Sensing Theory Of Multi-core Fiber And Its Application For Vital Signs Monitoring

The transmission capacity of single-mode fiber can not meet the requirements of the fast-growing bandwidth and transmission rate.As a result,space-division multiplexing(SDM)technology based on fiber is studied,which promotes the development of multi-core fiber.Currently,multi-core fiber has attracted much attention in the field of optical fiber sensing due to its high density,small size,and high sensitivity.The progress of photonic device manufacturing technology will provide better multi-core fiber and expand new applications.Therefore,the research of multi-core optical fiber is of great significance to the development of optical fiber sensors.This dissertation mainly studies the mathematical sensing theory and application of multi-core optical fiber(twin-core fiber,seven-core fiber).Firstly,the coupling-based and interferometer-based multi-core fiber sensors are proposed and studied theoretically.Secondly,the sensing characteristics of the two types of sensors are studied by numerical simulation and experiment,respectively.Finally,a multi-core fiber sensor is used for measuring vital signs(heart rate and respiration rate).The main innovations and achievements of this dissertation are as follows:1.We demonstrate the coupling-based multi-core(twin-core and seven-core)fiber sensors both theoretically and experimentally.The mode-coupling theory of these kinds of sensors is further developed and established.Then,the corresponding numerical simulations and experiments are carried out.(1)Based on the asymmetric two mode-coupling theory,the temperature and refractive index sensing formulas of twin-core fiber are derived.The corresponding experiments are carried out with coupling-based twin-core fiber sensors,which are in good agreement with the theoretical formula we derived.The corresponding sensitivities of refractive index and temperature are 595.33nm/RIU and-138pm/?,respectively.(2)The weak coupling theory of symmetrical seven-core fiber is established.Different from the available publications,it assumes that the phases of the center core and outer core are mismatched and affected by the external environment.The analytical solutions of the mode field amplitudes of the center and outer core are obtained based on the above assumption.The relationships between the wavelength shift and the sensing variables(temperature and refractive index,etc.)are derived by the differential method.Besides,a numerical simulation model is established to analyze the light field distribution of the samples with different waist diameters.The experiments of the surrounding refractive index sensing characteristic and temperature sensing characteristics are carried out.The corresponding sensitivities of refractive index and temperature are714.76nm/RIU and-124.63pm/?,respectively.The results are consistent with the theoretical analysis.The above results show that the coupling-based twin-core and seven-core fiber sensors can be used as high refractive index sensitive sensors.2.We demonstrate the interferometer-based twisted seven-core fiber sensor and composite air hole twin-core fiber sensor.(1)The intensity formula of Mach-Zehnder interferometer based on a twisted seven-core fiber is derived theoretically.The qualitative relationship between the induced twisted degree and the bending sensitivity is also derived.(2)The experiment is carried out for studying the relationship between twisted degree and bending sensitivity.Besides,the response of external torsion and the temperature of the sensor is also studied experimentally.The corresponding sensitivities of bending,torsion,and temperature are-25.16nm/m^-1,182.74nm·mm^-1·rad^-1,and 0.064nm/?,respectively.The experiment results show that the sensor can be used as a highly sensitive bending sensor.(3)Based on the beam propagation method,the influence of interference length on transmission spectra is simulated by using composite air hole twin-core fiber.The responses of the sensor to directional bending,external torsion,and external temperature are studied experimentally.It is proved that the composite air hole twin-core fiber can be used as a one-dimensional bending vector sensor.3.We demonstrate a dual biconical seven-core fiber sensor for measuring vital signs non-invasively.(1)The working principle and algorithm of the sensor are analyzed systematically.(2)The interferometer-based dual biconical seven-core fiber is fabricated.Both the heart rate and respiration rate measurement of 11 volunteers are carried out.The maximum error of the difference between the average heart rate measured by the two methods is less than 2.1 bpm,and the maximum error of the corresponding respiration rate is less than 3.6 rpm.The errors are within the acceptable range.(3)The stability experiment of the sensor is carried out.The experimental results show that the dual biconical seven-core fiber sensor has the advantages of good stability and a small standard deviation.Additionally,the sensor exhibits the advantages of small size,low cost,and light weight,which provides a new way for measuring vital signs non-invasively.