Current Status and Development Trend of Fiber Bragg Grating Sensing System

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Current Status and Development Trend of Fiber Bragg Grating Sensing System [Copy link]

Since 1978, when Hill et al. in Canada first discovered the photosensitive phenomenon in germanium-doped quartz fiber and used the standing wave method to produce the world's first fiber Bragg grating, and in 1989, Melt et al. in the United States realized the UV laser side writing technology of fiber Bragg grating (FBG), the manufacturing technology of fiber Bragg grating has been continuously improved, and people's research on fiber Bragg grating in optical sensing has become more extensive and in-depth. Fiber Bragg grating sensors have the advantages of anti-electromagnetic interference, high sensitivity, small size, light weight, low cost, and suitable for use in high temperature, corrosive environments, etc., as well as strong intrinsic self-coherence ability and the unique advantages of using multiplexing technology on one optical fiber to achieve multi-point multiplexing and multi-parameter distributed differentiation measurement. Therefore, fiber Bragg grating sensors have become a hot topic in current sensor research. How can the fiber Bragg grating system, which is mainly composed of light source, fiber Bragg grating sensor and signal demodulation system, achieve optimal matching of each part under the premise of reducing cost, improving measurement accuracy, and meeting real-time measurement, and meet the needs of practical application of fiber Bragg grating sensing system in various fields of modernization is also a key issue for researchers to consider.

This paper introduces the fiber Bragg grating sensing system, explains the broadband light source of the fiber Bragg grating system, focuses on analyzing the sensing principle of the fiber Bragg grating sensor and how to distinguish the measurement technology, summarizes the commonly used signal demodulation methods, and finally proposes optimization measures for various parts of the system to meet future needs.

1. Fiber Bragg grating sensing system

The fiber Bragg grating sensing system is mainly composed of broadband light source, fiber Bragg grating sensor, signal demodulation, etc. The broadband light source provides light energy for the system. The fiber Bragg grating sensor uses the light wave of the light source to sense the external measured information, and the external measured information is reflected in real time through the signal demodulation system.

1.1 Light source

The performance of the light source determines the quality of the optical signal sent by the entire system. In fiber Bragg grating sensing, since the sensing quantity is wavelength encoding, the light source must have a wide bandwidth and strong output power and stability to meet the needs of multi-point and multi-parameter measurement in distributed sensing systems. The commonly used light sources for fiber Bragg grating sensing systems are LED, LD and light sources doped with different concentrations and types of rare earth ions. LED light sources have a wide bandwidth, which can reach tens of nanometers, and have high reliability, but the output power of the light source is low, and it is difficult to couple with single-mode optical fiber. LD light sources have the characteristics of good monochromaticity, strong coherence, and high power. However, the stability of the LD spectrum is poor (4×10-4/℃). Therefore, the shortcomings of these two light sources restrict their application in optical sensing. The most widely studied light source doped with different types and concentrations of rare earth ions is the erbium-doped light source. Now the C-band erbium-doped light source has been successfully developed and used. With the requirements for communication capacity and speed in optical communications and the requirements for light source bandwidth for densely distributed optical fiber sensing, the research on the L band is becoming more and more important. Some researchers have proposed a C+L band development plan to increase the bandwidth and power of the light source. Erbium-doped light sources are 2 orders of magnitude higher in temperature stability than semiconductor light sources. At the same time, they can provide higher power, wider bandwidth and longer service life. Therefore, the measurement range of fiber grating sensors can be expanded and the signal-to-noise ratio of detection can be improved.

1.2 Fiber Bragg Grating Sensor

Fiber Bragg Grating sensor can realize direct measurement of physical quantities such as temperature and strain. Since the fiber Bragg grating wavelength is sensitive to both temperature and strain, that is, temperature and strain simultaneously cause the fiber Bragg grating coupling wavelength to move, it is impossible to distinguish temperature and strain by measuring the fiber Bragg grating coupling wavelength movement. Therefore, solving the cross-sensitivity problem and realizing the differentiated measurement of temperature and stress is the premise for the practical application of sensors. The differentiated measurement of temperature and stress is achieved by measuring stress and temperature changes through certain technologies. The basic principle of these technologies is to use two or two sections of fiber Bragg gratings with different temperature and strain response sensitivities to form a dual grating temperature and strain sensor, and to determine the temperature and strain response sensitivity coefficients of the two fiber Bragg gratings, and use two binary linear equations to solve the temperature and strain. Differentiation measurement technology can be roughly divided into two categories, namely, multi-fiber Bragg grating measurement and single fiber Bragg grating measurement.

