Chalcogenide glasses are primarily composed of elements from group VI of the periodic table, such as sulfur, selenium, and tellurium, and may include other non-oxide elements like germanium and arsenic. These glasses exhibit properties such as high refractive index (2-3.5), low phonon energy (<350 cm^-1), and tunable performance. As the most popular chalcogenide glasses, As-Se and As-S glasses are widely used in the fields of mid- and far-infrared imaging, supercontinuum generation, infrared laser power transmission, etc. However, the soluble impurity absorption and insoluble particle impurities inside the glass lead to a lot of trouble. In particular, particle-induced scattering is one of the key factors for the decrease of the glass transmittance. Such particles may be sourced from various conditions, such as the original presence of raw materials and introduction during glass preparation. So, it is urgent to find the real source and remove these impurities. For example, Nguyen et al. prepared As₂Se₃ fiber with the lowest loss below 1 dB/m, but found that there are a large number of submicron particles-Al₂SiO₃-inside the fiber, which enhances the scattering loss of the glass. To address this challenge, this work investigated the optical properties of glass by adding deoxidizers and filter sheets to the dynamic distillation process.

The glass was prepared using three-stage quartz ampoules. Filters with apertures of 250 μm, 160 μm, 100 μm and 10 μm were added to the multi-stage dynamic distillation process for removing particulate impurities from the glass. Metal Al as a deoxidizer was loaded in the ampoule during the distillation process for removing oxide impurities such as H₂O, O—H, etc. After the glass was ground and polished, the particles inside the glass samples were observed using a Dino Capture infrared camera and analyzed in the context of scattering theory to determine how the particles affect the transmittance of the glass; Then, transmittance maps were plotted to analyze the relationship between particles and transmittance of the glass prepared with different filter apertures.

The glass prepared using the same filter aperture was ground into thin slices of different thicknesses ranging from 2-20 mm to research the relationship between glass thickness, transmittance and cut-off wavelength. At the same time, As₂Se₃ glass with the highest purity in this study was loaded on the newly constructed CO₂ laser experimental platform for testing, and the relationship between the content of particles inside the glass and the CO₂ laser damage threshold of the glass was investigated.

High-purity, low-scattering As₂Se₃ glasses were prepared by adding filters as well as deoxidizers to the multistage dynamic distillation process. The transmittance of the glass samples obtained with a 250 μm pore size filter was only 43 % at 10.6 μm, while the transmittance obtained with a 160 μm pore size filter was increased to 57 %. When the pore size of the filter was further reduced to 100 μm, the transmittance reached to 59 %, and finally, the transmittance reached the highest level of 62 % at a pore size of 10 μm.

Additionally, the content of oxygen impurities (O—H, H₂O, Se—O) and heterogeneous impurities inside the glass was significantly reduced after purification. The difference in transmittance between samples with different thicknesses (from 2 to 20 mm) was not more than 2 %, suggesting that the glass has a high degree of homogeneity and reduced internal particle scattering.

Excluding the influence of external factors, the resistance of glass to laser damage is related to the glass component and the concentration of inclusions, under the premise of determining the glass component, it is necessary to improve the laser threshold by reducing the internal particles of the glass. The absorption of heat by particulate impurities within the glass affects its laser threshold, so the increase of the glass purity also increases the CO₂ laser damage threshold of the glass, specifically to 32.83 kW cm^-2 (Spot diameter of 0.025 cm).

The results of this study showed that the optical transmittance of the glass after filtration with a 10 μm pore size screen was close to the theoretical transmittance - 62.22 % - without taking into account any scattering losses. Meanwhile, the content of oxygen impurities (O—H, H₂O, Se—O) and heterogeneous impurities inside the glass has been significantly reduced. The difference in transmittance between samples with different thicknesses from 2 to 20 mm is not more than 2% with the reduced scattering inside the glass, while the preparation of ultra-low-scattering As₂Se₃ glass requires filters with smaller filter apertures. In summary, this study provides a new technique to further reduce the internal scattering of sulfur-based glass and to prepare ultra-low-loss sulfur-based optical fibers.