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.

http://ieeexplore.ieee.org/iel5/3/23015/01070639.pdf

Laser handbook

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.

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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

Measurement of Brillouin Gain Spectrum Distribution along an Optical Fiber Using a Correlation-Based Technique - Proposal, Experiment and Simulation -

Brillouin scattering in optical fibers offers a means for distributed strain and/or temperature sensor [1], [2]. Strain or temperature applied to an optical fiber causes a shift in Brillouin gain spectrum. Telecommunicationgrade fibers can be used as sensing medium, and no special process is required to make sensors [1], [2]. Because of these merits, this technique is attractive for application to smart structures [3], [4]. Optical fibers are attached to or embedded in structures for monitoring them. However, the technique developed so far has a spatial resolution limit of the order of several meters. This limit would prohibit application to fine structures such as aircraft’s wing. The resolution limit of the conventional technique is relevant to its pulse-based nature. An optical pulse for generating Brillouin gain has to be longer than the damping time of the acoustic wave. Otherwise, Brillouin gain spectrum (BGS) broadens out making it difficult to determine the spectral center precisely [5]. If we assume the linewidth of the BGS to be 30MHz, the broadening occurs when the pulse width is 3.0 m∗. To circumvent the resolution limit, we have proposed and developed a new technique [6]. It is based on the correlation between the pump and probe lightwaves that excite Stimulated Brillouin Scattering (SBS). SBS requires interference between the counterpropagating two lightwaves. In our technique we control their coherence so as to localize SBS at a specific position in

Measurement of Brillouin Gain Spectrum Distribution along an Optical Fiber Using a Correlation-Based Technique : Proposal, Experiment and Simulation (Special Issue on Optical Fiber Sensors)

The invention makes it possible to monitor an optical fibre circuit of a PON system with a practical construction. An optical fibre circuit monitoring system includes: a branched optical fibre circuit constituting a PON system and a monitoring device coupled on the OLT side of the branched circuit. The monitoring device includes a light source, beam splitter, detection unit and control unit. The light source outputs light having a comb-tooth shaped light-wave coherence function in which the optical frequency is modulated by a modulating signal of period p. The beam splitter splits the beam that is output from the light source into a monitoring beam and a reference beam. The detection unit detects the interference light produced by mutual interference of the reference beam and the reflected beam generated whilst the monitoring beam is propagated along the branched optical fibre circuit and converts this to an electrical signal. The control unit converts the period p and, based on this period p and the electrical signal that is output from the detection unit, finds the distribution of the coefficient of reflection along the direction of propagation of the monitoring beam in the branched optical fibre circuit.

Optical fibre circuit monitoring system and monitoring device included in this system

PROBLEM TO BE SOLVED: To provide a method and device capable of measuring the Brillouin gain spectrum by the space resolving power of 1 m or less. SOLUTION: A pump light and a probe light emitted from a light source 1 and a light source 2 respectively, and subjected to frequency modulation by a prescribed modulation frequency are incident on both ends of an optical fiber 6 to be measured, when the frequency difference between the central frequency of the pump light and that of the probe light is changed so that it becomes the vicinity of the brillan-frequency shift, the power movement from the pump light to the probe light is selectively caused in the position where the phases of both lights are synchronized. After emitted from the optical fiber 6 to be measured, the power of the probe light branched by an optical turnout 4 is detected by an optical detector 5, so that the Brillouin gain spectrum in the position where the phases of both lights are synchronized can be measured.

Brillouin gain spectrum measuring method and device

Provided is an optical fiber having holes extending along the axis whose transmission loss is substantially reduced and the manufacturing method thereof. First, a plurality of through-holes 9 are formed in a preform 5 extending along the preform axis. Subsequently, the preform 5 is heated by heating means 24 in the furnace preferably for 30 minutes or more at a temperature equal to or more than 800° C. while flowing a dry gas in the through-holes 9. As a result, the OH group which exists on the surfaces of the inner walls 5 a of the through-holes 9 of the preform 5 is discharged outside the preform. Subsequently, the preform 5 is drawn into an optical fiber.

Takemi Hasegawa

Papers

Measurement of Brillouin Gain Spectrum Distribution along an Optical Fiber Using a Correlation-Based Technique - Proposal, Experiment and Simulation -

Measurement of Brillouin Gain Spectrum Distribution along an Optical Fiber Using a Correlation-Based Technique : Proposal, Experiment and Simulation (Special Issue on Optical Fiber Sensors)

Optical fibre circuit monitoring system and monitoring device included in this system

Brillouin gain spectrum measuring method and device

Optical fiber and optical fiber transmission line, and manufacturing method therefor