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.     

Shack Hartmann wave-front measurement with a large F-number plastic microlens array

A new plastic microlens array, consisting of 900 lenslets, has been developed for the Shack Hartmann wave-front sensor.The individual lens is 300 μm × 300μm and has a focal length of 10 mm, which provides the same focal size, 60 μm in diameter, with a constant peak intensity. One can improve thewave-front measurement accuracy by reducing the spot centroiding error by averaging a few frame memories of an image processor. A deformable mirror for testing the wave-front sensor gives anappropriate defocus and astigmatism, and the laser wave front is measured with a Shack Hartmann wave-front sensor. The measurement accuracy and reproducibility of our wave-front sensor are better than λ/20 and λ/50 (λ = 632.8 nm),respectively, in rms.     

Variable Focusing Microlens Chip for Potential Sensing Applications

A variable focusing microlens chip, which has the capability of adjusting its focal length over a wide range without any mechanical driving parts, is reported in this paper. The packaged microlens chip consists of a flexible polymer lens, a fluidic chamber, an integrated sensor, and an actuator. A thermal actuator is introduced into this variable focusing microlens to obtain relatively large actuation force and displacement. The hemispheric convex polymer lens provides an initial focal point without being actuated. The focal length change is controlled by varying the voltage applied to the thermal actuator. A 1.9-mm-diameter polymer lens is made to test the performance of the device. The focal length of this chip varies from 14.658 to 2.782 mm, which corresponds to the change of numerical aperture from 0.078 to 0.412. Based on the working mechanism and constructing method of the single lens chip, a variable focusing microlenses array has been fabricated for future testing and application. Potential sensing applications for single lens and array include cell detection and immobilization, optical sensors, lab-on-a-chip, ophthalmic lens systems, microphotonics, high throughput scanning, and confocal imaging system     

Optical grating diffraction method: from strain microscope to strain gauge

The grating diffraction method for direct strain measurement is reviewed. Two systems which use this method are presented. The first system is a compact strain microscope. A Leitz optical transmitting microscope with white light source is reconstructed by developing a loading and recording system. Gratings with median density of between 40 and 200 lines/mm are used. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed with high image quality on the CCD sensor plane. Software is developed to precisely, quickly and automatically determine the diffraction spot centroids. The second system is a new strain sensor based on a high-frequency grating and two Position Sensor Detectors (PSDs). The grating, attached to the surface of the specimen, is illuminated by a focused laser beam, generally with a frequency of 1,200 lines/mm. The centroids of diffracted beam spots from the grating are automatically determined using two PSD sensors connected to a personal computer. The shift of diffracted beam spots due to specimen deformation is detected. Strain sensitivity of one micro-strain can be obtained, as can a 0.4 mm spatial resolution for strain measurement. The system can be used for both static and dynamic tests.     

大型结构应变场光纤分布监测系统

研究了一种用于大型工程结构应变、变形状态监测的基于光时域反射技术的分布式光纤应变传感系统。设计了一种新型的光纤微弯传感器结构,用于对沿传感光纤分布各测量点的应变信息进行提取;在此新结构的基础上,了全光纤型应变传感器串行阵列,以对构件应变测量和安全监测为主要目标,建立了起了分布光纤应变实时监测系统,进行了系统实验了得到了良好的结构。     

White-light-modified Talbot array illuminator with a variable density of light spots

A flexible array illuminator, comprising only two conventional optical elements, with a variable density of bright white-light spots is presented. The key to our method is to obtain with a single diffractive lens an achromatic version of different fractional Talbot images, produced by free-space propagation, of the amplitude distribution at the back focal plane of a periodic refractive microlens array under a broadband point-source illumination. Some experimental results of our optical procedure are also shown.     

