In situ monitoring of gratings' characteristics during continuous gamma irradiation to a dose of 9 MGy revealed by an
order of magnitude weaker Bragg wavelengths drift as compared to gratings written in Ge-doped silica-core fibres.
The field of optical fiber sensing is highly diverse and this diversity is perceived as a great advantage over more conventional sensors in that an optical sensor can be tailored to measure any of a myriad of physical parameters. In this paper we present a niche application for optical fiber sensors in the domain of biophotonics, namely the monitoring of stress build-up during the curing process of dental resin cements. We discuss the origin of this stress build-up and the problems it can cause when treating patients. Optical fiber sensors aim at excelling in two kind of applications: firstly to perform quality control on batch produced dental cements and measure their total material shrinkage, secondly to monitor the hardening of the cement during in-vivo measurements resulting in the dynamic measurement of the shrinkage and to control the stress in a facing based restoration. We therefore investigated two types of optical fiber sensors as alternatives to conventional measurement techniques; namely polarimetric optical fiber sensors and fiber Bragg gratings written in polarization maintaining fibers. After discussing the results obtained with both optical fiber sensors, we will conclude with a critical assessment of the suitability of the two proposed sensing configurations for multi-parameter stress monitoring.
In this paper we report on the measurement of the refractive index profile of optical fibers exposed to the
gamma radiation. The tool we used for determining the refractive index distribution is microinterferometric
tomography. Nuclear radiation is known to affect the guiding properties of optical fibers and it is therefore essential to
characterize these effects to assess the applicability of fiber-optic technology for communication and sensing in space
applications and in nuclear industry. We show that the fibres exhibit a slight refractive index increase which confirms
results reported earlier.
We investigate the possible use as on-line dosimeters of commercially-available PMMA plastic optical fibres. The dose measurement is derived from the radiation-induced attenuation. A novel interrogation scheme based on a ratiometric technique is proposed for real-time dosimetry applications.
Distributed fibre optic sensors are being evaluated by the nuclear industry for monitoring purpose. We evaluate the radiation tolerance of distributed Brillouin sensors up to very high total gamma doses.
The sensitivities of type I and IIA fibre Bragg gratings written to different reflectivities in SMF-28 and B/Ge fibres to ionizing radiation up to 0.54MGy are investigated. The Bragg wavelength shows a small and rapid increase at the start of irradiation followed by either a plateau (type I) or a decrease (type IIA).
Long-period gratings were written in a pure-silica-core fiber using the electric arc technique. To assess the influence of stress relaxation on grating formation, tomographic stress measurements were performed inside the grating. No stress annealing could be observed within the accuracy of the stress measurement, suggesting a negligible photo-elastic contribution to the arc-induced perturbation of the index profile. In contrast, stress relaxation was found inside a splice of pure-silica-core fiber where discharge conditions are different, indicating the general potential to alter the fibers stress and thus the index profile using an electric arc.
The study of the effect of high optical power on FBGs among different laboratories whitin the COST270 action required to verify the consistency and convergence of Bragg grating measurements obtained by the laboratories involved in COST270. This papers reports and discusses the results obtained during a round-robin for fibre Bragg grating metrology initiated within the COST270 action. This action was undertaken in order to minimize measurement scattering and to reduce the uncertainty in the interpretation of following COST270 round-robin measurements on the effect of high optical power on FBGs. This exercise dealed with the spectral characterization of FBGs written in standard telecommunication optical fibre (CORNING SMF-28) provided by the coordinating laboratory (Ecole Polytechnique Federale de Lausane, Institut d'Optique Appliquee).
We have studied the gamma dose-rate effect on the radiation sensitivity of a phosphorous doped optical fiber. The radiation sensitivity significantly increases with decreasing dose-rates.
We have studied the effect of gamma radiation on Long Period Fiber Grating (LPFG) temperature sensors fabricated in a pure-silica fiber, using the arc discharge technique. The temperature sensitivity of the LPFGs remains unchanged by the irradiation.
