# Plastic optical fiber sensors for mooring lines monitoring in floating wind turbines: A reliability study of OTDR measurement

Romain Grangeat, Marion Girard, Cyril Lupi, Dominique Leduc, Frédéric Jacquemin

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Romain Grangeat, Marion Girard, Cyril Lupi, Dominique Leduc, Frédéric Jacquemin. Plastic optical fiber sensors for mooring lines monitoring in floating wind turbines: A reliability study of OTDR measurement. Applied Ocean Research, 2025, 158, pp.104541. 10.1016/j.apor.2025.104541. hal-05054705

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Submitted on 5 May 2025

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Research paper

# Plastic optical fiber sensors for mooring lines monitoring in floating wind turbines: A reliability study of OTDR measurement

Romain Grangeat<sup>a, *</sup> <sup>Δ</sup>, Marion Girard<sup>b, Δ</sup>, Cyril Lupi<sup>c</sup>, Dominique Leduc<sup>c</sup>, Frédéric Jacquemin<sup>b, Δ</sup>

## ARTICLE INFO

### Keywords:

- Plastic optical fiber

- Monitoring wind turbines

- Polymer ageing

- Experimental measurements

## ABSTRACT

This study investigates the use of POF (Plastic Optical Fibers) for mooring lines monitoring in floating wind turbines. Focusing on their mechanical adaptability and optical performance in marine environments. Optical attenuation measurement are employed to determine integrity of POFs under mechanical stresses such as tension and torsion, as well as during prolonged water immersion at ambient temperature. Key findings demonstrate a high deformation capacity of POFs (up to 70 % strain), with negligible loss in light transmission under mechanical stresses in tension and torsion. Additionally, water absorption was found to have a limited impact on optical attenuation, even after extended immersion. These results confirm the suitability in POFs for mooring lines monitoring in floating wind turbines. Furthermore, elongation measurements performed on POFs of up to 100 meters in length validated the feasibility of monitoring strain using both transmission and reflection OTDR (Optical Time Domain Reflectometer) measurement. The results highlight POFs as cost-effective, durable sensors capable of addressing the high-strain demands of mooring line applications.

## 1. Introduction

The development of renewable energy is a contemporary imperative to address global environmental and energy challenges. Among renewable energy sources, offshore wind power has emerged as a promising solution, offering significant potential for generating electricity from ocean winds. In France, for example, the government has set a target of generating 45 GW of offshore wind power by 2050. However, with 80 % of the wind resource is located in areas where water depths exceeds 60 meters, fixed wind turbine structures cannot be installed, necessitating the development of floating structures. It is well recognized (EDF Group, 2024; Tong, 2010) that floating wind farms are particularly suited for deeper marine areas, providing stable and reliable energy production. Various types of floating offshore wind turbines exist, all of which rely on mooring lines to prevent structural drift (Fig. 1). These mooring lines typically consist of multiple braided polyamide 6 ropes, which offer substantial mechanical strength while allowing for flexibility and stretchability to accommodate sea movements.

Given their extensive length, often reaching several hundred meters,

and the significant deformations they experience due to marine forces, regular maintenance is essential. However, such maintenance is often costly and labor-intensive.

This study explores the potential of POFs as a solution for real-time mooring line monitoring, reducing maintenance efforts while ensuring structural integrity and operational efficiency in floating wind turbines.

To facilitate maintenance efforts, the advancement of *in situ* measurement techniques to assess the condition of mooring lines is essential. Mooring lines play a crucial role in ensuring the stability and safety of offshore structures. In this context, the use of nylon as a mooring line material has emerged as a preferred option due to its mechanical properties and resistance to marine conditions. Research has extensively studied the structural dynamics of offshore wind turbines under the influence of wind loads and waves, highlighting the fatigue and durability challenges of nylon mooring lines, which exhibit highly nonlinear mechanical behavior that remains poorly understood over the long term.

As demonstrated by (Flory et al., 2016; Pham et al., 2019) wind loads and waves significantly influence the structural dynamics of offshore wind turbines, particularly in terms of fatigue and durability issues in

<footnote><sup>a</sup> CESI LINEACT UR 7527, Le Paquebot, 24, 44600, Saint, Nazaire, France</footnote>

<footnote><sup>b</sup> GeM, UMR CNRS 6183, Nantes University, 58 Rue Michel Ange, 44600, Saint, Nazaire, France</footnote>

<footnote><sup>c</sup> GeM, UMR CNRS 6183, Nantes University, 2 Chemin de la Houssinière, 44300, Nantes, France</footnote>

<footnote>* Corresponding author.
E-mail address: rgrangeat@cesi.fr (R. Grangeat).</footnote>
nylon mooring lines. (Humeau et al., 2018) studied the influence of water on the mechanical behavior of polyamide 6 fibers, showing that while humidity has little impact on short-term tensile strength, it significantly affects long-term creep behavior. This result is further supported by (Civier et al., 2022). Continuous monitoring of mooring lines is therefore essential in assessing their performance and structural integrity.

