Optical fibre Bragg grating sensors for structure
monitoring within the nuclear power plants

Pierre Ferdinand, Sylvain Magne, Véronique Marty, Stéphane Rougeault, Pascal Bernage, Marc Douay, Éric Fertein, Franck Lahoreau, Pierre Niay, Jean-François Bayon, et al.

► To cite this version:

Pierre Ferdinand, Sylvain Magne, Véronique Marty, Stéphane Rougeault, Pascal Bernage, et al. .. Optical fibre Bragg grating sensors for structure monitoring within the nuclear power plants. Optical fibre sensing and systems in nuclear environments, Sep 1994, Mol, Belgium. 10.1117/12.198636. cea-04186576

HAL Id: cea-04186576

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# Optical Fibre Bragg Grating Sensors for Structure Monitoring within the Nuclear Power Plants

P. Ferdinand, S. Magne, V. Marty and S. Rougeault

LETI (CEA-Technologies Avancées) DEIN - SPE - CE/Saclay, 91191 - Gif-sur-Yvette Cedex,
FRANCE

Tel : 33 - 1 - 69.08.83.39, Fax: 33 - 1 - 69.08.83.95

P. Bernage, M. Douay, E. Fertein, F. Lahoreau and P. Niay

Laboratoire de Dynamique Moléculaire et Photonique, URA CNRS 779, Université des Sciences et
Technologies de Lille, UFR de Physique Bâtiment P5, 59655 - Villeneuve d'Ascq, FRANCE

Tel : 33 - 20.43.47.62, Fax: 33 - 20.47.40.84

J.F. Bayon, T. Georges and M. Monerie

France Telecom CNET LAB/RIO/TSO, Route de Trégastel, 22301 - Lannion Cedex, FRANCE

Tel : 33 - 96.05.28.28, Fax: 33 - 96.05.13.07

## ABSTRACT

A new kind of instrumentation based on Optical Fibre Bragg gating sensors is proposed for the main relevant monitoring needs in the electric power industry. The challenging metrological properties of these components are presented and their good resistance to γ-ray irradiations experimentally proved.

## INTRODUCTION

Instrumentation has ever been one of the essential aspects of the R&D in the nuclear power sector due to the large panel of physical parameters to determine (temperature, pressure, strain, electromagnetic field, radiation doses,...) and strength the need to improve both the safety of operations and the monitoring of equipments in Nuclear Power Plants (NPP) [1].

In the early eighties, guided-wave sensors based on already known but unexploited physics laws began to appear on an industrial ground. These demonstrators and products are better known today under the name of Optical Fibre Sensors (OFS); they can be used separately or together according to many topologies such as: serial, parallel, star, hybrid,... and so form Optical Fibre Sensor Networks (OFSN).

For several years, universities, industrial and military laboratories, have shown an extraordinary enthusiasm in developing OFS(s) and OFSN(s). In this ever-increasing and exciting field, LETI-CEA, one of the main French R&D laboratories, is largely involved in OFS(N) devoted to nuclear applications.

## 1. REVIEW OF THE POTENTIAL NEEDS FOR OFS(N) IN NUCLEAR POWER PLANTS

These factors have prompted LETI-CEA to maintain its activities in this field of fully innovative instrumentation. In this framework, during the two past years, LETI-CEA together with the end-user EDF (Electricité de France) and the nuclear firm FRAMATOI, IE wanted to evaluate this new technology of instrumentation, from an industrial point of view. Thus, a task force named CORA 2000, was set up to start a constructive dialogue between the nuclear energy actors and the fibre optic community, and to answer the following essential questions :

- what kind of OFS(N) are currently being developed or are actually available on the market?

- what are the main operating configurations for which OFS(N) can provide a real potential gain (technical and economical), either to retrofit the actual NPP, or for the new generation of NPP?

The initiatives under this programme were taken against a background of the “life cycle” and the “REP 2000” projects (REP: Réacteur à Eau Pressurisée, ie the french PWR (Pressurised Water
Reactor) technology). The "life cycle" project is devoted to the study of the technical implications of the decision to increase the lifetime of NPP from 20 to 40 years. This decision involved the technological upgrading of elementary systems, the replacement of major components,... and the installation of an improved monitoring system to successfully overcome the phase of obsolescence. The "REP 2000" project is looking into the technological outline of the next generation of REP NPP, which will appear in the years 2015-2020. Both projects require a high-capacity data processing and sensing, and an entry system offering high data throughput, safe and trouble-free operation.

