Focal Length Changes Research Articles

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO3.

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO3 . Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 μm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO3 permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO3 are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.

Calibration of focus-adjustable cameras for triangulation sensors by model-based camera parameter estimation

Abstract The working range of triangulation-based measuring systems is often limited by the depth of field of the applied optical system. Approaches such as increasing the depth of field range by reducing the aperture are limited in their effectiveness by a lower light yield and thus require a longer exposure time. Focus adjustable optics enable the depth of field to be modified and thus effect the location and shape of the working range. However, the validity of the model-based camera calibration is limited to the corresponding focus setting, especially with optics that exhibit the effect of focus breathing, i. e. whose focal length change with the focus setting. This paper presents an approach to model the calibration parameters over the entire focus range of a lens using function fitting of the camera parameters and the influence of different configurations on this process is examined.

Parallel CNN Network Learning-Based Video Object Recognition for UAV Ground Detection

Video object recognition for UAV ground detection is widely used in target search, daily patrol, environmental reconnaissance, and other fields. So, we propose the novel parallel deep learning network with the ability of the global and local joint feature extraction for the UAV video target detection. This paper focuses on solving the problems of feature extraction and target background discrimination required by target discovery to realize target discovery. Break through the key problems of real-time target recognition, such as multiscale targets, high background complexity, many small targets, dense target arrangement, and multidirection, and put forward an optimized network scheme, aiming at the problem of multiscale of image target and aiming at the problem of large change of target scale in image. In the network, the corresponding targets with different sizes and different aspect ratios are matched to make the different targets match the closest, and then, the position of the detection box is fine-tuned by regression. For the special problem of image viewing angle and for the rotation invariance of the airborne down looking image of the target, the usual solution is through data enhancement; that is, through the rotation transformation of the training data, the neural network can learn the rotation invariance of the target. Aiming at the problem of multi-directional image target and aiming at the problems of large target aspect ratio, large target tilt angle, and changeable direction in the target, we propose to use the tilt detection frame instead of the ordinary rectangular detection frame. Aiming at the problem of dense arrangement of image targets and aiming at a large number of densely arranged targets in the image, a feature refining module is proposed, which can effectively improve the detection performance of the detector for densely arranged targets. The experimental results shows that the proposed algorithm achieves more than 10% on the target detection accuracy with focal length change of 1-10 times. The detection accuracy meets the requirements of practical application.

Evaluating the Impact of Optical Axis Stability on Exoplanet Detection

For detecting exoplanets with high precision, using the angular distance between the two stars to detect the periodic motion of the star will be a better choice. This approach can avoid importing the position error of the reference catalog in the process that uses a traditional photographic plate to derive the star position suffers. At the precision level of microarcseconds, the error caused by optical axis deviation is not negligible. In this paper, we evaluate the impact of the stability of the optical axis on the relative angular distance measurement from the aspects of theoretical analysis and numerical simulation. When the angular distance error limit of 1 microarcsecond is given, the upper limit of optical axis deviation is estimated to be 68 milliarcsecond. In addition, when limiting the deviation of the optical axis, we give the corresponding error allowance of angular distance measurement. Moreover, we also discuss the way to resolve the problem of CCD distortion and focal length change on the measurement of angular distance. The work in this paper is of guiding significance to the design of a telescope.

Improved depth of field of the composite micro-lens arrays by electrically tunable focal lengths in the light field imaging system

Conventional micro-lens arrays (MLAs) for a light field imaging system can capture 3D images, which can be optically reconstructed. However, the sensed images frequently suffer from a fixed or limited depth of field (DOF) of MLAs. In this work, we propose composite micro-lens arrays (CMLAs), which are composed of electrically tunable liquid crystal MLAs (LC MLAs) and ultraviolet-curing MLAs (UVC MLAs). Moreover, we experimentally confirmed that the proposed CMLAs possess a tunable and improved DOF by electrically controlling the operating voltages applied to the LC MLAs in the light field imaging system. In the single LC MLA, the focal length changes from 1.62 mm to 2.53 mm, and the DOF is transferred from 0.83 mm to 23.85 mm under the influence of the operating voltage. The focal length of the CMLAs is tunable from 2.02 mm to 3.68 mm compared with LC MLAs, and the corresponding DOF could vary from 1.19 to 43.38 mm, which is much higher than that of the single LC MLAs. Furthermore, the CMLAs can capture clear images at large depth positions under lower operating voltages. Therefore, the CMLAs with the improved and adjustable DOF exhibit potential application for the light field imaging system and can be extended to the industrial and medical microscopy applications.

