| Refractive index?RI?is a fundamental parameter of the materials,it could determine the optical properties of materials such as concentration,strain and temperature.Therefore,its monitoring is essential in many application fields,including medical diagnostics,food safe control,chemical and biological analyses.Fiber-optic sensor is widely used due to their advantages such as high response speed,high sensitivity,immunity to electromagnetic interference and easy networking.Different types of fiber-optic RI sensors have been developed,such as surface plasmon resonance?SPR?,fiber Bragg grating?FBG?,fiber long-period grating?LPG?,interferometric fiber optic sensors,etc.Among the interferometric fiber optic sensors,the in-line fiber Mach–Zehnder interferometers?MZIs?are with the unique advantages of light weight,compactness,low cost and easy fabrication because of having one arm as core mode and the other in the cladding,eliminating the need for two physical fiber branches,the in-line fiber MZIs have attracted significant attention now.In this paper,in order to realize the low cost,easy fabrication and high sensitive measurement of RI,the core-offset in-line fiber Mach-Zehnder interferometers and peanut-shape structure in-line fiber Mach-Zehnder interferometers were fabricated by using a commercial fusion splicer and the sensing performances were investigated.The main results are as follow:The core-offset in-line fiber Mach-Zehnder interferometers were fabricated,and the effects of the sensor's parameters on the interference spectra and sensing performances were investigated.The research results showed that high order modes were more sensitive than low order modes,the RI sensitivity of sensor could be improved by exciting high order modes.As the arm length increased,the free spectral range?FSR?decreased and the number of the interference dips increased,different interference dips were with different RI sensitivities.The core-offset displacement had a significant influence on the mode power distribution and interference spectra,the spectra had the highest extinction ratio when the core-offset displacement is 6?m.The theoretical simulation and experimental results showed that the small core-offset structure less than 8.2?m could not excite high order modes,the interferometers was with low sensitivity which is only 43.147nm/RIU.The simple ways to enhance the core-offset in-line fiber Mach-Zehnder interferometer RI sensitivity by bending the interferometer and enlarging the core-offset displacement were proposed.The experimental results showed that after bending the core-offset in-line fiber MZI with core-offset displacement of 6?m and arm length of30mm,the interference spectra became more stable,and as the curvature radius was reduced,the RI sensitivity was increased.The RI sensitivity reached 358.039nm/RIU as the bending radius was 16.42mm,and the resolution was 5.586×10-5RIU.As the core-offset displacement was increased from 6 to 40?m,the interference between the core mode and cladding mode was suppressed,and meanwhile the interference within the cladding modes was dominated,the RI sensitivity was increased from 43.97 to123.40nm/RIU.The peanut-shape structure in-line fiber Mach-Zehnder interferometers were fabricated,the mode power distribution and the sensing performances were investigated.The wavelength-related and phase-related simultaneous measurements of refractive index?RI?and temperature were realized respectively by using a peanut-shape structure in-line fiber MZI with an arm length of 100mm.For the wavelength-related measurement,the highest temperature and RI sensitivities were 0.0709 nm/°C and-47.3620nm/RIU,respectively.For the phase-related measurement,the highest temperature and RI sensitivities were-0.0632rad/°C?0.0764 nm/°C?and77.0995rad/RIU?-93.2429nm/RIU?,and the phase-related measurement was with a relative lower measurement errors of 0.3800°C and 0.0004RIU respectively. |
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