Adjustable Focal Length Research Articles

Photo‐Active Soft Actuator Enabling Dynamic Manipulable Lenses with Surface Reshaping

AbstractManipulable lenses with tunable focal length have garnered great attention due to their unique advantages including strong adjustability, low power consumption, fast response times, and ease of integration. However, in the context of advancing intellectualization, weight reduction, and miniaturization of imaging systems, mere focal length adjustment is no longer sufficient. Herein, an innovative method is introduced for creating a dynamic light manipulable (DLM) lens with surface reshaping. This breakthrough is accomplished through the collaborative integration of a medium lens with an actuator based on photo‐active liquid crystal polymer (LCP) film, where the deformation of the photo‐active LCP film can be arbitrarily controlled by meticulously designing the orientation distributions of liquid crystal (LC) mesogens within the film. The lens with three distinct surfaces, including quasi‐cone, paraboloid, and 3 × 3 paraboloid array, are demonstrated with infrared stability, robust fatigue resistance, long natural recovery time, outstanding thermal stability, and excellent solvent resistance, further confirming the feasibility of the lens application in tunable display, imaging, infrared optics, and other systems. This work holds the promise of paving the way for a new generation of imaging systems.

Adaptive contour-tracking to aid wide-field swept-source optical coherence tomography imaging of large objects with uneven surface topology.

High-speed and wide-field optical coherence tomography (OCT) imaging is increasingly essential for clinical applications yet faces challenges due to its inherent sensitivity roll-off and limited depth of focus, particularly when imaging samples with significant variations in surface contour. Here, we propose one innovative solution of adaptive contour tracking and scanning methods to address these challenges. The strategy integrates an electrically tunable lens and adjustable optical delay line control with real-time surface contour information, enabling dynamic optimization of imaging protocols. It rapidly pre-scans the sample surface to acquire a comprehensive contour map. Using this map, it generates a tailored scanning protocol by partitioning the entire system ranging distance into depth-resolved segments determined by the optical Raleigh length of the objective lens, ensuring optimal imaging at each segment. Employing short-range imaging mode along the sample contour minimizes data storage and post-processing requirements, while adaptive adjustment of focal length and reference optical delay line maintains high imaging quality throughout. Experimental demonstrations show the effectiveness of the adaptive contour tracking OCT in maintaining high contrast and signal-to-noise ratio across the entire field of view, even in samples with significantly uneven surface curvatures. Notably, this approach achieves these results with reduced data volume compared to traditional OCT methods. This advancement holds promise for enhancing OCT imaging in clinical settings, particularly in applications requiring rapid, wide-field imaging of tissue structures and blood flow.

FOV adjustable liquid lens driven by electrowetting effect

In this paper, a FOV (field of view) adjustable liquid lens driven by electrowetting effect is demonstrated. The proposed lens consists of a window glass, a bottom electrode, four sidewall electrodes, two supporting shafts, and a deflectable aperture. The deflectable aperture is nested on the supporting shafts between the two liquids to limit the position of the liquid-liquid (L-L) interface. Different from the conventional FOV adjustable liquid lenses, the proposed lens can realize focal length adjustment and FOV deflection by applying voltages to the four sidewall electrodes, which has a simple structure and a miniaturized drive. The experiment shows that the aperture of the lens is 9 mm, the optical power range is -21 m-1 to -7 m-1 and the tilt angle of the L-L surface is ∼ 18° (± 9°). With a compact structure and easy drive, the proposed lens has great potential for applications in scanning, imaging, and inspection.

Microsphere lens array embedded microfluidic chip for SERS detection with simultaneous enhancement of sensitivity and stability

Surface enhanced Raman spectroscopy (SERS) utilizes the fingerprint features of molecular vibrations to identify and detect substances. However, in traditional single focus excitation scenarios, its signal collection efficiency of the objective is restricted. Furthermore, the uneven distribution of samples on the SERS substrate would result in poor signal stability, while the excitation power is limited to avoid sample damage. SERS detection system always requires precise adjustment of focal length and spot size, making it difficult for point-of-care testing applications. Here, we proposed a SERS microfluidic chip with barium titanate microspheres array (BTMA) embedded using vacuum self-assembled hot-pressing method for SERS detection with simultaneous enhancement of sensitivity and stability. Due to photonic nano-jets and directional antenna effects, high index microspheres are perfect micro-lens for effective light focusing and signal collecting. The BTMA can not only disperse excitation beam into an array of focal points covering the target uniformly with very low signal fluctuation, but enlarge the power threshold for higher signal intensity. We conducted a proof-of-principle experiment on chip for the detection of bacteria with immuno-magnetic tags and immuno-SERS tags. Together with magnetic and ultrasonic operations, the target bacteria in the flow were evenly congregated on the focal plane of BTMA. It demonstrated a limit of detection of 5 cells/mL, excellent signal reproducibility (error∼4.84%), and excellent position tolerance of 500 μm in X–Y plane (error∼5.375%). It can be seen that BTMA-SERS microfluidic chip can effectively solve the contradiction between sensitivity and stability in SERS detection.

