Pure Silica Core Optical Fibers Research Articles

Radiation-induced attenuation in single-mode pure-silica-core optical fibers two years after high-fluence reactor irradiation

Single-mode pure-silica-core optical fibers (PSFs) coated with different materials (aluminum, copper, polyimide, or acrylate) and reactor-irradiated 2 years ago to the fast-neutron fluence of 1.8·1020n/cm2 and γ-dose of 2.3·109 Gy with evacuation to 0.005–0.25 Pa were OTDR-tested at wavelengths λ = 1.31 and 1.55 µm. Metal-coated PSFs demonstrated virtually the same radiation-induced attenuation (RIA) at λ = 1.55 µm as was measured 1 day after the irradiation (∼2–4 dB/m), whereas RIA in the polyimide-coated PSF that survived the irradiation went down from ∼4.3 to 1.8 dB/m. Fiber microbending originating from radiation-induced coating decay (not mechanical failure of the fiber itself) was argued to be the source of the catastrophic RIA rise in the polyimide- and acrylate-coated PSFs during the reactor irradiation in vacuum.

Tuning of the Brillouin scattering properties in microstructured optical fibers by liquid infiltration

We demonstrate the possibility to modify the Brillouin scattering properties of a microstructured pure-silica core optical fiber, by infiltrating a liquid inside its holes. In particular, we show that the dependence of the Brillouin frequency shift (BFS) on the temperature can be reduced by infiltration, owing to the large negative thermo-optic coefficient of the liquid. Infiltrating a chloroform-acetonitrile mixture with a refractive index of 1.365 inside the holes of a suspended-core fiber with a core diameter of 3 µm, the BFS temperature sensing coefficient is reduced by ≈ 21%, while the strain sensitivity remains almost unaltered. Besides tuning the temperature sensing coefficient, the proposed platform could find other applications in Brillouin sensing, such as distributed electrical and magnetic measurements, or enhanced Brillouin gain in fibers infiltrated with high nonlinear optical media.

Radiation responses of ultra-low loss pure-silica-core optical fibers in the visible to infrared domains

–We investigated the origins of the steady state X-ray radiation-induced attenuation (RIA) in the visible to infrared spectral domains of an ultra-low loss Telecom-grade single-mode optical fiber. This pure-silica core optical fiber presents very high RIA in this spectral range, larger than so-called radiation sensitive optical fibers. The RIA is mainly caused by the appearance of two broad absorption features around 600 nm and 1200 nm, both leading to RIA levels of ∼2 dB/m at 2 kGy(SiO2) at room temperature. While the infrared-RIA may be attributed to the absorption bands at 0.93 eV and 1.2 eV of possible variants of Self-Trapped Holes, the exact nature of the large 600 nm absorption around remains unclear. It could not be only related to a very efficient generation of Non-Bridging Oxygen Hole Centers but our analysis suggests the presence of another defect structure associated with an absorption band around 2.14 eV (FWHM = 0.52 eV), stable at room temperature, but that can be passivated by post irradiation hydrogen-loading treatment.

Drawing- and radiation-induced color centers in pure-silica-core optical fibers in the near-IR range

Spectra of the initial loss and radiation-induced attenuation are compared in pure-silica-core optical fibers (PSFs) drawn from the same preform with the help of carbon-resistance heaters (CRH) or an oxyhydrogen flame torch (OFT). OFT-based drawing was found to increase the initial loss at λ = 1.31 and 1.55 µm by ∼2–3 dB/km owing to growth of the 1.38-μm OH-group and 1.53-μm SiH-group absorption bands. RIA was essentially reduced in the OFT-drawn PSF, because of suppression of self-trapped holes. Therefore, OFT-drawing of radiation-resistant PSF is argued to be promising, provided a tradeoff between the initial loss and RIA is found.

