Conventional Microlens Research Articles

A 2D/3D convertible integral imaging display with enhanced depth of field

Depth of field (DOF) enhancement and two-dimensional (2D)/three-dimensional (3D) convertible display are important research areas of integral imaging 3D display. However, there is no accurate definition of DOF in existing integral imaging 3D displays, and it is difficult to realize DOF enhancement and 2D/3D convertible display simultaneously. In this paper, we propose a DOF analysis method for integral imaging 3D display, and derive an accurate definition of DOF by analyzing the voxel formation mechanism and the minimum resolution angle of the human eyes. Then a 2D/3D convertible integral imaging display with an enhanced DOF is proposed based on the definition of the DOF. The DOF enhancement and different display modes of 2D and 3D are realized by using two sets of dual-state optical components, including polymer dispersed liquid crystal (PDLC) films and liquid crystal micro-lens arrays (LCMLAs). When PDLC film in one set is scattering and LCMLA has the optical properties of a micro-lens array, while PDLC film in the other set is transparent and LCMLA has the optical properties of a flat glass at the same time, the system operates in 3D display modes. Two sets of DOFs are formed in turns and spliced to enhance the whole DOF based on the persistence of vision. When only one PDLC film is scattering and two LCMLAs both have the optical properties of a flat glass, the system realizes 2D display. The experiments on the 2D/3D convertible integral imaging display prototype verify that the DOF is enhanced to 1.71 times and 2D/3D convertible display is realized simultaneously compared with the display based on a conventional micro-lens array. The proposed display has potential for applications in areas such as education and medical care.

Wide-field full-Stokes polarimetry for conical light based on all-dielectric metasurface

Polarization camera based on CMOS sensor and nano wire-grid technology have found widespread applications in medical diagnostics, remote sensing and industrial inspection. However, the limited filtering properties of wire-grid polarizers and the small field-of-view provided by conventional microlens restrict the energy efficiency of these systems while also increasing their cost, size and weight. In this study, we propose an innovative approach that integrates focusing and splitting of polarization states into a single-layer all-dielectric metasurface. This metasurface enables full-Stokes polarization imaging for a wide field-of-view conical light. The design of the metasurface utilizes a phase compensation method to effectively focus orthogonal polarized conical light onto the central pixel of the CMOS sensor. Theoretical analysis demonstrates that this metasurface can accurately detect full-Stokes parameters within ±20° incident cone angles with an average efficiency reaching 83.0%. The angle can be extended to ±90° with an average efficiency exceeding 80%. We fabricated a three super-pixel metasurface prototype, and experimental measurements reveal its ability to efficiently focus and split three pairs of orthogonal polarization states under ±11° conical angle incidence with an average focusing efficiency of 68.1%. This study presents a promising solution for achieving wide field-of-view and high-efficiency polarization detection in integrated CMOS systems.

Pitch‐Switchable Metalens Array for Wavefront Profiling at Multiwavelength

AbstractThe wavefront sensor is a crucial component in the adaptive optics system (AOS), responsible for detecting aberrated wavefronts. However, The Shack–Hartmann wavefront sensor with single‐pitch microlens array is limited in its adaptability to different observing conditions, affecting wavefront reconstruction accuracy. And conventional microlens arrays are wavelength‐specific. Here, a polarization‐dependent pitch‐switchable metalens array based on non‐interleaved silicon metasurfaces is proposed, capable of working at multiple wavelengths. By controlling the incident and outgoing light polarization, three metalens arrays with customized pitches can be switched at will. Furthermore, achromatic performance is achieved in the 950 nm and 1030 nm wavelengths through structural dispersion engineering. Finally, wavefront detection experiments are finally conducted to demonstrate the phase reconstruction with variable spatial resolution. The metalens array with high flexibility in wavefront sensor can further enhance the adaptability of AOS and has great potential for applications in light‐field imaging, and machine vision, etc.

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.

Fabrication of microoptical freeform arrays on wafer level for imaging applications.

Miniaturized imaging systems combining an ultra-compact form factor in combination with the ability of refocusing and depth imaging have gained much interest in the field of mobile imaging. Therefore, artificial compound eye cameras are an extremely promising approach for the realization of compact monolithic camera modules on wafer level. Up to now, their imaging performance was limited to low resolution in the range of VGA format according to fabrication constrains given by the established microoptical fabrication methods, namely the reflow of photoresist. In order to overcome these classical limitations, the use of refractive freeform arrays (RFFA) instead of conventional microlens arrays is inevitable. To enable high volume and cost efficient mass production of artificial compound eye cameras for mass markets like the consumer electronics industry, their fabrication on wafer level is essential, but has not been published up to now. We present a wafer level based process chain enabling the fabrication of these elements for the first time.

