Abstract

Subject of study. This paper discusses the main stages of the fabrication of porous silicate matrices composed of two-phase borosilicate glass and how they affect the optical quality of the fabricated samples, as well as how to quantitatively characterize the optical inhomogeneity of the porous matrices, caused by nonuniformity of the pore distribution over the volume of the sample. How the geometry of the chemical processing of the samples is associated with their optical quality is also studied. Method. Digital holographic interferometry implemented on a laboratory-fabrication test stand is used to monitor the optical homogeneity. A technique is worked out to obtain interferograms that characterize the state of the sample in the form of a plane-parallel plate at the preform stage and after acid and alkali processing, as well as a technique for analyzing the experimental data. The quantitative results obtained at the early fabrication stages of the matrices were compared on the same section of the sample (to within 0.1 mm). The spectral transmission of the samples was measured in the 350–100-nm region. The free volume of the sample occupied by pores was determined by weighing, using the mass of the sample in the air-dried state and with the pores filled with water. Main results. A technique was developed for monitoring the optical quality of nanoporous silicate matrices, using a laboratory test stand for digital holographic interferometry. The optical homogeneity of a porous sample was monitored by diagnosing the relative variations of the sample’s optical thickness (the product of the refractive index times the sample’s geometrical thickness). These variations in the blank occur because the sides of the sample are not parallel. They are caused in a porous matrix both by the nonparallelness of the sides of the blank and by the inhomogeneity of the spatial variations of the refractive index due to inhomogeneous distribution of the pores in the volume of the sample. Based on this, quantities are introduced that characterize the optical quality of the blank and of the porous matrix. The optical quality of a plane-parallel blank was determined in this paper from a sample with nonparallel sides, while the optical quality of the nanoporous structure was determined from the refractive-index variation on a given section. It is shown that the orientation of a sample relative to the direction of the gravitational field during chemical processing significantly affects its optical quality. It is experimentally established that, when plane-parallel samples are oriented perpendicular to the gravitational field during chemical processing, it is possible to guarantee the optical quality of the porous matrices in the region where they are transparent, compared with the optical quality of the blank. Practical significance. A technique is presented that makes it possible to quantitatively characterize the optical inhomogeneity on a given section of a porous sample in the region where it is transparent and to separate the amount by which the quality of the blank and the optical inhomogeneity of the porous structure contribute to the optical quality of the fabricated nanoporous matrices. Our experiments showed that nanoporous silicate matrices that are transparent in the visible and near-IR regions and that are horizontally oriented during chemical processing possess optical homogeneity comparable with the optical quality of the blank. The technique for monitoring the quality of the samples can be used when developing and optimizing regimes for obtaining nanoporous matrices from two-phase glasses, intended for creating optical elements based on them that meet specifications in optical homogeneity.

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