Low voltage driven microlens array based on ionic liquid doped polyvinyl chloride gels

Electrically responsive polyvinyl chloride (PVC) gels represent promising candidates for fabricating lenses with adjustable focal lengths. However, their practical use has been hampered by the elevated voltage requirements. In order to lower the driving voltage of PVC gels-based microlens array (MLA), trace ionic liquid (IL) −1-butyl-3-methylimidazole hexafluorophosphate ([BMIM]PF6) was doped. We employed a ring-array patterned electrode for the fabrication of PVC gels-based microlens arrays (MLAs). Our investigation revealed that the operating voltage for PVC gels-based MLAs doped with 0.01 wt% [BMIM]PF6 was nearly 1/22 of that required for MLAs created using undoped PVC gels. In the case of a microlens featuring a 250 μm aperture, the focal length varies from 4.2 mm to 2.5 mm as the voltage ranges from 5 V to 7 V. Importantly, this voltage range remains below the established safe limit for human exposure and obviates the need for intricate circuitry during manufacturing. Moreover, the extent of focal length variation can be expanded by augmenting the film thickness. Our approach offers valuable technical support for advancing the practical applications of PVC gel-based MLAs.

Design of a focusing system for micro-nano satellite remote sensing camera based on thermal control technology

Remote sensing cameras mounted on micro-nano satellite mare susceptible to optical system defocusing during launch and operation due to environmental influences. Traditional focusing system encounter issues such as large volume and mass, making them unsuitable for the design requirements of micro-nano satellite. Proposes a thermal control-based focusing system for compact systems with maintained imaging quality. The design is developed according to the characteristics of a compact coaxial four-mirror remote sensing camera optical system and the overall structure of the system. The focusing system utilizes structural thermal expansion to alter the distance between the primary and secondary mirrors, achieving the purpose of adjusting the system’s focal length. Finite element simulations were conducted to analyze the modal and thermal deformations of the overall structure, validating the rationality of the system design. Experimental verification of the thermal control focusing system demonstrated its adjustment accuracy and range, with clear imaging obtained after system focusing. The results indicate that when the temperature control accuracy reaches ±0.5 °C, the focal length adjustment accuracy can reach 0.0052 mm. This article proposed thermal control focusing system can adjust the camera’s focal length in orbit, exhibiting advantages such as a small volume, high stiffness, and high precision.

Phase distribution and circular dichroism switchable terahertz chiral metasurface.

Chiral metasurfaces have many applications in the terahertz (THz) band, but they still lack modulation flexibility and functionality expansion. This paper presents a terahertz chiral metasurface with switchable phase distribution and switchable circular dichroism (CD). The metasurface unit consists of a metallic inner ring embedded in vanadium oxide and a vanadium oxide outer ring, state switching by thermal control of vanadium oxide and a change in the frequency of the incident wave. Based on the switchable phase distribution, we designed a focusing vortex beam generator with adjustable focal lengths through simulation. Based on the switching CD capability, we simulate its mode switching in near-field imaging using numerical simulation, and innovatively propose an optical encryption method. Utilizing the chiral property, we also designed dual-channel switchable holographic imaging in the same frequency band, which combined with the state change of VO2 can realize a total of 4 holograms switching. Our proposed metasurface is expected to provide new ideas for the study of optical encryption and wavefront modulation of dynamics.

Reconfigurable three multi-mode terahertz metasurface.

We designed a three reconfigurable multi-mode terahertz metasurface based on a concentric elliptical ring structure. The proposed unit cell is a concentric elliptical ring composed of copper, vanadium oxide and photosensitive silicon from the inside ring to the outside ring. The conductivity of photosensitive silicon and vanadium oxide can be adjusted by changing the external operating temperature and pump light intensity. The same unit cell can reconstruct three kinds of states with different properties, and they have completely different transmission characteristics in various terahertz bands. By encoding the arrangement, through changing external stimulus and operating frequencies, the reconfigurable terahertz metasurface can achieve multiple functions including terahertz focusing with adjustable focal length, vortex beam with different topological charge, and near-field imaging with different patterns. It provides what we believe to be a new idea for the field of information security and the design of multifunctional and multifrequency terahertz devices.

