Fabrication Of Optical Fiber Michelson Interferometer Microcavity By Femtosecond Laser And Its Refractivie Index Sensing Property

Optical fiber refractive index(RI)sensors posses many unique advantages in marine monitoring compared with the traditional electrical sensor,such as high sensitivity,compact structure,corrosion resistance and immunity to electromagnetic interference,etc.In this paper,a novel optical fiber RI sensing structure based on Michelson interferometer(MI)was proposed through fabricating a microcavity in the optical fiber by using femtosecond laser-induced water breakdown technology.This structure not only exhibited ultrahigh RI sensitivity,but also obtained increased accuracy of RI measurement by compensating temperature crosstalk,meanwhile,compactness of reflection structures.The research work has been summarized as follows:Firstly,a fiber RI sensing structure with MI microcavity in the multi-mode optical fiber(MMF)was designed.Refractive index sensing property of this structure,which combined MI and Fabry-Pérot interferometer(FPI),was investigated theoretically,and the temperature crosstalk compensation function of FPI interference peak for refractive index measurement of MI interference peak was investigated,too.At the same time,the properties of refractive index and temperature sensitivity,and the influence of the size along with relative position of the microcavity to the reflection spectrum of this structure were simulated by using MATLAB software in order to determining the optimal parameters of the microcavity.Secondly,the optical fiber RI sensing structure was fabricated by using femtosecond laser-induced water breakdown technology and optical fiber fusion technology,Refractive index and temperature sensing properties were investigated experimentally.The MI interference peak exhibited extremely ultrahigh RI sensitivity of –11523.8nm/RIU in the range of 1.333-1.3387,and temperature sensitivity of 43.9pm/oC in the range of 18.75-55.38 oC in the air.At the same time,a increased accuracy of RI measurement could be obtained by compensating temperature crosstalk through measuring the RI and temperature sensitivity of FP interference peak.Finally,a optimized fiber sensing structure with MI microcavity in the optical fiber structure of single-multi-single(SMS)mode cascaded fiber was designed by using Beam PROP module of Rsoft software based on Beam Propagation Method(BPM)to optimized the fiber sensing structure above.Refractive index and temperature sensing properties were investigated exp erimentally,which exhibited the almost same high RI sensitivity with the structure above,furthermore,its spectral quality was improved.The result proved that the optimized structure not only ensured the extremely ultrahigh RI sensitivity,but also enhanced the role of MI effectively,which made the spectral quality and sensing function better.The encapsulation methods of fiber RI sensing structure with MI microcavity were studied at last.Two encapsulated fiber RI sensing probes by using cylindrical stainless steel tube and quartz glass tube,respectively,with length of 2cm and diameter of 2mm were obtained,which exhibited small volume,light weight,corrosion resistance and high mechanical strength,etc.The fiber RI sensing probe s will be adapt to the RI detection of the sea water and exhibit a widely practical application prospect.