In this context, the use of fiber optic sensors has gained attention as a promising solution, as demonstrated by (Lopez-Higuera et al., 2011) and (Min et al., 2021). These works examine the relevance of fiber optic sensors for monitoring structures in various environments, particularly in marine settings. In their review, (Min et al., 2021) discuss the use of fiber optic sensors for monitoring the health of marine structures, highlighting their resistance to electromagnetic interference, durability, and capability for distributed measurements. (Lopez-Higuera et al., 2011) explore the role of fiber optic sensors in structural health monitoring (SHM), emphasizing their usefulness for continuous monitoring of loading and degradation conditions in structures exposed to demanding environments. Other studies, such as those by (Di et al., 2018), demonstrate the ability of fiber optic sensors to measure mechanical deformations in structures subjected to environmental stressors. (Grangeat et al., 2020) examine the use of Fresnel reflection sensors to measure local water content in adhesive materials, enabling the assessment of moisture-induced degradation in marine contexts. (Bonilla Mora et al., 2018; Mieloszyk and Ostachowicz, 2017) have shown the capability of FBG (Fiber Bragg Grating) sensors to detect the presence of water within a bonded structure. This type of sensor enables measurements both during the water diffusion phenomenon and throughout the manufacturing process. Collectively, these studies demonstrate how fiber optic sensors can be employed to detect mechanical deformations in wet environments, providing indicators of ageing and degradation.

However, the vast majority of these studies have focused on silica optical fibers, the most widely developed and marketed fiber type today. Nevertheless, silica fibers have limitations in sensor applications due to their relatively low fracture deformation threshold (typically between 4 and 6 %) (Antunes et al., 2012; Kurkjian et al., 1989). Mooring lines, on the other hand, experience strain rates of up to 20 % (Chevillotte, 2020; Civier et al., 2024), far exceeding the deformation capacity of silica optical fibers. This highlights the potential of developing sensors POF-based sensors, which offer greater flexibility and deformation resistance in order to meet the demands of marine environments.

This study therefore focuses on the investigation of plastic optical

fibers for mooring lines monitoring using OTDR (Optical Time Domain Reflectometry) measurements. The OTDR method is a non-invasive measurement technique widely used to detect and locate faults in optical fibers, as extensively discussed in the review article by (Lu et al., 2019). This technique is based on sending light pulses along the fiber, followed by analyzing the backscattered light due to Rayleigh scattering and discontinuity points. When the light encounters a change in the refractive index, a small portion is reflected back to the source. By measuring the time for the light return, it is possible to precisely locate the point in the fiber where stress or a defect is detected. OTDR is particularly useful in marine environments, where the mechanical stresses on anchoring lines must be monitored to prevent any failure. However, the use of silica optical fibers, commonly associated with the OTDR method (Lu et al., 2019), as previously mentioned, presents a significant limitation, due to their low strain threshold, making them unsuitable for the high strain rates encountered in mooring lines.

On the other hand, POF have properties that may be better suited to this specific context (Kruszewski et al., 2004; Kuang et al., 2009). Typically, poly(methyl methacrylate) is used as the optical core material, while fluoropolymers are used for the optical cladding (Zubia and Arrue, 2001). It is worth noting that plastic optical fibers typically have a larger core diameter, optical aperture and higher attenuation coefficient than silica fibers.

Research on OTDR measurements applied to plastic optical fibers (POF) demonstrates their potential for various distributed monitoring applications. (Kuang, 2015) shows that POF, particularly GIPOF (Graded-Index Plastic Optical Fibers), can be used to locate and monitor crack propagation in offshore steel structures using a high-resolution photonic OTDR setup. (Saunders and Scully, 2005) explore the feasibility of a distributed pH sensor, illustrating how photonic OTDR overcomes the high losses of POF and ensures precise distributed measurements along the fiber. (Waluyo et al., 2022) demonstrate that OTDR can be applied to monitor temperature and humidity via POF by measuring variations in light intensity according to environmental changes, an approach suitable for dynamic conditions. Similarly, (Husdi et al., 2004) use OTDR to study the mechanical responses of POFs, such as micro-bending and axial strains, highlighting their potential for distributed structural health monitoring sensors. (Mizuno et al., 2021) synthesize these advances in a review, highlighting the benefits of POF in OTDR for supporting large deformations, a crucial advantage for geotechnical and civil infrastructures. Finally, (Liehr et al., 2009) study PMMA-based POFs and GIPOF for measuring deformations of up to 40 %, integrating them into technical textiles to monitor deformations in

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Typical mooring system components for floating offshore wind turbine (Guide to Floating Offshore Wind, 2024).</figcaption></figure>
geotechnical structures. Recent work (Leal-Junior et al., 2024) analyzed Rayleigh backscattering in perfluorinated POFs, demonstrating their feasibility as millimeter-scale force sensors and highlighting their potential for high-resolution structural monitoring. These studies confirm that, despite technical challenges related to attenuation and environmental constraints, POFs offer unique capabilities for distributed detection of cracks, pH, temperature, humidity, and deformations in diverse contexts.

This article explores the potential benefits of POFs for mooring lines monitoring in floating wind turbines will be explored, focusing on their flexibility and ability to adapt to the strains of nylon mooring lines. The opportunities presented by POFs in this area are examined, with emphasis on their resistance to mechanical stress (tension-torsion) and adaptability to marine environments. Concerns regarding the water sensitivity of POF, a critical factor in marine environments, will also be discussed. The aim of this article is to investigate the compatibility of POFs within the context of nylon mooring lines in floating wind turbines. The feasibility of strain measurement on a 100 m fiber is then assessed, considering technical constraints and precision requirements. This evaluation will be followed by a discussion of the implications of this work and potential challenges to be overcome. Finally, the article will conclude by highlighting the main lessons learned from this study and prospects for future research.

## 2. Compatibility of the POF sensor in the context of mooring lines for floating wind turbines

This section of the article focuses on a series of tests and measurements aimed at evaluating the performance and characteristics of POFs under different conditions. The POFs used in this study are the commercial Mitsubishi Eska CK-60 and CK-40 fibers, which are step-index fibers with a numerical aperture of 0.5. The optical core has a diameter of 1.470 mm for the CK-60 and 0.980 mm for the CK-40, made of in PMMA (Polymethyl-Methacrylate Resin), while the 30 µm thick cladding is composed of a fluorinated polymer (Fig. 2).