Glasses-Free Three-Dimensional Display Based on Microsphere-Lens Array

A 3D display based on a microsphere-lens (MSL) array is proposed. This 3D display uses the MSL array to project the parallax images on a 2D display panel into different spatial directions. Each MSL can work as a conventional micro-lens. However, the MSL has a much shorter focal length than the conventional micro-lens. The shorter focal length is helpful for increasing the viewing angle. A prototype of the 3D display is developed. Experiment result shows that the proposed display has a large viewing angle and the crosstalk is also limited in a low level.

Anomalous focusing characteristics of nanoscale metallic waveguide array under different incident wavelengths

The chromatic aberration effect of light focusing in a periodic nanoscale metallic waveguide array (NMWA) is investigated by using the coupled-mode theory and numerical simulations. The dispersion of the coupling coefficient in the periodic NMWA is calculated. The spreading angle of surface plasmon polaritons (SPPs) is derived from the distribution of the electric field density. The numerical results demonstrate that the light focusing in the periodic NMWA exhibits an anomalous chromatic property in an appropriate wavelength range compared to a conventional microlens.

Monolithically integrated glass microlens scanner using a thermal reflow process

A monolithically integrated glass microlens optical scanner is presented. A new wafer-level fabrication approach addresses the issue of microlens integration and alignment in conventional microlens integrated devices. A through-silicon plano-convex glass microlens has been fabricated on a silicon substrate prior to the formation of an integrated microlens actuator, realized through two thermal reflow processes at 850 °C. A lateral comb drive is adapted to demonstrate the glass microlens scanner. The 800 µm diameter microlens was laterally shifted up to ±51.6 µm when an ac voltage of 28.5 V in amplitude was applied at a resonant frequency of 1.749 kHz at atmospheric pressure. The optical scanning angle of ±2.0° has been obtained with the fabricated device.

Design and fabrication of a freeform microlens array for a compact large-field-of-view compound-eye camera

In this research, a unique freeform microlens array was designed and fabricated for a compact compound-eye camera to achieve a large field of view. This microlens array has a field of view of 48°×48°, with a thickness of only 1.6 mm. The freeform microlens array resides on a flat substrate, and thus can be directly mounted to a commercial 2D image sensor. Freeform surfaces were used to design the microlens profiles, thus allowing the microlenses to steer and focus incident rays simultaneously. The profiles of the freeform microlenses were represented using extended polynomials, the coefficients of which were optimized using ZEMAX. To reduce crosstalk among neighboring channels, a micro aperture array was machined using high-speed micromilling. The molded microlens array was assembled with the micro aperture array, an adjustable fixture, and a board-level image sensor to form a compact compound-eye camera system. The imaging tests using the compound-eye camera showed that the unique freeform microlens array was capable of forming proper images, as suggested by design. The measured field of view of ±23.5° also matches the initial design and is considerably larger compared with most similar camera designs using conventional microlens arrays. To achieve low manufacturing cost without sacrificing image quality, the freeform microlens array was fabricated using a combination of ultraprecision diamond broaching and a microinjection molding process.

Cross-lenticular lens array for full parallax 3-D display with crosstalk reduction

As a 3-D display technology, stereoscopic imaging with lenticular lens sheet can only offer viewers the horizontal parallax. To reconstruct the 3-D image with parallax in both horizontal and vertical directions, the full parallax technique with micro-lens array was proposed. But due to the fabrication constraints and cost concerns of the micro-lens array, application of the full parallax technique is restricted in practice. In this paper we revisited the lenticular sheet method and cross-lenticular lens array formed by two lenticular sheets overlapped orthogonally. By analyzing the optical properties of this cross-lenticular lens array, we found that it has reasonable imaging qualities as a conventional micro-lens array. Besides, due to the poor optical property of the outskirt area of the cross-lenticular lens, the cross-lenticular lens array has better crosstalk suppression capability than the conventional one. Based on the analysis, in this paper, we used the proposed lens array to reconstruct a 3-D image and verified its practicability. The cross-lenticular lens array was found feasible to take the full parallax technique into commercial applications, with comparable advantages in terms of low-cost and easy fabrication over a large area.

UV EXCIMER LASER BEAM HOMOGENIZATION FOR MICROMACHINING APPLICATIONS

A UV excimer laser beam delivery system is designed with conventional microlens arrays and diffractive diffuse. Ultraflat and ultrasmooth ablated surfaces with sharp edges are obtained on fused silica. Ablated surfaces of a depth of 5.9 μm with less than 10 nm surface roughness are obtained on fused silica, as measured by an atomic force microscope.