Today, the CORA 2000 task force prospective enquiry shows that, at least, two configurations make the use of the OFS(s) and OFSN(s) clearly advantageous [2]:

1) the very specific measurements required cannot be performed with another sensor technology,

2) an improvement in the monitoring of equipment is recognised sensitive in terms of availability (generator, transformer,...) as well as large structures (containment shield, cooling towers, primary and secondary pipes,... and dams within the hydraulic production), as described in Table 1.

These orientations are due to the fact that new doctrines of predictive maintenance and monitoring emerge. This trend demands better and better equipment-monitoring sensors and improved processing systems. As seen later on, in this document, OFSN, and more specifically optical sensor networks based on Bragg gratings, offer the possibility to carry out accurate and reliable measurements. These kinds of networks offer effective data feedback and improved equipment availability. They can also contribute to save on installations, maintenance and extension costs

In practice, while EDF has carried out some studies on some specific applications (e.g. monitoring of generators, pressurised water leak detection,...), LETI-CEA is obviously somewhat more involved in the R&D aspects of the tasks and more accurately in specific sensing such as advanced structure monitoring. From a technological point of view, developments in our Laboratory mainly deal with the Optical Fibre Sensing and Networking aspects. In fact, as we can see in the following table, the use of a new kind of transducer named **Bragg grating** is extremely interesting because it **allows to perform a lot of type of measurement, both in term of sensing (temperature, strain, pressure,...)** and **in term of network topology, due to an intrinsic spectral encoding** As a consequence, this approach looks very attractive, from the end user-point of view, because a part of the R&D costs could be shared between some different applications. The main applications selected by the CORA 2000 task force are depicted in Table 1.

<table><thead><tr><th>Application(s)</th><th>Domain/<br>Priority</th><th>Solution(s)</th><th>R&D time<br>before<br>industrialisation</th></tr></thead><tbody><tr><td>Concrete structure:</td><td>Monitoring</td><td></td><td></td></tr><tr><td>- Containment shield (structural integrity, nuclear safety during and/or after accident)</td><td>2</td><td><strong>Bragg grating</strong> networks embedded (or bonded) for temperature, pressure dans strain measurements</td><td>4 years<br>//<br>//</td></tr><tr><td>- Cooling towers, dams (structural integrity),</td><td>2</td><td></td><td></td></tr><tr><td>- Generators flooring (integrity, stability)...</td><td>2</td><td></td><td></td></tr><tr><td>Generator:</td><td>Monitoring</td><td></td><td></td></tr><tr><td>- Temperature (copper bars and/or stator)</td><td>1</td><td>OTDR, Spectral modulation encoding,</td><td>2 years<br>3 years</td></tr><tr><td>- Vibrations (front ends)</td><td>1</td><td>Fluorescence; Bragg gratings,...<br>Silicon cantilever beam,...</td><td>//</td></tr></tbody></table>
<figure><img src="image_2.png" /></figure>
<table><caption>Table 1: Main relevant needs for OFS(N) in Nuclear Power Plants</caption><thead><tr><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>Electrical equipments:</td><td>Monitoring</td><td></td><td></td></tr><tr><td>- Temperature in transformers (thermal gradients and hot spot detection)</td><td>1</td><td>Fluorescence, Raman Dist. Temp. Sensor</td><td>2 years</td></tr><tr><td>- Lines and cables (temperature distribution along the full length to optimise performance)</td><td>1</td><td>Temperature Bragg grating networks (serial, star,...)</td><td>//</td></tr><tr><td>Primary/secondary circuits:</td><td>Monitoring</td><td></td><td></td></tr><tr><td>- Steam leakage of large pipes (hot spots, leaks of pressurised water or steam)</td><td>1/2</td><td>DTS Raman,...<br>Temperature Bragg grating serial network</td><td>2 years<br>3 years</td></tr><tr><td>H<sub>2</sub> detection:</td><td>Safety</td><td></td><td></td></tr><tr><td>- Multipoint detection in containment shield (especially during and/or after accident)</td><td>2</td><td>Interferometry + Pd-coated fibre,<br>Spectroscopy</td><td>5 years<br>//</td></tr><tr><td>Fire detection:</td><td>Safety</td><td></td><td></td></tr><tr><td>- Multipoint detection in containment shield (in explosion-hazard areas, such as inside reactor coolant pump bunkers due to presence of large amount of oil)</td><td>2</td><td>Gas detection (combustion), IR pyrometry (fire), attenuation and/or scattering (dust), temperature Bragg grating networks (serial, hybrid,...)</td><td>5 years<br>//<br>//</td></tr></tbody></table>

The first applications listed above, that is to say the main applications selected by the task force CORA 2000, deal with monitoring of “structures” in the fullest meaning of the word: a line, a cable or a copper bar are 1D-structures while a transformer is a 1D coiled copper conductor and a concrete containment shield looks like a 2D-concave surface even if it is some meters thick...

