Comparison of Common Fiber Optic Sensors

Fabry-Birot (FP), Bragg grating (FBG) and fluorescent fiber sensors are all popular and technologically advanced sensors. Because they are all based on optical fiber, they have many common characteristics, such as anti-electromagnetic interference, can be used in harsh environments (no electromagnetic process), long transmission distance (slow light attenuation in optical fiber), long service life, compact structure, etc., which will not be repeated here. We will focus on their differences. Accuracy It should be said that they all have high accuracy and can meet most needs. But if we conduct in-depth discussion, theoretically, the accuracy that fiber grating sensors can achieve is higher. From the processing point of view, the sensing accuracy of FP is mainly determined by the processing accuracy of the cavity length, while the accuracy of FBG is mainly determined by the control of the grating period spacing and the effective refractive index. When the processing accuracy is guaranteed, FBG will win by virtue of its excellent linearity in its own measurement mechanism. From the sensing principle, it can be seen that the conversion of the cavity length change of FP into Δλ is achieved through phase change and interference, which is a nonlinear process, while FBG directly realizes the conversion of effective refractive index and grating period about Δλ through the formula λB=2neffΛ, which is completely linear and theoretically can provide better accuracy. In addition, the reflected light of fiber grating is sharper than the FP interference maximum wave packet in the frequency domain, so the measurement of its central spectrum should also be more accurate. The accuracy of fluorescence temperature measurement mainly depends on the characteristics of fluorescent substances being excited to emit fluorescence and the detection of changes in the intensity of fluorescent light. The current technical level makes its measurement accuracy equivalent to the first two technologies, and its cost will vary with accuracy and measurement range. However, in actual products, the measurement accuracy is affected by objective factors such as the material, process processing level, and signal demodulator resolution of the specific manufacturer's product itself, and specific comparisons need to be made for specific products. Integration and networking In this regard, FBG undoubtedly has obvious advantages. The characteristics of fiber grating itself make each probe point use only a very small light source component, and most of the light is transmitted and continues to propagate. According to the above introduction, up to 30 gratings can be used simultaneously on one optical fiber, and the transmission distance exceeds 45km. This feature undoubtedly brings great convenience to networking. At the same time, the use of technologies such as wavelength division multiplexing has also improved the feasibility of this technology. In general, FBG is very suitable for distributed measurement of multiple nodes in a large range. As for FP and fluorescence, it will be easier to implement for small-scale networks. Complexity The complexity of FP and fluorescence systems should be much lower than that of FBG, among which fluorescence is the simplest. As explained in the principle part, the first two sensor technologies ultimately come down to the measurement of Δλ. Obviously, because the signal of FBG is weak and is often accompanied by demultiplexing requirements, its system is much more complex than FP. Fluorescence belongs to light intensity detection, which is relatively simpler. Response frequency The response frequency depends more on the design of the network and the response speed of the filter demodulation equipment. FBG requires a high-performance demodulation and demultiplexing receiver, and the processing capacity of the receiver often affects its response frequency. FP and fluorescence are relatively simple, and the response frequency can generally be guaranteed. Light source According to the above discussion, FBG has very high requirements for light source, and needs high-power broadband light source or tunable light source. The requirements of FP and fluorescence are much lower, thanks to the strong reflection signal of FP and the light source of fluorescence only needs to excite fluorescence. Flexibility and scope of application The probes of the three are quite small and flexible, but FBG is obviously subject to its complex wavelength shift detection technology. In a high temperature environment (around 300 °C), the grating will be likely to be erased. Therefore, FBG is not suitable for a larger temperature range. Cost According to the above discussion, for a single measurement point (or a few measurement points, such as less than 50 measurement points), FP and fluorescence systems will undoubtedly have advantages due to their low complexity, simple wavelength shift detection technology, and low light source requirements. Fluorescence has the most cost advantage. However, for large systems with more than 50 nodes, FP and fluorescence will lead to a rapid increase in cost due to the difficulty of networking. In summary, it is generally believed that FBG sensors are suitable for large, complex, and high-precision low-temperature distributed sensing networks. FP and fluorescence have the advantages of fast response frequency (up to 200KHz), small probe size (micrometer level), long light source life, etc., which are suitable for flexible, small and simple sensing systems. Fluorescence has the advantages of high temperature measurement and low cost.