Focus-tunable Lens Research Articles

Line-field confocal optical coherence tomography based on tandem interferometry with a focus-tunable lens.

This article introduces an innovative line-field confocal optical coherence tomography (LC-OCT) system based on tandem interferometry, featuring a focus-tunable lens for dynamic focusing. The principle of tandem interferometry is first recalled, and an analytical expression of the interferometric signal detected is established in order to identify the influence of key experimental parameters. The LC-OCT system is based on a Linnik-type imaging interferometer with a focus-tunable lens for focus scanning, coupled to a Michelson-type compensating interferometer using a piezoelectric linear translation stage for coherence plane scanning. The system achieves axial and lateral image resolutions of approximately 1 µm over the entire imaging depth (400 µm), in line with conventional LC-OCT. Vertical section images (B-scans) of skin acquired at 14.3 fps reveal distinguishable structures within the epidermis and dermis. Using refocusing and stitching, images of a tissue phantom were obtained with an imaging depth superior to 1.4 mm. The system holds promise for LC-OCT miniaturization, along with enhanced imaging speed and extended imaging depth.

Design, modeling, fabrication, and characterization of 50 mm diameter focus tunable liquid crystal lens with enhanced optical performance

Design, modeling, fabrication, and characterization of 50 mm diameter focus tunable liquid crystal lens with enhanced optical performance

High-Magnification Object Tracking with Ultra-Fast View Adjustment and Continuous Autofocus Based on Dynamic-Range Focal Sweep.

Active vision systems (AVSs) have been widely used to obtain high-resolution images of objects of interest. However, tracking small objects in high-magnification scenes is challenging due to shallow depth of field (DoF) and narrow field of view (FoV). To address this, we introduce a novel high-speed AVS with a continuous autofocus (C-AF) approach based on dynamic-range focal sweep and a high-frame-rate (HFR) frame-by-frame tracking pipeline. Our AVS leverages an ultra-fast pan-tilt mechanism based on a Galvano mirror, enabling high-frequency view direction adjustment. Specifically, the proposed C-AF approach uses a 500 fps high-speed camera and a focus-tunable liquid lens operating at a sine wave, providing a 50 Hz focal sweep around the object's optimal focus. During each focal sweep, 10 images with varying focuses are captured, and the one with the highest focus value is selected, resulting in a stable output of well-focused images at 50 fps. Simultaneously, the object's depth is measured using the depth-from-focus (DFF) technique, allowing dynamic adjustment of the focal sweep range. Importantly, because the remaining images are only slightly less focused, all 500 fps images can be utilized for object tracking. The proposed tracking pipeline combines deep-learning-based object detection, K-means color clustering, and HFR tracking based on color filtering, achieving 500 fps frame-by-frame tracking. Experimental results demonstrate the effectiveness of the proposed C-AF approach and the advanced capabilities of the high-speed AVS for magnified object tracking.

Contrast sensitivity is resilient to induced fast periodic defocus oscillations.

This study investigates the potential effects of periodic defocus oscillations on contrast sensitivity. Sinusoidal fluctuations at 5, 15, and 25 Hz, with defocus peak-to-valley values ranging from 0.15 to 3 D, were induced by means of a focus-tunable lens after calibrating its dynamic behavior. Monocular contrast sensitivity was measured on five young emmetropic subjects. The experimental data shows that contrast sensitivity loss due to defocus fluctuations is low for a wide range of frequencies and amplitudes. Only for the more severe case studied (25 Hz, ± 1.5 D) contrast threshold showed a clear increase in most subjects. Qualitative comparison of the empirical data with a simulation of modulation loss due to time integration of defocused retinal point spread functions, suggests a short integration time by the eye for defocus blur, around or even below a hundredth of a second.

Spatially modulated ablation driven by chaotic attractors in human lung epithelial cancer cells

A significant modification in photoinduced energy transfer in cancer cells is reported by the assistance of a dynamic modulation of the beam size of laser irradiation. Human lung epithelial cancer cells in monolayer form were studied. In contrast to the quantum and thermal ablation effect promoted by a standard focused Gaussian beam, a spatially modulated beam can caused around 15% of decrease in the ablation threshold and formation of a ring-shaped distribution of the photothermal transfer effect. Optical irradiation was conducted in A549 cells by a 532 nm single-beam emerging from a Nd:YVO4 system. Ablation effects derived from spatially modulated convergent waves were controlled by an electrically focus-tunable lens. The proposed chaotic behavior of the spatial modulation followed an Arneodo chaotic oscillator. Fractional dynamic thermal transport was analyzed in order to describe photoenergy in propagation through the samples. Immediate applications of chaos theory for developing phototechnology devices driving biological functions or phototherapy treatments can be considered.

