Highly Sensitive Multimode Interference Mechanism For Coreless Side Polished Fibers In Sensing Refractive Index

Owning to the advantages of small size,electromagnetic immunity,and chemical inertness,refractive index(RI)fiber sensors have been widely applied into many fields,such as bio-medical detection,chemical detection,environmental monitoring,and food safety.Among refractive index fiber sensors,the multimode interference(MMI)based RI fiber sensors,such as single modemultimode-single mode fiber,single mode-photonic crystal fiber-single mode fiber and single mode-small core single mode fiber-single mode fiber,giving the additional advantages of ease of fabrication and low cost have been intensely studied recently.In this paper,a novel MMI based RI fiber sensor structure named coreless side polished fiber(CSPF)is proposed.The fabrication,operating mechanism and RI sensing characteristics of the CSPF structure are studied:Firstly,the CSPF structure are introduced,and the composition and operating mechanism of the wheel side polishing system for CSPF fabrication are discussed.Then,the MMI mechanism of the CSPF structure is analyzed by using the beam propagation method(BPM).Moreover,a method for verifying the interfered multiple modes is proposed by using the mode coupling theory,finite element method(FEM)and Fourier transformation.By using the simplified MMI theory and the theory of evanescent field,the RI sensitivity of the CSPF is analyzed and then derived as a function of surrounding RI,wavelength and difference of effective refractive indexes.Moreover,by using the results from Rsoft simulations and RI sensing experiments,the influences of surrounding RI and wavelength on refractive index sensitivity are studied.Furthermore,by comparing the performance of CSPF to that of other RI fiber sensors,the applications of CSPF are discussed.The experimental results show that the CSPF can achieve a highest sensitivity of 28000 nm/RIU in the RI range of 1.442-1.444.For the reason that the RI of common organic solution is about 1.44,the CSPF will serve as a good candidate for bio-chemical detection,environmental monitoring,and food safety.At the end,the main content and conclusions of this CSPF study are presented;the novelties,deficiencies and development direction of this study are discussed.Comparing to the typical side polished fiber,it's innovative to apply the MMI theory to the side polished fiber.In this paper,the coreless side polished fiber is theoretically,numerically,and experimentally investigated,and then the self-consistent conclusions are obtained.Meanwhile,the CSPF with a good repeatability and an ultra-high RI sensitivity will offer a good platform for applications in other fields.