Glass Optics Research Articles

Trace element analysis and luminescence behavior of quartz in pegmatites of the Tørdal Region, Norway

This publication presents a study on the mineral chemistry and luminescence properties of quartz samples from pegmatites of the Tørdal region in Norway. A total of 12 samples were analyzed using Secondary Ion Mass Spectrometry (SIMS), Electron Paramagnetic Resonance Spectroscopy (EPR), and Cathodoluminescence (CL) to gain insights into their trace element concentration and distribution as well as their luminescence behavior. The samples are characterized by different Cl emissions at 450 nm, 500 nm 650 nm and an additional shoulder at 390 nm, which is only partially visible due to the absorption of the glass optics. Of these luminescence bands, the 500 nm band is the most dominant in most samples and it is characterized by an initial blue-green luminescence, which is not stable under electron irradiation. Moreover, it is characterized by a heterogeneous distribution within the samples. This luminescence can be mostly assigned to [AlO4/M+]0 defects, with charge compensation mostly achieved by Li+. Analyses by EPR spectroscopy prove the dominance of structurally bound Al, Li, and Ti ions in the investigated samples. Further analyses using SIMS mapping demonstrate that Na and K are mainly bound to micro fractures or inclusions, suggesting a limited role in the compensation of the luminescence centers. Additionally, the SIMS mappings show that some samples contain Al-rich clusters of 10 to 20 μm in diameter, whereas other trace elements are characterized by a homogeneous distribution. These clusters correspond to bright luminescence areas in size and shape and could potentially indicate H+ compensated [AlO4/M+]0 defects.

Design and performance of indium seals for size-constrained tunable laser spectrometers.

Indium seals have been used extensively in ultra-high vacuum and cryogenic applications. Typically, these seals use indium alongside or in place of other metal gaskets in stainless-steel vacuum flanges, with some custom applications for flanges sealing directly with glass (optics or tubes). Here, we present the design and performance of three pressed indium seals (99.99% In) between aluminum and 0.5 in. diameter sapphire optics and aluminum and gold coated Kovar semiconductor packages (TO-66 and TO-39). Test fixtures were designed to mimic those of future tunable diode laser spectrometers for Earth, planetary, and manned spaceflight environmental monitoring applications. Successful high-hermeticity seals [<10-10 atm cc/s (He)] were achieved for all seals formed with sufficient pressure applied to allow indium to flow between mating surfaces. The hermeticity of the seals was maintained after temperature cycling (-10 to +80 °C, 20 cycles), with the optical seals surviving extended duration tests (-55 to +85 °C, per MIL-STD-883). Semiconductor packages (TO-39) subjected to these extended tests saw a moderate increase in leak rate [∼5 × 10-9 atm cc/s (He)]; however, further testing showed that either the glass-metal package seals or the indium were affected (the sample size was too small to draw firm conclusions for future applications). Overall, these results suggest long-term survivability of indium seals for Kovar-aluminum and sapphire-aluminum interfaces [>10 years at 10-10 atm cc/s (He)], where the coefficient of thermal expansion differs by approximately four times.

Printing, Characterizing, and Assessing Transparent 3D Printed Lenses for Optical Imaging

AbstractHigh‐quality lens production has involved subtractive manufacturing methods for centuries. These methods demand specialist equipment and expertise that often render custom high‐grade glass optics inaccessible. A low‐cost, accessible, and reproducible method is developed to manufacture high‐quality three dimensional (3D) printed lenses using consumer‐grade technology. Various planoconvex lenses are produced using a consumer‐grade 3D printer and low‐cost spin coating setup, and printed lenses are compared to commercial glass counterparts. A range of mechanical and optical methods are introduced to determine the surface quality and curvature of 3D printed lenses. Amongst others, high‐resolution interference reflection microscopy methods are used to reconstruct the convex surface of printed lenses and quantify their radius of curvature. The optical throughput and performance of 3D printed lenses are assessed using optical transmissivity measurements and classical beam characterization methods. It is determined that 3D printed lenses have comparable curvature and performance to commercial glass lenses. Finally, the application of 3D printed lenses is demonstrated for brightfield transmission microscopy, resolving sub‐cellular structures over a 2.3 mm field‐of‐view. The high reproducibility and comparable performance of 3D printed lenses present great opportunities for additive manufacturing of bespoke optics for low‐cost rapid prototyping and improved accessibility to high‐quality optics in low‐resource settings.

