Optical Microlens Arrays Research Articles

Least-squares phase-shifting algorithm of coherence scanning interferometry with windowed B-spline fitting, resampled and subdivided phase points for 3D topography metrology

3D topography metrology of optical micro-structured surfaces is critical for characterizing surface texture and form errors. The coherence scanning interferometry (CSI) faces many challenges for improving the accuracy of 3D topography metrology. In this paper, the least-squares (L-S) phase-shifting algorithm of coherence scanning interferometry with windowed B-spline fitting, resampled and subdivided phase points is proposed, which provides high accuracy of 3D surface topography. The proposed algorithm avoids the introduction of complex judgment criterion of fringe orders, and tedious phase unwrapping. Comparison with generalized phase-shifting algorithm, the wrong textures do not appear on the reconstructed 3D surfaces and 2D profiles for optical grating and Micro-lens array (MLA) surfaces using the proposed algorithm. Experimental results show that a VLSI step surface has high repeated measurement accuracy with a standard deviation of 1.6 nm, the periodic error of the proposed algorithm for PMMA MLA sub-aperture units is about 20 nm, and the mean value of PMMA MLA four random sub-aperture units are 6.66 nm (Sq), 77.63 nm (Sp), 36.38 nm (Sv), 114.18 nm (Sz) and 4.90 nm (Sa), respectively.

Three-dimensional gradient index microlens arrays for light-field and holographic imaging and displays.

The geometric, intensity, and chromatic distortions that are a result of the limitations of the material and processes used to fabricate micro-optical lens arrays (MLAs) degrade the performance of light-field systems. To address these limitations, inkjet print additive manufacturing is used to fabricate planar gradient index (GRIN) lenslet arrays, in which volumetric refractive index profiles are used to embed optical functions that would otherwise require multiple homogeneous index MLA surfaces. By tailoring the optical ink feedstock refractive index spectra, independent control over dispersion is achieved, and achromatic performance is made possible. Digital manufacturing is shown to be beneficial for optimizing individual micro-optical channels in arrays wherein the shape, size, aspect ratio, focal length, and optical axis orientation of the lenslets vary as a function of the position within the optical field. Print fabrication also allows opaque inter-lens baffling and aperture stops that reduce inter-channel cross talk, improve resolution, and enhance contrast. These benefits are demonstrated in a light-field display testbed.

Diffractive micro-lens array (DLA) for uniform and selective picosecond laser treatment.

Picosecond Nd:YAG lasers using diffractive optical elements (DOE) and micro-lens arrays (MLA) have widely been used in dermatology for the treatment of pigmented lesions and skin rejuvenation. This study designed and developed a new optical element of diffractive micro-lens array (DLA) by combing the features of DOE and MLA in order to achieve uniform and selective laser treatment. Both optical simulation and beam profile measurement demonstrated that DLA created a square macro-beam consisting of multiple micro-beams in a uniform distribution. Histological analysis confirmed that the DLA-assisted laser treatment generated micro-injuries at various skin depths from the epidermal layer to the deep dermal layer (up to 1200 µm) by adjusting the focal depths while DOE showed shallow penetration depths and MLA created non-uniform micro-injury zones. The DLA-assisted picosecond Nd:YAG laser irradiation can provide a potential benefit for pigment removal and skin rejuvenation via uniform and selective laser treatment.

Imaging Quality Prediction and Measurement for Optical Microlens Array

Imaging Quality Prediction and Measurement for Optical Microlens Array

Strain-tunable optical microlens arrays with deformable wrinkles for spatially coordinated image projection on a security substrate

As a new concept in materials design, a variety of strategies have been developed to fabricate optical microlens arrays (MLAs) that enable the miniaturization of optical systems on the micro/nanoscale to improve their characteristic performance with unique optical functionality. In this paper, we introduce a cost-effective and facile fabrication process on a large scale up to ~15 inches via sequential lithographic methods to produce thin and deformable hexagonally arranged MLAs consisting of polydimethylsiloxane (PDMS). Simple employment of oxygen plasma treatment on the prestrained MLAs effectively harnessed the spontaneous formation of highly uniform nanowrinkled structures all over the surface of the elastomeric microlenses. With strain-controlled tunability, unexpected optical diffraction patterns were characterized by the interference combination effect of the microlens and deformable nanowrinkles. Consequently, the hierarchically structured MLAs presented here have the potential to produce desirable spatial arrangements, which may provide easily accessible opportunities to realize microlens-based technology by tunable focal lengths for more advanced micro-optical devices and imaging projection elements on unconventional security substrates.

Three-dimensional see-through augmented-reality display system using a holographic micromirror array.

