Optical Fabrication Research Articles

Height reconstructions from geometric wavefronts using vision ray metrology

A recently reported vision ray metrology technique [Opt. Express 29, 43480 (2021)OPEXFF1094-408710.1364/OE.443550] measures geometric wavefronts with high precision. This paper introduces a method to convert these wavefront data into height information, focusing on the impact of back surface flatness and telecentricity errors on measurement accuracy. Systematic errors from these factors significantly affect height measurements. Using ray trace simulations, we estimate reconstruction errors with various plano-concave and plano-convex elements. We also developed a calibration technique to mitigate telecentricity errors, achieving submicron accuracy in surface reconstruction. This study provides practical insights into vision ray metrology systems, highlighting validity limits, emphasizing the importance of calibration for larger samples, and establishing system alignment tolerances. The reported technique for the conversion of geometric wavefronts to surface topography employs a direct non-iterative ray-tracing-free method. It is ideally suited for reference-free metrology with application to freeform optics manufacturing.

Four-level diffractive photon sieves by deep-UV femtosecond laser ablation

A growing demand for complex light manipulation and miniaturization of optics necessitates advanced optical elements, operating on light diffraction phenomena, capable not only of reshaping the intensity distribution but also integrating many optical functions in a compact, durable device. The prevailing fabrication methods for these elements often involve multi-step lithographic processes. In contrast, direct laser ablation offers a single-step, cost-effective, and maskless alternative. However, using solid-state laser systems’ fundamental wavelength (in the IR range) for ablation lacks the precise depth control required for multi-level diffractive optical element fabrication. In this paper, we present the first experimental proof that femtosecond direct laser ablation in the UV spectral range is a reliable method for fabricating diffractive optical elements. We demonstrate the high-quality production of compact photon sieve focusing elements with the shortest focal length ever reported at 9 mm. Furthermore, we report an efficiency of 3.3%, which, to our knowledge, is the highest for elements with such a small focal length and not far from the theoretical efficiency limit of 4.46% (considering perfect cylindrical ablated pits). Moreover, our fabricated elements focus light to a 2.3% smaller focal spot if compared to the refractive lens with the same parameters. This fabrication method shows great promise for advanced applications that require precise depth control in wide band gap materials, such as the fused quartz used in this study.

Density adaptive path based on stacked rotation convolution model in ultra-precision robotic optical polishing

Deterministic optics fabrication using sub-aperture tools has been vital for manufacturing precision optical surfaces, industrial robotic polishing, which is a more economical and intelligent method is required in modern fabrication process. However, the challenge of robotic polishing lies in the widely used spiral and raster paths, which may leave excess waviness from the tool path, and the unavoidable constant removal layer is added to obtain positive dwell time which cause low polishing accuracy. The waviness can be removed by either using smoothing tools sequentially or randomizing the tool path. However, process efficiency and accuracy are not well considered in the existing tool-path planning. A density adaptive path based on a stacked rotation convolution model to ensure polishing accuracy and efficiency while avoiding waviness generation is proposed in this study, and then the dwell time is calculated by anti-aliasing space-variant deconvolution. The robotic polisher experimental results confirm that the root mean square (RMS) of the final surface figure has been successfully reduced and stabilized at 7.355 nm, and the convergence effect at unit wavelength has been significantly augmented with an improvement of 367%, reducing the measurement from 28% to 131%; in addition, no obvious mid-spatial frequency (MSF) peak was generated in the PSD analysis of density adaptive path polishing results. Henceforth, the polishing accuracy, efficiency, and MSF error of robotic polishing can be greatly enhanced.

A methodical investigation on stilbazolium derivative substituted with 2,4-Dichlorobenzaldehyde as a donor: An effective crystal for optical device fabrication

For a couple of decades, nonlinear optics (NLO) has garnered significant attention due to its widespread application in optical systems. In this viewpoint, a new 2-[2(2,4-Dichloro-phenyl)vinyl]methyl-1-pyridinium iodide (DCPI) crystal was grown through slow evaporation process for NLO application. Single Crystal X-ray diffraction (SCXRD) study reveals that the DCPI crystallized centrosymmetric (P-1) space group. The CHN elemental analysis and NMR spectroscopy assess the purity of the material. The vinyl (CC) group formation in DCPI was asserted by the Fourier-transform infrared (FT-IR) spectrum. The Hirshfeld surface analysis confirms the presence of hydrogen bonds in the crystal structure through its contribution percentage (27 %) to the overall intermolecular interaction. DCPI crystal exhibits 80 % transparency in the visible region. Luminescence studies assess the green light-emitting characteristics of the DCPI crystal. The etching and Atomic Force Microscopy (AFM) studies confirm that the DCPI crystal has a flat surface with minimum dislocations. Z-scan studies found the third-order nonlinear characteristics of the crystal. Overall, the article illustrates the appropriateness of DCPI crystal in fabricating optical devices through its significant optical characteristics.

