Tunable Lens Research Articles

3D profile measurement for stepped microstructures using region-based transport of intensity equation

One of the major challenges in the semiconductor industry is to measure the three-dimensional (3D) profile of objects with large step height (>100 µm), as well as nanometer-level surface profiles. In this work, a novel technique called region-based transport of intensity equation (R-TIE) is explored to measure both large step heights as well as local surface profiles with nanometer resolution. The key factors of the R-TIE are the use of composite image segmentation, transport of intensity equation (TIE), and a smart imaging system. The imaging system incorporates an electrically tunable lens to record objects at different focus positions with large field of view, high speed, and no mechanical moving parts. The TIE part of the system allows for nanometer height measurement at areas of interest. We demonstrate the basic principle and feasibility of the proposed method by experimentally measuring different stepped specimens.

Antireflective structures on highly flexible and large area elastomer membrane for tunable liquid-filled endoscopic lens.

The antireflection coating on an optical lens plays a key role not only in minimizing specular reflection but also in maximizing light transmission. Antireflective structures (ARS) found in the eyes of moths have the same role as conventional antireflection coating, and they also provide many intriguing features such as broadband coverage, self-antireflection, and strong water repellence. In particular, ARS on a highly flexible membrane can effectively deliver antireflective properties, which are rarely achieved by conventional hard coating. Herein, we report antireflective structures on a highly flexible elastomeric lens membrane for tunable endoscopic imaging applications. A thin antireflective membrane was fabricated using a large-area nanohole template and replica molding. The thin membrane increases light transmittance up to 96.8% in the visible region while enhancing the image contrast up to 56%, higher than that of a smooth membrane during lens deformation. This antireflective, tunable, liquid-filled lens offers new opportunities for compact endoscopic laparoscopy enabling maximum transmittance and variable zoom imaging in narrow and low-illumination environments.

Movable electrowetting optofluidic lens for optical axial scanning in microscopy

Optical axial scanning is essential process to obtain 3D information of biological specimens. To realize optical axial scanning without moving, the tunable lens is a solution. However, the conventional tunable lenses usually induce non-uniform magnification and resolution issues. In this paper, we report a movable electrowetting optofluidic lens. Unlike the conventional tunable lens, our proposed optofluidic lens has two liquid-liquid (L-L) interfaces, which can move in the cell by an external voltage. The object distance and image distance are adjusted by shifting the L-L interface position. Therefore, the proposed lens can realize optical axial scanning with uniform magnification and resolution in microscopy. To prove the concept, we fabricate an optofluidic lens and use it in optical axial scanning. The scanning distance is more than 1 mm with uniform magnification and good imaging quality. Widespread application of such a new adaptive zoom lens is foreseeable.

View Expansion Microscope based on Viewpoint Movement using a Galvano Mirror and Focus Adjustment using an Electrically Tunable Lens

View Expansion Microscope based on Viewpoint Movement using a Galvano Mirror and Focus Adjustment using an Electrically Tunable Lens

Droplets Capped with an Elastic Film Can Be Round, Elliptical, or Nearly Square.

We present experiments that show that the partial wetting of droplets capped by taut elastic films is highly tunable. Adjusting the tension allows the contact angle and droplet morphology to be controlled. By exploiting these elastic boundaries, droplets can be made elliptical, with an adjustable aspect ratio, and can even be transformed into a nearly square shape. This system can be used to create tunable liquid lenses and, moreover, presents a unique approach to liquid patterning.

Dual functional optofluidic platform based on light-actuated air plug

Optical systems based on fluid-fluid interface are gaining great attention because they offer added advantages such as dynamic, reconfigurable, and self-healing capabilities with less complex fabrication steps compared to the solid based optical elements. Moreover, cost-effectiveness and ease of miniaturization pave a potential route towards the realization of portable optical and optofluidic devices. However, a single optofluidic device having remotely controlled multifunctional capabilities remains a challenge. We demonstrate an optofluidic multifunctional strategy, which could operate either as an optical switch or a tunable liquid lens. The principle relies on light-controlled volumetric change of an air plug confined in a minichannel filled with water. For the optical switch, volumetric change of the air plug modulates total internal reflection. A temperature difference of only 0.2 °C is sufficient for a subsecond switching time (0.6 ± 0.1 s). For the liquid lens, the change of plug size acts as an actuator to control the meniscus shape of the liquid. Upon heating or cooling, the meniscus develops a convex or a concave lens respectively. A temperature difference of 0.1–0.7 °C is sufficient for realizing tunability in the focal length. The demonstrated strategy is simple, cost-effective, non-contact and the device could be operated at room temperature.

