By Owais AliReviewed by Louis CastelJan 21 2026Monitoring large-scale civil and aerospace structures, including ageing bridges, composite aircraft, wind turbines, and space systems, poses significant challenges due to size, complexity, and harsh operating conditions. Conventional sensors are limited by drift, electromagnetic interference, and insufficient coverage for distributed measurements. Fiber optic sensors provide a transformative solution, offering high sensitivity, long-term stability, and the ability to perform remote, fully distributed monitoring of critical structural parameters.
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Core Fiber Optic Technologies Used in Structural Health Monitoring
Fiber Bragg Gratings
Fiber Bragg Grating (FBG) sensors are optical fiber devices in which a periodic modulation of the refractive index is inscribed along the doped silica core of a single-mode fiber. The fiber consists of a light-guiding core, a lower-index cladding for optical confinement, and a protective outer jacket. The Bragg grating reflects a narrow wavelength band, the Bragg wavelength, while other wavelengths pass through, with the reflected wavelength determined by the grating period and the effective refractive index of the core.
Any change in strain or temperature alters the grating period or effective refractive index, producing measurable shifts in the reflected wavelength through elastic deformation, the photoelastic effect, thermal expansion, and the thermo-optic response, enabling linear, repeatable, and precise sensing.
This wavelength-encoded capability allows FBG sensors to be deployed in demanding structural and aerospace environments.
In civil infrastructure, they are embedded or surface-bonded to bridges, tunnels, and pipelines to continuously monitor strain, temperature, and load-induced deformation. In aerospace structures, they are used to measure operational loads, vibrations, and thermal effects in aircraft wings and rotor blades, supporting structural health monitoring, predictive maintenance, and operational safety.1
Distributed Sensing
Distributed fiber optic sensing (DFOS) technologies use the optical fiber as a continuous sensor, providing spatially resolved measurements along its entire length rather than at discrete points.
These systems detect variations in naturally backscattered light caused by local changes in strain, temperature, or dynamic disturbances, relying on intrinsic scattering mechanisms such as Rayleigh, Brillouin, and Raman scattering, each sensitive to specific physical quantities.
Rayleigh-based sensing enables fully distributed measurement of static and dynamic strain and acoustic activity with millimeter-to-centimeter spatial resolution. Brillouin-based systems provide absolute strain and temperature measurements over kilometer-scale lengths via frequency shifts induced by acoustic phonons, making them suitable for large structures like bridges, pipelines, and tunnels. Raman-based sensing provides continuous temperature profiles by measuring intensity variations.
Collectively, these DFOS technologies enable real-time, continuous monitoring of strain, temperature, and acoustic events, supporting early damage detection, condition assessment, and long-term structural integrity management.2
Interferometric Sensors
Interferometric fiber-optic sensors detect changes in the optical phase difference between two coherent light waves caused by variations in a physical quantity, offering extremely high sensitivity with strain resolution of less than 1 microstrain.
Several interferometer configurations have been implemented, including Mach-Zehnder, Michelson, and Sagnac designs, as well as systems incorporating long-period gratings or photonic crystal fibers. Mach-Zehnder and Michelson interferometers are typically used to measure refractive index, temperature, and velocity, while Sagnac interferometers are primarily used for rotation sensing.
For structural health monitoring, Fabry-Perot and low-coherence interferometric sensors are most widely used for strain measurement. Fabry-Perot sensors achieve resolutions of ~0.15?microstrain over ranges of several thousand microstrain, remain compact for structural integration, and tolerate elevated temperatures.
Low-coherence sensors, such as SOFO systems, use long gauge lengths to provide highly stable, temperature-insensitive measurements and have been deployed in large infrastructure.
However, restricted multiplexing and low-frequency response limit their applicability for dynamic strain and impact monitoring.2
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Applications in Civil Engineering
Long-Term Monitoring of Bridges
Long-term monitoring of concrete bridges is critical for maintaining structural integrity, safety, and service life. FBG sensors are particularly suited for this purpose, as their optical fiber construction resists moisture, temperature variations, and chemical exposure, providing stable, high-resolution measurements over extended periods.
