Fabrication And Characterization Of M-Z Fiber Bending Sensor Based On Intermodal Interference

Among various optical sensors,sensors based on the intermodal interference principle have become a popular research direction due to their excellent performance,high integration and scalability.In this paper,the experiments and analysis of intermodal interference sensors based on the Mach-Zehnder（M-Z）principle are carried out,using two ways to achieve mode interference,namely,deformation of the fiber structure and enhanced core mismatch.（1）A slightly bent core can play the role of beam splitting and coupling in the fiber,and when the core is slightly bent part of the light in the core will be coupled into the cladding and coupled back to the core at the next slight bend.（2）Due to the mode-field mismatch effect,multiple fibers with different core diameters are fused together to excite higher order modes at the splice,which eventually cause inter-mode interference with the core base mode.The above two interference structures are more sensitive to changes in the external environment and can effectively improve the sensitivity of the fiber bend sensor.This paper proposes a single-mode-deformed multimode-single-mode（SDMS）structure,which consists of a deformed multimode sandwiched between two segments of a single-mode fiber.Mach-Zehnder Interferometer（MZI）into the structure,which in turn is superimposed on the Multimode Interference（MMI）inherent in multimode fibers to form a composite intermodal interference.Experimental results show that the asymmetric structure and the composite inter-mode interference improve the bending sensitivity of the structure,with the highest bending sensitivity of-23.2 nm/m^-1 and-11.64 nm/m^-1 for the two peaks,which is 10times that of ordinary SMS.the temperature sensitivity of the structure is-51.2 pm/℃and-19.7 pm/℃,and simultaneous two-parameter measurements of temperature and curvature can be achieved by matrix demodulation.In addition,this paper proposes a cascaded single-mode-multimode-no-core-multimode-single-mode（SMF-MMF-NCF-MMF-SMF,SMNMS）structure based on No-Core Fiber（NCF）,which is prepared by splicing fibers of different core diameters using a high precision fiber cutting and splicing system.The structure is made by using a high precision fusion splicing system to splice fibers with different core diameters.The multiple core diameter mismatch leads to multiple divergences of light at the interface of the fibers with different core diameters,thus filtering out the weak cladding modes and retaining the higher order modes,allowing the core mode to interfere with the higher order cladding modes.The experimental results show that the higher-order cladding mode interferes with the sensing structure,resulting in a high bending sensitivity of-114.74 nm/m^-1,-91.08 nm/m^-1 and-61.10 nm/m^-1 for the three interferometric peaks,respectively.The temperature sensitivities of the structures were lower,at 12 pm/℃,16 pm/℃and 16 pm/℃,respectively.By constructing a composite intermodal interference in the fiber as well as an intermodal interference with the involvement of higher order modes,the interference inside the fiber is more affected when subjected to bending action,thus effectively increasing the bending sensing sensitivity.In addition,the prepared sensor is highly explorable due to the flexibility of the modes and has great potential for application.