Raman investigation of germanium- and phosphorus-doping effects on the structure of sol–gel silica-based optical fiber preforms

Yb2+-Doped Silicate Glasses as Optical Sensor Materials for Cryogenic Thermometry

Optical sensors constitute attractive alternatives to resistive probes for the sensing and monitoring of temperature (T). In this work, we investigated, in the range from 2 to 300 K, the thermal behavior of Yb2+ ion photoluminescence (PL) in glass hosts for cryogenic thermometry. To that end, two kinds of Yb2+-doped preforms, with aluminosilicate and aluminophosphosilicate core glasses, were made using the modified chemical vapor deposition (MCVD) technique. The obtained preforms were then elongated, at about 2000 °C, to canes with an Yb2+-doped core of about 500 µm. Under UV excitation and independently of the core composition, all samples of preforms and their corresponding canes presented a wide visible emission band attributed to Yb2+ ions. Furthermore, PL kinetics measurements, recorded at two emission wavelengths (502 and 582 nm) under 355 nm pulsed excitation, showed an increase, at very low T, followed by a decrease in lifetime until room temperature (RT). A modified two-level model was proposed to interpret such a decay time dependence versus T. Based on the fit of lifetime data with this model, the absolute (Sa) and relative (Sr) sensitivities were determined for each sample. For both the preform and its corresponding cane, the aluminophosphosilicate glass composition featured the highest performances in the cryogenic domain, with values exceeding 28.3 µsK-1 and 94.4% K-1 at 30 K for Sa and Sr, respectively. The aluminophosphosilicate preform also exhibited the wider T operating range of 10-300 K. Our results show that Yb2+-doped silicate glasses are promising sensing materials for optical thermometry applications in the cryogenic domain.

Investigation of the Incorporation of Cerium Ions in MCVD-Silica Glass Preforms for Remote Optical Fiber Radiation Dosimetry

The incorporation of Ce³⁺ ions in silicate glasses is a crucial issue for luminescence-based sensing applications. In this article, we report on silica glass preforms doped with cerium ions fabricated by modified chemical vapor deposition (MCVD) under different atmospheres in order to favor the Ce³⁺ oxidation state. Structural analysis and photophysical investigations are performed on the obtained glass rods. The preform fabricated under reducing atmosphere presents the highest photoluminescence (PL) quantum yield (QY). This preform drawn into a 125 µm-optical fiber, with a Ce-doped core diameter of about 40 µm, is characterized to confirm the presence of Ce³⁺ ions inside this optical fiber core. The fiber is then tested in an all-fibered X-ray dosimeter configuration. We demonstrate that this fiber allows the remote monitoring of the X-ray dose rate (flux) through a radioluminescence (RL) signal generated around 460 nm. The response dependence of RL versus dose rate exhibits a linear behavior over five decades, at least from 330 µGy(SiO₂)/s up to 22.6 Gy(SiO₂)/s. These results attest the potentialities of the MCVD-made Ce-doped material, obtained under reducing atmosphere, for real-time remote ionizing radiation dosimetry.

F/Yb-codoped sol-gel silica glasses: toward tailoring the refractive index for the achievement of high-power fiber lasers

Accurate control of both the doping distribution inside the fiber core and the low refractive index contrast between the fiber core and cladding materials is essential for the development of high-power fiber lasers based on the use of single-mode large-mode-area (LMA) optical fibers. Herein, sol-gel monolithic F/Yb³⁺-codoped silica glasses were prepared from porous large silica xerogels doped with ytterbium salt solution, which had been subjected to fluorination with hexafluoroethane gas, before subsequent sintering. The fluorine content inside the doped glass has been varied by adjusting the fluorination duration. The space homogeneity of fluorine and ytterbium concentrations in the cylindrical preforms has been checked by chemical analysis and Raman spectroscopy. Moreover, the glass with the lowest fluorine content has been successfully integrated as a core material in a microstructured optical fiber made using the stack-and-draw method. This fiber was tested in an all-fiber cavity laser architecture to evaluate potential lasing performances of the F/Yb³⁺-codoped silica glass. It presents a maximum efficiency of 70.4%, achieved at 1031 nm from a 1.16 m length fiber. These results confirm the potentialities of the obtained F/Yb³⁺-codoped glasses for the fabrication of LMA optical fiber lasers.