Multilayered chalcogenide glass with gradient index for reduced SWaP IR optical system

Gradient index (GRIN) lens could promote the lightweight and miniaturization of optical imaging system, but the development of IR GRIN lens is still in its infancy. A new series of As–S–Se chalcogenide glasses (ChGs) possessing similar glass transition temperature, excellent thermal stability, and large refractive index variation was developed, and these properties enabled them to become a good glass material catalog for co-molding multilayered GRIN IR lens. By employing precision molding, layer-stacked GRIN ChG was co-molded with a maximum refractive index variation of 0.47 at 4 μm, which was correlated to the variation of Raman intensity and elemental content. A mid-IR optical imaging system was designed and fabricated using the GRIN ChGs, and IR images were obtained. This multilayered GRIN ChG could lead to 18% smaller and 35% lighter SWaP IR optical system.     

Finite Element Calculation of Refractive Index in Optical Glass Undergoing Viscous Relaxation and Analysis of the Effects of Cooling Rate and Material Properties

In this research, a methodology based on a numerical simulation model is presented to predict refractive index change introduced to two low T_g optical glasses, namely, P-SK57 and P-LASF47, by cooling. To model the structural relaxation behavior of glass around glass transition temperature, the Tool–Narayanaswamy–Moynihan (TNM) model is used. In addition, the fictive temperature of the glass samples during cooling is discussed. The effect of cooling rate on the fictive temperature of the glass samples is also presented. This study demonstrated that finite element method is capable of predicting refractive index of optical glass undergoing viscous relaxation. The simulated results in this study confirm that a higher cooling rate leads to a lower refractive index and a larger variation of refractive index in glass optics. The results also suggest that for glass, materials with high thermal conductivity and low heat capacity are preferred for compression molding process.     

LED非球面透镜注射压缩成型工艺稳健性优化分析

采用稳健设计方法,以发光二极管（LED）非球面透镜折射率变化为实验目标,以压缩距离、压缩速度、压缩力、保压压力、熔体温度、冷却时间、注射速率为影响因素,设计了L27(37)正交试验矩阵,并对透镜的注射压缩成型过程采用Moldflow2015软件模拟分析。结果表明,最优工艺参数组合为A3B1C2D3E3F2G2;熔体温度对透镜折射率的影响最大,注射速率次之,压缩距离、冷却时间、保压压力影响较小,压缩速度、压缩力影响最小;优化后采用更高的熔体温度和较快的注射速率,有利于减小透镜的体积收缩,改善透镜内部残余应力,折射率变化减小了45 %,翘曲变形量减小了4.3 %。     

Modeling of thermo-viscoelastic material behavior of glass over a wide temperature range in glass compression molding

In glass compression molding, most current modeling approaches of temperature-dependent viscoelastic behavior of glass materials are restricted to thermo-rheologically simple assumption. This research conducts a detailed study and demonstrates that this assumption, however, is not adequate for glass molding simulations over a wide range of molding temperatures. In this paper, we introduce a new method that eliminates the prerequisite of relaxation functions and shift factors for modeling of the thermo-viscoelastic material behavior. More specifically, the temperature effect is directly incorporated into each parameter of the mechanical model. The mechanical model parameters are derived from creep displacements using uniaxial compression experiments. Validations of the proposed method are conducted for three different glass categories, including borosilicate, aluminosilicate, and chalcogenide glasses. Excellent agreement between the creep experiments and simulation results is found in all glasses over long pressing time up to 900 seconds and a large temperature range that corresponds to the glass viscosity of log (η) = 9.5 – 6.8 Pas. The method eventually promises an enhancement of the glass molding simulation.     

注塑成型光学制品折射率分层模拟与实验研究

与玻璃制造光学制品相比,高分子注塑光学制品具有质量轻、易加工、抗冲击性好等优点,在航空航天、精密透镜等高端领域得到广泛应用。然而受注塑过程复杂压力场、温度场的耦合作用,注塑透明制品折射率通常呈非均匀的分布,存在角偏差、光畸变等光学缺陷。因此,开展注塑工艺对其折射行为影响的模拟与实验研究,对实现折射率可控光学制品的成型具有重要意义。基于Hele Shaw注塑理论和Lorentz Lorenz物理光学理论,构建了注塑光学制品厚度方向折射行为分层预测模型,开发了相关模拟程序,基于自主研发的注塑模软件Z-Mold实现了注塑过程与折射率分布的一体化模拟。以聚碳酸酯注塑平板件为例,利用Brewster法对折射率模拟结果进行了验证。该方法成功应用到神舟系列航天舱外服面窗的研制。     

