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.

This work presents an overview of progress and developments in the field of fiber optic sensor technology, highlighting the major issues underpinning recent research and illustrating a number of important applications and key areas of effective fiber optic sensor development.

http://snake.persiangig.com/document/sensor1.pdf

Fiber optic sensor technology: an overview

In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

High Power Fiber Lasers: A Review

This paper reviews the developments of a distributed strain and temperature sensing technique that uses Brillouin scattering in single-mode optical fibers. This technique is based on strain- and temperature-induced changes in the Brillouin frequency shift. Several approaches for measuring the weak Brillouin line are compared. >

Development of a distributed sensing technique using Brillouin scattering

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

It is demonstrated that stimulated Brillouin scattering (SBS) occurring in the core of an optical fibre may be used in the construction of a distributed temperature sensor. Initial experiments with a length of fibre have shown that the sensitivity of the Stokes frequency to temperature is approximately 5.5 MHz/ degrees C and that different Stokes frequencies may be observed according to the temperature distribution along the fibre.<
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Potential of stimulated Brillouin scattering as sensing mechanism for distributed temperature sensors

Observation of intermodal forward stimulated Brillouin scattering at 514.5 nm in dual-mode optical fibres is reported. The FSBS signal is shifted by 16.6 MHz and, unlike in normal background SBS, does not depend significantly on the laser linewidth. The Brillouin gain in backward and forward directions is compared.

Experimental observation of forward stimulated Brillouin scattering in dual-mode single-core fibre

Recently, stimulated Brillouin scattering in a forward direction (FSBS) in a dual-mode single-core optical fiber was reported by the authors (see Electron Lett., vol.26 p.1195-6, 1990). Frequency shifts on the order of 17 MHz were seen in fiber supporting LP/sub 01/ and LP/sub 11/ modes at 514.5 nm. The phenomenon is examined in detail, and the governing differential equations of the three-wave parametric process, (involving laser pump, Brillouin signal, and acoustic flexural-wave phonon) are derived and solved. FSBS is possible because although the overlap integral between a flexural fiber mode and the light is small, the phonon lifetime is much longer than in conventional stimulated Brillouin scattering (SBS). FSBS may also be the first example of a nonlinear effect which is enhanced by increasing the optical model area at constant pump power. >

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Theory of forward stimulated Brillouin scattering in dual-mode single-core fibers

Experimental data are presented which show for the first time that the back-scattered signals independently generated by stimulated Brillouin scattering in two separate optical fibres may be mixed to produce narrowband radio-frequency signals. This mechanism can be exploited for microwave generation and distributed temperature sensing.

Stimulated Brillouin scattering: a means to realise tunable microwave generator or distributed temperature sensor

A novel tunable microwave signal generation at a frequency of ∼ 12GHz based on mixing the output beam from an SBS ring resonator laser with the original pump source has been demonstrated. The gain profile of the SBS laser was measured to be 15 ± 1 MHz by increasing the input power of the pump such that the laser oscillated in several modes.

D. Culverhouse

Papers

Potential of stimulated Brillouin scattering as sensing mechanism for distributed temperature sensors

Experimental observation of forward stimulated Brillouin scattering in dual-mode single-core fibre

Theory of forward stimulated Brillouin scattering in dual-mode single-core fibers

Stimulated Brillouin scattering: a means to realise tunable microwave generator or distributed temperature sensor

Stimulated Brillouin scattering ring resonator laser for SBS gain studies and microwave generation