Research On Fiber-loop Ring-down Based Sensor And The Demodulation System

Fiber-loop ring-down(FLRD)technique was raised at the beginning of this century and has been widely used in the field of optical fiber sensing because of its flexible structure,high sensitivity and no influence from the fluctuation of light source.In this paper,the FLRD technique and its demodulation system are studied.The main research studied is as follows:1.The FLRD system was modeled and simulated.The theoretical model of FLRD sensor was established and the influence of cavity length,cavity's gain and cavity's loss was investigated by simulation,which lays the foundation for the design of FLRD system.2.The influence of the wavelength and pulse width on the performance of the FLRD system was studied.We found that the performance of the sensing system is strongly dependent on the interrogation wavelength and a narrower pulsewidth is helpful to avoid the disturbance of the relaxation oscillation.Furthermore,we found that the optimal interrogation wavelength can be determined by observing the lasing wavelength of the idle cavity.3.Self-imaging effect of the single-mode-no-core-single-mode(SNS)fiber structure was investigated,and based on this effect,we proposed an FLRD interrogated refractive index(RI)sensor based on an SNS fiber structure.We proposed a method to easily calculate the length of no-core fiber(NCF)required in the SNS structure when given external RI,SNS fiber core diameter and self-imaging peak wavelength.After determining the NCF length,the interrogation wavelength and the pulse width,experiments were performed in the RI range of 1.3330-1.3539.And a cubic polynomial fitting was established between RI and ring-down time.In the linear region,a sensitivity of-3271?s was achieved.4.A small package,low cos,real-time FLRD demodulation system was developed,which can realize real-time signal acquisition,calculate and display ring-down time.We also demonstrated experiments and simulations to verify the performance of each module in the system.This study lays the foundation for the miniaturization and instrumentation of FLRD demodulation system.