Silica Optical Fibers Research Articles

Evaluating and Minimizing Induced Microbending Losses in Optical Fiber Sensors Embedded Into Glass-Fiber Composites

Conventional silica optical fibers can be embedded into composite structures or packaging to provide structural monitoring capabilities. In this paper, the microbending optical losses induced by the packaging of a sensing optical fiber into a sandwiched glass-fiber reinforced structure are investigated experimentally and by simulations. Results show that the positioning of the optical fiber within a plain-weave glass fiber-reinforced structure has a critical effect on microbending optical losses due to potentially induced microdeformations of the optical fiber's cylindrical shape. Similar analysis is also carried out with the use of a 1D glass-fiber layer packaging, demonstrating lower residual microbending losses after manufacturing. By the proper positioning of an optical fiber containing a series of wavelength-multiplexed fiber Bragg grating (FBG) sensors over a 2D plain-weave glass fabric, a 10-fold improvement in the induced optical losses is demonstrated, increasing the optical power that reaches the interrogating unit and allowing the monitoring of hundreds of meters over glass-fiber structures. Although all the analysis is focused and verified using a series of FBG sensors, the results and conclusions related to the induced microbending losses are also valid and useful for distributed optical fiber sensing approaches.

Influence of ring structures on luminescence properties of trivalent cerium in Ge-doped silica optical fiber

The membered ring microstructures are considered to study the influences of the structure parameters, frontier molecular orbital, and luminescence properties in Ge-doped silica optical fiber material with trivalent cerium. The calculations indicate that Ce3+ ion is more likely to exist outside the membered ring structures than embedded into the membered rings. The results reveal that after the Ce incorporation, a strong absorption peak and an emission peak appear with a combination of Ce3+ ions and GeO2 matrix. Moreover, the energy levels for Ce-doped silica optical fiber are investigated, which explained the influence of different membered rings on the luminescence properties.

Self-assembled polyaniline nanostructures in situ deposited on silica optical fibers for ammonia gas sensing

Optical fibers coated with polyaniline (PANI) nanostructures were used to sense ammonia gas (NH3) concentration. The PANI nanostructures were synthesized using the oxidative polymerization process in a solution containing poly(methyl vinyl ether-alt-maleic acid) (PMVEA). The PANI nanostructures were in situ deposited on silica optical fibers by immersing the fibers in the solution during the polymerization process for specific durations. The morphology and the thickness of the PANI nanostructure coatings were characterized using Scanning Electron Microscopy (SEM). Raman and Fourier Transform Infra-red (FTIR) spectra confirmed that the PANI nanostructures were in the emeraldine salt (ES) state. The sensors were exposed to various concentrations of NH3 gas at room temperature. The sensor with a coating thickness of 639 nm shows the highest response with a cumulative absorbance response of 2.2 towards 1% NH3 in synthetic air. The response and recovery times are 2.82 min and 11.52 min, respectively.

Laser cooling of a Yb doped silica fiber by 18 Kelvin from room temperature.

An ytterbium doped silica optical fiber with a core diameter of 900µm has been cooled by 18.4 K below ambient temperature by pumping with 20 W of 1035 nm light in vacuum. In air, cooling by 3.6 K below ambient was observed with the same 20 W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be 1.2±0.1%. The core of the fiber was codoped with Al3+ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.

