Study On The Fabrication And Characteristics Of Long Period Fiber Grating Based On Perturbation Of Cladding

In this paper,the main job is to put forward two kinds of LPFG sensor based on the perturbation of cladding mode.One is a kind of composite structure made of three core optical fiber and LPFG.The tapered three core optical fiber with the features of wavelength division,a part of the energy directly inject into the grating cladding,destroyed the degeneracy properties of phase velocity between modes.The other is a composite structure made of fiber up-taper and sectional LPFG,the fiber up-taper formed by hot extrusion lead to fiber core deformation,destroyed the degeneracy properties of phase velocity between modes.Meanwhile,the paper propose a three-section LPFG on the basis of written to the LPFG in the traditional high frequency CO₂ laser pulses,and is can be used as a torsion sensor.Then the parameters of temperature and stress were measured for the long-period fiber grating sensors fabricated by the above-mentioned structures.First of all,a situation simulation was carried by Rsoft software on three taper optical fiber taper based on the filament heating method,create three core optical fiber taper waist diameter the suitable waist diameter of three core optical fiber taper was created according to the simulation results.Then it was concatenated with long period fiber grating,two interference peaks with different temperature and strain sensitivity,and they can be used for simultaneous measurement temperature and strain.The resolution of temperature and strain is 0.5°C and1merespectively,and it is higher than the most similar sensor.Secondly,this paper propose a method to improve the written LPFG method based on the high frequency CO₂ laser pulses.The write grid programs are divided into three equal parts,and this three parts have different space position,and they are perpendicular to each other.Experimental results show that when only change the writing direction not introduced the coupling length,the grating can distinguish the direction correlation with torsion,the clockwise torsion sensitivity is 0.127 nm/(rad m^-1),the counterclockwise torsion sensitivity is-0.119nm/(rad m^-1),it is higher than traditional LPFG for 3 to 4 times;When change the writing direction and introduce the coupling length,the clockwise torsion sensitivity is 0.21 nm/(rad m^-1),the counterclockwise torsion sensitivity is-0.5 nm/(rad m^-1),it is higher than traditional LPFG for 5 to 6 times.Finally,the fiber up-taper was made using fiber optical fiber welding mechanism,and will lead to optical fiber core deformation.When the fiber up-taper concatenated with LPFG,the deformation of optical fiber core will destroy the degeneracy properties of phase velocity between modes,which can lead to beat phenomenon and the new model coupling.So,the same order cladding mode propagation constant the grating is different,and the grating resonance peak was divided.Meanwhile,the other order cladding mode couple with fundamental mode and produce new resonance peak.The temperature and torsion characteristics of this structure was measured,its distortion measurement sensitivity has been greatly increased compared with sectional LPFG high torsion sensitivity.When the composite structure is anticlockwise torsion,torsion sensitivity reached 0.308 nm/(rad m^-1);when the composite structure is clockwise torsion,torsion sensitivity can reach 0.35 nm/(rad m^-1).