## 2. BRAGG GRATINGS FOR STRUCTURE MONITORING

It has been proved that in-fibre Bragg gratings show remarkable sensitivities when exposed to environmental stress conditions. As recently proposed by different authors [3 to 7], Bragg gratings offer very interesting characteristics which allow their use in structure monitoring applications. To a great extent, the upmost used feature of these in-fibre gratings is optical: they show a narrow-band reflectivity (~ 0.1 nm) at their resonant wavelength which can be accurately defined during the writing process (Fig.1). From the end-user point of view, these intrinsic sensors present a lot of advantages: they are small (some mm long and diameter < 0.2 mm), well localised (quasi-distributed networks), etc. Therefore, they can be embedded into materials and structures (smart structures). Moreover, they are sensitive to the main relevant parameters needed for structure monitoring within power plants (i.e. temperature, pressure and strain).
<figure><img src="image_3.png" /><figcaption>Figure 1: Bragg grating effect in a single mode fibre</figcaption></figure>

To achieve multiplexing and demultiplexing, different solutions have been proposed in the literature. The wavelength analysis for both sensing and multiplexing seems to be the simplest and the more pragmatic one, and also presents many advantages. As a matter of fact, we already know that any measurement based on a spectral signature gives absolute and inherently self-referencing results, even in the case of a discontinuous acquisition procedure, which is an advantage in practical monitoring applications where sensors are interrogated in time-sharing (no counting methods are required such as for interferometric fringe-counting procedure which indeed requires a continuous acquisition procedure). Moreover, in case of spectral analysis, the lead fibres used to transmit optical signals between the optoelectronic system and the Bragg gratings are insensitive to external perturbations because the spectral line determination does not depend on the level of the detected optical power (this is a crucial point for NPP applications where nuclear radiations darken fibres). So, remote sensing, up to kilometres, with any separation length between sensors (ranging from cm up to ~100 m) can be achieved for each specific application.

An extra economical advantage is linked to the possibility, with the same opto-electronic system (acquisition/demultiplexing), to implement and reconfigurate any specific network (star, series, parallel, fish-bone, hybrid) long after the setting-up, thus increasing the return on investments.

### 3. SENSITIVITY OF BRAGG GRATINGS TO PHYSICAL PARAMETERS

A well known characteristic of a Bragg grating: it reflects a specific wavelength $\lambda_g$, as defined by the classical Bragg relation: $\lambda_g = 2n \cdot \Lambda$ where $\Lambda$ is the spatial period of the photo-induced grating and $n$ the effective index of the propagating mode of the fibre. So, any modification of one of these two parameters modifies $\lambda_g$. Classically, the Bragg grating central wavelength $\lambda_g$ changes nearly linearly with limited changes in temperature (T), strain ($\epsilon = \Delta L/L$) and pressure (P) as follows: $\Delta\lambda_g/\lambda_g = a \cdot \Delta T + b \cdot \epsilon + c \cdot \Delta P$, where each parameter (a, b and c) depends on the thermal expansion coefficient, the photoelastic constant $P_c \sim 0.22$ and the Young modulus of the fibre materials. They are:

$$a = [\delta\Lambda/(\Lambda \cdot \delta T) + \delta n/(n \cdot \delta T)]^\circ; b = [1 - P_e] \text{ and } c = [\delta\Lambda/(\Lambda \cdot \delta P) + \delta n/(n \cdot \delta P)]$$

So, starting from typical data of silica, we can calculate temperature, strain and pressure sensitivities. At $\lambda = 1.3 \mu m$, near room temperature, they are respectively on the order of : 10 pm/K ; 1 pm/µε (with 1µε = 1µm/m) and - 5 pm/MPa.
## 4. MOTIVATIONS FOR STUDYING BRAGG GRATING BEHAVIOUR UNDER γ-RAY IRRADIATION

It is now clear that Bragg gratings are very attractive for sensing in NPP and especially for structure monitoring (in situ measurements with embedded sensors for instance). Nevertheless, before proposing such a new sensing technology and related equipments to the nuclear end-users, it is essential to know the behaviour under irradiation not only of the optical fibres but also of the sensing elements (Bragg gratings), both being exposed to an hostile environment during normal operations [8], and in even more dramatic conditions in case of nuclear accidents [9] [10]. A lot of papers have been published during the last decade concerning silica-based fibre behaviour during short or long-term steady-state gamma-ray exposures at low, or high dose rates.