Assessing the potential accuracy of a small-sized goniometer with extended dynamic range based on nuclear magnetic resonance

This paper evaluates potential accuracy characteristics of a small-sized goniometer based on nuclear magnetic resonance with an extended dynamic range. This required constructing a model of goniometer errors, estimating its accuracy based on this model, and formulating practical recommendations for the design of such a device based on the accuracy assessment. To evaluate the accuracy of a nuclear goniometer, a theoretical model was built that makes it possible to determine the optimal operating parameters of the cell gas mixture, the ranges of their permissible changes, the sensitivity of the goniometer, and the dependence of its characteristics on external and internal factors. In particular, the dependence of output signal of the device on the parameters of gas mixture and optical pumping has been determined. For a goniometer with a cell volume of 8 cm3, the optimum temperature is 130 °C, and the optimum intensity of the pumping radiation is 5 mW. The dependence of output signal on the measured angle of rotation was also established, as well as the noise and error dependence of the device on the permissible values of its parameters. Based on the model built, parameters of a goniometer with a cell volume of 8 cm3 were determined; the maximum angular sensitivity of such a goniometer with complete suppression of technical noise is δφsen=1.0 arcsec. The greatest contribution to the angle measurement error is from the instability of pumping power Ip (ΔIp/Ip=0.05) – 85 %, magnetic field B0 (ΔB0/B0=10-8) – 13 %, temperature T (ΔT/T=0,1) – 2 %. The goniometer under consideration corresponds to the medium accuracy class, δφtot≥10 arcsec. It could be used in optical manufacturing for operational control, calibration, and certification of optical products. To improve the angular accuracy of the goniometer, it is necessary to increase the stability of the laser pumping intensity

Calculation of Tool Offset and Tool Radius Errors Based on On-Machine Measurement and Least Squares Method in Ultra-Precision Diamond Turning

Metal mirrors will be widely used in the coming decades. Therefore, as one of the enabling technologies for metal optical freeform surface manufacturing, ultra-precision (UP) diamond turning error compensation has become a research hotspot. However, for the tool offset error and tool radius error, which are the main errors in UP diamond turning, no precise and efficient calculation method has been found in the literature. In this study, a more precise and efficient algorithm was developed and validated in three ways using on-machine measurement data and profilometer measurement data. After one compensation, the tool offset error can be reduced to below 0.1 μm, and the tool radius error can be reduced to below 1 micrometer, which will significantly improve the UP turning accuracy and efficiency of optical parts.

Crystal growth, linear optical, thermal, mechanical and third-order nonlinear optical properties of 1,3-diphenyl prop‑2-en-1-one single crystals

This work brings out the viability of the chalcone derivative (E)-1,3-diphenyl-2-propen-1-one (DPP) for industrial applications in nonlinear optical technology and device fabrication, through investigations on its crystal structure and nonlinear optical properties. The unit cell features of the DPP single crystals grown by slow evaporation solution growth method have been determined and hkl values indexed. The optical transparency of DPP in the near infrared and visible range is found to be admirable and an optical bandgap of 3.42 eV is revealed in the UV–Vis-NIR analysis. The work hardening coefficient (n) of DPP, determined as 2 from Vickers microhardness measurement, shows the moderate mechanical strength of the crystal. The Laser Damage Threshold value of 3.9 GW/cm2 sheds light on the high tolerance of DPP to optical damage and suggests its suitability in device fabrication. The potential of DPP crystal in nonlinear optical applications is suggested from its superior value of 1.16×10−7 esu for third order susceptibility and appealing values of other nonlinear optical parameters.

Infrared Thermographic Diagnostics of Wood Fire Resistance under Combined Thermal Effect Conditions from a Ground Fire Front and Firebrands

Experiments were conducted on certain types of combustible building materials and wood-based structures to determine the interaction between firebrands and forest fuel (FF). Using non-contact IR diagnostic methods in narrow spectral ranges of infrared wavelengths, the heat flux values generated by firebrands and the temperature field of the most heat-stressed sections of the structures under study were determined. Using a JADE J530SB thermal imager with an optical filter of 2,5—2,7 m, temperatures in the range 310—1500 K were measured in the infrared region. In order to interpret the recorded emission from the test sample, calibrations supplied by the narrowband optical filter manufacturer were used.