Deep learning-based correction of defocused fringe patterns for high-speed 3D measurement

Digital fringe projection profilometry often faces a trade-off between measurement accuracy and efficiency. Defocus technology is commonly employed to address this challenge and improve the efficiency of high-speed three-dimensional (3D) measurement. This technology uses 1-bit binary fringe patterns instead of traditional 8-bit sinusoidal patterns, but accurately measuring 3D shapes with both high speed and accuracy remains a challenge due to defocus errors. These errors are introduced by the manual adjustment of lens focal length and reduce both fringe pattern quality and measurement accuracy. To overcome this limitation, we propose a multi-stage generative adversarial network with a self-attention mechanism to correct inaccurate fringe patterns and transform them into more ideal sinusoidal fringe patterns. Our generation network comprises a multi-stage feature extraction network with a self-attention mechanism and an encoder-decoder network. A multi-stage network integrating residual and transformer modules is constructed to mine global feature information. The self-attention mechanism accurately detects key areas for correction, and the encoder-decoder network generates rectified sinusoidal fringe patterns by combining the feature information with the attention area. We use a discriminative network to evaluate whether the output of the generative network is good enough to be true. In our experiments, we considered different fringe widths and measured objects of various types and colors. The results show that our proposed method improves the quality of defocus fringe patterns and the accuracy of subsequent 3D reconstruction compared to existing direct defocus methods.

Calibration method for a multi-focus microscopic 3D imaging system.

This Letter presents a novel, to the best of our knowledge, method to calibrate multi-focus microscopic structured-light three-dimensional (3D) imaging systems with an electrically adjustable camera focal length. We first leverage the conventional method to calibrate the system with a reference focal length f0. Then we calibrate the system with other discrete focal lengths fi by determining virtual features on a reconstructed white plane using f0. Finally, we fit the polynomial function model using the discrete calibration results for fi. Experimental results demonstrate that our proposed method can calibrate the system consistently and accurately.

Flexible liquid crystal micro-lens arrays for curved integral imaging 2D/3D convertible display

ABSTRACT Glasses-free three-dimensional (3D) displays are being regarded as the key to the future of display that will redefine the display industry. Inspired by the curved screen, liquid crystal micro-lens arrays (LC MLAs) on flexible PET substrate with a radius of curvature of 5 cm was achieved. For demonstration, an integral imaging-based 2D/3D convertible display system is proposed by using the flexible LC MLAs (FLC MLAs) to switch its operation mode between 2D and 3D modes. When the FLC MLAs with an applied voltage are in the system, the prototype renders a matched 3D image and works in 3D mode. When the FLC MLAs do not have an applied voltage, the FLC MLAs are equivalent to glass, and the display system works in 2D mode. In addition, we also demonstrated that the FLC MLAs have a wider viewing angle than the flat integral imaging 3D display system. The depth of field ranges from 6.78 to 210.12 mm under low operating voltages of 3.4 to 3.1 Vrms, respectively. With its low voltage, thin structure, and adjustable focal length form factors, the developed display system can be integrated with off-the-shelf purchased flat panels, making it a promising option for portable electronics.

Blood-cerebrospinal fluid barrier opening by modified single pulse transcranial focused shockwave

Transcranial focused shockwave (FSW) is a novel noninvasive brain stimulation that can open blood-brain barriers (BBB) and blood-cerebrospinal fluid barriers (BCSFB) with a single low-energy (energy flux density 0.03 mJ/mm2) pulse and low-dose microbubbles (2 × 106/kg). Similar to focused ultrasound, FSW deliver highly precise stimulation of discrete brain regions with adjustable focal lengths that essentially covers the whole brain. By opening the BCSFB, it allows for rapid widespread drug delivery to the whole brain by cerebrospinal fluid (CSF) circulation. Although no definite adverse effect or permeant injury was noted in our previous study, microscopic hemorrhage was infrequently observed. Safety concerns remain the major obstacle to further application of FSW in brain. To enhance its applicability, a modified single pulse FSW technique was established that present 100% opening rate but much less risk of adverse effect than previous methods. By moving the targeting area 2.5 mm more superficially on the left lateral ventricle as compared with the previous methods, the microscopic hemorrhage rate was reduced to zero. We systemically examine the safety profiles of the modified FSW-BCSFB opening regarding abnormal behavior and brain injury or hemorrhage 72 hr after 0, 1, and 10 pulses of FSW-treatment. Animal behavior, physiological monitor, and brain MRI were examined and recorded. Brain section histology was examined for hemorrhage, apoptosis, inflammation, oxidative stress related immunohistochemistry and biomarkers. The single pulse FSW group demonstrated no mortality or gross/microscopic hemorrhage (N = 30), and no observable changes in all examined outcomes, while 10 pulses of FSW was found to be associated with microscopic and temporary RBC extravasation (N = 6/30), and abnormal immunohistochemistry biomarkers which showed a trend of recovery at 72 hrs. The results suggest that single pulse low-energy FSW-BCSFB opening is effective, safe and poses minimal risk of injury to brain tissue (Sprague Dawley, SD rats).

