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Fiber-optic Sensors

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Definition: optical sensors based on fiber devices

German: faseroptische Sensoren

Categories: fiber optics and waveguides, photonic devices

How to cite the article; suggest additional literature

Fiber-optic sensors (also called optical fiber sensors) are fiber-based devices for sensing some quantity, typically temperature or mechanical strain, but sometimes also displacements, vibrations, pressure, acceleration, rotations (measured with optical gyroscopes based on the Sagnac effect), or concentrations of chemical species. The general principle of such devices is that light from a laser (often a single-frequency fiber laser) or from a superluminescent source is sent through an optical fiber, experiences subtle changes of its parameters either in the fiber or in one or several fiber Bragg gratings, and then reaches a detector arrangement which measures these changes.

Compared with other types of sensors, fiber-optic sensors exhibit a number of advantages:

Bragg Grating Sensors

Fiber-optic sensors are often based on fiber Bragg gratings. The basic principle of many fiber-optic sensors is that the Bragg wavelength (i.e., the wavelength of maximum reflectivity) of a fiber Bragg grating depends not only on the Bragg grating period but also on temperature and mechanical strain. For silica fibers, the fractional response of the Bragg wavelength to strain is roughly 20% smaller than the strain itself, since the direct effect of strain is to some extent reduced by a decrease in refractive index. The temperature effect is close to that expected from thermal expansion alone. The effects of strain and temperature can be distinguished with various techniques (e.g. by using reference gratings which are not subject to the strain, or by combining different types of fiber gratings), so that both quantities are obtained at the same time. For pure strain sensing, the resolution can be the range of a few με (i.e., relative length changes of a few times 10−6), and the accuracy may not be much lower. For dynamic measurements (e.g. of acoustic phenomena), sensitivities better than 1 nε in a 1-Hz bandwidth are achievable.

Distributed Sensing

Other fiber-optic sensors do not use fiber Bragg gratings as sensors, but rather the fiber itself. The principle of sensing can then be based on Rayleigh scattering, Raman scattering or Brillouin scattering. For example, optical time domain reflectometry is a method where weak reflections can be localized using a pulsed probe signal. It is also possible, e.g., to exploit the temperature or strain dependence of the Brillouin frequency shift.

In some cases, the measured quantity is a kind of average over the full fiber length. This is the case for certain temperature sensors but also for Sagnac interferometers used as gyroscopes. In other cases, position-dependent quantities (e.g. temperatures or strains) are measured. This is called distributed sensing.

Quasi-distributed Sensing

A single fiber may contain many grating sensors (see above) in series to monitor the temperature and strain distribution along the whole fiber. This is called quasi-distributed sensing. There are different techniques to address the single gratings (and thus certain locations along the fiber):

Other Approaches

Apart from the approaches described above, there are many alternative techniques. Some examples are:

Applications

Even after a number of years of development, fiber-optic sensors have still not enjoyed great commercial success, since it is difficult to replace already well-established technologies, even if they exhibit certain limitations. For some application areas, however, fiber-optic sensors are increasingly recognized as a technology with very interesting possibilities. This holds particularly for harsh environments, such as sensing in high-voltage and high-power machinery, or in microwave ovens. Bragg grating sensors can also be used to monitor the conditions e.g. within the wings of airplanes, in wind turbines, bridges, large dams, oil wells and pipelines. Buildings with integrated fiber-optic sensors are sometimes called “smart structures”; they allow one to monitor the inside conditions and to gain important information on the strain to which different parts of the structure are subject, on aging phenomena, vibrations, etc. Smart structures are a main driver for the further development of fiber-optic sensors.

Bibliography

[1]D. Culverhouse et al., “Potential of stimulated Brillouin scattering as sensing mechanism for distributed temperature sensor”, Electron. Lett. 25, 913 (1989)
[2]A. D. Kersey, “A review on recent developments in fiber optic sensor technology”, Opt. Fiber Technol. 2, 291 (1996)
[3]A. D. Kersey et al., “Fiber grating sensors”, J. Lightwave Technol. 15 (8), 1442 (1997)
[4]B. Lee, “Review of the present status of optical fiber sensors”, Opt. Fiber Technol. 9 (2), 57 (2003)
[5]L. Zou et al., “Coherent probe-pump-based Brillouin sensor for centimeter-crack detection”, Opt. Lett. 30 (4), 370 (2005)
[6]F. M. Cox et al., “Opening up optical fibres”, Opt. Express 15 (19), 11843 (2007)
[7]O. Franzão et al., “Optical sensing with photonic crystal fibers”, Laser & Photon. Rev. 2 (6), 449 (2008)
[8]J. Albert et al., “Tilted Bragg grating sensors”, Laser & Photon. Rev. 7 (1), 83 (2013)
[9]P. Roriz et al., “Review of fiber-optic pressure sensors for biomedical and biomechanical applications”, J. Biomed. Opt. 18 (5), 050903 (2013)

(Suggest additional literature!)

See also: fibers, fiber Bragg gratings, single-frequency lasers, laser applications
and other articles in the categories fiber optics and waveguides, photonic devices

In the RP Photonics Buyer's Guide, 39 suppliers for fiber-optic sensors and related equipment are listed.

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