Search Results (12)

Over the past 18 months, we have performed hundreds of temperature characterizations of fiber Bragg gratings inscribed in different germanium-doped silica glass fibers. Under experimental conditions, the main conclusions are as follows: the temperature dependence of the “temperature gauge factor” or the normalized temperature sensitivity, K_T, was found to be quadratic in the −50–200 °C range, while it may be considered linear for the −20–100 °C range; K_T values at 20 °C vary from 5.176 × 10⁻⁶ K⁻¹, for a B/Ge co-doped fiber up to 6.724 × 10⁻⁶ K⁻¹, for a highly Ge-doped fiber; K_T does not depend on the hydrogen-loading process or the gratings coupling strength; K_T is essentially independent of wavelength in the 1500–1600 nm range, its value being accurately determined with a relative error ~0.2%; based on the accurate value of K_T = 6.165 × 10⁻⁶ K⁻¹, at 20 °C, obtained for gratings inscribed in the SMF-28 fiber, we calculated a value of 19.4 × 10⁻⁶ K⁻¹ for the thermo-optic coefficient of bulk germanium glass; and gratings produced by femtosecond-laser radiation and UV-laser radiation exhibit comparable values of K_T. The previous achievements allow, by having knowledge of K_T for a single grating, the accurate determination of the temperature dependence of the Bragg wavelength for any other grating inscribed in the same fiber; the presented methodology enables one to determine the “unknown” gratings’ temperature sensitivity, typically with an error of 0.01 pm/°C, being, therefore, very useful in research labs and computer simulations. Thus, expressions for the temperature dependence of K_T for gratings inscribed in several fibers are given, as well as an expression for K_T as a function of the effective refractive index. We have also fully analyzed the potential sources of error in K_T determination. Full article

Bi-doped glasses and fibers have been widely applied in solid-state and fiber lasers. However, the mechanism underlying near-infrared (NIR) luminescence remains unclear, and Bi-related luminescence centers (BLCs) are prone to alteration during fiber fabrication, making it challenging to achieve high-performance Bi-doped glass fibers. In this work, Bi-, Bi-Al-, and Bi-Ge-doped silica glasses were investigated to elucidate the origin of NIR luminescence. Two broad NIR fluorescence bands were observed in silica glasses, originating from distinct BLCs. The longer-wavelength fluorescence band at 1423 nm, demonstrating sensitivity to Bi doping concentration and homogeneity, is attributed to Bi clusters (aggregates of Bi⁺ ions), whereas the shorter-wavelength emission, independent of Bi concentration, originates from isolated Bi⁺ ions. A vacuum-assisted melting-in-tube method with a single-step heating process was employed to fabricate Bi-doped silica-based glasses and fibers. The fluorescence bands of the fibers remained consistent with those of the precursor glasses, indicating no new BLCs were formed during fiber fabrication. The modulation of fluorescence bands was primarily governed by Bi cluster formation. Suppressing Bi clustering through co-doping with Al/Ge or optimizing fabrication conditions offers an effective route to tailor the fluorescence properties of Bi-doped glasses and fibers. Full article

In this paper we derived an expression that allows the determination of the thermo-optic coefficient of weakly-guiding germanium-doped silica fibers, based on the thermal behavior of optical fiber devices, such as, fiber Bragg gratings (FBGs). The calculations rely on the full knowledge of the fiber parameters and on the temperature sensitivity of FBGs. In order to validate the results, we estimated the thermo-optic coefficient of bulk GeO₂ glass at 293 K and 1.55 μm to be 18.3 × 10⁻⁶ K⁻¹. The determination of this value required to calculate a correction factor which is based on the knowledge of the thermal expansion coefficient of the fiber core, the Pockels’ coefficients (p₁₁ = 0.125, p₁₂ = 0.258 and p₄₄ = −0.0662) and the Poisson ratio (ν = 0.161) of the SMF-28 fiber. To achieve that goal, we estimated the temperature dependence of the thermal expansion coefficient of GeO₂ and we discussed the dispersion and temperature dependence of Pockels’ coefficients. We have presented expressions for the dependence of the longitudinal and transverse acoustic velocities on the GeO₂ concentration used to calculate the Poisson ratio. We have also discussed the dispersion of the photoelastic constant. An estimate for the temperature dependence of the thermo-optic coefficient of bulk GeO₂ glass is presented for the 200–300 K temperature range. Full article

