Rayleigh, Brillouin and Raman scatterings in fibers result from the interaction of photons with local material characteristic features like density, temperature and strain. For example an acoustic/mechanical wave generates a dynamic density variation; such a variation may be affected by local temperature, strain, vibration and birefringence. By detecting changes in the amplitude, frequency and phase of light scattered along a fiber, one can realize a distributed fiber sensor for measuring localized temperature, strain, vibration and birefringence over lengths ranging from meters to one hundred kilometers. Such a measurement can be made in the time domain or frequency domain to resolve location information. With coherent detection of the scattered light one can observe changes in birefringence and beat length for fibers and devices. The progress on state of the art technology for sensing performance, in terms of spatial resolution and limitations on sensing length is reviewed. These distributed sensors can be used for disaster prevention in the civil structural monitoring of pipelines, bridges, dams and railroads. A sensor with centimeter spatial resolution and high precision measurement of temperature, strain, vibration and birefringence can find applications in aerospace smart structures, material processing, and the characterization of optical materials and devices.

Recent progress in distributed fiber optic sensors.

The application of structural health monitoring (SHM) systems to civil engineering structures has been a developing studied and practiced topic, that has allowed for a better understanding of structures’ conditions and increasingly lead to a more cost-effective management of those infrastructures In this field, the use of fiber optic sensors has been studied, discussed and practiced with encouraging results The possibility of understanding and monitor the distributed behavior of extensive stretches of critical structures it’s an enormous advantage that distributed fiber optic sensing provides to SHM systems In the past decade, several R & D studies have been performed with the goal of improving the knowledge and developing new techniques associated with the application of distributed optical fiber sensors (DOFS) in order to widen the range of applications of these sensors and also to obtain more correct and reliable data This paper presents, after a brief introduction to the theoretical background of DOFS, the latest developments related with the improvement of these products by presenting a wide range of laboratory experiments as well as an extended review of their diverse applications in civil engineering structures

A review of distributed optical fiber sensors for civil engineering applications

Brillouin scattering in optical fiber describes the interaction of an electro-magnetic field (photon) with a characteristic density variation of the fiber. When the electric field amplitude of an optical beam (so-called pump wave), and another wave is introduced at the downshifted Brillouin frequency (namely Stokes wave), the beating between the pump and Stokes waves creates a modified density change via the electrostriction effect, resulting in so-called the stimulated Brillouin scattering. The density variation is associated with a mechanical acoustic wave; and it may be affected by local temperature, strain, and vibration which induce changes in the fiber effective refractive index and sound velocity. Through the measurement of the static or dynamic changes in Brillouin frequency along the fiber one can realize a distributed fiber sensor for local temperature, strain and vibration over tens or hundreds of kilometers. This paper reviews the progress on improving sensing performance parameters like spatial resolution, sensing length limitation and simultaneous temperature and strain measurement. These kinds of sensors can be used in civil structural monitoring of pipelines, bridges, dams, and railroads for disaster prevention. Analogous to the static Bragg grating, one can write a moving Brillouin grating in fibers, with the lifetime of the acoustic wave. The length of the Brillouin grating can be controlled by the writing pulses at any position in fibers. Such gratings can be used to measure changes in birefringence, which is an important parameter in fiber communications. Applications for this kind of sensor can be found in aerospace, material processing and fine structures.

Recent progress in Brillouin scattering based fiber sensors.

In-service structural health monitoring of composite aircraft structures plays a key role in the assessment of their performance and integrity. In recent years, Fibre Optic Sensors (FOS) have proved to be a potentially excellent technique for real-time in-situ monitoring of these structures due to their numerous advantages, such as immunity to electromagnetic interference, small size, light weight, durability, and high bandwidth, which allows a great number of sensors to operate in the same system, and the possibility to be integrated within the material. However, more effort is still needed to bring the technology to a fully mature readiness level. In this paper, recent research and applications in structural health monitoring of composite aircraft structures using FOS have been critically reviewed, considering both the multi-point and distributed sensing techniques.

/pdf/fibre-optic-sensors-for-structural-health-monitoring-of-izgdedczqo.pdf

Fibre Optic Sensors for Structural Health Monitoring of Aircraft Composite Structures: Recent Advances and Applications.

Over the past few decades, optical fibers have been widely deployed to implement various applications in high-speed long-distance telecommunication, optical imaging, ultrafast lasers, and optical sensors. Distributed optical fiber sensors characterized by spatially resolved measurements along a single continuous strand of optical fiber have undergone significant improvements in underlying technologies and application scenarios, representing the highest state of the art in optical sensing. This work is focused on a review of three types of distributed optical fiber sensors which are based on Rayleigh, Brillouin, and Raman scattering, and use various demodulation schemes, including optical time-domain reflectometry, optical frequency-domain reflectometry, and related schemes. Recent developments of various distributed optical fiber sensors to provide simultaneous measurements of multiple parameters are analyzed based on their sensing performance, revealing an inherent trade-off between performance parameters such as sensing range, spatial resolution, and sensing resolution. This review highlights the latest progress in distributed optical fiber sensors with an emphasis on energy applications such as energy infrastructure monitoring, power generation system monitoring, oil and gas pipeline monitoring, and geothermal process monitoring. This review aims to clarify challenges and limitations of distributed optical fiber sensors with the goal of providing a pathway to push the limits in distributed optical fiber sensing for practical applications.