When fibres are exposed to nuclear radiations, color center formation in the optical core or cladding severely degrades their transmission [11 to 17]. A very wide variation in radiation responses of fibres occurs according to their compositions and fibres with pure silica core have demonstrated the best performances (Dainichi Nippon, Heraeus Fluosil for instance). In the visible and near IR domain the radiation-induced absorption decreases as the wavelength increases. 1.3 µm and 1.5 µm are thus the more useful wavelengths from the standpoint of both intrinsic and radiation-induced attenuations [11]. In many cases, this last damage is virtually permanent although a recovery is possible (mostly for pure silica core fibres) which kinetic strongly depends on temperature (several minutes at 80°C for instance) [11] [14]. Because of this temperature-dependent recovery phenomenon, the induced loss growth is resolved into two terms: a dose-dependent and a dose-rate dependent one. In normal operation (low dose rate), the dose rate dependent term is negligible while in nuclear accident or extreme conditions (high dose rate), both dose and dose-rate dependent terms are of importance, the first one being dominant at temperature higher than about 100 °C [14].

At LETI-CEA, the experience, based on more than ten years of experiments [16], allows us to determine which fibres, processing and materials are able to resist, in term of induced attenuation or bit error rate (BER) in numerical transmissions. It is now well established that the best results (~10⁻⁹ BER) are obtained with pure-core silica fibres, especially step-index polymer-clad optical fibres transmitting 850 nm light (~ 3 dB/km under 0.2 Gy/h over one year), graded-index fluorine-clad multimode fibres (4.5 dB/km in the same conditions) and pure silica single-mode optical fibres (both under normal operations and under accident) at 1.3 µm [17].

Unfortunately, up to now, grating growth has not been reported in fibres with pure silica core. However, photosensitivity has been demonstrated in a large variety of silica-based fibres doped either with germanium [18]-[19], europium [20] or cerium [21]-[22]. The exact mechanisms of the light-induced changes in refractive index are not yet fully understood but it seems that a common rule to all these dopants is to provide absorption bands within the fibre core. Absorption of light at a convenient wavelength is followed by photoionization of ions or atoms and then by charge transfers to trap sites inducing a photodarkening of the glass.

Various considerations show that the germanium-doped core fibres are, at this time, the most interesting photosensitive fibres [23]. Indeed, massive amounts of these fibres are produced at low cost for telecommunication applications. Writing a grating within these fibres does not induce significant excess loss at wavelengths interesting for optical sensor applications. Moreover, numerous methods of photosensitization of germanium-doped core fibres have been recently reported. They are reviewed in [24]. For example, low temperature high pressure hydrogen loading is able to induce large change in refractive index (~ 3.10⁻³) even in fibres which otherwise show poor photosensitivity [25]. Furthermore, long life span of gratings written in germane-silicate fibres has been reported by various groups [24]-[26]. However, although researches on radiation response of optical fibres doped with germanium have been undertaken for a long time, little is known about the effects of ionising radiations on gratings written within these fibres. The search for laws ruling the evolution of grating
characteristics (peak reflectivity, Bragg wavelength or Bragg wavelength variations versus temperature or strain) under γ-ray irradiation is highly desirable before planning any application of these grating devices to Nuclear Power Plants [27].

Paying attention to Bragg grating behaviour under ionising radiation is also pertinent from a theoretical point of view because we already know that a proposed mechanism of grating formation within germane-silicate fibres involves Ge-related colour centres, the latter being also created by ionising radiations [28]-[29].

# 5. EXPERIMENTS AND RESULTS

## 5.1 Methodology

Bragg gratings have been written in five single-mode GeO₂-fibres and exposed to γ-irradiation. The main characteristics of these five single-mode germane-silicate fibres are summarised in Table 2. The fibres labelled C, E, G are manufactured by the CNET laboratory in Lannion. Although fibres with a germanium-doped core and ultra low metal impurities concentration (< ppb) present a good stability under γ-ray irradiation [30], the radiation behaviour of our fibres is not so good, due to the presence of phosphorus in the cladding [31]. The fibres labelled K and L in Table 2 are commercially available polarisation-preserving fibres (High Birefringence "Hi-Bi" D Shape 820S2 and "Hi-Bi" D Shape 1300S2 from Andrew Corporation). The reason why we have chosen to write gratings within these five fibres results from the following grounds. Fibres E, K and L are well-matched to be used in Laser and OFS applications. With regard to the fibres C or E, we have previously reported that UV exposure leads to the photo-inscription of a Bragg grating (type I) followed, for a lasting exposure, by the erasure of its first order radiation spectrum and then by the formation of a new grating spectrum (type IIA) [32-33]. Various experimental results show that at least two photo-refractive effects could be involved in the complex dynamical evolution of grating transmission spectra observed in these fibres [32-33].