Potential of Liquid-Crystal Materials for Millimeter-Wave Application

In this study, we reviewed three topics regarding the application of liquid-crystal (LC) materials to millimeter-wave (MMW) devices. It is essential to develop useful measurement methods for refractive indices of LC materials in the MMW region. Herein, a novel measurement method using optical short is demonstrated using a Si semiconductor substrate. There are two approaches to develop MMW LC devices. One is the quasi-optical approach, which involves scaling up the optical components, and the other approach involves integrating the LC materials into high-frequency electric circuits. A three-dimensional (3D) printer is used to fabricate the Fresnel lens, which is a typical quasi-optical device useful in the MMW region, where we can develop the tunable lens by introducing LC materials. A planar-type MMW waveguide is advantageous for integrating the LC materials to develop LC MMW devices using the second approach. We investigated a useful microstrip-line-type LC phase shifter by developing a novel conversion circuit to introduce the LC material onto the dielectric substrate surface. A phase shifter is an important MMW component that is used to attain a phased array antenna system, and a minimal twin antenna array is demonstrated using the microstrip-line-type LC phase shifters.

Design studies of varifocal rotation optics

We present the design of a varifocal freeform optics consisting of two lens bodies each with a helical-type surface structure of azimuthally varying curvatures. This arrangement allows for tuning the optical refraction power by means of a mutual rotation of the lens bodies around the optical axis. Thus, the refraction power can be tuned continuously in a defined range. The shape of the helical-type surfaces is formed by a change in curvature subject to the azimuthal angle α. At the transition of the azimuthal angle from α = 2π to α = 0, a surface discontinuity appears. Since this discontinuity will seriously affect the imaging quality, it has to be obscured. In the initial state, i.e., zero-degree rotation, the curvatures of the opposing surfaces result in a specific refraction power, which is constant over the entire circular aperture. Rotating one of the lens bodies by an angle φ around the optical axis will change the opposing curvatures and result in a change of refraction power. Two circular sectors with different tunable optical refraction powers are formed, thus resulting in a tunable bifocal optics. Obscuring the minor sector will result in a tunable monofocal rotation optics. In contrast to conventional tunable lens systems, where additional space for axial or lateral lens movement has to be allocated in design, rotation optics allowing for a more compact design. A performance analysis of the rotation optics based on simulations is presented in dependence on aperture size as well as approaches to compensate for spherical aberrations.

고분자막과 다층구조를 이용한 가변렌즈기반 광학 줌 시스템

We have developed an optical zoom system based on tunable lens with polymer membrane and multilayered structure. The presented system employed two tunable lenses and two doublet lenses as the main refractive units. Each tunable lens had a delicately designed, solid-liquid mixed structure with multiple internal slim holes, which helped to improve the lens optical properties and stability against gravity. The optical structure and regulation principle of the zoom system is presented, as well as the detailed fabrication process of the tunable lens. Under different displacement loads, the lens deformation properties, and the adjustment ability and imaging characteristics of the zoom system were measured and analyzed. Besides, the spot diagram, field curvature and distortion of the system were simulated using the Zemax software. The magnification of the designed system could be regulated from 0.2X to 4.3X flexibly depending on the focal variation of each tunable lens.

A dual-mode variable optical power splitter using a Digital Spatial Light Modulator and a variable focus lens

This paper describes the design of a variable optical power splitter which splits input optical signal power into variable ratios between two output ports. The proposed design is a hybrid scheme which uses a conventional Digital Spatial Light Modulator (DSLM) and a tunable focus Electrically Tunable Lens (ETL). The proposed splitter has an inherent dual-mode of operation and is capable of operating in a DSLM-only, ETL-only or a hybrid DSLM/ETL mode depending on the desired accuracy of the power split ratio. We show that the improved power splitter design can potentially offer a fine split ratio control through additional ETL focal length control. Moreover, it eliminates the need of ascertaining an exact random image pattern through hit-and-trial for an exact split ratio in a DSLM-only splitter mode. The DSLM is used to obtain the closest value to the desired split ratio using a fixed circular pattern with a variable radius while the ETL is tuned for further fine-tuning the split ratio in order to get closer to the required value. We also show that a dual-mode control delivers multiple possible solutions for any desired split ratio. Experimental results validate the working of the proposed splitter design.