Surface modification of nickel-phosphorus plating by Ti ion implantation for chalcogenide glass lens molding

Nickel-phosphorus (Ni-P) plating has been developed as an excellent mold material in precision glass molding (PGM) for silicate optical lenses. However, the elemental diffusion and adhesion problems hinder the application of the Ni-P mold in the molding process for infrared chalcogenide glass (ChG) optics. In this study, Ti ion implantation and annealing were proposed for the surface modification of Ni-P plating. The modification mechanism of the Ti-Ni-P plating was studied, and the Ti ion distribution was simulated using the Monte Carlo method. Several characterization methods were conducted to analyze the surface composition of Ti-Ni-P plating. The mechanical properties of the plating were tested before and after implantation, proving that ion implantation improved the surface hardness of the plating. Finally, molding experiments were carried out using the Ti implanted flat and structured molds. The molded glass does not undergo interfacial reaction, and has high structural replication accuracy and higher infrared transmittance. It is validated that Ti ion implantation has a positive effect on the inhibition of the interfacial reaction in ChG molding.

Two-photon polymerization of silica glass diffractive micro-optics with minimal lateral shrinkage.

Three-dimensional printing enables the fabrication of silica glass optics with complex structures. However, shrinkage remains a significant obstacle to high-precision 3D printing of glass optics. Here we 3D-printed Dammann gratings (DGs) with low lateral shrinkage (<4%) using a two-photon polymerization (2PP) technique. The process consists of two steps: patterning two-photon polymerizable glass slurry with a 515 nm femtosecond laser to form desired structures and debinding/sintering the structures into transparent and dense silica glass. The sintered structures exhibited distinct shrinkage rates in the lateral against longitudinal directions. As the aspect ratio of the structures increased, the lateral shrinkage decreased, while the longitudinal shrinkage increased. Specifically, the structure with an aspect ratio of approximately 60 achieved a minimal lateral shrinkage of 1.1%, the corresponding longitudinal shrinkage was 61.7%. The printed DGs with a surface roughness below 20 nm demonstrated good beam-shaping performance. The presented technique opens up possibilities for rapid prototyping of silica diffractive optical elements.

Development of a fabrication process for production of diffractive optics.

In this study, a novel fabrication process, to the best of our knowledge, was developed to fabricate a glass harmonic diffractive lens. In this process, a polymethylmethacrylate master of the diffractive lens was machined using single-point diamond turning. Then an electrolytic plating process was conducted to grow a reverse nickel (Ni) mold. Precision compression molding was performed using the Ni mold to replicate the diffractive lens structures onto a glass surface. Surface measurements and optical testing show that the replicated diffractive lenses by the proposed method have high tolerances and require optical performance, demonstrating a high-volume, high-precision, and cost-effective process. The proposed method will be critical for consumer products where glass optics are increasingly used in lens assemblies.

Fiber-Fed 3D Printing of Germanate Glass Optics

In recent years, 3D printing glass optics has gained massive attention in industry and academia since glass could be an ideal material to make optical elements, including the lens. However, the limitation of materials and printing methods has prevented 3D printing glass optics progress. Therefore, we have developed a novel printing strategy for germanate glass printing instead of pure silica. Moreover, compared with traditional multi-component quartz glass, germanate glass has unmatched advantages for its mid-infrared (MIR) transparency and outstanding visible light imaging performance. Furthermore, compared with non-oxide glass (fluoride glass and chalcogenide glass), germanate glass has much better mechanical, physical, and chemical properties and a high refractive index. Germanate glass has been widely applied in remote sensing, ranging, environmental detection, and biomedical detection. However, it is difficult to shape, cast, polish, and grind for optical and photonics applications such as imaging optics and laser-collimation optics. These drawbacks have made germanate glass inaccessible to complex optical elements and greatly increased their cost. In this report, we use germanate glass fibers with a diameter of 125 µm based on fiber-fed laser heating technology to fabricate an mm-size optical application. In this paper, we combine the fiber-fed laser heating technology with an optimized temperature control process to manufacture high-precision optical elements. Germanate glass optics can be printed with excellent visible light and IR transparency and a smooth surface with roughness under 4 nm. By optimizing the layer-by-layer 3D printing process and the thermal feedback in the printing process, we avoid cracks and minimize surface deformation. This work shows the possibility of the mm-size glass optical elements 3D printing and widens its application for IR optics.

