Fabrication Of Optical Elements Research Articles

Comparative analysis of microlens array formation in fused silica glass by laser: Femtosecond versus picosecond pulses

The growing demand for flexible, high-quality fabrication of free-form micro-optics drives the development of laser-based fabrication techniques for both the shape formation and surface polishing of optical elements. In this paper, we performed a thorough and systematic study on fused silica glass ablation using 10 ps and 320 fs duration pulses. Ablation processes for both pulse durations were optimized based on the measurements of the removed material layer thickness and surface roughness, and by analyzing the topographies of ablated cavities to remove material layers as thin as possible with minimum surface damage. Our findings demonstrate higher process resolution and surface quality for femtosecond pulses. Ablation of pre-roughened glass reduced the minimal removable glass layer thickness well below the 1 μm mark for both pulse durations, improving the process resolution. The minimal removable glass layer thickness was 14 times smaller for the femtosecond pulses, with up to 4.5 times lower surface roughness compared to samples processed with picosecond pulses. On the other hand, results revealed faster glass removal rates with picosecond pulses. In the end, arrays of microlenses were fabricated with both pulse durations and subsequently polished with a CO2 laser. Results revealed higher performance of microlenses fabricated with femtosecond pulses, providing better focusing capabilities and lesser beam scattering. Finally, this study demonstrated the successful fabrication of free-form optical elements with femtosecond and picosecond pulses, demonstrating the versatility and the potential of laser-based techniques.

Advantages of Direct Laser Writing for Enhancing the Resolution of Diffractive Optical Element Fabrication Processes

A technology for direct laser writing of code sequences on modulation disks has been developed and implemented, ensuring high accuracy and reliability in the process of forming structural elements. The main advantages of applying direct laser writing compared to contact lithography for forming submicron-sized elements have been demonstrated. The proposed technology is characterized by high resolution and flexibility in configuring the parameters of the optical recording system, making it suitable for a wide range of applications in micro-optics. Direct optical writing is presented as a promising approach for enhancing the resolution of optical systems used in recording submicron-sized diffractive optical elements, as this technology enables the creation of complex optical structures. Additionally, a detailed classification of current approaches for further increasing the resolution of the optical recording system was conducted, including the application of a saturated absorber layer on the photosensitive surface, the use of a laser beam with intensity modeled by a Bessel function, and the synthesis of photosensitive materials with optimized exposure characteristics, ensuring high efficiency and accuracy in the process of direct laser writing.

Microstructuring of fused silica by laser-induced microplasma using a pyrographite target for phase optical elements fabrication

In this paper, we purpose to use laser-induced microplasma occurred from a pyrographite target by laser ablation for microstruturing of fused silica plate to fabricate phase optical elements. Laser parameters for erosive plasma torch formation and subsequent etching of fused silica were determined based on preliminary experimental studies and estimated energy calculations. The influence of the overlap coefficients in the focal spot on the reproducibility of etching depths results and the deviation accuracy of etching depths with the found laser parameters was experimentally studied. The best results in terms of accuracy (no more than 0.1 μm) in deviations of etching depths are achieved with the overlap coefficient of 0.5. Moreover, the paper presents estimated calculations of the speed and efficiency of ablation, as well as the coefficient of energy conversion from laser radiation to the ablation energy of fused silica, which was ∼ 0.25. Based on achieved results an attempt was made to fabricate the 6-sector SPP for the wavelength of 355 nm.

Polymer Complex Multilayers for Drug Delivery and Medical Devices.

Polymer complex multilayers (PCMs) can be engineered into various structures with tunable properties via layer-by-layer (LBL) assembly driven by noncovalent forces. Due to their ease of preparation, capability of integrating multiple functional components, and excellent substrate compliance, biocompatible PCMs as coating materials or individual entities have attracted extensive attention in biomedical applications. This Spotlight on Applications presents recent progress on PCMs applied for drug delivery and medical devices. We provide several examples to address the importance of using PCM platforms to achieve controlled drug delivery including stimuli-triggered release, sustained release, and spatiotemporal sequential release. The effects of PCM coatings on the bioresponse regulation and performance enhancement of implantable devices are also highlighted. Moreover, the design and fabrication of flexible electrical and optical elements modified with LBL PCMs have been discussed, which demonstrates the great potential to advance emerging wearable devices for disease monitoring and health management.

Design and fabrication of multifunctional holographic optical elements in laser Doppler velocimeter.

