Investigation of the refractive index distribution in precision compression glass molding by use of 3D tomography

Abstract

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. In the compression molding process, refractive index variation is often introduced in glass during cooling. The variation of the refractive index in a molded glass lens is an important parameter that can influence its optical performance, especially for lenses used in high precision optical systems. In this research, the three-dimensional (3D) refractive index distribution in molded glass lenses was studied using an optical setup based on the Mach–Zehnder interferometer and filtered back-projection reconstruction algorithm. Specifically, phase information was first evaluated by measuring interferometric fringes using a digital camera at pre-determined discrete locations. The 3D refractive index distribution inside the molded glass lens was then calculated from the phase information.

Annealing of Compression Molded Aspherical Glass Lenses

“…Refractive index of the glass lens was measured by an optical setup based on Mach-Zehnder interferometer [7,18]. The schematic of the optic setup is shown in Fig.…”

“…However, a small amount of residual stresses [5] and refractive index drop [6,7] were often observed in molded glass lenses. Residual stresses inside a glass lens can cause birefringence and refractive index variation.…”

“…The residual stresses were studied by a circular polariscope based on the property of birefringence of a glass lens [14][15][16][17]. The refractive index variation was measured and reconstructed by an optical setup based on MachZehnder interferometer [7,18]. By comparing the experimental results of pre-and postannealing experiments, it was discovered that both residual stresses and refractive index variation were reduced.…”

Alternative phase-shifting technique for measuring full-field refractive index

“…Currently, some methods have been proposed to measure full-field refractive index, such as the interferometric methods, [1][2][3][4][5][6] digital holographic microscopy, 7 and ellipsometric methods. 8 In the interferometric methods, the phase difference is relative to the refractive index.…”

Reducing Refractive Index Variations in Compression Molded Lenses by Annealing

“…Compression molding is a thermal forming process for precision glass optics [1,2]. However, refractive index variation was induced in glass during cooling when the glass material went through its glass transition region [3][4][5]. On the other hand, the variation of refractive index in a molded glass lens will introduce distortion to the wave-front passing through the glass lens, which leads to image quality degradation.…”