Cascaded Long-period Fiber Grating Design And Gas(Liquid) Environmental Monitoring

Environmental pollution is becoming more and more serious,and environmental protection issues are highly valued.For targeted control,accurate detection or monitoring of pollution conditions is required.Compared with other monitoring methods,the optical fiber sensing system has advantages such as corrosion resistance,harsh environment resistance,anti-electromagnetic lightning interference,compact structure,multi-channel reusable,low-cost mass production,and easy to build a sensor network.The sensing field has great advantages and good application prospects.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 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.Combined with nano-coating technology,the optimal coating parameters were obtained by simulation.The relative huminity and CH₄ gas environment were simulated,and the relationships between the central wavelenth of the transmission spectrum and the relative huminity gas concentration were established.And the temperature sensing sensitivity,strain sensing sensitivity and refravtive index sensing sensitivity were given.（2）The optical fiber simulation software OptiGrating was used to study the double LPG cascade grating.The influence of grating and fiber parameters on the transmission spectrum of the cascade LPG was investigated.It was applied to environmental monitoring and the sensor with larger resolution was obtained.Cascade of LPG-FBG grating simulation designed to explore the influence of a grating,the fiber parameters for LPG-FBG reflection spectrum of the grating cascade,and may propose using LPG-FBG grating cascade factors to eliminate cross-sensitivity more complex environmental problem（3）The MODE Solutions photonic design and analysis software was used to simulate and optimize the photonic crystal fiber cladding structural paramenters.For the circular pores,the optimal parameter is a 4-ring regular hexagonal arrangement with an aperture ratio of 0.4,a corresponding transmission loss of 8.0763 dB/cm,and a relative sensitivity of the sensor of 0.0391.For a square air hole,the optimal parameter is a4-ring regular hexagonal arrangement,the number of rings of the air hole is 4 rings,the aperture ratio is 0.4,the corresponding transmission loss is 7.3412 dB/cm,and the relative sensitivity of the sensor is 0.0376.