Ultrasonic vibration–assisted magnetorheological hybrid finishing process for glass optics

Ultrasonic vibration–assisted magnetorheological hybrid finishing process for glass optics

Towards freeform manufacturing of ultra-low expansion glass optics

Ultra-Low-Expansion glass (ULE®) has become an important technological enabler of advanced imaging for astronomy and for extreme-UV lithography. A major limitation though, is that ULE® cannot be poured from the fluid state unlike ZERODUR® which renders costly to produce large and/or complex shapes from it. Beside mirrors, optical components are rarely made of ULE® despite it sharing many properties of pure fused silica glass. Here we explore how femtosecond laser processing combined with laser induced reflow can be used to structure ULE® glass with the goal of producing miniature optical components. To fulfil optical roughness requirements, we adopt a strategy based on first producing elementary shapes, such as cubes or cylinders, that we further topologically transform into sphere, ellipsoids or curved surfaces, using a laser-reflow process. The structural modification of the glass matrix induced by the reflow were investigated using Raman spectroscopy. Our result points to a densification of the glass but no apparent sign of crystallization or devitrification. Furthermore, to understand whether the thermo-mechanical properties were affected or not, the thermal expansion coefficient was estimated using a dilatometry technic based on a pseudo-bimorph micro-cantilevers in a temperature-controlled chamber.

Precision replication of all types of optical surfaces—scientific and technological basis for the radical transformation of modern optical production

Subject of study. The developments and proposed innovations of the Scientific and Production Association “State Institute of Applied Optics” in the field of advancement of precision replication technology of all types of optical surfaces (planar, spherical, aspheric, and freeform) aimed at ensuring the possibility of series and mass production of the so-called combined optical elements (COEs) and the large-scale fabrication of objectives operating in a wide spectral range for various purposes based on these elements are discussed. Aim of study. This study is aimed at the introduction of new proposals for the radical transformation of modern optical production based on advanced precision replication technology of all types of optical surfaces of lenses and mirrors to the Russian and foreign scientific and technological community and the executives and process engineers of optical production facilities. Method. The technology for shaping the working surfaces of optical elements of all types (lenses, mirrors, and prisms) is based on precision replication of their working surfaces using highly precise and certified master details in relatively thin polymer layers on substrates made of glass or other optical materials. The use of cold-curing resins, predominantly photopolymerizable compositions, is advisable. Main results. We demonstrate that a radical solution to the problem of establishing the series and mass production of optical elements with working surfaces of any type can be based on the precision replication technology using cold-curable polymer materials. The COEs obtained using this method comprise a glass base with one or two relatively thin polymer layers that have outer surfaces of a specified shape. Using low-shrinkage cold-curing polymer materials ensures the absence of thermal deformations of the COEs and their high reproducibility in each batch. Such elements have high mechanical, thermal, and optical properties of glass optics, and the cost of their series production is manyfold lower. We established that (when the technological regimen is followed) the replicated surfaces are identical to the working surface of the original master detail in terms of surface shape (N, ΔN), surface finish class (P), and roughness parameters (Rz, Ra). Moreover, the storability and resistance to environmental factors of the COEs are practically equal to those of their purely glass counterparts, which is confirmed by the results of multiple complex tests of different COE types over a period of several years. A hierarchic system of master details comprising reference, control, and working matrices is proposed. The reference matrices must always be produced in pairs of convex and concave matrices (similar to spherical test plates). Practical significance. Owing to a rapid increase in the demand in optoelectronic instrumentation engineering for objectives for different applications in a wide spectral range (smartphones, tablets, digital cameras, video projectors, analytical instruments, thermal imaging equipment, video surveillance and security systems, and spaceborne optical surveillance instruments), there has been an urgent need for a significant increase in the production capacity and cost reduction of the manufacturing of basic optical elements. Precision replication technologies provide the fundamental and practical possibility for solving this problem.

Simulation of the Refractive Index Variation and Validation of the Form Deviation in Precisely Molded Chalcogenide Glass Lenses (IRG 26) Considering the Stress and Structure Relaxation.

Precise infrared (IR) optics are core elements of infrared cameras for thermal imaging and night vision applications and can be manufactured directly or using a replicative process. For instance, precision glass molding (PGM) is a replicative manufacturing method that meets the demand of producing precise and accurate glass optics in a cost-efficient manner. However, several iterations in the PGM process are applied to compensate the induced form deviation and the index drop after molding. The finite element method (FEM) is utilized to simulate the thermomechanical process, predicting the optical properties of molded chalcogenide lenses and thus preventing costly iterations. Prior to FEM modelling, self-developed glass characterization methods for the stress and structure relaxation of chalcogenide glass IRG 26 are implemented. Additionally, a ray-tracing method is developed in this work to calculate the optical path difference (OPD) based on the mesh structure results from the FEM simulation. The developed method is validated and conducted during the production of molded lenses.

