Polyimide-coated Optical Fiber Research Articles

Damage characterization of FRP embedded with different optical fibers under tensile load using acoustic emission and micrographic real-time inspection

Embedding optical fibers into fiber-reinforced plastics (FRPs) for composite health monitoring has attracted the interests of academia and industry recently though the impact of embedded optical fiber on FRP damage has not been clarified. In this study, the damage to the FRP structure caused by the embedded optical fibers was investigated. Five FRP samples incorporated with different types of optical fibers were subjected to tensile testing, and the crack initiation and propagation were in situ monitored using acoustic emission (AE) and micrographic inspection. After the AE signals were classified in terms of peak amplitude and peak frequency using K-means++ clustering, the damage progression from matrix cracking, fiber/matrix debonding, delamination, and fiber break were characterized. In-situ micrographic inspection clearly revealed the damage evolution around the embedded optical fiber. By correlating the stress-strain curves with the AE clustering and micrographic inspection results, it was verified that larger-diameter optical fibers reduce the modulus and ultimate strength of the FRP, while a good optical fiber/matrix interface mitigates these negative effects. The embedded optical fiber accelerated the crack propagation around it, leading to changes in the damage pattern. This phenomenon is the primary cause of FRP failure. Notably, polyimide-coated optical fibers with an outside diameter of 50 μm are ideal for embedded sensing as their incorporation into the matrix reduces the ultimate strength of FRP by only 1.8% with minimal impact on crack propagation.

Optical spectra in pure-silica-core single mode optical fibers after high-fluence reactor irradiation

The paper describes the study of radiation-induced attenuation (RIA) spectra in reactor-irradiated single mode at λ=1.55 μm optical fibers (OFs) with differente high-temperature coatings (polyimyde, aluminum and copper). The OFs were previously irradiated up to fast neutron fluence 1.8·1020 n/cm2 and gamma dose 2.32 GGy. After three years of storage at room temperature (RT), the RIA spectra were examined in the range of 650–1700 nm. All RIA mechanisms acting in this spectral range were identified: a tail of non-bridging oxygen hole center, a long-wavelength RIA (LWRIA) band with a maximum at λ > 1600 nm, unstructured “grey” loss due to coating microbendings or damage, and an absorption band peaking at 930 nm (E=1.33 eV). It was found out that the LWRIA can be significantly annealed at RT in polyimide-coated OFs. Grey loss were found to be more stable than those causing LWRIA.The LWRIA arising in reactor- and gamma-irradiated OFs have close thermal stability at RT and exhibit a similar shapes, which close to the absorption band of self-trapped holes (STH). This suggests a STH-like nature of the LWRIA.

High-accuracy high temperature measurement based on forward Brillouin scattering of polyimide-coated optical fiber

In this paper, a novel method for high-accuracy high temperature measurement is proposed, which is based on forward Brillouin scattering (FBS) of double-coated optical fiber with polyimide outer coating. The temperature variation changes the acoustic properties of polyimide-coated fiber that is suitable for high temperature measurement in the range of 120 ∼ 220 ℃. An interesting feature is that different radial acoustic modes have different frequency responses in the polyimide-coated fiber. Another interesting phenomenon is that the temperature linearly depends on the frequency shift of FBS spectrum with high sensitivity from 120 ℃ to 220 ℃. By using one acoustic mode with a high frequency shift-temperature sensitivity, we propose a novel high temperature measurement method based on FBS with high measurement accuracy. The experimental result shows that the low temperature measurement uncertainty of 0.02 ℃ can be achieved. The proposed method is expected to be used for high-accuracy temperature measurement applications within a temperature range of 100 ∼ 200 ℃, such as underground oil layer detection, power plant monitoring and high-power electrical systems monitoring.