Multi-fiber Bragg grating measurement mainly includes hybrid FBG/long period grating method, dual period fiber Bragg grating method, fiber Bragg grating/FP cavity integrated multiplexing method, and dual FBG overlapping writing method. Each method has its own advantages and disadvantages. The demodulation of the FBG/LPG method is simple, but it is difficult to ensure that the measurement is at the same point, with an accuracy of 9×10-6, 1.5℃. The dual-period fiber Bragg grating method can ensure the measurement position and improve the measurement accuracy, but the grating intensity is low and the signal demodulation is difficult. The fiber Bragg grating/FP cavity integrated multiplexing method sensor has good temperature stability, small size, high measurement accuracy, and an accuracy of up to 20×10-6, 1℃, but the FP cavity length adjustment is difficult and the signal demodulation is complex. The double FBG overlapping writing method has high accuracy, but the grating writing is difficult and the signal demodulation is also relatively complex.

Single fiber Bragg grating measurement mainly includes the single fiber Bragg grating method encapsulated with different polymer materials, the use of different FBG combinations and the prefabricated strain method. The single fiber Bragg grating method encapsulated with polymer materials uses the different responses of certain organic substances to temperature and stress to increase the sensitivity of the fiber Bragg grating to temperature or stress and overcome the cross-sensitivity effect. This method is simple to make, but it is difficult to select polymer materials. The method of using different FBG combinations is to write the grating at the connection of two optical fibers with different refractive indexes and temperature sensitivities or different temperature response sensitivities and doping material concentrations, and use different refractive indices and temperature sensitivities to achieve differentiated measurements. This method is simple to demodulate, and demodulation to wavelength encoding avoids stress concentration, but it has problems such as large loss, easy breakage at the fusion joint, and small measurement range. The prefabricated strain method is to first apply a certain pre-strain to the fiber Bragg grating, and firmly paste a part of the fiber Bragg grating on the cantilever beam under the pre-strain condition. After the stress is released, the deformation of the unpasted part of the fiber Bragg grating is restored, and its central reflection wavelength remains unchanged; while the deformation of the part pasted on the cantilever beam cannot be restored, resulting in a change in the central reflection wavelength of this part of the fiber Bragg grating. Therefore, this fiber Bragg grating has two reflection peaks, one reflection peak (the part pasted on the cantilever beam) is sensitive to both strain and temperature; the other reflection peak (the unpasted part) is only sensitive to temperature. By measuring the wavelength drift of these two reflection peaks, temperature and strain can be measured simultaneously.

1.3 Signal demodulation

In the fiber Bragg grating sensor system, signal demodulation is partly optical signal processing, which completes the conversion of optical signal wavelength information into electrical parameters; the other part is electrical signal processing, which completes the operation and processing of electrical parameters, extracts external information, and displays it in a familiar way. Among them, optical signal processing, that is, tracking and analyzing the central reflection wavelength of the sensor, is the key to demodulation. The most direct instrument for detecting the central reflection wavelength of the fiber Bragg grating sensor is a spectrometer. The advantages of this method are simple structure and easy use. The disadvantages are low accuracy, high price, large volume, and it cannot directly output electrical signals corresponding to wavelength changes. Therefore, it cannot meet the needs of practical automatic control. For this reason, people have studied and proposed a variety of demodulation methods to achieve fast and accurate signal extraction. It can be divided into filtering method, interference method, adjustable narrowband light source method and dispersion method.

