Aspheric Microlens Array Research Articles

Design of a Novel Microlens Array and Imaging System for Light Fields.

Light field cameras are unsuitable for further acquisition of high-quality images due to their small depth of field, insufficient spatial resolution, and poor imaging quality. To address these issues, we proposed a novel four-focal-square microlens and light field system. A square aspheric microlens array with four orthogonal focal lengths was designed, in which the aperture of a single lens was 100 μm. The square arrangement improves pixel utilization, the four focal lengths increase the depth of field, and the aspheric improves image quality. The simulations demonstrate pixel utilization rates exceeding 90%, depth-of-field ranges 6.57 times that of a single focal length, and image quality is significantly improved. We have provided a potential solution for improving the depth of field and image quality of the light field imaging system.

Scheme of flash LiDAR employing glass aspherical microlens array with large field of illumination for autonomous vehicles

This study demonstrates a new scheme of flash LiDAR using a glass aspherical microlens array (MLA) to achieve a large field of illumination (FOI) for autonomous vehicles. A wider FOI of up to 100° was obtained. In contrast to a spherical MLA, the FOI is 38.9° which indicates that the proposed aspherical MLA is 2.6 times wider than the spherical MLA. The wider FOI achieved for the glass MLA is due to a novel laser drilling technique that produces conical micro-holes with a high aspect ratio (depth: diameter = 1.8:1), forming elliptical-like aspherical microlenses through wet etching. An FOI estimation model to provide theoretical basis for designing aspherical MLA with wider FOI is presented, which is in good agreement with experimental results. Furthermore, the optical efficiency of 90% for the FOI was calculated. In this study, we have proposed a unique laser drilling technique to produce glass aspherical MLA with wider FOI and higher optical efficiency for flash LiDAR use in autonomous vehicle applications.

Irradiance-tailoring integral-illumination polarization homogenizer based on anamorphic aspheric microlens arrays.

In the realm of active polarization detection systems, the imperative for polarization illumination systems with high-uniformity and predefined-shape irradiance distribution is evident. This paper introduces a novel anamorphic aspheric (AAS) microlens array (MLA) integral polarization homogenizer, incorporating projection MLA (PMLA), condenser MLA (CMLA), polarization film (PF), and a sub-image array (SIA) mask based on Kohler illumination principles. Firstly, the optimal design of an AAS-based projection sub-lens is proposed to facilitate the creation of a short-working-distance, predefined-geometric and sharp polarization irradiance tailoring. The SIA mask is constituted by plenty of predistortion SIs, which are generated through a combination of chief ray tracing and the radial basis function (RBF) image warping method. In addition, accompanied with tolerance sensitivity analysis, detailed analysis of stray light generation factors and proposed elimination or suppression methods further ensure the engineering reliability and stability of the proposed system. A compact integral-illumination polarization homogenizer design example is realized with an overall irradiance uniformity exceeding 90% and a volume of 25 mm × 25 mm × 18.25 mm. Different predefined-geometrical-profile and high-uniformity polarization irradiance distribution can be achieved by substituting different SIA masks and PFs, without replacing MLA optical elements, which greatly saves cost. Substantial simulations and experiments corroborate the efficacy of our polarization homogenizer.

Design of elemental-image-predistortion and splicing seamless integral near-eye display based on aspherical microlens arrays

The near-eye display (NED) has long been a challenging and intriguing research focus in the fields of augmented reality (AR) and virtual reality (VR). In this paper, we propose a design method of elemental-image (EI) predistortion and splicing seamless integral NED based on microlens arrays (MLAs), which combines rigorous optical design with image preprocessing. The construction of MLA is based on sub-channel optimization and splicing, enabling the realization of both VR-NED and AR-NED. The calibration of each sub-channel in NED is conducted using a perfect lens that possesses an effective focal length equivalent to that of the human eye. To achieve a high-precision seamless integral NED, the precise location and acquisition of initial EIs are accomplished through chief ray tracing and image clipping. The predistortion EI array (EIA) is further obtained by combining chief ray tracing and radial basis function (RBF) image warping techniques. A seamless MLA-based integral AR-NED with a full field of view (FOV) of 60° is realized and verified. A prototype of a seamless MLA-based integral VR-NED is further implemented by integrating multiple discrete single-focal predistortion EIAs. The effectiveness of the proposed method has been validated through illumination and luminance simulations as well as experiments.