Analysis of axial scanning range and magnification variation in wide‐field microscope for measurement using an electrically tunable lens

Inserting an electrically tunable lens (ETL), such as liquid lens or tunable acoustic gradient lens, into a microscope can enable fast axial scanning, autofocusing, and extended depth of field. However, placing the ETL at different positions has different influences on image quality. Specially, in a wide-field microscope for measurement, the magnification has to be constant when introducing an ETL, otherwise it will affect measurement accuracy. To determine the best position of ETL, axial scanning range and magnification variation are quantitatively analyzed and discussed in finite and infinite microscopes through theoretical analysis, optical simulation, and experiment for four configurations: when ETL is placed at the back focal plane of objective, at the conjugate plane of objective's back focal plane between two relay lenses, or behind two relay lenses, and at imaging detector plane. The obtained results are as follows. When ETL is placed at the back focal plane, the system has a large scanning range, but the magnification varies because the back focal plane is inside the objective. When ETL is placed between two relay lenses, the magnification stays constant, but the scanning range is small. When ETL is placed behind two relay lenses, the magnification keeps invariant and the scanning range is large, but ETL and two relay lenses are inside the microscope and the system has to be customized. Finally, when ETL is placed at imaging detector plane, the magnification stays constant, but the scanning range is 0, which means the system has no axial scanning capability. RESEARCH HIGHLIGHTS: An electrically tunable lens (ETL) is introduced into a wide-field microscope for measurement. Axial scanning range and magnification variation are analyzed and discussed. Theoretical analysis, ZEMAX optical simulation and experiments are performed.

Experimental validations of a tunable-lens-based visual demonstrator of multifocal corrections.

The Simultaneous Vision simulator (SimVis) is a visual demonstrator of multifocal lens designs for prospective intraocular lens replacement surgery patients and contact lens wearers. This programmable device employs a fast tunable lens and works on the principle of temporal multiplexing. The SimVis input signal is tailored to mimic the optical quality of the multifocal lens using the theoretical SimVis temporal profile, which is evaluated from the through-focus Visual Strehl ratio metric of the multifocal lens. In this paper, for the first time, focimeter-verified on-bench validations of multifocal simulations using SimVis are presented. Two steps are identified as being critical to accurate SimVis simulations. Firstly, a new iterative approach is presented that improves the accuracy of the theoretical SimVis temporal profile for three different multifocal intraocular lens designs - diffractive trifocal, refractive segmented bifocal, and refractive extended depth of focus, while retaining a low sampling. Secondly, a fast focimeter is used to measure the step response of the tunable lens, and the input signal is corrected to include the effects of the transient behavior of the tunable lens. It was found that the root-mean-square of the difference between the estimated through-focus Visual Strehl ratio of the multifocal lens and SimVis is not greater than 0.02 for all the tested multifocal designs.

Magnification-invariant surface profiling technique for structured illumination imaging and microscopy

Abstract The structured illumination (SI) surface profiling method that is fast and has a depth-independent image magnification ratio is presented. Utilizing an electrically tunable lens (ETL) makes it possible to scan the object plane rapidly and motionlessly. The change in magnification ratio that generally occurs with the variation in the focal length of an objective lens is minimized by using a 4f relay system and placing the ETL at the confocal plane. The high reflection at the uncoated membrane surfaces of the ETL is bypassed by using polarization optics. It is also proposed that the fast response of the ETL can be fully exploited by switching the scanning sequence from that of a conventional SI imaging system; performing the depth scan before the pattern shift. It is shown that the scanning speed was increased up to 25 times. The principle and the related problems of SI profiling based on an ETL are fully analyzed, and the concept is confirmed experimentally. In the experiment, scanning depth variation of 35 mm could be achieved while keeping the magnification ratio variation below 0.03. The ability of 3D profiling is demonstrated by SI imaging a cone-shape 3D object and a face-shape plaster figure. In the fields requiring fast scanning, such as an intraoral scanner or biomedical imaging, the proposing SI imaging method can be fully utilized.

