The Construction Of Fiber Sensing Measurement Based On Long Period Fiber Grating

Optical fiber sensing technology takes light wave as carrier and optical fiber as medium.A new sensing technology senses and transmits the measured signal from outside.Compared with traditional electrical sensor,it has the advantages of high sensitivity,anti-electromagnetic interference,high temperature resistance,anti-corrosion and good flexibility,so it attracts wide attention.Long-period fiber grating(LPFG)appeared in the 1990 s,which is a passive optical fiber device with periodic refractive index modulation,its oversized period determines that it has a completely different coupling mechanism compared with traditional fiber Bragg grating(FBG).LPFG usually has the period of tens to hundreds of micrometers.The LPFG couples light from the fundamental mode to the high-order and forward-propagating cladding modes The central wavelength and coupling strength are closely related to the changes of the external environment.In addition to the advantages of low cost,simple fabrication and easy integration,it also has the advantages of low insertion loss,sensitivity to multiple external parameters and no backward reflection,so it has a very broad development prospects in the communication,sensing and other fields.In this paper,we mainly simulated the change of grating waveform and analyzed the sensitivity of temperature,stress and refractive index.We also researched the single parameter and multi-parameter sensing measurement of grating.The specific contents are as follows:(1)Combining with the coupling mode of LPFG,we simulated and analyzed the influence of grating parameters on waveform,the relationship between the parameter sensitivity and grating parameters.The influence of different etching methods on the grating parameters is analyzed.(2)The LPFG and FBG were compared to measure the temperature and the high sensitivity of LPFG with temperature was obtained.The LPFG was connected to the Sagnac and Machendel interferometric optical paths in temperature measurement.The same temperature sensitivity showed that the LPFG could maintain its sensing characteristics without the influence of different optical paths.Strain measurement of LPFG in micro-bending and axial direction was carried out,and the linear relationship between central wavelength and coupling depth was obtained.The refractive index of LPFG is measured and the linear relationship between the central wavelength and the refractive index was obtained.(3)We measured the temperature and strain sensing properties of polarization maintaining fiber(PMF)and realized that the temperature and strain were measured simultaneously for LPFG and PMF through Sagnac loop in cascading structure.This method could avoid the crosstalk of another parameter when we need measure only one parameter.We researched the temperature and strain sensing measurements of single-multi-single-mode fiber structures.It can be cascaded with the LPFG as a temperature compensator.The refractive index measurement with temperature compensated was realized.(4)Fiber Fabry-Perot(F-P)cavity is composed of single-mode optical fibers and the strain measurement in micron-sized is carried out.According to its compact structure,stable measurement and modulation only by the change of cavity length,it can be as a strain compensation in the temperature and refractive index measurement of LPFG.