Design Of Long-Period Fiber Grating Sensor For Underground Environment

Underground environmental polution is increasingly becoming serious,which affects the survival of animals and plants and the healthy life of human beings.For targeted prevention and control,it is necessary to accurately detect or monitor the polution situation.However,current detecting or monitoring methods can only determine the type of contaminated media,but not determine the amount of that.The long-period fiber grating has high sensitivity to the refractive index change of the external environment,and the nano-coating technology is combined to increase the selectivity and sensitivity of the sensor,so that the accurate measurement of the poluted gas and liquid media in the underground environment can be realized.In this paper,according to the situation of underground environment,the refrative index sensing characteristics of ordinary long-period fiber gratings and the evanescent wave sensing characteristics the long-period photonic crystal fiber are proposed to use for sensing.The simulations software is used to optimize the structural parameters of fiber grating.The main contents are as follow:(1)The main source of underground environmental pollution is the waste gas and waste liquid produced by human beings.According to the soil polution mechanism of underground environment and the corrosion mechanism of underground structures,it is concluded that the target of monitoring include gas concentration,ion concentration,pH value,and humidity.(2)The optical fiber simulation software Opticgrating was used to simulate the different structural paramenters of long-period fiber grating,such as the length,period and inclination of the grating,and the best writing parameters were obtained.The temperature and strain sensing characteristics of long-period fiber gratings were expored to provide a basis for setting temperature compensation and stress compensation,and to solve cross-sensing problems in the complex underground environment.The relative huminity,pH value and environment were simulated,and the relationships between the central wavelenth of the transmission spectrum and the relative huminity,pH value and gas concentration were established.And the temperature sensing sensitivity,strain sensing sensitivity and refravtive index sensing sensitivity were given.(3)The MODE Solutions photonic design and analysis software was used to simulate and optimize the photonic crystal fiber cladding structural paramenters.The best paramenters are 4 rings circular array of air holes,the diameter is 2.0 ?m,the pitch is 6.0 ?m,and the period of the grating is 705.3 ?m.The resulting sensor has an overlap power of 12.48%,a transmission loss of 10.39 dB/cm,and a sensitivity of 0.869.