In addition, multiplexing allows multiple sensing points along a single fiber, reducing cabling and installation complexity, while embedding sensors in critical tension zones of beams or bridge decks captures localized micro-strain variations, enabling early damage detection and preventive maintenance.1
A representative example is the Colle d’Isarco viaduct on the Italian Brenner Highway A22, where a 378?m bridge section with asymmetrical cantilevers was instrumented with FBG and conventional sensors. The distributed data acquisition system with wireless communication collects measurements from inaccessible locations, and analytical and nonlinear models interpret the data within a decision-support framework for long-term bridge management.3
Pipeline and Infrastructure Monitoring
A distributed fiber-optic monitoring system was deployed along a 500?m section of a 35-year-old buried gas pipeline in a landslide-prone area near Rimini, Italy.
Conventional vibrating wire gauges provided only localized strain measurements, so SMARTape fiber strain sensors and a temperature-sensing cable were installed in three parallel lines around the pipeline at 0°, 120°, and −120°, covering the entire length in segments of 71-132?m. The system measured average strain, curvature, and deformation with 20?µε strain resolution and 1.5?m spatial resolution, while temperature sensors enabled thermal compensation.
Over two years, the system provided continuous insight into burial- and landslide-induced deformation, was validated against vibrating wires, and detected changes during a simulated gas leak.4
Smart Concrete and Embedded Fiber Systems
Smart concrete incorporates embedded sensing capabilities to continuously monitor structural health, mitigating deterioration from environmental exposure, sustained loads, and chemical reactions. With global cement production exceeding 4?billion metric tons annually, even marginal improvements in monitoring can yield substantial economic and environmental benefits.5
Distributed optical fiber sensing technologies provide spatially continuous measurements of strain, cracking, and delamination, enabling early detection of damage.
A study using pulse-pre-pump Brillouin optical time-domain analysis tracked shrinkage and delamination in ultra-high-performance concrete overlays, detecting interfacial damage early. These measurements enable predictive maintenance, supporting data-driven lifecycle management and enhanced durability.6
Applications in Aerospace
Strain Monitoring in Wing Structures
Aerospace manufacturers are increasingly adopting composite materials for primary load-bearing structures such as aircraft wings and fuselages due to their high strength-to-weight ratio and fatigue resistance, necessitating continuous structural health monitoring to ensure safety and performance under operational loads.
FBG sensors have been widely deployed for this purpose, offering high-resolution, multiplexed measurements of dynamic strains in both laboratory and flight conditions, with placement guided by finite element analysis to cover high-strain regions while minimizing spectral distortion.
A study demonstrated a parallel processing interrogator capable of high-rate multiplexing of FBG sensors to capture both loading and low-frequency vibrations. A T38 quarter-scale wing model was instrumented with sensors beneath the top composite layer at FE-identified high-strain zones with low spatial gradients, and static and dynamic tests at 6 kHz confirmed precise and reliable strain measurements, validating the system for high-fidelity monitoring of composite wing structures.2
Real-Time Structural and Environmental Monitoring of Satellites
Space missions demand sensing systems that are robust, lightweight, and capable of operating under extreme environmental conditions, including wide temperature ranges, low pressure, and high radiation.
Traditional sensors are often limited by electromagnetic interference and harsh conditions, whereas FBG sensors offer high-resolution, multiplexed measurements that are immune to such disturbances, making them well-suited for aerospace applications.
Icarus, a prototype satellite developed by the student team at Eindhoven University of Technology, was equipped with FBG sensors connected to a PhotonFirst interrogator. These sensors simultaneously monitored temperature, pressure, radiation, and mechanical stress during a high-altitude balloon flight to 34.8 km.