Study on design and performance enhancement of Fresnel lens solar concentrator

Solar energy is being used at different applications and is proving to be a major renewable source of energy. The photovoltaic cells used in CPV are costly and less efficient. The recurrent method of the concentration of light by means of parabolic reflectors has a lot of drawbacks. The most efficient way to concentrate solar energy is to use a Fresnel lens for the full use of sunlight aspect ratio. In this work, Fresnel lens design parameters are studied and simulated. The temperature change in outside operation conditions has been recognised during design stages. After that, an analysis of the focal length change is carried out via simulation. To reduce the chromatic aberration behaviour due to the dispersion relationship between the material's refractive index and wavelength, it was assumed that each Fresnel lens groove has a varied wavelength design. The golden wavelength approach was used to determine the optimal wavelength design in the 0.54 m to 1.04 m range of the optimum solar spectrum. The results obtained are encouraging.It was found that the method of selecting different wavelengths for each concentric ring improves the flux distribution as well as the dispersion of the solar spectrum..

Method for evaluating spectral resolution based on a single-lens spectrometer.

Spectral resolution is a key parameter of a spectrometer. Typically, the Rayleigh criterion is used to evaluate spectral resolution; however, it is not applicable to a single-lens-based spectrometer, as its principle is different from that using a prism or grating. Therefore, this work proposes that a method resolution is a key parameter to evaluate spectral resolution by exploiting the concept of focus depth. Accordingly, the spectral resolution is determined using three factors, namely, aperture factor F of the lens, pixel size p of the charge-coupled device, and the derivative of the rate of change of focal length with respect to wavelength f'(λ). The proposed method is verified by simulations with the following lens parameters: a diameter of 50.8 mm, focal length of 200 mm at 587.6 nm, and F=3.94. The calculated and simulated spectral resolution values are, respectively, 1.7 nm and 1.2 nm at 480 nm. Based on an analysis of the influences of F, p, and f'(λ) on the spectral resolution, increasing f'(λ) or decreasing both F and p might improve the spectral resolution. Finally, the proposed method is validated via experiments for lenses with different F values as well as materials, and we determine their spectral resolutions; these results are observed to be similar to the calculated values.

Tunable Soft Lens of Large Focal Length Change.

Tunable lens technology inspired by the human eye has opened a new paradigm of smart optical devices for a variety of applications due to unique characteristics such as lightweight, low cost, and facile fabrication over conventional lens assemblies. The fast-growing demands for tunable optical lenses in consumer electronics, medical diagnostics, and optical communications require the lens to have a large focal length modulation range and high compactness. Herein, for the first time, an all-solid tunable soft lens driven by highly transparent dielectric elastomer actuators (DEAs) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and waterborne polyurethane (PEDOT:PSS/WPU) transparent electrodes is developed. The deformation of the tunable soft lens is achieved by the actuation of DEAs, mimicking the change of the surface profile of the human eye to achieve remarkable focal length variations. Upon electrical activation, this tunable soft lens can vary its original focal length by 209%, which is one of the highest among current tunable soft lenses and far beyond that of the human eye. This study demonstrates that transparent DEAs are capable of achieving focus-variation functions, and potentially useful in artificial robotic vision, visual prostheses, and adjustable glasses, which will induce significant effects on the future development of tunable optics.

Design of a compact varifocal panoramic system based on the mechanical zoom method.

