# Characterization and Radiation Testing of an Embedded, Distributed Fiber Dosimetry System for Space Applications

Arnaud Meyer, Adriana Morana, Martin Roche, Nourdine Kerboub, Julien Mekki, Gilles Mélin, Thierry Robin, André Champavère, Frédéric Saigné, Jérôme Boch, et al.

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Arnaud Meyer, Adriana Morana, Martin Roche, Nourdine Kerboub, Julien Mekki, et al. Characterization and Radiation Testing of an Embedded, Distributed Fiber Dosimetry System for Space Applications. Journal of Lightwave Technology, 2025, 43 (17), pp.8474-8483. 10.1109/JLT.2025.3587288. hal-05306138

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JOURNAL OF LIGHTWAVE TECHNOLOGY

# Characterization and Radiation Testing of an Embedded, Distributed Fiber Dosimetry System for Space Applications

Arnaud Meyer, Adriana Morana, Martin Roche, Nourdine Kerboub, Julien Mekki, Gilles Mélin, Thierry Robin, André Champavère, Frédéric Saigné, Jérôme Boch, Tadec Maraine, Cornelia Hoehr, Camille Bélanger-Champagne, Aziz Boukenter and Sylvain Girard

**Abstract**—A distributed dosimetry system based on phosphorus-doped optical fiber interrogated by an embedded Optical Time-Domain Reflectometer (OTDR) operated around 1610 nm is investigated for its performance, accuracy, repeatability and reliability in radiation environments corresponding to space applications. We analyzed the metrological properties of the system, showing an accuracy of ±6 % on dose measurement using the minimum pulse width, and characterized the evolution of measurement uncertainty according to the sampling length. We demonstrated the system ability to measure Total Ionizing Dose under γ- and X-rays, in a range of dose rates from 5.78 µGy(SiO₂)/s to 5038 µGy(SiO₂)/s, and with doses up to 344 Gy(SiO₂). Finally, the device reliability under 350 MeV and 480 MeV proton irradiation was tested. No Single-Event Latchups (SEL) have been observed up to a fluence of ~4.2×10⁹ cm⁻² and the Single-Event Functional Interrupt (SEFI) cross-section was on the order of ~10⁻⁸ cm².

**Index Terms**—Optical fiber, Radiation, Dosimetry, Optical Time-Domain Reflectometry.

## I. INTRODUCTION

DOSIMETRY, i.e. the measurement of radiation dose and dose rate, is a vital component to space missions, because on how such radiation affects living beings [1], electronic devices [2] and even materials [3]. Many technologies have therefore been developed for this purpose, ranging from luminescent detectors [4] to RadFETs [5]. However, these techniques only provide with punctual measurements, meaning that many different measuring units are needed to obtain a satisfactory dosimetry mapping of a spacecraft.

In this context, optical fibers (OFs) can provide with an interesting solution; first, because of their ability to sense radiation dose or dose rate through a variety of physical mechanisms leading to attenuation, light emission or refractive index change in the OF [6]; second, because of their ability to

be interrogated through distributed sensing techniques.

Such distributed sensing techniques, involving the evolution of a transmitted or scattered signal along a measurement line involving OFs, are able to provide real-time, linear mappings of a variety of measurands [7]. Therefore, application of such technologies to dosimetry could provide knowledge of radiation dose and dose rate at many locations of interest at once through a same OF.

Such distributed dosimetry techniques and setups have been investigated in the literature, mainly within the scope of particle accelerators, through the pioneer demonstration performed in the Deutsches Elektronen-Synchrotron (DESY) in Hamburg [8], and more recently with Distributed Optical Fiber Radiation Sensing (DOFRS) in CERN, which is still under active development to complement existing technology [9], [10], [11]. These environments present the advantages of large linear distances to be monitored, as well as the possibility to locate interrogation devices in separate, radiation-free areas to guarantee their performance and reliability.

In this latter example of the DOFRS system, the distributed dosimetry setup involves measurement lines using a single-mode, phosphorus-doped OF. Such types of OFs have been investigated in the literature for their dosimetric properties, especially regarding the behavior of the $P_1$ type of defects, generated in phosphosilicate glass under radiation [12]. These defects have the observable property of causing Radiation-Induced Attenuation (RIA), i.e. optical absorption in the near-infrared range, with a wide absorption band peaking at 0.79 eV [13], i.e. 1569 nm in the bulk material. The proximity of this absorption peak to the 1550 nm wavelength typically used in OF-based telecommunication makes interrogation of this type of defect accessible to common types of optical communication technologies, such as Optical Time-Domain Reflectometry (OTDR).

Moreover, the suitability of near-infrared RIA on phosphorus-doped fiber as a means of performing dosimetry has also been demonstrated in several other studies, highlighting a relatively low temperature sensitivity of ±15 %

This paper for Journal of Lightwave Technology was first submitted on xx xx 2025.

A Meyer, A. Morana, M. Roche, A. Boukenter, S. Girard are with Laboratoire Hubert Curien, Université Jean Monnet, 42000 Saint-Étienne, France (e-mail: sylvain.girard@univ-st-etienne.fr).

S. Girard is also with Institut Universitaire de France (IUF), Ministère de l'Enseignement Supérieur et de la Recherche, Paris, France.

