Focal Length Research Articles

Design and fabrication of a large-aperture electrowetting liquid zoom system

In this paper, a continuous zoom system based on large-aperture electrowetting liquid lenses is designed and fabricated. The inner diameter of the electrowetting lens is up to 20 mm, which can tune stably from a focal length of −328.49mm to −∞ with applied voltages of 0 to 115 V. The zoom system mainly consists of two large-aperture electrowetting liquid lenses and a glass lens, getting continuous zoom images by adjusting the voltages applied to the two liquid lenses without any mechanical movement, ensuring a more compact structure than conventional zoom systems. The theoretical formulas of the zoom ratio and image plane stability condition for this optical system are systematically derived based on the matrix optics, laying the foundation for guiding system regulation and finding methods to improve zoom ratio, and corresponding influence factors on the zoom ratio are verified by experiments. The system zoom ratio can reach 1.52 at a fixed working distance, close to the theoretical derivation result. The system is expected to be used for imaging in a variety of scenarios, such as special photography, dynamic measurement, etc.

Scaling Laws for Deployable Mesh Reflector Antennas

This paper begins with the formulation of a general, rapid design method for deployable mesh reflector antennas based on the AstroMesh architecture. This method is then used to obtain estimates of the total mass, stowed envelope size, and fundamental natural frequency of vibration for antennas with a range of aperture diameters and focal lengths, assuming an operational radio frequency of 10 GHz. A study of the scaling trends of this reflector design shows that the distribution of prestress in the inner tension structure has a major impact on the mass of the outer perimeter truss. Based on this result, a prestress optimization problem to design reflectors of minimum mass is formulated, and analytical scaling laws are obtained for the mass, stowed envelope, and natural frequency of optimally prestressed reflectors with aperture diameters up to 200 m. It is then shown that aperture diameters of 70–100 m are at the limit of launchers that are currently available or under development. A semianalytical homogenization model that accurately estimates the fundamental natural frequencies for batten-supported and free-free boundary conditions is also presented.

Perfect space-time vortices.

We introduce the concept of perfect space-time vortices (PSTVs) that can exist in media with anomalous dispersion. If the topological charge of a PSTV is not too large, the spatiotemporal intensity distribution of the vortex field does not depend on the magnitude of the topological charge. We show theoretically how a PSTV can be realized in the optical context through spatiotemporal focusing of a Bessel-Gaussian space-time optical vortex source that is placed in the focal plane of a space-time lens composed of an ordinary lens and a time lens with matched spatial and temporal focal lengths.

Ultra-short and highly efficient metamaterial Fresnel lens-assisted taper

This paper demonstrates the benefits of leveraging free-space optics concepts in the design of certain integrated photonic components, leading to a footprint reduction without compromising on performance. Specifically, we present ultra-short, highly efficient and fabrication-friendly mode-size converters based on metamaterial Fresnel lens-assisted tapers. This is achieved using a parameterized inverse-design approach, where the metamaterial phase shifters are realized using fabrication-friendly Manhattan geometries, by optimizing the width, length, and position of the phase shifters. This approach overcomes the limitations of the conventional method that uses local periodic approximation, which is not suitable for lenses with a short focal length and high numerical aperture. We also extend the free-space concept of compound lenses and demonstrate a doublet-based taper to further reduce the footprint. The devices are fabricated and experimentally characterized in terms of insertion loss and signal integrity at high data transmission rates, exhibiting high performance. For the singlet, it effectively achieves mode-size conversion from 15 μm to 0.5 μm within a 15 μm distance, leading to ×10 length reduction compared to a linear taper. The insertion loss is under 1 dB over the entire C-band. The doublet achieves the same mode-size reduction within a 10 μm distance, leading to ×15 length reduction compared to a linear taper. The insertion loss is near 1 dB over most of the C-band. In both cases, the signal integrity is maintained for up to 50 Gbit/s.