Fabrication of curved aspheric compound eye microlens array with high surface quality by precision glass molding

The curved compound eye microlens array (CEMLA) glass optical element is widely used in optoelectronic detectors because it affords a high image quality and a large field of view. However, fabrication of high-precision glass optical elements with a small cell size and many arrays on hard and brittle glass materials is challenging. Here, we report a fabrication method to address molding challenges for high quality CEMLA glass optical elements at molding temperatures up to 500 °C. First, an Ni-P with good turning performance and low adhesion to glass is chosen as the plating material, the CuNi is selected as the mold substrate for their close CTEs. To prevent the diffusion of P to the surface of optical elements and to improve the hardness and wear resistance of mold, a Ti-Ta-C coating is deposited on the Ni-P mold. By single-point diamond turning on the Ni-P surface using a fast tool servo, an aspherical CEMLA mold with a form accuracy PV less than 0.4 μm and surface roughness Sa less than 4 nm was machined. Finally, based on the creep testing of microdeformation compression, an accurate viscoelastic constitutive model of glass was established. Optimization of the molding process and accurate prediction of molding profile deviation were simulated. The aspheric CEMLA glass optical element with a form accuracy PV less than 0.5 μm and surface roughness Sa less than 4.5 nm was obtained by precision glass molding (PGM).

Fabrication of high aspect-ratio aspheric microlens array based on local spiral diamond milling

Ultraprecision mold machining combined with precision glass molding is the most promising technology for the mass production of glass microlens arrays (MLAs). However, fabricating an MLA with a high aspect-ratio (AR) (ratio of height to aperture), submicron scale profile accuracy, nanoscale surface finish, and good consistency remains a considerable challenge. A local spiral diamond milling (LSDM) method is proposed to generate a high AR MLA on a nickel-phosphorous (Ni-P) plated mold. A spiral toolpath generation algorithm that considers the tool edge radius and performs profile error (PV) compensation is developed. Each lenslet at any local position can be precisely machined by controlling the servo motions of the three translational axes. Then, the entire MLA can be generated by shifting the toolpath to go through the centers of all lenslets according to the MLA layout. The tool parameters are optimized to avoid local and global tool interference during machining. The generated toolpath provides steady slide axis movements to avoid dramatic changes in cutting speed and acceleration. An MLA with an AR of 0.166 is fabricated. The results show that the fabricated mold and the molded lenses have a profile accuracy (PV) of less than 1 μm and a surface roughness (Ra) of less than 14 nm. The cross-sectional profiles indicate that the fabricated MLA has high machining consistency.

自己整列微粒子の形状転写によるマイクロレンズアレイの製作とモールド変形を利用した非球面化の検討

Aspheric micro-lens arrays have been attracting great attention to improve light distribution control. This study proposes a simple and efficient method for fabricating close-packed aspheric micro-lens array. First, silica particles of ϕ10 μm were self-assembled over ϕ10 mm area using sedimentation method. Then the convex profile of the self-assembled particles was replicated to a spherical micro-lens array mold. The mold material was silicone elastomer or polydimethylsiloxane (PDMS). Then, the mold profile was replicated into ultraviolet (UV) curing resin. By applying uniaxial stretching to the mold during the secondary replication, aspheric micro-lens array was fabricated. Deformation of the PDMS mold under uniaxial stretching was analyzed by using finite element analysis (FEA) for rough estimation. Experimental results showed that intended lens profile was obtained though some defects were observed due to the nonuniformity of the particle diameters. Optical characteristic of elliptic micro-lens array was made clear.

Ultra-thin multifocal integral LED-projector based on aspherical microlens arrays.

Multifocal imaging has been a challenging and rewarding research focus in the field of imaging optics. In this paper, an ultra-thin multifocal integral LED-projector based on aspherical microlens array (MLA) is presented. A two-layer aspherical sub-lens with NA = 0.3 is proposed as a sub-channel projector and the optimization design ensures high optical integration precision and improves optical efficiency. To avoid the tailoring loss of the projected images between multi-plane projections, the central-projection constraints between size and projection distance for the multifocal projection are defined. The depth of focus (DOF) analysis for MLA and sub-lens is also introduced to proof the sufficiency of realizing multifocal projection. Combined with the radial basis function image warping method, multifocal sub-image arrays were acquired, and three types of multifocal integral projection were realized, breaking through the traditional limitations of the single-focal DOF. A prototype with thickness of less than 4 mm is developed. Substantial simulations and experiments are conducted to verify the effectiveness of the method and the design.

High-speed, large-area and high-precision fabrication of aspheric micro-lens array based on 12-bit direct laser writing lithography

Aspheric micro-lens array (AMLA), featured with low dispersion and diffraction-limited imaging quality, plays an important role in advanced optical imaging. Ideally, the fabrication of commercially applicable AMLAs should feature low cost, high precision, large area and high speed. However, these criteria have been achieved only partially with conventional fabrication process. Herein, we demonstrate the fabrication and characterization of AMLAs based on 12-bit direct laser writing lithography, which exhibits a high fabrication speed, large area, perfect lens shape control via a three-dimensional optical proximity correction and average surface roughness lower than 6 nm. In particular, the AMLAs can be flexibly designed with customized filling factor and arbitrary off-axis operation for each single micro-lens, and the proposed pattern transfer approach with polydimethylsiloxane (PDMS) suggests a low-cost way for mass manufacturing. An auto-stereoscopic-display flexible thin film with excellent display effect has been prepared by using above technology, which exhibits a new way to provide flexible auto-stereoscopic-display at low cost. In brief, the demonstrated fabrication of AMLAs based on direct laser writing lithography reduce the complexity of AMLA fabrication while significantly increasing their performance, suggesting a new route for high-quality three-dimentional optical manufacturing towards simplified fabrication process, high precision and large scale.

