RP Photonics logo
VL logo part of the
Virtual
Library

Encyclopedia of Laser Physics and Technology

Fiber-optic Sensors

previous  |  next  |  feedback

You can buy fiber-optic sensors and related equpiment from:


Ask RP Photonics for advice concerning principles and technical details of fiber-optic sensors.

Definition: optical sensors based on fiber devices

Fiber-optic sensors (also called optical fiber sensors) are fiber-based devices for sensing some quantity, typically temperature or mechanical strain, but sometimes also vibrations, pressure, acceleration, 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 Raman scattering or Brillouin scattering. It is 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, oil pipelines, or along electrical power lines. 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", Optical. 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]F. M. Cox et al., "Opening up optical fibres", Opt. Express 15 (19), 11843 (2007)

See also: fibers, fiber Bragg gratings, single-frequency lasers, laser applications

Categories: fibers and other waveguides, photonic devices

arrow
Home New articles Spotlight Feedback Advertising
Categories Search Quiz Links Pagehits
F

This encyclopedia is provided by
RP Photonics Consulting GmbH.

Utilize the expertise of the author, Dr. Rüdiger Paschotta, also in the form of technical consulting services!

TRUMPF logo

TRUMPF-Laser

a leading supplier of industrial diode pumped solid state lasers,
CO2 lasers, and laser systems for material processing.

Onefive logo

Onefive

Low-noise
femtosecond,
picosecond,
and tunable single-frequency lasers for OEM and R&D applications.

A.L.S. logo

A.L.S. GmbH

Picosecond laser diodes
<30 ps, 375 - 1600 nm, >1 Wp, single shot - 120 MHz

Field Guide to Lasers

This new book by Dr. Paschotta explains principles and types of lasers.

RP Fiber Power

This software helps to design and analyze fiber amplifiers and lasers.

RP Q-switch

A powerful software tool for designing
Q-switched lasers. See the details.

Your Advertisement at This Place

will be seen by many thousands of visitors per month. These banners receive far over 100'000 page hits per month. Check the details.