3D Nano‐Printed Bistable Microlens Actuator for Reconfigurable Micro‐Optical Systems

AbstractThe integration of multiple functional microscopy methods into a single, miniaturized instrument, known as multimodal endomicroscopy, presents significant challenges due to the often conflicting optical requirements of each imaging modality. In this study, the use of 3D nano‐printing based on two‐photon polymerization for the monolithic manufacturing of complete optical MEMS devices is investigated as an enabling platform for multimodal endoscopy. This approach offers unparalleled miniaturization, design flexibility, and alignment precision of micro‐optics and actuators. In particular, the design, fabrication, and characterization of a monolithically 3D nano‐printed bistable microlens actuator are discussed. Bistability allows the microlens to switch between two stable positions, thereby changing the focus distance and numerical aperture. The actuator features two coils with opposing current directions, which create a magnetic field gradient around a polymer micro‐magnet. This configuration allows for robust switching between stable positions with an average axial distance of 168.3 ± 7.3 µm. Experiments conducted to ascertain the optical characterization of the actuator demonstrate its capacity to transition between different optical modes. The beam widths are measured to be in the range of 4.1 ± 0.2 µm in the high‐resolution mode and 11.3 ± 1.1 µm in the extended depth of focus mode.

Improving detection accuracy of extreme-few-pixel Shack-Hartmann wavefront sensor based on tilt modulation

Conventional high spatial resolution Shack-Hartmann wavefront sensors have difficulty realizing high-precision wavefront detection under extreme-few-pixel conditions. A method of Shack-Hartmann wavefront sensor based on tilt modulation is proposed. The proposed method first increases the amount of sub-spot image information. Then, the corresponding wavefront detection accuracy can be improved by tuning parameters such as modulation amplitude, modulation angle, and modulation number. Finally, the detection accuracy for different parameter settings is fitted statistically. As observed in numerical simulations and experiments, the tilt modulation-based method effectively improves detection accuracy. Moreover, there are obvious advantages over the conventional Shack-Hartmann wavefront sensors in 4 × 4 sub-aperture pixels and a short focal length of the microlens array, which improves detection accuracy by about 84% in the experiments. Realization of the need for high spatial resolution and high detection accuracy of Shack-Hartmann wavefront sensors.

Ultrasonic aperture-tunable gel lens

Conventional camera modules require mechanical moving parts to move their lenses and to adjust their focal points. This paper examines optically tunable lenses with a focal length and lens aperture that can be controlled using ultrasound vibration and a transparent gel. The lens uses the acoustic radiation force, which induces changes in the lens profile; varifocal convex and concave lenses can be fabricated by adjusting the input signals. The optical characteristics of the lenses were evaluated using ray tracing simulations. The aperture can be controlled with the driving frequency, with higher frequencies leading to a wider range of focal length changes with a lower input voltage.

Optical design and development of an underwater dual-channel microlens array integral field snapshot hyperspectral imager

Compared to push-scan hyperspectral imagers, snapshot hyperspectral imagers offer an advantage by minimizing sensitivity to attitude jitter in underwater mobile platforms. Here we present the optical design and development of an underwater microlens array integral field hyperspectral imager. The system comprises a panchromatic imaging channel with a high spatial resolution and a spectral imaging channel with a lower spatial resolution. Through the fusion of high-resolution panchromatic images and low-resolution spectral images, we achieve high spatial resolution hyperspectral images. Both the panchromatic imaging channel and the spectral imaging channel share a common front objective, featuring a 25 mm focal length and a wide 36° field of view angle. Utilizing prism dispersion, the spectral imaging system spans a band range from 465 to 700 nm with a spectral resolution of less than 10 nm. Specialized algorithms for spectral image reconstruction and image fusion have been developed. The experimental results across diverse scenes confirm the exemplary spectral imaging performance of the system, positioning it as a robust solution for underwater snapshot hyperspectral imaging.