Thus, it was attractive for us to investigate the behaviour of the gratings written under UV exposure lasting enough to lead either to the first inscription or to the new grating formation. Refractive index modulations as high as $10^{-3}$ can be currently induced within the fibre G under UV exposure without any additional photo-sensitisation such as a hydrogenation treatment. Consequently, the photo-sensitivity of the fibre G is very high. We could expect that the fibre G was also highly sensitive to radiations.

<table><caption>Table 2: Characteristics of silica fibres used in the study</caption><thead><tr><td>Fibre type</td><td>Cladding dopants</td><td>Core dopants (mole %)</td><td>Core diameter</td><td>Inner cladding diameter</td><td>LP<sub>11</sub> cut-off wavelength</td></tr></thead><tbody><tr><td>C</td><td>F, P</td><td>Ge (28)</td><td>2.2 μm</td><td>20 μm</td><td>0.97 μm</td></tr><tr><td>G</td><td>B, P</td><td>Ge (11.5)</td><td>5 μm</td><td>42 μm</td><td>1.15 μm</td></tr><tr><td>E</td><td>F, P</td><td>Ge (15), Al (1.8), (Er<sup>3+</sup> 50 ppm)</td><td>2.2 μm</td><td>20 μm</td><td>0.96 μm</td></tr><tr><td>K</td><td>F</td><td>Ge, F</td><td>Elliptical core</td><td>D shape</td><td>0.7 μm</td></tr><tr><td>L</td><td>F</td><td>Ge, F, core cladding index difference ~0.035</td><td>Elliptical core</td><td>D shape</td><td>1.04 μm</td></tr></tbody></table>

The methods for producing the Bragg gratings and for monitoring their formation are related in details in [32]-[33]. The gratings were written using either a 243 nm pulsed source (LPX 105i - LPD 3002E system from LAMBDA Physics) or a 244 nm intracavity frequency-doubled Continuous-Wave (CW) Argon ion laser (FRED system from Coherent Corporation and an other home-made laser). The pulsed
or CW fringe patterns were obtained using a prism interferometer [34]. By a pulsed exposure of the fibres C, G and E at the fluence (per pulse) in the order of 250 mJ/cm², we have written 2l gratings near 1335 nm (grating length 2.5 mm). Various exposure times were used to obtain saturated and unsaturated grating reflectivities (first or second inscription spectra in the fibres C and E). Two gratings were written within the fibre K near 1010 nm at a fluence per pulse of ~ 170 mJ/cm². Two gratings R₁ and R₂ were written within the fibre L near 1335 nm at a fluence per pulse of 215 mJ/cm² (R₁ and R₂ were written by 1000 and 55000 pulses respectively). Near 1335 nm, 17 gratings were written in the fibres C and G by a CW exposure irradiance ranging from 4 W/cm² to 70 W/cm². The Bragg wavelength of each grating written within the fibres C, G, E, L was compared to an atmospheric line with an accuracy of 0.1 nm (0.1 nm = 2 x standard error). All the transmission spectrum measurements were performed at 21°C, 30 minutes after the UV exposure, each fibre being kept free of mechanical strain.

## 5.2 Experimental results

Three kinds of experiment have been performed to study the spectral behaviour of Bragg gratings under γ-irradiation dose, and two other experiments have been realised to determine the influence of γ-dose on the thermal and strain sensitivities of these gratings.

### Experiment #1

First of all, the transmission spectra of three gratings written in the fibre C were recorded in situ at room temperature under a steady state ⁶⁰Co irradiation at a dose rate of 113.8 Gy/h. Air dose rate measurements were made using a traditional alanine dosimetry technique. Few µW of optical power from a LED was injected within the fibre. After transmission through the fibre, the signal was analysed using a spectrometer of 30 pm typical resolution (Jobin-Yvon HR 640) in order to record the transmission spectrum of each grating as a function of the irradiation dose. The evolution in the fibre transmission was also measured far away from the Bragg wavelengths of the gratings as a function of the γ dose impinging on the fibre.

Figure 2 shows both the evolutions of the fibre transmission and the grating reflectivity versus γ-ray dose. This grating (type I) was written by 3200 pulses which corresponded to the experimental conditions of the inscription of the first spectrum. Figure 2 shows that, whereas the radiation-induced attenuation in the fibre steadily increases with the γ-ray dose, the filter response (transmittance T; spectrum shape and peak reflectivity) does not change significantly within the accuracy of the measurement (ΔT = ± 5 %). Similar behaviour could be observed from the spectra corresponding to the two other gratings written in the fibre. One can note that these two gratings were written for irradiation times long enough to get the grating erasures and the formation of new spectra [32].
<figure><img src="image_4.png" /><figcaption>Figure 2: Type I grating reflectivity and fibre transmittance at $\lambda \sim 1.3 \ \mu m$ versus $\gamma$-irradiation dose</figcaption></figure>