Distributed fiber optic radiation sensors

Abstract. The international research efforts focused on the development of radiation sensors based on optic fibers have their origins in the 1970s (Evans et al., 1978). Generally, the lightweight fiber optic sensors are immune to electromagnetic field interference and high voltages making them deployable in harsh environments at hard to reach areas where conventional sensors usually will not work at all. A further advantage of such radiation sensors is the possibility of remote and real-time monitoring (Huston et al., 2001). In this work, we present our results achieved in several research activities for development of fiber optic dosimeters. The findings show that both the measurement of the radiation-induced attenuation (RIA) along the entire sensing fiber and the accompanying change in the refractive index of the fiber core can be used for distributed radiation monitoring in the kGy and MGy range, respectively. Depending on the fiber type and material the RIA shows varying response to dose rates, environmental temperatures and the wavelength of the laser source used. Thereby, an operation with visible laser light provides most favorable performance in terms of high radiation sensitivity. Operating at these wavelengths, RIA monitoring could yield high-sensitivity dose measurement with sub-gray resolution and accuracy (Stajanca and Krebber, 2017b); however, conventional optical time-domain reflectometry (OTDR) systems for RIA measurements operating in the visible range suffer from low-spatial resolution, long measurement times and poor signal-to-noise (SNR) ratio. The limitations of the OTDR performance can be overcome by the incoherent optical frequency domain reflectometry (I-OFDR) developed by the Federal Institute of Materials Research and Testing (BAM, Liehr et al., 2009) with potential for dynamic real-time measurement. Over the years, several highly radiation sensitive fibers, such as perfluorinated polymer optical fibers (PF-POF, Stajanca and Krebber, 2017a), phosphorous-doped silica optical fibers (SOF, Paul et al., 2009), aluminium-doped SOF (Faustov et al., 2013) and erbium-doped SOF (Wosniok et al., 2016) have been identified and are commercially available. As mentioned before, the radiation-induced RIA increase is associated with an increase in the refractive index leading also to material compaction in the fiber core. The latter two effects can be used for measuring radiation distribution based on Brillouin scattering in the range of high radiation doses of several MGy (Phéron et al., 2012; Wosniok et al., 2016). When using fiber optic sensors for radiation monitoring, the existing post-irradiation annealing behavior of the optical fiber sensors must also be considered.

Electronic and optical properties of Yb/Al/P co-doped silica optical fiber

The corresponding relationship between energy levels and absorption spectra of Yb3+ ion in Yb/Al/P co-doped silica fiber is obtained by first-principle methods. Based on the calculation result that the ortho-position (OP) substitution model has the lowest average formation energy, Yb-OP substitution model with six coordination of Yb3+ is constructed, which is a possible local microstructure in Yb/Al/P co-doped silica fiber. Due to the spin-orbital coupling effect, two occupied impurity levels are introduced into the band gap located at 2.72 eV and 4.06 eV, representing 2F5/2 and 2F7/2 manifolds respectively. The absorption peaks of 971 nm, 922 nm and 881 nm are fitted by Gaussian decomposition, corresponding to the transition 2F7/2-1 - 2F5/2-5, 2F7/2-1 - 2F5/2-6 and 2F7/2-1 -2F5/2-7. We construct the local environment model of Yb3+ ion and provide the relationship between energy levels and optical absorption of Yb3+ ion from the perspective of theoretical calculation, which is helpful to understand the Yb/Al/P co-doped silica fiber theoretically.

Antenna-Coupled Electrode Electro-Optic Modulator for 5G Mobile Applications

A new device for conversion from 5G mobile wireless signals to optical signals, which is suitable for a passive wireless receiver in uplinks of 5G mobile front-haul in dense user environments, and a 5G-band sensor for precise antenna measurement, has been developed utilizing an array of antenna-coupled electrodes integrated on a high-speed LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> electro-optic (EO) modulator. The 5G wireless signal conversion and the high-speed data transfer over a silica optical fiber link for high-definition (HD) video streaming were demonstrated successfully using the fabricated device operating in the 28 GHz-band.

High Temperature Effects in Fused Silica Optical Fibers

High Temperature Effects in Fused Silica Optical Fibers

Quantitative in situ measurement of optical force along a strand of cleaved silica optical fiber induced by the light guided therewithin

We propose an optomechanical system to quantify the net force on a strand of cleaved silica optical fiber in situ as the laser light is being guided through it. Four strands of the fiber were bonded to both sides of a macroscopic oscillator, whose movements were accurately monitored by a Michelson interferometer. The laser light was propagating with variable optical powers and frequency modulations. Experimentally, we discovered that the driving force for the oscillator consisted of not only the optical force of the light exiting from the cleaved facets but also the tension along the fiber induced by the light guided therewithin. The net driving force was determined only by the optical power, refractive index of the fiber, and the speed of light, which pinpoints its fundamental origin.

