The Investigation On Optical Sensing Based On Tapered Multicore Fiber

Under the background of the development of optical communication and sensing technology,optical fiber sensing technology appeared immediately,and its rapid development has become a powerful tool for optical communication and sensing.Optical fiber not only has good optical transmission characteristics,but also can directly transmit information for researchers to use.Optical fiber sensors made of optical fibers can convert the advantages of optical fibers into anti-interference and low loss characteristics of optical fiber sensors.Since multi-core fiber and single-mode fiber have very different optical properties,applying multi-core fiber to sensors can not only expand space division multiplexing,but also make various interferometers,gratings,lasers,sensors,etc.In order to improve the sensitivity of multi-core fiber sensors,most sensors will be tapered.For example,the evanescent field distribution when the fiber transmits energy can be changed by tapering to sense changes in the external environment.In this study,a four-core fiber is tapered,and the two ends are connected to a single-mode fiber through a fan-in/fan-out coupler to input and output optical signals.After the taper is drawn,since the diameters of the cladding and the core become very small,and at the same time,the refractive index difference between the two will also decrease,resulting in the exposure of the evanescent field,which is very sensitive to changes in the external environment.High-performance sensors can be made.The angular core excitation scheme is adopted,that is,the light source is incident into one core of the tapered four-core fiber to excite asymmetric coupling modes.This asymmetric mode has a more sensitive response in sensing applications,and the specific studies are as follows:1.A simple four-core optical fiber sensor is proposed.The four-core optical fiber is melted and tapered under a hydrogen flame,and the light source is incident into one of the cores.During the process of the fiber’s tapering,the four cores gradually approach each other and enter each other’s mode field region to generate mode coupling.2.Use the beam analysis method to simulate the transmission of the internal light intensity after the fiber is tapered.The mode field distribution and supermode interference are numerically analyzed.First,it is found that the excited core will be close to its two adjacent cores,and the evanescent fields will overlap,resulting in mode coupling.Secondly,the cores at the diagonal positions that are originally far apart enter the mode field of the three cores,and the mode coupling of the four cores is generated.Demonstrate that asymmetric coupled modes,when excited,produce mode transitions,a physical phenomenon that has not been observed and discussed in multicore fibers.3.The results obtained through the experimental study are compared with the simulation results of the model field,which proves that the theoretical and practical experimental results are unified.That is to say,by photographing the cross-sectional views of the tapered four-core fiber with different diameters,the spatial distribution process of the asymmetric mode coupling is consistent with the numerical simulation results.On the other hand,the light source is input from one fiber core,and four different fiber cores at the output end output signals,and the output spectra of different output ports also reflect asymmetric modes.It is shown that the oscillation spectra of the two cores adjacent to the excitation core are highly similar to each other when the diameter of the taper is 26μm.The output spectral response of the diagonal core is somewhat out of phase with the spectral properties of the excitation core in the long wavelength range of 1450 nm,however,due to the effects of the asymmetric modes The diameter of the pull cone continued to decrease and soon disappeared.4.Tapered quad-core fiber is used for the measurement of liquid refractive index,fiber strain and curvature.The supermode interference spectrum generated by different taper diameters is studied.Using the supermode interference of the taper four-core fiber,when the refractive index and curvature increase,the transmission interference spectrum shifts to the long wavelength direction,and when the strain increases,the interference spectrum shifts to a shorter wavelength.wavelength drift.For refractive index,strain and curvature sensing,the measured sensitivities are 3844.53 nm/RIU,-185.18 pm/μ?,70.24nm/m^-1,respectively.