GAMMA-RADIATION-INDUCED ATTENUATION OF LIGHT IN PURE-SILICA CORE OPTICAL FIBER IN LONG-WAVELENGTH REGION

The dependences of the spectra of radiation-induced attenuation (RIA) of light in a pure-silica core (PSC) optical fiber (OF) during and after gamma-irradiation up to 590 kGy at a dose rate of 7.6 Gy/s in the near infrared range have been investigated. It was shown that starting with an absorbed dose of ~100 kGy, the RIA at 1550 nm becomes larger than at 1310 nm due to an increase in long-wavelength (LWL) RIA. The absorption band, with a maximum at a wavelength near 1800 nm, responsible for the LWL RIA is fully defined for the first time. At an absorbed dose of 590 kGy at wavelengths of 1310 and 1550 nm, the RIA is 14.1 and 23.3 dB/km, respectively. During 3.5 years of annealing of the OF at room temperature the RIA in the entire spectral range of 1100-1700 nm decreases by 40-50%.

Radiation Effects on Pure-Silica Multimode Optical Fibers in the Visible and Near-Infrared Domains: Influence of OH Groups

Signal transmission over optical fibers in the ultraviolet to near-infrared domains remains very challenging due to their high intrinsic losses. In radiation-rich environments, this is made even more difficult due to the radiation-induced attenuation (RIA) phenomenon. We investigated here how the number of hydroxyl groups (OH) present in multi-mode (MM) pure-silica core (PSC) optical fibers influences the RIA levels and kinetics. For this, we tested three different fiber samples: one “wet”, one “dry” and one with an intermediate “medium” OH content. The RIA of the three samples was measured in the 400–900 nm (~3 eV to ~1.4 eV) spectral range during and after an X-ray irradiation at a dose rate of 6 Gy(SiO2) s−1 up to a total accumulated dose of 300 kGy(SiO2). Furthermore, we evaluated the H2-pre-loading efficiency in the medium OH sample to permanently improve both its intrinsic losses and radiation response in the visible domain. Finally, the spectral decomposition of the various RIA responses allows us to better understand the basic mechanisms related to the point defects causing the excess of optical losses. Particularly, it reveals the relationship between the initial OH groups content and the generation of non-bridging oxygen hole centers (NBOHCs). Moreover, the presence of hydroxyl groups also affects the contribution from other intrinsic defects such as the self-trapped holes (STHs) to the RIA in this spectral domain.

Radiation induced absorption of hydrogen-loaded pure silica optical fibers with carbon coating for ITER diagnostics

We present results of gamma irradiation tests of 200 μm pure silica core optical fibers with a low and high-OH concentration, with and without hydrogen loading. All the fibers were manufactured by a candidate supplier of the fiber bundles for optical plasma diagnostics in ITER. The hydrogen-loaded fibers were stored for a one year before the irradiation start. The fibers were exposed at two Co-60 gamma facilities using dose-rates of 15 and 194 mGy/s up to 15 and 70 kGy absorbed doses, respectively. The optical transmission losses of the fibers were measured in-situ in the spectral range of 450–900 nm. The results are interpreted on the basis of available data on radiation defects in silica core fibers. It is demonstrated that hydrogen loading provides a radiation hardness improvement, which allows to satisfy the ITER requirements.

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.

Role of Inherent Radiation-Induced Self-Trapped Holes in Pulsed-Radiation Effect on Pure-Silica-Core Optical Fibers

Radiation-induced attenuation (RIA) is investigated in pure-silica-core optical fibers (PSFs) upon their pulsed-X-ray irradiation. RIA at small post-pulse times (t ≤ 0.001–0.1 s) is argued to be mainly due to inherent self-trapped holes (STHs), whereas at large times, due to strain-assisted STHs, as shown by Girard et al . Significant suppression of inherent STHs is found to occur in PSFs upon pulsed pre-irradiation with a dose of just ∼100–200 Gy. Thus, a radiation-hardening effect is revealed for the first time, which is of practical value for PSFs to be exposed to pulsed irradiation. Fiber drawing temperature Td is found to have an essential effect on RIA upon pulsed irradiation. The optimal Td-values are estimated for PSF applications at small (t 0.01 s) post-pulse times.

Developments of scintillator-based soft x-ray diagnostic in LHD with CsI:Tl and P47 scintillators.