Integrating Fiber Sensing for Spatially Resolved Temperature Measurement in Fuel Cells

Fiber optic sensors integrated into fuel cell stacks have the potential to significantly enhance the temperature control and health monitoring of fuel cells. Inhomogeneous loading, both within individual cells and across different cells in a stack, leads to the formation of local hotspots that accelerate aging and degrade performance. This study investigates the behavior and feasibility of incorporating polyimide-coated optical fiber sensors into bipolar plates for precise and spatially resolved temperature monitoring. The sensor is successfully integrated into a single cell of a fuel cell stack, positioned on the bipolar plate in direct contact with the membrane. Pre-tests are conducted to thoroughly evaluate the technical properties of the fiber in relation to specific cell requirements. Additionally, a physical prototype featuring the sensor is developed and employed to validate its effectiveness under realistic operating conditions. The temperature measurement obtained via the fiber exhibits a continuous profile throughout the entire length, covering both the active area and distributor region of the cell. Throughout the entire 60 min test period, the measuring system provided continuous and uninterrupted temperature measurements, encompassing the start of the stack, the heating phase, the subsequent stable operating point, and the cooling phase. However, some technical challenges are identified, as mechanical pressure exerted on the fiber influences the measured temperature. While this work demonstrates promising results, further advancements are necessary to address inhomogeneous loading within fuel cells and hotspot mitigation. The precise monitoring of temperature distribution enables early detection of potential damage, facilitating timely interventions to improve the service life and overall performance of fuel cells.

Evaluation of in-situ optical loss of polyimide-coated optical fiber under hydrothermal environments

When distributed fiber optic sensing (DFOS) devices are deployed in a geothermal borehole, the optical fiber as the key element, must be able to withstand the harsh environment because the temperature of the geothermal boreholes is higher than 250 °C in most cases. Understanding the deterioration behavior of optical fibers in hydrothermal conditions higher than 250 °C is important for predicting the lifetimes of DFOSs in operation.Geophysical equipment using optical fiber sensing was developed for in-situ measurement of the optical loss of optical fibers under hydrothermal conditions, and a survey method was established. Using this optical fiber sensing, we characterized the deterioration behavior of single-mode polyimide-coated optical fibers. Our experiments with carbon / polyimide-coated fibers under subcritical water from 250 to 350 °C revealed that the polyimide layer and silica cladding easily dissolved in water in that environment.

Distributed humidity fiber-optic sensor based on BOFDA using a simple machine learning approach.

We report, to our knowledge for the first time, on distributed relative humidity sensing in silica polyimide-coated optical fibers using Brillouin optical frequency domain analysis (BOFDA). Linear regression, which is a simple and well-interpretable algorithm in machine learning and statistics, is utilized. The algorithm is trained using as features the Brillouin frequency shifts and linewidths of the fiber's multipeak Brillouin spectrum. To assess and improve the effectiveness of the regression algorithm, we make use of machine learning concepts to estimate the model's uncertainties and select the features that contribute most to the model's performance. In addition to relative humidity, the model is also able to simultaneously provide distributed temperature information addressing the well-known cross-sensitivity effects.

Monitoring internal strains in asphalt pavements under static loads using embedded distributed optical fibers

This study explored the feasibility of using an embedded distributed optical-fiber sensors (DOFS) for providing a fully-distributed measurement in pavement structures, in particular that uses Rayleigh-scattering-based optical frequency-domain reflectometry (OFDR). Due to the novelty of this technology, and inherent heterogeneity, roughness and flexibility of pavements, the application of DOFS to asphalt pavements presents several uncertainties in terms of their durability, robustness and long-term stability. Therefore, a series of laboratory tests were performed to examine the performance of a packaged DOFS for measuring the responses under static loads for two types of pavement with different stiffnesses. The packaged DOFS uses a single-mode polyimide-coated optical fiber that is protected by a sheet-type sealing material and covered by a flexible smart mortar. The sensitivity of embedded DOFS was also assessed for three embedment depths (5, 10 and 20 mm). Although the raw data obtained from the DOFS contained a few outliers in some cases, it could obtain the full strain distribution with a high spatial resolution of 0.65 mm, and with a sensitivity similar to that of the traditional strain gauges. Moreover, the location of a load and the time-dependent strain response of the pavement could also be identified.