## Experiment #2

Gratings written within the fibres C, G and E were irradiated at room temperature by steady-state $\gamma$-ray irradiations according to the following experimental conditions : {110 Gy at 14 Gy/h, or 380 Gy at 47 Gy/h or 1080 Gy at 11 Gy/h or 3370 Gy at 34 Gy/h or 12.2 kGy at 122 Gy/h}. As for the two gratings written within fibre K, they were irradiated under 1080 Gy at 11 Gy/h. The difference between the two experiments is that the transmission spectra of these fibres were then recorded at room temperature, several days or more (up to months) after $\gamma$-ray exposure. For this purpose, we used the same experimental apparatus as the one used to record the grating spectra after their inscription. Within the range of parameters scanned in the course of our experiments, the $\gamma$-ray exposure did not significantly change each grating's transmission spectrum characteristics, whatever the writing conditions for the grating the $\gamma$-ray dose or dose rate might be.

## Experiment #3

Gratings written within the fibres C, G and L were irradiated at room temperature by a steady-state $\gamma$-ray irradiation performed at high dose rate up to a high dose (412 kGy at 2.45 kGy/h). The transmission spectra of these fibres were recorded, at room temperature, after several days following $\gamma$-ray exposure. For most Bragg gratings, evolutions in the grating reflectivity under the $\gamma$-ray exposure remained below our experimental accuracy (± 5 %). Figure 3 shows the spectra of two Bragg gratings written in an Hi-Bi fibre recorded before and after $\gamma$-ray irradiation at high doses and high dose rates; the left lines are for optical zero calibration.
<figure><img src="image_5.png" /><figcaption>Figure 3: Transmission spectra of two Bragg gratings (G₁ type I; G₂ type IIA) written in an Hi-Bi (Andrew D Shape) fibre</figcaption></figure>

The transmission spectra of the gratings written within fibre L, recorded before and after γ-ray exposure, is given in Fig. 3 as an example. However, three gratings written in fibres C and G using CW UV exposure at low irradiance (4 W/cm²) made an exception to this observation. The UV irradiation times used to write these gratings were tailored to obtain reflectivities of about 30 %, at the early stage of the grating formations. The γ-ray induced drop in the grating reflectivities were about 5 %. Starting from this figure, a maximum change in reflectivity of 5 % corresponds to a maximum refractive index modulation change δ(Δn) ~ 10⁻⁵. In the same way, the grating wavelengths did not change significantly under γ-ray irradiation whatever the dose or dose rate might be. For example, the maximum value of the Bragg wavelength shift, measured from all the recorded spectra, was 0.1 nm towards the blue. Thus, starting from an upper limit of 0.1 nm for the Bragg wavelength shift, we obtain a change in the mean refractive index induced by the γ-ray irradiation less than ~ 10⁻⁴.

## Experiments #4 and #5

LETI-CEA has developed an accurate experimental set-up to test the grating response versus temperature and strain. This set-up is made of an optical wide-band source (LED or SLD), several Bragg gratings placed in series, and a high resolution spectrophotometer (~ 30 pm). It is fully described in the associated OFS(10) Conference [35]. With such a set-up, the thermal and strain responses of Bragg gratings could be obtained with a relatively good accuracy. For instance, a non-irradiated Bragg grating written in an other germane-silicate fibre presents thermal and strain sensitivities which are respectively:

$\Delta\lambda_g/\Delta T = 8.63 \pm 0.13 \text{ pm/K}$; $\Delta\lambda_g/\Delta\epsilon = 1.027 \pm 0.025 \text{ pm/}\mu\epsilon$ [35]. These results, obtained with a relatively good accuracy and repeatability allows us to assume that all the Bragg gratings written in similar conditions have the same sensing behaviour. This is an experimental proof of the feasibility to compensate for the thermal shift via a reference unstrained grating. So, a pragmatic way to solve the classical problem of cross-sensitivity of the transducer consists in associating two Bragg gratings so as to realise a single transducer (one Bragg grating being exposed to the environment and the other one
being isolated from the parameter to determine, strain for instance). So, in the spectral approach, the measurement is simply given by the spectral separation between the two associated Bragg wavelengths, and do not require an absolute determination of these two Bragg wavelengths.

Table 3 shows the Bragg gratings thermal and strain responses before and after γ-irradiation of fibres types C and G. In these experiments, the temperature varies from 20°C to 45"C, and strain goes up to about 1000 με (500 μm over 0.5 m). These first results show that thermal and strain sensitivities obtained with these fibres (i.e. B or F, P and Ge-doped fibres) are not significantly affected by γ-irradiation, even at high doses (up to 412 kGy) and high dose rates (up to 2.45 kGy/h), within the standard error of our experiment. In any case, for the tested fibres, we assert that the γ-ray-induced increase of these parameters is weak and less than 5 % and 2 %, respectively, over a 412 kGy dose range. The behaviour of gratings written in other type of more specific fibres (rare earth-doped fibres (i.e. type E) and Hi-Bi fibres (i.e. type L) is still under evaluation and results of these experiments will be published later.

