Study On The Sensitization Characteristics Of PLD Coated Optical Fiber Sensor

Fiber Bragg grating(FBG)and long period fiber grating(LPFG)sensors have a series of advantages in widely application.A new method of preparing Sr Ti O₃(STO)and Zn S_0.7Co_0.3 nano films on cladding by pulsed laser deposition(PLD)was proposed.The sensitivity before and after coating was studied.The details are as follows:The bare FBG and the bare LPFG are relatively fragile and will be damaged in the harsh environments.Therefore,they need to have protective coatings.The transmission coupled mode theory of FBG and LPFG were analyzed,and the sensing principle of temperature,stress and refractive index were explored,and the corresponding sensitivity coefficient formulas were obtained,respectively.STO and Zn S_0.7-Co_0.3 have higher thermal optical coefficient and thermal expansion coefficient than the silicon dioxide material of grating itself,which can realize temperature sensitization.The high refractive index of the two materials can improve the coupling ability between the core mode and cladding modes,and realize the refractive index sensitization of the LPFG.The experiments of STO and Zn S_0.7Co_0.3 films on FBG and LPFG cladding were carried out by PLD.Temperature sensing results showed that the sensitivity of the STO coated FBG was about 9.97 pm/?,which is about 28%higher than that of the bare FBG(7.8 pm/?)before coating.In addition,several practical structures were proposed,among which the sensitivity range of Sagnac interferometric fiber optic sensor was larger,reaching 11.4 pm/?,increasing by 46.2%.In the cascaded optical path and Michelson interference structure,the sensitivity were 9.0 pm/?and 10.5pm/?respectively.The change of coupling wavelength and intensity of LPFG showed linear sensing characteristics in sections due to mode recombination.After coating,the sensitivity of temperature and refractive index sensing have improved in sections or as a whole.Among them,the sensitivity of LPFG coated with Zn S_0.7Co_0.3film in the range of 80?to 180?was 2.1 times that of bare grating.The sensitivity of STO coated LPFG was 3.1 times of the original in the range of 110?to 180?,1.98 times of the original in the range of 1.333 to 1.370 refractive index,and the range and sensitivity of intensity sensing were increased.Based on Ridge CV,Lasso CV and polynomial extended machine learning algorithms,the FBG coated with Zn S_0.7Co_0.3 film and all LPFG transmission spectra before and after coating were trained and predicted,which is for analyzing the characteristics of grating sensors in an unified way,not limited to the study of the change rule of a peak point,further to predict and analyze the change of spectral pattern around the peak,and improve the available range of spectral wavelength,and the model was evaluated by the determination coefficient².The prediction effect of Ridge CV regression was better than Lasso CV,and² improved with the increase of polynomial order,almost all of them had good robustness.The prediction effect of the coated gratings was generally better than that of the bare gratings,which also proves that the grating coating can achieve the sensitization effect,the linearity became larger,and the stability increased.These open up a new way for grating cladding ferroelectric thin film materials and transition metal doped chalcogenide semiconductor materials,further analysis of sensing performance by machine learning algorithm,prediction of spectral changes and characteristics,and prediction of complex environmental changes based on predicted spectra.