Signal Amplification of Fiber Integrated X-Ray Detector and Energy Independence

We developed a high spatial resolution and sensitive detector particularly suitable for radiation dose measurements in radiotherapy at small fields and X-ray imaging applications. The detector is based on a silica optical fiber whose sensitive terminal is functionalized by grafting Ag-doped ZnS scintillating materials that emit blue luminescence under X-ray irradiation. Due to the small scintillating head of the detector, the acquisition signal is quite low, and it is measured by a sensitive photon counter plugged at the other terminal of the fiber. The weaker signal can be enhanced significantly by adding a metal coating over the scintillator. Therefore, this study presents the effect of metal thickness and density on the signal amplification at high irradiation energies of 6 MeV and 15 MeV using a Linear Accelerator (LINAC). In addition, the stem effect was characterized for regular to small field beam size and shows that the detector is almost free from stem contribution during high energy irradiation.

Световоды типа кварц-кварц с малыми потерями и тонкой полимерной оболочкой для ближнего ИК-спектрального диапазона.

The issues of obtaining silica optical fibers with SiO2/SiO2-F composition (where SiO2 is a core, and SiO2-F is a light-reflecting cladding) with low optical losses for the near-IR spectral range are considered. This fiber type is widely used in various fields of science and technology, including laser medicine. The study results of the dependence of the numerical aperture and fluorine concentration in silica glass of the formed fiber preforms on the Freon (C3F8) consumption in the gas phase when using a waveguide-type microwave plasmotron at the H10 wavelength, are presented. It is showed that to increase the fluorimeters sensitivity in the near-IR spectral range as probe fibers for detecting luminescence, it is promising to use quartz-quartz fibers with the “Heraeus” glass core and the reflective SiO2-F cladding with an increased fluorine content in the glass (up to 7 wt. %). In this case, the numerical aperture of such fibers will reach values up to 0.32. For laser procedures in ophthalmology, surgery, phlebology and etc., optical fibers with a standard numerical aperture 0.20-0.24 can be used (with a fluorine concentration in the glass up to 3 wt. %).

Water enrichment/depletion of amorphous carbon coatings probed by temperature-dependent dc electrical conductivity and Raman scattering

Amorphous carbon (a-C) is an appealing disordered multi-phase material widely used as a cost-effective barrier coating (10–100 nm thick) for silica optical fibers to protect them from harsh environments. Those coatings prevent strength degradation of the optical fibers by moisture and, also block hydrogen diffusion into the fiber. In this paper, we experimentally study the in situ temperature-dependent dc electrical conductivity and Raman scattering of dried/wetted a-C coatings under multiple heating/cooling cycles within the range of 40–120 °C. At temperatures of above 80 °C, we observe hysteresis in both the dc electrical resistance and the Raman scattering intensity. One of the mechanisms is related to the escape of intercalated (or spatially confined) water from the a-C coating, causing the resistance decrease due to a closing of the bandgap. The opposite is observed when some water molecules experience dissociative absorption at the edge and basal defects that may be decorated with CH, COOH/COH and COC/CO functional groups. The content of water within the a-C coating is quantified through relative contributions of the water and edge defect related Raman bands. This study expands our knowledge about a-C ultrathin films, originally designed as barrier coatings, towards promising ultracompact humidity sensors and next-generation adsorbents.

Plasmonic Fiberoptic Absorbance Biosensor (P-FAB) for Rapid Detection of SARS-CoV-2 Nucleocapsid Protein.