Varifocal occlusion in an optical see-through near-eye display with a single phase-only liquid crystal on silicon

We propose a near-eye display optics system that supports three-dimensional mutual occlusion. By exploiting the polarization-control properties of a phase-only liquid crystal on silicon (LCoS), we achieve real see-through scene masking as well as virtual digital scene imaging using a single LCoS. Dynamic depth control of the real scene mask and virtual digital image is also achieved by using a focus tunable lens (FTL) pair of opposite curvatures. The proposed configuration using a single LCoS and opposite curvature FTL pair enables the self-alignment of the mask and image at an arbitrary depth without distorting the see-through view of the real scene. We verified the feasibility of the proposed optics using two optical benchtop setups: one with two off-the-shelf FTLs for continuous depth control, and the other with a single Pancharatnam–Berry phase-type FTL for the improved form factor.

Switchable visible and near-infrared light bifocal lens based on hybrid graphene-metasurface structures

We propose a wideband visible (VIS) and near infrared (NIR) light reflective flat lens with switching capability base on hybrid graphene-metasurface structure. This tunable flat lens structure consists of a slotted gold reflector substrate, a graphene monolayer and TiO2 nanofins that provide excellent focusing performance and tunability in the VIS and NIR range. In order to achieve the switching feature, a bi-functional device capable of switching reflection/absorption modes is required. To this end, the gold substrate with two cross slots has been used to create absorption properties and a graphene layer has been used to change the intensity of absorption and reflection. The switching property for incident light with circular polarization arises from uniformly changing the chemical potential of the graphene layer. By setting the chemical potential of graphene, μc, equal to 1.1 eV, we obtained the reflection property. The reflection mode exhibits a broadband (715–800 nm) optical response with high polarization conversion efficiency (more than 70 %). On the other hand, by choosing μc = 0.75 eV, the absorption feature is created at the wavelength of 700–780 nm. The reflected light can be focused on an arbitrary position above the structure by rotating the TiO2 nanofins with the help of Pancharatnam-Berry (PB) phase theory. In addition to obtaining the single and dual focus tunable flat lenses, we have also presented a multifunction configuration that can switch between different modes, such as non-focus, single and dual focus modes. Such switchable flat lenses can have various applications in imaging and sensing systems.

Design of the dual-band shared-aperture asynchronous zoom optical system using focus tunable lenses.

To meet the increasing demand of the current market and the diversity of application scenarios, combine the zoom system and the multi-band shared-aperture system, and fully harness their respective advantages, this paper proposes a dual-band shared-aperture asynchronous zoom optical system using focus tunable lenses (FTLs). To address the lack of available patents for such systems, we designed a sub-system simultaneous iterative optimization algorithm to calculate the initial structure parameters. This synchronous iterative optimization approach can strengthen the connection between sub-systems and compensate for the shortcomings of current mainstream design methods. The initial structure constructed in this way has a good performance in terms of structural stability and optimization potential. Based on these methods, we successfully designed an optical system that can work in both VIS and NIR bands, and the two sub-systems can zoom independently. The design results possess good performance in terms of distortion control, aberration correction, and volume control.

Split-Lohmann Multifocal Displays

This work provides the design of a multifocal display that can create a dense stack of focal planes in a single shot. We achieve this using a novel computational lens that provides spatial selectivity in its focal length, i.e, the lens appears to have different focal lengths across points on a display behind it. This enables a multifocal display via an appropriate selection of the spatially-varying focal length, thereby avoiding time multiplexing techniques that are associated with traditional focus tunable lenses. The idea central to this design is a modification of a Lohmann lens, a focus tunable lens created with two cubic phase plates that translate relative to each other. Using optical relays and a phase spatial light modulator, we replace the physical translation of the cubic plates with an optical one, while simultaneously allowing for different pixels on the display to undergo different amounts of translations and, consequently, different focal lengths. We refer to this design as a Split-Lohmann multifocal display. Split-Lohmann displays provide a large étendue as well as high spatial and depth resolutions; the absence of time multiplexing and the extremely light computational footprint for content processing makes it suitable for video and interactive experiences. Using a lab prototype, we show results over a wide range of static, dynamic, and interactive 3D scenes, showcasing high visual quality over a large working range.