M. Roche, N. Kerboub and J. Mekki are with CNES, 31400 Toulouse, France.

M. Roche, G. Mélin and T. Robin are with Exail, 22300 Lannion, France.

A. Champavère is with GuidOptix, 42330 Saint-Bonnet-les-Oules, France.

F. Saigné, J. Boch and T. Maraine are with IES, Université de Montpellier, CNRS, 34090 Montpellier, France.

C. Hoehr and C. Bélanger-Champagne are with TRIUMF, Vancouver, BC V6T 2A3, Canada.

Color versions of one or more of the figures in this article are available online at http://ieeexplore.ieee.org

Digital Object Identifier (DOI) is ...
## JOURNAL OF LIGHTWAVE TECHNOLOGY

within a range of -80 °C to +120 °C [14], a linear response up to a dose of 500 Gy(SiO₂) [10], and an ability to sense different natures of radiation, including γ and X-rays [15], protons [10], neutrons [16], or a mix of energetic particles generated at the CHARM facility at CERN [17].

This ability to detect Total Ionizing Dose (TID) with a same sensitivity regardless of the type of incoming particle enables accurate dimensioning of a dosimetry system for complex radiation environments such as space, through the help of calibration curves and Monte Carlo simulations [18].

OFs are used more and more frequently in spacecrafts, for applications such as communication, rotation measurement or inertial measurement; all with technological challenges relating to the particular constraints of the space environment and industry [19], [20].

Although most of the technology involved in distributed dosimetry is now well documented and described in the literature, its application to the space environment therefore requires a few more adjustments and investigations. First, the volume and mass constraints of space projects aim toward the use of miniaturized, embedded systems, whereas applications for particle accelerators do not include this type of constraint and therefore allow for standard, tabletop interrogation devices. Second, the space environmental constraints are much more demanding, and, among others, radiation tolerance is of primary concern because the whole interrogation system is submitted to radiation, whereas it can be safely isolated from the measuring part in ground applications.

Recent studies have focused on these specific topics, including our previous results obtained with an embedded OTDR device and a single-mode, phosphorus-doped OF, reduced in size and mass compared to standard telecom fibers [21]. Another notable work involves a similar kind of OF used for dosimetry inside the International Space Station, although only for a punctual, non-distributed measurement [22], [23].

For these reasons, we further investigate in this article the potential of a distributed dosimetry system tailored for space applications, involving an embedded OTDR for the interrogation part, and a single-mode, phosphorus-doped OF for the sensing part. First, we perform a metrological study of different interrogation systems to characterize their uncertainties according to different operating parameters. Then, we demonstrate the ability of a distributed dosimetry system of even more reduced dimensions compared to our previous study [21] to perform dose measurement and operate reliably under high Total Ionizing Dose (TID). Finally, we determine the system sensitivity to Single-Event Effects (SEE), caused by particles of very high energy.

Results from the past [21] and present studies are compiled in a Ph.D. thesis work [24], and the present article focuses on the characteristics of the investigated optical system, specifically on the embedded OTDRs.

## II. MATERIAL AND METHODS

### A. Optical fiber

The OF used in this work is a single-mode, phosphorus-doped OF manufactured by Exail. Its structure is close to a step-index fiber, with a core of 8 µm diameter made of silica doped with phosphorus at 6.6 wt%, a cladding of 125 µm made of pure

silica, and a coating of 245 µm diameter made of dual-coat acrylate.

This type of OF is very close to the one used in the DOFRS system at CERN, and was investigated in this study as a comparison with [21] that involved a similar type of fiber, but with reduced cladding and coating dimensions.

### B. OTDR devices

Three OTDR devices are considered in this study to perform interrogation of the OF attenuation. The first device, a JDSU MTS6000A portable OTDR device, equipped with a 8129 VLR module, is used in this study as a reference for a state-of-the art commercial device. The two other devices, namely the Viavi Micro iOTDR and Viavi Nano iOTDR, are embedded OTDR devices, reduced in volume and mass [25]. A brief comparison of their characteristics (dimensions, mass, operating wavelength and minimum pulse width) is presented in Table I.

<table><caption>TABLE I COMPARISON OF OTDR DEVICES USED IN THIS STUDY</caption><thead><tr><th>Device</th><th>MTS 6000A</th><th>Micro iOTDR</th><th>Nano iOTDR</th></tr></thead><tbody><tr><td>Dimensions [mm]</td><td>280×200×95</td><td>91×170×16.5</td><td>98.5×67.5×16.5</td></tr><tr><td>Mass</td><td>2620 g</td><td>200 g</td><td>110 g</td></tr><tr><td>Wavelength(s)</td><td>1550; 1625 nm</td><td>1610; 1625 nm</td><td>1610 nm</td></tr><tr><td>Min. pulse width</td><td>3 ns</td><td>5 ns</td><td>10 ns</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Fig. 1. Setup of metrological characterization of different OTDR systems (on the left), connected via an optical switch to a common measuring line (on the right).</figcaption></figure>

### C. Metrological study setup

The metrological characterization of the investigated systems was performed using the setup illustrated in Figure 1.

Four OTDR devices (one MTS6000A, two Micro iOTDRs named A and B, and one Nano iOTDR) were interrogating a same measuring line, in sequence, with varying parameters.

This measuring line was composed two optical fiber samples spliced together: first, a 1 km launch fiber, made of standard, telecom-grade single-mode fiber; second, an attenuating segment made of a 200 m-long single-mode, phosphorus-doped fiber that was previously irradiated, resulting in a total attenuation of ~12 dB at 1610 nm.

Each OTDR device performed acquisition one at a time on this common measuring line, through a 1x4 optical switch (LEONI eol 1x4), driven by the acquisition computer to connect only the active OTDR device to the sample before each reading.

Different OTDR pulse widths were experimented using this setup, to check the effect of this setting on the accuracy of the resulting attenuation measurement. Pulse width τ is typically linked to the spatial resolution Δz of an OTDR trace through the group width v_g using the following formula [26]:
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$$ \Delta z = \frac{v_g}{2} \tau $$

The investigated pulse widths ranged from the minimum available on each device (3 to 10 ns, cf. Table I) to 1 µs, corresponding to a theoretical spatial resolution range of approximately 0.3 m to 100 m.