The implementation of Wavelength Division Multiplexing (WDM) techniques in fiber optic data-communication links for nuclear installations requires to assess the radiation tolerance of passive WDM multiplexers. Nowadays, Fibre Bragg gratings (FBG) have become a key component in all-fibre WDM multiplexers. Up to now, only accelerated short term irradiation experiments of FBG filters have been discussed, suggesting possible ways to optimise their radiation tolerance. In this paper, we evaluate the long-term behaviour of FBGs filters, written in different types of optical fibres, after two years of continuous low-dose rate gamma-neutron irradiation. We compare these results with our previous accelerated tests. We evidence that FBG filters can withstand years in a nuclear environment, while keeping their filtering properties at an acceptable level.
The future thermonuclear fusion reactor ITER will require remote-handled equipment to monitor its operation and to allow hazard-free manipulations during its frequent maintenance periods. Heavy shielded umbilicals will be required to connect the sensors and the actuators with their instrumentation. Multiplexing sensor signals turns out to be essential to ease the umbilical management. We are considering fibre optic technology, with its intrinsic wavelength multiplexing (WDM) capabilities, to handle these ITER multiplexing issues. We propose a new analog data link design for low-bandwidth sensors and actuators based on commercial-off-the-shelf (COTS) fiber optic components. We rely on passive components such as WDM couplers and fibre Bragg gratings (FBG) to build a radiation-resistant analog data link. WDM couplers remain operational up to a 13 MGy gamma total dose. A radiation-induced channel drift is observed. The refractive index change under ionizing radiation is proposed as the degradation mechanism. FBG filters continue to operate satisfactorily up to a 150 MGy total gamma dose and a neutron fluence of about 1015 n/cm2. Our results on these COTS all-fibre passive components open perspectives to build a radiation-tolerant analog optical data link compatible with the ITER requirements.
In-core temperature measurment is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing capabilities of FThre Bragg Grating (FBG) temperature sensors to perform the in-core temperature measurements. We first report on our irradiation experiments on multiplexed FBGs, written in different fibres, employed to measure the in-pile temperature of an air-cooled graphite-moderated nuclear reactor. For some FBGs the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the difference saturated after 30 hours of reactor operation at about 5°C. Afterwards, we irradiated multiplexed FBG sensors in our material testing nuclear reactor, evidencing the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care and in well-specified conditions.
We have designed a compact multi-component force sensor using an elastic transducing body and eight multiplexed fibre Bragg gratings. We discuss its design and calibration issues.
Optical fiber technology is considered now for communication and sensing applications in various radiation environments, like space and nuclear industry. We report on results from an on-going experimental program, which aims at using multiplexed Fiber Bragg gratings (FBGs), essential photonic components, for in-pile temperature monitoring in a nuclear reactor. To the best of our knowledge, it is the first time that multiplexed FBG-sensors are used in such conditions.
Optical fiber sensors (OFSs) offer numerous advantages, which include immunity to electromagnetic interference, intrinsic safety, small size, a possibly high sensitivity, multiplexing capabilities, and the possibility of remote interrogation. However, OFSs have a relatively low penetration in the commercial market, which is still dominated by standard electromechanical sensors. Nuclear environments are an example where particular OFSs might have a distinct superiority in the competition, but the feasibility of using OFSs in radiation environments still needs to be assessed. In the present paper we report on irradiation experiments performed to provide a sound basis for the design of a fiber Bragg grating based sensor capable to operate even under high total dose exposure.
We report on our irradiation experiments on different types of fiber-optic sensors, including three types of commercially available temperature sensors, a multimode extrinsic Fabry-Perot cavity strain sensor and fiber Bragg- gratings. For the temperature sensors, results show that gamma radiation does not interfere with the basic sensing mechanism and that the most critical component turns out to be the optical fiber itself. Semiconductor absorption temperature sensor showed no degradation up to total doses of 250 kGy, whereas the specifications of Fabry-Perot type sensor and fluorescence temperature sensors were already dramatically influenced below the kGy-level. Replacing the optical fiber by a more radiation resistant version allowed to increase the radiation hardness of the fluorescence sensors system by orders of magnitude. The use of fiber- optic sensors in the presence of neutron radiation remains compromised. Similar conclusions are valid for the Fabry- Perot type fiber-optic strain sensors. We finally show that the Bragg-grating resonance wavelength can shift with radiation dose, but that the temperature sensitivity remains unaltered.
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