Multifocal tornado beams carrying chirality

The significance of abruptly autofocusing vortex beams with a unique helical phase structure lies in particle trapping and optical spanner applications. This paper presents a novel modulation of a circular Pearcey beam utilizing a multicyclic helical phase and a chirp factor. Both theoretical analyses and experimental results demonstrate that we have successfully designed a beam with tornado properties and optically chiral structure. This beam has multidimensionally tunable parameters, allowing not only different focusing intensities, focal lengths and focal spot shape distributions, but also controllable rotational propagation directions. Furthermore, it has the ability to produce two dual focusing modes with dissimilar focal spot distributions. We believe that such a beam holds potential applications in the realisation of optical spanners, multi-region trapping of particles and the fabrication of chiral metamaterials.

Bloch-surface-wave resonance in a cavity resonator for focusing retroreflection.

A wavelength-selective retroreflector for a diverging wave will be an attractive external mirror for compact wavelength-stabilized semiconductor lasers and consists of a focusing grating coupler and a cavity resonator. In this Letter, the retroreflector utilizing a Bloch surface wave (BSW) resonance was theoretically investigated for improving the retro-reflectance in comparison to a previously reported structure utilizing guided-mode resonance (GMR). A retroreflector with an aperture size of 31 µm and focal length of 67 µm was designed for an operation wavelength of 1550 nm. The maximum retro-reflectance is predicted by numerical simulation to be higher by 11% than that of the GMR-based retroreflector.

Optical performance of a solar dish concentrator formed by the same size mirror located on parabolic frame

Solar dish concentrator with low cost and meeting the flux distribution is always pursued for the efficient utilization of concentrated solar energy. This paper investigates the optical performance of a solar dish concentrator formed by using a number of identical square mirror units arranged on a parabolic surface frame. The manufacturing of the mirror surface of this concentrator requires only one kind of mold, which has the significant advantage of low-cost manufacturing. Three common types of mirror units including the parabolic mirror (focal length fm), spherical mirror (radius Rm) and plane mirror are considered. The influence of key geometric parameters including the mirror width d (250–750 mm) and dimensionless parameters such as f1/D, fm/f1 and Rm/f1 (f1 is the focal length of parabolic frame) on its optical performance indexes such as the focused spot size (i.e., intercept width w), average local concentrator ratio (LCR), peak LCR and flux uniformity is analyzed using ray-tracing method, and the correctness of optical model and concentrator optical function is verified by outdoor concentrating experiments. The results show that the concentrator composed of parabolic or spherical mirrors can easily obtain a small circular focusing spot with high LCR and the intercept width can be easily controlled within 200 mm, the peak LCR can reach 34457 (average LCR is 12898), and average LCR reaches 14794 (peak LCR is 29497) at the smallest spot with w = 38.4 mm, which can be a low-cost alternative to parabolic dish concentrator. The concentrator with plane mirrors are easier to obtain square focusing spots with excellent flux uniformity, w can be controlled from 180 to 380 mm and uniform LCR is between 100 and 1400, which is very suitable for concentrating photovoltaic field.

Design framework of a computer-generated hologram that performs volumetric beam shaping for advanced laser processing

Axial beam shaping is very effective for material laser processing, typically laser cutting, drilling, and grooving. We demonstrate a framework for designing a computer-generated hologram (CGH) that performs volumetric beam shaping. The procedure performs axial beam shaping with a continuous intensity distribution, unlike our previous research in which only discrete focal points were arranged three-dimensionally. This research is the more general approach for volumetric beam shaping. An important point in this research is finding an optimal interval in the optical axis direction and in calculating the CGH design. The design interval is half of the focusing length (the full width at half-maximum of the laser beam profile in the axial direction) given by the diffraction limit of the optical system. The optimal value is obtained using an axially shaped beam that is the reconstruction of the CGH calculated from Zernike polynomials. We also demonstrate that the optimal interval for evaluating the axially shaped beam is also half of the beam length. Following the CGH design procedure, we demonstrate CGHs that generate long-focus beams with an arbitrary axially shaped beam. We found a tradeoff relation between the focusing length and the intensity of the long-focus beam, suggesting that the use of a focused beam with an appropriate length according to the purpose will lead to improved processing efficiency.

