Investigation Of Vector Sensing Performance Of Non-centrosymmetric Single-mode-multimode-single-mode Fiber Structure

SMS fiber structures,which classified as interferometric sensors,offer the advantages of easy and low-cost fabrication and relative simplicity of integration with other optical devices.In this thesis,vector sensors based on non-centrosymmetric SMS structure were fabricated focused on the recent surge in vector sensor demand.The specific condition of SMS structure for detecting vector parameters was investigated.The wheel polishing method and femtosecond laser direct writing technique were adopted to modulate the external shape and internal refractive index distribution of the MMF,which makes the improved non-centrosymmetric SMS structure capable of measuring directional twist and vector bending.The key contributions of this thesis include:1.Mode propagation analysis method is utilized in order to investigate the MMI effect as well as the self-imaging phenomenon that occurs in the MMF of the SMS fiber structure.The mode field distributions and coupling losses at varying propagating distances were theoretically simulating and calculating using mode propagation analysis method.Classical beam propagation method was used to simulate and calculate the propagation modes in optical fibers as well as to analyze the effects of MMF length,MMF radius and emitted wavelength on the MMI phenomenon.2.The external shape of the MMF was modulated effectively and the birefringence was introduced to the MMF.Five distinct angled side polishing MMF configurations were proposed and designed.The polishing length and depth of angled side polishing MMFs were optimized by simulation.A wheel polishing technique was used to fabricate the designed angled side polishing MMFs,and their polarization characteristics were examined using the polarization scanning technique.The experimental results demonstrated that birefringence has been introduced into the MMFs by asymmetrically modulating the shape of the fiber,and the magnitude of the birefringence is related to the polishing parameters.3.The performance of angled side polishing SMS fiber structures for measuring torsion was investigated.The investigation implies that vector measurement capacity of the SMS fiber structure relies on the existence of birefringence and non-centrosymmetric refractive index distribution of the MMF.The appearance of elliptical birefringence is caused by the combination of circular birefringence and the intrinsic linear birefringence of the fiber,which occurs when the angled side polishing MMFs were twisted.The torsion characteristics of SMS fiber structures based on angled polishing MMF were explored.Experimental results illustrated that the torsional sensitivity is related to the magnitude of birefringence of angled side polishing MMF.And the maximum sensitivity of the SMS structures based on angled polishing MMFs is0.566 nm/（rad/m）.4.Refractive index modification is performed on the core of the MMF using femtosecond laser direct writing technology.The vector bending measurement capacity of the refractive index modulated SMS fiber structure was investigated.The refractive index distribution of the MMF is non-centrally symmetrical because of the spatial refractive index effect caused by femtosecond laser.The polarization characteristics of the refractive index modulated MMF were examined,the experimental result demonstrated that the internal birefringence effect of the MMF was enhanced after femtosecond laser modification.The bending characteristics of the refractive index modulated SMS fiber structure at multiple orientations were also measured.The experimental results show that the proposed structure is capable of vector bending measurement and the maximum bending sensitivity is-45.8 nm/m^-1,discernable bending orientations are 0^°、45^°、90^°、135^°and 180^°.In this thesis,two types of non-centrally symmetrical SMS fiber structures were designed and fabricated by external shape modulation and internal refractive index modulation.The improved structures were applied in vector parameters measurement which broadens the potential applications for traditional SMS fiber structures.The research demonstrates that SMS fiber structures based on refractive index non-centrally symmetric MMFs are expected to apply in novel sensing fields.