It is difficult to find the micromirror array with desired specifications for augmented-reality displays, and the custom fabricating methods are complicated and unstable. We propose a novel, to our knowledge, three-dimensional see-through augmented-reality display system using the holographic micromirror array. Unlike the conventional holographic waveguide-type augmented-reality displays, the proposed system utilizes the holographic micromirror array as an in-coupler, without any additional elements. The holographic micromirror array is fabricated through the simple, effective, and stable method of applying the total internal reflection-based hologram recording using a dual-prism. The optical mirror and microlens array are set as references, and the specifications can be customized. It reconstructs a three-dimensional image from a displayed elemental image set without using any additional device, and the user can observe a three-dimensional virtual image while viewing the real-world objects. Thus, the principal advantages of the existing holographic waveguide-type augmented-reality system are retained. An optical experiment confirmed that the proposed system displays three-dimensional images exploiting the augmented-reality system simply and effectively.

Manufacturing of 3D Microlens Array Mold on Bulk Metallic Glass by Self-Aligned Multi-Ball Hot Embossing

Bulk metallic glasses (amorphous alloys) exhibiting both high hardness and extraordinary thermoplastic forming ability, are perfect mold material for 3D optical microlens array. However, conventional 3D micro fabrication for metallic glass requires Si templates which are extremely costly and eco-unfriendly to machine and demold. In this study, we report a green and low-cost self-aligned multi-ball hot embossing process to create 3D micro dimple array on metallic glass by replicating spherical crowns of self-aligned precision balls. A 3D metallic glass micro dimple array with aperture of 867.2 μm and surface roughness of 8.2 nm was fabricated in 30 s. Different from conventional hot embossing that material full-filled into mold, the new process enables micro dimple fabrication with different specifications by freely controlling the down displacement of balls. Therefore, the relationship between down displacement and depth of micro dimple was revealed through exploring the material flow behavior by a verified FE model. In addition, to test the feasibility of the fabricated metallic glass mold, a PMMA compound eye structure was embossed and its focusing and imaging performance were subsequently evaluated. This research demonstrates a new manufacturing process for rapid fabrication of 3D microlens arrays metallic glass mold in a neat and economic way.

Flexible fabrication of optical glass micro-lens array by using contactless hot embossing process

Abstract This paper reported the fabrication of optical glass micro-lens array (MLA) by using contactless micro-embossing technology. Firstly, the micro-hole array in the tungsten carbide (WC) substrate was made by the micro-electrical discharge machining (micro-EDM). Then, to improve the oxidation resistance, anti-sticking properties and chemical inertness of mold surface, Cr-W-N coating was deposited on the mold insert by plasma-enhanced sputtering technique. Finally, controllable MLA were fabricated with the coated EDM-mold by using the precision glass molding machine from Shenzhen University (SZU’s PGMM30). The surface roughness, three-dimensional morphology and optical performance of glass MLA were measured by ContourGT-X 3D optical profiler, ultraviolet spectrophotometer, and optical imaging system, respectively. The experimental results indicated that the contactless hot embossing process has the capability of fabricating high-quality and multi-scale glass MLA. This work provides an effective method for the fabrication of optical glass MLA.

Challenges in the Fabrication of Microstructured Polymer Optics

The fabrication of microstructured polymer optics enables a multitude of new options in the design of technical optics. However, challenges arise along the varying process chains of mold insert fabrication, integration into molding tools, replication by means of injection compression molding and metrology. In order to study the effects, diffractive optical elements (DOE) and microlens arrays (MLA) are fabricated using two different process chains. DOEs are fabricated using a laser direct writing (LDW) based mold insert fabrication. The MLA mold insert is produced using ultra-precision milling (UP-milling). Both optical parts are replicated using injection compression molding. The occurring effects are discussed and the results show, that with complete process control high quality microstructured polymer optical parts can be produced and characterized.

Ultra-fast adaptive optical micro-lens arrays based on stressed liquid crystals

We characterized the optical properties of a photoresist-based micro-lens using a stressed liquid crystal matrix for high-speed variation of the focal distance. We also determined the operational parameters for the adaptive optical spherical and cylindrical micro-lens arrays. The resulting optical elements feature fast response times, around 1 ms, with a focal length range of 2600-970 μm.