OPTICAL GLASS FROM THE VIEWPOINT OF THE LENS DESIGNER*

The glass used in the manufacture of lenses must be chemically stable and substantially free from bubbles, color, and striae. The lens designer is then concerned only with the refractive index and the dispersive power of the available glasses. Because practically every new lens must be achromatic, the positive elements used in it must be made from glass having a low dispersive power; the negative elements must have a relatively high dispersive power. The choice of refractive index is determined by many conflicting factors because the only “tools” available to the designer for the correction of many different aberrations are the radii of curvature of the lens surfaces, the thicknesses and air spaces, and the refractive indices of the glasses. The range of optical glasses commercially available and the reasons underlying the designer's selection of glass for different kinds of lenses are discussed.     

Refractive Index Drop Observed After Precision Molding of Optical Elements: A Quantitative Understanding Based on the Tool–Narayanaswamy–Moynihan Model

The room-temperature refractive index is measured for three different prior cooling rates (approximately 10, 50, and 250 K/h) for two glasses especially developed for precision molding. The empirical logarithmic relationship between the cooling rate and the refractive index is also reproduced for the comparatively high cooling rate of ca. 250 K/h. The same relationship is found in a simulation of these cooling rates by the semiempirical Tool–Narayanaswamy–Moynihan model for structural relaxation, with the necessary parameters obtained from differential scanning calorimetry and temperature jump experiments. The measured and the simulated refractive indices coincide within the limits of experimental error. The results demonstrate that the index drop, which is observed when these glasses are first cooled at a regular optical cooling rate (e.g., 2 K/h), and then precision molded (typical cooling rate 1000 K/h), can be understood considering the concepts of structural relaxation.     

Study on the blackening phenomenon of leaded glass during microgroove molding using nickel phosphorous mold

Precision glass molding (PGM) is a recently developed method to fabricate glass microgroove components. Lead glass is commonly used as an optical material due to its high refractive index and low transition temperature. A nickel-phosphorous (Ni–P) plated mold is traditionally employed in the PGM process for microstructures optics. However, leaded glass is subject to color change and can blacken during the PGM process, reducing the light transmittance of microgrooves. In this paper, an equation for the redox reaction between Ni and Pb is proposed, which is based on the diffusion of inner Ni atoms to the surface of the mold and the standard electrode potential of the Pb ions in leaded glass. A viscoelastic constitutive model of the glass is established to simulate the compression stress distribution during molding. Finally, the effects of molding pressure, molding temperature, and mold material on glass blackening are studied. The results show that the blackening of leaded glass is caused by Pb enriching the surface. The rise in molding stress and temperature increases the deformation of Ni–P plating, which promotes the diffusion of Ni atoms. By adding a titanium incorporated diamond-like carbon (Ti-DLC) coating, the deformation of the Ni–P plating during molding is suppressed, and the diffusion of Ni atoms can be prevented. In this way, the blackening of leaded glass can be prevented.     

Predicting and interpreting oxide glass properties by machine learning using large datasets

With the advent of powerful computer simulation techniques, it is time to move from the widely used knowledge-guided empirical methods to approaches driven by data science, mainly machine learning algorithms. We investigated the predictive performance of three machine learning algorithms for six different glass properties. For such, we used an extensive dataset of about 150,000 oxide glasses, which was segmented into smaller datasets for each property investigated. Using the decision tree induction, k-nearest neighbors, and random forest algorithms, selected from a previous study of six algorithms, we induced predictive models for glass transition temperature, liquidus temperature, elastic modulus, thermal expansion coefficient, refractive index, and Abbe number. Moreover, each model was induced with default and tuned hyperparameter values. We demonstrate that, apart from the elastic modulus (which had the smallest training dataset), the induced predictive models for the other five properties yield a comparable uncertainty to the usual data spread. However, for glasses with extremely low or high values of these properties, the prediction uncertainty is significantly higher. Finally, as expected, glasses containing chemical elements that are poorly represented in the training set yielded higher prediction errors. The method developed here calls attention to the success and possible pitfalls of machine learning algorithms. The analysis of the SHAP values indicated the key elements that increase or decrease the value of the modeled properties. It also estimated the maximum possible increase or decrease. Insights gained by this analysis can help empirical compositional tuning and computer-aided inverse design of glass formulations.     

