Research Of Microwave Demodulation Techniques For Interfermetric Fiber Sensing Of Strain

In recent years,optical fiber sensing technology has attracted extensive interests of the academe and the industry,due to its great advantages such as small volume,light weight,resistance from electromagnetic interference and so on.Especially,the fiber Bragg grating(FBG)sensor,due to its remarkable sensing ability and multiplexing capacity,plays an important role in various applications,which makes the creation of highly-efficient demodulation techniques of FBG a research hotspot among the academe.At present,several FBG demodulation schemes have been proposed,however,these approaches have some intrinsic problems such as high cost,slow responding speed or poor stability,which significantly limits the wide applications of FBG sensors in engineering field.Microwave photonics is an emerging interdisciplinary,which combines the advantages of optical fiber technology and mature microwave technology.It has been applied in many aspects such as signal generation,sensing technology,novel radar and so on.Compared to the conventional optic fiber sensing schemes,microwave photonics based fiber sensing system has better disturbance-resisting capacity.When combined with highly-mature microwave technology,this novel optic fiber sensing system can achieve much fast signal processing speed,and lower cost at the same time.In such background,novel fiber sensing systems based on microwave demodulation technology are proposed in this paper.Deep theoretical and experimental researches on how to apply such sensing systems to the sensing of fiber parameters are investigated.The main achievements are as follows:(1)A strain demodulation system of FBG sensor based on an M-Z interferometer structure in microwave domain is proposed.In this novel system,microwave demodulation technology and the chromatic dispersion of the M-Z interferometer are used to transfer the wavelength shift of FBG into the intensity change of the interference between microwave signals,so the wavelength shift of FBG can be demodulated by interrogating the intensity change of the output signal.This FBG demodulation system has advantages of great sensitivity,besides,it realizes over 2000 ?? strain sensing on the FBG.(2)A vibration FBG sensing system based on the microwave M-Z interferometer structure is designed and introduced.The interferometer makes a vibration signal modulated to the output interference signal,and the vibration frequency can be directly monitored.This vibration sensing system has wide sensing range,achieving the vibration interrogation of the frequency from 80 Hz to 6 kHz.(3)The novel optical fiber sensing system based on an optoelectronic oscillator(OEO)is verified.By monitoring the frequency shift of the output oscillation signal in the OEO,the parameter that is applied on the transmission fiber can be demodulated,which realizes the optical fiber sensing characteristic of the OEO.This OEO based fiber sensing system has realized temperature monitoring with a sensitivity of 2.05 kHz/? and the linearity reaches 0.9914.Besides,a novel fiber strain sensor based on an OEO system with two FBG is put forward,and the feasibility of this sensing system to be used as a FBG strain sensor is theoretically analyzed.The novel microwave photonic sensing systems proposed in this paper can efficiently realize measurement of several parameters which are of great stability and flexibility,and the sensitivity can be easily tuned by changing the frequency of microwave signal.Therefore,the novel sensing technique can meet the demands of many applications.