Research Of Optical Fiber Sensor Based On Modal Interference And Stimulated Brillouin Scattering

The sensing technology is the core of the Internet of Things.Its development level determines the development level of the Internet of Things.It is also an important indicator to measure the degree of informationization,technological innovation and the development of country.In recent years,optical fiber sensors have played import roles in biological,chemical,environmental testing and military security,including the single or simultaneous measurements of the temperature,strain,refractive index(RI),liquid level and magnetic field.Compared to the traditional electric sensors,the optical fiber sensor has the advantages of anti-electromagnetic interference,high temperature resistance,corrosion resistance,high sensitivity,etc.It has great advantages in miniaturized and intelligent sensor networks.With the advancement of optical fiber manufacturing technology,fiber-optic sensors based on special optical fibers have demonstrated excellent performance,and have been developed in the direction of high sensitivity,miniaturization,and multi-parameter integration.In this paper,based on the development requirements of high sensitivity,miniaturization and multi-parameter integration of fiber optic sensors and carried out on the basis of previous work,the interferometric fiber-optic sensor based on special fiber and optical fiber sensor based on stimulated Brillouin scattering have been studied theoretically and experimentally.The main research results include:1.A few mode D-shaped fiber is fabricated and its mode characteristics is analyzed.The influence of the external refractive index and fiber grinding depth on the effective refractive index of modes of the D-shaped fiber is investigated,which provide a way to create high sensitivity fiber optic sensors.A novel D-shaped fiber structure combined with a fiber Bragg grating(FBG)for RI and temperature sensing is proposed.Combined with the termperature sensing property of the FBG,the possibility of simultaneously measuring the RI and temperature relies on monitoring the resonance dip of the modal interferometer and the Bragg wavelength of the FBG.Experimental results showed that the RI sensitivity of-31.79 nm/RIU within the range of 1.333-1.428.When the wavelength measurement resolution of OSA is 0.01 nm,the RI and temperature measuremeut resolution could reach 1.4×10-3 RIU and 1.7 ?,respectively.2.A coreless-D-shaped-coreless(CDC)fiber structure for liquid-level sensing is proposed and experimentally investigated.Using the principle that the cladding modes of D-shape fiber is sensitive to the ambient refractive index change,the CDC fiber structure transmission dip wavelength shifts related to the length of D-shape fiber immersed in the liquid.High sensitivity measurement of the liquid level is achieved by monitoring the dip wavelength.The sensitivities for three different liquids with the RIs of 1.333,1.355,and 1.377 are 191.89 pm/mm,208.11 pm/mm,and 213.80 pm/mm,respectively.The temperature influence is further studied.The sensor exhibits a low temperature cross-sensitivity of-0.128 mm/?The proposed liquid level sensor with low cost and easy fabrication is suitable to high sensitivity liquid-level sensing applications3.A optical fiber magnetic field sensor based on D-shaped fiber modal interferometer and magnetic fluid is proposed.The refractive index of the magnetic fluid changes with the external magnetic field intensity,which causes the transmission spectrum of the mode interference structure to shift to realize the magnetic field measurement Simultaneous measurement of the magnetic field and temperature was realized by monitoring the wavelength shift of the two resonance dips at the same time.In the experiment,the magnetic field and temperature sensitivity can reach 99.68 pm/Oe and-77.49 pm/?,respectively.Comparing the sensor with the reported fiber-optic magnetic field sensor,the proposed sensor based on D-shaped fiber is featured with high sensitivity,low cost and compact sturcture,and has application prospects in the field of magnetic field and temperature simultaneous sensing4.A coreless-single mode-polarization-maintain-single mode-coreless fiber structure for twist sensing is proposed.The influence of the length of the coreless fiber on the transmission spectrum of the sensing structure is theoretically analyzed,and the experimental verification is carried out to select the optimal length of the coreless fiber.Experimental results indicate that the extinction ratio of the fiber structure comb spectrum various with twist rate.The maximum sensitivity of 0.34 dB/(rad/m)for range from-240° to 360° is achieved.Temperature measurement was further achieved by monitoring the dip wavelength shift of the transmission spectrum,and the temperature sensitivity is 41.89 pm/?5.We propose a novel M-shaped single mode fiber(M-SMF)sensor based on stimulated Brillouin scattering for simultaneous temperature and strain measurement The longitudinal acoustic modes in this fiber are studied by simulation.Compared with the standard single-mode fiber,the calculation results shows that multi-peak Brillouin spectrum in M-SMF is contributed by the multiple acoustic modes.The sensing characteristics of the M-SMF under different temperature and strain conditions are further studied.The proposed fiber enables discriminative sensing of temperature and strain at an accuracy of 0.47 ? and 12.3??,respectively.