Advances in predicting surface shape changes of mirror blanks through elliptical nozzle gas jet forming

To facilitate the formation of bifocal aspheric optical surfaces, we present an approach for predicting the surface profile of bifocal aspheric surfaces formed by the gas-liquid interface when an elliptical nozzle gas jet is employed. Through an analysis of the gas flow field morphology emanating from the elliptical nozzle, we inferred the impact of gas jet parameters on the gas-liquid interface surface shape within the core region of the gas jet. By analyzing the variation in the gas flow field morphology emitted from an elliptical nozzle, we deduced the influence patterns of gas jet parameters on the gas-liquid interface surface shape within the gas jet's core region. Theoretical analysis is substantiated by numerical simulations, confirming regular changes in the vertex curvature and conic constant of mirror blanks concerning variations in jet initial velocity and nozzle aspect ratio. A comparison between experimental data and numerical simulation results reveals an average prediction deviation of 0.0083 mm−1 for the vertex curvature and a prediction deviation of 10.7 % for the conic constant, challenging to rectify within numerical simulations. Hence, an empirical model, incorporating jet parameters, is developed based on experimental data to predict the vertex curvature and conic constant of mirror blanks. This model demonstrates an average prediction error of 2.901 × 10−3 mm−1 for the vertex curvature and 7.64 % for the conic constant, surpassing the predictive accuracy of the numerical simulation model.

Low-damage optical manufacturing via plasma finishing and figuring

Precision optical components have stringent requirements on surface roughness, form error, and subsurface damage for superior performance. However, conventional grinding, lapping, and polishing processes of fused silica inevitably introduce subsurface damage (SSD) due to the use of abrasives. Thus, this paper proposes an abrasive-free, low-damage manufacturing process for fused silica optical components, which combines inductively coupled plasma (ICP) for SSD recovery and capacitively coupled plasma (CCP) for form error correction. This paper mainly aims to reveal the advantages and challenges of the combined plasma process. The SSD recovery capability of ICP finishing was first verified. The comparison of surface morphology after buffered oxide etch (BOE) etching and CCP etching revealed that extensive surface roughening is caused by plasma etching rather than SSD. Experimental studies on the combination of ICP and CCP demonstrated that ICP finishing can not only recover SSD but also inhibit the surface roughening by plasma etching. The investigation of form error after ICP finishing revealed that the induced form error consists of workpiece distortion and localized deformation with a crater-like structure, affecting the precision and duration of CCP figuring. The combined plasma process was conducted and a low-damage surface with roughness less than Sa 0.3 nm and form error less than RMS 20 nm was achieved.

Theoretical exploration of binary mixtures derived from hydrogen bond liquid crystalline soft materials for optical device fabrication

Binary mixtures are obtained from 4-Nitrobenzaldehyde (NBA), 4-(heptyloxy)benzoic acid (7OBA) and 4-(octyloxy)benzoic acid (8OBA) abbreviated as NBA + 7OBA and NBA + 8OBA. Density Functional Theory (DFT) is used to optimize the resultant molecular structures. HOMO–LUMO and NBO analysis are adapted in obtaining the charge transfer phenomenon of the binaries. Molecular electrostatic Potential (MEP) analysis reveals the active sites of liquid crystals. Atomic charge distribution of the binary mixture is investigated theoretically. Topological analysis is carried out to understand the intermolecular interaction between non mesogenic NBA and mesogenic precursors along with nonlinear optical properties. The results obtained are correlated with the experimental data obtained.

Research on the generation mechanism of surface and subsurface damage in loose abrasive lapping of RB-SiC ceramics

Reaction-bonded silicon carbide ceramics (RB-SiC) are extensively utilized in aerospace, space optics, and other fields due to their superior physical and chemical properties. Loose abrasive lapping plays a crucial role in the optical manufacturing of large-diameter RB-SiC mirrors. Previous research predominantly focused on grinding processes and overlooked the removal mechanism during lapping and their impact on surface and subsurface damage. In this study, a three-body brittle fracture removal model was established to explore the removal mechanisms of RB-SiC. Additionally, experiments were carried out to investigate the influence of abrasive particle size on the surface and subsurface damage. Experimental results confirm the theoretical model and indicate that for RB-SiC, different particle sizes correspond to distinct removal mechanisms, causing abrupt changes in surface roughness, while the layer under SiC acts as a buffer against the propagation of subsurface damage. These findings help optimize the manufacturing process, improve lapping efficiency, and enhance mirror performance.