To understand POF behavior under ideal conditions, attenuation measurements in the dry state at three different wavelengths: 405.4 nm, 519.4 nm, and 635.6 nm are examined and compared with supplier data. Next, the effects of water absorption on POF and its influence on signal attenuation are explored. The tensile test of POF is also addressed to understand its strength and durability under stress. Furthermore, the effect of twisting on POF and its consequences on its optical performance are studied. These measurements and tests will provide insight into the mechanical and optical properties of POFs under varying conditions, contributing to potential applications in various fields, particularly in the monitoring of nylon mooring lines for floating wind turbines.

### 2.1. Optical attenuation: cut-back measurements in dry state

Attenuation refers to the loss of light that a waveguide can experience considering the effects of diffusion, reflection, and absorption.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Mitsubishi Eska CK-60 and CK-40 fibers.</figcaption></figure>

Typically, cut-back measurements can be performed to measure this attenuation. The cut-back method is standardized in the IEC 60,793–1–40:2019 norm and attenuation measurements for POFs are discussed in the handbook by (Ziemann et al., 2008). The measurement principle used in this study is illustrated in Fig. 3. A collimator was used to ensure identical injection conditions for the different POFs.

Cut-back measurements involve measuring the power transmitted through a waveguide at various distances. In this study, the power transmitted through a 12 m Mitsubishi Eska CK-40 POF is initially measured. This optical power was indirectly measured by capturing the voltage provided by a biased free-space detector. The same fiber was then cut to different lengths (10 m, 7 m, and 1 m), and the transmitted power was measured for each of these lengths. Eq. (1) was then used to calculate the linear attenuation $\alpha$ of the fiber that represents the loss of light transmission in the optical fiber.

$$ \alpha (\text{dB.m}^{-1}) = \frac{10}{L_1 - L_2} \log_{10} \left( \frac{P_1}{P_2} \right) \quad (1) $$

Where $\alpha$ the linear attenuation in dB.m$^{-1}$, $P_1$ and $P_2$ the transmitted powers in watts (W) for the respective POF lengths $L_1$ and $L_2$ in meters (m).

These cut-back measurements were performed on three different fiber samples at three different wavelengths: 405.4 nm, 519.4 nm, and 635.6 nm. The obtained results were then compared with supplier data (Mitsubishi Eska CK-60) (Fig. 4).

Cut-back measurements allow for verifying the results obtained by the supplier. Validating the measurement principle is the first step. The current objective is to determine the linear attenuation when the fiber is immersed in water.

### 2.2. Water diffusion in immersion

Before conducting optical measurements, it is essential to verify the impact of humid aging on the attenuation of POFs. Indeed, humidity can strongly influence attenuation effects due to water absorption. These effects, studied by (Kaino, 1985), result in increased losses at specific wavelengths in the visible and near-infrared regions through the activation of OH bond vibrations in PMMA. (Liehr et al., 2017) confirm that this absorption influences attenuation properties but can be exploited for distributed humidity detection. For the application case of this study, the impact of immersion in distilled water at 25 °C will be examined on Mitsubishi Eska CK-60 POF (1.5 mm in diameter). It should be noted that, in the context of this study, the real environment is seawater. The literature has shown that a polymer absorbs slightly more water when aged in distilled water than seawater (Deroiné et al., 2014; Shaafaey et al., 2023). In practice, working with distilled water is simpler and allows for a more conservative approach compared to seawater. Before aging, POFs were placed in a desiccator for one week at an average humidity level of 17 % to ensure a homogeneous water content that was as low as possible. Initially, the water diffusion kinetics were determined using a gravimetric test on 1.5 mm diameter POFs (Mitsubishi Eska CK-60). Gravimetric tests involve tracking changes in the mass of a material exposed to a wet environment. The mass $m(t)$ of three samples was periodically measured with a precision balance (resolution $10^{-5}$ g), allowing us to calculate the absorbed water content $C(t)$ (Eq. (2)).

$$ C(t) = \frac{m(t) - m_0}{m_0} \quad (2) $$

Where $C(t)$ the water content absorbed, $m(t)$ the mass of sample over time in grams (g) and $m_0$ the initial mass before ageing in grams (g).

The results are shown in Fig. 5 as a function of the square root of time divided by the POF radius. This unit is used to verify the linearity of the initial diffusion phase, which is characteristic of Fickian diffusion, and to normalize the data with respect to thickness, allowing for the determination of the material’s behavior rather than the influence of geometry.
<figure><img src="image_4.png" /><figcaption>Fig. 3. Cut-back measurement principle to determine the linear attenuation of POF.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Evolution of the optical attenuation as a function of the wavelength determine with cut-back measurements.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Water kinetic diffusion of POF in water immersion at 25 °C.</figcaption></figure>

Moreover, considering Crank's solution equation (Crank, 1975), dividing by the thickness renders the equation to be dimensionless.

The results reveal a linear increase in the water content (up to about 1.5 %) during the initial immersion period. Subsequently, this water

content continues to increase at a slower rate until it stabilizes at around 1.75 %. This result is consistent with those obtained in the literature (Smith and Schmitz, 1988; Unemori, 2003). The difference may be attributed to the presence of the fluoropolymer optical cladding. These experimental results can be effectively modeled using a cylindrical solution of the Dual-Fick model (22). Parameter identification was achieved through the method of least squares minimization, and these parameters are listed in Table 1, where D and $C_{sat}$ denote the diffusion coefficient and water uptake saturation respectively.