The scan time was set in all cases to 10 s, the minimum allowed by the devices, in order to check device performance at the highest available temporal resolution. Both operating wavelengths of 1550 nm and 1625 nm were analyzed in the case of the MTS6000A, and the Micro iOTDRs tested in this setup were only able of a single interrogation wavelength of 1610 nm.

This setup was left running during approximately 1 month, totalizing about 1200 similar OTDR traces acquired for each combination of device and parameters. In addition, the ambient temperature was recorded using a thermocouple reader.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Setup of γ ray irradiation. The dose rate of each irradiated element is indicated in gray above their corresponding position.</figcaption></figure>

### D. γ ray irradiation setup

To characterize the ability of the Nano iOTDR to perform distributed dosimetry, radiation tests were performed under γ rays at the PRESERVE facility in IES, Université de Montpellier (France). The ⁶⁰Co radioactive source installed in the irradiation chamber provides an isotropic flux of high-energy photons, with two main energy peaks located at 1.173 MeV and 1.332 MeV, equivalent to a mean photon energy of 1.25 MeV. The dose rate received by the samples is set by adjusting their distance from the source, and the total dose by adjusting the irradiation time.

In our experiment, illustrated in Figure 2, we set up two 100 m samples of phosphorus-doped OF (described in section II.A), wound as coils of ~10 cm diameter. These samples (named A and B) were installed in the irradiation chamber, inside 3D-printed, plastic boxes isolating them from ambient light to avoid extrinsic photobleaching effects that could affect attenuation measurements in this type of experiment [27]. One end of each sample was spliced to a standard, single-mode, telecom-grade pigtail to connect them to their respective measuring device.

Five embedded OTDR devices were involved in this experiment. One Nano iOTDR was connected to the first OF sample and placed inside the irradiation chamber, to monitor the ability of the device to operate under radiation and withstand high levels of TID, i.e. high amounts of accumulated dose. Four additional devices (one Nano iOTDR, two Micro iOTDR at 1610 nm and one Micro iOTDR at 1625 nm) were placed

outside the irradiation chamber, and connected through the optical switch (also placed outside the chamber) to the second OF sample.

The irradiation time was 84 days, during which both samples were continuously interrogated by the OTDR devices with a scan time of 10 s and their minimum available pulse width, and the corresponding OTDR traces recorded for further analysis. In the case of the second measuring line, the OTDR devices were actuated one at a time, in a repeating sequence, the acquisition software operating the optical switch accordingly. The whole experiment was performed at room temperature, within 14 °C to 21 °C as recorded by a thermocouple reader.

On-site dosimetry performed by the facility operator indicated a dose rate of 5.78 µGy(SiO₂)/s for both OF samples and a dose rate of 15.28 µGy(SiO₂)/s for the irradiated Nano iOTDR, all with a relative uncertainty of ±5 %.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Setup of X-ray irradiation.</figcaption></figure>

### E. X-ray irradiation setup

To extend the range of doses and dose rates explored with γ ray irradiation, additional radiation tests were performed under X-rays using the MOPERIX facility in Laboratoire Hubert Curien, Université Jean Monnet Saint-Étienne (France). This irradiator involves an X-ray tube with a tungsten anode, operated in our case with a voltage of 100 kV, yielding a continuous spectrum of high-energy photons up to 100 keV with a mean energy-fluence of 40 keV [28]. The dose rate is set by adjusting the distance between the sample and the source, as well as adjusting the tube current of the source. The total dose is set by adjusting the irradiation time. Dosimetry was performed using a plane-parallel irradiation chamber with a relative uncertainty of ±10 %.

This experimental setup is illustrated in Figure 3: a coil of phosphorus-doped OF (cf. section II.A) was installed inside the irradiation chamber, and spliced to a radiation-hard transport fiber going outside the chamber. The transport fiber was then spliced to a standard, single-mode, telecom-grade pigtail and connected to a Nano iOTDR device performing continuous acquisition with a scan time of 10 s and a pulse width of 10 ns.

Two similar experiments were conducted at room temperature using this setup: first, an OF sample of 40 m was irradiated at a dose rate of 475 µGy(SiO₂)/s up to a dose of 100 Gy(SiO₂); then, another OF sample of 20 m was exposed to a dose rate of 5038 µGy(SiO₂)/s up to a dose of 344 Gy(SiO₂). These dose rates were measured on-site using an ionization chamber. In both cases, OF samples were installed as flat, single-layered coils to ensure homogeneous penetration of X-rays within their volume, because of the relatively lower energy of photons produced in this irradiator.
# JOURNAL OF LIGHTWAVE TECHNOLOGY

## F. Proton irradiation / SEE testing setup

In order to assess the sensitivity of the distributed dosimetry system to SEE, radiation tests were performed, through the RADNEXT European project [29], at the PIF facility in the TRIUMF particle accelerator center, Vancouver (Canada). The beam line 1B used for this experiment provides protons at very high energies, either 350 MeV or 480 MeV [30].

<figure><img src="image_5.png" /><figcaption>Fig. 4. Setup of proton irradiation for SEE testing.</figcaption></figure>

As shown in Figure 4, the tested device (either Micro iOTDR at 1610 nm or Nano iOTDR) was connected to a 50 m coil of phosphorus-doped OF), and placed in the trajectory of the high energy proton beam in order to target specific system components.

