Aperture Microlens Research Articles

Large-depth-of-field imaging system using a square aperture overlapped micro-lens array

The application of a light-field camera based on an overlapped square aperture micro-lens array (MLA) is reported. The location and direction of the incident light rays are recorded and reconstructed with a large depth of field by this imaging system. The optimized square MLA for light-field imaging, which increases the fill factor and utilization efficiency of the image sensor, is proposed. The array is fabricated by ultra-precision machining and micro-injection molding, which is economical and ensures that the array remains in high accuracy and consistency. Finally, a light-field imaging system is built with lens groups, an MLA, and an adjustable square stop. A comparative study of the field in-depth and super-resolution image is conducted. The experimental results show that the system significantly increases the range of the depth of the field. The reconstructed image is also enhanced with higher resolution and clarity.

Design and fabrication of ultrathin lighting responsive security device based on moiré imaging phenomenon

Security authentication of high- and low-value products is a pressing issue. Security technologies should be upgraded before they are counterfeited. In this study, an ultrathin lighting responsive moiré imaging device, which is different from traditional security device is developed for security authentication. Some interesting lighting responsive moiré imaging phenomena including light-activated three-dimensional floating imaging and light-controlled image transformation have been developed as covert security features for advanced authentication. These security features can be verified instantly using a point light source such as a white light-emitting diode configured in a smart phone. A compact defocus optical structure consisting of a micro lens-mirror array (MLMA) and a micro-pattern array is designed to fabricate the lighting responsive moiré imaging device. The structure makes the device ultrathin, which enables the easy integration of the device. The MLMA also overcame the difficulty in the fabrication of high numerical aperture micro-lens array required in traditional moiré imaging system. Flexible devices with 78-μm thickness were fabricated by photolithography, imprinting, and gravure-like techniques. Such thin device allows its potential integration with banknotes, currency, or other valuable documents/products.

基于方形孔径微透镜阵列二维叠栅条纹的微小角度测量

基于方形孔径微透镜阵列二维叠栅条纹的微小角度测量

Analysis of the depth of field in hexagonal array integral imaging systems based on modulation transfer function and Strehl ratio.

Integral imaging is a technique for displaying three-dimensional images using microlens arrays. In this paper, a method for calculating root mean squared wavefront error and modulation transfer function (MTF) of a defocused integral imaging capture system with hexagonal aperture microlens arrays is introduced. Also, maximum allowable depth of field with Century MTF analyzing and Strehl criterion are obtained.

Fabrication of high numerical aperture micro-lens array based on drop-on-demand generating of water-based molds

Abstract This letter presented a simple fabrication method of micro-lens arrays with high numerical aperture based on a new drop-on-demand generating of water-based mold. The micro-droplet array which acted as the water-based mold was generated on the polypropylene substrate using a drop-on-demand generator, and then a replication process was followed for fabricating the micro-lens arrays with high numerical aperture. A spherical-like micro-lens array with the numerical aperture of 0.62 was fabricated. The surface of the formed micro-lens was demonstrated super smooth (Ra~0.5 nm, Rq~0.7 nm, Rmax~13.2 nm, 2×2 μm2 scanned area) by the atomic force microscope, which can reduce the optical noise. The qualities of imaging and light gathering characteristics of the MLAs were evaluated by the optical microscope.

Precision alignment of integrated optics in hybrid microsystems.

We achieve submicrometer precision in the integration of micro-optics with surface electrode ion traps. The high-precision alignment is accomplished using off-axis linear Fresnel zone plates (FZPs). Four pairs of FZPs are fabricated on the optics chip that contains the high numerical aperture microlens, a diffractive optical element (DOE). The four pairs of FZPs enable alignment in six translational and rotational degrees of freedom. Four corresponding alignment rulers are etched in the top metal layer of the ion trap, enabling quantification of misalignment. The integration of optics for efficient light delivery and the collection of fluorescence from trapped ions are key to achieving scalability in quantum information processing. An accurate and precise approach to the integration of DOEs advances the scalability of surface electrode ion traps and many other hybrid microsystems.

Research on Fabricating of Square Aperture Spherical Micro-Lens Array through the Laser Lithography

Due to the practical problems nowadays that the duty cycle is small and the utility rate of solar energy is low of photoelectric sensor light sensitive array, and having analyzed the advantages on solar energy utilization of square aperture spherical micro-lens than traditional circular aperture micro-lens array, we conduct a research on the fabricating of square aperture spherical micro-lens and testing technology. With the help of light spread theory, we analyses the relationship between light amplitude distributing rules, exposure length and exposure energy, establish a mathematical model of light distributing rules and use computer software to emulate the model. The results indicate solar energy distributes regularly in adhesive film. If the energy is fixed, exposure length will gradually become saturated following time. At the same time, we fabricate infrared quartz micro-lens array with array 256×256, single element lens 40×40μm, spherical radius 60μm, single element spacing 1μm.The experiment data are the same as emulation results.

