| As an important branch of the optical fiber industry,optical fiber sensing technology combines sensing and transmission.It can realize long-distance,large-scale sensing and networking,and carry out accurate and fast detection of different parameters such as temperature,vibration,and strain in the multiple measuring points.Therefore,it was widely used in structural engineering inspection,bridge vibration monitoring,aerospace,medicine and many other fields.Fiber Bragg Grating(FBG)has many remarkable advantages compared to other traditional sensor such as smaller volume,preservative,anti-electromagnetic interference and so on.At the same time,increasingly complex testing environment and increasingly demanding engineering needs have put forward higher requirements for the demodulation distance,multiplexing capacity,and cost of use of FBG sensing systems.In this paper,based on incoherent optical frequency domain reflectometry(IOFDR)and optical feedback technology,we combine the generation principle and laser characteristics of chaotic laser and study the demodulation principle,algorithm and other related issues,realize a high spatial resolution,large capacity,identical weak grating demodulation system.The main research contents are as follows:(1)Analyze the sensing principle of FBG and the principle and characteristics of chaotic laser.Study the effects of various factors such as Rayleigh scattering and multipath reflection on the multiplexing capacity of optical fiber sensing system.It is proved by simulation that the system multiplexing capacity can be maximized by selecting the grating reflectivity appropriately.Analyzing the principle of chaotic laser,Simulating and verifying the linewidth characteristics of distributed feedback lasers in chaotic state.Finally draw a conclusion that by using chaotic laser to broaden the linewidth of the laser,then compared with linewidth of the laser with noise modulation,and choose a better solution.(2)Building a weak fiber Bragg grating demodulation system based on incoherent optical frequency domain reflection technology.In the aspect of optical path,driving the DFB laser output chaotic laser by using optical feedback technology.Using continuous frequency-sweep signal whose sweep cycle is 1ms frequency bandwidth is 0.5GHz~1.5GHz to modulate the chaotic laser.Erbium-doped Optical Fiber Amplifier(EDFA)is used to amplify the intensity of laser.The sensing part is composed of an optical circulator and FBG array that is responsible for collecting the light reflected from gratings.In the aspect of circuit,designing the thermostat and drive circuit of DFB laser,the reflected light will be converted into electrical signal,and then a wideband mixer and low-pass filter are designed to process the final signal.(3)Researching the weak FBG demodulation algorithm based on incoherent optical frequency domain reflectometry to obtain the location information and wavelength information of every grating.Wavelength-temperature characteristic curve of DFB laser is applied to standardize the center wavelength of the output light.Gaussian fitting algorithm is used to splicing and peak finding the center wavelength of the grating.(4)Main performance test of the demodulation system.Research on the relationship between the length of fiber front of the gratings and interval between adjacent gratings is implemented.Eliminating the influence of fence effect on FBGs demodulation.The long distance demodulation capability of the system is verified by adding 450 m delay-fiber before FBGs.The large capacity demodulation capability of the system is verified by demodulate 3640 identical weak FBGs with 0.01% Reflectivity and space interval 10 cm.In a static temperature experiment,we choose 10 FBGs from the front end and tail end of the 3640 FBG arrays with the same wavelength of 1550.5nm,reflectivity of 0.01%,space interval 10 cm,the demodulation linearity,demodulation accuracy and stability of the demodulation system are analyzed and verified. |
PDF Full Text Request