Investigation Of Mach-Zehnder Fiber Interference Sensors Based On Thin Diameter Fiber

With the more wide application of fiber-optic interferometric sensors,the sensitive optical fiber sensors are needed.Therefore,it is of great practical significance to investigate high sensitive M-Z interferometers.In this paper,the in-Line M-Z fiber optic interferometer is designed based on a novel 40?m diameter fiber(thin diameter fiber,TDF)and used in temperature,refractive index and humidity detection.The main work is summaried as the following three aspects:(1)An in-Line M-Z interferometer based on singlemode-multimode-thinned-multimode-singlemode(SMTMS)fiber structure was proposed.We innovatively employ multi mode fibers in the structure,theoretically and experimentally study the mechanism of the improving interference fringe visibility.It can not only improve the detection accuracy of the sensor,but also make the sensor have smaller size,thus applying to more sensing fields,such as biochemical fields.Based on the theory of M-Z interference,the theoretical and experimental research for the proposed interferometer based on the SMTMS structure is carried out.It can be obtained that the interference of the SMTMS structure occurs between the core fundamental mode and the cladding mode by analyzing the spatial frequency spectrum of the transmission spectrum.(2)The M-Z interferometers based on the TDF are applied in temperature sensing.The sensitivity measured in the temperature range of 20?100? is 0.065 nm/?.To study the relationship between the temperature sensitivity and the diameter of the fiber,the sensing portion of the SMTMS structure was etched and the temperature was experimentally measured.When the TDF of interferometer is etched to the diameter of 18.7?m,the high sensitivity of 2.49 nm/? is obtained,which is approximately 40 times what the sensitivity was before etching.In addition,the high temperature sensing characteristics of the TDF are also studied in the paper,and the length of the interferometric arm is controlled shorter than 5mm to obtain the relatively large detection range,which is suitable for high temperature detection from 100 ? to 1000 ?.Different from the sensors based on interference between core modes,the sensor proposed in this paper has the cladding mode with pure silica as the main waveguide and does not exhibit inflection points when measuring high temperature.The temperature sensitivity was respectively 0.102 nm/? and 0.285 nm/? before or after 850 ?.The experiment shows that the sensor has high sensitivity,compact structure and anti-interference ability.It is suitable for multi-point distributed high temperature detection and has great application value for temperature detection in the extreme environment.(3)The MZIs based on the TDFs were also applied in the refractive index and humidity measurement.The high sensitive refractive index sensor was obtained by etching the TDF part of the SMTMS structures,and the relationship between the sensitivity and the cladding diameter was theoretically simulated and experimentally measured.In the current experiment conditions,the refractive index sensor has the highest sensitivity of 21292.2 nm/RIU in the refractive index range of 1.4?1.4025,and 3436.8 nm/RIU in the refractive index range of 1.336?1.344.According to the referenced literature,the highest sensitivity measured in the range of 1.4304?1.4320 is 19212.5 nm/RIU,when the sensor structure of single mode-thin core-single mode fiber is tapered to 12.5?m.For the high refractive index sensitivity of the micro MZI based on TDF,it can also be applied in the field of humidity sensing.The experimental result shows that the sensor has the sensitivity of 0.112nm/%RH in the range of 40%?70%relative humidity without any moisture sensitive material,and increases to 0.452nm/%RH in the range of 70%?90%,after the high relative humidity of 90%,the sensitivity increases to 1.07nm/%RH.Thus,the sensor presented in the paper has good application prospects in the humidity sensing.In conclusion,the Mach-Zehnder interferometer based on the SMTMS structure is of great application value in the detection field of ocean temperature,salinity,and humidity.