Micro-optical Arrays Research Articles

Fabrication of Large-Area Micro-Hexagonal Cube Corner Retroreflectors on Three-Linear-Axis Ultraprecision Lathes

Hexagonal cube corner retroreflectors (HCCRs) are the micro-optics arrays with the highest reflectivity. However, these are composed of prismatic micro-cavities with sharp edges, and conventional diamond cutting is considered unmachinable. Besides, 3-linear-axis ultraprecision lathes were considered unfeasible to fabricate HCCRs due to the lack of a rotation axis. Therefore, a new machining method is proposed as a viable option to manufacture HCCRs on the 3-linear-axis ultraprecision lathes in this paper. For the mass production of HCCRs, the dedicated diamond tool is designed and optimized. The toolpaths are proposed and optimized to further increase tool life and machining efficiency. The Diamond Shifting Cutting (DSC) method is analyzed in-depth both theoretically and experimentally. By using the optimized methods, the large-area HCCRs with a structure size of 300 µm covering an area of 10 × 12 mm2 are successfully machined on 3-linear-axis ultraprecision lathes. The experimental results show that the whole array is highly uniform, and the surface roughness Sa of three cube corner facets is all less than 10 nm. More importantly, the machining time is reduced to 19 h, which is far less than the previous processing methods (95 h). This work will significantly reduce the production threshold and costs, which is important to promote the industrial application of HCCRs.

Two-photon grayscale lithography for free-form micro-optical arrays.

Compared to standard rotationally symmetric macroscopic optical components, free-form micro-optical arrays (FMOAs), sometimes termed microstructured optical surfaces, offer greater design freedom and a smaller footprint. Hence, they are used in optical devices to deliver new functionalities, enhanced device performance, and/or a greater degree of miniaturization. But their more complex surface shape is a challenge for traditional manufacturing technologies, and this has triggered a substantial effort by research institutes and industry to develop alternative fabrication solutions. Two-photon polymerization (2PP) is a promising additive manufacturing technology to manufacture 3D optical (micro)structures. The manufacturing times involved are, however, often impractically long, especially for the excellent surface quality required for optical applications. Recently, Nanoscribe GmbH has reduced manufacturing times substantially with the introduction of so-called two-photon grayscale lithography (2GL). However, its acceleration potential and consequent impact on surface quality have, to the best of our knowledge, yet to be reported. A direct comparison between 2PP and 2GL indicates that, for the investigated FMOA, 2GL is around five times faster than 2PP and also delivers better surface quality. This study therefore confirms the potential of 2GL to manufacture complexly shaped FMOAs.

Self-tuned ultrasonic elliptical vibration cutting for high-efficient machining of micro-optics arrays on brittle materials

Ultrasonic elliptical vibration cutting is a very promising technique for the machining of brittle materials. However, its machining performance is currently limited by the ductile machining model and the machining strategy with a constant feed rate, leading to low machining efficiency. To overcome this defect, this paper presents a novel self-tuned ultrasonic elliptical vibration cutting (SUEVC) technique to achieve high-efficient ductile-regime machining of the micro-optics array on brittle materials. The proposed SUEVC includes a ductile-regime machining model and a tool path generation method. In SUEVC, the feed rate adaptively changes with respect to the local shape variation of the desired surface along the feeding direction to ensure both crack-free surface and high machining efficiency. Finally, two 1 × 3 spherical micro-optics arrays were successfully fabricated on single-crystal MgF2 by SUEVC and the traditional machining strategy respectively. Results demonstrated that the SUEVC could enhance the machining efficiency by 30% relative to the traditional machining strategy, while maintaining similar surface roughness and a crack-free surface.

Wavelength-Tuning Common-Path Digital Holographic Microscopy for Quantitative Phase Imaging of Functional Micro-Optics Components

This study proposes a novel wavelength-tuning common-path digital holographic microscopy technique for quantitative phase imaging of functional micro-optics components. The proposed technique is immune to vibration and can reduce system error. In the proposed configuration, a parallel glass plate was inserted into the light path to create two identical test beams, which passed through a specially designed window filter. In this process, one beam serves as the object beam, while the other is diffracted to produce an ideal spherical wave front (the reference beam). A wavelength tunable laser was used as the light source to generate phase-shifting digital holograms. Structural information for the functional micro-optics components was then extracted using a classical four-step phase-shift algorithm. The viability of the proposed technique was assessed by measuring a micro-optics array.

Development of self-tuned diamond milling system for fabricating infrared micro-optics arrays with enhanced surface uniformity and machining efficiency.

