Silica-based Optical Fibers Research Articles

Coupled temperature and γ-radiation effect on silica-based optical fiber strain sensors based on Rayleigh and Brillouin scatterings.

Coupled temperature and γ-ray influence on Brillouin (PPP-BOTDA) and Rayleigh (TW-COTDR) scatterings are quantified. Aging tests of these distributed strain measuring systems are performed on-line, up to 1 MGy, at room temperature, 80 ∘C, 100 ∘C and 120 ∘C. Brillouin and Rayleigh frequency shifts remain identical regardless of the temperature: 3 MHz (2 MHz) and 7 GHz (3 GHz) for Ge-doped (respectively F-doped) fiber at 1 MGy. Meanwhile, radiation-induced attenuation is diminished because of the higher temperature; hence, the maximal distance range is less deteriorated. These tests help to explain the origin of the Brillouin frequency shift under γ-rays, with an acoustic velocity variation of about 1 m/s in 1 MGy irradiated samples.

V-P2O5 micro-clustering in P-doped silica studied by a first-principles Raman investigation

Synthetic vitreous silica is currently the preferred material for the production of optical fibres because of the several excellent properties of this glass, e.g. high transmission in the visible and IR domains, high mechanical strength, chemical durability, and ease of doping with various materials. For instance, fiber lasers and amplifiers exploit the light amplification properties provided by rare-earth ions employed as dopants in the core of silica-based optical fibers. The structure and composition of the nearest neighbor shell surrounding rare-earth ions in silica-based optical fibers and amplifiers have been intensively debated in the last decade. To reduce aggregation effects between rare-earth ions, co-dopants such as phosphorus and aluminium are added as structural modifiers; phosphorus-doping, in particular, has proved to be very efficient in dissolving rare-earth ions. In this work, we provide further insights concerning the embedding of P atoms into the silica network, which may be relevant for explaining the ease of formation of a phosphorus pentoxide nearest-neighbor shell around a rare-earth dopant. In particular, by means of first-principles calculations, we discuss alternative models for an irradiation (UV, x–, γ-rays) induced paramagnetic center, i.e. the so called room-temperature phosphorus-oxygen-hole center, and its precursors. We report that the most likely precursor of a room-temperature phosphorus-oxygen-hole center comprises of a micro-cluster of a few (at least two) neighboring phosphate tetrahedra, and correspondingly that the occurrence of isolated [(O-)2P(=O)2]− units is unlikely even at low P-doping concentrations. In fact, this work predicts that the symmetric stretching of P=O bonds in isolated [(O-)2P(=O)2]− units appears as a Raman band at a frequency of ~1110 cm−1, and only by including at least another corner-sharing phosphate tetrahedron, it is shown to shift to higher frequencies (up to ~40 cm−1) due to the shortening of P=O bonds, thereby leading to an improved agreement with the observed Raman band located at ~1145 cm−1.

Overview of radiation induced point defects in silica-based optical fibers

Abstract Silica-based optical fibers, fiber-based devices and optical fiber sensors are today integrated in a variety of harsh environments associated with radiation constraints. Under irradiation, the macroscopic properties of the optical fibers are modified through three main basic mechanisms: the radiation induced attenuation, the radiation induced emission and the radiation induced refractive index change. Depending on the fiber profile of use, these phenomena differently contribute to the degradation of the fiber performances and then have to be either mitigated for radiation tolerant systems or exploited to design radiation detectors and dosimeters. Considering the strong impact of radiation on key applications such as data transfer or sensing in space, fusion and fission-related facilities or high energy physics facilities, since 1970′s numerous experimental and theoretical studies have been conducted to identify the microscopic origins of these changes. The observed degradation can be explained through the generation by ionization or displacement damages of point defects in the differently doped amorphous glass (SiO2) of the fiber's core and cladding layers. Indeed, the fiber chemical composition (dopants/concentrations) and elaboration processes play an important role. Consequently, identifying the nature, the properties and the generation and bleaching mechanisms of these point defects is mandatory in order to imagine ways to control the fiber radiation behaviors. In this review paper, the responses of the main classes of silica-based optical fibers are presented: radiation tolerant pure-silica core or fluorine doped optical fibers, germanosilicate optical fibers and radiation sensitive phosphosilicate and aluminosilicate optical fibers. Our current knowledge about the nature and optical properties of the point defects related to silica and these main dopants is presented. The efficiency of the known defects to reproduce the transient and steady state radiation induced attenuation between 300 nm and 2 µm wavelength range is discussed. The main parameters, related to the fibers themselves or extrinsic - harsh environments, profile of use - affecting the concentration, growth and decay kinetics of those defects are also reviewed. Finally, the main remaining challenges are discussed, including the increasing needs for accurate and multi-physics modeling tools.

