Research On Sensing Characteristics Based On Inter-Mode Interference Fiber Structure

In recent years,the optical fiber sensors based on the modal interference have received more and more attention from scientists because they can realize the functions of both detection arm and reference arm by themselves and only need to use fibers to realize the interference between different modes of light,which have the advantages of compact structure,simple optical path and easy fabrication.In this paper,two kinds of inter-mode interferometric optical fiber sensors are fabricated mainly by using special fiber and singlemode fiber splicing method and hydrogen-oxygen flame fusion taper method,and their sensing characteristics are analyzed theoretically and studied experimentally.The main research contents of this paper are:1.Two kinds of sensors based on modal interference fiber structures are designed and fabricated.Their basic structures and preparation methods are described,and their sensing principles are analyzed theoretically,which lays the foundation for the subsequent experimental research.Firstly,an inter-mode interferometric optical fiber sensor based on the single-mode fiber–hollow-core fiber–four-core fiber–hollow-core fiber–single-mode fiber(SMF–HCF–FCF–HCF–SMF)structure is fabricated by direct fusion splicing technology.By analyzing the RSOFT simulation results and spectra,the influence of different fiber lengths on the transmission spectrum of the sensor is explored.Then,a tapered sensor based on single-mode fiber–multimode fiber–tapered four-core fiber–multimode fiber–single-mode fiber(SMF–MMF–TFCF–MMF–SMF)structure is fabricated by fiber fusion tapering technology.In order to further improve the sensing performance of the sensor,a method is proposed to combine the tapered area with the temperature-sensitive material PDMS,and the basic parameters and preparation of PDMS materials are investigated.2.The refractive index(RI)and temperature sensitivity of the sensor based on SMF–HCF–FCF–HCF–SMF structure are measured experimentally.The experimental results show that the sensor has a maximum RI sensitivity of 275.30 nm/RIU in the RI range of1.332～1.412,and a maximum temperature sensitivity of 94.4 pm/℃ in the temperature range of 25 ℃～55 ℃.At the same time,since different interference valleys have different responses to external RI and temperature sensitivity,the sensitivity matrix is used to demodulate,and the simultaneous measurement of RI and temperature is realized.3.Temperature sensing experiments are carried out using a sensor based on SMF–MMF–TFCF–MMF–SMF structure,and the effect of wrapping PDMS on the temperature sensing characteristics of the sensor is also studied through the experiment.The following important conclusions are obtained: In the temperature range of 25 ℃～55 ℃,the maximum temperature sensitivity of the sensor without PDMS wrapped is 72.2 pm/℃,while the maximum temperature sensitivity of the sensor with PDMS wrapped is 186.2 pm/℃,which is 2.6 times higher than that of the sensor without PDMS.At the same time,using PDMS to wrap the tapered fiber can also protect the tapered structure and improve the mechanical strength of the sensor.