Investigation Of Sensing Based On Fiber Loop Ring-down Spectroscopy Technique

Fiber optic sensing technology has many advantages such as its low price,lightweight and extended period of use.However,the power fluctuation and low precision sighificantly limit its application.As a spectral absorption measurement technique in the time domain,the cavity ring-down spectroscopy makes up the defect of wave instability in optical fiber sensing measurement.In particular,the fiber cavity ring-down spectroscopy,which combines the cavity ring-down and the optical fiber sensing technology,has become a new method in the field of optical fiber sensing.At present,it has been reported in the fields of micro-detection,temperature and magnetic field sensings.In this thesis,the Cavity Ring-down Spectroscopy(CRDS)and Fiber Loop Ring-down Spectroscopy(FLRDS)are demonstrated.Meanwhile,their development history,research progress and classification are also introduced.The merit and demerit of these technologies are discussed.Then the Beer-Lambert law and the operating principle of the FLRDS system are presented,and the advangtages of the FLRDS and factors that affect the sensitivity are studied.The main devices of the FLRDS and their principles are introduced.The amplification principle,gain and noise characteristics of erbium-doped fiber amplifier are discussed in detail.On the basis of theory above,a measurement system based on fiber ring-down cavity with tunable cavity gain is proposed,and the parameters of the measurement system is optimized experimentally.It solves the problem that the polarization state is difficult to measure,and achieves high sensitivity and stability effectively.It provides a feasible and practical solution for the Optical rotationmeasurement of water solutions.A misplaced interference structure based on the optical fiber cavity ring-down is also designed and prepared.Its structural parameters are optimized experimentally.The Mach-zender sensing system based on misplaced interference structure is established.The solution and temperature characteristics of sensing system are analyzed by sensing experiments.The innovations of this thesis:(1)A low-gain,low-noise erbium-doped fiber with a length of 2 m is used to make an erbium-doped fiber amplifier,compensates the loss in the cavity.The significance advangtage of this method is that it is not necessary to choose erbium fiber as EDFA for communication band because of its tunable intracavity gain.Then the influences of the pulse curve and quantity in the case of intracavity,external magnification and no amplification are studied.The results show that the pulse intensity for the intracavity signal amplification is 2 times of that for the cavity amplification,and is 4 times of that for the no-signal amplification.Meanwhile,the number of the pulse peak for the intracavity signal amplification is more than that for the other two cases which benifits the data acquisition and improvement of the measurement accuracy and efficiency.(2)Three different interferometer structures with different misplacement(D)and interference lengths(L)are designed and prepared.The experimental results show that the interference waveforms vary with the changes of D and L.The optimal values of D and L used in this paper are 3.75 ?m and 4 cm,respectively.(3)The measurement system detects the attenuation time of the signal rather than the light intensity,so the stability of the light source has little effect on the signal detection of.The ring-down time(?)decreases wiht the increasing of the polarization angle from 0° to 90° with a step of 9°,and the sensitivity of 4.05 ?s/° is achieves.According to the solution concentration and temperature sensing experiments,the concentration detection limit of 0.0014 g/mL and the temperature sensitivity at 1.83 ?s/? are realized.