Research On Interferometric Sensor Based On Optical Fiber Taper Structure

With the improvement of optical fiber sensor technology,many kinds of intermodal interference based sensors has become the hot topic of research because of their features such as simple structure,flexible design,high sensitivity,fitting for hard conditions.Interferometric fiber optic sensors have great practical significance and potential applications.In this paper,new structures of interferometric fiber optic sensor are designed based on intermodal interference principle.The interference and sensitivity to outside parameters are studied from both the theoretical and experimental aspects.The main researching works are listed as the following.(1)A novel dual-parameter optical fiber sensor based on a single-mode fiber(SMF)down-taper and multimode fiber(MMF)is proposed and demonstrated.When the external environment changes,the interference dips formed between core and different cladding modes exhibit different response.Select two interference dips as the measurement point,simultaneous measurement of temperature and liquid level can be achieved with double wavelength matrix.(2)Magnetic field sensing is realized by sensor based on the cascaded fiber taper and MMF.By using the tunable refractive index(RI)property of magnetic fluid(MF),magnetic field measurement can be realized by detecting the spectrum under different magnetic field intensities.The results show that there is a good linear relationship between wavelength shift and magnetic field for a range of 0 to 20 mT.In the range of 0 to 12 mT,the transmission loss change linearly with magnetic field.When the magnetic field continues to increase,the transmission loss variation diminishes gradually and tends to be constant finally.(3)A dual-parameter fiber sensor based on an intermodal interferometer(IMI)with an inline embedded fiber Bragg grating(FBG)is proposed.The IMI is formed by cascading a down-taper structure and a spherical-shaped structure.Due to the different wavelength shifts of the IMI and FBG to temperature,liquid level and RI,simultaneous measurement can be achieved.Experimental results indicate a good linear relation between the wavelength shift and external parameters and a relatively high sensitivity can be realized.(4)An optical fiber magnetic field sensor using intermodal interferometer coated by MF is proposed.The interferometer consists of down-taper and spherical structure.Two interference valleys of the interferometer under different magnetic field intensities have been experimentally analyzed.The experimental results show that the sensor has high magnetic field sensitivity.