Research On Sensing Technology Based On Fiber Loop Ring-Down Spectroscopy

Optical fiber sensing is widely used in aerospace,petrochemical and other fields due to its advantages such as strong resistance to electromagnetic interference and simple structure.However,fiber sensing technology also has certain shortcomings,for example,sensitivity is easily affected by optical power fluctuations and absorption distances,so it is of great significance to develop new fiber sensors.Cavity ring-down spectroscopy was originally used to measure the reflectivity of high-reflectance mirrors.The repeated propagation of light through the cavity increased the absorption length and thus the sensitivity of the system.Fiber loop cavity ring-down spectroscopy technology combines fiber with cavity ring-down spectroscopy technology,uses fiber as the optical transmission medium,and replaces the highreflection mirror with a fiber ring to form a resonant cavity.It has the advantages of both optical fiber and cavity ring-down spectroscopy such as not being affected by light source fluctuations,high sensitivity,and simple construction.For a fiber ring cavity ring-down system,the number of oscillating peaks output directly affects the accuracy of the final measurement results when the system is stable.Researchers have introduced erbium-doped fiber amplifiers to offset some of the inherent losses in the system,thereby increasing the number of ringing peaks.In current fiber cavity ring-down systems,the common practice is to insert an erbiumdoped fiber amplifier into the ring cavity.However,the way of amplification in the cavity will be affected by gain power fluctuations,and there will be excessive gain and noise interference.Therefore,this thesis attempts to simplify the system by placing the erbium-doped fiber amplifier outside the cavity,and explores the sensitivity of the fiber sensor in this case.In this thesis,the fiber cavity ring-down spectroscopy technology is mainly applied to the measurement of liquid refractive index and optical fiber bending loss.The main research contents include:(1)The principle of fiber loop ring-down spectroscopy technology is described.The development history,basic principles and structural classification of fiber cavity ring-down spectroscopy are introduced.The advantages of fiber loop ring-down spectroscopy technology are analyzed,and the main physical parameter in optical fiber loop ring-down spectroscopy system,ring-down time is emphasized.(2)A fiber loop ring-down system was set up for experiment.Focused on solving the problem of parameter selection for building system components;system parameters were determined through experiments.It is determined that the splitter ratio of the coupler in the ring-down cavity is 1:99,the pulse signal frequency is set to 4.49 k Hz,the duty cycle is 1%,and the system works at the best state when the ring cavity length is about 1160 meters.(3)The corresponding refractive index sensor is designed and manufactured based on the principle of evanescent wave absorption,and the sensor is tested by using fiber loop ring-down system.The experimental results show that when the refractive index is between 1.332 and 1.363,the decay time has a good linear relationship with the refractive index.The sensitivity of the sensor is 50.205?s/RI,the resolution is 1.22E-2RI,the system stability is 0.381%,and the repeat error is 3.89%.(4)The bending loss of single-mode fiber is studied by the fiber ring cavity ringdown system.By winding a single-mode fiber into the form of a ring,the relationship between the diameter of the optical fiber ring and the decay time is studied.The linear relationship between the bending diameter of the optical fiber ring is 13mm—21mm.The fiber ring diameter error measured by the decay time is within 2.01%.