Research On Modal Interference Sensor Based On Multi-core Fiber

Inter-mode interference optical fiber sensor has attracted widespread attention due to its inherent advantages of simple structure,low cost,and high accuracy,and it has become an important part in the field of optical fiber sensing.In this thesis,two kinds of inter-mode interference-type optical fiber sensing structures have been designed and manufactured by using multi-core optical fiber.The characteristics of the designed sensing structure in the measurement of physical parameters such as temperature,strain,curvature and torsion have been experimentally studied in detail.The main content of dissertation includes the following aspects:1.The development and classification of multi-core optical fiber are presented,and the research status of multi-core optical fiber sensor is described in detail.Based on Maxwell's equations,the mode theories in ideal waveguide and optical fiber are analyzed.The sensing principles two common types of intermodal interferometric sensors based on Sagnac and Mach-Zehnder are introduced,which would provide a theoretical basis for the design of sensors.2.A Sagnac interference sensor based on a multi-core-polarization-maintenance fiber?MCF-PMF?structure is designed and manufactured.The sensing principle of the fiber-optic sensor is described.The measurements of temperature,strain and curvature by using this MCF-PMF structure were achieved and the influence of the lengths of polarization-maintaining fiber?PMF?and multi-core fiber?MCF?on the sensing sensitivity are also studied in experiment.The results show that both the lengths of the MCF and the PMF have a slight influence on the temperature sensitivity of the sensor,but there is no regularity.In temperature measurements,the temperature sensitivity fluctuations caused by the changes of the PMF and MCF lengths were0.36nm/?and 0.77nm/?,respectively.The maximum temperature sensitivity achieves-2.38nm/?and the measurement accuracy is 0.08°C when the lengths of PMF and MCF are10cm and 12cm,respectively.The variations of the PMF and MCF lengths have greater impact on the sensor's strain sensitivity.The strain sensitivity increases with the decrease of the length of PMF and the increase of the length of MCF.When the length of the PMF and MCF are respectively 4cm and 8cm,the sensor has a maximum strain sensitivity of 39.2pm/??.When the length of the multi-core fiber is constant,the shorter PMF was used,the higher strain sensitivity can be obtained.Curvature experiments show that the changes of PMF and MCF lengths have a certain effect on the curvature sensitivity,but there is no obvious linear relationship between fiber length and curvature sensitivity.The fluctuations are 3.48nm/m^-1 and 3.85nm/m^-1,respectively.The sensor achieves a maximum curvature sensitivity of 17.58nm/m^-1 when the PMF and MCF lengths are 6cm and 8cm,respectively.The sensor shows a high temperature sensitivity and good linear response in strain and curvature measurement.3.A multi-mode fiber--multi-core fiber--multi-mode fiber?MMF-MCF-MMF?structure-based Mach-Zehnder sensor is fabrication and experimentally demonstrated.The strain,torsion and temperature sensing characteristics of different resonant peaks of the sensor were experimentally studied.The simultaneous measurement of temperature and torsion was realized by using the different temperature and torsion responses characteristics of the same resonant dip in the transmission spectra.In torsion experiment,the sensor shows the maximum wavelength and intensity sensitivities of 13.8 pm/?rad/m?and 1.837dB/?rad/m?with the clockwise twisting.In counterclockwise twisting,the maximum wavelength and intensity sensitivities of the sensor are-41pm/?rad/m?and-1.881dB/?rad/m?,respectively.The intensity of the resonant dip in the transmission spectra of the sensor increases with the increase of clockwise twisting and decreases with the increase of counterclockwise torsion.Therefore,the twist direction discrimination can be realized by analyzing the intensity change of the resonant dip.In the temperature measurement,the transmission spectra shift to the longer wavelength?red-shift?with the temperature increase,the maximum wavelength and intensity sensitivities of temperature are 70pm/?and-0.296dB/?,respectively.The obtained strain sensitivities are about two to three orders of magnitude lower than those for torsion and temperature responses,the influence of strain is negligible.In addition,combining with the sensitivity matrix,the two-parameter measurement i.e.temperature-clockwise torsion or temperature-counterclockwise torsion can be realized by using the different wavelength and intensity response characteristics to temperature and torsion.