A method for converting the existing mechanical zoom optical system to the focus tunable lens zoom optical system

The focus tunable lens (FTL), an emerging device, has brought a new development direction for the research of zoom optical systems, and the zoom lens designed with this lens has more advantages in response speed and mechanical vibration control than the conventional mechanical zoom system. However, there is a relative lack of design methods and computational theories for this type of system at present, and there is a lack of initial structure patents for reference. To solve this problem, this paper proposes a method that can convert the existing conventional mechanical zoom system into an FTL zoom system, and we first introduce the equivalent group element substitution theory for optical systems, then we design a computational screening algorithm for the equivalent FTL group element. We performed an example demonstration to illustrate the details of this method, and we finally obtain one FTL zoom optical system with an F/# of 4, a total length of 120 mm, and an 8 mm image plane. The whole design process is smooth, and the system structure is stable.

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

Simultaneous thickness and phase index measurements with a motion-free actively tunable Twyman-Green interferometer.

In this paper, we present a scheme to simultaneously measure the thickness and refractive index of parallel plate samples, involving no bulk mechanical motion, by deploying an electronically tunable Twyman-Green interferometer configuration. The active electronic control with no bulk mechanical motion is realized via the introduction of a tunable focus lens within the classical motion-based Twyman-Green interferometer configuration. The resulting interferometer is repeatable and delivers accurate estimates of the thickness and refractive index of a sample under test. Elimination of bulk motion also promises a potential for miniaturization. We develop a theoretical model for estimating sample thickness and index values using this reconfigurable interferometer setup and present detailed experimental results that demonstrate the working principle of the proposed interferometer.

Robust, motion-free optical characterization of samples using actively-tunable Twyman–Green interferometry

Optical interferometry-based techniques are ubiquitous in various measurement, imaging, calibration, metrological, and astronomical applications. Repeatability, simplicity, and reliability of measurements have ensured that interferometry in its various forms remains popular—and in fact continues to grow—in almost every branch of measurement science. In this paper, we propose a novel actively-controlled optical interferometer in the Twyman–Green configuration. The active beam control within the interferometer is a result of using an actively-controlled tunable focus lens in the sample arm of the interferometer. This innovation allows us to characterize transparent samples cut in the cubical geometry without the need for bulk mechanical motion within the interferometer. Unlike thickness/refractive index measurements with conventional Twyman–Green interferometers, the actively-tunable interferometer enables bulk-motion free thickness or refractive index sample measurements. With experimental demonstrations, we show excellent results for various samples that we characterized. The elimination of bulk motion from the measurement process promises to enable miniaturization of actively-tunable Twyman–Green interferometers for various applications.

Diffractive Alvarez-Lohmann lenses for correcting aberrations of tunable membrane lenses

Focus-tunable lenses, e.g., liquid filled membrane lenses (MLs), have found increasingly widespread application in optical systems. If a large refractive power range is to be used, the correction of chromatic aberrations is particularly challenging: a group containing a single ML cannot be corrected over the whole refractive power range. In analogy to hybrid achromats for lenses with constant focal lengths, we present the combination of an ML and a diffractive Alvarez-Lohmann-lens (ALL) for the compensation of axial color over a large refractive power range. In contrast to the combination of multiple MLs, this does not increase the axial length of the system significantly. At the same time, the flexible adaption of the phase function of the diffractive ALL can reduce spherical aberration over the whole focal range. Design examples with ray-tracing and wave-optical simulations demonstrate the performance of the resulting hybrid tunable element. Experimental data from fabricated sample lenses provide a proof of principle.

Design and theoretical analysis of the image-side telecentric zoom system using focus tunable lenses based on Gaussian brackets and lens modules

For now, telecentric system and zoom system have been extensively implemented in machine vision, industrial inspection, etc. However, studies on optical systems with both telecentric and zoom functions are still rare. To improve this situation, we present the design method of the telecentric zoom system with four focus tunable lenses (FTL) and a moving aperture stop (STOP) by applying Gaussian brackets and lens modules. We first improve the Gaussian brackets to achieve the initial parameter solution of the telecentric zoom FTL system, then we introduce a selection method to find the initial structural parameters with sufficient potential for optical design. After that, we use the lens module method to design the initial structure of the optical system, we also expand the combination method of the lens modules which contains an FTL. Finally, the initial structures of a 4X image-side telecentric zoom FTL system are constructed as an example and then optimized using CODEV. As a result, we get an image-side telecentric zoom FTL system with an F/# of 4, a zoom range of 20 mm to 80 mm, an image plane size of 8 mm, and a total length of 218 mm, which performs well in telecentricity control, distortion control, and aberration correction.

