In this work we present a study on teh Super Luminescent LIght Emitting Diodes (SLEDs) performance under high doses of gamma radiation. We investigate GaAs SLEDs with emission wavelengths around 830 nm. The devices were exposed to ionising radiation at a dose rate of about 4.7 Gy/s, up to a cumulated dose of 10.1 MGy in the CMF facility of the Belgian nuclear research centre SCK•CEN. We measured the device characteristics before adn after irradiation. We show that the SLED performance is only marginally affected.
We discuss on-going reliability studies of micro-optical components and assemblies as conducted in the EU FP6 Network of Excellence on Micro-Optics "NEMO". We focus on three case studies including first biaxial fatigue testing of micro-optical components, second reliability testing and quality control of MEMS and third micro-interferometric tomography for measuring optical fibre refractive index changes. For each of these case studies we discuss the dedicated measurement and characterization methods as well as first results and the perspectives for future research.
In addition to the numerous assessments presented in literature of the radiation-induced optical property changes in
optical fibres, we investigate the impact of MGy dose levels on the mechanical properties of different single and
multimode fibres. We compare the tensile test and two-point bending test results obtained by two different laboratories
with commercially available optical fibres before and after irradiation up to 15 MGy. Our results show a significant
strength reduction of about 50 % at high dose levels, relevant for particular applications in nuclear power facilities and in
large nuclear physics experiments. This strength reduction seems to depend on both the coating materials and the test
conditions, as suggested by the roughening of the outer glass surface of the optical fibre, observed with atomic force
microscope (AFM) images.
Maintenance tasks of the future International Experimental Thermonuclear fusion Reactor (ITER) will require communication links between the remotely operated equipment in the reactor vessel and the control room, some of which need to be radiation tolerant up to MGy dose levels. As a key element of opto-electronic transceivers, we therefore assessed the DC behavior of a commercial-off-the-shelf (COTS) SiGe heterojunction bipolar transistor (HBT) under gamma radiation up to 15 MGy, with dose rates from 160 Gy/h to 27 kGy/h. Our in-situ measurements of the forward DC current gain (hfe) present a limited loss of about 30 % for a base current of 100 μA, with a dependence on the biasing conditions and a thermally activated recovery. These first ever reported results up to MGy levels allow us to design circuit-hardened driving electronics for both photonic transmitters and receivers, enabling high bandwidth communications applied in a fusion reactor environment.
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.
Microinterferometric tomography allows for determination of three-dimensional refractive index distributions in phase isotropic microelements. In this paper we present the measurement of the refractive index profile of the optical fibers exposed to the gamma radiation. 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 nuclear industry. It is already well known that radiation affects the absorption of the fiber. Here, we investigated whether we could quantify the effect of radiation on the refractive index of core and cladding of an optical fiber.
Future thermonuclear fusion reactors need remote-handled equipment for maintenance tasks, since the stringent environmental conditions prohibit direct human interventions. Fiber-optic technology is considered since many years as a potential reliable alternative to conventional electronic transmission lines. Recently we demonstrated the feasibility of transmitting analog data with a hybrid opto-electronic link at 850 nm, up to total doses of several MGy. However, for bidirectional communications under these severe conditions, we still need to characterize the corresponding photo detector response and design an adapted radiation tolerant amplifier. We therefore assessed in-situ the radiation response of commercially available p-i-n type Si-detectors, at a constant temperature of about 60°C, first under gammas rays up to a total dose of about 10 MGy and under neutrons up to a fluence of about 7•1015/cm2. We also performed similar tests with InGaAs photodiodes at different wavelengths, in order to assess their use in radiation tolerant coarse wavelength division multiplexing (CWDM) transmission architectures. Our results indicate that the wavelength dependence of the detectors' response under gamma radiation remains almost unchanged. We observed no catastrophic failure for these InGaAs devices, nor for the Si devices up to 10 MGy. The increase of the dark current is the most obvious radiation effect, particularly under neutrons. In this paper we also present temperature dependent measurements and hence assess to what extend both ionizing and particle radiation affect the photodiodes reliability.
