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

Publisher Summary Polarization mode dispersion (PMD) is a linear effect that can be compensated in principle. In an ideal circularly symmetric fiber, the two orthogonally polarized modes have the same group delay. However, in reality, fibers exhibit a certain amount of birefringence because of imperfections in the manufacturing process or mechanical stress on the fiber after manufacture. It is noted that fluctuations in the polarization mode and fiber birefringence produced by the environment lead to dispersion that varies statistically with time and frequency. PMD causes different delays for different polarizations and when the difference in the delays approaches a significant fraction of the bit period, it leads to pulse distortion and system penalties. Environmental changes— including temperature and stress—cause the fiber PMD to vary stochastically in time. PMD, illustrating the basic concepts, the measurement techniques, the PMD measurement, the PMD statistics for first- and higher orders, the PMD simulation and emulation, the system impairments, and the mitigation methods has been summarized in the chapter. Both the optical and the electrical PMD compensations are considered.

Polarization-Mode Dispersion

Distributed optical fibre sensing, employing either Rayleigh, Raman, or Brillouin scattering, is the only physical-contact sensor technology capable of accurately estimating physical fields with spatial continuity along the fibre. This unique feature and the other features of standard optical fibre sensors (e.g., minimal invasiveness and lightweight, remote powering/interrogating capabilities) have for many years promoted the technology to be a promising candidate for geo-hydrological monitoring. Relentless research efforts are being undertaken to bring the technology to complete maturity through laboratory, physical models, and in-situ tests. The application of distributed optical fibre sensors to geo-hydrological monitoring is here reviewed and discussed, along with basic principles and main acquisition techniques. Among the many existing geo-hydrological processes, the emphasis is placed on those related to soil levees, slopes/landslide, and ground subsidence that constitute a significant percentage of current geohazards.

A Review of Distributed Fibre Optic Sensors for Geo-Hydrological Applications

In recent years, railway infrastructures and systems have played a significant role as a highly efficient transportation mode to meet the growing demand in transporting both cargo and passengers. Application of these structures in extreme environmental situation under severe working and loading conditions, caused by the traffic growth, heavier axles and vehicles and increase in speed makes it extremely susceptible to degradation and failure. In the last two decades, a significant number of innovative sensing technologies based on fiber optic sensors (FOS) have been utilized for structural health monitoring (SHM) due to their inherent distinctive advantages, such as small size, light weight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability. Fiber optic-based monitoring systems use quasi-distributed and continuously distributed sensing techniques for real time measurement and long term assessment of structural properties. This allows for early stage damage detection and characterization, leading to timely remediation and prevention of catastrophic failures. In this scenario, FOS have been proved to be a powerful tool for meticulous assessment of railway systems including train and track behavior by enabling real-time data collection, inspection and detection of structural degradation. This article reviews the current state-of-the-art of fiber optic sensing/monitoring technologies, including the basic principles of various optical fiber sensors, novel sensing and computational methodologies, and practical applications for railway infrastructure monitoring. Additionally, application of these technologies to monitor temperature, stresses, displacements, strain measurements, train speed, mass and location, axle counting, wheel imperfections, rail settlements, wear and tear and health assessment of railway bridges and tunnels will be thoroughly discussed.

A review of railway infrastructure monitoring using fiber optic sensors

In this letter, a coded, Raman-based distributed temperature sensor system using 255-bit Simplex coded optical time domain reflectometry (OTDR) and optimized sensing link composed of cascaded fibers with different Raman coefficients, is proposed. This system is compared to a system with an uncoded OTDR on standard single-mode fiber, demonstrating significant enhancement in the interrogation distance (19.5 km from coding gain, and 9.6 km from link optimization). Total sensing range of 37 km at 17-m/3-K spatial/temperature resolution were achieved, while employing conventional low power (80 mW) laser diodes

Raman-based distributed temperature sensor with simplex coding and link optimization

This paper demonstrates signal-to-noise ratio (SNR) improvement of an optical time-domain reflectometer (OTDR) using simplex codes (scs) for the first time. By developing a generalized procedure for the analysis of noise transfer in the coded OTDR employing L-bit scs, the SNR dependence on the receiver bandwidth is also investigated. Experimental results obtained from a constructed sc-OTDR showed excellent agreement with theoretical results over entire code lengths. Improvement of the SNR up to 9.2 dB has been demonstrated using 255-bit scs.

Optimization of SNR improvement in the noncoherent OTDR based on simplex codes

We have analyzed the effectiveness of two different polarization-mode dispersion (PMD) compensators in high-speed transmission systems suffering from polarization-dependent loss (PDL). When the link PDL value increases, the system performance improvement from the compensators decreases. Numerical analysis of a 10-Gb/s transmission system shows that even a small average link PDL of 0.5 dB could seriously limit the effectiveness of the PMD compensations.

Limitation of PMD compensation due to polarization-dependent loss in high-speed optical transmission links

We examine the performance of optical 8-ary differential phase-shift keying transmission systems according to the type of receiver structure and modulation format. Compared with the approach based on a multilevel decision, we found that a bilevel receiver provides 3-dB gain in optical signal-to-noise ratio sensitivity and is more robust against chromatic dispersion for either nonreturn-to-zero or return-to-zero modulation.

Performance comparison of optical 8-ary differential phase-shift keying systems with different electrical decision schemes

We propose and experimentally demonstrate a novel technique which significantly improves the performance of optical time-domain reflectometry (OTDR). Applying biorthogonal codes and Moore-Penrose generalized inverses, a signal-to-noise ratio enhancement of up to 9.04 dB, namely a coding gain, was achieved over conventional OTDR.

/pdf/snr-enhancement-of-otdr-using-biorthogonal-codes-and-3iu1z2bcsk.pdf

SNR enhancement of OTDR using biorthogonal codes and generalized inverses

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