Research On Frequency-shifted Interferometry Based Tapered Fiber Cavity Ring-down Sensor

Fiber Cavity Ring-down Spectroscopy(FCRDS)technology is a new fiber sensing method that combines fiber sensing and cavity ring-down spectroscopy technology.It is widely used in many fields for measuring many key parameters and has attached great attention among researchers.However,the traditional FCRDS technology has high requirements on the response speed of the detection equipment,which limits its application and development.As a novel fiber sensing technology,Frequency-shifted interferometry(FSI)technology has higher sensitivity and lower requirements of hardware,and has proved its feasibility in many fields.The FSI-CRD technology combines the advantages of FCRDS and FSI technology to provide a new sensing method with low cost and high precision.Tapered fiber sensor has the characteristics of simple preparation and strong applicability and can be applied to many measurement occasions.However,it often needs to be combined with other sensing technologies to improve measurement sensitivity due to its low sensitivity.In this dissertation,we combined tapered fiber sensor and FSI-CRD technology and studied its principle and application fields.The main contents and results of the dissertation include the following:(1)The development status of tapered fiber sensors is introduced.The principle of fiber cavity ring-down spectroscopy and frequency-shifted fiber cavity ring-down technology are introduced.(2)The principle of the fiber evanescent field is analyzed.The common fiber evanescent field sensor is compared and analyzed.The fusion taper method is adopted as the evanescent field sensor in this experiment because of its simple operation and reproducibility.(3)The sensing principle of frequency shift interference is studied and the theoretical formulas involved in the system are deduced,which provides a theoretical basis for the construction of experimental systems.(4)The FSI-CRD strain sensing system was designed and built.The strain sensing probe was fabricated by the method of melting taper and placed in the ring-down cavity.In the strain variation range of 0-80 ??,the strain measurement sensitivity of 0.00111 dB/?? was obtained.The feasibility of measuring the weak strain by the combination of tapered fiber and FSI-CRD technology was verified.(5)By using a section of fused fiber taper in the fiber ring-down cavity,refractive index of liquids at different concentrations were measured.Since the evanescent field excited by the tapered fiber is very sensitive to surrounding medium changes,the sensitivity of the system is improved.The experimental results show that sensitivities of NaCl and glucose solutions are 3.7798 dB/RIU and 4.2297 dB/RIU,respectively,in the low concentration range.The relative deviation of measured cavity loss is less than 0.4% over 40 min continuous observation,and the system shows good stability.Based on the experimental study of strain and refractive index,the frequencyshifted interference cavity ring-down technique shows good system sensitivity and stability,which proves the feasibility of the technology in these fields.Based on the existing research results,frequency-shifted interference cavity ring-down technology has been widely used in many fields and has important research value and application prospects.