Infrared micro-optics arrays (MOAs) featuring large numbers of micro-freeform lenslet are required increasingly in advanced infrared optical systems. Ultra-precision diamond cutting technologies have been widely used to fabricate MOAs with high form accuracy. However, the existing technologies can easily cause the non-uniformly fractured surface of infrared MOAs, due to the inherent low fracture toughness and high anisotropy of infrared materials as well as the time-varying chip thickness induced by ever-changing height and slope of the desired MOAs. In this study, a novel self-tuned diamond milling (STDM) system is proposed to achieve the ductile cutting of infrared MOAs with enhanced the surface uniformity and machining efficiency, and the corresponding toolpath planning algorithm is developed. In STDM system, a dual-axial fast servo motion platform is integrated into a raster milling system to self-adaptively match the maximum chip thickness for each tool rotational cycle with the critical depth of cut of the infrared material according to the local surface topography, thereby obtaining crack-free lenslet with high surface uniformity. Practically, micro-aspheric MOAs free from fractures are successfully machined on single-crystal silicon, a typical infrared material, to validate the proposed cutting concept. Compared with the conventional diamond milling, the proposed STDM is demonstrated to be able to avoid the non-uniform fractures without needing to reduce feed rate, and a smaller surface roughness of 4 nm and nearly double machining efficiency are achieved.

Flexible fabrication of micro-optics arrays with high-aspect-ratio by an offset-tool-servo diamond machining system.

Micro-optics arrays (MOAs) with high aspect ratio (AR) have unique advantages in realizing the minimization of optical systems by reducing the focal distance. Fast or slow tool servo (F/STS) is widely regarded as an outperforming technique for the fabrication of MOAs featuring high form accuracy. However, in the machining of MOAs with high AR, the non-smooth cutting trajectory of F/STS inevitably leads to intensive tool vibrations and the interference between the tool flank face and the finished surface, thereby deteriorating surface roughness. In this study, a novel offset-tool-servo (OTS) diamond machining technology and the corresponding toolpath generation algorithm are proposed to achieve the flexible fabrication of micro-freeform lens arrays with high AR. In OTS, with the assistance of four-axis servo motions, a spiral toolpath is generated for each single lenslet, which effectively avoids the tool interference induced by the steep descending movement of the tool in F/STS. Besides, the proposed machining strategy well ensures the smoothness of the generated toolpath for each lenslet, thereby effectively avoiding the destruction of the surface quality induced by the tool vibrations. In practice, this method is validated by fabricating different MOAs with aspheric and freeform structures. Compared with F/STS, the OTS method is demonstrated to be able to achieve two times larger AR values, and smoother and more uniform surface quality are simultaneously achieved.

基于微光学阵列差分真时延网络的光学多波束合成系统

基于微光学阵列差分真时延网络的光学多波束合成系统

Virtual spindle based tool servo diamond turning of discontinuously structured microoptics arrays

Abstract We propose a virtual spindle based tool servo (VSTS) diamond turning method for the generation of discontinuously structured microoptics arrays. Uniqueness of the VSTS is that a virtual spindle axis (VSA) is constructed at any specified position by combining the multiple translational and rotational servo motions of the machine tool. Thereby, we can array the microstructure cell with a shape that can be generated in fast- or slow tool servo by controlling the VSA to sequentially pass through the centre of each cell. Feasibility of this technique is demonstrated by fabricating discontinuous microoptics arrays on both planar and freeform surfaces.

Construction of an instant structured illumination microscope

A challenge in biological imaging is to capture high-resolution images at fast frame rates in live cells. The “instant structured illumination microscope” (iSIM) is a system designed for this purpose. Similarly to standard structured illumination microscopy (SIM), an iSIM provides a twofold improvement over widefield microscopy, in x, y and z, but also allows much faster image acquisition, with real-time display of super-resolution images.The assembly of an iSIM is reasonably complex, involving the combination and alignment of many optical components, including three micro-optics arrays (two lenslet arrays and an array of pinholes, all with a pitch of 222μm) and a double-sided scanning mirror. In addition, a number of electronic components must be correctly controlled. Construction of the system is therefore not trivial, but is highly desirable, particularly for live-cell imaging.We report, and provide instructions for, the construction of an iSIM, including minor modifications to a previous design in both hardware and software. The final instrument allows us to rapidly acquire fluorescence images at rates faster than 100fps, with approximately twofold improvement in resolution in both x–y and z; sub-diffractive biological features have an apparent size (full width at half maximum) of 145nm (lateral) and 320nm (axial), using a 1.49 NA objective and 488nm excitation.

Miniaturization of a micro-optics array for highly sensitive and parallel detection on an injection moulded lab-on-a-chip.