Distributed Fiberoptic Sensor for Simultaneous Temperature and Strain Monitoring Based on Brillouin Scattering Effect in Polyimide-Coated Fibers

A unique multiparameter sensor for distributed measurement of temperature and strain based on spontaneous Brillouin scattering in polyimide-coated optical fiber is proposed, which is an excellent candidate for the cross-sensitivity problem in conventional Brillouin sensing network. In the experimental section, the discrimination of strain and temperature is successfully demonstrated by analysing the unequal sensing coefficients of the Brillouin frequency shifts generated by different acoustic modes. The Brillouin frequency shifts of the main two peaks are successfully measured to discriminate the strain and temperature with an accuracy 19.68 με and 1.02°C in 2.5 km sensing range. The proposed distributed Brillouin optical fiber sensor allows simultaneous measurement of temperature and strain, thus opening a door for practical application such as oil explorations.

Distributed Fiberoptic Sensor for Simultaneous Humidity and Temperature Monitoring Based on Polyimide-Coated Optical Fibers.

Along temperature, humidity is one of the principal environmental factors that plays an important role in various application areas. Presented work investigates possibility of distributed fiberoptic humidity monitoring based on humidity-induced strain measurement in polyimide (PI)-coated optical fibers. Characterization of relative humidity (RH) and temperature response of four different commercial PI- and one acrylate-coated fiber was performed using optical backscattering reflectometry (OBR). The study addresses issues of temperature-humidity cross-sensitivity, fiber response stability, repeatability, and the influence of annealing. Acrylate-coated fiber exhibited rather unfavorable nonlinear RH response with strong temperature dependence, which makes it unsuitable for humidity sensing applications. On the other hand, humidity response of PI-coated fibers showed good linearity with fiber sensitivity slightly decreasing at rising temperatures. In the tested range, temperature sensitivity of the fibers remained humidity independent. Thermal annealing was shown to considerably improve and stabilize fiber RH response. Based on performed analysis, a 20 m sensor using the optimal PI-coated fibers was proposed and constructed. The sensor uses dual sensing fiber configuration for mutual decoupling and simultaneous measurement of temperature and RH variations. Using OBR, distributed dual temperature-RH monitoring with cm spatial resolution was demonstrated for the first time.

Effect of Below-Freezing Temperature on Optical Loss of Polyimide-Coated Optical Fibers

An increase in the optical loss in polyimide-coated optical fibers, caused by microbendings is detected at temperatures below -25 °C. It is shown that the observed increase in the optical loss is due to the presence of a residual N-methyl-2-pyrrolidone solvent in the polyimide coating.

Spatial and Temporal Resolution Optimization on Raman Distributed Temperature Sensor System for Quench Detection in a No-Insulated Coil

Quench detection and protection technologies are extremely important for high-temperature superconducting (HTS) magnet applications. In a previous work, we have verified the feasibility of using a Raman-based distributed temperature sensor (RmDTS) system to measure a no insulation (NI) HTS coil temperature changes in an overcurrent induced quench event. However, rough and slow temperature measurement is unsuitable for actual application on quench detection and protection. Therefore, we designed a new RmDTS system consisting of an RmDTS device whose temporal resolution is 15 ms and a thinner polyimide-coated optical fiber with a diameter of 150 μ m. Then, the optimized RmDTS system and an original one were, respectively, applied to detect the NI coil temperature during quenches. The comparative results show that the highest heat was generated on the 22nd turn of the NI coil, and the temperature response time of the original DTS system is 2 s slower than that of the optimized one. By using an optimization method, the spatial and temporal resolutions of the RmDTS system have been enhanced triple and 66 times than before.

