Basic principles of fiber optic sensors
Optical fiber is the abbreviation of optical fiber. The main component of optical fiber is silica, which is composed of a core with a higher refractive index, a cladding with a lower refractive index, and a protective layer. The core is a thin glass filament with a diameter of about 0.1 mm, and a guided wave structure that encloses light in it and propagates in the axial direction. The discovery of the optical fiber sensor originated from the practice of detecting the external disturbance of the optical fiber. In practice, people have found that when the optical fiber is subjected to changes in the external environment, it will cause changes in the optical wave parameters transmitted inside the optical fiber, and these changes are in a certain law with external factors. Developed optical fiber sensing technology.
Optical fiber has a certain sensitive effect on many external parameters. To study the principle of optical fiber sensing is to study how to apply these effects of optical fibers, to study the interaction of light in the modulation area with the external measured parameters, and to realize the function of "transmitting" and "sensing" the external measured parameters. This is an optical fiber sensor. Core.
In optical communication systems, optical fibers are used as a medium for long-distance transmission of light wave signals. Obviously, in this type of application, the less external interference the optical signal transmitted by the optical fiber is, the better. However, in the actual optical transmission process, optical fibers are susceptible to external environmental factors, such as temperature, pressure, electromagnetic field and other external conditions, which will cause changes in optical fiber parameters such as light intensity, phase, frequency, polarization, and wavelength. Therefore, people found that if the change of the light wave parameter can be measured, the size of the various physical quantities that cause the change of the light wave parameter can be known, and thus the optical fiber sensing technology is produced. Optical fiber sensing technology is a technology that uses optical fiber's sensitivity to certain physical quantities to convert external physical quantities into signals that can be directly measured. Because optical fibers can not only be used as the propagation medium of light waves, but also because the characteristic parameters (amplitude, phase, polarization state, wavelength, etc.) that characterize light waves when light waves propagate in optical fibers are due to external factors (such as temperature, pressure, strain, magnetic field, electric field, and displacement). , Rotation, etc.) changes directly or indirectly, so that the optical fiber can also be used as a sensing element to detect various physical quantities.
The figure above is the schematic diagram of the optical fiber sensor. Fiber optic sensors are usually composed of light source, transmission fiber, sensing element or modulation area, light detection and other parts. Parameters such as light intensity, wavelength, amplitude, phase, polarization state, and mode distribution may be changed by external influences during fiber transmission, especially temperature, pressure, acceleration, voltage, current, displacement, vibration, rotation, bending, When strain, chemical quantity and biochemical quantity affect the optical path, these parameters will change accordingly. The optical fiber sensor detects the size of each corresponding physical quantity according to the relationship of these parameters with the change of external factors.
