse measurements using Shack-Hartmann sensors.

A concept for the fast measurement and reconstruction of optical wavefronts using Shack-Hartmann sensors (SHSs) is presented. For wavefronts with a diameter at the scale of several tens of millimeters, hundreds of measurements with an SHS may be necessary to cover the wavefront. In the proposed concept, a few SHSs are used to measure about 2% of the entire wavefront, providing sufficient measurement data for its successful reconstruction. The small number of SHSs mounted in parallel makes the concept suitable for time-critical applications. A simulation analysis is performed, and an experimental validation of the concept is presented, demonstrating that the wavefront can be reconstructed with an RMS error of about 100nm.

Estimation of Crystalline Lens Material Properties From Patient Accommodation Data and Finite Element Models.

The mechanical properties of the crystalline lens are related to its optical function of accommodation, and their changes with age are one of the potential causes for presbyopia. We estimated the mechanical parameters of the crystalline lens using quantitative optical coherence tomography (OCT) imaging and wavefront sensing data from accommodating participants and computer modeling. Full-lens shape data (from quantitative swept-source OCT and eigenlens representation) and optical power data (from Hartmann-Shack wavefront sensor) were obtained from 11 participants (22-30 years old) for relaxed accommodation at infinity and -4.5 D accommodative demand. Finite element models of lens, capsular bag, zonulae, and ciliary body were constructed using measured lens geometry and literature data, assuming a 60-mN radial force. An inverse modeling scheme was used to determine the shear moduli of the nucleus and cortex of the lens, such that the simulated deformed (maximally stretched) lens matched the participant's lens at -4.5 D. The shear moduli of the nucleus and cortex were 1.62 ± 1.32 and 8.18 ± 5.63 kPa, on average, respectively. The shear modulus of the nucleus was lower than that of the cortex for all participants evaluated. The average of the two moduli per participant was statistically significantly correlated with age (R2 = 0.76, P = 0.0049). In vivo imaging and mechanical modeling of the crystalline lens allow estimations of the crystalline lens' mechanical properties. Differences between nuclear and cortical moduli and their dependency with age appear to be critical in accommodative function and likely in its impairment in presbyopia.

Synchronous optical intensity and phase measurements to characterize Rayleigh-Bénard convection.

Propagation of a laser beam through the Rayleigh-Bénard (RB) convection is experimentally investigated using synchronous optical wavefront and intensity measurements. Experimental results characterize the turbulence strength and length scales, which are used to inform numerical wave optic simulations employing phase screens. Experimentally found parameters are the refractive index structure constant, mean flow rate, kinetic and thermal dissipation rates, Kolmogorov microscale, outer scale, and shape of the refractive index power spectrum using known models. Synchronization of the wavefront and intensity measurements provide statistics of each metric at the same instance in time, allowing for two methods of comparison with numerical simulations. Numerical simulations prove to be within agreement of experimental and published results. Synchronized measurements provided more insight to develop reliable propagation models. It is determined that the RB test bed is applicable for simulating realistic undersea environments.

Chirped pulse phase-shifting digital holography for capturing the sequence of ultrafast optical wavefronts

For capturing the sequence of ultrafast optical wavefronts, chirped pulse phase-shifting digital holography, where a chirped reference pulse and multiple object pulses for probing an ultrafast phenomenon are used in an on-axis optical setup, is proposed and analyzed. In this study, holograms with different polarization directions at different times are obtained using a single-shot optical pulse. The procedure for reconstructing time-separated multiple amplitude images and phase images are derived and verified by numerical simulations.

Arbitrary Optical Wavefront Shaping with Holographic Plasmonic Gap Waveguides

AbstractManipulations of free‐space light are usually achieved through various metasurfaces and spatial light modulators. However, an external light source is required to excite these devices, making complete on‐chip integrations difficult. Integrated photonics enables the miniaturization and multi‐functionalization of optical systems by densely packing numerous optical components on a single chip. Particularly, plasmons have recently attracted extensive attention due to their unique abilities to enable the routing and manipulation of light at the nanoscale. Here optical wavefront shaping achieved by holographic metal‐insulator‐metal plasmonic gap waveguides is reported. An arbitrary free‐space wave's amplitude and phase information can be recorded in the waveguide by the amplitude variation of the guided wave. By elaborately designing the plasmonic gap waveguide, the guided waves can be molded into any desired free‐space light field, thus enabling complex free‐space functions including highly directional beams, dual beams with arbitrarily tailorable power ratio and radiation angles, focusing beams, and Airy beam generation. This study opens a new route for optical wavefront shaping via modulating guided waves. It paves the way for optical interconnects across multifunctional photonic integrated devices and free space, holding potential in optical communications, light detection and ranging, imaging, and displays.

On‐Chip Electrically Driven Tunable Meta‐Lens for Dynamic Focusing and Beam Steering

AbstractThe on‐chip meta‐lenses consisting of subwavelength slot arrays with lateral gradients have shown great potential for parallel signal processing and optical computing. The dynamic tuning mechanisms which are necessary steps toward exploiting the applications of the on‐chip meta‐system have been rarely mentioned yet. Here, a free‐form functional microheater via topology design is proposed to manipulate the refractive index distribution, allowing for flexible optical wavefront shaping. For a proof‐of‐concept, wide range of dynamic focusing and beam steering are demonstrated via a tunable meta‐lens on silicon photonic platform. The continuous scanning of the focused beam is experimentally achieved with a maximum travel range of 100 µm in the longitudinal direction (≈95 nm mW−1) and 25 µm in the lateral direction (≈20 nm mW−1). The thermal‐optic tuning elements can response the electrical control within ≈41.2 µs. A waveguide array at the focal plane is used to characterize the device. The on‐chip loss of the tunable meta‐lens is measured to be <1 dB including the coupling loss from slab to single‐mode waveguide. The proposed tunable meta‐lenses are fabricated via a standard silicon photonic process and allow for arbitrarily two dimensional (2D) beam steering. It offers great versatility for on‐chip meta‐systems with a wider scope of functionalities and applications.

