The working principle of FBG sensor and different measurement methods of temperature and stress

Photocoupler

This paper introduces the structure of fiber Bragg grating sensing system, analyzes the performance of three different light source LEDs, LD and erbium-doped light source used in fiber Bragg grating sensing system, and explains the working principle of FBG sensor and various temperature and The method of distinguishing stress is described by several common signal demodulation techniques such as filtering method, interference method and adjustable narrow-band source method. Finally, it is proposed to adapt to the future needs of the fiber grating sensing system light source, fiber grating sensor and Signal demodulation is optimized.

Since 1978, Hill et al. of Canada first discovered photosensitivity in erbium-doped quartz fibers and used the standing wave method to fabricate the world's first fiber grating. In 1989, Melt et al. in the United States realized fiber Bragg gratings (FBG). Since the UV laser side-write technology, the manufacturing technology of fiber grating has been continuously improved, and the research on optical grating in optical sensing has become more extensive and in-depth. The fiber Bragg grating sensor has the advantages of high resistance to electromagnetic interference, high sensitivity, small size, light weight, low cost, suitable for use in high temperature, corrosive environment, etc. It also has intrinsic self-coherence capability and is in one fiber. The unique advantage of multi-point multiplexing and multi-parameter distributed discrimination measurement is realized by using multiplexing technology. Therefore, fiber Bragg grating sensors have become a research hotspot of current sensors. How can a fiber Bragg grating system consisting of a light source, a fiber grating sensor and a signal demodulation system be able to achieve optimal matching of various parts under the premise of reducing cost, improving measurement accuracy, and satisfying real-time measurement, etc., to satisfy the fiber grating sensing The need for the system to be practical in all areas of modernization is also a key consideration for researchers.

In this paper, the fiber Bragg grating sensing system is introduced. The broadband light source of the fiber Bragg grating system is described. The sensing principle of the fiber Bragg grating sensor and how to distinguish the measurement technology are analyzed. The signal demodulation methods commonly used for signals are summarized. Finally, the optimization measures for various parts of the system to meet the needs of the future are proposed.

1. Fiber Bragg Grating Sensing System

The fiber grating sensing system is mainly composed of a broadband light source, a fiber grating sensor, and signal demodulation. The broadband light source provides light energy to the system. The fiber grating sensor uses the light wave of the light source to sense the information measured by the outside world, and the information measured by the outside world is reflected in real time by the signal demodulation system.

1.1 Light source

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

1.2 Fiber Bragg Grating Sensor

Fiber Bragg Grating sensors enable direct measurement of physical quantities such as temperature and strain. Since the wavelength of the fiber grating is sensitive to both temperature and strain, that is, temperature and strain simultaneously cause the wavelength of the fiber grating coupling to move, so that the temperature and strain can not be distinguished by measuring the wavelength shift of the fiber grating coupling. Therefore, solving the cross-sensitive problem and realizing the differential measurement of temperature and stress is the premise of the practical use of the sensor. Temperature and stress differential measurements are achieved by certain techniques to determine stress and temperature changes. The basic principle of these techniques is to use two or two fiber gratings with different temperature and strain response sensitivities to form a double grating temperature and strain sensor. By determining the temperature and strain response sensitivity coefficients of two fiber gratings, two binary The equation solves the temperature and strain. Differentiating measurement techniques can be broadly classified into two categories, namely, multi-fiber grating measurement and single fiber grating measurement.

Multi-fiber grating measurement mainly includes hybrid FBG/long period grating method, double-cycle fiber grating method, fiber grating/FP cavity integrated multiplexing method, and dual FBG overlap writing method. Each method has its own advantages and disadvantages. The FBG/LPG method is simple to demodulate, but it is difficult to ensure that the measurement is the same point with an accuracy of 9×10-6 and 1.5°C. The dual-period fiber grating method can ensure the measurement position and improve the measurement accuracy, but the grating intensity is low and the signal demodulation is difficult. Fiber Bragg Grating/FP Cavity Integrated Multi-Purpose Sensor has good temperature stability, small volume and high measurement accuracy. The accuracy can reach 20×10-6, 1°C, but the cavity length adjustment of FP is difficult and the signal demodulation is complicated. The double FBG overlap writing method has higher precision, but the raster writing is difficult and the signal demodulation is complicated.

Single fiber grating measurements mainly include single fiber grating methods with different polymer materials, different FBG combinations and pre-strain methods. The single fiber grating method of encapsulating a polymer material is to increase the sensitivity of the fiber grating to temperature or stress by using different organic materials to respond to temperature and stress, and overcome the cross sensitivity effect. This method is simple to make, but it is difficult to select a polymer material. The different FBG combination method is to write the grating at the junction of two kinds of optical fibers with different refractive index and temperature sensitivity or different temperature response sensitivity and doping material concentration, and different measurement is realized by using different refractive index and temperature sensitivity. This method is simple in demodulation, and demodulation into wavelength coding avoids stress concentration, but has problems such as large loss, easy fracture of the fusion joint, and small measurement range. The pre-straining method first applies a certain pre-strain to the fiber grating, and a part of the fiber grating is firmly adhered to the cantilever beam under the condition of pre-strain. After the stress is released, the unbonded portion of the fiber grating is recovered and its center reflection wavelength is constant; and the deformation of the portion pasted on the cantilever beam cannot be recovered, resulting in a change in the central reflection wavelength of the portion of the fiber grating. Therefore, the fiber grating is changed. There are 2 reflection peaks, one reflection peak (the part pasted on the cantilever beam) is sensitive to both change and temperature; the other reflection peak (unapplied part) is only sensitive to temperature, by measuring the wavelength shift of these two reflection peaks Simultaneously measure temperature and strain.