The efficiency of thin film photoelectric devices can be improved by redirecting incident electromagnetic radiation along their surface layers. Redirection can be achieved using antireflection coatings made of subwavelength dielectric particle arrays. In this study, we fabricate such coatings, consisting of Ge particles on quartz glass substrates via solid-state dewetting, transforming thin Ge films into compact particles. Using optically transparent substrates, we measure reflection and transmission spectra and determine absorption spectra, showing that substrates coated with Ge particles absorb much more strongly than substrates coated with continuous Ge films. The spectra obtained using objective lenses with different aperture angles indicate that scattered radiation is predominantly directed at glancing angles to the substrate surface. The lateral propagation of scattered radiation is the result of destructive interference, which suppresses both reflected and transmitted radiation. Full article

In recent years, the use of optical methods for temperature measurements has been attracting increased attention. High-performance miniature sensors can be based on glass microspheres with whispering gallery modes (WGMs), as their resonant frequencies shift in response to the ambient parameter variations. In this work, we present a systematic comprehensive numerical analysis of temperature microsensors with a realistic design based on standard silica fibers, as well as commercially available special soft glass fibers (GeO₂, tellurite, As₂S₃, and As₂Se₃). Possible experimental implementation and some practical recommendations are discussed in detail. We developed a realistic numerical model that takes into account the spectral and temperature dependence of basic glass characteristics in a wide parameter range. To the best of our knowledge, spherical temperature microsensors based on the majority of the considered glass fibers have been investigated for the first time. The highest sensitivity dλ/dT was obtained for the chalcogenide As₂Se₃ and As₂S₃ microspheres: for measurements at room temperature conditions at a wavelength of λ = 1.55 μm, it was as high as 57 pm/K and 36 pm/K, correspondingly, which is several times larger than for common silica glass (9.4 pm/K). Importantly, dλ/dT was almost independent of microresonator size, WGM polarization and structure; this is a practically crucial feature showing the robustness of the sensing devices of the proposed design. Full article

Nanogratings (NGs) are self-assembled subwavelength and birefringent nanostructures created by femtosecond laser direct writing (FLDW) in glass, which are of high interest for photonics, sensing, five-dimensional (5D) optical data storage, or microfluidics applications. In this work, NG formation windows were investigated in nine commercial glasses and as a function of glass viscosity and chemical composition. The NG windows were studied in an energy—frequency laser parameter landscape and characterized by polarizing optical microscopy and scanning electron microscopy (SEM). Pure silica glass (Suprasil) exhibits the largest NG window, whereas alkali borosilicate glasses (7059 and BK7) present the smallest one. Moreover, the NG formation windows progressively reduced in the following order: ULE, GeO₂, B33, AF32, and Eagle XG. The NG formation window in glasses was found to decrease with the increase of alkali and alkaline earth content and was correlated to the temperature dependence of the viscosity in these glasses. This work provides guidelines to the formation of NGs in commercial oxide glasses by FLDW. Full article