High-speed all-in-focus 3D imaging method based on liquid lens focus scanning

A large open aperture in an optical system can capture high-resolution images but yields a shallow depth of field. To overcome this issue, we propose a method for retrofitting microscopy imaging systems by using a variable-focus liquid lens to achieve 3D focus scanning. Specifically, the focal length of the imaging system was changed by controlling the liquid lens, and a sequence of images was captured at different focal planes in milliseconds. The image scale and phase were corrected. Then the in-focus pixels were abstracted by employing the Laplacian operator. These pixels were marked in the index table. According to the index table, the focused parts of multiple images were merged, and an all-in-focus image was generated. At the same time, a depth map was constructed based on the image number in the index table and the extracted depth information. Additionally, we have optimized the image processing flow; the processing speed was improved to around 6.5 fps.

First spectroscopic study of HFRC plasma

An advanced spectral diagnostic system was developed to measure the electron temperature (T e), electron density (N e), and ion temperature (T i) of the Huazhong University of Science and Technology field-reversed configuration plasma. The system consists of an optic fiber spectrometer with a wide spectral band and a 670 mm focal length high throughout Czerny–Turner monochromator equipped with both a 3600 g mm−1 grating and a 2400 g mm−1 grating to measure the line spectrum. Accompanying these components is an electron-multiplying charge-coupled device camera to capture spectral data. The relative intensity of the optical fiber spectrometer was calibrated using a standard luminance source, and the wavelength calibration of the spectrometer was accomplished using a Hg/Ar lamp. This diagnostic setup was configured to measure electron density based on the Stark effect of Hγ (n = 5 → n = 2; 434.04 nm). Doppler broadening of an O III (2s22p(2P0)3p → 2s22p(2P0)3s; 375.988 nm) emission line was measured and analyzed to obtain the ion temperature, and electron temperatures were estimated from the relative strength of Hβ (n = 4 → n = 2; 486.14 nm) (Dβ) and Hγ (Dγ) spectral lines when the electron density was obtained from Stark effect measurements. The initial experimental results indicate that the highest electron temperature of the formation region was approximately 8 eV, the electron density of the colliding-and-merging region was approaching 1020 m−3, and the ion temperature reached about 40 eV.

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.

Maritime Electro-Optical Image Object Matching Based on Improved YOLOv9

The offshore environment is complex during automatic target annotation at sea, and the difference between the focal lengths of visible and infrared sensors is large, thereby causing difficulties in matching multitarget electro-optical images at sea. This study proposes a target-matching method for visible and infrared images at sea based on decision-level topological relations. First, YOLOv9 is used to detect targets. To obtain markedly accurate target positions to establish accurate topological relations, the YOLOv9 model is improved for its poor accuracy for small targets, high computational complexity, and difficulty in deployment. To improve the detection accuracy of small targets, an additional small target detection head is added to detect shallow feature maps. From the perspective of reducing network size and achieving lightweight deployment, the Conv module in the model is replaced with DWConv, and the RepNCSPELAN4 module in the backbone network is replaced with the C3Ghost module. The replacements significantly reduce the number of parameters and computation volume of the model while retaining the feature extraction capability of the backbone network. Experimental results of the photovoltaic dataset show that the proposed method improves detection accuracy by 8%, while the computation and number of parameters of the model are reduced by 5.7% and 44.1%, respectively. Lastly, topological relationships are established for the target results, and targets in visible and infrared images are matched based on topological similarity.