Power supply and communication with the device were relayed by a current monitoring device, enabling to track abnormal rises in the power consumption of the tested OTDR, characteristic of the phenomenon of Single-Event Latchup (SEL), and power cycle the device if necessary.

In each run, the tested OTDR device performed continuous acquisition with a scan time of 10 s, until appearance of a failure. For each observed failure, the proton fluence, given by the radiation monitor of the facility, was recorded and the device was power cycled to resume acquisition.

All proton irradiations reported in this study were operated in a total duration of 24 h, exploring different combinations of devices, beam energies and targeted components.

## III. RESULTS AND DISCUSSION

### A. Dynamic range

Dynamic range, as applied to OTDR devices, can be defined differently according to application. Whereas some definitions focus on ability to detect strong reflecting events for line breakage detection [31], our application of distributed dosimetry requires to accurately measure fiber attenuation down to low backscattered signal levels.

Moreover, the dynamic range values given by OTDR manufacturer are usually measured at the largest available pulse width, and can anyway not be fully used for attenuation measurement because a significant part of this dynamic range is reserved for detecting bright reflection events without overloading the device receiver [26].

Because measurement of attenuation from an OTDR trace is based on a linear fit from a segment of this trace, we estimated the dynamic range of the Micro and Nano iOTDR using a statistical method that evaluates the quality of such a linear fit as it evolves with the absolute value of the backscattered signal. The metric used for this qualification is the coefficient of determination R² (with 0 ≤ R² ≤ 1), that evaluates the fraction of variability in the data y (of mean value ŷ) that is explained by the linear model ŷ [32]:

$$R^2 = 1 - \frac{\sum_i (y_i - \hat{y}_i)^2}{\sum_i (y_i - \bar{y})^2}$$

We calculated this coefficient on OTDR traces acquired using the metrology setup (cf. section II.C) at different pulse widths. In each case, we considered the portion of signal between the start of the attenuating segment (determined manually for each configuration) and the first occurrence of a data point below a cutoff threshold C.

With a decreasing value of C, this coefficient typically evolves in two stages, as illustrated in Figure 5: it first increases, because more data points with low noise are integrated into the model, decreasing the uncertainty on the fit; then it decreases, because the added data points are obtained at lower backscattered signal values, and therefore higher noise, increasing the uncertainty on the fit. Between these two phases, a R² maximum is obtained at the cutoff threshold value Cmax. This value corresponds to the backscattered signal threshold below which the quality of the linear fit starts to decrease, and therefore reflects the dynamic range of the device, in a manner suitable to our application, at the investigated pulse width.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Evolution of the coefficient of determination R² on the linear fit applied to the attenuating segment of a Micro iOTDR A trace acquired with a pulse width of 5 ns and a scan time of 10 s, as a function of the cutoff threshold. The location Cmax of the maximum value is shown by the vertical dashed line.</figcaption></figure>

The value of Cmax obtained by the calculations described here above, on all investigated OTDR devices and pulse widths and a scan time of 10 s, is shown in Figure 6.

A first noticeable characteristic is the overall decrease of Cmax (and therefore increase of dynamic range) with increasing pulse width, up to 100 ns. This feature is commonly observed on OTDR devices [26] and explained by a larger amount of energy being injected into the probe pulse, increasing the “signal” part of the signal-to-noise ratio.

However, this tendency appears to be reversed with the largest investigated pulse widths of 300 ns and 1 µs. This behavior is explained by the relatively short length (200 m) of the sample probed here, compared to the reduced spatial sampling, respectively 0.64 m and 1.28 m, automatically applied for these pulse width values by the OTDR devices investigated here. In addition to the fact that reflection peaks at the segment start and end are also broadened as a result of convolution with a pulse of larger width, the overall amount of data points available for linear fitting is significantly reduced compared to smaller pulse widths, decreasing their statistical quality. With longer samples (≥ 1 km), the dynamic range is expected to continue increasing with increased pulse width.

These results are quite consistent between devices, with some noticeable tendencies: the MTS6000A, with both operating wavelengths, has overall the largest dynamic range at equivalent pulse width; both Micro iOTDR devices show
equivalent performance across all investigated pulse widths; finally, the Nano iOTDR exhibits the smallest dynamic range for all pulse widths lesser or equal to 100 ns.

<figure><img src="image_7.png" /><figcaption>Fig. 6. Dynamic range, as estimated by the $C_{max}$ method described in section III.A, as a function of pulse width for all investigated OTDR devices with a scan time of 10 s.</figcaption></figure>

A remarkable result, for further analyses presented in this article, is the value of -6.5 dB obtained with the lowest pulse width available to both Micro and Nano iOTDRs.

## B. Measurement accuracy and repeatability

Measurement uncertainties are one of the most important values to determine on any measuring system, in order to assess the level of achievable precision. Using statistical analysis of the data acquired using the metrology setup (cf. section II.C), we evaluated the systematic error (linked to accuracy) and the random error (linked to repeatability) of all investigated devices and pulse widths, with a scan time of 10 s.

For each acquired OTDR trace, a linear fit was performed between the start of the attenuating segment (defined manually according to the device and pulse width) and the first occurrence of a backscattered signal value lesser or equal to the $C_{max}$ value obtained for these settings in section III.A. The slope of the obtained linear fit is the measurement of the segment attenuation.

Figure 7 shows the histograms of all such measurements, for all investigated devices and pulse widths up to 300 ns. In these graphical representations, the accuracy of the measurement can be estimated from the difference between histogram centers, and the repeatability from the spread of each distribution.

Figure 8 focuses on the mean value (top graph) and standard deviation (bottom graph) of each set of measurements, to represent evolution of these values with increasing pulse width for each device.