Carbon nanotubes based methanol sensor for fuel cells application

An electrochemical sensor is built using vertically grown multi-walled carbon nanotubes (MWNTs) micro-array to detect methanol concentration in water. This study is done for the potential use of the array as methanol sensor for portable units of direct methanol fuel cells (DMFCs). Platinum (Pt) nanoparticles electro-deposited CNTs (Pt/CNTs) electrode shows high sensitivity in the measurement of methanol concentration in water with cyclic voltammetry (CV) measurement at room temperature. Further investigation has also been undertaken to measure the concentration by changing the amount of the mixture of methanol and formic acid in water. We compared the performance of our micro array sensor built with Pt/CNTs electrodes versus that of Pt wire electrode using CV measurement. We found that our Pt/CNTs array sensor shows high sensitivity and detects methanol concentrations in the range of 0.04 M to 0.10 M. In addition, we found that co-use of formic acid as electrolyte enables us to measure up to 1.0 M methanol concentration.     

微透镜阵列对Hartmann—Shack波前探测器探测精度的影响

微透镜阵列是Ｈａｒｔｍａｎｎ－Ｓｈａｃｋ波前探测器的核心元件之一。本文从理论上分析了微透镜阵列的性能对Ｈａｒｔｍａｎｎ－Ｓｈａｃｋ波前探测器的波前探测精度的影响，并根据理论分析进行了一系列实验，得到了有益的结果。     

微透镜列阵成像光刻调焦方法

本文分析了离焦量对微透镜列阵成像光刻图形质量的影响,给出了系统离焦量的容差.同时提出了一种结构简单、可应用于微透镜列阵成像光刻系统调焦的新方法.并将基于该调焦方法的实验装置应用于微透镜列阵成像光刻系统,进行了光刻实验.实验表明,利用该方法时微透镜列阵成像光刻系统调焦,可得到接近微透镜列阵极限像质的光刻图形.     

微透镜阵列与针孔阵列合成的微光学器件的研制

给出用于并行共焦显微镜中的微光学阵列器件的制作工艺。特别设计了微透镜阵列与针孔阵列对准粘接的机械和观察系统。实验证明系统是简易可行的,并给出了实验结果。     

Misalignment effects of the Shack-Hartmann sensor

The Shack-Hartmann sensor uses a microlens array and a CCD camera for wave-front measurements. To obtain wave-front measurements with high accuracy, an accurate relative alignment of both is essential. The different states of misalignment of the Shack-Hartmann sensor are divided into groups and are treated theoretically and experimentally. Their effect on the accuracy of wave-front measurements is evaluated. In addition, a practical method for proper alignment of the Shack-Hartmann sensor is proposed.     

Effective formation method for an aspherical microlens array based on an aperiodic moving mask during exposure

An aperiodic mask design method for fabricating a microlens array with an aspherical profile is proposed. The nonlinear relationship between exposure doses and lens profile is considered, and the select criteria of quantization interval and fabrication range of the method are given. The mask function of a quadrangle microlens array with a hyperboloid profile used in the infrared was constructed by using this method. The microlens array can be effectively fabricated during a one time exposure process using the mask. Reactive ion etching was carried out to transfer the structure into the substrate of germanium. The measurement results indicate that the roughness is less than 10 nm (pv), and the profile error is less than 40 nm (rms).     

Novel methods to improve the measurement accuracy and the dynamic range of Shack–Hartmann wavefront sensor

As a wavefront sensor, the Shack–Hartmann wavefront sensor plays an important role in the wavefront measurement of human eyes. However, the low measurement accuracy and the small dynamic range of Shack–Hartmann wavefront sensor limit its application. In this paper, we present a matched-filter algorithm to improve the measurement accuracy by more than an order of magnitude. Moreover, we also introduce a new algorithm to extend the dynamic range of Shack–Hartmann wavefront sensor. With this method, the recorded spots of Shack–Hartmann wavefront sensor are not constrained to stay in the corresponding pixel area of the microlens. The result shows that the dynamic range can be extended from 57.1 to 160% for the first 24 items of Zernike wavefronts, respectively. The improvement by our methods makes the Shack–Hartmann more suitable for the measurement of highly aberrated eyes.     

Implementation of field lens arrays in beam-deflecting microlens array telescopes

Laterally displaceable microlens array telescopes allow for variable and fast beam deflection. The generation of spurious light usually leads to a reduction of transfer efficiency with increasing displacement. We present the introduction of an array of field lenses on the back side of a recollimating microlens array that results in a reduced deflection angle dependency of transfer efficiency. A paraxial matrix formalism is used to prove the theoretical elimination of spurious light by use of a field lens array. The fabrication of well-aligned double-sided lens arrays by UV replication is discussed. Measurements of transfer efficiency with and without the use of field lens arrays are compared with the results of numerical wave-optic simulations.     