Enabling Sustainability in Glass Optics Manufacturing by Wafer Scale Molding

Numerous optical applications have rising demands for ever increasing quantities from lighting and projection optics for modern vehicles to home or street lighting using LED technology. Glass is the material of choice for most of those application fields. It has several advantages over polymers, including heat and scratch resistance as well as longevity and recyclability. Non-isothermal glass molding has become a viable hot forming technology for mass production of optics. The major challenge is enabling a scalable replication process allowing the optical glass elements to be manufactured with high form accuracy and at low-cost production with low reject rates. This work introduces recent developments in glass optics manufacturing that allow the fulfilment of seemingly contradicting criteria: the economic growth and the need for less consumption of resources and energy. While single cavity non-isothermal molding is state-of-the-art, a manufacturing innovation through wafer-scale molding enables an exponentially increasing number of optics to be produced per production shift, allowing a significant reduction of unit costs. In parallel, as multiple optics are produced in one manufacturing cycle, the energy consumption and the consequent CO2 emission can be reduced. In contrast, the technological development arises several challenges that will be discussed in this work. Besides the selection of suitable mold concepts and materials, the challenges also include the temperature control of the mold and the blank up to the optimization of flow and shrinking mechanisms of the glass during rapid forming. Another difficulty in the non-isothermal glass molding is to maintain the low form deviation required for precision optics, repeatability, and low failure rates through process optimization. Finally, detail calculations of cost, energy and CO2 consumption, in comparison with conventional fabrication of glass components using grinding and polishing as well as single cavity molding, will be demonstrated. The non-isothermal wafer-level glass molding is a new technological solution for the sustainable manufacturing of optics at large-scaled production.

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.

High-Precision Printing of Complex Glass Imaging Optics with Precondensed Liquid Silica Resin.

3D printing of optics has gained significant attention in optical industry, but most of the research has been focused on organic polymers. In spite of recent progress in 3D printing glass, 3D printing of precision glass optics for imaging applications still faces challenges from shrinkage during printing and thermal processing, and from inadequate surface shape and quality to meet the requirements for imaging applications. This paper reports a new liquid silica resin (LSR) with higher curing speed, better mechanical properties, lower sintering temperature, and reduced shrinkage, as well as the printing process for high‐precision glass optics for imaging applications. It is demonstrated that the proposed material and printing process can print almost all types of optical surfaces, including flat, spherical, aspherical, freeform, and discontinuous surfaces, with accurate surface shape and high surface quality for imaging applications. It is also demonstrated that the proposed method can print complex optical systems with multiple optical elements, completely removing the time‐consuming and error‐prone alignment process. Most importantly, the proposed printing method is able to print optical systems with active moving elements, significantly improving system flexibility and functionality. The printing method will enable the much‐needed transformational manufacturing of complex freeform glass optics that are currently inaccessible with conventional processes.

Understanding and reducing mid-spatial frequency ripples during hemispherical sub-aperture tool glass polishing

During sub-aperture tool polishing of glass optics, mid-spatial surface ripples are generated because of material removal non-uniformities during tool linear translation (resulting in feed ripples) and tool pathway step overlaps (resulting in pitch ripples). A variety of tool influence function (TIF) spots, trenches, and patches were created to understand and minimize such ripples on fused silica workpieces after polishing with cerium oxide slurry using a rotating hemispherical pad-foam tool. The feed ripple amplitude can be decreased by reducing the non-uniformities in the pad texture and/or by minimizing a derived feed ripple metric (rf=Vmax0.5Vf/Rt) via adjustments in processing parameters. Pitch ripples can be minimized by reducing relative step distance to spot radius ratio (xs/at) and by achieving a flat bottom trench shape cross section or by reducing the material removal per pass. Using the combined methods, an overall ripple error of ∼1.2nm rms has been achieved.