Multi-channel soft x-ray (SX) diagnostic has been used in the large helical device (LHD) to research magnetohydrodynamic equilibria and activities. However, in the coming deuterium plasma experiments of LHD, it will be difficult to use semiconductor systems near LHD. Therefore, a new type of SX diagnostic, a scintillator-based type diagnostic, has been investigated in order to avoid damage from the radiation. A fiber optic plate coated by P47 scintillator will be used to detect SX emission. Scintillation light will be transferred by pure silica core optical fibers and detected by photomultiplier tubes. A vertically elongated section of LHD will be covered by a 13 ch. array. Effects from the Deuterium Deuterium neutrons can be negligible when the scintillator is covered by a Pb plate 4 cm in thickness to avoid gamma-rays.

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm − 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H 2. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H 2 on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.

Influence of γ-Radiation Dose Growth and Dose Rates on Luminescence Properties of Pure Silica Core Fibers with High-OH Group Content

Luminescence properties of pure silica core optical fibers (OFs) with a high-OH group content are studied in the visible region at different doses and dose rates of 60Co γ-radiation. It is shown that the γ-induced light emission (GLE) spectra consist of Cherenkov emission along the luminescence bands. Different increase rates of the luminescence bands at 450 and 650 nm and the absorption band at 610 nm with increasing γ-radiation dose are observed. The 450 nm luminescence band is caused by the recombination of holes with the trapped electrons of E'-centers. The dose dependence of the 650 nm luminescence band, caused by the nonbridging oxygen hole center (NBOHC), does not coincide with that of the 610 nm absorption band related to this center. It is shown that a considerable part of NBOHC participates in nonradiative relaxation. Also, there is an absorption band caused by color centers different from NBOHC, that contributes to the formation of the 610 nm absorption band. A linear dependence of GLE intensity (at wavelengths of 450 and 650 nm) on the dose rate increase is observed. Such a dependence is caused by a substantial increase in the Cherenkov emission intensity and comparatively small increases in luminescence band intensities at 450 and 650 nm.

Study on pure silica core optical fibers

An optimal refractive index profile of pure silica core optical fiber (PSCF) was designed, in combination with the characters of the modified chemical vapor deposition (MCVD) process. Techniques of preform fabrication by a new furnace round heating MCVD process and fiber drawing process were reviewed. Difficulties in doping fluorine in silica, widening the depressed-index cladding and maintaining the index of fiber core were discussed. Methods used to overcome these difficulties were given at the same time. Additionally, the optimal refractive index profiles of PSCF were presented.

Measurement Method of Radiation Induced Emission Spectra of Optical Fibers

Radiation-induced light emission (RLE) spectrum of pure silica core optical fiber is measured under intense γ-irradiation of a source 60Co. It is shown that the spectrum is subjected to deformation due to high degree of reabsorption caused by γ-induced color centers. The method of measurement and calculation of true spectrum of emission in optical fibers is created. The formula of true intensity I(λ) obtained within the method takes into account the optical losses coefficient from registered RLE intensity value and allows to plot the true spectrum. The true RLE spectra of pure-silica core fiber irradiated at different integrated doses are measured and plotted using this method. It is shown that the method can be used to obtain the true RLE spectra of optical fibers under various types of irradiation.

Gamma-rays and pulsed X-ray radiation responses of nitrogen-, germanium-doped and pure silica core optical fibers

The sensitivities of three single-mode optical fiber types were characterized under γ(∼1 MeV)-rays and pulsed X(∼1 MeV)-ray radiation environments at room temperature. The radiation-induced attenuation (RIA) time dependent changes were measured at 1.55 and 1.31 μm for undoped, germanium- and nitrogen-doped core fibers. The nitrogen-doped core fiber exhibited the lowest RIA level for the time range 10 −6 to 10 +2 s after pulse and a non-linear RIA dose dependence for our tested dose range 0.01–1 kGy (SiO 2). This fiber showed also a good radiation response under steady-state γ-irradiation. The tested pure silica core fiber was the most resistant fiber under γ-rays, but had the highest RIA levels after a pulsed X-ray one. The radiation-properties of the germanosilicate fibers depend greatly on their cladding composition. The phosphorus-codoping of these fibers suppressed their pulsed X-ray RIA peaks but was responsible for the highest permanent RIA levels after both types of irradiation. Our results showed that the two environments lead to the same RIA levels in germanosilicate and nitrogen-doped fibers, implying that the same mechanisms and color centers are involved at the different dose rates. We proposed some explanations, based on spectroscopic measurements, concerning the influence of the different tested core dopants.