The filtering method includes body filtering method, matched grating filtering method and tunable FP filtering method. The element of body filtering method is wavelength division multiplexer. The working principle is that the light emitted from the coupler is divided into two beams of equal intensity, one beam is filtered by a filter related to the wavelength; the other beam is used as a reference beam, and the two beams of emitted light are converted into electrical signals through a photodetector. After processing, the influence of optical power change is eliminated, and finally, the output value related to the central wavelength of the fiber grating is obtained. This method can realize the measurement of dynamic and static parameters. The resolution is 375x10-6, and the dynamic strain measurement response speed does not exceed 100Hz. The matched grating filtering method uses other FBG or bandpass filter optical devices to track the wavelength change of FBG under the action of the driving element, and then obtains the measured stress or temperature by measuring the driving signal of the driving element. This method has a simple structure, good linearity, and a resolution of up to 0.4×10-6. This method can realize static measurement. However, the disadvantage of this method is that the two gratings must be strictly matched, and the measurement range of the sensor grating is not large. The tunable FP filter method is that the reflected signal of the sensor array FBG enters the tunable fiber FP filter (FFP). When the transmission wavelength of the FFP is adjusted to the reflection peak wavelength of the FBG, the filtered transmission light intensity reaches the maximum value. The reflection peak wavelength of the FBG can be obtained from the FFP driving voltage-transmission wavelength relationship. The scanning plus the disturbance signal constitutes a wavelength locked closed loop, and its stress resolution can reach 0.3×10-6. This demodulation method can realize dynamic and static measurements. Since the tuning range of the FFP filter cavity is very wide, multi-sensor demodulation can be realized. However, the cost of high-precision FFP is relatively high.

The filter demodulation method has a simple structure, but it is difficult to further improve its sensing accuracy. The interference method has higher accuracy and can greatly improve the sensing resolution. The tunable narrow-band light source demodulation method can obtain a very high signal-to-noise ratio and resolution. The experimental minimum wavelength resolution is about 2.3pm, and the corresponding temperature resolution is about 0.2℃. However, due to the unsatisfactory stability and tunable range of the current fiber laser, the number and scope of use of fiber grating sensors are limited to a certain extent.

2. Development trend of fiber Bragg grating sensing system

In order to adapt to the networking, large-scale and quasi-distributed measurement of future fiber Bragg grating sensing systems. Many researchers are continuously studying various aspects of fiber Bragg grating sensing systems to optimize the system. The optimization of fiber Bragg grating sensing systems mainly considers three aspects, namely, light source, fiber Bragg grating sensor and signal demodulation. For the optimization of the sensing system, it is mainly based on the number of sensors, the sensitivity of the sensors and the resolution of the demodulation system. According to the actual measurement needs, different light sources, sensors and demodulation systems are configured to achieve low cost, small measurement error and high measurement accuracy. In view of the requirements of the networking of future fiber Bragg grating sensing systems, light sources with good stability, broadband and high output power should be used. Erbium-doped, neodymium-doped, ytterbium-doped plasma light sources are the focus of future development. Fiber Bragg grating sensors can realize both single parameter measurement and multi-parameter measurement. When measuring a single parameter, the sensitivity and test accuracy of the sensor should be improved. In practical applications, attention should be paid to the compromise between the sensitivity and range of the sensor. With high sensitivity, the range is naturally small. This is because the strain of the fiber Bragg grating has a limit value, and the grating will be destroyed if it exceeds this limit value. In order to achieve quasi-distributed measurement, the number of sensors reused is large. When arranging sensors, sometimes multiple sensors with different sensitivities are arranged at one point to achieve a wide range of temperature and pressure measurements. Since the sensing quantity is mainly carried by a small wavelength offset, a practical signal demodulation scheme must have extremely high wavelength resolution. Secondly, it is necessary to solve the detection problem of dynamic and static signals, especially the combined detection of the two has become a difficulty in the practical demodulation technology of grating sensors. The biggest advantage of the application of fiber Bragg grating sensing system is that it can well reuse sensors to achieve distributed sensing. For example, the newly launched FBGSLI of Micron Optics in the United States adopts an adjustable laser scanning method and uses time division technology to query up to 256 Bragg gratings of four optical fibers at the same time. Therefore, the future fiber Bragg grating sensing system will be able to meet the high-precision real-time measurement of a single point, and can adapt to the networked quasi-distributed multi-point and multi-parameter testing requirements, and play a greater role in the future sensing field.

3. Conclusion

With the in-depth study of fiber Bragg grating sensing system, the research focuses are: first, the research on the sensor's ability to sense strain and temperature changes at the same time; second, the research on the signal demodulation system; third, the research on the practical application of fiber Bragg grating sensor packaging technology, temperature compensation technology, light source stability, and sensor system networking. Especially with the development of all-optical networks, fiber Bragg grating sensing systems can apply mature wavelength division multiplexing, time division multiplexing, and space division multiplexing technologies to achieve quasi-distributed fiber sensing. Fiber Bragg grating system networks with a large number of multiplexing, high measurement accuracy, and high sensitivity will have a wider range of applications in the production field.