Fiber Sensors Based On Eccentric Hole-assisted Dual-core Fiber

Sensors based on the traditional optical fiber platform have become the major role in the fiber sensors because of their low cost,simple structure and easy fabrication.However,with the increase of application fields,sensors based on traditional optical fibers face many challenges,such as low sensitivity,being unable to build in-fiber microfluidic channel,and so on,which have limited the application of optical fiber sensors in new fields.Therefore,the researchers have turned their attentions to special optical fibers with flexible structure and special performance.Using special optical fibers to realize optical fiber sensors with unique advantages or superior performance have become a research hotspot.In this thesis,an eccentric hole-assisted dual-core fiber?EHADCF?is designed,and several distinctive optical fiber sensors based on the EHADCF are realized.This study is of great significance to promote the development of optical fiber sensing technology.The main works of this thesis are listed as follows:An eccentric hole-assisted dual-core fiber?EHADCF?with multi-core,few-mode and air-hole is designed,and the coupling characteristics between the cores and the mode characteristics of the fiber are studied detailedly using the coupled mode theory to determine the parameters of the EHADCF,and the fiber samples are characterized.This fiber have strong resonance coupling between the two cores,and can support both single-mode and dual-mode transmission in the 1310 nm band.Its large air-hole structure is suited to the construction of microfluidic channel in the fiber.An in-fiber directional coupler based on the EHADCF is realized,and a vector bending sensor is realized by using the EHADCF-based directional coupler.The refractive index of the EHADCF is modulated by bending to cause the change of the coupling coefficient between the two cores,which in turn leads to the shift of the transmission spectrum.The bending sensitivities of the sensor can reach-15.95 nm/m^-1 at 0obending direction and 14.87 nm/m^-1 at180obending direction,respectively.Moreover,the bending sensor is insensitive to the change of the surrounding refractive index.A microfluidic fiber refractive index sensor based on the EHADCF is realized.A high-frequency CO₂ laser is used to fabricate microholes on the EHADCF-based directional coupler to build a microfluidic channel.The liquid is injected into the microfluidic channel to modulate the effective refractive index of the fundamental mode in suspended core.Due to the liquid,the coupling coefficient and the phase matching wavelength between the two cores are changed,which cause the shift of the resonance dip.The refractive index sensitivity can reach 627.5 nm/RIU in the refractive index range of 1.335?1.385.The EHADCF-based resonance type temperature and humidity sensors are respectively realized by integrating functional materials into the air-hole of EHADCF.Due to the high thermo-optic coefficient of the refractive index matching liquid or the humidity-sensitive characteristic of the gelatin,the effective refractive index of the mode in suspended core can be modulated by temperature or humidity,which causes the transmission spectrum to shift.The proposed temperature sensor has a larger measurement range than other liquid-filled resonance coupling type temperature sensors,and its temperature sensitivity is-562.4 pm/oC in the temperature range of 10oC?100oC.The average and maximum sensitivities of the proposed humidity sensor repectively are-7.005 nm/%RH and-18.94 nm/%RH in the relative humidity range of 70%?90%.A Mach-Zehnder interferometer refractive index sensor based on EHADCF is realized by offset splicing.Relying on the microfluidic channel,the liquid can modulate the phase of the beam in the suspended fiber core,which causes the shift of the transmission spectrum.Since the suspended core supports multiple modes,the Mach-Zehnder interference is generated by different modes in different wavelength ranges.The order of the interference dips can be identified depending on the interference discontinuous zone.It can help solve the problem that the target interference peak cannot be tracked during the demodulation process of the interferometric fiber sensor.