Growth of optical transmission loss at 850 nm in silica core optical fibers during fission reactor irradiation

Pure, OH-doped and F-doped silica core optical fibers were irradiated in a fission reactor at 400±10 K using an electric heater at a reactor power greater than 10 MW (20% of the full power). The temperature was not controlled well at the early stage of the reactor startup, when the temperature was about 320–340 K. The optical fibers were irradiated with a fast neutron ( E>1 MeV) flux of 3.2×10 17 n/cm 2s and a gamma dose rate of 3×10 3 Gy/s for 527 h. Optical transmission loss at 850 nm was measured in situ during irradiation. A prompt increase in optical transmission loss was observed as irradiation started, which was probably due to dynamic irradiation effects caused by short-lived and transient defects and is probably recoverable when irradiation ceases. After the prompt increase in optical transmission loss, a so-called radiation hardening was observed in fibers containing OH. Radiation hardening was also observed in 900 ppm OH-doped fiber at the second startup. The optical transmission loss increased linearly with irradiation dose, denoted as the accumulated loss, which we believe is due to irradiation-induced long-lived defects. Accumulated loss dominates radiation-induced optical transmission loss in a fission reactor irradiation.

Nonuniform distribution of oxygen hole centers in silica optical fibers

This letter reports spatially variant distributions of nonbridging oxygen hole centers (NBOHCs) in pure synthetic silica core optical fibers with fluorine doped silica claddings. Results of low-temperature x-band electron spin resonance (ESR) etchback measurements have established the presence of NBOHCs in increased concentration at the core-clad interfacial region. The observations on a low-OH content silica core fiber are contrasted with that on a high-OH fiber. The correlation between the ESR results and the spatially variant luminescent patterns observed in these fibers during excimer laser irradiation is also discussed.

A comparison of neutron and gamma damage effects on silica glass in a nuclear reactor radiation environment

A study of the relative damage effects of neutrons and gamma rays on silica glass in a nuclear reactor radiation environment is reported. The neutron and gamma energy spectra of the Ohio State University Research Reactor beam port #1 were applied to silica glass to obtain primary knock-on charged particle energy spectra. The resultant charged particle spectra were then applied to the polyatomic forms of the Lindhard et al. integrodifferential equation for damage energy and the Parkin and Coulter integrodifferential equation for net atomic displacement. The results show that near a nuclear reactor core the vast majority of the dose to silica is due to gamma rays (factor of roughly 40) and that neutrons cause much more displacement damage than gamma rays (35 times the oxygen displacement rate and 500 times the silicon displacement rate). However, pure silica core optical fibers irradiated in a nuclear reactor's mixed neutron/gamma environment exhibit little difference in transmission loss on an equal dose basis compared to fibers irradiated in a gamma only environment, indicating that atomic displacement is not a significant damage mechanism.

Radiation Response of Pure Silica Core Optical Fibers. IV

ABSTRACTThe radiation response of a series of optical fibers with pure silica cores, that differed as to residual hydroxide and chloride content and as to preform formation, have been measured after exposure to pulsed electron beam radiation to 10 krads [100 Gys]. The irradiations were carried out at both ambient temperature and at −50°C and with two different operating power levels at each temperature. The responses have been analyzed seeking to identify correlations between the differences in the residual chemical contents or preform formation and the observed differences in the initial response and the recovery behavior among the optical fibers in the series.

Radiation Effects on Pure Silica Core Optical Fibers by γ-Rays: Relation between 2 eV Band and Non-Bridging Oxygen Hole Centers

The defect center causing the 2 eV absorption band induced in pure silica core fibers by 60Co γ-rays is discussed. The 2 eV band and non-bridging oxygen hole center (NBOHC, SI–O·) show almost the same thermal annealing characteristics. The peak wavelength of the 2 eV band in low-OH (≦6 ppm) fibers is 630 nm, but shifts to shorter wavelengths according to the amount of OH group the sample contains, and appears at 600 nm in high-OH (≧700 ppm) fibers. The peak shape is broader in high-OH fibers than in low-OH fibers. This shift and broadening is attributable to a hydrogen bond between NBOHC and a hydrogen atom. Based on the above results, the defect center responsible for the 2 eV band is considered to be NBOHC.