SARS-CoV-2 nucleocapsid protein-based COVID-19 diagnosis is a promising alternative to the high-priced, time-consuming, and labor-intensive RT-PCR tests. Here, we developed a rapid, dip-type, wash-free plasmonic fiber optic absorbance biosensor (P-FAB) strategy for the point-of-care detection of SARS-CoV-2 N-protein, expressed abundantly during the infection. P-FAB involves a sandwich assay with plasmonic labels on the surface of a U-bent fiber optic sensor probe with a high evanescent wave absorbance (EWA) sensitivity. The SARS-CoV-2 N-protein is quantified in terms of the change in the intensity of the light propagating through the U-bent sensor probe coupled to a green LED and a photodetector. Firstly, the optical fiber material (silica vs. polymeric optical fiber), was evaluated to realize a sensitive sensor platform. The optimal size of AuNP labels (20, 40, and 60 nm) to achieve high sensitivity and a lower limit of detection (LoD) was investigated. Following the P-FAB strategy, fused silica/glass optical fiber (GOF) U-bent senor probe and citrate-capped AuNP labels (size ~40 nm) gave rise to an LoD down to ~2.5 ng/mL within 10 mins of read-out time. Further, studies on development and validation of a point of care (PoC) read-out device, and preclinical studies are in progress.

Dosimetric Application of Phosphorus Doped Fibre for X-ray and Proton Therapy.

Phosphorous-doped silica optical fibres with a core diameter of 4 µm were tested in X-ray and proton fields for application in cancer therapy dosimetry. Specifically, the radiation-induced attenuation was investigated in terms of linearity in deposited dose in 15 MV and 6 MV photons and 74 MeV protons, as well as Bragg-peak detection along the proton track. Fibres were found to demonstrate linear relative dose response in both radiation modalities, but possible saturation did occur at the high linear energy transfer of the Bragg peak. This demonstrates the possibility to use these fibres as a relative dosimeter for radiation therapy applications.

Influence of the Cooling Rate of Silica Glass on Its Surface Relief

The method of atomic force microscopy explores a roughness of a surface of the silica glass cooled with different velocity from temperature 2300 °C. It is compared two modes of cooling of a glass rods with a diameter of 16mm: convective heat exchange with the surrounding air and quenching with dropping the sample into a vessel with water. The roughness of a surface of a silica glass in the second mode of cooling was essentially less that can promote magnification of its strength. The results of these studies may turn out to be promising in terms of increasing the strength and reliability of silica optical fibers used in laser surgery.

Ge-doped silica optical fibre for Time Resolved Radiation Dosimetry

Time-resolved radiation dosimetry in radiotherapy helps determine the quality of delivery of patient prescribed doses. Earlier reports on doped silica optical fibres as scintillators in radioluminescence (RL) based radiation dosimetry have indicated particular merits, including versatility, robustness, high spatial resolution, wide dynamic range, and the ability for real-time measurements. Time-resolved radiation dosimetry requires high temporal resolution, enabled using a suitable scintillator and high-speed electronics. We report on the potential of Ge-doped optical fibres as a suitable scintillator to be adopted for use in a time-resolved radiation dosimetry system. High-energy clinical X-ray beams (6 MV) were used to irradiate the Ge-doped optical fibre samples. RL responses were recorded for six dose-rates (between 100 MU/min and 600 MU/min) delivered by a Varian 2100 C/D linear accelerator. The photon-counting circuit gating time was set at its shortest capability of 100 μs? The Ge-doped optical-fibre scintillator showed linear RL response, with minimal observable memory and afterglow and plateau effects. The fluorescence lifetime analysis, demonstrate a calculated rise time of 590.1 ns and a decay time of 0.423 μs, indicating superior performance compared to other scintillator forms. These results demonstrate the potential for the Ge-doped optical-fibre scintillator to be used in a time-resolved radiation dosimetry system.

