Experimental Study Of Weak Fiber Grating Sensor And Demodulation Based On Chaotic Fiber Laser

Fiber optic sensing has been widely used in power industry, aerospace, chemical environment, biomedicine, intelligent structure, military and other fields, because of its good transmission and sensing properties. The distributed optical fiber sensing system based on back scattering has the advantages of simple installation, convenient trend monitoring and so on. The quasi-distributed optical fiber sensing system with fiber grating as the sensing element has the advantages of single point measurement precision, easy networking, low cost and so on. It has an extensive application prospect in the industrial inspection system. Compared with the distributed optical fiber sensing system, quasi-distributed optical fiber sensing system has higher resolution and signal-to-noise ratio and a higher value in the practical application of optical fiber sensing. However, the multiplexing capability, resolution and demodulation difficulty of fiber Bragg grating sensing system has restricted the fiber grating for large-scale network applications.In order to improve the multiplexing capacity and spatial resolution of the quasi-distributed fiber Bragg grating sensor system, a scheme of high precision quasi-distributed optical fiber sensing system based on chaotic fiber laser and the same weak reflection grating are proposed. The scheme is based on the chaotic laser properties that high bandwidth, similar to noise in time domain, autocorrelation curve similar to delta function shape. A new method of grating sensing information demodulation is proposed. With the new sensor system and the demodulation scheme, the precision and the multiplexing ability of the sensing system improve at the same time while reducing the cost and the difficulty of demodulation.The main work of this dissertation is as follows:(1) The types and application fields of optical fiber sensor is introduced based on the brief introduction of the principle of the fiber optic sensing system. According to the fiber grating coupled mode theory, the application field of fiber grating sensors is introduced. The development status and problems of the grating multiplexing scheme and demodulation technology in the current grating sensing system are analyzed.(2)In view of the problems that exist in the current sensing system: the low sensing accuracy and multiplexing ability, the high demodulation cost, we introduce the emergence and development of chaotic optical fiber laser, application in secure communication, laser ranging, etc. We proposed an experiment to realize the quasi-distributed optical fiber Bragg grating sensing scheme based on chaotic laser. According to the characteristics of the sensing system, the emphasis is to figure out the system principle of optical fiber sensing scheme improving the multiplexing capability and accuracy. The simulation using the MATLAB software for the proposed system of the multiplexing and demodulation method are carried out. The multiplexing capacity can reach 700 when the grating reflectivity is 0.001%. The simulation results accorded with the theoretical conclusion. It proved that the sensing system is feasible.(3)The strain sensing system based on chaotic fiber laser is experimentally demonstrated. The laser signal generated by chaotic fiber laser is divided into 90:10 two parts. The 10% of the light is received by photoelectric detector is the reference optical signal, and the other 90% light enter fiber Bragg grating through the optical circulator, and the detector receives optical signal reflected from the grating. Both optical signals are used to carry out the cross-correlation algorithm, the grating strain information and precise positioning can be achieved through cross-correlation peak amplitude and delay for synchronization. Experimental procedure is simple, and experimental strain measurement accuracy is 12u?, the strain sensitivity 1.21 pm / u?.(4) The experiment setup of fiber optic sensing experiment system is composed of the same weak reflection grating, and five fiber grating with 10 cm interval and the reflectivity is 10%. Multi-point synchronous measurement experiment carried out corresponding to variation information. The correlation results by the sample reference signal and sensor signal in the time domain can synchronous demodulation all grating wavelength information. The demodulation technology converts the measurement for wavelength in the frequency domain to the time serious in the time domain. The difficulty of the sensing cost and demodulation’s difficulty greatly reduced. Multiple-points synchronous detection for the strain measurement has been achieving. The accuracy can reached 14u?, and the spatial resolution reached 7mm.