Preparation method of silicon-based aspheric cylindrical microlens array

Preparation method of silicon-based aspheric cylindrical microlens array

Fabrication and Optical Characterization of Polymeric Aspherical Microlens Array Using Hot Embossing Technology

Hot embossing has been widely used in fabricating microlens arrays because of its low cost, high efficiency, and high quality. The process parameters such as molding temperature, molding pressure, and holding temperature affect the microlens array’s replication quality. This work selected the stainless steel S136H tool steel as the mold material to process an aspheric microlens array structure through ultra-precision milling. Polymethyl methacrylate (PMMA) microlens arrays with different surface replication were prepared by controlling the molding temperature, molding pressure, and holding temperature. By analyzing the surface quality, contour replication, and optical imaging of hot-embossed samples, the optimal molding temperature of PMMA for optimal replication of aspheric lens arrays was determined as 130 °C. Besides, the internal elastic recovery of PMMA affected the dimensional accuracy and optical performance of the lens. The results showed that, at the molding pressure of 400 N and the holding temperature of 60 °C, the surface defects were eliminated, and the aspheric lens array had perfect replication with a profile deviation of only 4 μm. The aspheric microlens array with good quality was eventually achieved by these optimal process parameters, which provides a foundation for producing aspheric microlens arrays in a low-cost and high-efficiency way.

Improvement in light out-coupling efficiency of OLED by using high fill factor parabola curve microlens arrays

Aspheric microlens arrays (MLAs) are introduced on glass substrate to improve the light out-coupling efficiency (LOCE) of organic light-emitting diode (OLED). Geometric ray tracing was employed to simulate the improved effects and optimize the structure parameters of the MLAs pattern. The simulation results show that when the parameters of MLAs as follows, the curved profile is parabola, the base shape is hexagon, the fill factor is 1, the aspect ratio is 0.6, and the material refractive index should be match to that of the substrate of the OLED, which can improve LOCE maximum.Poly-dimethyl-siloxane (PDMS) parabola curve MLAs films ware fabricated by maskless lithography technique and template replication. The experimental results show that the MLAs can improve the LOCE and the trends of the experimental results agree with the simulation results. A maximum increase of 49 % in the coupling efficiency was achieved.

Off-spindle-axis spiral grinding of aspheric microlens array mold inserts.

A novel approach named off-spindle-axis (OSA) spiral grinding for fabricating aspheric microlens array (AMLA) mold inserts for precision glass molding (PGM) is presented. In OSA spiral grinding, three translational motions of the grinding wheel are synchronized with the rotation of the workpiece to form a local spiral wheel path for individual lens-lets. With this approach, the form accuracy of lens-lets can be compensated within sub-micrometer by means of the on-machine measurement. The determination of wheel path and form error compensation via on-machine measurement are systematically studied. A tungsten carbide mold insert with four convex aspheric lens-lets is fabricated to evaluate the grinding performance. PGM experiments are performed to produce glass AMLA using the ground insert. The experimental results indicate that both the ground and molded AMLA with homogeneous quality are achieved. The form accuracy and surface roughness of both the mold insert and the molded AMLA were less than 0.3 µm in PV and 10 nm in Sa, respectively.

Fabrication of high fill-factor aspheric microlens array by dose-modulated lithography and low temperature thermal reflow

A cost-effective fabrication method for high quality and high fill-factor aspheric microlens arrays (MLAs) is developed. In this method, the complex shape of aspheric microlens is pre-modeled via dose modulation in a digital micromirror device (DMD) based maskless projection lithography system. Digital masks for several bottom layers are replaced from circle to hexagon for the purpose of enhancing the fill-factor of MLAs, then a low temperature thermal reflow process is conducted, after which the average surface roughness of microlens is improved to ~ 0.427 nm while the pre-modeled profile keeps unchanged. Experimental results show that the fabricated aspheric MLAs have almost 100% fill-factor, high shape accuracy and high surface quality. The presented method may provide a promising approach for rapidly fabricating high quality and high fill-factor aspheric microlens in a simple and low-cost way.