To anticipate optical measurements on aged fibers in ambient conditions, desorption measurements were carried out. Desorption tests were performed on a fiber that had previously undergone ageing under water immersion. This fiber was placed in ambient conditions at 25 °C and 70 % RH, and its mass was measured over time (Fig. 6).

The results obtained indicate a significant decrease in water content over time. After just 10 minutes under ambient conditions, the fiber had lost >50 % of its water content. The material's desorption is therefore not negligible.

The objective now is to determine the evolution of the optical attenuation of the POF during water absorption. This result will help assess the effect of water absorption by the material on light propagation.

## 2.3. *In situ* optical attenuation measurement set-up during water ageing

The results obtained so far on the rapid desorption of fibers in ambient conditions highlight the necessity of implementing *in situ* measurements during the POF water absorption. An experimental setup for in-situ measurement of the optical attenuation of Mitsubishi Eska CK-40 POF (1 mm in diameter) during immersion has been established. The concept involves placing several fibers of different lengths in a tank filled with water and to measure the transmitted power for each of these fibers. During the various measurements, only the tank moves to align with and inject light into the different POFs (Fig. 7).

The measurement principle is illustrated in Fig. 8. It was decided to work with three fibers of each length: 1 m, 6 m and 12 m. Each of the lengths was tested with three different lasers, each with a power of 4.5 mW, at wavelengths of 405.4 nm, 519.4 nm and 635.6 nm.

Before performing these immersion measurements, it is important to verify the performance of the experimental setup. To achieve this, measurements were conducted prior to ageing and compared with the conventional cut-back measurements carried out previously (Fig. 4) on

<table><caption>**Table 1**

Diffusion parameters identified on POF.</caption><tr><td>D<sub>1</sub> (mm<sup>2</sup>.s<sup>-1</sup>)</td><td>(2,52 ± 0,17).10<sup>-7</sup></td></tr><tr><td>C<sub>sat1</sub> (%)</td><td>1,24 ± 0,05</td></tr><tr><td>D<sub>2</sub> (mm<sup>2</sup>.s<sup>-1</sup>)</td><td>(1,53 ± 0,06).10<sup>-8</sup></td></tr><tr><td>C<sub>sat2</sub> (%)</td><td>0,51 ± 0,10</td></tr></table>
<figure><img src="image_7.png" /><figcaption>Fig. 6. Water desorption on POF aged.</figcaption></figure>

Mitsubishi Eska CK-40 POFs (1 mm in diameter). Repeatability tests were performed with more than five measurements for each fiber length (1 m, 6 m, and 12 m) at a wavelength of 635.6 nm (Fig. 9).

Firstly, a low standard deviation (a maximum of 2.1 % for the 1 m fiber) was observed in the transmitted power measurements between fibers of the same length. This transmitted power was measured indirectly by capturing the voltage provided by a biased free-space detector. However, it was noted that the longer the fiber, the lower the transmitted power, mainly due to attenuation effects within the fibers. By averaging the results obtained for each fiber length, the optical attenuation between the different lengths can be calculated. For a wavelength of 635.6 nm, a value of 0.175 ± 0.012 dB.m⁻¹ was obtained using Eq. (1). This value closely matches that previously obtained using conventional

cut-back measurements (0.177 ± 0.019 dB.m⁻¹). The strong agreement between these results validates the performance of the experimental setup developed in this study (Fig. 7). This validation enables the implementation of *in situ* optical attenuation measurements of the POF during immersion.

Using Eq. (1), it is possible to plot the evolution of optical attenuation for each wavelength and different fiber length pairs (1 m and 6 m; 1 m and 12 m; 6 m and 12 m) during immersion. The mean values and standard deviations are shown in Fig. 10.

The results show a slight change in optical attenuation as the POF ages during immersion. This slight increase is probably due to water absorption by the material, as previously observed (Fig. 5). The comparison between the supplier's data (without ageing) and the data after 5 months of immersion is shown in Fig. 11.

An increase in optical attenuation of approximately 10% is observed after 5 months of immersion. In his study, (Kaino, 1985) demonstrated equivalent results during aging at 90 % RH and 50 °C for the wavelengths used here. The behavior during immersion appears to be similar to that observed under high humidity conditions. Although the variation is modest, this result should be taken into consideration when using an optical fiber as a sensor to set up a monitoring system for a structure exposed to a wet environment. It is now necessary to verify the impact of mechanical stresses on the optical properties and determine whether POFs can be used for monitoring mooring lines in floating wind turbines.

## 2.4. Mechanical tests: tension and torsion

Tensile tests were first performed on Mitsubishi Eska CK-40 POFs (1 mm in diameter) with a useful length of 20 cm. The aim of this section is to verify the maximum elongation a POF can withstand to determine whether this deformation is compatible with a mooring line for a floating wind turbine, which can experience elongations of up to 20 %. To better characterize the POF, adapted jaws were used. They consisted of winding the fibers around a wheel (Fig. 12).

These tests were conducted using a ZWICK machine equipped with a

<figure><img src="image_8.png" /><figcaption>Fig. 7. Setting up experimental bench to measure optical attenuation during water uptake.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Principle of *in situ* measurement of optical attenuation of fibers in immersion.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 9. Repeatability on transmitted power measured on different fiber lengths.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10. Optical attenuation evolution as a function of immersion time for three different wavelengths.</figcaption></figure>

500 N load cell, applying a displacement rate of 10 mm/min. Fiber deformation was measured throughout the tests using an optical extensometer. To facilitate this measurement, marks were placed on the samples to track their behavior during testing. Three tests were carried out on three different POF samples (Fig. 13).