Recent developments in novel silica-based optical fibers

We have summarized our recent work in the area of novel silica-based optical fibers, which can be classified into two types: silica optical fiber doped with special elements including Bi, Al, and Ce, and micro-structured multi-core fibers. For element-doped optical fiber, the Bi/Al co-doped silica fibers could exhibit a fluorescence spectrum covering the wavelength range between 1000 and 1400 nm with a full width at half maximum (FWHM) of about 150 nm, which enables its use in fiber amplifiers and laser systems. The Ce-doped fiber’s center wavelengths of excitation and emission are about 340 and 430 nm, respectively. The sapphire-derived fiber (SDF) with high alumina dopant concentration in the core can form mullite through heating and cooling with arc-discharge treatment. This SDF can be further developed for an intrinsic Fabry-Perot interferometric that can withstand 1200 °C, which allows it to be used in high-temperature sensing applications. Owing to the strong evanescent field, micro-structured multi-core fiber can be used in a wide range of applications in biological fiber optic sensing, chemical measurement, and interference-related devices. Coaxial-core optical fiber is another novel kind of silica-based optical fiber that has two coaxial waveguide cores and can be used for optical trapping and micro-particle manipulation by generating a highly focused conical optical field. The recent developments of these novel fibers are discussed.

Observation of Dirac mode in modified honeycomb hollow core photonic crystal fiber

The Dirac mode in the hollow core photonic crystal fibers (HC-PCFs) traditionally rely on high refractive index glasses with a high air-filling factor. In this paper, the Dirac mode was observed in the modified honeycomb lattice HC-PCFs, which breaks the constraint of refractive index, and can be realized in pure silica glasses for the first time. The band structure of the modified honeycomb lattice was calculated to elaborate the formation process of the Dirac point. The Dirac mode in the HC-PCFs was established at the Dirac frequency outside the bandgaps. In addition, the relationship between the Dirac frequency and the air-filling fraction and the refractive index were also analyzed. This design provides a modified honeycomb lattice HC-PCFs with substantially reduced fabrication effort and expands the application of Dirac mode in the conventional silica-based optical fiber devices.

X-Rays, <inline-formula> <tex-math notation="LaTeX">$\gamma$ </tex-math> </inline-formula>-Rays, and Proton Beam Monitoring With Multimode Nitrogen-Doped Optical Fiber

We investigated the potential of a multimode (50-μm core diameter) nitrogen doped silica-based optical fiber for X-rays, protons, and y-rays radiation detection and dosimetry. X-rays results confirm that this N-doped fiber presents a strong radioluminescence (RL) around 550 nm when exposed to radiation. The RL observed from a few centimeter long samples linearly depend on the dose rate at least in the 1-mGy/s-50-Gy/s range. Low dose rate y-ray t ests demonstrate the radiation detection at dose rate as low as 40 μGy/s with a 2-m-long fiber sample. This fiber is also associated with an optically stimulated luminescence (OSL) when exposed to a 1064-nm laser light postirradiation. This OSL signature provides shortly after the irradiation a precise estimation of the accumulated dose at least in the 1-Gy-10-kGy range. Under proton exposure, we benchmarked the performances of the N-doped fiber with the ones of a conventional Markus chamber used by the TRIUMF proton-therapy team. This comparison highlights the potential of the small-size N-doped fiber to partially overcome the known limitations of commercial fiber-based dosimeters for medical applications. The obtained results are very promising for the future design of more complex spatially resolved beam monitoring systems.

