Fiber-optic sensor

A fiber-optic sensor measures a physical quantity through its effect on light guided in an optical fiber. Strain, temperature, pressure, rotation, vibration, magnetic field, refractive index and chemical concentration all change some property of the guided light: its phase, its wavelength, its intensity or its polarization. An interrogator at the end of the fiber reads that change. Because the sensing element is glass and the signal is optical, the sensors are immune to electromagnetic interference, need no electrical power at the measurement point, tolerate high temperatures and corrosive environments, and can be read from kilometers away.

Classification

Sensors are grouped in two ways. In an intrinsic sensor the fiber itself is the transducer, as in a grating or an interferometer coil; in an extrinsic sensor the fiber only carries light to a separate element, such as a Fabry-Perot cavity on the fiber tip or a fluorescent temperature probe. By geometry they are point sensors, measuring at one location; quasi-distributed sensors, with many discrete points along one fiber; and distributed sensors, which measure continuously along the fiber length.

The main families are:

• Interferometric sensors (Mach-Zehnder, Michelson, Fabry-Perot, Sagnac), which convert changes in optical path into phase. They are the most sensitive and include the fiber-optic gyroscope and fiber hydrophones.
• Grating sensors, chiefly FBG sensors, which encode the measurand in a reflected wavelength and are the dominant quasi-distributed technology.
• Distributed sensors based on backscattering: Rayleigh (DAS and OFDR-based strain sensing), Brillouin (strain and temperature) and Raman (temperature only).
• Intensity and polarimetric sensors, which are simple and inexpensive but more sensitive to source drift and bend loss. Current sensors based on the Faraday effect belong to the polarimetric group.

Sensitivity of a fiber interferometer

The phase accumulated along a length $L$ of fiber is $\varphi = 2\pi n L/\lambda$. Both $n$ and $L$ respond to temperature and strain. For temperature,

$$\frac{1}{L}\frac{d\varphi}{dT} = \frac{2\pi}{\lambda} \left(\frac{dn}{dT} + n\,\alpha\right)$$

With $dn/dT \approx 1 \times 10^{-5}$ K⁻¹ for silica, thermal expansion $\alpha = 0.55 \times 10^{-6}$ K⁻¹ and $n = 1.45$, this is about 44 rad per meter per kelvin at 1550 nm. For axial strain the elongation is partly offset by the strain-optic effect, leaving a factor of about 0.78, so the phase changes by about 4.6 rad per meter per microstrain. Interferometric demodulation resolves microradians, which is why a few meters of fiber can detect nanostrain and why these sensors must be isolated from temperature when strain is the target.

Distributed techniques in brief

Distributed sensors use time-of-flight, as in optical time-domain reflectometry, or frequency-domain ranging, as in OFDR, to assign each backscattered signal to a position. Raman distributed temperature sensing compares the anti-Stokes and Stokes backscatter, shifted by about 13 THz, whose ratio depends mainly on temperature, after correction for the different attenuation at the two wavelengths; typical systems resolve about 1 m over several kilometers. Brillouin systems measure the frequency shift of acoustic-phonon backscatter, about 10.8 GHz at 1550 nm in standard fiber, which moves by roughly 1 MHz/K and 0.05 MHz/µε. Rayleigh-based OFDR reaches millimeter resolution over tens of meters. DAS uses coherent Rayleigh backscatter to follow dynamic strain along tens of kilometers.

Where they are used

Structural health monitoring of bridges, dams, tunnels, wind-turbine blades and aircraft composites uses FBG arrays and distributed strain sensing. Oil and gas wells use Raman temperature profiling, DAS and fiber pressure gauges. Power utilities monitor cable and transformer temperatures with fiber probes that are safe in high-voltage fields. Navigation uses gyroscopes; medicine uses fiber pressure and temperature catheters and shape-sensing fibers; security uses fence and pipeline vibration sensing.

Pitfalls

• Cross-sensitivity. Almost every optical fiber sensor responds to both strain and temperature. A strain measurement without temperature compensation is only as good as the thermal stability of the installation.
• Strain transfer. The fiber measures its own strain. Coatings, adhesives and cable structures transfer only part of the host strain to the glass, so a sensor must be calibrated in its final mounting.
• Fading and polarization. Interferometric signals fade when the polarization states of the two arms become orthogonal, and coherent Rayleigh signals fade at random points; interrogators use polarization diversity and multiple frequencies to counter both.

Common questions

What does a fiber-optic sensor measure?

Any quantity that changes the optical path, loss or polarization of the fiber, or of an element attached to it. Strain and temperature are the most common; pressure, acceleration, rotation, acoustic waves, electric current, liquid level and chemical species are measured by converting them into strain, phase or absorption.

How far can fiber-optic sensors be from the interrogator?

Tens of kilometers is routine for distributed systems and for FBG arrays at 1550 nm, where the fiber loss is about 0.2 dB/km. The limit is the returned signal, which falls with the round-trip fiber attenuation, and for time-domain methods the pulse repetition rate, which falls as the fiber lengthens.

Are fiber-optic sensors better than electrical sensors?

They are preferred where electrical sensors struggle: strong electromagnetic fields, explosive atmospheres, long cable runs, high temperature, or many measurement points on one cable. For a single measurement in a benign environment an electrical strain gauge or thermocouple is usually simpler and cheaper.

References: A. H. Hartog, An Introduction to Distributed Optical Fibre Sensors (CRC Press, 2017); A. D. Kersey et al., J. Lightwave Technol. 15, 1442 (1997); B. Culshaw, J. Dakin (eds.), Optical Fiber Sensors, Vols. 1–2 (Artech House, 1988–1989).