Zonal shape reconstruction for Shack-Hartmann sensors and deflectometry

Some metrological means, such as Shack-Hartmann, deflectometry sensors or fringe projection profilometry, measure the shape of an optical surface indirectly from slope measurements. Zonal shape reconstruction, a method to reconstruct shape with a high number of degrees of freedom, is used for all of these applications. It has risen in interest with the use of deflectometers for the acquisition of high resolution slope data for optical manufacturing, especially because shape reconstruction is limiting in terms of shape estimation error.Zonal reconstruction methods all rely on the choice of a data formation model, a basis on which the shape will be decomposed, and an estimator. In this paper, we first study the canonical Fried and Southwell models of the literature and analyze their limitations. We show that modeling the slope measurement by a point-wise derivative as they both do can induce a bias on the shape estimation, and that the bases on which the shape is decomposed are imposed because of this assumption.In the second part of this paper, we propose to build an unbiased model of the data formation, without constraints on the choice of the decomposition basis. We then compare these models to the canonical models of Fried and Southwell.Lastly, we perform a regularized MAP reconstruction, and compare the performance in terms of total shape error of this method to the state of the art for the Southwell and Fried models, first by simulation, then on experimental data. We demonstrate that the suggested method outperforms the canonical models in terms of total shape reconstruction error on a deflectometry measurement of the high-frequency content of a freeform mirror.

A novel optical acoustic fabric based on intensity-modulated flexible air-filled fiber adapter for sound sensing

This study proposes and demonstrates an optical acoustic fabric design method to detect sound, which is based on intensity-modulated flexible air-filled fiber adapter (FAFFA). A FAFFA was woven into the weft center of the polyamide fabric as weft, and the FAFFA was realized by connecting polymer optical fibers at each end of an flexible tube. The principle of the optical acoustic fabric is to allow the flexible tube to deform with the resonance of the polyamide fabric, subsequently changing the light intensity in the FAFFA. The macro-scale finite element method and ray tracing optical simulation investigated the operating principle of optical acoustic fabric and performance of FAFFA structures, and the influence of fabric parameters was evaluated by meso-scale finite element simulation. The yarn of the optical fiber acoustic fabric has low Young’s modulus and high flexibility. The proposed FAFFA structure is simple to prepare and has good maneuverability, which breaks away from the constraints of the traditional optical acoustic sensor membrane structure. This proposed optical acoustic fabric is easy to prepare, simple in principle and highly flexible. Along with wireless devices, the use of the optical acoustic fabric can be further extended to “hearing aids” for special individuals in wearable applications.

Recent Patents for Optical Component Polishing Technology

Background: Fluid jet polishing, Airbag polishing, Magnetorheological polishing, and Ion beam polishing are all emerging polishing technologies commonly used for processing optical components, which can be used to manufacture various high-precision and high-quality optical components. Their processing mechanisms are to remove materials by high-speed jet impacting the surface of the workpiece; using a spherical flexible airbag as a polishing tool to obtain a super-smooth surface by adding polishing fluid containing fine abrasives; controlling the magnetic field to form a flexible polishing belt of Magnetorheological fluid, which undergoes relative motion to achieve material removal; using a neutral ion beam to bombard the surface of the workpiece to remove workpiece surface atoms and obtain a super-smooth surface through energy and momentum transfer between ions and workpiece atoms. The characteristics and removal mechanisms of various polishing methods are briefly described, and the advantages and key issues that need to be addressed for each polishing technology are pointed out, and suggestions and prospects for future research, application, and development are also given. Objective: In order to meet the growing demand for high-precision optical components and mass-manufactured optical components, this paper aims to study the various polishing technologies that are continuously improved and suggestions are made for future research and development directions. Method: This paper reviews the current patents related to various polishing technologies, such as Fluid jet polishing, Airbag polishing, Magnetorheological polishing and Ion beam polishing and other representative polishing technologies. Results: Various types of polishing technologies have been studied, and the main problem at the current stage is the lack of accuracy. The problem of the polishing device, such as fluid jet polishing, only considers the change of fluid velocity and pressure to construct the removal function model for analysis and summary, and suggestions for the latest research progress of various polishing technologies are given. Conclusion: The improvement and optimization of polishing technology are beneficial to improving the polishing accuracy and quality of optical components and have great help for the future development of related patents.

