Study On Fabrication And Sensing Characteristics Of Microtapered Long-period Fiber Grating

Microtapered long-period fiber grating(MLPFG)not only has good torsion sensing properties,but also has stable optical properties in high temperature environment.In recent years,MLPFG has attracted the attention of many scholars because of its low fabrication cost,good compatibility with common communication system and extensive application in the field of sensing and communication.In this paper,the fabrication and sensing characteristics of MLPFG were studied.The main research contents are as follows:1?The transmission characteristics of MLPFG were analyzed by mode coupling theory.Combining with previous studies,the mathematical expressions of the related physical quantities were calculated,which provides theoretical support and guidance for future experiments.2?An experimental system which can be simultaneously used to fabricate MLPFG and observe it's spectra online is designed and built.The MLPFGs based on two different fibers were fabricated,one of them is based on standard single-mode fiber,the other is based on microfiber.Under certain parameters,the spectra of these two MLPFGs are relatively ideal,the transmission loss is about 0.5 dB,and the extinction ratio of the resonant peak also exceeds 30 dB.The effect of fabrication parameters on the grating spectra was investigated.For MLPFG based on standard single-mode fiber,under the premise of weak coupling,when the number of taper increases,the extinction ratio of the spectra will also increase,the wavelength of the resonant peak will drift to the long wave direction,and the bandwidth will become narrower and narrower.For MLPFG based on microfiber,under the premise of weak coupling,the extinction ratio also increases and the bandwidth also becomes narrower and narrower as the number of taper increases.However,the wavelength of the resonant peak drifts to the shortwave direction with the increase of the number of taper.In addition,when the grating period increases,the resonant peak will drift to the shortwave direction.3?The high temperature characteristics of MLPFG based on microfiber were investigated,and the related annealing experiments were also carried out.In the experiment of exploring the high temperature characteristics of the grating,due to the release of residual stress and the rearrangement of fiber structure,the two gratings with different periods have similar performance,that is,when the temperature does not exceed 850?,with the increase of temperature,the resonant peaks drift towards the long wave direction,sensitivities are 0.0192nm?-1 and 0.0194nm?-1.In the annealing experiment,by comparing the change of grating spectra before and after annealing,it is found that the critical temperature at which the grating produces irreversible change is between 600? and 800?,which is at least 100? higher than that reported previously.4?The torsion sensing characteristics of the above two types of MLPFGs were explored.In the experiment,the grating was fixed between two points separated by a certain distance,and one end of the grating was twisted while the other end remains unchanged.The sensing characteristics of the gratings were explored by observing the changes of the spectra.For MLPFG fabricated on standard single-mode fiber,the torsion sensitivity is 0.10 nm·m·rad-1,which is twice as much as that of the chiral long·period fiber gratings(CLPG)written on photonic crystal fibers and three times as much as that of the long period fiber gratings(LPFG)written with CO2 lasers.And when the torsion angle is greater than 120°,the shift of the resonant peak to the short wave direction means that the grating is twisted in the forward direction,and the shift of the resonant peak to the long wave direction means that the grating is twisted in the reverse direction,and the magnitude of the torsion force can be determined according to the sensitivity;when the torsion angle is less than 120°,the basis for judging the torsion direction is still the drift direction of the resonant peak,but to judge the magnitude of the torsion force,both the drift direction and the sensitivity of the resonant peak should be considered.For MLPFG fabricated on microfiber,when the mechanical torsion rate is in the range of 0?550 rad/m,the resonant peaks of spectra drift towards the long wave direction with the increase of the mechanical torsion rate,and the sensitivity of the torsion sensing is 0.01276nm·m·rad-1.