Study On The Structure Design And Application Of Few Mode Long Period Fiber Grating

As a novel type of long period fiber grating,few mode long period fiber grating(FM-LPFG)is widely concerned in recent years.It not only has the advantages of single-mode long period fiber grating(SM-LPFG),such as light weight,small volume,immunity from electromagnetic interference,but also has the advantages of FMF,such as limited number of modes,low mode crosstalk and large mode field area.So it has a good application prospect in the fields of fiber communication,fiber sensing and device manufacturing.Aiming at the structure design and application of FM-LPFG,the communication devices(mode filters and filters)and sensors with various FM-LPFG are studied in this paper.Based on the existing theoretical research of FM-LPFG,the sensing characteristics with various FM-LPFG structures,such as coated FM-LPFG,FM-LPFG based on the dual-peak resonance and cladding-reduced FM-LPFG,are analyzed.Furthermore,the FM-LPFGs with excellent performance are designed,which expands the application potential of FM-LPFG in multi-parameter measurement and cross sensitive problem solving.In addition,the spectral characteristics of few-mode tilted long period fiber grating(FM-TLPFG)and cascaded FM-LPFG are further studied,and applied to the design of mode filters and filters.Through the discussion of grating parameters,the optimization method for device performance is proposed.This research focuses on the application of FM-LPFG in sensing field and communication field,which promotes the development of sensing and communication technology to the practical utilization.Chart 1 is the introduction of this paper,which briefly reviews the development process of FM-LPFG.The mode filed analysis of FMF and the coupled-mode theory of FM-LPFG are mainly introduced.It summarizes the research status and application of FM-LPFG,and further introduces the research of all kinds of new fiber grating sensors and communication devices.Through the inspiration of those work,the background and significance of this paper are presented.The paper is divided into three parts.Part I:The research on sensing characteristics of FM-LPFG,including Chart 2,3and 4In Chart 2,the sensing properties of coated FM-LPFG are studied.Firstly,the response characteristics of the effective refractive index with overlay thickness are analyzed.The mode barrier region and non-mode barrier region are divided,and the optimal overlay thickness is given.Secondly,the resonant characteristics of FM-LPFG are given when the couplings of different core modes and cladding modes work in the mode barrier region and non-mode barrier region.The sensitivity of FM-LPFG sensor working in mode barrier region is higher,and the sensitivity of film thickness is increased by 14.4 times.Besides,in the SRI sensing field,the film thickness range corresponding to the mode barrier region moves to the direction of large-film thickness with the increase of SRI,and the resonance peak in the mode barrier region has higher SRI sensitivity,which is about 5 times higher than that in the non-mode barrier region.In Chart 3,the SRI sensor based on cladding-reduced FM-LPFG is proposed.The effective refractive indexes of two core modes in FMF is calculated by two-layer and three-layer structure models.The influences of cladding radius on resonant wavelength,effective refractive indexes of core modes and cladding modes are analyzed.The distribution of cut-off cladding radius of different orders cladding modes is discussed.In addition,based on the coupled-mode theory,the transmission spectrum and SRI sensitivity of the cladding-reduced FM-LPFG are analyzed numerically,and the effect of reducing cladding on SRI sensitivity is discussed.The results show that the resonant peaks corresponding to the core mode LP₀₁ have higher response to the cladding radius and SRI than that of the higher-order core mode.And the sensitivity of the resonant peak corresponding to LP₀₁-HE_1,7 to the cladding radius is about 5.76nm?m.When the radius is reduced to 50?m,the SRI sensitivity of the such peak is as high as1690.00 nm RIU,which is about 6 times higher than that of the same peak with the cladding radius of 62.5?m.In Chart 4,a novel FM-LPFG sensor based on the dual-peak resonance for simultaneous detection of SRI and temperature is proposed.Firstly,the coupled-mode theory and the phase-matching condition of FM-LPFG are introduced.Secondly,the dual-peak resonance near phase-match turning point(PMTP)of core mode LP₀₁ and transmission characteristics of FM-LPFG are analyzed.Then,it is found that the FM-LPFGs working near PMTP with different grating periods show the change of wavelength and intensity to the change of environment parameters.Because of the different sensing characteristics,combined with the dual-wavelength method and the dual-parameter method,the sensor can be used to measure SRI and temperature simultaneously.This kind of sensor not only can realize the simultaneous measurement of two environmental parameters,but also has higher response sensitivity.In the dual-wavelength method,the maximum sensitivity of SRI and temperature can