Replacing the bulky traditional optical elements in the optical system with a holographic optical element (HOE) is conducive to the functional integration and volume miniaturization. However, when the HOE is used in the infrared system, the mismatch between the recording wavelength and the working wavelength will lead to the reduction of diffraction efficiency and the introduction of aberration, which will seriously affect the performance of the optical system. This paper proposes a design and fabrication method of multifunctional infrared HOEs that can be used in laser Doppler velocimeter (LDV), which can reduce the effect of wavelength mismatch on HOE performance while integrating the functions of the optical system. The restriction relationship and selection method of parameters in typical LDV are summarized; the decrease of diffraction efficiency due to the mismatch between recording and working wavelengths is compensated by designing the angle of signal and reference wave of the HOE; and the aberration caused by wavelength mismatch is compensated by cylindrical lens. The optical experiment shows that the HOE can produce two groups of fringes with opposite gradient, which proves the feasibility of the proposed method. Moreover, this method has a certain degree of universality, and it is expected to design and fabricate HOEs for any working wavelength in the near-infrared band.

Study on the Surface Generation Mechanism during Ultra-Precision Parallel Grinding of SiC Ceramics

Silicon carbide (SiC) is a typical, difficult-to-machine material that has been widely used in the fabrication of optical elements and structural and heat-resistant materials. Parallel grinding has been frequently adopted to produce a high-quality surface finish. Surface generation is a vital issue for assessing surface quality, and extensive modeling has been developed. However, most of the models were based on a disc wheel with a cylindrical surface, whereas the surface topography generation based on an arc-shaped tool has been paid relatively little attention. In this study, a new theoretical model for surface generation in ultra-precision parallel grinding has been established by considering the arc-shaped effect, synchronous vibration of the wheel, and cutting profile interference in the tool feed direction. Finally, the ground surface generation mechanism and grinding ductility were analyzed in the grinding of SiC ceramics. The results showed that the spiral and straight-line mode vibration patterns were the main feature of the machined surface, and its continuity was mainly affected by the phase shift. Furthermore, for the in-phase shift condition, the grinding ductility was more significant than for the out-of-phase shift due to the continuously decreasing relative linear speed between the wheel and workpiece.

Fabrication of curved aspheric compound eye microlens array with high surface quality by precision glass molding

The curved compound eye microlens array (CEMLA) glass optical element is widely used in optoelectronic detectors because it affords a high image quality and a large field of view. However, fabrication of high-precision glass optical elements with a small cell size and many arrays on hard and brittle glass materials is challenging. Here, we report a fabrication method to address molding challenges for high quality CEMLA glass optical elements at molding temperatures up to 500 °C. First, an Ni-P with good turning performance and low adhesion to glass is chosen as the plating material, the CuNi is selected as the mold substrate for their close CTEs. To prevent the diffusion of P to the surface of optical elements and to improve the hardness and wear resistance of mold, a Ti-Ta-C coating is deposited on the Ni-P mold. By single-point diamond turning on the Ni-P surface using a fast tool servo, an aspherical CEMLA mold with a form accuracy PV less than 0.4 μm and surface roughness Sa less than 4 nm was machined. Finally, based on the creep testing of microdeformation compression, an accurate viscoelastic constitutive model of glass was established. Optimization of the molding process and accurate prediction of molding profile deviation were simulated. The aspheric CEMLA glass optical element with a form accuracy PV less than 0.5 μm and surface roughness Sa less than 4.5 nm was obtained by precision glass molding (PGM).

Hardness and Young's modulus of BaGa4S7 and BaGa4Se7 nonlinear optical crystals

We report hardness and Young's modulus measurements of two recently developed non-chalcopyrite, non-centrosymmetric crystals, BaGa4S7 and BaGa4Se7, which seem very promising for three-wave parametric interactions in the mid-IR part of the spectrum between 5 and 18 μm. The obtained averaged values for the hardness from nanoindentation tests, converted into Vickers hardness number, are 556 kg/mm2 for the sulphide and 336 kg/mm2 for the selenide compound, respectively. Knowledge of these values will be essential for proper polishing and fabrication of optical elements, the wide use and commercialization of these novel non-oxide nonlinear crystals, which possess a number of other advantages over classical chalcopyrites operating in the same spectral window.

