Phase Shift Grating Sensing Technology And Application Based On Photoelectric Oscillator

Fiber Bragg Grating(FBG)sensors have attracted much attention because of their important application prospects in medical,civil engineering,power industry and homeland security.Fiber Bragg Grating sensors have the advantages of high sensitivity,good stability,low cost and so on.In recent years,the measurement of temperature,pressure,axial strain and other parameters have important research significance.The refractive index is an important index to characterize the optical properties of materials.At present,refractive index sensing technology is increasingly widely used in petrochemical industry,water quality monitoring,food industry,biomedicine and other fields,therefore,FBG has great research and application value in the field of refractive index sensing.In general,the signal demodulation of the FBG refractive index sensors is mostly realized by detecting the changes of optical power or wavelength.However,due to the relatively low resolution and the slow measurement speed of spectrum analyzer,a wide line width of ordinary FBG,and insensitivity to the change of refractive index,the performance of traditional FBG refractive index sensor is not high.In order to improve the sensing resolution and measurement accuracy of the refractive index,phase-shifted fiber grating(PS-FBG)has been developed and used.Due to the existence of phase shift,a very narrow gap appears in the reflection spectrum.Due to its extremely narrow notch bandwidth(<1pm),the sensor bandwidth is smaller,the sensor measurement accuracy will be higher in theory.Therefore,PS-FBG has a greater advantage than ordinary FBG in refractive index sensing.However,most OSA cannot distinguish the spectral details of PS-FBG and its small wavelength changes as a result of the low spectral resolution,so its applications are also limited.In order to solve this problem,the rapid development of microwave photonics technology in recent years has made it possible to demodulate the wavelength of PS-FBG.The core idea is to convert the wavelength change in the optical domain into the frequency change in the microwave domain.Since the small wavelength change will cause a large frequency change in the electrical domain,the high-speed and high-resolution sensing measurements can be realized by more mature electrical spectrum analysis technology.Among many sensing applications of microwave photonics,the sensor based on optoelectronic oscillator(OEO)is undoubtedly one of the most attractive technologies.In this paper,a high sensitivity refractive index sensor based on an optoelectronic oscillator is proposed and verified by experiments.The sensing unit of the OEO refractive index sensor is a PS-FBG after cladding etching,which has higher sensitivity than the OEO structure using a common grating as a sensing unit.Since the electrical spectrum analyzer has a higher scanning speed and resolution than the optical spectrum analyzer,so the resolution of the sensor can be significantly improved,the results of measurement are more accurate.In addition,a high-sensitivity axial strain sensor based on double-loop OEO is proposed and verified by experiments in this paper.The axial strain is uniformly applied to the PS-FBG by the stepping motor,and the change of the axial strain applied to the PS-FBG is converted into the change of the OEO oscillation frequency.The double-loop structure of the optoelectronic oscillator can effectively suppress a part of side mode of the optoelectronic oscillation signal and reduce the threshold condition of the loop gain to make the oscillator vibrate more easily.At the same time,the system improves the side mode suppression ratio of the oscillator and the spectral purity of the oscillator signal.