Research On Correlation Demodulation Technology Based On Fiber Loop Ring Down Spectroscopy

In recent years,optical fiber sensor has been widely used in the fields of aerospace,resource exploration,medical diagnosis,biochemical detection due to its advantages of compactness,light mass,anti-electromagnetic interference,etc.With the increasing requirements for measurement accuracy,single transmission-type or reflection-type traditional optical fiber sensor has been unable to satisfy the demands for high sensitivity sensing.Fiber loop ring down（FLRD）spectroscopy technology can amplify the sensitivity of the sensor unit with the advantage of mult-pass cyclic absorption,and has been widely used in micro-detection.However,the traditional pulse-FLRD needs to consider the tradeoff between the pulse width and the fiber loop length,which greatly limits the performance of the system.In this paper,utilizing the combination of the correlation demodulation technology and the FLRD system,the loss in the fiber loop can be measured by detecting the correlated ring down time of the amplified spontaneous emission（ASE）source.With the help of the high autocorrelation of ASE source,the length of fiber loop can be shorten to meters and the response speed is improved by two orders of magnitude compared with the pulse-FLRD.It means that the correlated FLRD system is more suitable for high sensitivity and real-time detection.The research contents and achievements of this paper are as follows:（1）Aiming to solve the problems of the current FLRD system demodulation scheme,the FLRD based on correlation demodulation technology is proposed.The sensing mechanism is derived by Beer-Lambert law and correlation function.The influences of the parameters such as loss,fiber loop length,coupling ratio on the output of the system are also modeled and simulated,which lays a theoretical foundation for the subsequent experimental research.（2）A high sensitive and fast response correlated FLRD system with an ultra short fiber loop is demonstrated,and its demodulation ability of loss is discussed.Firstly,the hardware components of the system are optimized to maximize the performance of the correlated FLRD.A data acquisition and processing algorithm program is further designed.Finally,the loss sensing characteristics of the correlated FLRD system are deeply studied.The response speed of the system is in the order of nanoseconds.The sensitivity and resolution are obtained as0.0152 ns^-1/dB and 2.7×10^-3 dB,respectively.It reveals that the FLRD based on correlation demodulation technology can improve the sensitivity and response speed simultaneously.（3）Utilizing the linear response of the correlated FLRD to the loss,three kinds of intensity-type optical fiber sensors based on bending loss,evanescent field effect and interference principle are demodulated,which focuses on high sensitivity and real-time measurements of both macro-bending parameters and temperature.The resolutions of the system are 0.028 turns and 0.016℃.The sensitivities reach up to 0.0017 ns^-1/turn and 4.025×10^-4 ns^-1/℃,respectively.Besides,the system shows excellent stability.The research results indicate that the FLRD based on correlation demodulation has the advantages of short fiber loop length,high sensitivity,fast response speed,and good repeatability,which has great potential in real-time detection of micro-loss.