Analysis of Low-Dark-Current, High-Sensitivity, Low-Optical-Crosstalk, and Small-Pixel-Size Pinned Photodiode With No-Gap Microlens

A low-dark-current, high-sensitivity, low-optical-crosstalk, and small-pixel-size pinned photodiode with no-gap microlens is designed, fabricated, and measured. Compared with nonpinned photodiodes of the same size, the dark current of a 2μm × 2μm pinned photodiode is reduced about 41.2%, while the photocurrent is reduced less than only 1% at the low reverse biased voltage Vpd=2 V under illumination of the same light source (470 nm, 450 μw/cm2). Due to the use of the pinning layer, the equivalent capacitance of the 2-m pinned photodiode is increased about 1.1 fF. In addition, the no-gap microlens technology has been developed here to improve the photodiode performance. Prototype complementary metal-oxide-semiconductor (CMOS) imagers consisting of a 640 (row) by 480 (column) array of pinned photodiodes with no-gap or conventional microlens have been designed and fabricated based on 0.18-μm CMOS process. The test results show that, compared with the photodiode with a conventional microlens, the sensitivity is increased about 74% and the optical crosstalk is reduced up to 7.2% for the photodiode with a no-gap microlens. Thus, the small-pixel-size pinned photodiode with no-gap microlens shows advantages in applications of high-resolution optical detectors and high-quality CMOS image sensors.

Fast switching characteristics of a microlens array using the electroclinic effect of SmA^* liquid crystals

We present a microlens array characterized by the electroclinic effect of chiral smectic A (SmA(*)) liquid crystals, which show the very fast dynamic switching characteristics required in high-speed optical devices. In order to easily control the intensity at the focal length of the proposed dynamic microlens structure, we adopt a solid-type liquid crystal polymer with optical anisotropy, which can split the beam intensity into two directions, depending on the vectorial portion of the polarization state of the light. The proposed microlens shows a focal intensity tunable by controlling the polarization of light at the SmA(*) liquid crystal. The lens has a very fast switching time of about 24 micros, which is several times faster than conventional microlens arrays with surface-stabilized ferroelectric liquid crystals.

Micro gradient-index conical lenses: simulation and fabrication methods

In this paper, a micro gradient-index conical lens, which has a larger acceptance angle than a conventional microlens, is presented. Methods on how to simulate these lenses in commercial optical design software CodeV are introduced, and the effects of several index profiles and cone shapes are compared in simulation. Results show that a micro gradient-index conical lens has a four times larger acceptance angle compared with a microlens. Additionally, conical lenses with a Gaussian-index profile show a larger acceptance angle than those with a solid refractive index. Fabricated conical lenses show an acceptance angle of more than 27 degrees for a detection threshold of 50%, which agrees with the simulation result.

Analysis of a closed-boundary axilens with long focal depth and high transverse resolution based on rigorous electromagnetic theory.

We find that a microcylindrical axilens with a closed boundary and with an f-number less than 1 still can achieve the properties of long focal depth and high transverse resolution, unlike a microcylindrical axilens with an open boundary, which fails to maintain those properties for low f-numbers. The focusing characteristics of the closed-boundary axilens and the open-boundary axilens are numerically investigated based on the boundary integral method. The numerical results show that the ratio of the extended focal depth of the closed-boundary axilens to the focal depth of the conventional microlens can reach up to 1.26 and 2.12 for the preset focal depths 3 and 5 microm, respectively, even though the f-number is reduced to 1/3.

Wavelength-division microlens interconnection using weakly diffracted Gaussian beam

Free-space digital optics is a new technology that exploits the ability of optics to handle thousands of light beams or information channels at once. This and other features of optics complement the strengths and weaknesses of purely electronic systems. A compact free-space optical system is proposed that uses an array of microlenses for chip-to-chip and board-to-board interconnections. Here the weakly diffracted Gaussian beam and wavelength-division architecture are utilized to improve the channel density and reduce crosstalk in a microlens interconnection system. Based on the simulation, we improve the channel capacity by 3.47 times (or reduce crosstalk by 93.1 dB) while maintaining the same crosstalk (or channel density) compared with the conventional microlens interconnection system. The nonperfect filtering effect of different wavelengths at the detector plane is also studied to fully investigate the properties of the proposed scheme. The parameter sensitivity of the proposed system is studied for completeness. From the simulations, the relationship between the interconnection distance and spot size at the detector plane is wavelength-independent. In addition, the spot size is more sensitive to change of microlens diameter than to other system parameters.

Elliptical micro‐fresnel lenses fabricated by electron‐beam writing technique

AbstractElliptical micro‐Fresnel lenses can exhibit astigmatic characteristics; and hence, an astigmatic optical system made of a conventional combination lens can be made much thinner by their use. The authors have developed an electron‐beam writing system which has a function suitable for fabrication of thin‐film micro‐optical elements. The function of this writing system has been expanded so that elliptical micro‐Fresnel lenses can be fabricated. It has been confirmed that the fabricated lenses have a good sawtooth cross section. For the focusing characteristics of the lens, the eccentric ratios and the observation position dependence have been investigated theoretically and experimentally. The obtained focused spots almost reach the theoretical diffraction limit in both the major and minor directions. The intensity distribution of the focusing spots agree with the theoretical prediction. Hence, the fabricated lenses have focusing characteristics in agreement with the designed ones. Compared to the conventional microlens, these elliptical Fresnel lenses have a size smaller by one order of magnitude and their focusing efficiency is as high as 72 percent. Hence, it is expected that these lenses are new elements for micro‐optics.