On the Enlargement of the Emission Spectra from the 4I13/2 Level of Er3+ in Silica-Based Optical Fibers through Lanthanum or Magnesium Co-Doping

Improving optical fiber amplifiers requires the elaboration and use of new materials and new compositions. In this sense, we prepared erbium-doped optical fiber samples that were co-doped with magnesium or lanthanum by gradual-time solution doping. Doping concentrations and thermal processes induce the formation of nanoparticles. The effect of lanthanum and magnesium contents on the width of the spontaneous emission of the 4 I 13 / 2 level of Er 3 + was characterized in the nanoparticle-rich fiber samples. For that purpose, the width was characterized by the effective linewidth and the full-width at half-maximum (FWHM). The results indicate the robustness of the effective linewidth to strong variations in the intensity profiles of the 4 I 13 / 2 spontaneous emission. Increasing the doping concentrations of both magnesium and lanthanum increases the FWHM and the effective linewidth, along with optical losses. Results show that the fabrication of nanoparticle-rich optical fibers through lanthanum or magnesium doping induces an FHWM broadening of 54% and 64%, respectively, or an effective linewidth broadening of 59% (for both elements) while maintaining a transparency that is compatible with fiber laser and amplifier applications.

Specific Features of Fiber Optics Application in ITER

Silica-based optical fibers have high transmission in the visible range and are widely used for transmitting light from plasma to detectors in modern fusion devices. Pure silica-core/F-doped silica-clad fibers are mainly applied for plasma diagnostics. The possible problems that can be encountered with fiber optics in ITER diagnostics are discussed. The neutron fluence to be accumulated in the bundles during the ITER lifetime is estimated. The results of the irradiation tests of the optical fibers at the fission reactor and the 14 MeV neutron generator are used for analysis of radiation resistance of the optical fibers. The results of radiation tests and neutron calculations prove the requirement to apply silica fibers with high radiation resistance in ITER. It is recommended to place the fiber bundles in the port cell behind the biological shielding, where the neutron flux from the plasma is attenuated to a value of ~107 n/(cm2 s). The conclusion is made that it is possible to use the modern optical fibers in the plasma optical diagnostics (in a spectral range of 450–700 nm), the optical fibers being capable of surviving the radiation load during the whole ITER lifetime.

Researching the Interface of Polymer Matrices with Optical Fibers in Smart Materials

The interface of a polymer matrix with silica-based optical fibers covered with a protective layer is studied using the methods of microstructural analysis. The compatibility of the protective layer with the polymer matrix is investigated by IR spectroscopy taking into account the modes of curing.

Thulium-Doped Silica Fibers with Enhanced Fluorescence Lifetime and Their Application in Ultrafast Fiber Lasers

In this work we report on the thulium-doped silica-based optical fibers with increased fluorescence lifetime of the 3F4 level thanks to the modification of the local environment of thulium ions by high content of alumina. The determination of the cross-relaxation energy-transfer coefficients from the measurements of the fluorescence lifetimes of the 3F4 and 3H4 energy levels of Tm3+ ions in the experimentally prepared optical fiber is provided as well. Preforms of optical fibers were prepared either by conventional solution-doping of Tm3+ and Al3+ ions or by dispersion-doping of Tm3+ ions with alumina nanoparticles. Optical fibers were characterized by means of Tm, Al, and Ge concentrations, refractive index profiles, optical spectral absorption and luminescence, and by time-resolved fluorescence spectroscopy. Highly aluminium-codoped thulium silicate optical fibers exhibited fluorescence lifetimes of over ~500 μs with maximum value of 756 μs, which means a fluorescence lifetime enhancement when compared to the thulium-doped fibers reported elsewhere. We show an application of the thulium-doped fiber in a compact all-fiber ring laser that is passively mode-locked by using graphene-based saturable absorber. The output pulsewidth and repetition rate were 905 fs and 32.67 MHz, respectively.

X-rays, γ-rays, electrons and protons radiation-induced changes on the lifetimes of Er3+ and Yb3+ ions in silica-based optical fibers

The evolution of Ytterbium 2F5/2 and Erbium 4I13/2 energy level lifetimes versus doses of various radiation types (40keV X-rays, 480MeV protons, 1.2MeV γ-rays and 6MeV electrons) were investigated on samples of silica-based Rare-Earth Doped optical Fibers (REDFs). For each studied sample (Er-doped and Er/Yb-doped), a strong dependence of the lifetime value on the irradiation dose (for equivalent doses ranging from 10Gy(SiO2) to 10MGy(SiO2)) is observed regardless of the radiation nature. For both fiber types and luminescent ions, complex dose dependences are observed: a limited decrease of the lifetime at the lower doses, whereas a large reduction is reported at doses exceeding 100kGy(SiO2). The results highlighted the vulnerability of REDF based systems, such as optical amplifiers and sources, for space and nuclear applications. In such harsh environments, Radiation Induced Attenuation (RIA) is also a key issue. The positive effect of Ce co-doping on both the RIA and the lifetimes is reported even at the highest dose of 10MGy(SiO2). The basic mechanisms involved during the interaction between radiation and fiber material were also investigated through low temperature spectroscopic measurements that revealed the fundamental role of radiation-induced point defects.

