FBG sensors give the opportunity to measure a variety of parameters in conditions where other sensor technologies fail or simply cannot operate. Such FBG sensors have intrinsic advantages, including resistance to electromagnetic interference, non-electrical conductivity, passive measurements, small size and small weight, and the option of multipoint measurements. the reflection wavelength of the FBG (Bragg wavelength) depends on the grating characteristics (period, modulation) and is influenced by the ambient conditions such as strain and temperature. Development of fiber optic devices based on fiber optic sensors for operation in harsh environments (such as for temperatures of up to 1000°С) is becoming an increasingly important field. In the case of temperature sensing the Bragg wavelength is a function of the temperature. This temperature dependence results from changes of the refractive index of the fiber as well as from thermal expansion of the glass material. Many material properties show strong temperature dependence. Examples of such temperature dependencies are dew point, density, electrical conductivity, refractive index, rigidity, and diffusion. Temperature measurement also plays an important role in health monitoring of electric circuits or civil structures.
The main advantages of FBG sensors are their measurement of reflected light, wavelength-encoded sensing, and multiplexing capability. Nowadays there are many types of FBG sensors used for measuring temperature: intrinsic and extrinsic.
Three major types exist:
- The intensity modulated sensors
Intensity modulated FBG sensors are based on the principle of letting a physical disturbance such as temperature cause a change in the received light through an optical fiber;
- The phase modulated sensors
Phase modulated FBG sensors are based on the principle of comparing the phase of light in the sensing fiber with a reference fiber in an interferometer;
- The wavelength modulated sensors
Wavelength modulated FBG sensors are based on the principle that a physical disturbance such as temperature or strain changes the reflected wavelength of the light.
In general, it should be noted, phase modulated and wave modulated FBG sensors are providing much more accurate measurements than intensity modulated sensors but at the cost of much more expensive interrogators.
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