A compact varifocal panoramic annular lens (PAL) system based on the four-component mechanical zoom method is proposed, which solves the problem that the traditional PAL system cannot zoom in to the region of interest. By moving the zoom group and the compensation group, our design achieves continuous zooming, in which the focal length changes from 3.8 to 6 mm. It can keep the position of the image surface unchanged while maintaining a compact structure. The system has a field of view (FoV) of 25°-100° in wide-angle mode and an FoV of 25°-65° in telephoto mode. The modulation transfer function of the wide-angle view is higher than 0.22 at 147 lp/mm. The F-theta distortion is less than 3%, and the relative illuminance is higher than 0.9 in the zoom process. Compared with the zoom PAL system with multiple free-form aspheric surfaces, the proposed system uses multiple spherical lenses and only one Q-type asphere lens to achieve outstanding panoramic zoom imaging results. It is practical and straightforward, easy to manufacture, detect, and mass produce.

Depth estimation using spectrally varying defocus blur.

This paper proposes a method to estimate depth from a single multispectral image by using a lens property known as chromatic aberration. Chromatic aberration causes light passing through a lens to be refracted depending on the wavelength. The refraction causes the angle of rays to vary depending on their wavelength and a change in focal length, which leads to a defocus blur for different wavelengths. We propose a theory to recover a continuous depth map from the blur in a single multispectral image that includes chromatic aberration. The proposed method needs only a standard wide-aperture lens, which naturally exhibits chromatic aberration, and a multispectral camera. Moreover, we use a simple yet effective depth-of-field synthesis method to calculate the derivatives and obtain all-in-focus images necessary to approximate spectral derivatives. We verified the effectiveness of the proposed method on various real-world scenes.

Electrohydrodynamic printing of a dielectric elastomer actuator and its application in tunable lenses

Optical lenses driven by dielectric elastomer (DE) actuators with tunable focal lengths are presented here. They are inspired by the architecture of the crystalline lens and the ciliary muscle of the human eye and have prompted a growing interest. The most commonly used DEs in tunable lenses have often required highly transparent films and also the need to encapsulate clear liquid silicone to act as the lens. There is a restriction on the properties of the tunable lens imposed by materials limitations. Here, the fabrication of a fully 3D printed tunable lens with an inhomogeneous structure is described. It exhibited a 29% change in focal length from 33.6 mm to 26.1 mm under a dynamic driving voltage signal control. Furthermore, it displayed excellent stability when the focal length was tuned from far to near (30.1 mm to 25.3 mm) for 200 cycles. The tunable lens obtained mimics the working principle of the human eye in auto adjusting the focal length and has evident potential applications in imaging, information storage, beam steering and bifocal technology.

Soft Tunable Lenses Based on Zipping Electroactive Polymer Actuators.

Compact and entirely soft optics with tunable and adaptive properties drive the development of life‐like soft robotic systems. Yet, existing approaches are either slow, require rigid components, or use high operating voltages of several kilovolts. Here, soft focus‐tunable lenses are introduced, which operate at practical voltages, cover a high range of adjustable focal lengths, and feature response times in the milliseconds range. The nature‐inspired design comprises a liquid‐filled elastomeric lens membrane, which is inflated by zipping electroactive polymers to tune the focal length. An analytic description of the tunable lens supports optimized designs and accurate prediction of the lens characteristics. Focal length changes between 22 and 550 mm (numerical aperture 0.14–0.005) within 260 ms, equal in performance to human eyes, are demonstrated for a lens with 3 mm aperture radius, while applying voltages below 500 V. The presented model, design rules, and fabrication methods address central challenges of soft electrostatic actuators and optical systems, and pave the way toward autonomous bio‐inspired robots and machines.

Construct Achromatic Polymer Microlens for High‐Transmission Full‐Color Imaging

AbstractTraditional microlens focuses the beam with spherical convex surface, which achieves high transmission by restraining the scattering loss yet brings spherical aberration and chromatic aberration. While recently‐developed metalens, which relies on phase modulation by elaborately designing local resonators, has realized diffraction‐limited focusing. Yet, the issues of scattering loss of nanoresonators and chromatic aberration remain serious. Here, the design strategies of both metalens and traditional microlens are combined by introducing accurate phase modulation for the wavefront into microlens designing; and a broadband, polarization‐independent, and achromatic microlens with high efficiency is realized. With concentric‐circular polymer (phenolic resin) terrace as basic building blocks, a precise thickness profile is constructed and a high‐index polymer microlens is formed by electron‐beam grayscale lithography. The diffraction‐limited focusing possesses less than 5% change of focal length when the wavelength varies from 425 to 700 nm, showing the full‐color imaging and detection with a focusing efficiency of 80% (at 700 nm wavelength). Moreover, the rotation symmetry of the microstructures of microlens makes it work for arbitrary polarization. The achromatic imaging capability of the microlens is verified by whitelight imaging. It is expected that this high‐efficiency polarization‐independent broadband achromatic polymer microlens may have wide applications in high‐efficient imaging and sensing.

