Research On All-fiber Temperature And Torsion Sensors Based On Modal Interference

Fiber optic sensor based on the modal interference principle is one kind of optical fiber sensors and being concerned by the researchers in recent years. It has the advantages of high sensitivity, corrosion resistant ability, light weight, small size, anti-electromagnetic interference, integrated sensing and transmission, and can be compatible with optical fiber communication systems etc. Compared with the traditional interferometric fiber optic sensors, this kind of sensors is only composed of one optical fiber. Its structure is more compact, and is not strict requirements for the light source. With the development of the optical fiber sensing technology, the fiber optic sensor based on the modal interference principle will have potential application in large generators, border security, large civil engineering structures monitoring, space shuttle monitoring and other fields.In this paper, two fiber optic sensors based on two different types of modal interference principle are designed, and the characteristics of the sensors are investigated. The main content of this paper are summarized as follows:The first chapter is the introduction part. The development history, application, classification and the research status of the modal interference technology are introduced. In addition, the main content of this paper as well as the innovation of the research are summarized too.In the second chapter, the basic principle of optical interference technology, the modal interference theory of optical fiber and the interference theory of two orthogonal polarization modes in polarization maintaining fiber are presented.The third chapter introduces a modal interference fiber optic sensor based on the "no core optical fiber-multimode optical fiber-no core optical fiber (NMN)" structure. Firstly, the sensing principle of the sensor is analyzed. Then, we simulate the internal energy distribution of the sensor, and the effect of the no-core-optical-fiber length on the energy distribution of the sensor is also investigated by the simulation. Finally, we investigate the temperature and refractive index response characteristics of the sensor.The fourth chapter proposes a kind of polarization modal interference optical fiber sensor to realize the torsion measurement. One segment of single mode fiber is used in the sensor. Because the electric field vector in the single mode fiber which is full circular-symmetric geometry is not affected by the twist, the sensor can be used to measure the torsion based on the Sloe interference principle. According to the theoretical analysis, we simulate the torsional characteristics of the sensor. In the experiment, the torsion measurement results of the sensor are in agreement with the simulated results after the correction. The maximum torsional sensitivity of the sensor is -0.79dB/degree, and the direction of torsion can be achieved at the same time. The temperature response of this sensor have also been researched in the experiment, and the maximum error of the torsion measurement caused by temperature change is only 0.04 degrees/℃.