The properties of specialty optical fiber technology are determined by many aspects, such as the choice of material from which optical fibers are made, the refractive index profile, or the optical fiber manufacturing method. Typical optical fibers are made from ultrapure silicon dioxide (SiO₂), called fused silica, in a process called chemical vapor deposition (CVD). The differences between refractive indexes are most often obtained by doping silica glass with selected inorganic compounds, mainly germanium dioxide (GeO₂), which increases the refractive index, and fluorine, which lowers it accordingly. The proper design of the dopant profile in the optical fiber core and in the layer surrounding the core is crucial for nonlinear optical fibers with shaped dispersion characteristics. Such optical fibers can be used for the generation of nonlinear phenomena, such as supercontinuum generation (broadband light source) or soliton self-frequency shift. As part of the research, structures of nonlinear optical fibers with flattened normal dispersion in the near-infrared range were designed through the use of numerical simulations in COMSOL Multiphysics. The theoretical chromatic dispersion characteristics and dependence of the effective mode field area on the wavelength were obtained from the theoretical structures. Based on the designed optical fiber structures, a series of nonlinear optical fibers were produced, which were characterized by a high concentration of GeO₂ in the core and the presence of a fluorine-doped layer around the core. The influence of geometrical parameters, e.g., the width of the fluorine-doped layer (ratio of the radius of the fluorosilicate layer to the radius of the core), and the imperfections resulting from the technological aspects on the optical properties of manufactured optical fibers, with a particular emphasis on chromatic dispersion and the effective mode field area, was determined experimentally. Theoretical optical fiber models, along with their calculated properties (chromatic dispersion and effective mode field area), were compared with real measurements. Full article

LaF₃ is commonly added to oxide glass, in particular to silica, to form oxyfluoride glass. After appropriate thermal treatment at a temperature lower than 800 °C, usually, glass ceramics are obtained. Recently, LaF₃ nanoparticles have been used as precursors to obtain amorphous nanoparticles of undefined composition in optical fiber. However, fiber fabrication necessitates temperature much higher (typically up to 2000 °C) than the one required for bulk glass. In this article, we report on the reactivity of fluoride ions in LaF₃ with SiO₂ and GeO₂ (a common dopant used to dope optical fiber) powders at high temperature. TGA, EDX-SEM, XRD and Raman analyses were performed. Above 1000 °C, LaF₃ starts to react, preferentially with SiO₂, to form SiF₄ gaseous species. The remaining lanthanum ions form La₂Si₂O₇ and La₂Ge₂O₇ phases. These results could contribute to improve material development for the fiber optics community. Full article

The quality of treatment delivery as prescribed in radiotherapy is exceptionally important. One element that helps provide quality assurance is the ability to carry out time-resolved radiotherapy dose measurements. Reports on doped silica optical fibers scintillators using radioluminescence (RL) based radiotherapy dosimetry have indicated merits, especially regarding robustness, versatility, wide dynamic range, and high spatial resolution. Topping the list is the ability to provide time-resolved measurements, alluding to pulse-by-pulse dosimetry. For effective time-resolved dose measurements, high temporal resolution is enabled by high-speed electronics and scintillator material offering sufficiently fast rise and decay time. In the present work, we examine the influence of Ge doping on the RL response of Ge-doped silica optical fiber scintillators. We particularly look at the size of the Ge-doped core relative to the fiber diameter, and its associated effects as it is adjusted from single-mode fiber geometry to a large core-to-cladding ratio structure. The primary objective is to produce a structure that facilitates short decay times with a sufficiently large yield for time-resolved dosimetry. RL characterization was carried out using a high-energy clinical X-ray beam (6 MV), delivered by an Elekta Synergy linear accelerator located at the Advanced Medical and Dental Institute, Universiti Sains Malaysia (USM). The Ge-doped silica optical fiber scintillator samples, fabricated using chemical vapor deposition methods, comprised of large core and small core optical fiber scintillators with high and low core-to-cladding ratios, respectively. Accordingly, these samples having different Ge-dopant contents offer distinct numbers of defects in the amorphous silica network. Responses were recorded for six dose-rates (between 35 MU/min and 590 MU/min), using a photomultiplier tube setup with the photon-counting circuit capable of gating time as small as 1 μs. The samples showed linear RL response, with differing memory and afterglow effects depending on its geometry. Samples with a large core-to-cladding ratio showed a relatively short decay time (<1 ms). The results suggest a contribution of Ge-doping in affecting the triplet states of the SiO₂ matrix, thereby reducing phosphorescence effects. This is a desirable feature of scintillating glass materials that enables avoiding the pulse pile-up effect, especially in high dose-rate applications. These results demonstrate the potential of Ge-doped optical-fiber scintillators, with a large core-to-cladding ratio for use in time-resolved radiation dosimetry. Full article

