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.     

Electrowetting-driven variable-focus microlens on flexible surfaces

We demonstrate a flexible, electrowetting-driven, variable-focus liquid microlens. The microlens is fabricated using a soft polymer polydimethylsiloxane. The lens can be smoothly wrapped onto a curved surface. A low-temperature fabrication process was developed to reduce the stress on and to avoid any damage to the polymer. The focal length of the microlens varies between -15.0?mm to +28.0?mm, depending on the applied voltage. The resolving power of the microlens is 25.39 line pairs per mm using a 1951 United States Air Force resolution chart. The typical response time of the lens is around 50?ms.     

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.     

All-optical controlling of the focal intensity of a liquid crystal polymer microlens array

The current work demonstrates a liquid crystalline polymer microlens array (LCP MLA) with an all-optically tunable and multistable focal intensity through photochemical phase transition. The operational mechanism of the optical tuning is associated with the photoisomerization effect. The proposed LCP MLA device has a focusing unit based on a birefringence LCP and a tuning unit with a light responsive material to control the polarization state of the incident probe beam. The optically variable refractive indices of LCP enable a positive or negative MLA that can control the polarization of incident light to be realized.     

新颖的微流控光学变焦透镜阵列集成器件

提出一种微流控光学变焦透镜阵列芯片,以解决变焦透镜阵列的电控调谐和集成化制造方法问题.该器件的基本形式为"上盖片 内芯 下盖片"的三明治夹心结构,采用导电材料制作内芯,使得器件的制作简化,易于集成.透镜材料由折射率不同的两种不混合液体组成,利用电润湿效应控制两种液体的界面曲率,从而实现微透镜阵列焦距的电控调节.     

Combined Twyman-Green and Mach-Zehnder interferometer for microlens testing

A new interferometer design for microlens testing is presented. The phase-shifting system combines the advantages of a Twyman-Green and a Mach-Zehnder interferometer and permits full characterization of the aberrations of microlenses as well as radius of curvature and focal length measurements. The Twyman-Green system is applied to surface testing in reflection (single reflection), whereas the Mach-Zehnder system is used for lens testing in transmission (single pass). Both measurements are performed without removal of the test part, allowing for combination of the results without confusion of the actual lens and without an azimuthal orientation error. The interferometer setup is explained, the test procedure is described, and experimental results are given.     

Polymer-based flexible microlens arrays with hermaphroditic focusing properties

Polymer microlens arrays with hermaphroditic focusing behaviors are demonstrated. Each microlens in an arrays exhibits either converging or diverging focus, depending on the polarization direction of the incident light. A polymer film with patterned microlens arrays is flexible, lightweight, and ultrathin (approximately 50 microm). Details of the lens structure, device fabrication, and lens performance are described.     

Fabrication of polymer-based reflowed microlenses on optical fibre with control of focal length using differential coating technique

Thermal reflow of polymer to generate spherical profile has been used to fabricate microlenses in this paper. A simple model based on the volume conservation (before and after reflow) and geometrical consideration of lens profile, shows that the focal length of the microlens produced by reflow technique is a function of the initial geometry of microcylinders, i.e. diameter and thickness. This relationship of focal length with diameter and thickness is used as a basis to control focal length. A simple spin coating technique on dual surface is used to achieve differential thickness, to control the focal length of microlenses produced on the same substrate. A biomedical application of such combination of microlenses is endoscopy where the lenses of varying diameter and equal focal length are needed on top of optical fibre bundles to provide independent function of illumination and imaging. This paper incorporates the differential thickness technique to show a micro fabrication process to produce the polymer reflowed microlenses, with a control of focal length based on thickness. The design also helps to integrate these microlenses on top an optical fibre with accurate alignment.     

Design and fabrication of a microlens on the sidewall of an optical fiber with a metallized 45 degrees end face

We propose a method for designing a self-aligned microlens. We have improved its fabrication by employing metallization on a 45 degrees angled surface of the optical fiber. We designed the focal length of the microlens to be 14.0 microm, considering the dimensions of a scanning near-field optical microscopy (SNOM) probe, and we calculated possible dimensions of diameter and height by the ray-tracing method. The modeling of lens formation was also carried out with two assumptions: no volume change and no movement of peripheral parts of the photoresist (PR) on the substrate during reflow. To fabricate a microlens of diameter 16.0 microm and height 5.0 microm we exposed a coated PR to UV light guided into the optical fiber, followed by optimized reflow of 150 degrees C for 2 min. For this microlens the focal length and the beam waist were 14.0 and 1.4 microm, respectively. This lens can be used for compact optical data storage.     

微透镜列阵与红外探测器列阵集成芯片的研究

在分析微透镜列阵光聚能原理的基础上,针对背照式256290铂硅红外焦平面探测器列阵 的结构参数,设计了衍射微透镜列阵,使入射光通过硅基底聚焦至探测器的各个光敏面上, 提高光能利用率从而增强探测能力。实验获得了微透镜列阵与红外焦平面集成芯片,并在热成像中取得了良好的结果。     

Adaptive beam shaping by controlled thermal lensing in optical elements

We describe an adaptive optical system for use as a tunable focusing element. The system provides adaptive beam shaping via controlled thermal lensing in the optical elements. The system is agile, remotely controllable, touch free, and vacuum compatible; it offers a wide dynamic range, aberration-free focal length tuning, and can provide both positive and negative lensing effects. Focusing is obtained through dynamic heating of an optical element by an external pump beam. The system is especially suitable for use in interferometric gravitational wave interferometers employing high laser power, allowing for in situ control of the laser modal properties and compensation for thermal lensing of the primary laser. Using CO(2) laser heating of fused-silica substrates, we demonstrate a focal length variable from infinity to 4.0 m, with a slope of 0.082 diopter/W of absorbed heat. For on-axis operation, no higher-order modes are introduced by the adaptive optical element. Theoretical modeling of the induced optical path change and predicted thermal lens agrees well with measurement.     