Accuracy, evaluated from the average of all values measured with a same set of parameters, appears to be globally consistent between devices, with a mean measured attenuation of ~64 dB/km. We can however observe a tendency, for all devices, of larger measured attenuation values when the pulse width increases.

Repeatability, on the opposite, is evaluated from the standard deviation of all values measured with a same set of parameters; a smaller standard deviation corresponding to an increased repeatability. We observe different types of behavior among investigated devices: the MTS6000A exhibits a high repeatability, consistently through all investigated pulse widths; both Micro iOTDRs show a lower repeatability only at their lowest pulse width, then a quite constant value for pulse widths

greater than 10 ns; finally, the Nano iOTDR repeatability consistently increases with increasing pulse width up to 100 ns.

From these repeatability results, we can estimate the random uncertainty on the attenuation measurement performed by each combination of device and pulse width. Noticeably, the random uncertainty at a 95 % confidence interval is ±6 % for both Micro iOTDRs operating at 5 ns, and ±5 % for both Nano iOTDRs operating at 10 ns.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Histograms of attenuation values measured on the attenuating segment of the metrology setup, on all investigated devices and pulse widths.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Evolution of mean attenuation (top, characterizing accuracy) and standard deviation (bottom, characterizing repeatability) as a function of pulse width for all investigated devices.</figcaption></figure>

## C. Influence of the sampling length on repeatability

The main interest of distributed dosimetry lies in its ability to perform localized dose or dose rate measurement with a large spatial resolution. However, the higher this spatial resolution, the shorter the sampling length used for attenuation measurement will become. This could result in potentially increased uncertainties, as less data points are available to perform the linear fit used for attenuation measurement.

To evaluate the influence of the sampling length on measurement repeatability, a new analysis was performed on data acquired using the metrology setup (cf. section II.C). For each device, we measured the attenuation of the segment ranging from the start of the attenuating sample (determined manually for each device) to the investigated sampling length,
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from 1 m to 50 m, after which the value threshold $C_{max}$, as
defined in section III.A, is reached for all devices.

Figure 9 shows the evolution of relative uncertainty on attenuation measurement, for all investigated devices operating at their minimum pulse width and a scan time of 10 s, for sampling lengths of increasing values. Uncertainties were evaluated at a 95 % confidence interval, as described in section III.B.

All investigated devices exhibit a similar trend, with decreasing uncertainties as the sampling length increases, until reaching a minimum level around 50 m. For the shortest investigated sampling lengths, the very high uncertainty level can be explained by small-scale fluctuations of the backscattered signal, which can in the most extreme cases make the linear fit algorithm yield an attenuation value of the wrong sign.

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Evolution of relative uncertainty on attenuation measurement (at a confidence level of 95 %) according to sampling length for all investigated OTDR devices operating at their minimum pulse width and a 10 s scan time.</figcaption></figure>

D. *γ and X-ray irradiation results*

Figure 10 depicts the time evolution of the OTDR traces on the Nano iOTDR A line in the γ ray irradiation setup (cf. Section II.D). This graph clearly illustrates the RIA phenomenon, as the slope of the OTDR trace decreases over irradiation time, corresponding to an increase of OF attenuation.

By applying the slope calculation method described in Section III.A (with the $C_{max}$ value of -6.5 dB determined for the Nano iOTDR operating at pulse with of 10 ns), the RIA of the irradiated segment can be determined for each acquired OTDR trace. In addition, using the date and time of each sample, along with the dosimetry data of the irradiation facility (and other parameters such as the shutdown times and decay of the $^{60}$Co source), we can also determine the corresponding dose received by the irradiated fiber.

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Evolution of OTDR traces acquired by the Nano iOTDR A line during the first 75 days of γ ray irradiation. The data contained in the area with the white background are the ones considered for further calculations.</figcaption></figure>

These RIA vs. dose data are shown in Figure 11 for both Nano iOTDR A and B measurement lines. Two parts can be distinguished in this plot: above a dose of ~3×10⁻² Gy(SiO₂), RIA follows a linear trend up the maximum experimented dose of 40.16 Gy(SiO₂), with a sensitivity of ~3.8 dB km⁻¹ Gy(SiO₂)⁻¹; however, below this dose, evolution of RIA is poorly correlated with the increasing dose. We explain this behavior by the difficulty of the linear fit to accurately measure low attenuation values on lowly-attenuating segment (which OTDR traces are therefore practically flat). This constitutes a low detection limit of the measurement method, determined to be at ~3×10⁻² Gy(SiO₂) for the Nano iOTDRs scanning a fiber length of 100 m.

<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Log-log plot of the evolution of RIA measured by Nano iOTDR A and B lines during the complete duration of γ ray irradiation. Dashed lines show the linear fit applied to each data set, which details are given in legend.</figcaption></figure>

A comparison between the RIA measured by the different embedded OTDR devices involved in the γ ray irradiation experiment is shown in Figure 12. We observe that all acquired data exhibit very good linearity in the experimented range, and a good consistency between measurements made on different devices. Combining the results obtained on measurement line B (the four last graphs), we obtain a sensitivity of 4.02 dB km⁻¹ Gy(SiO₂)⁻¹ with a relative uncertainty of ±15.3 % at a 95 % confidence level. This spread cannot be explained by the different wavelengths used by the devices, as Micro iOTDR B (operating at 1610 nm) and C (operating at 1625 nm) exhibit very similar slopes, as expected from the very small variation of the absorption band of the P₁ defects in this wavelength range [21]. The mean reason for these deviations is rather explained by the presence of noise in the attenuation measurements, which are the combined result of lesser exploitable lengths at higher doses and the measurement uncertainty induced by using short pulse widths.

