Fabrication of sol-gel derived homogeneously doped Er3+/Yb3+:SiO2 microspheres using laser melting

Silica microspheres have been demonstrated as the optical micro-cavity for laser generation based on whispering-gallery modes (WGM). They can achieve a high-quality factor (Q) within a tiny volume. Traditionally, complex processes are needed to process a coating containing photoluminescent elements, with enough thickness and uniformity, for laser generation. We developed a novel sol-gel fabrication method of Er³⁺/Yb³⁺doped silica microspheres with a homogeneous doping concentration over the entire microsphere volume. The sol-gel precursors were doped with 1–2 mol% Er³⁺ and/or Yb³⁺. The viscosity of the precursor was controlled at around 4000 cP, and gel fibers can be conveniently drawn from the precursor. After firing at 1000 °C for 1 h, transparent fibers with diameters of 40–180 µm were obtained. The fiber tips were quickly melted into microspheres using a CO₂ laser. The diameters of microspheres were determined by the fiber diameter and laser parameters, such as laser power and irradiation time duration. Typically, the microspheres had diameters between 90 to 160 µm. The emission spectrum under 357 nm and 527 nm excitation showed characteristic Er³⁺ emission peaks, that match the literature well. The UV-VIS spectra confirm the photoluminescence results and showed both Er³⁺ and Yb³⁺ characteristic absorptions. The optical behaviors of the microspheres indicate that the Er³⁺ and Yb³⁺ were well dispersed in the silica matrix and the microspheres had typical optical activities of Er³⁺/Yb³⁺ glasses.

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