| Optical fiber refractive index sensor is widely used in the field of refractive index sensor because of its advantages of small size,light weight,fast response,immunity to electromagnetic interference and so on.Among them,the refractive index sensor based on in-line fiber Mach-Zehnder interferometer(MZI),unlike the traditional MZI,which requires two independent physical interference arms,only needs one fiber to realize the mode interference between the core and the cladding.Among various structures of these interferometers,the core-offset in-line fiber MZI has simple structure and good mechanical strength.And it only needs to fuse single-mode fiber with a core-offset distance by a fiber fusion splicer,which makes it possess the advantages of easy fabrication and low cost.However,optical fiber sensor will be affected by the temperature fluctuation in the sensing measurement,which will cause the cross-sensitive problem between temperature and refractive index,thus affecting the accuracy of refractive index measurement.In order to solve the cross sensitivity between refractive index and temperature,the temperature sensing characteristics of the core-offset in-line fiber MZI and its temperature dependence on refractive index sensing was studied.The research contents and obtained experimental results are as follows:A core-offset in-line fiber MZI was fabricated by a fiber fusion splicer.By observing the interference spectrum of the interferometer,the best core-offset displacement and the arm length of the interferometer were determined to be 6μm and30 mm,respectively.Then,the refractive index sensing characteristics of straight coreoffset in-line fiber MZI at room temperature were studied,and the highest refractive index sensitivity of-30.77nm/RIU was obtained.In order to improve the refractive index sensitivity,we bent the straight interferometer.The results showed that the refractive index sensitivity was significantly improved after bending.When the curvature radius is reduced to 25.42 mm,the refractive index sensitivity can be improved to-115.56nm/RIU.The temperature sensing characteristics of the core-offset in-line fiber MZI and its temperature dependence on refractive index sensing was studied when the temperature changes from 25 to 60℃.The temperature dependence on refractive index sensing of the straight core-offset in-line fiber MZI was analyzed.By subtracting the variation of the temperature dependent refractive index of glycerin solution from the variation of total temperature dependent refractive index measured in the experiment,the variation of the temperature dependent refractive index of the sensor itself was separated,and the equivalent thermal-optical coefficient of the sensor was obtained,and the correction method of the deviation of the temperature dependent refractive index measurement was given.The temperature sensing characteristics of the bent core-offset in-line fiber MZI and its temperature dependence on refractive index sensing was studied.A core-offset in-line fiber MZI with a core-offset displacement of 6μm and an interference arm length of 30 mm was fabricated and bent.The results showed that not only the refractive index sensitivity,but also the temperature sensitivity was improved by bending.As the temperature increased from 25 to 60℃,it was found that the temperature sensitivity increased gradually with the decrease of the curvature radius.When the curvature radius decreased to 25.42 mm,the temperature sensitivity increased from 0.0499nm/℃ to0.1175nm/℃.Within the range of the measurement error,the variation of refractive index of the liquid has little effect on the temperature sensitivity.At the same time,the variation of temperature hardly affects the refractive index sensitivity.Bending the core-offset in-line fiber MZI to a certain curvature radius,the refractive index and the temperature were measured simultaneously by using two interference dips with different sensitivities in the interference spectrum.When the curvature radius was small,the number of interference dips decreased to only one due to the increase of free spectral range.By introducing the interferometer cascaded with FBG,the problem that the number of interference dips is not enough was solved.For an interferometer with a curvature radius of 25.42 mm,the temperature sensitivity of0.1175 nm/°C and the refractive index sensitivity of-115.5646 nm/RIU were obtained,respectively. |
