Variable Focal Length Research Articles

Tunable Lens Driven by Electrohydrodynamic Pumping

Developing soft actuators has emerged as one of the most promising areas in robotics, with a growing demand for tunable lenses driven by the rapid advancements in intelligent mobile devices and cameras. Traditional mechanical lens systems are characterized by their heaviness, complexity, and rigidity. To address these limitations, this research introduces a novel approach: a dual‐chamber liquid lens actuated by an electrohydrodynamic (EHD) pump. The study involves the fabrication of a planar EHD pump, with an experimental investigation into the impact of electrode gap, electrode width, and electrode‐pair gap on the pump's pressure. Following the optimization of the EHD pump, a dual‐chamber lens is constructed, comprising two chambers, a rigid transparent plate, and two prestretched elastic membranes, which are connected to the EHD pump via tubes. Additionally, a mathematical model is developed to predict the focal length of the lens. Both experimental and theoretical analyses are conducted to explore the effects of the prestretch ratio of the elastic membranes and the radius ratio of the two chambers on focal length variation. The theoretical predictions of the focal length–voltage curves align closely with the experimental findings, providing validation for the proposed theoretical model.

Analysis of the zoom system of a microscope condenser and its modification using variable focus lenses

A detailed theoretical analysis and optimization of the classical three-element zoom (pancratic) microscope condenser according to a patent from the 1930s [Reichspatentamt Nr. 713188 (29 10 1936)] is performed and formulas are derived for calculating basic parameters and the displacement of lenses during zooming. Furthermore, the modification of the classical zoom microscope condenser is investigated using a simpler optical system of two lenses with variable focal lengths and fixed positions. The relations for the calculation of the focal lengths of both variable focus lenses and the basic parameters of the zoom system have been described. The proposed two-element zoom system consisting of a system of two lenses with variable focal lengths maintains a constant distance between the object and image planes and fixed position of both lenses during zooming. The basic parameters and third-order aberration coefficients of such a system are calculated using an example.

Simultaneous Multifocal Plane Fourier Ptychographic Microscopy Utilizing a Standard RGB Camera.

Fourier ptychographic microscopy (FPM) is a computational imaging technology that can acquire high-resolution large-area images for applications ranging from biology to microelectronics. In this study, we utilize multifocal plane imaging to enhance the existing FPM technology. Using an RGB light emitting diode (LED) array to illuminate the sample, raw images are captured using a color camera. Then, exploiting the basic optical principle of wavelength-dependent focal length variation, three focal plane images are extracted from the raw image through simple R, G, and B channel separation. Herein, a single aspherical lens with a numerical aperture (NA) of 0.15 was used as the objective lens, and the illumination NA used for FPM image reconstruction was 0.08. Therefore, simultaneous multifocal plane FPM with a synthetic NA of 0.23 was achieved. The multifocal imaging performance of the enhanced FPM system was then evaluated by inspecting a transparent organic light-emitting diode (OLED) sample. The FPM system was able to simultaneously inspect the individual OLED pixels as well as the surface of the encapsulating glass substrate by separating R, G, and B channel images from the raw image, which was taken in one shot.

An electromechanically driven dielectric elastomer based tunable reflector

Deformable optics offer numerous advantages over conventional optical assemblies, including compactness, cost-effectiveness, efficiency, and flexibility. This study focuses on a reflector based on dielectric elastomer actuators (DEAs) with an internal fluid (air) coupling. DEAs are a class of electroactive materials adept at accommodating substantial actuation strains and rapid responses. Fluid distributed between the active and passive parts remains entirely enclosed by the device and transmits actuation pneumatically. Dynamic maneuvers conducted through a series of controlled electrical signals demonstrate proper control over optical characteristics. However, DEs exhibit inherent flaws in dynamic actuation, referred to as instabilities, which are mitigated by applying an initial pre-stretch. The study identifies optimal parameters that confer stability to the reflector: minimum to no creep, zero residual vibrations, and low viscous losses. An analytical framework is developed to assess device performance, focusing on the spherical curvature assumption that closely resembles the behavior of tunable spherical reflectors. Additionally, an optical bench setup is employed to demonstrate the relationship between focal length and applied pressure. Notably, this paper underscores the potential of a DE-based variable focal length reflector to function effectively within a dynamic environment.

All-silicon metalens for broadband achromatic polarization multiplexing in long-wave infrared wavelengths

Traditional long-wave infrared polarimetry usually relies on complex optical setups, making it challenging to meet the increasing demand for system miniaturization. To address this problem, we design an all-silicon broadband achromatic polarization-multiplexing metalens (BAPM) operating at the wavelength range of 9–12 µm. A machine-learning-based design method is developed to replace the tedious and computationally intensive simulation of a large number of meta-atoms. The results indicate that the coefficients of variation in focal length of the BAPM are 3.95% and 3.71%, and the average focusing efficiencies are 41.3% and 40.5% under broadband light incidence with x- and y-polarizations, respectively.