Multiwavelength high-order optical vortex detection and demultiplexing coding using a metasurface

Orbital angular momentum (OAM) of an optical vortex has attracted great interest from the scientific community due to its significant values in high-capacity optical communications such as mode or wavelength division multiplexer/demultiplexer. Although several configurations have been developed to demultiplex an optical vortex, the multiwavelength high-order optical vortex (HOOV) demultiplexer remains elusive due to lack of effective control technologies. In this study, we present the design, fabrication, and test of metasurface optical elements for multiwavelength HOOV demultiplexing based on optical gyrator transformation transformations in the visible light range. Its realization in a metasurface form enables the combined measurement of OAM, the radial index p, and wavelength using a single optical component. Each wavelength channel HOOV can be independently converted to a high-order Hermitian–Gaussian beam mode, and each of the OAM beams is demultiplexed at the converter output. Furthermore, we extend the scheme to realize encoding of the three-digit gray code by controlling the wavelength or polarization state. Experimental results obtained at three wavelengths in the visible band exhibit good agreement with the numerical modeling. With the merits of ultracompact device size, simple optical configuration, and HOOV recognition ability, our approach may provide great potential applications in photonic integrated devices and systems for high-capacity and demultiplex-channel OAM communication.

Modeling nonequilibrium thermoviscoelastic material behaviors of glass in nonisothermal glass molding

AbstractNonisothermal glass molding (NGM) is developed aiming to satisfy the fabrication of complex and precision optical elements at high production volumes. The technology, however, was encountered by the high form deviation of manufactured glass optics. During the nonisothermal compression molding, glass undergoes a rapid temperature change making it a transition from equilibrium to nonequilibrium nature of matter. Owing to this characteristic, viscoelastic material behaviors of glass depend on not only temperature but also thermal history. Understanding the nature of nonequilibrium thermoviscoelasticity, the central importance to control the final shape and optical characteristics of the manufactured glass optics is the focus of this research. First, this study presents preliminary experimental investigations indicating the difference in viscoelastic deformation responses under the equilibrated and nonequilibrated glass specimens. In the following, modeling the temperature‐dependent viscoelasticity over a wide temperature range in glass transition region is described, where temperature is directly coupled in each parameter of a rheological constitutive model. This concept allows us to bypass the thermorheologically simple assumption used to tolerate the description of thermoviscoelastic responses in most of the earlier works. Finally, we propose a modeling method that incorporates temperature and thermal history into the rheological model parameters, each of which is described by employing the Mauro–Allan–Potuzak viscosity equation. Viscoelastic experiments were carried out to validate the model. The results show that the time‐dependent responses varying with temperature are well predicted in both equilibrium and nonequilibrium regimes of glass‐forming systems. The benefit of this research is that a unique material model is entirely applicable for numerical studies of various hot forming processes such as annealing when glass undergoes pure thermal loads, as well as precision glass molding and NGM when glass is deformed by fully coupled thermal–mechanical loads under either its equilibrium or nonequilibrium state of materials, respectively.

Precise wavefront characterization of x-ray optical elements using a laboratory source.

Improvements in x-ray optics critically depend on the measurement of their optical performance. The knowledge of wavefront aberrations, for example, can be used to improve the fabrication of optical elements or to design phase correctors to compensate for these errors. At present, the characterization of such optics is made using intense x-ray sources, such as synchrotrons. However, the limited access to these facilities can substantially slow down the development process. Improvements in the brightness of lab-based x-ray micro-sources in combination with the development of new metrology methods, particularly ptychographic x-ray speckle tracking, enable characterization of x-ray optics in the lab with a precision and sensitivity not possible before. Here, we present a laboratory setup that utilizes a commercially available x-ray source and can be used to characterize different types of x-ray optics. The setup is used in our laboratory on a routine basis to characterize multilayer Laue lenses of high numerical aperture and other optical elements. This typically includes measurements of the wavefront distortions, optimum operating photon energy, and focal length of the lens. To check the sensitivity and accuracy of this laboratory setup, we compared the results to those obtained at the synchrotron and saw no significant difference. To illustrate the feedback of measurements on performance, we demonstrated the correction of the phase errors of a particular multilayer Laue lens using a 3D printed compound refractive phase plate.

Multiparameter laser performance characterization of liquid crystals for polarization control devices in the nanosecond regime

Interactions of liquid crystals (LC’s) with polarized light have been studied widely and have spawned numerous device applications, including the fabrication of optical elements for high-power and large-aperture laser systems. Currently, little is known about both the effect of incident polarization state on laser-induced–damage threshold (LIDT) and laser-induced functional threshold (LIFT) behavior at sub-LIDT fluences under multipulse irradiation conditions. This work reports on the first study of the nanosecond-pulsed LIDT’s dependence on incident polarization for several optical devices employing oriented nematic and chiral-nematic LC’s oriented by surface alignment layers. Accelerated lifetime testing was also performed to characterize the ability of these devices to maintain their functional performance under multipulse irradiation as a function of the laser fluence at both 1053 nm and 351 nm. Results show that the LIDT varies as a function of input polarization by 30–80% within the same device, while the multipulse LIFT (which can differ from the nominal LIDT) depends on irradiation conditions such as laser fluence and wavelength.