Multimaterial polarization maintaining optical fibers fabricated with the powder-in-tube technology

We present the fabrication of multimaterial polarization maintaining optical fibers. We exploit the flexibility of the powder-in-tube process for fabricating silica-based optical fibers composed of two rods of glass material on the sides of the core. We demonstrate the capability of this process to use glass material with properties sorely different from the ones of silica for developing high-birefringence optical fiber. This proof-of-concept paves the way for the use of different materials with specific properties for improving the performances of polarization maintaining optical fibers.

Real time monitoring of water level and temperature in storage fuel pools through optical fibre sensors

We present an innovative architecture of a Rayleigh-based optical fibre sensor for the monitoring of water level and temperature inside storage nuclear fuel pools. This sensor, able to withstand the harsh constraints encountered under accidental conditions such as those pointed-out during the Fukushima-Daiichi event (temperature up to 100 °C and radiation dose level up to ~20 kGy), exploits the Optical Frequency Domain Reflectometry technique to remotely monitor a radiation resistant silica-based optical fibre i.e. its sensing probe. We validate the efficiency and the robustness of water level measurements, which are extrapolated from the temperature profile along the fibre length, in a dedicated test bench allowing the simulation of the environmental operating and accidental conditions. The conceived prototype ensures an easy, practical and no invasive integration into existing nuclear facilities. The obtained results represent a significant breakthrough and comfort the ability of the developed system to overcome both operating and accidental constraints providing the distributed profiles of the water level (0–to–5 m) and temperature (20–to–100 °C) with a resolution that in accidental condition is better than 3 cm and of ~0.5 °C respectively. These new sensors will be able, as safeguards, to contribute and reinforce the safety in existing and future nuclear power plants.

Oxyfluoride Core Silica-Based Optical Fiber With Intrinsically Low Nonlinearities for High Energy Laser Applications

A few-moded silica-based optical fiber fabricated from core materials that possess intrinsically low optical nonlinearities is reported. Specifically, the 8-μm core, 125-μm cladding diameter silicate fiber was composed of a strontium aluminosilicate oxyfluoride core with a fused silica cladding and was fabricated using the molten core method. Relative to conventional optical fibers, reductions of ∼6.3 dB in Brillouin gain coefficient (gB ), ∼0.9 dB in Raman gain coefficient (gR), and ∼2.2 dB in thermo-optic coefficient were realized as was a “silica-like” nonlinear refractive index (n2) with a value of ∼3 × 10 –20 m2/W. The role of each core material constituent on parameters that drive optical nonlinearities is discussed to provide a materials solution route for low nonlinearity fiber systems. Materially addressing optical nonlinearities represent a simpler and more effective approach to mitigating power-scaling limits in high energy fiber laser systems compared to the geometric approaches employed using microstructured fibers.

Nanoparticle doping for high power fiber lasers at eye-safer wavelengths.

A nanoparticle (NP) doping technique was developed for fabricating erbium (Er)- and holmium (Ho)-doped silica-based optical fibers for high energy lasers. Slope efficiencies in excess of 74% were realized for Er NP doping in a single mode fiber based master oscillator power amplifier (MOPA) and 53% with multi-Watt-level output in a resonantly cladding-pumped power oscillator laser configuration based on a double-clad fiber. Cores comprising Ho doped LaF3 and Lu2O3 nanoparticles exhibited slope efficiencies as high as 85% at 2.09 µm in a laser configuration. To the best of the authors' knowledge, this is the first report of a holmium nanoparticle doped fiber laser as well as the highest efficiency and power output reported from an erbium nanoparticle doped fiber laser.

