Fiber-optic 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 reflectance) of a fiber Bragg grating depends not only on the Bragg grating period but also on temperature and mechanical strain.

For optical strain sensors based on 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.

There are also Bragg grating laser sensors, where fiber lasers are realized, consisting of two gratings and a rare-earth doped fiber in between. Alternatively, there can be one FBG and a broadband reflector on the other side. When supplied with pump light, such a device produces an output with a wavelength close to the Bragg wavelength.

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):

• In one technique, called wavelength division multiplexing (WDM) or optical frequency-domain reflectometry (OFDR), the gratings have slightly different Bragg wavelengths. A wavelength-tunable laser in the interrogator unit can be tuned to the wavelength belonging to a particular grating, and the wavelength of maximum reflectance indicates the influences of strain or temperature, for example. Alternatively, a broadband light source (e.g. a superluminescent source) may be used together with a wavelength-swept photodetector (e.g. based on a fiber Fabry–Pérot) or a CCD-based spectrometer. In any case, the maximum number of gratings is typically between 10 and 50, limited by the tuning range or bandwidth of the light source and the required wavelength interval per fiber grating.
• Another technique, called time division multiplexing (TDM), uses identical weakly reflecting gratings, interrogated with short light pulses. The reflections from different gratings are then distinguished via their arrival times. Time division multiplexing is often combined with wavelength division multiplexing in order to multiply the number of different channels to hundreds or even thousands.
• An optical switch allows one to select between different fiber lines, further multiplying the possible number of sensors.

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.

See the articles on optical temperature sensors and optical strain sensors for more details.

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

• Fiber Bragg gratings may be used in interferometric fiber sensors, where they merely serve as reflectors, and the measured phase shift results from fiber spans between them.
• There are Bragg grating laser sensors, where a sensor grating forms the end mirror of a fiber laser resonator, containing e.g. some erbium-doped fiber, which receives some 980-nm pump light via the fiber line. The Bragg wavelength, which depends on e.g. temperature or strain, determines the lasing wavelength. This approach, which has many further variations, promises very high resolution due to the small linewidth of such a fiber laser, and very high sensitivity.
• In some cases, pairs of Bragg gratings are used as fiber Fabry–Pérot interferometers, which can react particularly sensitively to external influences. The Fabry–Pérot interferometer can also be made with other means, e.g. with a variable air gap in the fiber.
• Long-period fiber gratings are particularly interesting for multi-parameter sensing (e.g. of temperature and strain), and alternatively for strain sensing with very low sensitivity to temperature changes.

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.

Your question or comment:

Spam check:

  (Please enter the sum of thirteen and three in the form of digits!)

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.

[1]	D. Culverhouse et al., “Potential of stimulated Brillouin scattering as sensing mechanism for distributed temperature sensor”, Electron. Lett. 25, 913 (1989), doi:10.1049/el:19890612
[2]	A. D. Kersey, “A review on recent developments in fiber optic sensor technology”, Opt. Fiber Technol. 2, 291 (1996), doi:10.1006/ofte.1996.0036
[3]	A. D. Kersey et al., “Fiber grating sensors”, IEEE J. Lightwave Technol. 15 (8), 1442 (1997), doi:10.1109/50.618377
[4]	B. Lee, “Review of the present status of optical fiber sensors”, Opt. Fiber Technol. 9 (2), 57 (2003), doi:10.1016/S1068-5200(02)00527-8
[5]	L. Zou et al., “Coherent probe-pump-based Brillouin sensor for centimeter-crack detection”, Opt. Lett. 30 (4), 370 (2005), doi:10.1364/OL.30.000370
[6]	F. M. Cox et al., “Opening up optical fibres”, Opt. Express 15 (19), 11843 (2007), doi:10.1364/OE.15.011843
[7]	O. Franzão et al., “Optical sensing with photonic crystal fibers”, Laser & Photon. Rev. 2 (6), 449 (2008), doi:10.1002/lpor.200810034
[8]	J. Albert et al., “Tilted Bragg grating sensors”, Laser & Photon. Rev. 7 (1), 83 (2013), doi:10.1002/lpor.201100039
[9]	P. Roriz et al., “Review of fiber-optic pressure sensors for biomedical and biomechanical applications”, J. Biomed. Opt. 18 (5), 050903 (2013), doi:10.1117/1.JBO.18.5.050903
[10]	L. Mescia and F. Prudenzano, “Advances on optical fiber sensors”, Fibers 2 (1), 1 (2014), doi:10.3390/fib2010001

(Suggest additional literature!)