Optimal design of parabolic through solar collector networks: A design approach for year-round operation

In this article, area optimization in parabolic trough solar collector (PTC) networks used in industrial processes is explored. The Particle Swarm Optimization (PSO) technique to maximise economic benefits while minimising total operating costs is employed. The objective also considers flexibility across wide temperature ranges and process heat loads. To achieve this the relationship between solar collection area and associated costs, including initial investment, maintenance, and energy efficiency is meticulously analysed. This work proposes strategies for the optimal adjustment of PTC collector area, accounting for thermal variations and specific demands of industrial processes. The results indicate that the optimised geometry of a PTC, which maximizes the present value of annual energy savings, has the following dimensions: length: 12.5 m; aperture: 5.8 m; tube diameter: 0.014 m; focal distance: 1.449 m; glass envelope inner diameter: 0.111 m. For a process thermal load ranging from 0.4 MW to 4 MW, the optimal network structure that maximizes energy capture in both summer and winter consists of 12 collectors in series (125 m long), with the number of parallel lines proportional to the heat duty. In this article, area optimization in parabolic trough solar collector (PTC) networks used in industrial processes is explored. The Particle Swarm Optimization (PSO) technique to maximise economic benefits while minimising total operating costs is employed. The objective also considers flexibility across wide temperature ranges and process heat loads. To achieve this the relationship between solar collection area and associated costs, including initial investment, maintenance, and energy efficiency is meticulously analysed. This work proposes strategies for the optimal adjustment of PTC collector area, accounting for thermal variations and specific demands of industrial processes. The results indicate that the optimised geometry of a PTC, which maximizes the present value of annual energy savings, has the following dimensions: length: 12.5 m; aperture: 5.8 m; tube diameter: 0.014 m; focal distance: 1.449 m; glass envelope inner diameter: 0.111 m. For a process thermal load ranging from 0.4 MW to 4 MW, the optimal network structure that maximizes energy capture in both summer and winter consists of 12 collectors in series (125 m long), with the number of parallel lines proportional to the heat duty. The network size is determined by the required heat load more than the inlet and required process temperature.

Multi-lens component used for a LWIR field-integral gas spectral imager

This paper reports a miniature multi-lens component used for a LWIR field-integral gas spectral imager. The component is designed for crosstalk minimization and consists of a 4∗3 array of micro-lenses and a detector. To reduce the cost, the detector is uncooled and the optical parameters of the 12 micro-imaging lenses are identical. These include 3.38 mm focal length and 35° field-of-view. The F/# is designed to be 1.2 to increase the luminous flux of the imager. After image quality evaluation and tolerance analysis, the micro-imaging lens has good imaging quality, is insensitive to tolerances, and is easy to fabricate. To enhance the environmental stability of the component, a temperature control system was designed to depress the temperature drift. In addition, the athermalization of the micro-imaging lens was beneficial. The performance test of the component shows that the NETD of the 12 micro-imaging lenses is less than 90 mK and most of them are less than 65 mK, and an image of an ethylene gas cloud observed by the component is also shown. The component has been integrated to the spectral imagers, and crosstalk is analyzed.

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.

Thermo-sensitive Poloxamer based antibacterial anti-inflammatory and photothermal conductive multifunctional hydrogel as injectable, in situ curable and adjustable intraocular lens

Cataract patients look forwards to fewer postoperative complications and higher vision quality after surgery. However, the current intraocular lens (IOL) implanted after cataract surgery neither can adjust focal length in response to ciliary muscle contraction as natural lens nor have the ability to prevent postoperative complications. Herein, a thermosensitve Poloxamer based hybrid hydrogel with antibacterial anti-inflammatory and photothermal functional elements doping was designed and used as injectable, in situ curable, and adjustable IOL (FHTAB IOL). The FHTAB IOL was composed of thermosensitve triblock-polymer F127DA and a small amount of HAMA, combined with BP NS, TA, and Ag NPs. FHTAB IOL can be injected into the empty lens capsule after cataract surgery via an injectable thermos-gel under NIR illumination and then be rapidly cured to form a full-size IOL under short-time blue light irradiation. The designed injectable FHTAB IOL possesses high transparency and transmittance, with a refractive index similar to the natural lens and adjustable properties. It was stabilized as a refractive medium without any leakage in the eye. In addition, the TA and Ag NPs loaded in the FHTAB IOL displayed significant antibacterial and anti-inflammatory effects in vitro and vivo. This study presents a potentially effective new strategy for the development of multifunctional adjustable IOLs.