The structure of the fiber Bragg grating sensing system is analyzed. The functions of three different light source LEDs, LDs and erbium-doped light sources used in the fiber Bragg grating sensing system are analyzed. The operating principle of the FBG sensor and various temperature and stress are described. The difference measurement method describes several commonly used signal demodulation skills such as filtering method, dry and method, and adjustable narrow-band light source method. Finally, it is proposed to customize the future requirements 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 a photosensitive scene in an erbium-doped quartz fiber and used the standing wave method to make the world‘s first fiber grating. In 1989, Melt et al. completed the fiber Bragg grating (FBG). Since the writing skills of the UV laser side, the manufacturing skills of fiber grating have been continuously improved, and the research on fiber 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, and is suitable for use in high temperature and corrosive environments. It also has intrinsic self-coherence capability and is in an optical fiber. The use of multiplexing skills to achieve the unique advantages of multi-point multiplexing, multi-parameter distributed differential measurement. Therefore, the fiber grating sensor has become a hot topic in the discussion of sensors at that time. How can the fiber grating system composed of light source, fiber grating sensor and signal demodulation system be able to achieve optimal matching under the premise of reducing cost, improving measurement accuracy and satisfying real-time measurement, and satisfying fiber grating sensing The need for the system to be practical in all areas of modernization is also a question for the researchers to think about.

This article introduces the fiber Bragg grating sensing system, and clarifies the broadband light source of the fiber Bragg grating system. The main points are analyzed the sensing principle of the fiber Bragg grating sensor and how to distinguish the measurement skills. The signal demodulation method commonly used for signals is carried out. To sum up, in the end, propose some optimization methods for the system that are accustomed to future needs.

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 supplies 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 is reflected in the signal demodulation system in real time.

1.1 Light source

The function of the light source determines whether the light signal sent by the whole system is good or bad. In fiber Bragg grating sensing, because the sensing amount is wavelength encoding, the light source must have a wide bandwidth and strong output power and stability to satisfy the multi-point and multi-parameter measurement requirements of the distributed sensing system. The light source commonly used in fiber Bragg grating sensing systems is LED, LD and a source of rare earth ions doped with different concentrations and different varieties. LED light source has a wide bandwidth, can reach dozens of nanometers, has high reliability, but the output power of the light source is low, and it is difficult

Coupled with a 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 defects of these two kinds of light sources themselves restrict their application in light sensing. The most widely studied light source with doped rare earth ions of different varieties and different concentrations is an erbium-doped light source.

Nowadays, the C-band erbium-doped light source has been successfully developed and used. With the request for communication capacity and speed in optical communication and the distributed fiber-optic sensing point-to-source bandwidth request, the L-band research is getting more and more serious.

Yes. Some researchers have proposed a C+L band development plan to improve the bandwidth and power of the light source. The erbium-doped light source is two orders of magnitude better than the semiconductor light source in terms of temperature stability. Together, it can supply higher power, wider bandwidth and longer service life. Therefore, it can expand the measurement scale of the fiber grating sensor and improve the inspection letter. Noise ratio.

1.2 Fiber Bragg Grating Sensor

Fiber Bragg Grating sensors can directly measure physical quantities such as temperature and strain. Because the fiber grating wavelength is sensitive to temperature and strain, that is, the temperature and strain together cause the fiber grating coupling wavelength to move, so that the temperature and strain can not be distinguished by measuring the wavelength shift of the fiber grating coupling. Therefore, the handling of interspersed sensitive questions, the completion of the difference between temperature and stress measurement is the premise of the practical use of the sensor. After a certain skill to determine the stress and temperature changes to complete the temperature and stress differential measurement. The basic principle of these skills is to use two or two fiber gratings with different temperature and strain response sensitivity to form a double grating temperature and strain sensor. After determining the temperature and strain response sensitivity coefficients of two fiber gratings, two two are used. The elementary equation solves the temperature and strain. The difference measurement skills can be roughly divided 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, dual-cycle fiber grating method, fiber grating/F-P cavity integrated multiplexing method, and dual FBG stack 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 two-period fiber grating method can ensure the measurement of the orientation and improve the measurement accuracy, but the grating intensity is low and the signal demodulation is difficult. Fiber Bragg Grating/F-P cavity integrated dual-use sensor has good temperature stability, small volume and high measurement accuracy. The precision can reach 20×10-6, 1°C, but the cavity length adjustment of F-P is difficult and the signal demodulation is disorderly. The dual FBG stack writing method has higher precision, but the raster writing is difficult and the signal demodulation is messy.

The single fiber grating measurement mainly includes the single fiber grating method with different polymer materials, different FBG combinations and prefabricated strain methods. The single-fiber grating method of encapsulating polymer materials is to use some organic materials to add temperature and stress to the temperature and stress sensitivity, and to overcome the sensitivity of insertion and insertion. The manufacturing of this method is simple, but the selection of polymer information is difficult. The use of 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 data concentration. Different refractive index and temperature sensitivity are not used. Do the same to measure the difference.

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 welded joint, and small measurement scale. The prefabricated strain method is to first apply a certain pre-strain to the fiber grating, and a part of the fiber grating is firmly attached to the cantilever beam under the condition of pre-strain. After the stress is released, some of the fiber gratings are not repaired, and the center reflection wavelength is constant; while some deformations posted on the cantilever beam cannot be recovered, and then the center reflection wavelength of some of the fiber gratings is changed. Therefore, the fiber grating is modified. There are 2 reflection peaks, one reflection peak (some of which is posted on the cantilever beam) is sensitive to both change and temperature; the other reflection peak (not posted some) is only sensitive to temperature, and the wavelength shift of the two reflection peaks can be measured. Measure the temperature and strain together.