The linear trend of RIA increase with dose has been further tested under X-rays, using the setup described in Section II.E, in order to access higher dose and dose rates values. These results are summarized in Figure 13, along with the ones obtained under γ rays on the Nano iOTDR A line.

The evolution of RIA still globally follows a linear trend, but deviations can be observed at higher doses : for instance, fitting the 475 Gy(SiO₂)/s data up to a dose of 40 Gy(SiO₂) yields a sensitivity of 3.80 dB km⁻¹ Gy(SiO₂)⁻¹, whereas fitting the same data up to a dose of 100 Gy(SiO₂) yields a sensitivity of
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3.55 dB km⁻¹ Gy(SiO₂)⁻¹, both subject to the ±10 % uncertainty on dose measurement described in Section II.E. These limits on the RIA linearity with dose are known and described in the literature [33].

Finally, the Nano iOTDR A device, which was continuously irradiated during the 84 days of the γ ray irradiation, at a dose rate of 15.28 Gy(SiO₂)/s, underwent a TID of 106.20 Gy(SiO₂). During the whole duration of the experiment, no sign of hardware or software failure has been detected, as well as no trace of a significant difference in the measurement quality, as shown in the comparison between Nano iOTDR A and B data in Figures 11 and 12. These results can be combined with the ones obtained in [21] on a Micro iOTDR irradiated under similar conditions up to a TID of 81.05 Gy(SiO₂), also without any noticeable failure.

<figure><img src="image_13.png" /><figcaption>**Fig. 12.** Linear plot of the evolution of RIA measured by all embedded OTDR devices involved in the γ ray irradiation experiment. In each graph is shown the reference of the measured sample (A or B), its length, the total dose received, and a linear fit of the corresponding data.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 13.** Log-log plot of the evolution of RIA measured by Nano iOTDR A under γ rays and X-rays. γ rays data are shown in a lighter color to highlight X-ray data.</figcaption></figure>

E. SEE testing results

A first, important, result of the proton irradiation testing was the absence of any observed rise in the power consumption for both embedded OTDR devices during the whole experiment. The reached proton fluences without occurrence of SEL were therefore 2.921×10¹⁰ cm⁻² at 355 MeV and 4.212×10⁹ cm⁻² at 480 MeV for the Micro iOTDR device, and 1.406×10¹⁰ cm⁻² at 480 MeV for the Nano iOTDR device. Comparing to an estimated fluence rate of ~10² cm⁻² s⁻¹ of ~300 MeV protons in lower Earth orbit (LEO) [34], these results would translate to ~10.6 yr without observation of SEL for the Micro iOTDR and ~4.46 yr for the Nano iOTDR.

The second result was the observation of software failures, manifesting through loss of communication with the device. These Single-Event Functional Interrupts (SEFIs) were recorded, along with the corresponding proton fluences at which they occurred. From these data, the observed SEFI cross-section was calculated for each targeted component in each device, and shown in Figure 14 for the Micro iOTDR, and in Figure 15 for the Nano iOTDR. Also, while the Micro iOTDR retained full functionality after power cycling following a SEFI, the Nano iOTDR unit under test started systematically displaying a warning message after proton irradiation. The origin of this failure is still under investigation.

These results highlight different sensitivities to SEFI depending on the targeted component; in particular, the CPU of the Micro iOTDR device appears to be its most sensitive component, while on the opposite side the analog/digital converter and DC/DC converters appear significantly less sensitive. Despite these variabilities, the median SEFI cross-section is globally on the order of ~10⁸ cm², corresponding to one SEFI every ~11.5 d in LEO.
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<figure><img src="image_15.png" /><figcaption>**Fig. 14.** Box plot of SEFI cross-sections observed on the Micro iOTDR, categorized into each type of targeted component. Different colors indicate different proton energies.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 15.** Box plot of SEFI cross-sections observed on the Nano iOTDR, categorized into each type of targeted component, for a proton energy of 480 MeV.</figcaption></figure>

In order to test whether the high sensitivity of the Micro iOTDR CPU to SEFI can be mitigated, two complementary tests were performed at 355 MeV with additional aluminum (Al) shielding: one with 19 mm-thick shielding, targeting CPU only, and one with 13 mm-thick shielding, targeting both CPU and DC/DC converter. In both cases, SEFI cross-sections are comparable with the ones obtained without shielding.

IV. CONCLUSION

Two types of embedded OTDR devices (iOTDR Micro and iOTDR Nano) were characterized for their suitability to perform attenuation measurements targeting dosimetry applications in the space environment.

Evaluation of their metrological properties, conducted in the absence of radiation, concluded first to an expected high influence of pulse width on measurement uncertainties, although application requirements may dictate the use of the shortest available pulse widths to increase spatial resolution. Study of the dynamic range, applied to this particular application of attenuation measurement, determined this parameter for a wide range of pulse widths on all investigated standard and embedded OTDR devices. In particular, a cutoff value of -6.5 dB, after which the linear fit quality starts to decrease, was determined for both Micro and Nano iODTR devices operating at their minimum scan time and pulse width. Influence of sampling length with regard to uncertainties was also evaluated, showing a minimum viable sampling length of 10 m for the Micro iOTDR and 20 m for the Nano iOTDR in

order to reach a measurement uncertainty lesser or equal to an already significant value of 25 %.

Radiation testing and calibration of the distributed dosimetry system, under γ rays showed a linear evolution of RIA with ionizing dose, with a sensitivity of ~4 dB km⁻¹ Gy(SiO₂)⁻¹ up to a dose of 40.16 Gy(SiO₂). X-ray irradiation tests, performed at higher doses and dose rates, are well consistent with these results, despite an observed slight decrease of the linearity.