Design of the microlens arrays coupling with imaging fiber bundle

To ameliorate the disadvantages of imaging system coupled with imaging fiber bundle, a method by adding square aperture microlens arrays at both entrance and exit ends of the imaging fiber bundle is proposed to increase the system’s coupling efficiency. The expressions for solving the parameters of both ends’ microlens units are deducted particularly. The microlens arrays used for an infrared imaging fiber bundle with the single fiber diameter of 100 μm and core diameter of 70 μm are designed by this method. The simulation results show that compared with the system without microlens arrays, the fill factor of the imaging fiber bundle coupled microlens arrays system is increased from 44.4% to more than 90%, and the coupling efficiency is doubled too. So the design method is correct, and the introduction of microlens arrays into imaging fiber bundle coupled system is feasible and superior.

A novel boundary-confined method for high numerical aperture microlens array fabrication

In this paper, we present a technique to improve microlens array (MLA) uniformity after the thermal reflow process. It achieved high uniformity and high-numerical aperture (NA) simultaneously without sacrificing the fill factor. In order to improve the fill factor, a residual photoresist (PR) between the photoresist cylinders is used to make photoresist flow outward in standard thermal reflow processes. The PR cylinders, however, merge together easily due to an inexact reflow time and temperature distribution. This results in low uniformity and small lens height or low NA. We propose a boundary-confined method to pattern thin PR holes to prevent PR microlenses from merging together even after a long reflow time. Thick PR cylinders are patterned inside thin PR holes serving as boundaries. PR microlenses are formed after reflowing the thick photoresist cylinders. Both the uniformity and the height of a microlens can be well controlled. Besides, the fill factor is high due to the high resolution at the thin photoresist layer in photolithography. Our results show that the microlens is approximately a hemispherical profile. The gap between microlenses with a 48 µm diameter in hexagonal arrangement is 2 µm and the height of the microlens is 22 µm.

Microlens array fabrication by enhanced thermal reflow process: Towards efficient collection of fluorescence light from microarrays

This paper describes two enhanced resist reflow methods for the fabrication of microlens arrays and demonstrates their use for integrated biomolecular fluorescence detection on printed microarrays. A PDMS (polydimethylsiloxane) microlens array was fabricated by a double soft lithography approach using a photoresist microlens array as master mold. Additionally, by using both a careful control of the surface wettability and thermal treatments, we demonstrate the possibility to extend the resist reflow process in order to tune the diameters of microlens array over a large range by using a unique photomask pattern. We introduce an enhanced reflow on hydrophobic surfaces obtained by fluorosilane treatment and identify a threshold shrinkage temperature (T shrinkage) of 140 °C, above which the diameter of microlenses can be then reduced down to 40% compared with the initial pattern on the photomask. Furthermore, on hydrophilic substrates, achieved by an accurate incomplete development of the photoresist, we demonstrate a nearly perfect linear dependency (1.4 μm/°C) of microlens diameter spreading up to 70% the initial diameter inside a temperature reflow window of 110–140 °C. For both approaches, above a freezing temperature (T freezing) of 170 °C, the microlens profile characteristics are temperature independent. By using high numerical aperture microlens array, we provide a proof of concept for the integration and enhanced light collection of the fluorescent signals collected form a microarray of fluorescent spots thus showing the potential of the concept for biophotonic integration.

Fabrication of microlens array with graduated sags using UV proximity printing method

A graduated microlens array is presented in this paper. The proposed device has the same aperture microlens with a gradually increasing sag in the substrate. The design produces gradual decrease in the focal length and intensity when the light passes through the graduated microlens array. This paper presents a new graduated microlens array fabrication method that uses a variable printing gap in the UV lithography process. This method can precisely control the geometric profile of each microlens array without using the thermal reflow process. The angles between the mask and photoresist were placed at 5°, 8°, 10°, 15°, and 20° using a fixture designed in this study. The mask patterns were ellipses with an isosceles triangle arrangement to compensate for the partial geometry.

Soft-lithography-enabled fabrication of large numerical aperture refractive microlens array in hybrid SiO2–TiO2 sol-gel glass

This paper describes a low-cost soft-lithography method for building large-numerical-aperture microlenses in hybrid sol-gel glass. The fabrication processes comprise three steps, namely fabrication of large numerical aperture microlens array in photoresist as a master, replication of the master in poly-dimethylsiloxane (PDMS) as elastomeric molds and reproduction of the PDMS replica onto the hybrid sol-gel glass as an end structure. Comparing with the direct UV fabrication in the photosensitive sol-gel glass, the proposed method provides a unique advantage in terms of fabrication freedom in shape and thickness of micro-optical elements in the sol-gel glass. This eliminates the difficulties in direct photolithography in sol-gel glass for arbitrary shapes and great thickness due to ever increasing demands.