These tests (Fig. 13) allowed for the calculation of an average elastic modulus of 3019 ± 114 MPa. In addition, a strain at break of ε<sub>break</sub> = 69.7 ± 5.3 % was observed. Therefore, PMMA optical fibers exhibit a remarkably high elongation at break. It is worth noting that bulk PMMA has an elongation at break of approximately 4 %. In the case of fibers, the manufacturing process, likely involving extrusion, leads to a significant increase in this elongation at break during tensile tests (Yang et al., 2004). To verify the impact of mechanical stresses on the propagation properties of the POF, the jaws were modified to allow light propagation along the fiber by reducing pinching. A connector made of PLA (Polylactic Acid) was 3D-printed. The concept was to place the fiber inside this connector and inject epoxy adhesive to secure the fiber without creating pinching that could interfere with light propagation.

<figure><img src="image_12.png" /><figcaption>Fig. 11. Optical attenuation measured for different wavelengths: supplier data, no ageing and 5 months ageing.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. Traction jaws for fibers.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 13. Tensile tests on three POFs of 1 mm diameter.</figcaption></figure>

This connector was placed between self-tightening jaws (Fig. 14). During the tensile test, light was injected into the POF, and the transmitted power was measured to determine the influence of mechanical stresses on light propagation.
<figure><img src="image_15.png" /><figcaption>Fig. 14. Placement in self-clamping fiber jaws with the connector.</figcaption></figure>

It is important to verify that the presence of the connector does not cause a change in the behavior of the fibers compared to what was previously determined (Fig. 13). To achieve this, tensile tests were carried out under the same conditions as before to compare the results. Samples 1–3 are those tested with traction jaws for fibers (Fig. 12), and samples 4–6 were tested with self-clamping jaws and the connector (Fig. 14).

Equivalent results were observed (Fig. 15) in the presence of the connector. The connector did not have any mechanical effect on the tensile test results. During these three tensile tests, the transmitted light power was measured using a photodiode detector (Fig. 16). The measured voltage was normalized to compare the different tests.

Hence, there is no loss of light before the fiber breaks during a tensile test. As radial deformation remains uniform throughout the test, the fibers retain their waveguiding properties.

When tensioning a mooring line, torsion effects may occur. The tension-torsion interaction has been discussed in the articles by (Chaplin et al., 2000) and (Davies et al., 2006). They emphasize the importance of understanding and controlling tension-torsion interactions in anchor lines to prevent problems related to torsion (fatigue, permanent deformation). Solutions such as "Torque Match" polyester ropes or the selection of components with compatible torsional properties are proposed to minimize these effects. In this context, torsion tests were also performed in this study. The fiber was positioned within the previously used jaws. During the torsion test, the bottom jaw was rotated to induce twisting of the fiber on itself (Fig. 17).

Three tests were carried out on three different fibers (Fig. 18). During

the tests, the power transmitted through each fiber was measured at each rotation by capturing the voltage provided by a biased free-space detector. It is possible to plot the evolution of the transmitted power as a function of the unit angle $\theta$ (Eq. (3)).

$$ \theta = \frac{\mu}{L} \qquad (3) $$

Where $\theta$ the unit angle in turns.m⁻¹, $\mu$ the number of turns and L the fiber length in meters (m).

For a 1 m fiber, a drop in the transmitted power was observed when it was rotated on itself by almost 295 turns. Three different phases can be identified during the tests (Fig. 18). In the first phase, there is no light loss, and the transmitted power remains unchanged. This power loss therefore appears for a unit angle of 295 turns/m, which is significantly higher than what was observed in the work of (Davies et al., 2006). Indeed, rotations of up to 9 turns were observed for 46 m mooring lines, corresponding to a unit angle of 0.20 turns/m. In the context of this study's application, torsion effects will have no impact on POF measurements. In the second phase, a significant reduction in transmitted power was observed, with a loss of approximately 50 % measured. This loss is mainly due to the scattering effects caused by the torsion of the fiber. Finally, the fiber undergoes complete breakage at around 800 turns for a 1 m length of fiber.

In conclusion, our results indicate that tensile and torsional stresses have an almost negligible effect on the light propagation properties of POFs.

These results show that there does not appear to be any major obstacle in the instrumentation of a floating wind turbine mooring line using POFs. Indeed, mechanical tension-torsion stresses, as well as immersion, have a negligible impact on POFs without significantly increasing attenuation. Measurements on mooring lines of 100 m appear feasible.

### 3. Feasibility of measuring elongation on a 100 m fiber

This section of the article is dedicated to exploring the performance of Mitsubishi Eska CK-40 POF (1 mm in diameter) using two different OTDR measurement modalities: transmission measurement and reflection measurement. These two measurement modalities provide complementary perspectives on the performance and characteristics of POF, highlighting its capabilities and limitations in different situations. By exploring transmission and reflection measurements, the potential applications of POF in various fields, including the nylon mooring line

<figure><img src="image_16.png" /><figcaption>Fig. 15. Comparison of tensile tests with and without the connector.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. 16. Tensile test performed on three POFs by injecting light during tests.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 17. Torsion test on POF.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 18. Division into three phases of the evolution of the transmitted power during the torsion tests.</figcaption></figure>

monitoring for floating wind turbines, can be identified by measuring their elongations.