Distributed optical fiber sensing: Review and perspective

A differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) for centimeter spatial resolution sensing using meter equivalent pulses is proposed. This scheme uses the time domain waveform subtraction at the same scanned Brillouin frequency obtained from pulse lights with different pulse-widths (e.g. 50ns and 49ns) to form the differential Brillouin gain spectrum (BGS) at each fiber location. The spatial resolution is defined by the average of the rise and fall time equivalent fiber length for a small stress section rather than the pulse-width difference equivalent length. The spatial resolution of 0.18m for the 50/49ns pulse pair and 0.15m for 20/19ns pulse pair over 1km sensing length with Brillouin frequency shift accuracy of 2.6MHz are demonstrated.

Differential pulse-width pair BOTDA for high spatial resolution sensing

A novel approach to realize long-range distributed temperature and strain measurement with high spatial resolution, as well as high temperature and strain resolution, is proposed based on optical frequency-domain reflectometry (OFDR). To maintain the high measurement resolution and accuracy while increasing the sensing length, an optimized nonlinearity compensation algorithm is implemented to ensure a large wavelength tuning range. The compensated OFDR trace exhibits improved sensing resolution at a short distance, and the spatial resolution gradually deteriorates at the far end due to accumulated phase noise induced by fast tuning of the laser wavelength. We demonstrated the spatial resolution of 0.3 mm over a single-mode fiber sensing length of over 300 m, and temperature and strain resolution of 0.7 $^{\circ}\hbox{C}$ and 2.3 $\mu\varepsilon$ with spatial resolution of up to 7 cm, respectively.

Long-Range High Spatial Resolution Distributed Temperature and Strain Sensing Based on Optical Frequency-Domain Reflectometry

The distributed vibration or dynamic strain information can be obtained using time-resolved optical frequency-domain reflectometry. Time-domain information is resolved by measuring Rayleigh backscatter spectrum in different wavelength ranges which fall in successive time sequence due to the linear wavelength sweep of the tunable laser source with a constant sweeping rate. The local Rayleigh backscatter spectrum shift of the vibrated state with respect to that of the non-vibrated state in time sequence can be used to determine dynamic strain information at a specific position along the fiber length. Standard single-mode fibers can be used as sensing head, while the measurable frequency range of 0-32 Hz with the spatial resolution of 10 cm can be achieved up to the total length of 17 m.

Distributed vibration sensing with time-resolved optical frequency-domain reflectometry

Coded optical probe pulses in return-to-zero/non-return-to-zero (RZ/NRZ) formats are used for long-range distance sensing based on a differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) in order to enhance the spatial resolution and measurement accuracy. It is found that using the RZ format maintains the Brillouin spectral shape, enhances the sensing range and leads to a higher signal-to-noise ratio compared to a single-pulse Brillouin optical time-domain analysis. With 512 bit RZ-coded pulse pairs of 60/55 ns for the DPP-BOTDA, a spatial resolution of approximately 0.5 m and a strain resolution of 12 microepsilon (which is equivalent to a 0.7 MHz Brillouin frequency shift) have been achieved over a 50 km large effective area fiber.

High-resolution DPP-BOTDA over 50 km LEAF using return-to-zero coded pulses.

A distributed optical fiber sensor with the capability of simultaneously measuring temperature and strain is proposed using a large effective area non-zero dispersion shifted fiber (LEAF) with sub-meter spatial resolution. The Brillouin frequency shift is measured using Brillouin optical time-domain analysis (BOTDA) with differential pulse-width pair technique, while the spectrum shift of the Rayleigh backscatter is measured using optical frequency-domain reflectometry (OFDR). These shifts are the functions of both temperature and strain, and can be used as two independent parameters for the discrimination of temperature and strain. A 92 m measurable range with the spatial resolution of 50 cm is demonstrated experimentally, and accuracies of ±1.2 °C in temperature and ±15 μe in strain could be achieved.

https://www.mdpi.com/1424-8220/13/2/1836/pdf

Wenhai Li

Papers

Differential pulse-width pair BOTDA for high spatial resolution sensing

Long-Range High Spatial Resolution Distributed Temperature and Strain Sensing Based on Optical Frequency-Domain Reflectometry

Distributed vibration sensing with time-resolved optical frequency-domain reflectometry

High-resolution DPP-BOTDA over 50 km LEAF using return-to-zero coded pulses.

Distributed temperature and strain discrimination with stimulated brillouin scattering and rayleigh backscatter in an optical fiber.