1.3 Signal Demodulation

In the fiber Bragg grating sensing system, part of the signal demodulation is optical signal processing, which completes the conversion of the optical signal wavelength information to the electrical parameter; the other part is the electrical signal processing, completes the operation processing of the electrical parameter, extracts the external information, and takes people The familiar way is shown. Among them, optical signal processing, that is, tracking analysis of the center reflection wavelength of the sensor is the key to demodulation. The most direct detection instrument for the reflection wavelength of the center of the FBG sensor is the spectrometer. The advantage of this method is that the structure is simple and convenient to use. The disadvantages are the accuracy of the bottom, the high price, the large volume, and the direct output of the electrical signal corresponding to the wavelength change. Therefore, the need for practical automatic control cannot be met. To this end, people have studied and proposed a variety of demodulation methods to achieve fast, accurate extraction of signals. Can be divided into filtering method, interference method, adjustable narrow-band light source method and dispersion method.

The filtering method includes a bulk filtering method, a matched grating filtering method, and a tunable FP filtering method. The components of the bulk filtering method are wavelength division multiplexers. 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 wavelength-dependent filter; the other beam is used as a reference beam, and the two beams of outgoing light are converted into electrical signals by the photodetector, and are eliminated by processing. The effect of the change in optical power, and finally, the output value associated with the center wavelength of the fiber grating is obtained. This method enables 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 band-pass filter optical devices to track the wavelength variation of the FBG under the action of the driving components, and then, by measuring the driving components. Drive the signal to obtain the measured stress or temperature. The method has the advantages of simple structure, good linearity and resolution of 0.4×10-6. This method can achieve static measurements. However, the shortcoming of this method is that the two gratings are strictly matched, and the measurement range of the sensing grating is not large. The tunable FP filter method is that the reflected signal of the sensing array FBG enters the tunable fiber FP filter (FFP), and when the transmission wavelength of the FFP is adjusted to the reflection peak wavelength of the FBG, the filtered transmitted light intensity reaches a maximum value by the FFP. The driving voltage - the transmission wavelength relationship gives the reflection peak wavelength of the FBG. The scanning plus the disturbance signal forms a wavelength-locked closed loop with a stress resolution of 0.3×10-6. This demodulation method enables dynamic and static measurements. Due to the wide tuning range of the FFP filter cavity, multi-sensor demodulation can be achieved. However, high precision FFP costs are high.

The filter demodulation method has a simple structure, but it is difficult to further improve its sensing accuracy. The interferometry method has higher precision and can greatly improve the sensing resolution. The adjustable narrowband source demodulation method can obtain high signal-to-noise ratio and resolution. The minimum wavelength resolution obtained by the experiment is about 2.3 pm, and the temperature resolution is about 0.2 °C, but due to the stability of the current fiber laser. The tuning range is not ideal, which limits the number and range of fiber grating sensors to a certain extent.

2. Development trend of fiber grating sensing system

In order to adapt to the future, fiber-optic grating sensing system network, large-scale, quasi-distributed measurement. Many researchers are constantly researching various aspects of fiber Bragg grating sensing systems to optimize the system. The optimization of the fiber Bragg grating sensing system is mainly considered from three aspects, namely, the light source, the fiber grating sensor and the signal demodulation. For the optimization of the sensing system, mainly according to the number of sensors, the sensitivity of the sensor and the resolution of the demodulation system, according to the actual measurement needs, different light sources, sensors and demodulation systems are configured, which makes the cost low and the measurement error small. High measurement accuracy. For the future requirements of the networked fiber grating sensing system, a light source with good stability, wide bandwidth and high output power should be used. The light source of erbium-doped, erbium-doped or erbium-doped plasma is the focus of future development. Fiber Bragg Grating sensors enable both single-parameter measurements and multi-parameter measurements. When measuring single parameters, the sensitivity and test accuracy of the sensor should be improved. In practical applications, pay attention to the compromise between sensitivity and range of the sensor. The sensitivity is high and the range is naturally small. This is because the strain of the fiber grating has a limit beyond which the grating is destroyed. In order to achieve quasi-distributed measurement, the number of sensor multiplexing is large. When the sensor is arranged, sometimes a plurality of sensors with different sensitivities are arranged at one point to realize a wide range of temperature and pressure measurement. Since the sensing amount is mainly a small wavelength shift as a carrier, a practical signal demodulation scheme must have extremely high wavelength resolution. Secondly, it is necessary to solve the problem of detecting dynamic and static signals, especially the combination detection of the two has become a difficult point in the practical demodulation technology of grating sensing. The biggest advantage of fiber Bragg grating sensing system application is the good reuse of sensors for distributed sensing. For example, MICROP Optics of the United States, the new FBGSLI adopts the tunable laser scanning method, which can simultaneously use the time division technology. Four channels of fiber are queried for up to 256 Bragg gratings. Therefore, the future fiber Bragg grating sensing system will be able to meet the single-point high-precision real-time measurement, and can adapt to the networked quasi-distributed multi-point, multi-parameter test requirements, and play a greater role in the future sensing field. .

3, the conclusion

With the in-depth study of fiber Bragg grating sensing systems, the research focuses on the following: first, the sensor can simultaneously sense strain and temperature changes; second, the research on signal demodulation system; third, the packaging of fiber Bragg grating sensors Practical applications such as 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 technology to achieve quasi-distributed optical fiber sensing, multiple multiplexing, and measurement accuracy. The high-sensitivity fiber-optic grating system network will have wider application in the production field.