The Sensing Characteristic Of Cascaded Tilted Long Period And Tilted Fiber Bragg Gratings

In the rencent years,fiber grating senors have been a hot area of research and been used to a vast range of industry,biomedicine,petrochemical industry and so on.As a kind of the fiber grating,the tilted fiber gratings has unique advantages and the studies of the tilted fiber grating is increasing.The major research of the tilted fiber grating is about the tilted short period fiber gratings(TFBG,also called tilted fiber Bragg grating or blazed fiber Bragg grating).The studies of the tilted long period fiber gatings(TLPFG)is fewer in comparison with the tilted Bragg grating.In this dissertation,the structure based on the cascaded TLPFG and TFBG(CTLBFG)is presented and the properties of this structure is different from the single TLPFG and TFBG.The studies of CTLBFG have a great meaning to the titled fiber gratings.This dissertation focuses on the coupled theory,simulation,experiment and sensing applications of the TLPFG,TFBG and CTLBFG.In chapter one,a brief overview on the classification and overseas and domestic research status of the titled fiber grating is provided.Many structures cascaded with TFBG are introduced and the theory analysis and properties of the cascaded long peiod fiber grating and Bragg grating is the important points.At last,the research background,content and frame are presented.In chapter two,the coupled mode theory of the tilted fiber grating is studied.Then the transfer matrix of TLPFG and TFBG can be deduced from the coupled mode equations of TLPFG and TFBG.According to the coupling mechanism of the CTLBFG,the transfer matrix of the CTLBFG is obtained by adapting the original transfer matrix of TLPFG and TFBG.Chapter three is the simulation of the properties of the reflection spectrum of the CTLBFG about grating parameters,film parameters and surrounding refractive index.Simultaneously,the approach of the sensitivity optimization is proposed.Finally,the influence of the temperature and bending to the CTLBFG is theoretically analyzed.The TLPFG and TFBG which satisfy the demand of the CTLBFG are fabricated in chapter four.The reflection spectrum of the experiment well meets the theoretical simulation of chapter two.In the experiment,the response of the spectrum of TLPFG,TFBG and CTLBFG to the temperature,bending and surrounding refractive index is explored.The results show the l=2 cladding mode owns a better sensitivity to surrounding refractive index and a lower sensitivity to temperature in comparison with l=1 cladding mode in TLPFG.Both of the l=1 and l=2 cladding mode have a good sensitivity to bending.The two reflection peaks of the CTLBFG show the response to the temperature,bending and surrounding refractive index to a certain extent,but the response degree is different.The mutual-coupling reflection peak have a good performance in sensing these variables compared with Bragg reflection peak.In order to expanding the application of the CTLPFG in the multi-parameter sensing,the cascaded CTLBFG and FBG is put forward in chapter five.The reflection spectrum is simulated by the transfer matrix of the cascaded CTLBFG and FBG.Multi-point pressure sensor and multi-parameter sensor based on wavelength division multipiexing is presented by analyzing the coupling mechanism of the cascaded CTLBFG and FBG.CTLBFG has the characteristics of small volume,wide sensing range,high sensitivity,multi-parameter sensing and so on.The CTLBFG which combines the advantages of the TLPFG and TFBG is believed to have an important application in the area of the fiber sensors.