A miniaturised array of supercritical angle fluorescence (SAF) micro-optics embedded in a microfluidic chamber was fabricated by injection moulding. The fabricated chip could enhance the fluorescence signal around 46 times compared to a conventional microscope. Collection of the fluorescence signal from the SAF array is almost independent of the numerical aperture, and the limit of detection was improved 36-fold using a simple and inexpensive optical detection system.

Design of micro-optics array to realize two dimensional perfect shuffle transform

Two-dimensional (2D) perfect shuffle transform (PST) plays an important role in optical communication and optical information processing due to very high space-bandwidth product (SBWP) and spatial degrees of freedom. The planar-array of micro-blazed grating has been proposed to realize 2D PST based on its interesting properties such as high diffraction efficiency, small size, high degree of integration, and easy to fabricate. 2D PST could be realized in free space through controling the etching direction and the period of micro-blazed grating, which results in signal beams can reach the desired spatial position. Unlike the conventional method depending on the operation of dividing, magnification, interlacing, and superimposing, this approach, which mainly relies on the diffractive properties of signal beams and the distribution of light intensity, presents a single diffractive element to perform 2D PST. Thus, this approach has the advantages of high energy efficiency, high feasibility, compact in structure, and easy to integrate. The theoretical analyses and the experimental results show that it should be helpful in optical interconnection network, especially, in PS omega network.

Topographic optical profilometry by absorption in liquids

Optical absorbance within a liquid is used as a photometric probe to measure the topography of optical surfaces relative to a reference. The liquid fills the gap between the reference surface and the measuring surface. By comparing two transmission images at different wavelengths we can profile the height distribution in a simple and reliable way. The presented method handles steep surface slopes (<90°) without difficulty. It adapts well to any field of view and height range (peak to valley). A height resolution in the order of the nanometer may be achieved and the height range can be tailored by adapting the concentration of water soluble dyes. It is especially appropriate for 3D profiling of transparent complex optical surfaces, like those found in micro-optic arrays and for Fresnel, aspheric or free-form lenses, which are very difficult to measure by other optical methods. We show some experimental results to validate its capabilities as a metrological tool and handling of steep surface slopes.

Fabrication of Einzel Lens Array with One-Mask Reactive Ion Etching Process for Electron Micro-Optics

In this paper, we report the fabrication of electron micro-optics by using deep reactive ion etching of three stacked silicon wafers. The three 120-µm-thick silicon wafers stacked with cavities are etched by a direct cavity through etching technique, where each wafer is bonded with a 200-µm-thick glass with through holes, thus separated by a space in part. An array of the electron micro-optics consisting of three electrodes can be fabricated by a one-mask process without assembling and alignment process. The dimensional profiles of a sample etched using this technique are also investigated. This micro-optics fabrication technology is useful for making future multiple electron beam devices with electron optics.

Dynamically adjustable three-dimensional gray masks operated by electrostatic force modulation for the fabrication of microlens arrays in microchannels

We propose a new approach to fabricating micro-optical elements, in particular, microlenses and microlens arrays using an adjustable gray-scale mask and a microfluidic conveyor module. A liquid with acrylate oligomer is photopolymerized while it flows through this module, thus forming the micro-optical components. The adjustable gray-scale mask contains a layer filled with a UV-absorbing liquid and transparent elastomer structures in the shape of microlenses. By applying a high voltage, the shape of these microlenses can be altered, enabling one to change the intensity transmission distribution of the gray-scale mask and, as a consequence, the surface profile of the corresponding microlens structures in the microfluidic conveyor module below. The produced microlens structures are washed out of the conveyor module without adhesion on the walls of the PDMS microchannel surfaces due to the existence of an oxygen-aided inhibition layer. The dependence of the produced microlens morphology on the index-matching oil, the concentration of UV-absorbing liquids, and the electrostatic force modulation of the adjustable gray-scale mask are characterized. The adjustable gray-scale mask combined with a suitable flow/stop sequence enables one to fabricate microlenses and microlens arrays in a continuous and high-throughput mode.

Investigation of an optical method for determining the average radius of curvature of micro-optical lenticular lens arrays

Micro-optical elements are of great importance in areas of optoelectronics and information processing. Establishing fast, reliable methods for characterization and quality control of these elements is important in order to maintain the optical performance in a high volume production process. We investigate an optical technique, applied to a polymer-based, injection moulded, lenticular array, but the method is also applicable for the tooling for these elements. The cylindrical lenses have feature sizes of 1–15 µm. The method is based on observation of the intensity distribution, which can be obtained in the far field of surface reflections resulting from a plane wave incident on the lenticular array. The intensity distribution of the diffraction orders is highly correlated with the shape of the illuminated lenslets. This is exploited to attain information about possible defects and shape variations that have arisen in the moulding process. The experimental results are compared with a model, based on the Fraunhofer approximation to the Huygens–Fresnel principle. Furthermore, the optical elements under investigation are probed using a scanning probe microscope. Hence, access to accurate topological data of the elements is attained and used to confirm the validity of the proposed optical technique.

