Fiber optic sensor

from Wikipedia, the free encyclopedia

A fiber optic sensor (FOS) is a special measuring transducer (sensor) for optical measuring methods based on optical fibers (LWL). With fiber optic sensors, the measured variable is not represented or transmitted by an electrical variable, but by an optical one.

Fiber-optic cables are used in telecommunications and communication technology for data transmission, or in automation technology, and, in addition to the high bandwidth, are primarily characterized by the fact that they enable transmission that is not susceptible to external influences, for example to electromagnetic fields . Despite these generally good and interference-free transmission properties, the optical signals in optical waveguides can also be attenuated or changed by internal and external influences, cf. Article optical fiber . Among other things, they react strongly to geometrical changes in the waveguide due to bending, tension, pressure or torsion and generally to changes in the light-guiding properties, such as damage to the cladding material. This is where fiber optic sensors come into play. With them, influencing the light signals through external parameters is explicitly desired.

The change in various parameters of the light used is evaluated, including above all the intensity , wavelength (color) and polarization as well as the transit time of the signals. The easiest way to determine the change in intensity and thus the transmission properties is to measure the attenuation. The other parameters usually require a somewhat more complex measurement setup, so a spectrometer is required to record the change in wavelength and corresponding polarizers and modulators are necessary to record the polarization .

There are two classes of fiber optic sensors:

  1. intrinsic : Here the glass fiber serves directly as a measuring sensor and is therefore both a sensor and a cable. Examples are:
    • fiber-optic pressure sensors , in which bending losses induced by pressure lead to changes in transmission in the glass fiber.
    • Fiber optic temperature measurement for spatially resolved temperature measurement through temperature-dependent Raman scattering in the glass fiber.
    • Measurement of strain or temperature through a fiber Bragg grating integrated into the glass fiber (determination of the reflected wavelength by a spectroscope)
    • Fiber gyro for measuring the angular velocity by means of the interference of two light beams rotating in opposite directions in a coiled glass fiber (based on the Sagnac interferometer ).
    • The fiber-optic hydrophone for measuring pressure fluctuations in water (underwater microphone) can be constructed according to the principle of the Mach-Zehnder interferometer . One of two fiber spools is protected from environmental influences, the other is in the water. Pressure fluctuations in the water change the optical length of this fiber coil. The sensitivity increases with the number of windings. Alternative designs of the fiber-optic hydrophone are coated end faces of the fiber, which also allow an interferometric measurement via a change in length when the pressure changes, or the use of the piezo-optical effect (refractive index of the water is pressure-dependent).
    • In fiber optic dosimeters the effect is exploited that by ionizing radiation defects occur in the glass that lead to a reduced transmission. Since the effect is cumulative and almost irreversible , these dosimeters have a larger linearity range and the display is more stable over the long term than with other designs.
  2. extrinsic : Here the glass fiber only serves as a transmitter for the measured variable recorded by the sensor, which the sensor must provide as an optical signal. Examples are:
    • Glass fiber pyrometers in which the infrared radiation used for temperature measurement can be transported to measuring locations that are difficult to access ( blast furnace etc.).
    • fiber-optic temperature probes in which the temperature-dependent kinetics of the phosphorescence of magnesium fluorine manate attached to the tip of the glass fiber is used for temperature measurement.
    • Optical microphones ( glass fiber sound transducers ) in which the changes in sound pressure on the membrane are converted into optical intensity changes by reflecting laser light on the moving membrane.
    • fiber optic light barriers are used for object detection in automation technology. In this case, optics and electronics are arranged separately in favor of the application.
    • fiber optic temperature sensor , the glass fiber tip of which is equipped with a gallium arsenide crystal, which is evaluated with regard to its property of changing the position of the strip edge under the influence of temperature.

See also

Individual evidence

  1. ↑ Fedor Mitschke: Glass fibers - physics and technology . Elsevier - Spektrum Akademischer Verlag, 2005, ISBN 3-8274-1629-9 , pp. 249-257 . ( 12 fiber optic sensors )
  2. ↑ Marc Blumentritt: Fiber optic sensors for monitoring chemical corrosion processes in reinforced concrete structures . Cuvillier Verlag, 2007, ISBN 3-86727-399-5 , pp. 72 f . ( limited preview in Google Book search).
  3. ↑ Martin Löffler-Mang: Optical sensors: laser technology, experiments, light barriers . Vieweg + Teubner Verlag, 2012, ISBN 3-8348-1449-0 , p. 186–190 ( limited preview in Google Book search).
  4. ↑ Wolf-Dieter Roth: The fiber optic transducer . In: Heise Online . April 18, 2005. Retrieved August 13, 2011.
  5. ↑ Fi e berthermometer. Principles and applications of fiber optic temperature measurement (PDF; 214 kB)