Birefringence measurement for validation of simulation of precision glass molding process

During fabrication of glass lens by precision glass molding (PGM), residual stresses are setup, which adversely affect the optical performance of lens. Residual stresses can be obtained by measuring the residual birefringence. Numerical simulation is used in the industry to optimize the manufacturing process. Material properties of glass, contact conductance and friction coefficient at the glass‐mold interface are important parameters needed for simulations. In literature, these values are usually assumed without enough experimental justifications. Here, the viscoelastic thermo‐rheological simple (TRS) behavior of glass is experimentally characterized by the four‐point bending test. Contact conductance and friction coefficient at P‐SK57? glass and Pt‐Ir coated WC mold interface are experimentally measured. A plano‐convex lens of P‐SK57? glass is fabricated by PGM for two different cooling rates and whole field birefringence of the finished lens is measured by digital photoelasticity. The fabrication process is simulated using finite element method. The simulation is validated, for different stages of PGM process, by comparing the load acting on the mold and displacement of the molds. At the end of the process, the birefringence distribution is compared with the experimental data. A novel plotting scheme is developed for computing birefringence from FE simulation for any shape of lens.     

注射压缩成型LED透镜光学性能分析

针对注射压缩成型的发光二极管（LED）透镜,采用L18(37)正交实验矩阵及模拟优化获得A3B2C2D2E2F3G1最佳成型工艺参数组合,并对透镜进行了光学性能测试。结果表明,工艺参数中的压缩速度增大和注射速率降低,透镜的内部残余应力和折射率变化减小而光学性能提高;采用热流道注射压缩成型的透镜与冷流道的相比,折射率变化降低,冷流道透镜的透光率和最大光强为93.1 %和590.3 cd,热流道透镜的的透光率和最大光强为97.5 %和665.3 cd,热流道透镜的透光率和光强分别比冷流道的提高4.4 %和75 cd,热流道注射压缩成型加工的LED透镜在光学性能方面比冷流道的优越。     

Optical properties,thermal stability,and forming region of high refractive index La2O3–TiO2–Nb2O5 glasses

The high refractive index La₂O₃–TiO₂–Nb₂O₅ glasses were prepared by containerless processing, and the glass‐forming region was determined. The refractive index showed the range from 2.20 to 2.32, and the values were much higher than those of most optical glasses. The completely miscible 30LaO_3/2–(70?x)TiO₂–xNbO_5/2 (0 ≤ x ≤70) system was fabricated to study the compositional dependence of refractive index and optical transmittance. The crucial determinants of the refractive index of oxide glasses, oxygen molar volume, and electronic polarizability of oxygen ions were calculated. The principle of additivity of glass properties was suitable for the calculation of refractive index between glass and compositional oxides. All the glasses were colorless and transparent in the visible to 6.5 μm middle infrared (MIR) region. These results are useful for designing new optical glasses with high refractive index and low wavelength dispersion in wide optical window.     

Stress Relaxation and Refractive Index Change of As2S3 in Compression Molding

As₂S₃ is one of the chalcogenide glasses that have attracted increasing interests for compression molding applications. This article aimed to evaluate the stress relaxation behavior of As₂S₃ above its glass transition temperature and calculate its refractive index change during cooling. First, creep tests were conducted with cylinder glass specimens at three different temperatures, in order to deduce the shear stress relaxation function by using the relationship with creep compliance function. In addition, the shift factor for thermo-rheological simplicity using Williams–Landel–Ferry equation was obtained. Then, finite element simulation was implemented to verify the calculated shear stress relaxation function. The acquired shear stress relaxation function needs to be modified to compensate the influence of friction on the thickness change in the experiments, so that the simulation results using the modified shear stress relaxation would match the experiments better. Finally, the refractive index changes of As₂S₃ at different cooling rates were modeled by using the Tool–Narayanaswamy–Moynihan (TNM) model for structural relaxation behavior. It is confirmed that the slower the cooling rate is, the less the refractive index drop will be. It was also demonstrated that the refractive index drop is strictly dependent on the cooling rate logarithmically by using TNM model. In summary, the results presented in this article can provide reliable references for viscoelastic characterization of As₂S₃ glass, crucial for compression molding or similar applications.     