Dynamic Tunable Meta-Lens Based on a Single-Layer Metal Microstructure

Ultra-thin focusing meta-lenses based on the metasurface structure with adjustable focal length show important applicant value in compact systems, especially in on-chip terahertz spectroscopy, imaging systems, and communication systems. A stretchable substrate, dynamic focusing meta-lens based on the cross-polarized metal C-shaped split ring resonators (SRRs) is designed and investigated. At the operation frequency of 0.1 THz, the operation characteristics of the unit cell structure and the formed meta-lens are investigated. The phase of the unit cell structures can be modulated by changing the rotation angle, width, and symmetry axis of the C-shaped metal SRRs. When the terahertz wave is incident vertically, the focusing performance can be achieved based on the specific arrangement of the metasurface unit cells. By stretching the flexible substrate of the meta-lens, the dynamic focusing effect can be realized. When the substrate stretches from 100% to 120%, the focal length changes from 59.8 mm to 125.2 mm, the dynamic focusing range is 109.4% of the minimum focal length, and the focusing efficiency changes between 5.5% and 10.5%.

Holographic optical elements with a large adjustable focal length and an aberration correction.

Holographic optical element (HOE) has the advantages of light weight, small volume and multiple functions, but the fixed focal length limits its application. The current methods of adjusting focal length suffer from the disadvantages of small adjustable range and the introduction of aberration. This paper proposes a design method for HOE, and it can compensate the aberrations of the reconstructed image for each distance in a large adjustable range. The focal length of HOE is adjusted by modulating the incident light through the wavefront modulator, and the aberration of the reconstructed image is corrected by optimizing the phase distribution of HOE and superimposing different compensation phases for different reconstruction distances. The reconstructed image experimentally moves without aberration in a large range, which demonstrates the feasibility of the proposed method. It is expected to be widely used in various optical fields where the focal length of HOE needs to be dynamically tuned.

Energy‐efficient compressive sensing for multi‐target tracking in wireless visual sensor networks

SummaryWireless visual sensor networks (WVSN) have vital roles in surveillance applications. In these networks, wireless visual sensors include camera and transceiver module and collect visual information. However, energy consumption and coverage of the tracked targets are important challenges in WVSNs since increasing the coverage leads to increasing energy consumption. Therefore, energy optimization and satisfying the quality of experience (QoE) of the tracked targets are essential issues in these networks. In fact, the appropriate focal length setting leads to increasing the target coverage and quality of the captured targets' images and energy consumption. Therefore, selection of the suitable visual sensors and setting their focal length can overcome the energy consumption challenges and improve QoE of the tracked targets. In this case, compressive sensing is also expected to overcome the battery constraints of the WVSN resources. In this paper, the problem is to minimize the energy consumption of the multi‐target tracking with high reliability, while the coverage and also the quality of the received image of the targets are satisfied by selection of the proper visual sensors and the focal length adjustment. The convex optimization method is used to solve the problem. Also, based on the Karush‐Kuhn‐Tucker conditions, the optimal solution for the problem is obtained. Simulation results validate the efficiency of the proposed method in comparison with the other bench mark algorithms.