Future thermonuclear fusion reactors need remote-handled equipment for maintenance tasks, since the stringent environmental conditions prohibit direct human interventions. Fiber-optic technology is considered since many years as a potential reliable alternative to conventional electronic transmission lines. Recently, we demonstrated the feasibility of transmitting analog data with a hybrid opto-electronic link at 850 nm, up to total doses of several MGy. For bidirectional communications under these severe conditions, however, we still need to characterize the corresponding photo detector' response and design an adapted radiation tolerant amplifier. Moreover, the neutron sensitivity needs to be assessed as well, since this could prevent them to be stored inside the bioshield during the plasma burns. We therefore assessed the radiation response of commercially available p-i-n type Si-detectors, on line, at a constant temperature of about 60°C, first under gammas rays up to a total dose of about 10 MGy, and also under neutrons up to a fluence of about 7•1015/cm2. Similar tests were also performed with InGaAs photodiodes, in order to assess their use in radiation tolerant coarse wavelength division multiplexing (CWDM) transmission architectures. Our results indicate that the response of the detectors under gamma radiation remains almost unchanged. The displacement damage caused by the neutrons steadily increases with the neutron fluence, both in terms of an increased dark current and a reduced photo response. In this paper we also present temperature dependent measurements and hence assess to what extend both ionizing and particle radiation affect the photodiodes reliability.
KEYWORDS: Optical fibers, Temperature metrology, Single mode fibers, Environmental sensing, Environmental monitoring, Multimode fibers, Standards development, Reflectometry, Signal attenuation, Signal to noise ratio
Optical fibers stimulate much interest since many years for their potential use in various nuclear environments, both for radiation tolerant and EMI-free data communication as well as for distributed sensing. Besides monitoring temperature and stress, measuring ionizing doses with optical fibers is particularly essential in applications such as long-term nuclear waste disposal monitoring, and for real-time aging monitoring of power and signal cables installed inside a reactor containment building. Two distinct options exist to perform optical fiber dosimetry. First, find an accurate model for a restricted application field that accounts for all the parameters that influence the radiation response of a standard fiber, or second, develop a dedicated fiber with a response that will solely depend on the deposited energy. Using various models presented in literature, we evaluate both standard commercially available and custom-made optical fibers under gamma radiation, particularly for distributed dosimetry applications with an optical time domain reflectometer (OTDR). We therefore present the radiation induced attenuation at near-infrared telecom wavelengths up to MGy total dose levels, with dose rates ranging from about 1 Gy/h up to 1 kGy/h, whereas temperature was raised step-wise from 25 °C to 85 °C. Our results allow to determine and compare the practical limitations of distributed dose measurements with both fiber types in terms of temperature sensitivity, dose estimation accuracy and spatial resolution.
Optical fiber technology is seriously considered for communication and monitoring applications during the operation and maintenance of future thermonuclear fusion reactors. Their environment is characterized, in particular, by possibly high gamma dose-rates and total doses in excess of 10 MGy. In addition, the maintenance equipment might be stored in close vicinity of the reactor during its operation and therefore the communication devices might also be exposed to a substantial neutron fluence. The feasibility of applying photonic technology in these radiation fields therefore needs to be assessed. Whereas many reports deal with the radiation behavior of a variety of fiber-optic devices, only little information is available on the radiation tolerance at high total dose (e.g. > 1 MGy). We describe our recent results obtained on vertical-cavity surface-emitting laser (VCSEL) assemblies. We have conducted high total dose (up to 20 MGy) irradiation experiments on such devices, which confirmed their excellent gamma radiation hardness. The optical power loss at nominal forward current was less than 2 dB and the threshold current remained unaltered. We have also irradiated these devices with neutrons inside the BR1 reactor (SCK•CEN, Mol, Belgium) up to a total fluence on the order of 1015 n∙cm-2. The response of VCSELs to neutrons is, as expected, different from that to gamma radiation. VCSELs previously exposed to gamma rays exhibited an accelerated degradation under neutron radiation compared to not pre-irradiated devices. The beneficial effect of applying a continuous forward bias to the VCSELs is also evidenced.