Elongation measurements have already been performed on POFs using the OTDR method. Indeed, (Durana et al., 2009) explored a POF-based sensor to measure the elongation of an aircraft flap subjected to bending loads. The method relies on detecting the phase shift between two fibers, one under tension and the other under compression. (Poisel, 2008) proposed a POF elongation sensor based on measuring the phase

shift of modulated light signals. The fibers, integrated into lightweight structures such as wind turbine blades or aircraft wings, allow the detection of length changes of approximately 50 µm. The results high-light sufficient sensitivity for structural monitoring applications and significant cost advantages compared to FBG sensors. (Liehr and Kreb-ber, 2010) proposed a method based on Optical Frequency Domain Reflectometry (OFDR) to dynamically measure length changes between multiple reflection points. This approach achieves a resolution below 1 µm and is suitable for both static and dynamic measurements, up to 2 kHz.

This study differs from the literature by proposing the use of OTDR measurements to perform tests on long fiber lengths ranging from 10 to 100 m undergoing significant deformations.

### 3.1. OTDR measurement

The measurement principle is based on the use of a pulsed laser. The pulsed light is first injected into a POF in free space. The time required for the pulse to travel along the fiber can be measured by determining the time interval between the injection of the pulse into the fiber and its exit. This time interval is called the time of flight and denoted $t_{fly}$ (Fig. 19).

The light speed propagation noted $v_{fiber}$, can be calculated using Eq. (4).

$$v_{\text{fiber}} = \frac{c_0}{n_{\text{group}}} \qquad (4)$$

Where $v_{fiber}$ the light speed propagation in m.s⁻¹, $c_0$ = 299 792 458 m.s⁻¹ the light speed in vacuum and $n_{group}$ the group index of POF. In this study, the group index of Mitsubishi Esak POFs empirically determined by (Ayesta et al., 2017) is used: $n_{group}$ = 1.48 at 1550 nm.

By measuring the flight time and calculating the light speed, the fiber length $l_{fiber}$ can be determined (Eq. (5)).

$$l_{\text{fiber}} = \frac{c_0 \cdot t_{\text{fly}}}{n_{\text{group}}} \qquad (5)$$

Where $l_{fiber}$ the fiber length in meters (m), $c_0$ is the light speed in vacuum in m.s⁻¹ and $t_{fly}$ the flight time in seconds (s).

In order to carry out this measurement, an optical bench was developed which includes the following components:

- A pulsed laser from Thorlabs (NPL64B) trigger by a low frequency generator between 0 and 2.2 MHz at 640 nm;

- A 2 to 2 plastic optical coupler from I-FiberOptics (IF 540) with 50 cm each arms;

- Two high frequency detectors from Thorlabs (FPD 610-FS VIS) up to 1.5 GHz;
<figure><img src="image_20.png" /><figcaption>Fig. 19. OTDR measurement principle.</figcaption></figure>

- A 1 mm diameter PMMA optical fiber (Mitsubishi Eska CK-40) from I-FiberOptics;

- The optical connection is provided by SMA connectors from I-FiberOptics (SMA 905) and a high frequency oscilloscope.

It should be noted that the results in Fig. 3 show lower attenuation at a wavelength of 519.4 nm. However, for economic and practical reasons, a wavelength of 640 nm was chosen. Optical equipment for this wavelength is generally more affordable and widely available.

SMA connectors are used to connect the fiber and the optical coupler. These connectors were pre-installed. To achieve this, the optical fiber was bonded in place using cyanoacrylate adhesive and then polished to create a surface as flat as possible. This bench allows measurements in transmission (at the fiber tip) and in reflection (using the reflection at the fiber tip) (Fig. 20).

In the early stages, it is essential to check the pulses directly at the laser output. These pulses are used as a reference for calculating the time of flight. The setup allows two parameters to be adjusted: the pulse width directly on the laser (ranging from 5 ns to 39 ns, according to supplier) (Fig. 21), and the pulse repetition frequency (triggered externally by the low-frequency generator) (Fig. 22). Variations in pulse

width result in a significant change in amplitude (Fig. 21). This variation is a limitation of the laser or the pulse generator. It is therefore necessary to match this width to the specific fiber length being tested, taking into account any optical losses that may occur.

The repetition frequency adjusts the time interval between each pulse (Fig. 22). This frequency is controlled by the low-frequency generator and can be set between 0 and 2.2 MHz. The selection of this frequency depends on the specific fiber length being tested to avoid overlap between the reference pulse and the output pulse. For example, with a frequency of 500 kHz, measurements can be conducted on fiber lengths of <400 m without interference.

## 3.2. Static measurement: length fiber measurement

Fiber length measurements were first carried out on the test bench. Fibers of approximately 10 m (POF 1), 50 m (POF 2), and 100 m (POF 3) in length were measured. These lengths were estimated by unwinding the fiber strand, so the measurements are approximate. The repetition rate was set to 500 kHz, and the minimum pulse width (5 ns) was used for the 10 m, 50 m and 100 m fibers. However, the maximum pulse width (39 ns) was used for the 100 m fiber during tip reflection due to

<figure><img src="image_21.png" /><figcaption>Fig. 20. General set-up to make pulsed measurement.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>Fig. 21. Maximum and minimum pulse width.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>Fig. 22. - Frequency of pulse repetition.</figcaption></figure>

significant optical losses.

Initially, all measurements were made in transmission, i.e. with the detector shown in Fig. 20. The measurement performed on the POF 1 fiber is shown in Fig. 23.

The black curve represents the reference pulse, and the red curve represents the pulse measured at the fiber tip. By measuring the time between these two pulses, it is possible to calculate the time of flight. To achieve this, the time at the rising edge of the pulses was used, corresponding to 10% of the pulse's maximum amplitude. The detectors used enable acquisitions at a frequency of 1.5 GHz (Thorlabs - FPD 610-FS VIS), corresponding to one acquisition every 0.67 ns. This time interval corresponds to a light propagation distance of 14 cm within the POF (Eq. (5)), resulting in a spatial resolution for length measurements of ±7 cm. Equivalent measurements were performed on POF 2 and POF 3. The results obtained are listed in Table 2 together with the calculated fiber length (Eq. (5)).