reach 1940nm RIU and-0.42 nm?,which are 6 times and 4 times higher than that of the same type of sensor respectively.In the dual-parameter method,the sensitivity of intensity response to SRI and temperature is-682.42d B RIU and 0.072d B?,respectively.Part II:The design method of FM-LPFG communication device,including Chart 5and 6In Chart 5,a novel FM-TLPFG mode filter based the dual-peak resonance near PMPT is presented.On the basis of the coupled-mode theory,the resonant wavelength can be obtained,and the working wavelength range can be determined.The numerical simulation gives that only resonant peaks corresponding to LP₁₁-HE_1,23 occurs near the resonant peaks corresponding to LP₁₁-HE_1,19.By analyzing the transmission spectrum under different grating period,tilt angle and grating length,the grating parameters corresponding toe maximum coupling between core mode LP₀₁ and cladding mode HE_1,19,as well as the wavelength bandwidth and higher-order core mode loss are obtained.As the results show,when the grating length is 34-46mm and tilt angle is 80-82°,the FM-TLPFG mode filter can effectively eliminate the fundamental mode LP₀₁,and the LP₁₁mode maintain a low loss in the wavelength range of 1480-1550nm.The bandwidth of such mode filter is about 2.7 times than that of the reported mode filter.It is believed that the FM-TLPFG can be applied as mode filter in MDM system.In Chart 6,a novel comb filter based on cascaded FM-LPFG is proposed firstly.The mode interference mechanism of cascaded FM-LPFG is described in detail.The influences of the number of cascaded grating segments and the grating parameters on the characteristics of interference fringes are discussed.The results show that cascaded fiber length can be used to optimize the performance of comb filter.By reducing the bandwidth of comb filter,not only the symmetrical comb filter with the bandwidth ratio of 1:1 is proposed,but also asymmetrical comb filters with the bandwidth ratio of 1:4,1:6,1:9 and1:12 are introduced.Compared with cascaded SM-LPFG comb filter,cascaded FM-LPFG mode filter has more filter channels,so multi-channel comb filter can be designed.Part III:Experimental study of FM-LPFG,including Chart 7 and 8In Chart 7,the mode excitation and intermodal interference of FMF are studied.Firstly,based on the theoretical analysis of step refractive index fiber,the mathematical model of mode transmission in FMF is established.The characteristic equation is calculated by scalar method,and the modes of FMF are analyzed in detail.Secondly,the influence of FMF length and fiber connection method on intermodal interference is discussed by using the beam propagation method(BPM).The longer the FMF or the larger the dislocation between SMF and FMF becomes,the more obvious interference phenomenon in the transmission spectrum is.Based on the mode excitation theory,the high-order core mode excited by the light from SMF to FMF is simulated and analyzed.With the increase of dislocation,the core fundamental mode energy reduces,and it is continuously converted to the high-order core mode.It is a theoretical analysis for the spectral measurement of FM-LPFG.In Chart 8,the writing process,spectral measurement and sensing characteristics of FM-LPFG are put forward.Firstly,the influences of FMF length and fiber connection method on intermodal interference are analyzed in experiment.The results show that with the increase of dislocation,the core fundamental mode energy is continuously coupled to the higher-order core mode,and the intermodal interference can also be eliminated by adjusting the FMF length.Secondly,FM-LPFG is written by CO₂ lasers,which is used for sensing glycerin solution.Based on it,the dual-parameter measuring system of SRI and temperature is constructed.It can measure SRI and temperature simultaneously.The result shows that the highest SRI and temperature sensitivities are about263.257 nm RIU and 0.058 nm?,respectively.Thirdly,so as to improve the SRI sensitivity,the cladding radius of FM-LPFG is etched by HF solution.By comparing the sensing characteristics of the resonant peak before and after etching,it is found that the SRI sensitivity can be improved.And the SRI sensitivity after etching is about-850.80 nm RIU,which is 1.64 times higher than that before etching.Finally,cascaded FM-LPFG with different structures are written,and the influence of grating parameters on transmission spectrum are discussed.Through the glycerol solution sensing experiment,the SRI sensing characteristics of cascaded FM-LPFG is explored.The result show that the SRI sensitivities of all peaks formed by interference between the same core mode and cladding mode are approximately the same.In Chart 9,it summarizes the performance of the novel FM-LPFG structure sensor,mode filter,comb filter and other communication devices.It also points out the shortcomings of this paper and looks forward to the future research direction of FM-LPFG.Although the research of FM-LPFG in this paper is still in a relatively basic exploration stage,more types of FM-LPFG sensors and communication devices can be designed through the theoretical design method in the future research work.