Fiber-optic sensors (also called optical fiber sensors) are fiber-based optical sensors for 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.
One distinguishes intrinsic and extrinsic sensors. Intrinsic sensors are those where a fiber itself (possibly in a modified form, e.g. containing a Bragg grating) acts as the sensor. Extrinsic sensors use fibers only for transporting light to and from the actual sensor.
Many fiber-optic sensors are based on single fibers, but others are made with fiber bundles. For example, there are extrinsic sensors where some illumination light is sent to a sample through some of the fibers of a bundle, and reflected light or into used fluorescence light is sent back through other fibers.
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 (e.g. fused silica) 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
Many fiber-optic sensors are based on fiber Bragg gratings. The basic operation principle is often 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 subject to the same temperature but not to the strain, or by combining different types of fiber gratings), so that strain and temperature 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 be of the order. 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 small 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. That emission wavelength can then be measured, and notably it can hardly be influenced even during propagation in a rather long fiber – in contrast to amplitude-coded signals, which may be affected by attenuation.
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. A single fiber may then e.g. replace a large number of electronic temperature sensors.
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. Pairs of fiber Bragg gratings are used as fiber Fabry–Pérot interferometers, which can react particularly sensitively to external influences. (A Fabry–Pérot interferometer can also be made with other means, e.g. with a variable air gap in the fiber.) Such techniques can also be implemented with multiple fiber segments; the method (usable for dynamic sensing) is then called fiber segment interferometry . Using a sinusoidal optical frequency modulation, which can be easily achieved with current modulation of a single-frequency laser diode, in combination with suitable signal processing (e.g. using a field programmable gate array (FPGA)), one can separately measure the average strain in any particular fiber segment with high temporal resolution .
- 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 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.
- 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 substantial number of years of development, fiber-optic sensors have still not become very wide-spread, 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.
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