Points for Attention in High Temperature Fiber Optic Sensors


Points for Attention in High Temperature Fiber Optic Sensors
Issues that should be paid attention to for high temperature resistant fiber optic sensors:
The basic structure of the high temperature fiber optic sensor includes: light source, transmission fiber and light detection part, which has been widely used in various fields. Now, let's talk about the application precautions of high temperature fiber optic sensor.
1. Optical fiber.
Common fibers include step type, gradient type multimode fiber and single mode fiber. The following factors should be considered when selecting fiber:
(1) Optical fiber numerical aperture NA. When improving the coupling efficiency of the light source and the optical fiber, a larger NA needs to be used, but the larger the NA, the greater the modal dispersion of the optical fiber, and the smaller the information transmission capacity. However, for most optical fiber sensors, there is no problem of information capacity. The general requirement of optical fiber: 0.2≤NA "0.4" maximum aperture.
(2) Optical fiber transmission loss. Sensing loss is the most important optical characteristic of optical fiber. It largely determines the span of long-distance optical fiber communication relay station. Most of the optical fiber sensing systems have short distances, with long distances less than 4M and short distances only a few millimeters. Especially the special optical fiber as a sensitive element has low transmission loss requirements. Generally, the optical fiber with a loss below 10dB/km can be used.
(3) Dispersion phenomenon. Dispersion is an important parameter that affects the ability of optical fiber to transmit information. As mentioned earlier, this requirement can be relaxed.
(4) Fiber strength. For fiber optic sensors, stronger strength is required without exception.
2. Light source.
Due to the large number of light sources, the basic requirements for the light source are the same. It is necessary to make light with appropriate characteristics and sufficient power reach the detector to ensure that the detection system has a large enough signal-to-noise ratio. The principle is: choose a radiation It is strong enough and requires a light source with the maximum radiant power at the working wavelength of the sensitive element; the light source must be matched with the optical fiber to obtain a better coupling rate; the light source has good stability and can work at room temperature for a long time.
  (1) Hot incandescent lamps. The white light source radiation is similar to black body radiation. Its advantages are: cheap, easy to obtain, and convenient to use, but when it is used in sensors, because of its low radiation density, it is only suitable for fiber bundles and thick-core step fibers. The disadvantages are poor stability and short life.
(2) Gas laser device. Highly consistent light source, easy to achieve single-mode operation, very narrow linearity; higher radiation density, high coupling efficiency; lower noise.
(3) Solid-state laser device. At present, the solid-state rubidium ion laser is mainly used, which has the advantages of small size, sturdiness, high efficiency, and high radiation density. Its disadvantage is that it has poor coherence and frequency stability compared with gas lasers.
(4) Semiconductor laser. As an important light source for optical fiber sensors, LED has the advantages of small size, high intensity, long life, high reliability, moderate radiation density, and simple power supply.