Bend Limitation of a Polyimide-Coated Optical Fiber at Cryogenic Temperature of 77 K

A Raman-based distributed temperature sensor (RmDTS) system is a promising method for quench detection in high-temperature superconducting (HTS) magnets. To sense the temperature distribution of HTS magnets, optical fibers need to be wound with HTS wires synchronously. Accordingly, the bend limitation of the sensing optical fibers at 77 K is very important for an HTS coil temperature measurement. In this study, a segment with a 2.5-m-long polyimide-coated multimode optical fiber was bent as the curvature radii of 3, 5, 10, 15 and 25 mm, respectively, and applied to sense a room and a liquid nitrogen temperature. The experimental results of bending and without bending optical fiber show that when the curvature radius of the optical fiber is smaller than 10 mm, the temperature measurement is unreliable. The recommended minimal curvature radius of the used optical fiber for applying the RmDTS system to monitor the HTS coil temperature is 10 mm.

Strain pattern detection of composite cylinders using optical fibers after low velocity impacts

The strain pattern distributed on composite cylinders after impacts was detected using optical fibers for the first time. The sensing optical fiber was implemented on the composite cylinders using aluminum (Al)-coated optical fiber, polyimide-coated optical fiber, or standard single mode fiber (SSMF) of polymer-coating. The residual strain of this sensing fiber was measured by a Brillouin optical correlation domain analysis (BOCDA) sensor system, using phase modulation and single side band modulation methods. Impact events of 10, 20, and 40 J energies as barely visible impact damages (BVID) were applied on the cylinders, and these fibers were deployed on the cylinder surface, or were embedded in the cylinders. For the surface deployment, Al-coated fiber exhibited the highest residual strain due to Al plastic deformation, and the strain was three times higher than the lowest SSMF value. For the embedment deployment, these fibers were individually embedded in the cylinder, which was composed of sixteen carbon fiber reinforced polymer layers using a filament winding process. In contrast to the surface deployment, the embedded SSMF gave as high residual strain as that of the Al-coated fiber. These similar residual strains of the three embedded optical fibers suggested that they came from permanent material damage inside the material. Impact events with fiber embedment caused the optical signal in the Al-coated fiber to suffer serious additional insertion loss, but the SSMF did not show loss. So the general fiber for optics communications was successfully demonstrated as a distributed residual strain sensor to detect BVID impacts of composite materials. The strain values were maintained for at least 15 months after impact events. This residual strain sensor is economical, because special fibers like Al-coated fiber were not needed, and the sensor is efficient, because it could sense the strain for long distance, without additional impact-driven insertion loss.

Analysis of Polyimide-Coated Optical Fiber Long-Period Grating-Based Relative Humidity Sensor

A long-period grating (LPG)-based relative humidity (RH) sensor is developed, based on using a tailored layered polyimide coating on the grating region at the distal end of the fiber probe and allowing it to respond to humidity changes. The attenuation band of the coated LPG, on which the sensor is based, responds to the varying RH (in this case, over the region from 20% to 80% RH) through changes in the characteristic wavelength of the LPG. The experimental results confirm that the sensor created shows a linear response to the variation of RH levels. The sensitivity of the sensor is estimated to be 0.10 nm/%RH with negligible hysteresis (<; 1% RH). An advantage of the sensor design is that the probe is configured in reflection mode where the distal end of the fiber (containing the LPG) is coated with a silver mirror to monitor the wavelength shift at the same end of the fiber at which the light is launched (and to directly relate this to the RH change). The results obtained are compared with those from other probe designs (which often are in the less convenient transmission mode) and are seen to exhibit excellent overall performance and repeatability, showing the value of the probe for practical RH measurement, for example, in structural monitoring applications, a key area where effective monitoring of moisture and humidity change is important.

Characterization of polyimide-coated optical fibers

Optical fiber coatings play an important role in fiber strength, fatigue, and attenuation. Polyimide coatings on optical fiber are in growing demand because of their excellent high-temperature properties. Mechanical and optical characterizations of polymide-coated optical fibers were carried out at different temperatures. The strength was virtually unchanged at room temperature and at 300°C in air. Fatigue parameters (<i>n</i> values) were greater than 30 when tested in water at 80°C. Optical attenuation was unchanged when the fibers were tested at 340°C for 48 h. When the fibers were subjected to - 40°C to +70°C temperature cycling, the change in attenuation was less than 0.1 dB/km at 850 nm.