Research On Multimode Interference And Refractive Index Sensing In Microstructure Fiber

With the advancement of society,sensors are becoming more and more common in life.Among them,fiber optical sensors have the characteristics of small size,low power consumption,anti-electromagnetic interference,etc.,so,they have a wide range of applications in temperature,refractive index,displacement,stress,and acoustics.At present,sensors are mainly divided into 3 types on the market,including interference type,grating type,and resonant cavity type.Fiber optic sensors based on interference have the advantages of easy integration and fast response speed,and have been more widely used in applications.Nowadays,the interferometer-based sensors with simple structure,such as single mode-multimode-single mode,core offset splicing,etc.,have many reports in the references.Most of the structures are made by fusion splicing three sections of optical fibers,and there are few structures made by fusion splicing more than three sections of optical fibers in the reports.Therefore,we simulate the multi-segment fiber core offset splicing structure and the multi-polymer microsphere structure,and studies the high-sensitivity refractive index sensing of these two structures in this paper.The main work of this paper is as follows:Firstly,we propose and numerically investigate a fiber-optic sensor comprised by core-offset splicing many equal-length single-mode optical fiber segments.In principle,due to the core-offset between fiber segments,an open cavity structure is formed,it is beneficial to enhance the interaction between the transmitted light and the external environment.The emergence of high-order modes in the cladding and air enhances the multi-mode interference in the structure.So,the structure can be applied to the field of sensing.The light scattering is enhanced in the structure because of a small core-offset distance between adjacent fibers(in this paper,it is the distance between the core center line of the first section of the optical fiber and the boundary line of the outer surface of the second section of the optical fiber),so,the degeneracy is reduced and the free-spectral-range limitation is overcome,resulting in the large wavelength tuning range.Using the controlled variable method,we optimize the length of the fiber,the core-offset distance,and the number of fiber segments.Finally,up to 832 nm/RIU could be obtained based on fiber-optic sensor in which the core-offset distance is 4 ?m and the length of fiber is 120 ?m with 10 fiber segments.Secondly,we propose a fiber optic sensor based on a silica taper-assisted multiple polymer microspheres.High-sensitivity refractive index sensing can be realized owing to the multi-beam interference and whispering gallery mode in each microsphere.Scattering and reflection of light in each microsphere enhance the multimode interference;the whispering gallery mode with certain resonant wavelength excited in the microsphere futher enhances the transmission spectrum with high contrast.In addition,the irregular transmission spectrum corresponds to a larger wavelength tuning range,and there is a large shift in the transmission spectrum,because the transmitted light between the microspheres is greatly affected by environmental factors,thereby the high sensitivity of refractive index sensing can be realized.Using the controlled variable method to optimize the diameter of the microspheres,the distance between the microspheres and the number of microspheres.Up to 846 nm/RIU can be obtained based on the taper-assisted 2-sphere sensor in which the microsphere diameter is 15 ?m and the distance between the microspheres is 6 ?m.We know that part of the scattering light in transmission causes more loss due to a larger scattering angle in the case of a double microsphere sensor.If the scattering light which have been lost were converged into the structure,the multi-beam interference and whispering gallery mode would be enhanced.It is beneficial to enhance the refractive index sensitivity.Therefore,another two microspheres are placed on both sides of 2-microsphere sensor to recollect those large-angle scattering light.After optimization,the refractive index sensitivity is increased to 1296.3 nm/RIU.