Multi-value phase grating fabrication using direct laser writing for generating a two-dimensional focal spot array

As a beam splitter, multi-value phase grating (MVPG) has a higher diffraction efficiency than the traditional Damman grating (DG) due to its increased number of phase values within one period of the grating. In this paper, two MVPGs are numerically designed within a 120 μm × 120 μm area, which generate 4 * 4 and 5 * 5 focal spot arrays in the far field. Both gratings are fabricated by direct laser writing (DLW) technology. Their diffraction efficiencies reach 68.58% and 63.4%, respectively. To compare, DGs with the same size and focal spot arrays are designed and fabricated, whose diffraction efficiencies are tested to be 29.55% and 35.04%, respectively. The results demonstrate the better optical properties of multi-value phase gratings and the capability of DLW in three-dimensional nano-scale diffractive optical element fabrication.

Quill-free additive manufacturing of fused silica glass

Additive manufacturing of high-quality macroscopic fused silica glass structures, with deposition rates of up to 1.2 mm3/s, is presented. Three co-axial nozzles were used to avoid the so-called quill effect. Homogeneous, crack-free, multilayer, as well as free-standing objects were printed using cluster-free sub-µm powders delivered to a CO2 laser-induced melt pool. Structures with an overhang of up to 45° were possible to print. Laser post-processing was used to improve the surface roughness and transparency. This system can be suitable for fabrication of advanced optical elements and devices, such as waveguides or fiber preforms.

Fabrication of liquid crystal optical elements based on SLM projection splicing technology

Fabrication of liquid crystal optical elements based on SLM projection splicing technology

Material removal model of magnetorheological finishing based on dense granular flow theory

Magnetorheological finishing (MRF) technology is widely used in the fabrication of high-precision optical elements. The material removal mechanism of MRF has not been fully understood because MRF technology involves the integration of electromagnetics, contact mechanics, and materials science. In this study, the rheological properties of the MR polishing fluid in oscillation model have been investigated. We propose that the shear-thinned MR polishing fluid over the polishing area should be considered a dense granular flow, based on which a new contact model of MRF over the polishing area has been constructed. Removal function and processing force test experiments were conducted under different working gaps. The normal pressure and effective friction equations over the polishing area were built based on the continuous medium and dense granular flow theories. Then, a novel MRF material removal model was established. A comparison of the results of the theoretical model with actual polishing results demonstrated the accuracy of the established model. The novel model proposed herein reveals the generation mechanism of shear force over a polished workpiece and realizes effective decoupling of the main processing parameters that influence the material removal of MRF. The results of this study will provide new and effective theoretical guidance for the process optimization and technology improvement of MRF.

Complex optical elements for scanning helium microscopy through 3D printing

Developing the next generation of scanning helium microscopes requires the fabrication of optical elements with complex internal geometries. We show that resin stereolithography (SLA) 3D printing produces low-cost components with the requisite convoluted structures whilst achieving the required vacuum properties, even without in situ baking. As a case study, a redesigned pinhole plate optical element of an existing scanning helium microscope was fabricated using SLA 3D printing. In comparison to the original machined component, the new optical element minimised the key sources of background signal, in particular multiple scattering and the secondary effusive beam.

2D Perovskite Micro-optics Enabled by Direct Femtosecond-Laser Projection Lithography

Using direct femtosecond-laser projection lithography in chemically synthesized CsPbBr3 perovskite microcrystals we demonstrate high-throughput fabrication of advanced binary microscale optical elements for nanofocusing as well as generation of high-order optical vortex beams. The obtained results highlight the CsPbBr3 microcrystals as a promising material for realization of various complicated 2D micro-optical elements and holograms that can be directly imprinted using non-destructive and practically relevant laser technologies.

Direct (3+1)D laser writing of graded-index optical elements

We propose single-step additive fabrication of graded-index optical elements by introducing the light exposure as the additional dimension to three-dimensional (3D) laser writing, hence (3 + 1)D writing. We use a commercial printer and photoresist to realize the proposed single-step fabrication method that can be swiftly adopted for research and engineering. After presenting the characterization of the graded-index profiles via basic structures, we demonstrate two different optical devices: volume holograms that are superimposed using angular and peristrophic multiplexing, and optical waveguides with well-defined refractive index profiles. In the latter, we precisely control the propagating modes via tuning the (3 + 1)D-printed waveguide parameters and report step-index and graded-index core–cladding transitions.

Ion-beam lithography for fabrication of diffractive optical phase elements in silver-ion-exchanged glasses

Ion-beam lithography for fabrication of diffractive optical phase elements in silver-ion-exchanged glasses