The results obtained are close to the estimated values but significantly more precise. It is therefore possible to measure fiber lengths using pulsed light measurements.

POFs, due to their relatively large core, are multimode fibers. This property leads to modal and chromatic dispersion effects, which can

<figure><img src="image_24.png" /><figcaption>Fig. 23. Pulsed measurement in transmission for the POF 1 (around 10 m).</figcaption></figure>

<caption>Table 2</caption>

Results obtained on three different length fibers for transmission pulsed measurement.

<table><thead><tr><td></td><td>Fly time (ns)</td><td>Calculated fiber length (m)</td></tr></thead><tbody><tr><td>POF 1</td><td>49.8</td><td>10.09 ± 0.07</td></tr><tr><td>POF 2</td><td>251.7</td><td>50.98 ± 0.07</td></tr><tr><td>POF 3</td><td>511.3</td><td>103.57 ± 0.07</td></tr></tbody></table>

degrade the quality of transmitted signals and, consequently, the accuracy of measurements. To assess these impacts, the profile of a light pulse injected at the fiber's input can be compared to that observed at the output, after propagation over a 100-meter length. The amplitudes of the two pulses have been normalized relative to their maximum amplitude to enable comparison (Fig. 24).

There is very little change in the shape of the pulse between its injection into the POF and its measurement after 100 m of propagation. This suggests that the effects of chromatic dispersion and modal dispersion are secondary order relative to our intended application.

In this study, reflection measurements were also conducted using the

<figure><img src="image_25.png" /><figcaption>Fig. 24. Comparison between the input pulse and the one at the end of a 100 m plastic optical fiber.</figcaption></figure>
reflection detector shown in Fig. 20. The repetition rate was set to 500 kHz, and the maximum pulse width (39 ns) was used. The aim was to make measurements when direct access to the fiber tip was not possible. However, the first measurements (on POF 1) indicated the need to position a mirror at the optical fiber tip. Without the mirror, the reflection at the fiber tip was too weak to be observed by the reflection detector (Fig. 25). After mounting an SMA 905 connector at the fiber tip, it was positioned in a polishing puck to create an optical interface as flat as possible to maximize reflection. The polishing puck was then placed onto a mirror. This simple method for creating reflection is discussed critically in the conclusion.

The black curve represents the reference pulse, while the blue (without mirror) and red (with mirror) curves represent the reflected pulse. Initially, both the blue and red curves show a pulse. This pulse is at the same position as the reference pulse and is due to the reflection that occurs at the SMA connector between the optical coupler and the POF. In both cases, the distance traveled by the light is the same, with 1 m representing the 2 p.m. of the optical coupler between the injection of the light into the fiber and the detectors. A second reflection is observed, but only in the case of the red curve (with mirror). Using this reflected pulse and the reference pulse, the time of flight can be calculated, which in this case is 107.5 ns. When calculating the fiber length (Eq. (5)), it must be taken into account that the light travels twice the length of the fiber in reflection.

In conclusion, the calculated fiber length for reflection measurements is 10.09 ± 0.07 m. This result is equivalent to the calculated fiber length for transmission measurements (Table 2). Therefore, it is possible to perform both transmission and reflection measurements.

### 3.3. Dynamic measurement: elongation fiber measurement

The current objective is to perform measurements while the fiber is under tension. To achieve this, tensile tests were set up in which the elongation of the fiber was monitored by a camera. Markers were applied to the optical fiber, and point tracking was carried out during the test (Fig. 26) using the optical images captured by the camera. It should be noted that the setup does not allow measurement for the measurement of the force applied to the fiber during the tensile test.

Point tracking involves measuring the distance between the two markers on the optical fiber. By measuring this length, it is possible to calculate the deformation of the effective length (1.040 ± 0,005 m between the two jaws) and thus the change in length of the fiber during the tensile test. The controlled displacement of the movable jaw is shown in Fig. 27, with data acquired every 20 s.

To ensure a secure grip in the self-tightening jaws, 3D printed

connectors were developed to minimize fiber pinching during testing. These connectors were assembled using epoxy adhesive. Measurements were performed on an 11.3 m long fiber, and the results obtained are shown in Fig. 29. Pulsed measurements in transmission were also carried simultaneously (Fig. 28).

A pulse shift is observed during the tensile test. This shift, as well as the time at the rising edge, results in an increase in the time of flight, and by using (Eq. (5)), the evolution of fiber length can be plotted and compared with the results obtained.

Both measurement methods yield comparable results. It is therefore possible to perform fiber elongation measurements using pulsed light measurements. Additional measurements were performed on different fiber lengths (approximately 20 m, 30 m, 50 m, and 100 m) and compared with the point tracking method (Fig. 30).

The results obtained with pulsed light measurements are very similar to those obtained with point tracking. In conclusion, regardless of the fiber length, it is possible to track its elongation using pulsed light measurements in transmission.

With the effective length known, it becomes possible to determine the fiber deformation in this region using pulsed light measurements (Fig. 31).

As the effective length is independent of the total fiber length, the measured deformations remain consistent. Reflection measurements were also conducted by placing a mirror at the fiber tip. The results are comparable to those observed previously in transmission. For instance, the Fig. 32 displays the result obtained from reflection for a fiber length of 100.4 m.