Radioluminescence of silica optical fibre scintillators for real-time industrial radiation dosimetry

Dosimetry systems are important in any radiation facility to ensure quality, safety, validation, qualification and quality control of established and new processes, including at industrial radiation facilities. We present a study of silica optical fibre as scintillators for industrial radiation dosimetry using Radioluminescence (RL). The RL technique provides for a remote and real-time dosimetry measurement in such a high dose environment. This study establishes the dosimetric characteristics of two variants of pure silica optical fibre, low and high hydroxyl (OH) when exposed to industrial radiation levels. The samples were exposed to a linear accelerator, with doses in the order of tens of kGy (from 10 kGy through to 70 kGy in individual exposures). The performance parameters observed during the study include the RL emission spectrum, emission intensity and emission linearity with exposure dose. The RL spectrum demonstrated a central wavelength of 650 nm and 550 nm for the high and low OH scintillators, respectively. RL intensity was observed to increase with exposed dose, a result of recombination of ionizing radiation generated electron-hole pairs. The RL intensity relation with exposed dose indicated a linear response, with the higher OH sample showing the greater slope, indicating greater sensitivity. The overall results indicate the potential use of pure silica optical fibre as scintillators for remote and real-time industrial radiation dosimetry.

Numerical model for enhancing stimulated Brillouin scattering in optical microfibers

Stimulated Brillouin scattering (SBS) is useful, among others for generating slow light, sensing and amplification. SBS was previously viewed as a poor method due to the limitation on optical power in high-powered photonic applications. However, considering the many possible applications using SBS, it is now of interest to enhance SBS in areas of Brillouin frequency shift together with Brillouin Gain. A numerical model, using a fully vectorial approach, by employing the finite element method, was developed to investigate methods for enhancing SBS in optical fiber. This paper describes the method related to the numerical model and discusses the analysis between the interactions of horizontal, shear and hybrid acoustic modes; and optical modes in optical fiber. Two case studies were used to demonstrate this. Based on this numerical model, we report the influence of core radius, clad radius and effective refractive index on the Brillouin frequency shift and gain. We observe the difference of Brillouin shift frequency between a normal silica optical fiber and that of a tapered fiber where nonlinearities are higher. Also observed, the different core radii used and their respective Brillouin shift. For future work, the COMSOL model can also be used for the following areas of research, including simulating “surface Brillouin shift” and also to provide in-sights to the Brillouin shift frequency vB of various structures of waveguides, e.g circular, and triangular, and also to examine specialty fibers, e.g. Thulium and Chalcogenide doped fibers, and their effects on Brillouin shift frequency.

Comprehensive approach to determination of space proton-induced displacement defects in silica optical fiber

Silica-based optical fibers presents a variety of applications used in radiation environments such as space, fusion facilities, accelerators and nuclear power plants. The radiation-induced displacement damage in optical fibers resulting in point defects may lead to attenuation signals that is a major concern for these applications. The present study proposes a computational approach to the calculation of the proton-induced displacement damage in vitreous silica. Therefore, Geant4 as a Monte Carlo particle transport code has been used to obtain the knock-on atom distributions caused by the interaction of space trapped proton with vitreous silica during an ISS mission. Moreover, molecular dynamics simulations using ReaxFF potential have been performed to produce the initial vitreous silica structure to evaluate the displacement damage cascades by LAMMPS package. The results show that ReaxFF has an appropriate potential to produce and evaluate the vitreous silica structure that provides better agreement with experimental data at both short-range and medium-range order. Furthermore, ODC(Si3) and NBOHC(O1) are dominant defect species created in the vitreous silica after trapped proton irradiation, where the total number of defects have increased on average by 94 for each keV increasing in PKA energy approximately.

Rainbow Archimedean spiral emission from optical fibres

We demonstrate a new practical approach for generating multicolour spiral-shaped beams. It makes use of a standard silica optical fibre, combined with a tilted input laser beam. The resulting breaking of the fibre axial symmetry leads to the propagation of a helical beam. The associated output far-field has a spiral shape, independently of the input laser power value. Whereas, with a high-power near-infrared femtosecond laser, a visible supercontinuum spiral emission is generated. With appropriate control of the input laser coupling conditions, the colours of the spiral spatially self-organize in a rainbow distribution. Our method is independent of the laser source wavelength and polarization. Therefore, standard optical fibres may be used for generating spiral beams in many applications, ranging from communications to optical tweezers and quantum optics.