In this study, Yb/Al/Ge co-doped silica fiber core glasses with different GeO₂ contents (0–6.03 mol%) were prepared using the sol–gel method combined with high-temperature sintering. The absorption, fluorescence, radiation-induced absorption, continuous-wave electron paramagnetic resonance spectra, and fluorescence decay curves were recorded and analyzed systematically before and after X-ray irradiation. The effects of GeO₂ content on the valence variations of Yb³⁺/Yb²⁺ ions, spectral properties of Yb³⁺ ions, and radiation resistance of Yb/Al/Ge co-doped silica glasses were systematically studied. The results show that even if the GeO₂ content of the sample is relatively low (0.62 mol%), it can inhibit the generation of Yb²⁺ ions with slight improvement in the spectral properties of Yb³⁺ ions in the pristine samples and effectively improve its radiation resistance. Direct evidence confirms that the generation of trapped-electron centers (Yb²⁺/Si-E’/Al-E’) and trapped-hole centers (Al-OHC) was effectively inhibited by Ge co-doping. This study provides a theoretical reference for the development of high-performance, radiation-r esistant Yb-doped silica fibers. Full article

A procedure based on energy-dispersive X-ray spectroscopy in a scanning electron microscope (SEM-EDXS) is proposed to measure ultra-thin oxide layer thicknesses to atomic scale precision in top-down instead of cross-sectional geometry. The approach is based on modelling the variation of the electron beam penetration depth and hence the depth of X-ray generation in the sample as a function of the acceleration voltage. This has been tested for the simple case of silica on silicon (SiO₂/Si) which can serve as a model system to study gate oxides in metal-on-semiconductor field-effect transistors (MOS-FETs). Two possible implementations exist both of which rely on pairs of measurements to be made: in method A, the wafer piece of interest and a reference sample (here: ultra-clean fused quartz glass for calibration of the effective k-factors of X-ray lines from elements O and Si) are analysed at the same acceleration voltage. In method B, two measurements of the apparent O/Si ratio of the same wafer sample need to be made at different acceleration voltages and from their comparison to simulations the SiO₂ layer thickness of the sample can be inferred. The precision attainable is ultimately shown to be limited by surface contamination during the experiments, as very thin carbonaceous surface layers can alter the results at very low acceleration voltages, while the sensitivity to ultra-thin surface oxides is much reduced at higher acceleration voltages. The optimal operation voltage is estimated to lie in the range of 3–15 kV. Method A has been experimentally verified to work well for test structures of thin oxides on Si-Ge/Si. Full article

Femtosecond (fs) laser written fiber Bragg gratings (FBGs) are excellent candidates for ultra-high temperature (>800 °C) monitoring. More specifically, Type II modifications in silicate glass fibers, characterized by the formation of self-organized birefringent nanostructures, are known to exhibit remarkable thermal stability around 1000 °C for several hours. However, to date there is no clear understanding on how both laser writing parameters and glass composition impact the overall thermal stability of these fiber-based sensors. In this context, this work investigates thermal stability of Type II modifications in various conventional glass systems (including pure silica glasses with various Cl and OH contents, GeO₂-SiO₂ binary glasses, TiO₂- and B₂O₃-doped commercial glasses) and with varying laser parameters (writing speed, pulse energy). In order to monitor thermal stability, isochronal annealing experiments (Δt⁓ 30 min, ΔT⁓ 50 °C) up to 1400 °C were performed on the irradiated samples, along with quantitative retardance measurements. Among the findings to highlight, it was established that ppm levels of Cl and OH can drastically reduce thermal stability (by about 200 °C in this study). Moreover, GeO₂ doping up to 17 mole% only has a limited impact on thermal stability. Finally, the relationships between glass viscosity, dopants/impurities, and thermal stability, are discussed. Full article