Comprehensive refractive manipulation of water waves using electrostriction

The control of wave propagation based on refraction principles offers unparalleled possibilities as shown by the striking example of optics. This approach is unfortunately limited for water waves as it relies mainly on variations of the liquid depth which, while controlling the wave velocity, also trigger nonlinearities and damping. In this article, we show experimentally that electrostriction allows to implement extensive refraction-based control of water waves in a precise and contactless manner. The setup consists of an electrode under high voltage placed above the grounded conductive water. The waves propagating under the electrode can be slowed down up to approximately half their speed compared to free propagation. We characterize the Snell-Descartes laws of refraction and the total internal reflection for the water waves. We implement emblematic refraction-based devices such as electrically tunable focusing lenses, waveguides without obstacles, and beam splitters based on frustrated internal reflection to perform interference experiments.

Compact electromagnetically driven liquid-filled lens with bidirectional wide range focus tuning capability

A novel compact focus tunable liquid-filled lens using electromagnetic actuation method is presented. It consists of a specially designed elastic membrane that incorporates an annular ridge structure and non-uniform thickness contour for actuator integration and dynamically improving optical quality over the whole focal length tuning range, respectively. Moreover, a custom-designed moving coil type electromagnetic actuator together with a refractive index matching strategy between the filling liquid and the membrane are adopted for providing bidirectional, wide-range focus tuning capability. For proof-of-concept demonstration, a new process combining injection molding with 3D printing is developed to fabricate the proposed lens with effective aperture size of 4 mm with high reliability and repeatability. Nearly symmetric focal length tuning from infinity to ±40 mm and reduced spherical aberration characteristic are experimentally disclosed for the as-fabricated device, and its application in dynamic focusing imaging is also successfully performed. © 2022 Elsevier B.V. All rights reserved. • A customized electromagnetic actuation mechanism is designed • A refractive index matching strategy is used for wide-range focus tuning • A specially designed elastic membrane is used for improving dynamic optical quality • Nearly symmetric focal length tuning from infinity to ±40 mm is achieved

Deep Reinforcement Learning Based System for Intraoperative Hyperspectral Video Autofocusing.

Hyperspectral imaging (HSI) captures a greater level of spectral detail than traditional optical imaging, making it a potentially valuable intraoperative tool when precise tissue differentiation is essential. Hardware limitations of current optical systems used for handheld realtime video HSI result in a limited focal depth, thereby posing usability issues for integration of the technology into the operating room. This work integrates a focus-tunable liquid lens into a video HSI exoscope, and proposes novel video autofocusing methods based on deep reinforcement learning. A first-of-its-kind robotic focal-time scan was performed to create a realistic and reproducible testing dataset. We benchmarked our proposed autofocus algorithm against traditional policies, and found our novel approach to perform significantly (p < 0.05) better than traditional techniques (0.070 ±.098 mean absolute focal error compared to 0.146 ±.148). In addition, we performed a blinded usability trial by having two neurosurgeons compare the system with different autofocus policies, and found our novel approach to be the most favourable, making our system a desirable addition for intraoperative HSI.

Design and fabrication of a focus-tunable liquid cylindrical lens based on electrowetting.

In this study, a focus-tunable liquid cylindrical lens based on electrowetting was designed and fabricated. The cylindrical cavity usually used in common electrowetting zoom spherical lenses was replaced by a 20 mm × 10 mm × 8 mm cuboid cavity, in which the interface of two liquids formed a toroid owing to the electrowetting effect. The proposed liquid cylindrical lens can serve as either a converging or diverging lens with the response time under 110 ms by changing the supplied voltage. The zoom lens we fabricated worked stably under 0-110 V voltage for a long time, guaranteeing that the focal length of the liquid cylindrical lens can range within (-∞, -148.36 mm) ∪ (697.21 mm, +∞). By combining the liquid lens that we designed with a simple fixed cylindrical lens, a cylindrical lens system with an arbitrary focal length suitable for various tasks in beam manipulation can be realized.

Adaptive Optics Compensation for Orbital Angular Momentum Optical Wireless Communications

Adaptive optics (AO) can efficiently compensate for turbulence-induced distortion in orbital angular momentum (OAM)-based optical wireless communication (OWC) systems. In this paper, we design a modified phase diversity algorithm (MPDA)-based wavefront sensor to enhance the reconstruction accuracy of distorted OAM wavefront information. Aiming to further strike a compelling trade-off between AO system complexity and compensation accuracy, we first construct a novel AO system that applies a quickly and electronically controlled focus-tunable lens (FTL). It decontaminates distorted OAM signaling beams while having a low systemic complexity and superior convergence performance. Furthermore, we propose the 3-modified phase diversity algorithm (3-MPDA) AO scheme relying upon a Fourier intensity and two defocused intensities as the prior information, which beneficially balances the compensation effect and the number of defocused intensities and exhibits good noise robustness against charge-coupled device (CCD) detectors. In summary, this paper provides new insight for designing AO schemes with high compensation performance in communication links.