Long-period Fiber Grating Technology For Environmental Sensing

In the past few years,the durability of structures becomes a major issue because of the degradation and damages caused by corrosion.Geotechnical underground environment tends to suffer suverely from corrosion problem which are more prominent compared with on ground civil engineering structures due to the characteristics of complexity,diversity,dynamics and various geotechnical environment around underground environment.These threatens the durability and safety of most buildings and underground structures.There are many technique used for corrosion monitoring of steel bars such as traditional method,electronic sensor etc.but these techniques has huge technical difficulties because of the complexity of underground environment and it also transmit short and weak signals which has serious impact on effectiveness of the monitoring signal and stability.Due to the excellent performance of Long-period fiber grating sensors,it has been proposed in this research for underground geotechnical environment sensing,it has the advantage of being immune to electromagnetic interference.The sensing principal and various characteristics of LPG were described.The transmission spectrum simulations and model analysis are performed using simulation software,the parameters for writing the Long period grating were obtained and the photonic crystal structure combined with Long period grating displaying the sensing performance are obtained.The main foundings of the research are as follows:The Long period grating target and critical values for the corrosion monitoring were studied based on the mechanism of chloride ion erosion and conrete carbonation.It was determine that when p H = 9 the passive film on the surface of rebar in concrete begins to be formed and p H = 15 complete destruction of the passivation film as per carbonation of concrete.In case of chloride ion erosion,the critical value of Cl—/OH— is given by the fitting function.The LPG spectral characteristics in different environment were simulated and analyzed by commercial simulation software Opti Grating.Using the refractive index,The LPG refractive index,temperature,strain and were simulated respectively.The CO2 concerntration of the LPG was simulated and the writing parameters of LPG were obtained.The photonic crystal fiber combined with long-period grating was analyzed using the optical finite element software Lumerical MODE Solutions.The most sensitive IGPCF geometric parameters were obtained which shows the the power overlap,highest sensitivity,and sensitivity coefficient of model H4-2.8-7.0.