Tungsten carbide molds for precision glass molding process: Mechanism of high-temperature degradation

Tungsten carbide (WC) is a well-established material used in glass molding field due to its low thermal expansion and high compressive strength at elevated temperatures. During glass molding process, the WC mold has to withstand thousands of thermo-mechanical coupled interactions in the aerobic environment. Understanding the degradation mechanism of WC substrate during precision glass molding is very important for the design and preparation of protective coatings and the optimization of glass molding process. In this study, surface failure analysis of WC substrate was carried out using barrel compression test and ball-to-disk friction test. The microstructures, element compositions, surface morphology of the WC substrates were measured by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and white light interferometer (WLI). Damaging phenomena such oxidation, interdiffusion, spallation, glass sticking, and surface roughening on the WC substrate were observed. The experimental results showed that the WC substrate degraded very rapidly at above 600 °Cand air environment. The fundamental degradable reason of WC substrate is the oxidation, which directly leads to the decrease of the material strength, the aggravation of the wear, occurring of the glass sticking, and the increase of surface roughness. Furthermore, the WC mold microhole array without protective coatings can be used to fabricate the glass microlens array in a suitable glass molding environment. This work provides engineering guidance for the fabrication of glass optics during glass molding process.

Line contact ring magnetorheological finishing process for precision polishing of optics.

A line contact ring magnetorheological finishing (RMRF) process is conceptualized and developed for the finishing of precision optical surfaces. This tool is analogous to the cup wheel tool widely used in the optical industry for forming and fine grinding but semi-rigid in nature, thus removing high irregularities preferentially. This paper presents detailed simulations of the development of the new, to the best of our knowledge, RMRF tool and its experimental use and suitability in the polishing process. The line contact interaction between the workpiece and magnetorheological fluid present in the annular region is responsible for the finishing mechanism. The material removal rate and influence function of the RMRF process were calculated, which is essential and used for corrective polishing. The volumetric material removal rate was 0.0010406mm3/min with surface roughness of 5.94 nm; however, it can be increased with optimization of machine parameters such as gap, tool rotation, current, etc. Using this RMRF process, a polishing experiment was done over an NBK-7 glass workpiece of 40 mm aperture having 70 mm radius of curvature. Surface roughness improved down to 7 nm from 28.5 nm, and figure error improved down to 295 nm from 849 nm after four cycles of polishing. This investigation demonstrates the suitability of the RMRF process for precision polishing technology for glass optics.

Digitized, networked optics production

The increasing demands on the quality of high-precision optical components mean that established manufacturing processes are reaching their technical limits. While individual processes can therefore hardly be improved any more, networking production machines digitally throughout the entire process chain offers high potential for meeting the tight tolerances in the optics industry and increasing the production output.To gain these benefits, first, an infrastructure is needed, containing a flexible and module data structure to satisfy different requirements from the processes. Second, the idea of offering services such as initial data analytics, artificial intelligence (AI) and machine learning (ML) must be integrated. In this paper, we present a digital infrastructure, that contains data from the entire process chain of glass and polymer optics production including simulation, process and measurement data. Services to optimize the production process are integrated and provide an outlook on how AI/ML can be added.

Three-dimensional printing of glass micro-optics

To meet the increasing needs of high-precision glass micro-optics and address the major limitations of current three-dimensional (3D) printing optics, we have developed a liquid, solvent-free, silica precursor and two-photon 3D printing process. The printed optical elements can be fully converted to transparent inorganic glass at temperatures as low as 600 C with a shrinkage rate of 17%. We have demonstrated the whole process, from material development, printing, and performance evaluation of the printed glass micro-optics. 3D printing of glass micro-optics with isotropic shrinkage, micrometer resolution, low peak-to-valley deviation ( &lt; 100 n m ), and low surface roughness ( &lt; 6 n m ) has been achieved. The reported technique will enable the rapid prototyping of complex glass micro-optics previously impossible using conventional glass optics fabrication processes.

Omnidirectional high reflector at 650, 850, 1300 and 1550 nm for optical fiber communication by fused silica/YBCO-superconductor Octonacci photonic crystal

The transfer matrix method (TMM) is deployed to determine the reflectance spectra of Octonacci photonic structures. First, the structures are designed using the TiO2 and SiO2 slabs materials. This design allows the obtainment of limited high reflectors for some optical communication wavelengths and for both TE and TM polarizations. After that, the superconductor YBa2Cu3O7 (YBCO) and the silica slabs replace the previous materials in Octonacci structures. This modification permits to extend the omni-directional reflectance bands for both polarizations. The iteration number of the Octonacci sequence has an important effect on these bands. Later the geometric thickness of the photonic structure is optimized by changing the reference wavelength λ0 and omnidirectional high reflectors that cover all optical communication wavelengths (650, 850, 1300 and 1550 nm) are obtained. In addition, for these wavelengths, a tiny effect of the ambient temperature is noticed. These omnidirectional high reflectors can find application in glass (GFO) and plastic (PFO) fiber optics and may be used in sensing applications.