<table><caption>Table 3: Strain and thermal sensitivities of Bragg gratings before and after γ-irradiation</caption><thead><tr><th>Fibre type</th><th>Type of UV exposure</th><th>γ-ray dose (kGy)</th><th>Grating thermal sensitivity [pm/K] versus γ-ray irradiation</th><th>Grating strain sensitivity [pm/με] versus γ-ray irradiation</th></tr></thead><tbody><tr><td rowspan="3">C</td><td>Pulsed</td><td>0</td><td>8.83 ± 0.23</td><td>0.978 ± 0.007</td></tr><tr><td>//</td><td>412</td><td>9.40 ± 0.12</td><td>1.001 ± 0.008</td></tr><tr><td>CW</td><td>412</td><td>9.08 ± 0.18</td><td>1.080 ± 0.021</td></tr><tr><td rowspan="5">G</td><td>Pulsed</td><td>0</td><td>8.61 ± 0.29</td><td>0.988 ± 0.003</td></tr><tr><td>//</td><td>0.3</td><td>8.49 ± 0.27</td><td>1.020 ± 0.002</td></tr><tr><td>//</td><td>1</td><td>8.41 ± 0.38</td><td>1.045 ± 0.003</td></tr><tr><td>//</td><td>3</td><td>8.38 ± 0.25</td><td>1.026 ± 0.002</td></tr><tr><td>CW</td><td>412</td><td>8.93 ± 0.34</td><td>1.013 ± 0.008</td></tr></tbody></table>

The relatively poor accuracy in the above thermal sensitivity measurements is related to the small temperature range and to the weak accuracy in the determination of this parameter (not better than ± 0.2"C) due to the lack of perfect reproducibility of the grating position in the temperature control area.

## 6. DISCUSSION

The poor sensitivity of the Bragg gratings over a γ-ray irradiation is, at first glance, rather surprising. Indeed, V.B. Neustruev et al., have shown that there is a strong analogy between the γ-ray and the 248 nm irradiation effects [29]. A γ-ray irradiation, performed by V.B. Neustruev et al. on a silica glass doped with 11% mole GeO₂, at a dose rate of 9.72 kGy/h created the same paramagnetic defect centres Ge(1), Ge(2) and Ge(E') as the 248 nm exposure did [29].

Furthermore, concentrations of defects created by the γ-ray irradiation at a dose of 740 kGy were comparable to those induced under pulsed UV exposure at a dose of 10²² photons/cm³ [29]. These doses are similar to those used in our experiments.

The colour centre model has been first proposed by D.P. Hand and P.S¹.J. Russell to account for the fibre photosensitivity [28]. This model assumes that the change in refractive index is related to the change in the UV spectrum resulting from the creation of these paramagnetic centres [29]. For example, using a Kramers-Kronig analysis, RM. Atkins et al. have recently shown that this change in absorption in the wavelength range from 165 nm to 300 nm could account for the UV index change
[36]. In the experiment realised by R.M. Atkins et al., the fibre was doped with 3 mole % germania
[36] and the refractive index change was as low as ~ 3.10⁻⁵.

Starting from these results, let us assume for a time that the colour centre model is convenient to account for the photosensitivity of our fibres, that the defects produced by γ-ray irradiation come from the breakage of the same wrong bonds as those involved when irradiating the glass by UV light and that the saturation levels of defect concentration are similar whatever the nature of the irradiation may be. These last hypotheses are consistent with the observations reported by V.B. Neustruev et al. [29]. As photobleaching of gratings is currently produced under UV fringeless exposure [33]-[37], we could expect such bleaching under γ-ray exposure. But, as a complete contrast, we did not observe any bleaching of gratings under γ-ray irradiation. To clarify this issue, we have performed the following experiments:

Firstly, the core of a slice of preform G (thickness = 147 µm) was irradiated by 10⁵ pulses of UV light at a fluence per pulse of 200 mJ/cm². These irradiation conditions correspond to the saturation of photosensitivity within this glass. The UV-induced absorption spectrum was then recorded in the spectral range from 225 nm to 290 nm. Afterwards, the preform slice was exposed to γ-ray irradiation by 3.37 kGy at a dose rate of 34 Gy/h. The excess loss spectrum was recorded one month after the γ-ray irradiation. The γ-ray exposure led to an unstructured increase in the background of the UV-induced loss spectrum. The γ-ray induced attenuation increases monotonously from ~ 15 cm⁻¹ (~ 0.95 dB) at λ = 280 nm to ~40 cm⁻¹ (~ 2.55 dB) near 230 nm. Such a shape of spectral attenuation could be produced by the wing of a γ-ray induced absorption band peaking in the vacuum UV.