In conclusion, these results demonstrate that it is possible to measure fiber elongation in both transmission and reflection, even for long fibers (up to 100 m in this study).

## 4. Conclusion and discussion

This study highlights the use of POFs and OTDR measurements for monitoring the elongation of long-length POFs (up to 100 m in this case). The proposed system enables precise measurements using low-cost and widely available technologies. Unlike silica fibers, POFs exhibit high deformation thresholds, making them particularly suitable for the significant deformations encountered in the anchoring lines of floating wind turbines. Initially, the robustness of POFs under tension and torsion was demonstrated. Indeed, they retain their light propagation properties under these various mechanical stresses. Furthermore, the device enables in-situ measurements, even in marine environments. The study validates that optical attenuation and deformations can be reliably monitored, even during fiber immersion. Experimental results

<figure><img src="image_26.png" /><figcaption>Fig. 25. Pulsed measurement in reflection for the POF 1 (around 10 m) with and without mirror reflection at the fiber tip.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>Fig. 26. Measurement principle by point tracking method during tensile test.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>Fig. 27. Set point displacement of mobile jaw.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>Fig. 29. Fiber elongation with two different methods measurement: point tracking with optical picture and pulsed measurement.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>Fig. 28. Pulse evolution at the fiber tip during tensile test.</figcaption></figure>

also show that increases in attenuation due to water absorption remain minor and manageable over extended immersion periods. However, tests combining different types of stress (tension, torsion, and immersion) over extended periods have not been conducted. Based on individual stress results and considering typical deformation levels in real-

life conditions, confidence remains high in the fiber's under combined stresses. Additionally, water absorption induces a plasticization effect, decreasing the rigidity of polymers and increasing elongation at break, which is beneficial in this application. Nonetheless, further studies are required to precisely quantify potential cumulative effects over the long term. After validating the feasibility of using POFs as sensors within anchoring lines, the use of OTDR measurements with pulsed lasers enabled precise and reproducible measurements of the lengths and elongations of fibers up to 100 m long. These results confirm the relevance of the system for real marine applications, although it could be improved in future research.

Indeed, in this study OTDR transmission measurements were performed on 100 m long POFs, demonstrating the feasibility of such measurements for monitoring fiber elongations and lengths under realistic conditions. In an operational context, the mooring lines of floating wind turbines can reach lengths of up to 300 m. Based on the data obtained regarding optical attenuation, it is reasonable to consider that transmission measurements will remain feasible for these distances, validating the applicability of the system to large-scale marine configurations. However, OTDR reflection measurements revealed relatively low fiber tip reflections, even when positioning the fiber tip with an SMA 905 connector on a mirror. This makes extending this method to fibers longer than 100 m particularly challenging. Enhancing the optical system or optimizing fiber tip reflection is therefore essential to overcome this limitation. A promising approach involves metallizing the fiber tip to create total internal reflection, which could significantly improve the reflected signal and extend the range of reflection measurements. This
<figure><img src="image_31.png" /><figcaption>Fig. 30. Fiber elongation for three different fiber length with two different methods measurement.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>Fig. 31. Used length deformation as a function of the imposed displacement for different fiber lengths (measured by pulsed measurement).</figcaption></figure>

technical development represents a critical step toward strengthening the robustness and versatility of this approach for marine applications. Another point for improvement would be to deepen the analysis of measurement uncertainties. Although the initial validations exhibit low variability, further exploration of error sources, particularly in dynamic and multi-parameter environments, could enhance the system's reliability and precision.

It is also important to note that photoelastic effects were not considered in the scope of this study. However, validation of the optical measurement through external point tracking demonstrated that these effects are not necessarily predominant. This can be attributed to the fact

<figure><img src="image_33.png" /><figcaption>Fig. 32. Fiber elongation with pulsed measurement in reflection.</figcaption></figure>

that the fiber elongation was performed over a short length (approximately 1 meter) of the total fiber length. For a fiber spanning several tens of meters undergoing elongation along its entire length, it would be essential to account for photoelastic effects, leveraging Becker's work (Becker et al., 2018), which provides the photoelastic constants for PMMA. It would also be crucial to examine the potential plasticization effects that may occur in POF during large deformations, as observed in the present study.

To complete this work and increase TRL (Technology Readiness Levels) of the nylon mooring line monitoring method, future research will need to focus on the integration of POFs directly within the mooring lines during their manufacturing process. This step represents a
technological challenge, as it requires ensuring mechanical compatibility between POFs and mooring lines while maintaining the fiber's optical and structural performance under marine conditions. One approach would involve developing specific manufacturing processes to encapsulate the POF without compromising its optical integrity, while ensuring a uniform distribution within the mooring lines to withstand tension, torsion, and prolonged immersion. Overcoming this challenge would also require mechanical and optical testing under real conditions to validate the robustness and durability of this system. Furthermore, integrating the POF during the manufacturing phase could simplify installation and maintenance by implementing an integrated, reliable, and cost-effective monitoring systems for floating wind turbines.

## CRediT authorship contribution statement

**Romain Grangeat:** Writing – original draft, Methodology, Conceptualization. **Marion Girard:** Writing – review & editing, Methodology, Conceptualization. **Cyril Lupi:** Methodology, Conceptualization. **Dominique Leduc:** Formal analysis, Conceptualization. **Frédéric Jacquemin:** Supervision.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

This work was performed within the MONAMOOR project with funding from France Energies Marines and its members and partners, as well as French State funding managed by the National Research Agency under the France 2030 investment plan (ANR-10-IEED-06-34). It also benefited from the financial support of Bretagne, Pays de la Loire and SUD Provence-Alpes-Côte d'Azur regions.

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