| Optical fiber sensors have many advantages,such as electromagnetic interference resistance,long-distance operation,corrosion resistance,fast response,high sensitivity Therefore,they are widely used in many fields,such as food safety,medicine and environmental monitoring.The fiber sensor based on macrobending fiber has the advantages of simple structure and easy fabrication,and can be used as a point measuring device.In order to realize the fiber refractive index sensor with simple structure,low preparation cost,high sensitivity and temperature compensation function,this paper cascades the macrobending structure fiber and the core-offset structure,single-mode-multi-mode(SMFMMF)alternating structure fiber respectively.A highly sensitive refractive index sensor and an optical fiber sensor that can simultaneously measure refractive index and temperature are obtained respectively.The main contents are as follows:(1)A Mach-Zehnder interferometer based on fiber core-offset and macrobending cascade structure is proposed to achieve the measurement of refractive index with high sensitivity.A Mach-Zehnder fiber interferometer is constructed by using the offset structure of the fiber,and part of the light transmitted in the fiber core will enter the cladding layer,and part of the light energy will be re-coupled back to the fiber core at the end of the bending structure of the fiber.The changing refractive index can be detected by monitoring the drift of the interference dip.The effects of different bending radius and core offset on refractive index sensor performance are investigated experimentally.Experimental results show that the wavelength dip changes nonlinear with the RI variations.The results also show that as the macrobending radius decreases,the transmission loss will increase,and the RI measurement range will decrease for the probes with the macrobending radius of 5.5 mm and core offsets of 8 μm and 12 μm.The sensitivity of the sensor is up to 699.95 nm/RIU at refractive index1.43 when the core of the fiber is offset 2 μm and the radius of the macrobending structure is 5.5 mm.In the refractive index range of 1.35-1.38,when the optical fiber core offset is 8μm and the macrobending fiber radius is 5.5mm,the sensor can also be used for intensity modulation,and its sensitivity is 366.78 d B/RIU.Finally,the temperature characteristics,hysteresis and stability of the sensor probe are analyzed,and the temperature sensitivity of the optical fiber sensor is 0.112 nm/℃,the hysteresis of the sensor is small,and it has good stability.(2)An alternating SMF-MMF structure fiber sensor encapsulated by a cascade macrobending fiber and a heat-shrinkable sheath is proposed,which can measure refractive index and temperature simultaneously.The alternating SMF-MMF structure has the transmission characteristics of long period fiber grating(LPFG),and its temperature sensitivity can be improved by sealing it in a heat-shrinkable tube.When four multi-mode fiber segments are embedded in the single-mode fiber segment,the length of the multi-mode fiber segment is 250 μm and the length of the single-mode fiber segment is 400 μm,respectively,an obvious LPFG resonance dip can be generated in the transmission spectrum(1217.8 nm),and the extinction ratio is 36.9 d B.A Mach-Zehnder interference peak at 1621.2nm is generated by cascading a 9 mm diameter macroscopically curved fiber structure.Temperature can be measured by monitoring the drift of LPFG characteristic dip,and refractive index can be measured by monitoring the drift of interference dip.The experimental results show that the temperature sensitivity of the device is-255.52 pm/℃ in the range of 35-80 ℃,and the refractive index sensitivity is 165.04 nm/RIU in the range of1.335-1.38. |
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