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.

Femtosecond‐Laser Direct‐Writing Hydrogel‐Based Microlens Arrays Customized with Multi‐Dimensional Optical Configurations for Information Encryption

AbstractFemtosecond‐laser direct‐writing lithography prints tunable 3D micro/nano‐scale optical devices based on hydrogel materials have garnered significant attention owing to their use in optical applications. Although the refractive index (RI) is an important parameter of optical devices, a quantitative measurement to determine that of femtosecond‐laser‐induced microstructures is lacking. This study proposes a novel method to measure the RI change of a tunable hydrogel device in different environments using a femtosecond‐laser‐customized multidimensional grating. As hydrogels can be tuned by changing the pH values, the specific RI changes of a hydrogel are first quantitatively investigated with different pH values such that the proposed method can be employed to precisely control the dynamic and tunable optical performance of hydrogel optical devices. To this end, a 3D hydrogel microlens array (HMA) with variable focal lengths is designed and manufactured to verify its dynamic and tunable optical performances. The HMA displays reversible and tunable imaging capabilities, confirming its application in information encryption. As a proof of concept, a customized sequence of letters “CFAE” is successfully obtained as the “output” by assigning the pre‐set pH values as the “code” and the original letter sequence as the “input.”

A Fully Metaoptical Zoom Lens with a Wide Range.

Metalenses are typically designed for a fixed focal length, restricting their functionality to static scenarios. Various methods have been introduced to achieve the zoom function in metalenses. These methods, however, have a very limited zoom range, or they require additional lenses to achieve direct imaging. Here, we demonstrate a zoom metalens based on axial movement that performs both the imaging and the zoom function. The key innovation is the use of a polynomial phase profile that mimics an aspheric lens, which allows an extended depth of focus, enabling a large zoom range. Experimental results show that this focal length variation, combined with the extended depth of focus, translates into an impressive zoom range of 11.9× while maintaining good imaging quality. We see applications for such a zoom metalens in surveillance cameras of drones or microrobots to reduce their weight and volume, thus enabling more flexible application scenarios.

Highly sensitive refractometric sensing system based on the combination of lensing and laser speckle correlation

A simple optical sensing system based on laser speckle and lensing for monitoring refractive index variations of fluids is proposed and demonstrated. The fluid of interest is contained in a cylindrical glass vessel, which acts as an optical lens whose focal length depends on the refractive index of the fluid. Focal length variations imply optical wavefront alterations that are revealed through laser speckle pattern correlation. The device exhibits a refractive index resolution on the order of 10−6, which is comparable to those of more complex and costly refractometric sensors and demonstrates the benefits of combining these two optical techniques. The simplicity and high sensitivity of the presented configuration make it greatly suitable for a variety of applications ranging from chemical sensing to specimen detection.

Investigation into fabrication and optical characteristics of tunable optofluidic microlenses using two-photon polymerization.

Optofluidic systems, integrating microfluidic and micro-optical technologies, have emerged as transformative tools for various applications, from molecular detection to flow cytometry. However, existing optofluidic microlenses often rely on external forces for tunability, hindering seamless integration into systems. This work presents an approach using two-photon polymerization (TPP) to fabricate inherently tunable microlens arrays, eliminating the need for supplementary equipment. The optofluidic design incorporates a three-layered structure enabling dynamic manipulation of refractive indices within microchannels, leading to tunable focusing characteristics. It is shown that the TPP fabricated optofluidic microlenses exhibit inherent tunable focal lengths, numerical apertures, and spot sizes without reliance on external forces. This work signifies some advancements in optofluidic technology, offering precise and tunable microlenses with potential applications in adaptive imaging and variable focal length microscopy.

Dish spliced concentrator with both uniform andfocused performance through a variable focallength.

We present a dish spliced concentrator (DSC) featuring hexagonal spherical sub-mirrors of uniform size. The DSC offers advantages over traditional parabolic dish concentrators, including a compact layout, cost-effectiveness, higher concentration ratio, and improved light uniformity. Its versatility allows for both uniform and focused light concentration by adjusting parameters like the focal length of the DSC, making it suitable for concentrating photovoltaic (CPV) and concentrating solar thermal (CST) applications. We design the DSC using three-dimensional (3D) vector rotation theory, implementing ray tracing and transmission characteristic analysis based on three-dimensional vector reflection theory. We establish a simulation model to evaluate the impact of geometric parameters on the DSC's optical performance.

