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 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:

• They consist of electrically insulating materials (no electric cables are required), which makes possible their use e.g. in high-voltage environments.
• They can be safely used in explosive environments, because there is no risk of electrical sparks, even in the case of defects.
• They are immune to electromagnetic interference (EMI), even to nearby lightning strikes, and do not themselves electrically disturb other devices.
• Their materials can be chemically passive, i.e., do not contaminate their surroundings and are not subject to corrosion.
• They have a very wide operating temperature range (much wider than is possible for many electronic devices).
• They have multiplexing capabilities: multiple sensors in a single fiber line can be interrogated with a single optical source (see below).

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):

• 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 reflectivity 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 Approaches

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.

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

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

Categories: fibers and other waveguides, photonic devices