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

The authors report a new approach to obtain single-transverse-mode operation of a multimode fiber amplifier, in which the gain fiber is coiled to induce significant bend loss for all but the lowest-order mode. They have demonstrated this method by constructing a coiled amplifier using Yb-doped, double-clad fiber with a core diameter of 25 {micro}m and NA of {minus}0.1 (V {approx} 7.4). When operated as an ASE source, the output beam had an M{sup 2} value of 1.09 {+-} 0.09; when seeded at 1,064 nm, the slope efficiency was similar to that of an uncoiled amplifier. This technique does not require exotic fiber designs or increase system complexity and is inexpensive to implement. It will allow scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.

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Single-mode operation of a coiled multimode fiber amplifier

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.

A distributed optical fibre sensor is introduced which is capable of quantifying multiple dynamic strain perturbations along 1 km of a sensing fibre simultaneously using a standard telecommunication single-mode optical fibre The technique is based on measuring the phase between the Rayleigh scattered light from two sections of the fibre which define the gauge length The phase is spatially determined along the entire length of the fibre with a single pulse This allows multiple moving strain perturbation to be tracked and quantified along the entire length of the fibre The demonstrated setup has a spatial resolution of 2 m with a frequency range of 500-5000 Hz The minimum detectable strain perturbation of the sensor was measured to be 80 ne

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A distributed optical fibre dynamic strain sensor based on phase-OTDR

We demonstrate a low-loss, long-range, single-ended distributed optical fiber sensor to measure both temperature and strain simultaneously and unambiguously By using the Landau-Placzek ratio and cascaded Mach-Zehnder interferometric filters, we measure both the intensity and the frequency changes in the Brillouin backscattered signal Strain and temperature measurements can then be independently resolved A temperature resolution of 4°C, a strain resolution of 290 µepsilon, and a spatial resolution of 10m have been achieved for a sensing length of 15 km

All-fiber system for simultaneous interrogation of distributed strain and temperature sensing by spontaneous Brillouin scattering

A distributed acoustic wave detection system and method is provided. The system may include a fiber optic cable deployed in a well and configured to react to pressure changes resulting from a propagating acoustic wave and an optical source configured to launch interrogating pulses into the fiber optic cable. In addition, the system may include a receiver configured to detect coherent Rayleigh noise produced in response to the interrogating pulses. The CRN signal may be use to track the propagation of the acoustic wave in the well.

Distributed acoustic wave detection

A technique facilitates the monitoring of elongate structures. An elongate structure is combined with an optical fiber deployed along the structure. An interrogation system is operatively joined with the optical fiber to input and monitor optical signals to determine any changes in parameters related to the structure.

System and method for monitoring structures

A high power Q-switched erbium doped fibre laser is demonstrated using a novel, large mode area, single transverse mode fibre. Peak powers in excess of 4 kW, and pulsewidths of 10 ns are reported at a repetition rate of 500 Hz. These results represent the highest peak powers obtained from an actively Q-switched erbium doped fibre laser.

/pdf/q-switched-erbium-doped-fibre-laser-utilising-a-novel-large-44roejze65.pdf

Q-switched erbium doped fibre laser utilising a novel large mode area fibre

To perform distributed sensing with an optical fiber using Brillouin scattering, a light pulse is transmitted into the optical fiber, where the transmitted light pulse has a first frequency. Backscattered light and optical local oscillator light are combined, where the backscattered light is received from the optical fiber in response to the transmitted light pulse, and where the optical local oscillator light has a second frequency. A frequency offset is caused to be present between the first frequency of the transmitted light pulse and the second frequency of the optical local oscillator light, where the frequency offset is at least 1 GHz less than a Brillouin frequency shift of the backscattered light. Spectra representing Stokes and anti Stokes components of the backscattered light are acquired, where the Stokes and anti Stokes components are separated by a frequency span that is based on the frequency offset.

Gareth P. Lees

Papers

All-fiber system for simultaneous interrogation of distributed strain and temperature sensing by spontaneous Brillouin scattering

Distributed acoustic wave detection

System and method for monitoring structures

Q-switched erbium doped fibre laser utilising a novel large mode area fibre

Distributed sensing in an optical fiber using brillouin scattering