Research Of Refractive Index Variation And Its Correction In Precision Molded Glass Optics

Precision glass compression molding is an alternative and creative manufacturing method of fabricating high-quality glass components. It has numerous advantages such as high efficiency, low cost, and being environmentally conscious. It is also an inherent manufacturing process for glass aspherical lenses. As a critical part of compression molding process for precision glass optics, the molds are required to have extremely accurate surface topography and low surface roughness, from which the molded glass can be used directly without finishing process. Therefore, this technology has attracted increasing attention in both scientific and industrial communities.During molding, particularly during cooling, some properties of glass materials undergo minute change, thus the optical characteristics of molded glass lens will be affected. A series of studies on precision molding process of optical glasses are described in this dissertation. By combining experiments with simulation, refractive index and density in molded are explored, specifically, how these parameters change, what mechanism behind these changes and how these changes influence the optical performance are studied in detail. A possible correction method of refractive index variation in molded glass lenses is also explored.In order to study the distribution of refractive index in precision molded glass lens after cooling, precision thermal treating experiments with different conditions are carried out in a precision glass molding machine, and a group of glass samples are obtained. Following, the computed tomography is employed as a nondestructive measurement method for refractive index measurement in precision molded glass lenses, and a 3D refractive index distribution map of the index variation is identified.An optical measurement system based on Mach-Zehnder interferometer is built in this research. A series of fringe patterns that are measured using lasers of different wavelengths are collected. An image processing system is programmed to extract the wavefront information from the fringe patterns collected. From the wavefront information, the refractive index distribution in the glass lenses is reconstructed slice by slice using filtered back-projection algorithm, and a 3D refractive index distribution in molded glass is obtained finally. This is a new technology with high accuracy of measuring refractive index in glass lenses. In this research, the standard deviation of refractive index reconstruction can be as low as 5.5X10-5.Based on the measurement and reconstruction methodology, distribution rules of refractive index in precision molded glass are studied, and the factors that affected refractive index variations are also analyzed. These studies confirmed that refractive index variation in molded glass is mainly influenced by cooling rate. Higher cooling rate and shorter wavelength result in larger refractive index variation and lower index dispersion. When glass lens is heated up above Tg, the influence of soaking temperature on refractive index variation is insignificant. Experiments on glass lenses with different sizes that are processed under the same thermal conditions are also carried out, and the results showed that refractive index variation is higher in larger size glass lenses, demonstrating a slight size affect on index variation.Finite element analysis of precision glass molding process is also carried out based on Tool-Narayanaswamy model to understand the mechanism of refractive index variation in molded glass optics. Thermal deformation of glass lens during thermal processing is investigated, and "density drop" and "density variation" phenomenon are discovered by numerical simulation. Contrast analysis on density distribution and refractive index distribution indicate that density variation is a main inducing factor of refractive index variation in molded glass.Influence of refractive index drop and refractive index variation on optical performance, such as spherical aberration, maximum optical path difference and optical modulation transfer function, are studied also. A possible correction method of refractive index is also introduced. Based on ray-tracing simulation, a method of pre-compensation on precision molded glass lens geometry is proposed. Simulation results show that, optical aberration influenced by refractive index variation can be corrected effectively by optimizing the conic surface parameters of the glass lens in design stage.