Planar Waveguide-Based Fiber Spectrum Analyzer Mountable to Commercial Camera

We present the design of a planar spectrometer that separates the wavelength channels from an input fiber and focuses the spectral lines onto a camera without any free-space optical elements. The geometric arrangements of the waveguides to achieve different spectroscopic parameters are explained in detail, allowing adjustable focal lengths, high spectral resolution, and broad free spectral range. The optical chip is fabricated on a low-cost polymer platform as proof of concept. The optical spectrum of a multiwavelength laser is measured by the proposed device, and the result is in good agreement with a commercial optical spectrum analyzer. The large focal depth of the chip allows an optical assembly of much relaxed alignment accuracy. We demonstrate a tube design to encapsulate the chip fixed with the input fiber. The assembly is then mounted to a commercial camera with standard C-mount threading as a convenient fiber spectrum analyzer without customized detectors and circuits. Our design may provide a low-cost and versatile solution for the development of compact spectroscopic equipment.

Customizable Sophisticated Three-Dimensional Shape Changes of Large-Size Liquid Crystal Elastomer Actuators.

Stimuli-responsive liquid crystal elastomers (LCEs), which exhibit sophisticated and versatile shape variations and functions upon stimulations, have constantly interested material science researchers. To date, many challenges still exist in scaling up orientated LCEs with sophisticated physical shapes and multi-functions. Herein, LCEs with various customizable conventional and exotic three-dimensional (3D) shapes and with sizes larger than those previously reported have been prepared by combining magnetic field alignment and soft lithography technology. These LCEs have film, cylinder, ellipsoid, hemispheroid, tube, pyramid, triangle and rectangle frame, grid pattern, cubic frame, and spring shapes. Meanwhile, diversified deformation behaviors such as contraction, expansion, bending, and twisting have been achieved by effectively controlling the alignment directions. Finally, the LCE actuator with hemispheroid shape has been explored for its possible applications in dynamic Braille displays or lenses with adjustable focal length. The simple strategy reported here provides a convenient way to customize multimorphological large-size 3D LCE actuators and their stimuli-responsive deformations. These systems will considerably enlarge the potential applications of LCEs and benefit the development of LCE soft robots and the future special bionic systems.

Low-Cost Endoscope Camera System for Neurosurgical Cadaveric Laboratory Dissections

Many training institutions in low-income countries do not have the resources to purchase and maintain a clinical-grade endoscopy tower dedicated to the laboratory. This project aimed to create a low-cost endoscope camera system using online-sourced materials to allow the operators to practice endoscopic surgical techniques in a cadaver laboratory setting. A low-cost endoscope system was created using a 34MP camera with recording capabilities and direct streaming to high-definition multimedia interface in full high resolution, with an adjustable focal length coupler and a light-emitting diode light source. The system cost was $443, as the endoscope and the monitor were already in the laboratory. The system was successfully employed to practice endoscopic dissections in 3 cadaveric specimens with good visualization of relevant structures. This article demonstrated how to produce a low-cost endoscope camera system for laboratory training in neuroendoscopy.

Combined diffractive optical elements with adjustable optical properties controlled by a relative rotation: tutorial.

A pair of adjacent transmissive diffractive optical elements (DOEs) forms a combined DOE with tunable optical properties, as, for example, a diffractive lens with an adjustable focal length. The optical properties are controlled by a relative movement of the two DOEs, such as a translation or a rotation around the optical axis. Here we discuss various implementations of this principle, such as tunable diffractive lenses, axicons, vortex plates, and aberration correction devices. We discuss the limits of the tuning range and of diffraction efficiency. Furthermore, it is demonstrated how chromatic aberrations can be suppressed by using multi-order DOEs.

Curvature Induced by Deflection in Thick Meta-Plates.

The design of advanced functional devices often requires the use of intrinsically curved geometries that belong to the realm of non-Euclidean geometry and remain a challenge for traditional engineering approaches. Here, it is shown how the simple deflection of thick meta-plates based on hexagonal cellular mesostructures can be used to achieve a wide range of intrinsic (i.e., Gaussian) curvatures, including dome-like and saddle-like shapes. Depending on the unit cell structure, non-auxetic (i.e., positive Poisson ratio) or auxetic (i.e., negative Poisson ratio) plates can be obtained, leading to a negative or positive value of the Gaussian curvature upon bending, respectively. It is found that bending such meta-plates along their longitudinal direction induces a curvature along their transverse direction. Experimentally and numerically, it is shown how the amplitude of this induced curvature is related to the longitudinal bending and the geometry of the meta-plate. The approach proposed here constitutes a general route for the rational design of advanced functional devices with intrinsically curved geometries. To demonstrate the merits of this approach, a scaling relationship is presented, and its validity is demonstrated by applying it to 3D-printed microscale meta-plates. Several applications for adaptive optical devices with adjustable focal length and soft wearable robotics are presented.