Investigation On SMS Optical Fiber Sensing Based On Interference Effect In Multimode Optical Fiber

The optical sensors possess their inherent advantages, including small size, light weight, low cost, high sensitivity, easy fabrication, and immunity to electromagnetic interference. They have important applications in the fields of aerospace, military, national defense. The sensing mechanism of optical fiber sensors is commonly based on multimode interference and self-imaging effect. The single-mode-multimode-singlemode(SMS) structure is a new type of multimode interference sensor that has been extensively researched by experts. Based on the analyses on the theory and applications of the SMS structure, we had studied refractive index Sensor and stress sensor which are based on multimode fiber. The numerical simulation and experiment have been analyzed. The followings are the main content:(1) On the one hand, the basic principle, the classification of optical fiber sensors, and the application are depicted. On the other hand, the development and research progress of multimode interference sensor and the SMS structure are described in detail.(2) The basic theory of step type multimode fiber is analyzed, which includes the optical field and patterns,and the sensing mechanism.(3) A novel valley detection method of refractive index sensor based on Singlemode-multimode-single mode(SMS) fiber structure is proposed in this chapter. A higher sensitivity than previous findings has been obtained with this method. It shows that the mode number has vital influence on the output. There are always inerratic transmission curves when the mode number is definite by keeping the normalized frequency of the fibers same. The sensitivity is higher with the increase of the diameter of the multimode fiber. The results show that the sensitivity is 3.725×105 nm/RIU and the detection limit is 3.92×10-6RIU when the diameter of the multimode fiber is 70μm.(4) It is found that the shift of the output spectrum is sensitive to the length of multimode fiber, which means it the output spectrum would change with the stress on it. We used the method of transverse stretching to achieve stress sensing based on the theory of stress on the optical fiber. When the diameter of the multimode fiber is 30μm, the simulation result of the average value of sensitivity with several length of multimode fiber is 0.3942nm/g. The result is 4 times of that of stress sensor based on Bragg Grating Fiber.