Design and analysis of medium wave infrared miniature static Fourier transform spectrometer

This paper presents a spatial modulation Fourier transform micro-spectrometer based on micro-optical elements. The infrared microstructure diffractive optical elements, multi-step micro-mirrors and microlens array are introduced to realize the miniaturization of the instrument. In addition, the structure and basic principle of Fourier transform infrared micro-spectrometer are introduced. The design theory of micro-collimation system is analyzed based on the negative dispersion, the abberation correction and the arbitrary phase modulation characteristics of diffractive optical element. Combined with the characteristics of micro-static interference system, the micro-focusing coupled optical system is analyzed and designed. Based on the wave aberration theory and the Sellmeier dispersion formula, the influence of residual aberration on spectral recovery and the diffraction efficiency of diffraction surface in single-chip hybrid diffractive-refractive collimating lens are studied. The effects of diffraction of multi-stage micro-mirrors and the aperture diffraction of microlens array on spectral recovery are studied by using the scalar diffraction theory. Furthermore, the influence of axial assembly error of relay system on the whole system performance is studied. The results show that the diffraction efficiency of the diffraction surface, the diffraction of the multistage micro-mirror and the microlens array have no effect on the recovery spectrum when the working band of the system is 3.7-4.8 μm. Finally, in order to verify the accuracy of the system design results, an optical simulation software is used to simulate the infrared micro-Fourier transform spectrum. The accuracy of the system model is verified by the simulation that the reconstructed spectrum is in agreement with the ideal spectral curve and the actual spectral recovery error is 2.89%. The medium-wave infrared micro-static Fourier transform spectrometer has no movable parts and adopts micro-optics element to replace the traditional infrared lens. Therefore, it has the advantages of not only good stability, but also small size and light weight so that it is helpful in on-line monitoring applications and provides a new design idea about the micro-Fourier transform spectrometer.

Rapid localized heating of graphene coating on a silicon mold by induction for precision molding of polymer optics.

In compression molding of polymer optical components with micro/nanoscale surface features, rapid heating of the mold surface is critical for the implementation of this technology for large-scale applications. In this Letter, a novel method of a localized rapid heating process is reported. This process is based on induction heating of a thin conductive coating deposited on a silicon mold. Since the graphene coating is very thin (∼45 nm), a high heating rate of 10∼20°C/s can be achieved by employing a 1200 W 30 kHz electrical power unit. Under this condition, the graphene-coated surface and the polymer substrate can be heated above the polymer's glass transition temperature within 30 s and subsequently cooled down to room temperature within several tens of seconds after molding, resulting in an overall thermal cycle of about 3 min or shorter. The feasibility of this process was validated by fabrication of optical gratings, micropillar matrices, and microlens arrays on polymethylmethacrylate (PMMA) substrates with very high precision. The uniformity and surface geometries of the replicated optical elements are evaluated using an optical profilometer, a diffraction test setup, and a Shack-Hartmann wavefront sensor built with a molded PMMA microlens array. Compared with the conventional bulk heating molding process, this novel rapid localized induction heating process could improve replication efficiency with better geometrical fidelity.

光学微透镜阵列模压成形研究进展与展望

光学微透镜阵列模压成形研究进展与展望

Replication of optical microlens array using photoresist coated molds.

A cost reduced method of producing injection molding tools is reported and demonstrated for the fabrication of optical microlens arrays. A standard computer-numerical-control (CNC) milling machine was used to make a rough mold in steel. Surface treatment of the steel mold by spray coating with photoresist is used to smooth the mold surface providing good optical quality. The tool and process are demonstrated for the fabrication of an ø50 mm beam homogenizer for a color mixing LED light engine. The acceptance angle of the microlens array is optimized, in order to maximize the optical efficiency from the light engine. Polymer injection molded microlens arrays were produced from both the rough and coated molds and have been characterized for lenslet parameters, surface quality, light scattering, and acceptance angle. The surface roughness (Ra) is improved approximately by a factor of two after the coating process and the light scattering is reduced so that the molded microlens array can be used for the color mixing application. The measured accepted angle of the microlens array is 40° which is in agreement with simulations.

Fill-factor improvement of Si CMOS single-photon avalanche diode detector arrays by integration of diffractive microlens arrays.

Single-photon avalanche diode (SPAD) detector arrays generally suffer from having a low fill-factor, in which the photo-sensitive area of each pixel is small compared to the overall area of the pixel. This paper describes the integration of different configurations of high efficiency diffractive optical microlens arrays onto a 32 × 32 SPAD array, fabricated using a 0.35 µm CMOS technology process. The characterization of SPAD arrays with integrated microlens arrays is reported over the spectral range of 500-900 nm, and a range of f-numbers from f/2 to f/22. We report an average concentration factor of 15 measured for the entire SPAD array with integrated microlens array. The integrated SPAD and microlens array demonstrated a very high uniformity in overall efficiency.