Acoustic Bessel-like beam generation using phononic crystals

Diffraction-free beams with large depth-of-field have a lot of potential in the field of acoustics, such as imaging, sensing, and particle manipulation. In this study, an acoustic Bessel-like beam is produced using an axicon-sonic crystal lens. The sonic crystal is created using cylindrical glass rods arranged in a triangular shape with a centered square lattice configuration. The numerical simulation between 4 and 8 kHz indicates that the axicon-sonic crystal converts the plane acoustic wave into a Bessel-like beam. The analysis of the beam indicates that the depth of field of this beam depends on the size and periodicity (lattice parameter) of the sonic crystal. The axicon lens also displays variable focal lengths at different frequencies. A graded index layer was implemented to mitigate the reflection caused by the significant impedance mismatch. Experimental validation of acoustic Bessel-like beam formation is also reported for the working frequencies. At 8 kHz, the measured range to the 50% on-axis intensity was 34λ, while the focus width at the same frequency was measured to be 2λ. The integration of three distinct design strategies—axicon shape, sonic crystal, and graded index—expands the possibilities for sound focusing applications.

Dynamic polarization-regulated metasurface with variable focal length

Polarization and focal length are both critical optical parameters with many applications in many fields, such as optical communications and imaging. The development of metasurfaces provides a new realization of optical systems. In this paper, based on metasurfaces’ powerful electromagnetic modulation capability, we integrate polarization conversion with continuous zoom function and propose a dynamic polarization-regulated metasurface with variable focal length. It realizes the reversible conversion of polarization state, which can convert linearly polarized light into elliptically polarized light and circularly polarized light and convert circularly polarized light to linearly polarized light. At the same time, it achieves a 4.4× zoom range, with a constant focal length variation from 70 µm to 309 µm. The metasurface has the advantages of small size, easy integration, and reconfigurability, providing a new design idea for complex functional optical systems.

2OC: A General Automated Orientation and Orthorectification Method for Corona KH-4B Panoramic Imagery

Due to a lack of geographical reference information, complex panoramic camera models, and intricate distortions, including radiation, geometric, and land cover changes, it can be challenging to effectively apply the large number (800,000+) of high-resolution Corona KH-4B panoramic images from the 1960s and 1970s for surveying-related tasks. This limitation hampers their significant potential in the remote sensing of the environment, urban planning, and other applications. This study proposes a method called 2OC for the automatic and accurate orientation and orthorectification of Corona KH-4B images, which is based on generalized control information from reference images such as Google Earth orthophoto. (1) For the Corona KH-4B panoramic camera, we propose an adaptive focal length variation model that ensures accuracy and consistency. (2) We introduce a robust multi-source remote sensing image matching algorithm, which includes an accurate primary orientation estimation method, a multi-threshold matching enhancement strategy based on scale, orientation, and texture (MTE), and a model-guided matching strategy. These techniques are employed to extract high-accuracy generalized control information for Corona images with significant geometric distortions and numerous weak texture areas. (3) A time-iterative Corona panoramic digital differential correction method is proposed. The orientation and orthorectification results of KH-4B images from multiple regions, including the United States, Russia, Austria, Burkina Faso, Beijing, Chongqing, Gansu, and the Qinghai–Tibet Plateau in China, demonstrate that 2OC not only achieves automation but also attains a state-of-the-art level of generality and accuracy. Specifically, the standard deviation of the orientation is less than 2 pixels, the mosaic error of orthorectified images is approximately 1 pixel, and the standard deviation of ground checkpoints is better than 4 m. In addition, 2OC can provide a longer time series analysis of data from 1962 to 1972, benefiting various fields such as environmental remote sensing and archaeology.

Thermal effect analysis on cuboid Pr:YLF crystals pumped by blue laser diodes.

Using blue laser diodes (LDs) to pump Pr:YLF crystals can directly realize visible-band laser output. Compared with the traditional frequency doubling and LD direct output method, it has the advantages of simple design, compact structure, and high beam quality. For solid-state lasers, pump-induced thermal effects of gain media are the principal limiting factors for the desired high-power output. In this paper, internal temperature space model distribution of a rectangular cross-section Pr:YLF crystal is established. On this basis, the temperature distribution, thermal stress distribution, and thermal focal length variation of single-end pumped and double-end pumped laser crystals are analyzed. The results are verified by COMSOL simulations and experimental measurements. To our knowledge, this analysis is the first to examine the thermal effect of a rectangular cross-section Pr:YLF crystal, analyzing the limit power that the crystal can withstand, which paves the way for better performances of visible lasers with stable and high-power output.