分层反压注射成型厚壁光学透镜的工艺分析

为了研究反压压力对成型厚壁光学透镜的影响,利用自行开发的分层反压注射成型厚壁光学透镜的实验方案,通过正交实验优化设计开展了工艺参数优化实验,研究了在不同分层方式下内外层反压压力对对厚壁光学透镜光学性能的影响,从而根据正交优化参数组合成型具有优异光学性能的理想厚壁光学透镜。 正交实验研究结果表明,使用聚甲基丙烯酸甲酯（PMMA）为厚壁光学透镜的原材料进行分层反压注射成型,能够有效减少厚壁光学透镜收缩率,同时随着内外层反压压力的提高,厚壁光学透镜的透光率和折射率均不同程度的得到提高,透镜光学性能优异。     

A New Class of Glasses for Double-Crucible Optical Fibers with High Numerical Apertures

A series of boron- and germanium-free silicate glasses was developed for the core glass of high-numerical-aperture double-crucible optical fibers. When combined with a boro-silicate cladding glass from a series also investigated in this study, fibers with a numerical aperture in the range 0.3 to 0.5 can be produced. The relatively high refractive indices in the core glasses are achieved by including large mole fractions of BaO, CaO, and ZnO, accompanied in some cases by smaller additions of ZrO₂ and/or Y₂O₃ in the glass composition. Measurements of the refractive index, density, Rayleigh scattering coefficient, and viscosities and observations of the devitrification behavior and chemical durability of the glasses are presented. Attenuation spectra of suitable core glasses and of double-crucible optical fibers comprised of different core and cladding combinations yielding high numerical apertures are given. The fibers have attenuations near 10 dB/km at 850 nm.     

Optical properties of bismuth tellurite based glass

A series of binary tellurite based glasses (Bi(2)O(3))(x) (TeO(2))(100-) (x) was prepared by melt quenching method. The density, molar volume and refractive index increase when bismuth ions Bi(3+) increase, this is due to the increased polarization of the ions Bi(3+) and the enhanced formation of non-bridging oxygen (NBO). The Fourier transform infrared spectroscopy (FTIR) results show the bonding of the glass sample and the optical band gap, E(opt) decreases while the refractive index increases when the ion Bi(3+) content increases.     

Investigation of europium-doped aluminium phosphate glass for red light generation

Europium doped phosphate glasses (KPEu) with various concentrations of Eu₂O₃ were prepared by a melt-quenching method for red light application. The prepared samples were characterized using physical, absorption, photoluminescence, X-ray induced luminescence (XEL) and J-O analysis techniques. Density, molar volume and refractive index of the investigated glasses increased with increasing concentration of Eu₂O₃. Eight absorption peaks were observed, with six peaks in the visible region (350–550) nm and two in the infrared region (2000–2200). Four emission peaks were observed in the visible region when the glass was excited at 394 nm. Judd-Ofelt (JO) intensity parameters (Ω_λ λ = 2, 4, 6) were determined using the JO-theory for the prepared glass samples. The radiative properties of the glasses were determined using JO-parameter, refractive index and emission spectra. The trend of JO parameters for the present KPEu₆ glass sample were Ω₂ > Ω₄ > Ω₆. These glasses also showed higher branching ratios and stimulated emission cross section (σ_e) values, making these suitable for red laser applications. XEL showed the same peaks and trend as emission spectra but its peaks were shifted and had high intensity. The luminescence colours of these glass samples entirely fall in the reddish orange region (x = 0.653–0.654, y = 0.346–0.347) as confirmed by CIE 1931 diagram. The CCT values of the prepared glass samples were in the range of 2394–2431 K.     

Fine Annealing of Optical Glass

Since the most important characteristic of optical glass is the uniformity of its refractive index, which is itself a function of fictive temperature, any fine annealing treatment must be such as to bring about substantial uniformity in fictive temperature. Methods of accomplishing this are reviewed. In terms of total required time in the annealing process, the most efficient method consists of soaking the glass for a short time at a temperature near the annealing point and cooling at a constant rate to a temperature below the annealing range and at an increased rate to room temperature. Formulas are given for specifying the parameters in the annealing schedule in terms of the final condition of the glass as to stress and index. The several outstanding advantages of this method of annealing are discussed.     

Optical glass with refractive index of 1.75 and high specific refraction for spectacle optics

The interrelation between the chemical composition of spectacle glass and its optical characteristics is considered. A correlation dependence of different cations on the refractive index is established. The possibility of directed synthesis of new glasses with specified optical characteristics is established.Translated from Steklo i Keramika, No. 6, pp. 13–16, June, 1995.