Fabrication of micro-lens array using surface acoustic wave

A technique to form an optical micro-lens array using surface acoustic wave (SAW) was investigated. The lens has no mechanical moving parts, such as gearing systems, and is composed of a simple structure. A viscoelastic transparent silicone gel film with the thickness of 20 μm was formed on a 128°-rotated Y-cut X-propagation LiNbO3 (LN) substrate between two interdigitated transducers (IDTs). The IDT electrodes consisted of 30 finger pairs with the aperture length of 9.1 mm and the periodic pitch of 200 μm were fabricated on the LN substrate. The IDTs was excited with a continuous sine wave with the amplitude of 0 to 15 Vpp at the resonance frequency (20 MHz) to generate the SAW. The leaky SAW propagated into the gel, and the acoustic radiation force acted to the surface of the gel film so that the surface profile of the gel could change and a micro-lens array could be fabricated on the gel. The lens height could be controlled by varying the voltage applied to the IDTs.

Noncontact microembossing technology for fabricating thermoplastic optical polymer microlens array sheets.

Thermoplastic optical polymers have replaced traditional optical glass for many applications, due to their superior optical performance, mechanical characteristics, low cost, and efficient production process. This paper investigates noncontact microembossing technology used for producing microlens arrays made out of PMMA (polymethyl methacrylate), PS (polyStyrene), and PC (polycarbonate) from a quartz mold, with microhole arrays. An array of planoconvex microlenses are formed because of surface tension caused by applying pressure to the edge of a hole at a certain glass transition temperature. We studied the principle of noncontact microembossing techniques using finite element analysis, in addition to the thermal and mechanical properties of the three polymers. Then, the independently developed hot-embossing equipment was used to fabricate microlens arrays on PMMA, PS, and PC sheets. This is a promising technique for fabricating diverse thermoplastic optical polymer microlens array sheets, with a simple technological process and low production costs.

Polymer/LC Phase Separation Assisted in Fabrication of Self-Assembled Optical Microlens Arrays

Here we report on the development of the low-temperature self-assembly microlens arrays via liquid crystal/photopolymer blends phase separation method. In this method, liquid crystal/photopolymer blends phase separate well to form a pioneer structure for the subsequently photopolymerized mircolens arrays. The morphology of the microlens arrays has been observed by SEM, AFM and scanning white light interferometer. The obtained focal length and f-number are 95 μm and 7.6, respectively. The results obtained from our experiments show a comparable light-gathering capability and can be applied in optical systems.

Geometrical characterization of micro-optical cylindrical lens arrays using angular resolved diffraction

The versatility of micro-optical lens arrays (MLA) is giving rise to many new applications often to be included in mass-produced parts and systems. The need for fast and robust methods of characterization and quality control, addressing an industrial manufacturing environment, is evident. It has been shown (Iversen et al 2009 J. Opt. A: Pure Appl. Opt. 11 054014 (6pp), Büttner and Zeitner 2002 Appl. Opt. 41 6841–8) that the average radius of curvature of an MLA structure can be extracted by observing the far-field diffraction pattern intensity distribution obtained from illumination of the MLA structure with a coherent light source. This method is based on a priori knowledge of the grating period. We here present a method that fully characterizes the geometrical properties, i.e. the grating period and the average radius of curvature of an MLA structure in a single measurement cycle. By scanning the angle of incidence of the coherent illumination and simultaneously observing the diffracted intensity, information about the grating period and the radius of curvature can be extracted. The method is implemented with emphasis on further development for compact, high-speed dedicated systems. We present experimental data extracted from two different MLA structures using the proposed method. The results are compared with atomic force microscopy measurements of the MLA geometry.

Light Extraction in InGaN-based Light-emitting Diodes Integratedwith UV-curable Optical Adhesive Microlens Arrays of High RefractiveIndex and Packaged with Molding Epoxy

We fabricated InGaN-based 460-nm light-emitting diodes (LEDs) integrated with microlens arrays made of UV-curable optical adhesive to achieve enhanced light output [Son et al., J. Korean Phys. Soc. 51, 377 (2007)]. This enhanced output due to the integrated microlens obtained from a chiplevel LED may not be present in commercial LEDs after packaging because the refractive index of the molding epoxy used in the packaging process is similar to that of the UV-curable optical adhesive. In this paper, we describe the integration of microlens arrays with InGaN LEDs by using an UV-curable optical adhesive with a relatively high refractive index and the packaging of those LEDs in molding epoxy with a normal refractive index. We observed a greater light output in the packaged LEDs with the microlenses than in the conventional packaged LEDs, thereby confirming that the enhancement effect of the integrated microlens was preserved in the final LED structure after packaging. We performed raytracing simulations on our LED structures to confirm that the numerical results support the experimental findings.