Implementation of multilens micro-optical systems with large numerical aperture by stacking of microlenses

In the field of micro-optics there is a demand for objectives with large numerical aperture (NA). One example is optical storage in which a NA greater than 0.5 is required. For planar microlenses the NA is determined by means of the maximal index difference and the degree of exchange and reaches typical values of 0.13-0.2. Thus stacking is needed to build high NA objectives from planar microlenses. An additional benefit of stacking lenses is the possibility to correct for different types of aberrations. We realized two stacked systems: an array of micro-objectives with a NA of 0.45 from three microlens arrays and a confocal sensor head from four microlens arrays and one pinhole array mask.     

Sensitivity enhancement of grating interferometer based two-dimensional sensor arrays using two-wavelength readout

Diffraction gratings integrated with microelectromechanical systems (MEMS) sensors offer displacement measurements with subnanometer sensitivity. However, the sensitivity of the interferometric readout may drop significantly based on the gap between the grating and the reference surface. A two-wavelength (2-λ) readout method was previously tested using a single MEMS sensor for illustrating increased displacement measurement capability. This work demonstrates sensitivity enhancement on a sensor array with large scale parallelization (~20,000 sensors). The statistical representation, which is developed to model sensitivity enhancement within a grating based sensor array, is supported by experimental results using a thermal sensor array. In the experiments, two lasers at different wavelengths (633 and 650 nm) illuminate the thermal sensor array from the backside, time-sequentially. The diffracted first order light from the array is imaged onto a single CCD camera. The target scene is reconstructed by observing the change in the first diffracted order diffraction intensity for both wavelengths. Merging of the data from two measurements with two lasers was performed by taking the larger of the two CCD measurements with respect to the reference image for each sensor. ~30% increase in the average sensitivity was demonstrated for a 160×120 pixel IR sensor array. Proposed architecture is also applicable to a variety of sensing applications, such as parallel biosensing and atomic force microscopy, for improved displacement measurements and enhanced sensitivity.     

电涡流传感器阵列测试技术

针对采用扁平柔性电涡流传感器阵列实现大面积金属曲面部件位置实时监测,对电涡流传感器的阵列测试技术进行了研究.采用一种基于时分多路的电涡流阵列测试的方法,通过对传感器探头和测试电路的合理设计,使系统电路得到简化,减小阵列单元之间的串扰,提高传感器系统的测试性能,实现了电涡流传感器阵列的快速、高精度测量.     

衍射微透镜列阵质量评价方法研究

根据实际工程需要 ,对用于萨克 -哈特曼波前传感器的二元衍射微透镜列阵衍射效率的评价方法进行了研究。导出了制作误差与衍射效率的关系式 ,研究了由测得的制作误差评估衍射效率的方法 ,并建立了一套测量系统。     

Nanophotonic three-dimensional microscope

Three-dimensional (3D) optical microscopy based on integral imaging techniques is limited mainly by diffraction effects and the pitch of the microlens array used to sample the specimen. We integrate nanotechnology to the integral imaging technique and demonstrate a nanophotonic 3D microscope, where a nanophotonic lens array is used to finely sample the specimen. The resolution limitation due to diffraction is reduced by capturing images before the diffraction effects predominate and hence overcomes the bottleneck of achieving high resolution in an integral imaging 3D microscope.     

Optical Properties of an UV-Cured Liquid-Crystal Microlens Array

We propose and demonstrate, for the first time to our knowledge, a microlens array of the gradient-index type using a novel liquid-crystalline material that possesses the property ofphotopolymerization by UV irradiation. Optical and electrical properties of the UV-curable liquid crystal are investigated to optimize UV curing conditions. The microlens array is prepared by use of an UV-curable liquid crystal, and gradient-index profiles are monitored during photopolymerization. As a result, relatively good focusing and imaging properties can be obtained even after photopolymerization. This technique affords us a very controllable way to fabricate the micro-optic components.