The successful operation under near-space conditions demonstrates the reliability and versatility of FBG sensors for real-time monitoring in extreme environments, highlighting their potential for future aerospace missions.7
Automatic In-Flight Structural Monitoring of UAVs
A study implemented an in-flight structural health and usage monitoring system on a UAV by embedding twenty FBG sensors into the composite front spar of the wing, coupled with a miniaturized optical interrogation unit and wireless data transmission.
Sixteen flight tests, including both pristine and artificially damaged configurations, were conducted to acquire real-time strain data, which were analyzed using strain field pattern recognition combining self-organizing maps for clustering operational conditions and principal component analysis with damage indices for classification.
The system achieved a maximum damage detection accuracy of 0.981 and an F1 score of 0.978, demonstrating that the FBG-based monitoring system can reliably provide automatic in-flight monitoring, remote damage detection, and assessment of operational load variations in UAV composite structures.8
Advantages, Challenges, and Future Directions
Photonic sensors for structural health monitoring offer distinct advantages over conventional technologies, including multi-parameter sensing of strain, temperature, vibration, and acoustic emissions along a single fiber, high-resolution wavelength-encoded measurements with long-term stability, and the ability to interrogate distributed structures over long distances without local power.
Their small size, flexibility, and rugged dielectric construction enable embedded integration into composite materials and operation in extreme environments, from deep-sea and high-temperature industrial settings to aerospace applications, while providing immunity to electromagnetic interference, chemical exposure, and mechanical stress.
However, challenges such as complex installation, fragility, long-term durability in harsh conditions, potential interface failures, lack of standardized interrogation hardware, and higher upfront costs limit broader adoption.
Ongoing developments focus on chip-scale interrogators, AI-enhanced data analysis, hybrid sensor networks, integration with digital twins for predictive maintenance, and advanced fiber materials and miniaturized sensors to improve performance, reliability, and application scope.
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References and Further Reading
- Alhussein, A. N., Qaid, M. R., Agliullin, T., Valeev, B., Morozov, O., & Sakhabutdinov, A. (2024). Fiber Bragg Grating Sensors: Design, Applications, and Comparison with Other Sensing Technologies. Sensors, 25(7), 2289. https://doi.org/10.3390/s25072289
- Di Sante, R. (2015). Fibre Optic Sensors for Structural Health Monitoring of Aircraft Composite Structures: Recent Advances and Applications. Sensors, 15(8), 18666-18713. https://doi.org/10.3390/s150818666
- A. Beltempo, Cappello, C., D. Zonta, Bonelli, A., Bursi, O. S., Costa, C., & W. Pardatscher. (2015). Structural health monitoring of the Colle Isarco Viaduct. 28, 7–11. https://doi.org/10.1109/eesms.2015.7175843
- Bado, M. F., & Casas, J. R. (2020). A Review of Recent Distributed Optical Fiber Sensors Applications for Civil Engineering Structural Health Monitoring. Sensors, 21(5), 1818. https://doi.org/10.3390/s21051818
- Ritchie, H., & Rosado, P. (2025). Global cement production has plateaued over the last decade. https://ourworldindata.org/data-insights/global-cement-production-has-plateaued-over-the-last-decade
- Qiao, H., Lin, Z., Sun, X., Li, W., Zhao, Y., & Guo, C. (2022). Fiber Optic-Based Durability Monitoring in Smart Concrete: A State-of-Art Review. Sensors, 23(18), 7810. https://doi.org/10.3390/s23187810
- Jacco Overdulve. (2025). Aster's Space Mission with PhotonFirst. https://www.photonfirst.com/blog/aster-space-mission-with-photonfirst
- Alvarez-Montoya, J., Carvajal-Castrillón, A., & Sierra-Pérez, J. (2020). In-flight and wireless damage detection in a UAV composite wing using fiber optic sensors and strain field pattern recognition. Mechanical Systems and Signal Processing, 136, 106526. https://doi.org/10.1016/j.ymssp.2019.106526
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