Focus‐Tunable Planar Lenses by Controlled Carriers over Exciton

AbstractWhile recent studies on nanoscale diffractive lenses demonstrate their potential as possible candidates for thin‐film display applications, their narrow focal ranges limit their application. Graphene, however, may realize focal controllability for its unique optoelectric property; due to its unique band structure among 2D materials, its carriers can be controlled by adjusting the Fermi level. Furthermore, due to the bandgap property of graphene, the intraband excitation of carriers is dominant over the interband excitation of carriers, which results in enhanced photonic transmission and reduced absorbance. Utilizing this property, graphene‐based ultrathin focusing device is fabricated that alters its optical characteristics when direct‐current voltage is applied producing vertical fringe‐specific electric field. The proposed device demonstrates 8.6% change in focal length and 48.85% focusing efficiency at wavelength of 405 nm. Overall, this study on electrically tunable ultrathin microlens introduces potential for holographic displays and expands the research scope in future display technologies.

Cascaded nonlinearity induced spatial domain effects in a high power femtosecond optical parametric oscillator.

We have investigated the effect of cascaded optical nonlinearity on the spatial beam properties of a femtosecond optical parametric oscillator (OPO). The OPO was operated with a tunable phase mismatch by varying the angle of the nonlinear crystal. The cascaded nonlinearity induced self-focusing and defocusing changed resonator's stability and impacted mode properties. With tuning of a phase mismatch, the calculated parabolic part of cascaded nonlinearity lens focal length changes from f ∼ 30 mm (D ∼ 33 m-1 at Δθ ∼ -0.5o) to infinity and back to f ∼ -110 mm (D ∼ -9 m-1 at Δθ ∼ 0.9o) in the LBO nonlinear crystal. Such high power nonlinear lenses in a cavity operated near its stability limit promoted the generation of axially asymmetric or pass-to-pass unstable resonator modes. It was shown that phase mismatched optical parametric oscillation changes the physical character of the resonator from linear to ring-like with two nonlinear crystals having two different focusing powers. Calculations showed that the QCN induced spatial nonlinear phase should lead to severe longitudinal chromatic aberrations for broad spectrum pulses. A numerical simulation in XYZ spatial domain and calculations using ABCD matrix approach confirmed the physical mechanisms underlying the experimental results and allowed for the interpretation of the observed effects.

Evaluation of Heat Flux Distribution Uniformity around the Receiver Tube of Parabolic Trough Collector based on Six Statistical and Geometrical Indices

In addition to the total amount of heat flux (HF), its distribution around the receiver tube has an outstanding effect on the outlet temperature (To) and thermal efficiency (ηth) of parabolic trough collectors (PTCs). According to former researches, whatever the heat flux around the receiver tube is more uniform, the collector efficiency increases. But no index has yet been provided for the evaluation of HF distribution. In this paper, the quality of HF distribution around the receiver tube of LS-3 PTC is investigated by definition of six statistical and geometrical indices, including S.D, A.E, Int, Avg, Ust, and Bst. To investigate these indices, first, the HF around the receiver tube should be varied by the change of focal length (f) and using Compound Parabolic Concentrator (CPC). Second, the above-mentioned indices are calculated based on the statistical formulations. Third, the receiver tubes affected by the miscellaneous HF distributions (as boundary conditions) are simulated using one validated numerical method. Finally, the correlations between the specified indices and To are investigated. According to the results, the index of Int indicates the number of sun rays hit on the receiver tube. Thus this index can represent optical efficiency (ηopt). Also, the inverse of both Ust and Bst parameters are introduced as the indices of uniform HF around the receiving tube. In other words, the statistical and circumferential uniformities of HF values are two effective parameters in the definition of the HF uniformity. Meanwhile, the regression expresses the effect of Ust is 84 times more than Bst on To and ηth. Also, the overall efficiency can increase up to 2.61% using the CPC-type reflector.