A computational model of bio-inspired tunable lenses

Bio-inspired tunable lenses are composed of soft dielectric elastomer membranes, clear liquids and rigid frames. Being a new type of optical lens, its focus can be adjusted by the applied voltage. For this type of micro-lens to be practically used in real life, the functional relationship between the voltage and the focus needs to be determined, together with the impact of lens parameters such as the pre-stretching of soft dielectric elastomer membranes, the scale and the volume of liquids on this relationship, as well as on the maximum change of lens focus. In this paper, a compact computational model is proposed based on the assumption of homogeneous deformation and application of energy methods. The model results fit well with the experimental data. By using this model, the relationship curve between the applied voltage and the focus has been given, and it has been verified that such lens will not fail in operation. Finally, by analyzing the three parameters of lens, we can also demonstrate that the pre-stretching of dielectric membranes and the scale affect the relationship between the applied voltage and the focus the most, with the volume of liquids having the least impact.

Numerical study of an electrowetting liquid microlens

We construct a numerical model for a liquid microlens formed by filling a microwell with two immiscible liquids, namely oil and water. The water–oil interface can be actuated as a tunable liquid lens because its curvature is well controlled by applying an external electric field. The contact angle of the oil droplet has the opposite tendency to that of a traditional water-filled microwell because of the repulsion force from the water phase. We study the dynamic interface deformation as functions of time and applied voltage, which typically within 10 ms is good enough as an imaging system for human eyes. In addition, using two different filling liquids provides more possibilities for tuning the focal length.

Liquid crystal lens with optimized wavefront across the entire clear aperture

Abstract Comparative theoretical analysis of performances of three electrically variable liquid crystal lens designs is conducted. Traditional hole patterned electrode, additional floating and inner-ring electrode configurations are considered within the framework of a modal control lens. The control mode of those electrodes and the choice of the voltage and frequency values of the driving electric signals are optimized to obtain spherical profile of the optical phase retardation across the entire clear aperture of the lens. This would allow the effective use of the tunable lens in combination with larger aperture fixed focus lenses. It is shown that we can improve significantly the acceptable (with low aberrations) dynamic variation range of the optical power of the lens by switching the control mode of electrodes.

Graphene aperture-based metalens for dynamic focusing of terahertz waves.

We theoretically study a tunable reflective focusing lens, based on graphene metasurface, which consists of rectangle aperture array. Dynamic control of either the focal intensity or focal length for terahertz circular polarized waves can be achieved by uniformly tuning the graphene Fermi energy. We demonstrate the graphene apertures with the same geometry; however, spatially varying orientations can only control the focal intensity. To change the focal length, the spatially varying aperture lengths are also required. A comparative study between the metalenses, which generate only geometric or both gradient and geometric phase changes, has shown that the apertures' spatially varying length distribution is the key factor for determining the modulation level, rather than the focal length's modulation range. This kind of metalens provides tunable, high-efficiency, broadband, and wide-angle off-axis focusing, thereby offering great application potential in lightweight and integrated terahertz devices.

Trial functions for reduced-order models of piezoelectrically actuated microelectromechanical systems tunable lenses

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Tunable fluidic lenses with high dioptric power

We report a complete theoretical model and supporting experimental results on the fabrication and characterization of macroscopic adaptive fluidic lenses with high dioptric power, tunable focal distance, and aperture shape. The lens is 17 mm wide and is made of an elastic polydimethylsiloxane (PDMS) polymer, which can adaptively restore accommodation distance within several cm according to the fluidic volume mechanically pumped in. Moreover, the lens can provide for magnification in the range of +25 diopter to +100 diopter with optical aberrations within a fraction of a wavelength, and overall lens weight of less than 2 g. The agreement between the non-linear theoretical model describing the elastic membrane deformation and the experimental results is apparent. We stress that these features make the proposed lenses appropriate for the low vision segment, as well as for applications in video magnifiers, camera zooms, telescope and microscopes objectives, and other machine vision applications where large magnification is required.

A Robust Soft Lens for Tunable Camera Application Using Dielectric Elastomer Actuators.

Developing tunable lenses, an expansion-based mechanism for dynamic focus adjustment can provide a larger focal length tuning range than a contraction-based mechanism. Here, we develop an expansion-tunable soft lens module using a disk-type dielectric elastomer actuator (DEA) that creates axially symmetric pulling forces on a soft lens. Adopted from a biological accommodation mechanism in human eyes, a soft lens at the annular center of a disk-type DEA pair is efficiently stretched to change the focal length in a highly reliable manner. A soft lens with a diameter of 3 mm shows a 65.7% change in the focal length (14.3-23.7 mm) under a dynamic driving voltage signal control. We confirm a quadratic relation between lens expansion and focal length that leads to large focal length tunability obtainable in the proposed approach. The fabricated tunable lens module can be used for soft, lightweight, and compact vision components in robots, drones, vehicles, and so on.