Finally, reliability of the device itself in a radiation environment was also experimented, first through γ ray irradiation of the Nano iOTDR device, with no sign of failure or degradation of the measurement quality observed up to a TID of 106.20 Gy(SiO₂), although this type of device was permanently affected by proton irradiation, hinting for critical component sensitivity that is still under evaluation. SEE testing, through proton irradiation, did not cause any observed SELs up to 2.921×10¹⁰ cm⁻² at a proton energy of 355 MeV (equivalent to ~10.6 yr in LEO) on the Micro iOTDR device. On the other hand, SEFIs were observed with a median cross section of ~10⁸ cm² (equivalent to ~11.5 d⁻¹ in LEO) when targeting different components, with an extra sensitivity of the CPU, which does not appear to be mitigated with the use of external shielding. To ensure reliable operation in space environment, these devices should therefore be remotely power-cyclable in case of error. The rate of such resets can be minimized by studying a possible SEE hardening of the most sensitive components identified in this study (such as the CPU).

In combination with the radiation testing of the Micro iOTDR presented in [21], these results demonstrate the potential of embedded OTDR devices to perform distributed dosimetry measurements in radiation environments such as space. The next steps to further explore this topic could be environmental studies, such as temperature and operation under vacuum, as well as system-scale radiation effects simulation in order to provide hardened architectures with increased reliability.

ACKNOWLEDGMENTS

We thank the company Viavi Solutions France SAS for lending the OTDR devices for this research.

The PRESERVE Platform has been funded by the Occitanie Region and the European Union via ERDF funds.

TRIUMF receives funding via a contribution agreement with the National Research Council of Canada.

This activity has received funding from the European Union's 2020 research and innovation programme under grant agreement No 101008126, corresponding to the RADNEXT project.

REFERENCES

[1] J. Restier-Verlet et al., «Radiation on Earth or in Space: What Does It Change? », *International Journal of Molecular Sciences*, vol. 22, n° 7, Art. n° 7, janv. 2021, doi: 10.3390/ijms22073739.

[2] D. M. Fleetwood, P. S. Winokur, et P. E. Dodd, « An overview of radiation effects on electronics in the space telecommunications environment », *Microelectronics Reliability*, vol. 40, n° 1, Art. n° 1, janv. 2000, doi: 10.1016/S0026-2714(99)00225-5.

[3] M. Ferrari et al., « "Radiation to Materials" at CERN », *IEEE Transactions on Nuclear Science*, vol. 70, n° 8, Art. n° 8, août 2023, doi: 10.1109/TNS.2023.3241785.
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[4] A. Nagamatsu, K. Murakami, K. Kitajo, K. Shimada, H. Kumagai, et H. Tawara, « Area radiation monitoring on ISS Increments 17 to 22 using PADLES in the Japanese Experiment Module Kibo », *Radiation Measurements*, vol. 59, p. 84-93, déc. 2013, doi: 10.1016/j.radmeas.2013.05.008.

[5] U. Straube, T. Berger, et M. Dieckmann, « The ESA Active Dosimeter (EAD) system onboard the International Space Station (ISS) », *Zeitschrift für Medizinische Physik*, mai 2023, doi: 10.1016/j.zemedi.2023.03.001.

[6] S. Girard et al., « Overview of Radiation Effects on Silica-Based Optical Fibers and Fiber Sensors », *IEEE Transactions on Nuclear Science*, p. 1-1, 2024, doi: 10.1109/TNS.2024.3511455.

[7] X. Bao et L. Chen, « Recent Progress in Distributed Fiber Optic Sensors », *Sensors*, vol. 12, n° 7, Art. n° 7, juill. 2012, doi: 10.3390/s120708601.

[8] H. Henschel, M. Körfer, J. Kuhnhenn, U. Weinand, et F. Wulf, « Fibre optic radiation sensor systems for particle accelerators », *Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment*, vol. 526, n° 3, Art. n° 3, juill. 2004, doi: 10.1016/j.nima.2004.02.030.

[9] D. Di Francesca et al., « Distributed Optical Fiber Radiation Sensing in the Proton Synchrotron Booster at CERN », *IEEE Transactions on Nuclear Science*, vol. 65, n° 8, Art. n° 8, août 2018, doi: 10.1109/TNS.2018.2818760.

[10] D. Di Francesca et al., « Qualification and Calibration of Single-Mode Phosphosilicate Optical Fiber for Dosimetry at CERN », *J. Lightwave Technol.*, JLT, vol. 37, n° 18, Art. n° 18, sept. 2019.

[11] K. Bilko et al., « CERN Super Proton Synchrotron Radiation Environment and Related Radiation Hardness Assurance Implications », *IEEE Transactions on Nuclear Science*, vol. 70, n° 8, Art. n° 8, août 2023, doi: 10.1109/TNS.2023.3261181.

[12] D. L. Griscom, E. J. Friebele, K. J. Long, et J. W. Fleming, « Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus-doped silica glass and optical fibers », *Journal of Applied Physics*, vol. 54, n° 7, Art. n° 7, juill. 1983, doi: 10.1063/1.332591.

[13] S. Girard et al., « Overview of radiation induced point defects in silica-based optical fibers », *Reviews in Physics*, vol. 4, p. 100032, nov. 2019, doi: 10.1016/j.revip.2019.100032.

[14] A. Morana et al., « Operating Temperature Range of Phosphorous-Doped Optical Fiber Dosimeters Exploiting Infrared Radiation-Induced Attenuation », *IEEE Transactions on Nuclear Science*, vol. 68, n° 5, Art. n° 5, mai 2021, doi: 10.1109/TNS.2021.3053164.

