Fiber optic
Common fiber types include stepped and gradient multimode fiber and single mode fiber. The following factors must be considered when selecting fiber:
(1) Numerical aperture NA of optical fiber
From the viewpoint of improving the coupling efficiency between the light source and the optical fiber, a large NA is required, but the larger the NA, the more severe the mode dispersion of the optical fiber and the smaller the capacity for transmitting information. However, for most optical fiber sensors, there is no problem of information capacity. The maximum aperture of the optical fiber is suitable. The general requirement is: 0.2 ≤ NA < 0.4.
(2) Optical fiber transmission loss
Transmission loss is the most important optical characteristic of optical fiber, which largely determines the span of long-distance optical fiber communication relay station. However, most of the optical fiber sensor system is short, the elder is less than 4M, and the short is only a few millimeters. In particular, special optical fibers as sensitive components can relax transmission loss requirements, and optical fibers with a general loss of <10 dB/km can be used.
(3) Dispersion
Dispersion is an important parameter affecting the information capacity of optical fibers. As mentioned before, this requirement can be relaxed.
(4) Fiber strength
For sensors, strong strength is required without exception.
2. Light source
(1) incandescent light source
The radiation of an incandescent source is approximately blackbody radiation. The advantages are: low price, easy to obtain, easy to use, but used in the sensor, because the radiation density is relatively small, it can only be used together with the fiber bundle and the thick core step fiber. The disadvantage is poor stability and short life.
(2) Gas laser
High-coherence light source, easy to achieve single-mode operation, very narrow linearity; high radiation density, high coupling efficiency with single-mode fiber; less noise.
(3) Solid state laser
Nowadays, solid-state helium ion lasers are mainly used, and the advantages are small size, durability, high efficiency, and high radiation density. The spectrum is uniform and narrow, with the disadvantage that coherence and frequency stability are not as good as gas lasers.
(4) Semiconductor laser
It is an important light source for fiber optic sensors. The main LEDs are small in size, rugged, long-lasting, high-reliability, moderate radiation density, and simple power supply.
There are many light sources, and the basic requirements for the light source are the same. The light with sufficient power must be enough to reach the detector to ensure that the detection system has a large enough signal-to-noise ratio. The principle is to select a light source with sufficient radiation. It is required to have the maximum radiant power at the working wavelength of the sensitive component; the light source must be matched with the fiber to obtain the best coupling ratio; the stability of the light source is good and can work at long-term room temperature.
3. Photodetector
Photodetectors are indispensable components in photoelectric detection, converting optical signals into electrical signals. Selection criteria: high sensitivity in the working band; the noise introduced by the detector must be small, so the dark current, leakage current and parallel conductance should be selected as small as possible; high reliability and good stability; small size and easy Assembled, easy to couple with fiber; bias or bias current should not be too high; low price.