An optimization method of dynamic patterns based on aspheric microlens array

An optimization method is proposed to improve the image quality of dynamic patterns in this paper. In order to obtain parallel emergent beam of high quality, aspheric microlens array is used in the optical system. Besides, an analytical model of the substrate and a single aspheric lens of MLA is presented based on the principle of ray tracing which is used in optical design for virtually tracing rays through an optical system. Moreover, how do the parameters influence the image quality is analyzed and the relationship between the conic constant and the divergence angle of emergent beam is derived. Preliminary optimization results with a second order aspheric lens are compared with the simulation results obtained by ZEMAX. The ray tracing method in this article helps improve the image quality of dynamic patterns and has the potential to be used in designing aspheric surface because it offers a way to improve the parallel rays and optimize the parameters of the optical system.

Fabrication method of quartz aspheric microlens array for turning mask

In order to solve the two difficult problems of the poor processing controllability and the low surface accuracy of quartz aspheric microlens array processing, a fabrication method of quartz aspheric microlens array for turning mask is proposed. This method mainly uses single point diamond turning technology and reactive ion etching technology, studies the turning and etching properties of the mask material, and optimizes the mask material by experiment. Finally, the fabrication of an aspherical glass microlens array with an area of 5 mm×5 mm was carried out. The experimental results are compared with the expected parameters. The analysis shows that the error root mean square of the quartz glass component is 1.155 nm, and the surface accuracy error is 0.47%.

Fabrication of high quality aspheric microlens array by dose-modulated lithography and surface thermal reflow

A novel fabrication method for high quality aspheric microlens array (MLA) was developed by combining the dose-modulated DMD-based lithography and surface thermal reflow process. In this method, the complex shape of aspheric microlens is pre-modeled via dose modulation in a digital micromirror device (DMD) based maskless projection lithography. And the dose modulation mainly depends on the distribution of exposure dose of photoresist. Then the pre-shaped aspheric microlens is polished by a following non-contact thermal reflow (NCTR) process. Different from the normal process, the reflow process here is investigated to improve the surface quality while keeping the pre-modeled shape unchanged, and thus will avoid the difficulties in generating the aspheric surface during reflow. Fabrication of a designed aspheric MLA with this method was demonstrated in experiments. Results showed that the obtained aspheric MLA was good in both shape accuracy and surface quality. The presented method may be a promising approach in rapidly fabricating high quality aspheric microlens with complex surface.

Fabrication of high fill-factor aspheric microlens array by digital maskless lithography

A simple and effective method for fabricating high fill-factor aspheric microlens array using digital maskless lithography is reported. The method use the contour planes of the designed profile as binary image, which replace physical mask by DMD display. On the lithography system, the dose accumulated over multiple exposures and the exposure dose reconstructed in wafer, then the designed profile developed. Aspheric microlens arrays with hexagonal base and square base were produced by the method, and they were gapless at each microlens periphery, so the microlens array with fill-factor of 100%.

Improved slicing strategy for digital micromirror device‐based three‐dimensional lithography with a single scan

To improve the profile quality of three-dimensional microstructures, an improved method was proposed in real-time maskless lithography based on digital micromirror device. First, to calculate the spatial distribution of exposure dose for the target profile in design, the relations of the exposure dose and the development depth were investigated. Moreover, a suitable development time was obtained. Second, an improved slicing strategy based on equal-arc algorithm was proposed to reduce the profile error. Different from traditional equal-expose-dose method, this method generated slicing layers with equal arc-length instead of equal-exposure dose. Such an approach has advantages in obtaining a series of suitable section patterns adapting to the curvature variation of the design profile. Finally, an aspheric microlens array was fabricated with this method and the equal-exposure-dose method. Results showed that the resulting microstructure profile by their method was more consistent with the design profile with the same slicing numbers. Moreover, results also showed that the method used can obtain as good profile quality as that of the equal-exposure-dose method with only a half of slicing numbers. This indicated that the proposed method is feasible for achieving good profile quality with slicing efficiency improved remarkably.

Adjusting light distribution for generating microlens arrays with a controllable profile and fill factor

This paper presents a simple but effective process to fabricate high quality aspheric microlens arrays (MLA) and bifocal microlens arrays (BMLA) by predicting and adjusting the UV light distribution. The profile-controllable BMLA has two different focal lengths simultaneous along the optical axis, measured at f1 = 26 ± 1 μm (the top lens) and f2 = 35 ± 1 μm (the base lens), respectively. The fill-factor and curvature of the obtained profiles have been well optimized by controlling the air gap and exposure time, for instance, the fill factor is topped up to 99.7% from an initial 25.8% (the corresponding aperture diameter is enlarged to 15 μm from an initial 10 μm) and the micro lens height ranges from 2.66 μm to 9.03 μm. Optical tests indicate that MLAs and BMLAs have a uniform shape and a good consistency of the focal length. By adjusting the light distribution, this proposed strategy possesses the ability and flexibility to fabricate profile-controllable MLAs and BMLAs.