Nonlinear Optical Wavefront Engineering with Geometric Phase Controlled All‐Dielectric Metadevice

AbstractOptical wavefront engineering with artificial nanostructures has important applications in laser manufacturing, bioimaging, optical communications, etc. In the linear optical regime, metasurfaces are widely utilized to realize wavefront engineering functionalities, such as imaging with a flat lens and generation of vortex beams and holographic images. On the other hand, it is highly desirable to directly control the wavefront of light during nonlinear optical processes. Here, the nonlinear optical wavefront engineering with geometric phase controlled all‐dielectric metadevices is experimentally demonstrated. This is realized by integrating the multifunctional dielectric metasurface with the conventional nonlinear optical crystals. It is demonstrated that the nonlinear metadevice can be used to generate optical vortices and nonreciprocal holographic images at second harmonic frequencies. The proposed all‐dielectric metadevice represents an important strategy to develop compact and multifunctional nonlinear optical sources.

Three-Channel Near-Field Display and Encryption Based on a Polarization Multiplexed Metasurface.

Multichannel metasurfaces are becoming a significant trend in the field of optical encryption due to their excellent manipulation of optical wavefronts. However, existent multichannel metasurfaces for optical encryption mostly implement only two channels in the near-field, or three channels by combining the near- and far-field. In this paper, we propose and simulate a three-channel metasurface that works entirely in the near-field and uses the polarization state of the incident light, left circularly polarized (LCP) light, right circularly polarized (RCP) light, and linearly polarized (LP) light as the security key. The metasurface consists of two types of nanostructures that work as a polarizer and a quarter-wave plate, providing an additional degree of freedom for encoding that enables independent near-field display at 633 nm wavelength incident light. The proposed three-channel metasurface has the advantages of high information density and high security, which will pave the way for multi-channel applications such as ultracompact displays, optical encryption, and information storage.

Directional, multi-beam, and compact wavefront transformers based on 3-D near-zero refractive index metamaterial

Optical wavefront transformation is essential for communication and power transfer applications across all electromagnetic spectra. In this work, we report directional multi-beam compact wavefront transformers which convert cylindrical wavefronts into finite-aperture directional beams based on near-zero refraction in 3-D metamaterial at terahertz frequencies. For this purpose, a near-zero-index metamaterial (NZIM) with three-dimensional functionality is realized, and its effective material parameters are retrieved. Near-zero refraction in NZIM is verified through the uniform phase maintenance inside the multiple stacked NZIM layers. By embedding a cylindrical wavefront source inside NZIM configurations, uni-, bi-, and quad-directional wavefront transformers are realized. The results were compared and verified against the ideal cases.

Towards ideal focusing of diffused light via optical wavefront shaping

Controllable optical propagation, such as forming diffraction-limited optical focusing, beyond the diffusion limit in biological tissue or tissue-like scattering media, has been desired for long yet considered challenging. In the past two decades, optical wavefront shaping (WFS) has been proposed and has progressed, demonstrating its remarkable potential. That said, inherent tradeoffs still exist among optimization speed, control degree of freedom, and energy gain, which has hindered wide applications of the technology. Most recently, an analogue optical phase conjugation system was developed, equipped with stimulated emission light amplification that effectively achieves the least tradeoff ever, yielding high-gain and high-speed performance of optical focusing through dynamic thick media.

Generalizable turbulent flow forecasting for adaptive optics control.

In this work, we characterize the capability of artificial neural network predictive models for generalizable turbulence forecasting, particularly for use in predictive adaptive optics (AO) applications. Predictive AO control, which utilizes future state predictions of an optical wavefront propagated through a turbulent medium to drive correction, is a promising technology for optical propagation in high-disturbance and low-signal environments. The dynamics describing the evolution of turbulent flow can vary greatly. Accordingly, a generalizable approach to turbulence forecasting has key benefits in allowing for prediction across a range of conditions, thus enabling continuous predictive AO operation in dynamic environments and having reduced sensitivity to changes in conditions. We present a model for generalizable turbulence forecasting, which demonstrated consistent high performance over a range of compressible flow conditions outside those included in the training sample, with only a minimal increase in prediction error compared with a hypothetical baseline model, which assumes perfect a priori characterization. These results demonstrate a clear ability to extract useful dynamics from a limited domain of turbulent conditions and apply these appropriately for forecasting, which could inform future design of predictive AO systems.

Near-Eye Holographic 3D Display and Advanced Amplitude-Modulating Encoding Scheme for Extended Reality

Electronic holographic displays can reconstruct the optical wavefront of object light, exhibiting the most realistic three-dimensional (3D) images, in contrast to conventional stereoscopic displays. In this paper, we propose a novel, near-eye holographic 3D display (NEHD) applicable to AR/MR/holographic devices and experimentally demonstrate the proposed module’s performance with 360° full-viewed holographic 3D movie at 30 fps. To realize high-quality of reconstructed holographic 3D (H3D) images, we also propose an advanced amplitude-modulating (AM) encoding scheme suited for the proposed amplitude-modulating NEHD. We experimentally verify that the new hologram-encoding approach can improve the image quality of H3D reconstructions through quantitative statistical analyses, by using evaluation methods for H3D images that are suggested in the paper. Two holograms at different viewing directions of the same 3D scene are designed to be displayed onto the proposed NEHD prototype for two eyes of an observer, respectively. The presented techniques for the proposed NEHD enable the observer to experience the depth cue, a realistic accommodation effect, and high-quality H3D movies at each eye.