Flexible focal array engineering with a binary array vector optical field

In recent years, the vector optical field (VOF) with space-variant polarization distribution has attracted great attention due to its unexpected effects and applications in a wide range of areas. The focal engineering plays the most important role, as the focused VOF provides various interesting properties. Here, we propose a kind of binary array VOF (BA-VOF), which can be applied in focal array engineering. The BA-VOF comprises an array of the first base field of radially polarized VOF and an array of the second base field of superposed subfields with phase modulations. We theoretically design and experimentally generate the BA-VOF. Based on the BA-VOF, we present a flexible method to manipulate the amount of the focal spots in the focal array. Moreover, the polarization state and spin angular momentum of each focal spot in the focal spot array can also be flexibly manipulated. The BA-VOF and the flexibly manipulated focal array are inspirable in the area of structured light, which can be applied in regions needing focal engineering, such as optical tweezers, optical fabrication, optical imaging, and so on.

Design and Manufacture of 30-Degree Projection Lens for Augmented Reality Waveguide.

A projection lens with a 30-degree field of view is developed for use in augmented reality (AR) glasses, including a waveguide combiner designed for a 0.35-inch LCoS panel. The entrance pupil diameter of the lens is 14 mm and the lens has an effective focal length of 16.443 mm; an F-number of 1.175. This paper has four key issues: optical projection lens design, lens manufacturing and assembly tolerance analysis, projection lens resolution testing, and AR glasses system resolution testing of panel images projected by the projection lens. After lens manufacture, the lens was tested, achieving a central field image quality of 57 cycles/mm, an angular resolution of 33 pixels per degree (PPD), a 0.7 field image quality of 40.3 cycles/mm, and an angular resolution of 23 pixels per degree (PPD). Imaging performance testing based on a diffraction-type waveguide shows a resolution of 57 cycles/mm in the center area and an angular resolution of 33 PPD.

Gravitational effects on optical lens fabrication: First insides on Earth- and microgravity experiments

Abstract Understanding the effects of gravity on manufacturing processes is a pioneering extension of the process parameter space used to date. Until now, the improvement of manufacturing technologies has mainly focused on process parameters such as temperature, pressure, and material composition, as access to variable gravity environments is limited. The Einstein-Elevator opens up new possibilities for the variation of these process parameters and the development of in-space manufacturing technologies. Together with the research of innovative production processes for optical components within the PhoenixD Cluster of Excellence, this creates an entirely new field of research. The research presented here focuses on investigating gravity's effects on dispensed optical lens production. Using a jet dispenser, sessile droplets are produced during a flight phase in the Einstein-Elevator and cured directly by UV polymerization. As part of this study, optical lenses were produced and compared under microgravity and Earth's gravitational conditions. Geometric properties such as height and contact angle of the lenses produced were analyzed. It was found that lenses fabricated under microgravity have a larger contact angle than those fabricated under Earth gravity. Similarly, the height increases with decreasing gravity. These results are consistent with the theoretical assumptions described, although generalized theories to describe the morphology of a sessile droplet are not yet available. The case study findings on the influence of gravity as a process parameter on drop morphology represent a fundamental improvement for additive manufacturing technologies, especially for in-space manufacturing.

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.

Nanophotonic structure inverse design for switching application using deep learning

Switching functionality is pivotal in advancing communication systems, serving as a paramount mechanism. Despite numerous innovations in this field, optical switch design, fabrication, and characterization have traditionally followed an iterative approach. Within this paradigm, the designer formulates an informed conjecture regarding the switch's structural configuration and subsequently resolves Maxwell's equations to ascertain its performance. Conversely, the inverse problem, which entails deriving a switch geometry to achieve a targeted electromagnetic response, continues to pose formidable challenges and necessitates substantial time and effort, particularly under the constraints of specific assumptions. In this work, we propose a deep neural network-based method to approximate the spectral transmittance of all-optical switches. The findings substantiate the efficacy of deep learning in the design of all-optical plasmonic switches, which are renowned as the fastest switches at the nanoscale. The nonlinear Kerr effect in square resonators is leveraged to demonstrate the switching performance. Juxtaposed with conventional simulations, the proposed model showcases a remarkable improvement in computational efficiency. Furthermore, deep learning can resolve nanophotonic inverse design problems without reliance on trial-and-error or empirical strategies. Compared to simulations, the mean squared error for both forward and inverse models is meager, with values of around 0.03 and 0.02, respectively. The deep learning-proposed switches exhibit excellent suitability for integration into photonic integrated circuits, substantially influencing the progression of all-optical signal processing.