Secondly, the core of another slice of preform G (thickness = 126 µm) was irradiated by a γ-ray dose of 3.37 kGy at a dose rate of 34 Gy/h. The γ-ray induced absorption spectrum was then recorded between 225 nm and 290 nm. Its shape looked like the one corresponding to the γ-ray induced loss spectrum recorded using the first slice of preform G, on the exception that a slight structure peaking near

λ = 260 nm could now be detected. The induced loss increases from ~ 10 cm⁻¹ (~ 0.55 dB) at 280 nm to
~ 30 cm⁻¹ (~ 1.65 dB) near 230 nm.

Obviously, the γ-ray induced production of colour centres responsible for the excess loss observed between 230 nm and 280 nm was not affected by the preliminary UV exposure. Accordingly, the order of magnitude of the concentration of the γ-ray induced defects can be assumed to be similar at the locations of the bright UV fringes and at those of the dark fringes. This can explain why the grating reflectivities (which depend only on the modulation of the refractive index, not on its mean value) did not change significantly under γ-ray exposure. Moreover, as there is no evident saturation of γ-ray induced defects at the gating place, this also shows that the γ-ray induced loss spectrum did not contribute to a significant change in the mean refractive index at the grating position. Indeed, the upper limit of the mean refractive index change following a γ-ray exposure is 10⁻⁴ whereas a mean index change as high as 2.10⁻³ can be currently observed when fibre G is irradiated by UV light. If we assume that the conclusion by V.B. Neustruev et al. about the defect concentrations created by the γ-ray irradiation and by UV light [29] can also be drawn in our experiments, this means that the colour centre model only accounts for a small part of the index changes observed in our heavily doped fibres. A recent observation of UV light-induced compaction in the core of preform G shows that other mechanisms different from a simple redistribution of charges can significantly contribute to the UV-induced change in refractive index [38].

7. CONCLUSION

We have reported the configurations for which the use of OFS(N)(s) gives clear industrial advantages and shown, in the first part of this paper, that the main applications are related to “structure”
monitoring. Moreover, the good adequation of the very promising field of applications of Bragg gratings, with the requirements of these applications has been shown. Presence of ionising radiations in the vicinity of the nuclear reactor (in the containment shield of a NPP) imposes us to understand the behaviour of Bragg gratings exposed to these environments. That is why the behaviour of Bragg gratings written in different kinds of Ge-doped silica fibres has been tested, to both low and high gamma-ray energies and dose rates.

As a conclusion, gamma-ray exposure does not significantly change the spectral characteristics of the tested Bragg gratings (peak reflectivity and Bragg wavelength). An upper limit of ~ 0.1 nm for the Bragg grating wavelength shift has been obtained in two fibres, corresponding to a change in mean refractive index < 10⁻⁴. Thermal and strain sensitivities of two kinds of fibres have also been determined in order to test their reliability under nuclear environments.

From a theoretical point of view, these experiments and others based on gamma-ray induced absorption measurements in the UV spectral range on pure and pre-UV irradiated slices of preforms, seem to show that the colour centre mechanism classically proposed to explain grating formation within germano-silicate fibres only accounts for a small part of the UV-induced index changes observed in our highly doped fibres. As further experimentations, equivalent tests will have to be done on gratings written within more typical fibres such as “telecom” ones, for instance, fibres with cladding free of phosphorous or highly purified fibre cores.

From the instrumentalist point of view, the quasi-lack of γ-ray radiation influence on both thermal and strain sensitivities of the fibres tested, even at high doses (up to 412 kGy) and high dose rates (up to 2.45 kGy/h), are very good news for nuclear end-users. It proves clearly the large potential of this new kind of instrumentation for any nuclear area (NPP, submarine, spatial,...). To complete the study, further evaluations will be done on more specific fibres (rare earth-doped core fibres and polarisation maintaining fibres). The fact that the spectral behaviour of Bragg gratings is not affected by γ irradiation even in case of high doses and high dose rates, means that a Bragg-grating based instrumentation can be used without any spectral recalibration or system modification, within an hostile nuclear environment; for instance in a NPP during normal operations until installation dismantling, and also in case of a nuclear accident.

## 8. ACKNOWLEDGEMENTS

The authors wish to thank their partners from Electricité de France (EDF) and FRAMATOME for helpful discussions during the CORA 2000 workshops.

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