[15] J. Perrot et al., « Distributed Optical Fiber-Based Dosimetry With Optical Frequency Domain Reflectometry », *IEEE Sensors Journal*, vol. 24, n° 13, Art. n° 13, juill. 2024, doi: 10.1109/JSEN.2024.3401249.

[16] J. Bisutti, S. Girard, et J. Baggio, « Radiation effects of 14MeV neutrons on germanosilicate and phosphorus-doped multimode optical fibers », *Journal of Non-Crystalline Solids*, vol. 353, n° 5, Art. n° 5, avr. 2007, doi: 10.1016/j.jnoncryol.2006.10.013.

[17] D. Di Francesca et al., « Dosimetry Mapping of Mixed-Field Radiation Environment Through Combined Distributed Optical Fiber Sensing and FLUKA Simulation », *IEEE Transactions on Nuclear Science*, vol. 66, n° 1, Art. n° 1, janv. 2019, doi: 10.1109/TNS.2018.2882135.

[18] D. Lambert et al., « Simulation-Assisted Methodology for the Design of Fiber-Based Dosimeters for a Variety of Radiation Environments », *IEEE Transactions on Nuclear Science*, vol. 71, n° 8, Art. n° 8, août 2024, doi: 10.1109/TNS.2024.3380318.

[19] S. Girard et al., « Recent advances in radiation-hardened fiber-based technologies for space applications », *J. Opt.*, vol. 20, n° 9, Art. n° 9, août 2018, doi: 10.1088/2040-8986/aad271.

[20] I. McKenzie, S. Ibrahim, E. Haddad, S. Abad, A. Hurni, et L. K. Cheng, « Fiber Optic Sensing in Spacecraft Engineering: An Historical Perspective From the European Space Agency », *Front. Phys.*, vol. 9, nov. 2021, doi: 10.3389/fphy.2021.719441.

[21] A. Meyer, « Towards an Embedded and Distributed Optical Fiber-based Dosimeter for Space Applications », présenté à IEEE Nuclear & Space Radiation Effects Conference (NSREC), juill. 2022.

[22] D. Di Francesca et al., « Low radiation dose calibration and theoretical model of an optical fiber dosimeter for the International Space Station », *Appl. Opt.*, AO, vol. 62, n° 16, Art. n° 16, juin 2023, doi: 10.1364/AO.483560.

[23] M. Roche et al., « Solar Particle Event Detection With the LUMINA Optical Fiber Dosimeter Aboard the International Space Station », *IEEE Transactions on Nuclear Science*, vol. 71, n° 8, Art. n° 8, août 2024, doi: 10.1109/TNS.2024.3368137.

[24] A. Meyer, « Optical-fiber-based distributed dosimetry for space applications », Ph.D. thesis, Université Jean Monnet - Saint-Etienne, 2023. https://theses.hal.science/tel-04515101

[25] Viavi Solutions, « Nano and Micro iOTDR Cards datasheets », https://www.viavisolutions.com/en-us/literature/nano-and-micro-iotdr-cards-data-sheets-en.pdf

[26] A. H. Hartog, « Principles of Optical Time-Domain Reflectometry (OTDR) for Distributed Sensing », in *An Introduction to Distributed Optical Fibre Sensors*, CRC Press, 2017, p. 55-106. doi: 10.1201/9781315119014.

[27] C. Campanella et al., « Photobleaching Effect on Infrared Radiation-Induced Attenuation of Germanosilicate Optical Fibers at MGy Dose Levels », *IEEE Transactions on Nuclear Science*, vol. 68, n° 8, Art. n° 8, août 2021, doi: 10.1109/TNS.2021.3068829.

[28] A. Meyer, D. Lambert, A. Morana, P. Paillet, A. Boukenter, et S. Girard, « Simulation and Optimization of Optical Fiber Irradiation with X-rays at Different Energies », *Radiation*, vol. 3, n° 1, Art. n° 1, mars 2023, doi: 10.3390/radiation3010006.

[29] R. G. Alía et al., « Heavy Ion Energy Deposition and SEE Intercomparison Within the RADNEXT Irradiation Facility Network », *IEEE Transactions on Nuclear Science*, vol. 70, n° 8, Art. n° 8, août 2023, doi: 10.1109/TNS.2023.3260309.

[30] E. W. Blackmore, P. E. Dodd, et M. R. Shaneyfelt, « Improved capabilities for proton and neutron irradiations at TRIUMF », in *2003 IEEE Radiation Effects Data Workshop*, juill. 2003, p. 149-155. doi: 10.1109/REDW.2003.1281368.

[31] D. L. Philen, I. A. White, J. F. Kuhl, et S. C. Mettler, « Single-Mode Fiber OTDR: Experiment and Theory », *IEEE Transactions on Microwave Theory and Techniques*, vol. 30, n° 10, Art. n° 10, oct. 1982, doi: 10.1109/TMTT.1982.1131282.

[32] D. C. Montgomery et G. C. Runger, *Applied Statistics and Probability for Engineers*. John Wiley & Sons, 2010.

[33] A. Gallet et al., « Novel methodology for interpolation of radiation-induced attenuation in optical fibers », *Optical Fiber Technology*, vol. 80, p. 103389, oct. 2023, doi: 10.1016/j.yofte.2023.103389.

[34] J. Řipa, G. Dilillo, R. Campana, et G. Galgóczi, « A comparison of trapped particle models in low Earth orbit », in *Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray*, SPIE, déc. 2020, p. 597-606. doi: 10.1117/12.2561011.