Graphene-based metalens with rotatable radiation pattern composed of cross-shaped apertures

A method for designing a metalens with electrically tunable focal length is proposed and analyzed. Metasurfaces are defined as low-thickness devices to initiate and generate the abrupt electromagnetic wavefront variation. However, there is no way to control them actively. Our research proposes the design of a metalens that consists of cross-shaped apertures (CSAs) in a gold film and a graphene layer. The CSAs have different lengths and rotation angles. By applying bias voltage to graphene, the focal length can be varied and controlled electrically. When the Fermi energy of the graphene changes from 0.1 to 0.5 eV, the focal length changes from 10.5 to 12.67 mm. The structure is simulated by the finite-difference time-domain numerical method. By splitting graphene layers to several parts and biasing each of them with different bias voltages, the transmitted focused beam could be rotated up to 35 deg. As an application of the metalens device, a refractive index sensor with sensitivity of 80 μm per refractive index unit (μm / RIU) is proposed.

Analysis on an optimal transmittance of Fresnel lens as solar concentrator

This paper presents a novel method approaches to simplify refractions on prisms to get the ideal shape of the Fresnel lens. A simplification is about to use one refraction on the upper surface to replace the two refractions on the upper and the lower surfaces. A differential equation that describes the ideal shape of Fresnel lens has been derived out. It shows that the particular solution of the differential equation is a bunch of confocal curves. Especially, the ideal curves are closing to their basic circle when total refracted angle less is than 30°, which makes it reasonable to use the basic circle as the profile of Fresnel lens. Then a Fresnel lens was designed with considering material and manufacturing technology. Then an analysis on the focal length change during the tilt incidence was mainly carried via simulation and experiment. It is found that the effective tilt incident angle can be enhanced from 7° to 32° by lifting up the position of the receiver and adding a simple mirror secondary reflector. The experimental result shows that the Fresnel solar concentrator is working efficiently during the time from about 10:00 to 14:30 a day. The steady output temperatures are 125 °C, 150 °C and 170 °C, with collection efficiencies are about 0.53, 0.48 and 0.44. It can well satisfy mid-temperature solar thermal application.

Experimental observation of flat focusing mirror based on photonic jet effect

In this work, we experimentally demonstrate that a thin rectangle dielectric-metal structure can have a function of a flat focusing mirror based on photonic jet effect in reflection mode. Using polydimethylsiloxane (PDMS) rectangle with size length of 10 μm and wavelength-scale thickness of 1 μm on the top of a silicon wafer, we have built a flat mirror which focuses an incident beam at the focal length changing from 1.38 μm to 11.67 μm upon tuning the beam incidence angle from 30° to 75°. The focusing properties of such a mirror persist in the wavelength range of 405 nm to 671 nm. Our approach can be extended to realize other optical functionalities by properly controlling rectangle dimensions and materials. This flat focusing mirror is able to guide the incident beam in free space without perceptible diffraction at the distance equal to the photonic jet length and suitable for small-scale photonic circuits.

Computational Model and Design of the Soft Tunable Lens Actuated by Dielectric Elastomer

Abstract Inspired by the accommodation mechanism of the human eye, several soft tunable lenses have been fabricated and demonstrated the capability of controllable focus tuning. This paper presents a computational model of a dielectric elastomer-based soft tunable lens with a compact structure that is composed of a lens frame, two soft films, and the optically transparent fluid enclosed inside. The two soft films, respectively, serve as the active film and passive film. The active film is a dielectric elastomer film and can be coated with the annular electrode or circular electrode. The deformation of the lenses with both electrode configurations can all be formulated by a boundary value problem with different boundary conditions and be solved as the initial value problem using the shooting method. Two common failure modes of loss of tension and electrical breakdown are considered in the calculation of the lens. The computational results can well fit the experimental data. The focus tuning performances as well as the distributions of stretches, stresses, and electric field in the active films of the lenses with two different electrode configurations are compared. The influences of several parameters on the performances of the lenses are discussed, such that the tunable lens can be designed to have maximum focal length change or to be optimized based on different application requirements.