Fiber optics for plasma diagnostics in ITER

The work is related to literature survey and study to find out the proper fiber optics for visible plasma diagnostics in ITER. The silica-based optical fibers have high transmission in the visible range and they are widely used for transmitting light from plasma to detectors in the modern thermonuclear facilities. The fiber bundle consists as a rule of several tens or hundreds optical fibers of the 100–600μm diameter. In ITER the fiber bundles extend from radiation field area of the region near the plasma up to the diagnostic room at the distance about 100m. The lifetime of the optical fiber in ITER should be more than 15 years and the radiation hardness is a major problem for the fiber optics. The estimations of the neutron fluxes at the expected places of the fiber bundle location are given. A brief review is presented for the irradiation tests of the optical fibers. The fiber optics will suffer from radiation-induced optical absorption and radioluminescence. It is shown that the pure silica-core/F-doped silica-clad fibers and H2 doped silica core fibers are the best for ITER application. The analysis of the irradiation tests and the neutron calculations shows that radiation resistance of the modern optical fibers provides the opportunity for its application in ITER in the spectral range 450–1300nm.

Use of thulium-doped LaF3 nanoparticles to lower the phonon energy of the thulium's environment in silica-based optical fibres

New lasers and amplifiers based on rare-earth (RE)-doped silica optical fibres need improved spectroscopic performance. In this context, a route of interest consists of embedding RE ions within nanoparticles of composition and structure different from those of silica. In this report, optical fibres were prepared by doping the preform with LaF3:Tm3+ nanoparticles. Particles react through the process (Modified Chemical Vapor Deposition) to form fluoride-free amorphous nanoparticles that encapsulate the thulium ions. Nanoparticle doping permits enhanced 3H4 lifetimes up to 58 μs with good optical transparency and only 3000 at. ppm lanthanum. We also report on the advantage of doping with nanoparticles in order to control energy transfer.

Cathodoluminescence investigation of Ge-point defects in silica-based optical fibers

Cathodoluminescence (CL) measurements have been performed on Ge doped and Ge/F co-doped optical fibers, in the aim of studying the spatial distributions of the emitting precursor defects present in the as-drawn optical fiber as well as those of the radiation induced centers generated by the 10keV electron exposure. Using the CL instrument, we recorded different emission bands located in the visible spectral domain (300–750nm) as well as CL imaging of associated defects, with a spatial resolution of about 1µm, along the fiber transverse cross sections. In the pristine fiber, Germanium Lone Pair Centers (GLPCs) emitting at 400nm are the main precursor sites observed in both fibers. Whereas during electron exposure, these centers are converted into other Ge-related defects. In this paper, we studied in situ their bleaching kinetic using CL monochromatic imaging. As expected, our results show that the GLPC signal decreases with the electron fluence, confirming its precursor role. Thanks to the CL abilities, we also demonstrate that the GLPC conversion into radiation induced defects (and then its bleaching kinetic) depends on the germanium concentration, opening the way to a better control of the radiation sensitivity of germanosilicate glass.

Formation and applications of nanoparticles in silica optical fibers

Optical fibers are the basis for applications that have grown considerably in recent years (telecommunications, sensors, fiber lasers, etc). Despite undeniable successes, it is necessary to develop new generations of amplifying optical fibers that will overcome some limitations typical of silica glass. In this sense, the amplifying Transparent Glass Ceramics (TGC), and particularly the fibers based on this technology, open new perspectives that combine the mechanical and chemical properties of a glass host with the augmented spectroscopic properties of embedded nanoparticles. This paper is an opportunity to make a state of the art on silica-based optical fibers containing nanoparticles of various types, particularly rare-earth-doped oxide nanoparticles, and on the methods for making such fibers. In the first section of this article, we will review basics on standard optical fibers and on nanoparticle-doped fibers. In the second section we will recall some fabrication methods used for standard optical fibers, and in the third section we will describe how TGC fibers can be obtained. This new generation of fibers, initiated a decade ago, already show very positive outlook and results departing from those obtained previously in homogeneous silica fibers. The next few years should see the emergence of new components based on these optical fibers.

Damage of silica-based optical fibers in laser supported detonation

The study of detonation-like mode of laser induced damage propagation is presented. This mode is new investigation object of laser destruction of silica-based optical fibers. The fiber destruction images were obtained in evolution and in static (on saved samples).