Radiation tolerant fiber optic transmission links are desired for their potential use in future thermonuclear fusion reactors and other harsh radiation fields. As part of it, we design and test a radiation opto-electronic transmitter based on vertical-cavity surface-emitting lasers (VCSELs) and dedicated driver electronics consisting of discrete components. VCSELs have already demonstrated their high radiation acceptance level. We confirm this by on-line irradiation experiments on such devices up to 10 MGy total dose. For the design of the driver circuit, we rely on discrete commercial-off-the-shelf (COTS) bipolar transistors. When the radiation induced degradation of these components is considered within the time design of the circuits, total dose levels larger than 1 MGy can be tolerated. The driver uses standard TTL input signals and delivers a forward current of 12 mA to a pigtailed 840 nm VCSEL. SPICE simulations show that the driver still delivers a sufficient forward current to the VCSEL, in spite of the radiation induced degradation of the Hfe and VCEsat values of the transistors. These simulations are verified by our experiments. At a total dose of 1 MGy, the measured decrease of the forward current is only about 8%. This induces an optical output power decrease that can still be tolerated with irradiated VCSELs, as shown by our experiments. We conclude that a high total dose hardened optical transmitter for use in nuclear instrumentation systems can be fabricated using discrete COTS bipolar transistors, COTS vertical-cavity surface-emitting lasers and COTS optical fiber.
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.
KEYWORDS: Optical fibers, Temperature metrology, Single mode fibers, Data modeling, Color centers, Gamma radiation, Multimode fibers, Phase only filters, Environmental sensing, Thermal effects
The many advantages of optical fibers for their use in various nuclear environments ushered the respective communities to extensive studies over the last decades. Forecasting the behavior of fiber-optic links exposed to ionizing radiation still remains an important issue. We have developed an industry-aimed pragmatic method based on a simple model for the prediction of radiation induced losses in commercially available optical fibers exposed to 60Co gamma rays. When environmental and measurement conditions are well defined, long-term losses could be predicted with a precision of about 15%, for dose rates ranging between 100 Gy/h and 3 kGy/h and total doses up to MGy levels. Thermally induced effects were also considered, between ambient temperature and 80 degree(s)C. From an interpretation of these results, we discuss its applicability and potential further improvements.
Optical fiber technology is seriously considered for communication and monitoring applications during the operation and maintenance of future thermonuclear fusion reactors. Their environment is characterized, in particular, by possibly high gamma dose-rates and total doses up to 100 MGy. The feasibility of applying photonic technique in such intense radiation fields therefore needs to be assessed. Whereas many reports deal with the radiation behavior of a variety of fiber-optic devices, only little information is available on the radiation tolerance at high total dose (e.g. > 1 MGy). We describe our recent results obtained at fiber-optic components intended for ITER (International thermonuclear Experimental Reactor) remote-handling applications. We have conducted high total dose (up to 15 MGy) irradiation experiments on a variety of COTS fiber- optic devices, including edge-emitting laser diodes, vertical-cavity surface-emitting lasers, PIN photodiodes and single-mode optical fibers. A remarkably low radiation induced loss was obtained on a single-mode pure silica core optical fiber, whereas VCSELs confirmed their excellent radiation hardness. With the exception of photodiodes, the optical characteristics of selected fiber-optic devices seem to be able to cope with high total gamma doses. However, our results also indicate that radiation induced degradation of connector assemblies might limit their use in severe radiation environments.
Whenever equipment is intended for use in hostile environmental conditions or when severe reliability demands constrain its functional specifications, very specific research is required to assess the successful operation of individual devices and subsystems. This statement is certainly valid as far as both space and nuclear applications are concerned. Indeed, space and nuclear environments show several similar characteristics, among which the presence of ionizing and particle radiation as well as extreme pressure or temperature conditions. Even if the basic environmental conditions such as dose rates, total doses, particle types, pressure and temperature ranges may differ, the fundamental effects which influence or degrade the specifications of devices often remain the same. Hence, we are convinced that space and nuclear communities have to learn from each other's complementary experience. This perspective serves as a frame for the results reviewed in this paper, as they are issued from research and development efforts of (European) nuclear actors on one hand and space industry on the other. After an introductory comparison of nuclear and space radiation environments, three complementary contributions are highlighted. The first two focus on radiation testing and include radiation effects on two modem individual photonic devices as well as standard irradiation test procedure definitions for fiber-optic systems. Then we turn to a major part of this paper which describes advanced photonic systems especially designed for space applications.
Recent developments of stability control in mines, essentially based on Ge-doped Fibre Bragg Gratings
(FBG) are reported including results about the different aspects of the system : accurate characterisations of FBG,
sensor network topology and multiplexing method, user interface design and sensor packaging.
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