Realization of optical perfect shuffle with microoptical array element

A new method to realize the optical perfect shuffle (PS) with a microoptical array element is presented in this paper. The whole process is simulated by computer, and parameters of the structure to fabricate the experimental component are given. The microoptical array element has been fabricated by introducing very large scale integration (VLSI), stepping photolithography and reactive iron etching (RIE), which can realize 8-channel PS transformation. Experiments, tests and analysis have been done using the array element. The experimental results show that the method proposed in this paper agrees well with theoretical expectation. This success of the experiment lays a good foundation for us to do further research on realization of optical switching and communication through cascade multilevel PS interconnection.

Array of micro-optical traps for cold atoms or cold molecules using a Damman grating

We propose a novel scheme to produce an array of micro-optical traps by using an optical system composed of a Damman grating and a spherical lens, which is illuminated by a red detuned Gaussian laser beam. We derive formulas to calculate the periodical structures of the micro-optical well arrays, and characteristics of each well, such as the spatial intensity distribution, intensity gradient, and geometrical parameters. Applications of our micro-optical traps in the fields of atom and molecule optics are briefly discussed. Our study shows that the proposed array of micro-optical wells can be used to trap cold atoms or cold molecules more efficiently than previous optical traps, even to prepare novel optical lattices.

Very large-scale integral imaging (VLSII) for 3-D display

Very large-scale integral imaging (VLSII) systems utilizing multiple micro-optics arrays and display devices can be used for wide viewing angle 3-D displays. We analyze the distortion of 3-D integral images when micro-optics arrays are placed on a spherical surface and are used in direct pickup integral imaging. We show that 3-D orthoscopic real images with non-negligible depth always suffer from image distor- tion, while orthoscopic virtual images do not. Also, nonplanar (curved) display panels and image sensors are not currently available. We pro- pose a solution to remedy this problem by placing a spherical lens in front of a planar lenslet array. © 2005 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1824907) tion on VLSII using spherical surfaces to determine what kind of integral images are formed and how direct camera pickup can be used to obtain elemental images. We attempt to provide analytical answers to some of these questions. Also, nonplanar ~curved! display panels and image sensors are not currently available. We propose a solution to remedy this problem by modeling the VLSII with lenslet arrays arranged on spherical surfaces as an II system with planar lenslet arrays, on which thin lenses with proper focal lengths are attached. Our analysis illustrates that 3-D orthoscopic real images with large depth always suffer from image distortion, while orthoscopic virtual im- ages do not. Preliminary experiments are presented. 2 Analysis

Ultrafast spatiotemporal processing with thin-film micro-optics

For spatiotemporal transformation and processing of ultrashort-pulse laser beams, serious design constraints arise from dispersion and diffraction. At pulse durations in 10-fs range, temporal and spatial parameters of propagating wave packets are coupled and significant inhomogeneities appear. To enable a controlled shaping or encoding and a reliable detection or decoding with 2-D spatial resolution, specific advantages of thin-film micro-optical arrays can be exploited. Transmitting and reflecting components of extremely small conical angles are used to generate multiple nondiffracting beams and self-imaging phase patterns. With novel-type metal-dielectric microaxicons, low-dispersion reflective devices are realized. Beam propagation is simulated numerically with Rayleigh-Sommerfeld diffraction theory. For ultrafast time-space conversion, matrix processors consisting of dielectric thin-film microaxicons are tested. Transversally resolving linear and nonlinear autocorrelation techniques are applied to characterize the space-time structure of localized few-cycle wave packets shaped from Ti:sapphire laser beams at pulse durations down to 8 fs. Bessel-like X waves are generated and their propagation is studied. In combination with autocorrelation, wavefront analysis of ultrashort-pulse lasers with Bessel-Shack-Hartmann sensors operated in reflection setup is demonstrated.

Ultrashort-pulse wave-front autocorrelation.

Combined spatially resolved collinear autocorrelation and Shack-Hartmann wave-front sensing of femtosecond laser pulses is demonstrated for the first time to our knowledge. The beam is divided into multiple nondiffracting subbeams by thin-film micro-optical arrays. With hybrid refractive-reflective silica/silver microaxicons, wave-front autocorrelation is performed in oblique-angle reflection. Simultaneous two-dimensional detection of local temporal structure and wave-front tilt of propagating few-cycle wave packets is demonstrated.