The Effect Of Supermode Interference In Tapered Multicore Fiber And Its Sensing Applications

An important way of optical fiber sensing is to realize the detection of environmental physical quantity through the interaction between optical fiber evanescent field and environmental physical quantity to change the characteristics of the transmitted light field.By tapered the fiber,evanescent field is generated around the fiber to increase the sensitivity of fiber sensing technology,which is a hot spot in the research of fiber sensing technology at present.In addition,evanescent field can capture and manipulate micro and nanoparticles,which plays an important role in the study of microscopic particles.Therefore,it is of great significance to study the characteristics of fiber evanescent field.For standard single-mode fiber(SMF),the mode field distribution outside the core and cladded boundary is generally rotationally isotropic after tapering,and the electric field intensity decays exponentially along the transverse direction.However,multi-core fiber(MCF)has very different waveguide structures after tapering.For seven-core fiber(SCF),during the taper process,weakly coupled SCF changes to strongly coupled SCF when the periodically distributed side core enters the effective mode region of the central core,weakly coupled SCFs become strongly coupled SCFs,the hexagonal evanescent field generated by supermode superposition in the taper multicore fiber is obtained by theoretical analysis and numerical simulation,using weakly coupled seven-core fiber,we fabricated strongly-coupled tapered seven-core fiber with different diameters by hydrogen heating and taper method.A laser with a central wavelength of 1550 nm was inserted into the fiber,and the upconversion nanoparticles were adsorbed by the optical attraction generated by the strongly-coupled seven-core fiber.This hexagonal evanescent field phenomenon can be used to increase the sensing parameters of fiber integrated devices.On this basis,based on the principle of supermodel interference,the ambient temperature and refractive index were detected by supermodel interference.The specific research contents are as follows:1.A multi-core optical fiber sensor with simple structure is proposed.The distribution of the mode field and the mechanism of supermode interference spectrum are studied numerically and verified experimentally.In the experiment,a weakly coupled seven-core fiber was fused at both ends of the single-mode fiber,and the single-mode fiber was uniformly taper by hydrogen flame heating,the area of the middle core and the side core is superimposed to produce supermodel interference,at this point,the weakly coupled multicore fiber becomes strongly coupled multicore fiber,which has the advantage of parameter structure adjustability compared with the strongly coupled multicore fiber prepared directly.2.The SCF evanescent field was found to be split.Supermodel interference produces a hexagonal distribution of evanescent field,so its optical attraction should also be unevenly distributed.The upconversion nanoparticles were successfully adsorbed onto the surface of the tapered seven-core fiber by optical attraction,976 nm laser was fed into the fiber to excite the emission peaks of nanoparticles at 582.59 nm and 782.93 nm,using1000 X CCD,the nanocrystals on the surface of SCF were observed to form 60° symmetric linear arrays,the particle clusters with a period of about 1.53 μm and a size of about 3 μm were measured3.Tapered SCF is used to measure the ambient temperature and refractive index.The supermode interference spectrum generated by different taper diameter is studied.It is found that with taper SCF supermode interference,when the temperature and refractive index increase,the transmission interference peaks drift towards the long wavelength direction.The optimum extinction ratio of the spectrum is 47.4 d B,and the sensitivity of the temperature sensor is 840 pm/℃;for the refractive index sensor,the sensitivity is 392nm/RIU.