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.     

Liquid-crystal lens with a focal length that is variable in a wide range

A liquid-crystal (LC) lens driven by two voltages is reported. The lens has a focal length that is electrically tunable. The range of the variable focusing power is very wide, covering approximately 0.8-10.7 D. In the entire focal range the LC lens maintains high optical quality. The LC lens can be driven in a simple way to prevent the occurrence of a disclination line. The use of the LC lens in image formation is demonstrated.     

Electrically switchable holographic liquid crystal/polymer Fresnel lens using a Michelson interferometer

A holographic technique for fabricating an electrically switchable liquid crystal/polymer composite Fresnel lens is reported. A Michelson interferometer is used to produce the required Fresnel pattern, by placing a convex lens into one path of the interferometer. Simplicity of the method and the possibility of fabricating different focal length lenses in a single arrangement are advantages of the method. The performance of the fabricated lens was demonstrated and its electro-optical properties were investigated for its primary focal length.     

Drop‐on‐Demand Inkjet Printing of Thermally Tunable Liquid Crystal Microlenses

In this letter, the authors demonstrate Drop‐on‐Demand printing of variable focus, polarization‐independent, liquid crystal (LC) microlenses. By carefully selecting the surface treatment applied to a glass substrate, the authors are able to deposit droplets with a well‐defined curvature and contact angle, which result in micron‐sized lenses with focal lengths on the order of 300–900 µm. Observations with an optical polarizing microscope confirm the homeotopic alignment of the LC director in the droplets, which is in accordance with the polarization independent focal length. Results show that microlenses of different focal lengths can be fabricated by depositing successive droplets onto the same location on the substrate, which can then be used to build up programmable and arbitrary arrays of microlenses of various lens sizes and focal lengths. Finally, the authors utilize the thermal dependency of the order parameter of the LC to demonstrate facile tuning of the focal length. This technique has the potential to offer a low‐cost solution to the production of variable focus, arbitrary, microlens arrays.

     

Tunable microfluidic microlenses

A novel type of liquid microlens, bounded by a microfabricated, distensible membrane and activated by a microfluidic liquid-handling system, is presented. By use of an elastomer membrane fabricated by spin coating onto a dry-etched silicon substrate, the liquid-filled cavity acts as a lens whereby applied pressure changes the membrane distension and thus the focal length. Both plano-convex and plano-concave lenses, individual elements as well as arrays, were fabricated and tested. The lens surface roughness was seen to be approximately 9 nm rms, and the focal length could be tuned from 1 to 18 mm. This lens represents a robust, self-contained tunable optical structure suitable for use in, for example, a medical environment.     

Subwavelength focus of light by a planar microlens

We numerically show that using a planar gradient-index (GRIN) microlens it is possible to focus coherent light of wavelength λ into a narrow line of width 0.026 λ and length 0.182 λ. The efficiency of focusing amounts to 22%. Replacement of the GRIN lens with a photonic-crystal analogue fabricated in silicon is shown practically not to impair the lens performance. In fact, the replacement is shown to result in decreasing side-lobes in the focal spot, while increasing the efficiency up to 30%.     

Analysis of a cylindrical microlens array with long focal depth by a rigorous boundary-element method and scalar approximations

We investigated the focal characteristics of open-regional cylindrical microlens arrays with long focal depth by using a rigorous boundary-element method (BEM) and three scalar methods, i.e., a Kirchhoff and two Rayleigh-Sommerfeld diffraction integral forms. Numerical analysis clearly shows that the model cylindrical microlens arrays with different f-numbers can generate focusing beams with both long focal depth and high transverse resolution. The performance of the cylindrical microlens arrays, such as extended focal depth, relative extended focal depth, diffraction efficiency, and focal spot size, is appraised and analyzed. From a comparison of the results obtained by the rigorous BEM and by scalar approximations, we found that the results are quite similar when the f-number equals f/1.6; however, they are quite different for f/0.8. We conclude that the BEM should be adopted to analyze the performance of a microlens array system whose f-number is less than f/1.0.     

Analysis of a micro-optical light modulator

A light modulator with microlens arrays in a confocal arrangement and with various filters in the common focal plane of the arrays, which are translated with the help of piezoelectric actuators, is proposed. The theoretical analysis deals with the influence of the lens arrays on the performance of the modulator. The system is investigated for spatially incoherent beams. It is shown that the configuration is suited for efficient modulation of radiation emitted by multimode fibers. A choice of the proper focal length of the microlens arrays and lens pitch d results in a good transmission efficiency (above 90%) combined with a large number of possible switching states.     

Zonal wavefront sensor with reduced number of rows in the detector array

In this paper, we describe a zonal wavefront sensor in which the photodetector array can have a smaller number of rows. The test wavefront is incident on a two-dimensional array of diffraction gratings followed by a single focusing lens. The periodicity and the orientation of the grating rulings of each grating can be chosen such that the +1 order beam from the gratings forms an array of focal spots in the detector plane. We show that by using a square array of zones, it is possible to generate an array of +1 order focal spots having a smaller number of rows, thus reducing the height of the required detector array. The phase profile of the test wavefront can be estimated by measuring the displacements of the +1 order focal spots for the test wavefront relative to the +1 order focal spots for a plane reference wavefront. The narrower width of the photodetector array can offer several advantages, such as a faster frame rate of the wavefront sensor, a reduced amount of cross talk between the nearby detector zones, and a decrease in the maximum thermal noise. We also present experimental results of a proof-of-concept experimental arrangement using the proposed wavefront sensing scheme.