Passive millimeter-wave imaging system with variable focal length optical configuration

Passive millimeter-wave imaging system with variable focal length optical configuration

Laser-Induced Fabrication of Micro-Optics on Bioresorbable Calcium Phosphate Glass for Implantable Devices.

In this study, a single-step nanosecond laser-induced generation of micro-optical features is demonstrated on an antibacterial bioresorbable Cu-doped calcium phosphate glass. The inverse Marangoni flow of the laser-generated melt is exploited for the fabrication of microlens arrays and diffraction gratings. The process is realized in a matter of few seconds and, by optimizing the laser parameters, micro-optical features with a smooth surface are obtained showing a good optical quality. The tunability of the microlens' dimensions is achieved by varying the laser power, allowing the obtaining of multi-focal microlenses that are of great interest for three-dimensional (3D) imaging. Furthermore, the microlens' shape can be tuned between hyperboloid and spherical. The fabricated microlenses exhibited good focusing and imaging performance and the variable focal lengths were measured experimentally, showing good agreement with the calculated values. The diffraction gratings obtained by this method showed the typical periodic pattern with a first-order efficiency of about 5.1%. Finally, the dissolution characteristics of the fabricated micropatterns were studied in a phosphate-buffered saline solution (PBS, pH = 7.4) demonstrating the bioresorbability of the micro-optical components. This study offers a new approach for the fabrication of micro-optics on bioresorbable glass, which could enable the manufacturing of new implantable optical sensing components for biomedical applications.

Intraoperative zoom lens calibration for high magnification surgical microscope

Background and objectivesAn augmented reality (AR)-based surgical guidance system is often used with high-magnification zoom lens systems such as a surgical microscope, particularly in neurology or otolaryngology. To superimpose the internal structures of relevant organs on the microscopy image, an accurate calibration process to obtain the camera intrinsic and hand–eye parameters of the microscope is essential. However, conventional calibration methods are unsuitable for surgical microscopes because of their narrow depth of focus at high magnifications. To realize AR-based surgical guidance with a high-magnification surgical microscope, we herein propose a new calibration method that is applicable to the highest magnification levels as well as low magnifications. MethodsThe key idea of the proposed method is to find the relationship between the focal length and the hand–eye parameters, which remains constant regardless of the magnification level. Based on this, even if the magnification changes arbitrarily during surgery, the intrinsic and hand–eye parameters are recalculated quickly and accurately with one or two pictures of the pattern. We also developed a dedicated calibration tool with a prism to take focused pattern images without interfering with the surgery. ResultsThe proposed calibration method ensured an AR error of < 1 mm for all magnification levels. In addition, the variation of focal length was within 1% regardless of the magnification level, and the corresponding variation with the conventional calibration method exceeded 20% at high magnification levels. ConclusionsThe comparative study showed that the proposed method has outstanding accuracy and reproducibility for a high-magnification surgical microscope. The proposed calibration method is applicable to various endoscope or microscope systems with zoom lens.

Electro‐Reconfigurable Adaptive Microlens with Simultaneous Multidirectional Focal Adjustment and Zooming (Adv. Mater. Technol. 8/2023)

Adaptive Microlens In article number 2201988, Jin Woo Bae and co-workers present a PVC gel-based electro-reconfigurable adaptive microlens that enables multidirectional focal adjustment and zooming without mechanical parts or fluid leakage like human eyes. It has an excellent variable focal length at low input voltages, high transparency, and fast response, showing great potential for future electric optical modules.

Electro‐Reconfigurable Adaptive Microlens with Simultaneous Multidirectional Focal Adjustment and Zooming

AbstractElectrically reconfigurable lenses capable of focal adjustment and zooming require deformable adaptive optical components. However, existing electroactive optical devices that perform these functions are limited by fluid leakage or require complex mechanical parts. Although polyvinyl chloride (PVC) gel‐based lenses with variable focal lengths and zooming have recently been developed, focal adjustment can only be made in the horizontal axis. Herein, a PVC gel‐based adaptive microlens capable of controlling the focal length and focal point simultaneously in the vertical, horizontal, and diagonal directions without mechanical gears or liquid leakage is presented. By optimizing the characteristics of PVC gels plasticized with three different structured plasticizers, a PVC gel‐based adaptive microlens is fabricated. The produced microlens demonstrates the properties of multidirectional focal adjustment, variable focal length (+33.7 to −15.1 mm) at low input voltages (&lt;300 V), excellent transparency (&gt;90%), fast response (0.10 s at 100 V), silent operation, low power consumption (0.39 mW), and excellent potential for further miniaturization.