Considerations for Fiber Optic Sensor Applications
1. Optical fiber
Common optical fibers include stepped and gradient multimode fibers and single-mode fibers, and the following factors must be considered when selecting optical fibers:
(1) The numerical aperture NA of the optical fiber
From the viewpoint of improving the coupling efficiency between the light source and the optical fiber, it is necessary to use a large NA, but the larger the NA, the more severe the mode dispersion of the optical fiber, and the smaller the capacity to transmit information. However, for most optical fiber sensors, there is no problem of information capacity, and the maximum aperture of optical fiber is appropriate, and the general requirement is: 0.2≤NA "0.4.
(2) Optical fiber transmission loss
Transmission loss is the most important optical feature of optical fiber, which largely determines the span of long-distance optical fiber communication relay stations, but in the optical fiber sensor system, most of the distances are relatively short, the elderly lack 4M, and the short ones are only a few millimeters. In particular, special optical fibers, which are sensitive components, can relax the requirements for transmission loss, and can be used for optical fibers with a general loss of < 10 dB/km.
(3) Dispersion
Dispersion is an important parameter that affects the information capacity of optical fibers, and as mentioned above, the requirements in this regard can be relaxed.
(4) The strength of the optical fiber
For sensors, without exception, high strength is required.
2. Light source
(1) Incandescent light source
The radiation of an incandescent light source is approximately black-body radiation. Its advantages are: low price, easy to obtain, easy to use, but in the sensor, because the radiation density is relatively small, so it can only be used with fiber bundles and thick core step fibers. The defect is that the stability is relatively poor and the life is short.
(2) Gas lasers
High coherence light source, easy to complete single-mode operation, linearity is very narrow; The radiation density is relatively high, and the coupling efficiency with single-mode fiber is high; The noise is relatively small.
(3) Solid-state lasers
At present, solid-state rubidium-ion lasers are mainly used, which have the advantages of small size, durability, high efficiency and high radiation density. The spectrum is uniform and narrow, and the defects are not as coherent and frequency stable as gas lasers.
(4) Semiconductor lasers
It is an important light source for optical fiber sensors, the primary LED, which has the advantages of small size, durability, long life, high reliability, moderate radiation density and simple power supply.
There are many light sources, the basic requirements for the light source are common, it is necessary to make the detector with appropriate characteristics and the power to meet the large light to ensure that the detection system has a large signal-to-noise ratio, and the principle of following is: select the light source with strong radiation, and require the maximum radiation power at the operating wavelength of the sensitive element; It is necessary to match the light source to the optical fiber to achieve the best coupling ratio; The stability of the light source should be good, and it can operate at room temperature for a long time.
3. Photodetectors
Photodetector is an indispensable device in photoelectric detection, which converts optical signals into electrical signals. Selection principle: high sensitivity in the operating band; The noise introduced by the detector must be small, so the equipment with dark current, leakage current and parallel conductance as small as possible should be selected; High reliability and stability; Small size, easy to assemble, easy to couple with optical fiber; The bias or bias should not be too high; The price is low.
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