Fabrication And Sensing Application Of Unplanar Grid Long-period Fiber Grating

Fiber grating is a kind of transmission optical fiber device with abundant structures and excellent characteristics,and it has been widely applied in the sensing field including bending,torsion,temperature,refractive index,etc.Limited by the fiber diameter scale and fabrication method,the grid structure of traditional fiber grating is planar,which limited the design of fiber grating.Compared with the traditional planar grid,the unplanar grid provides higher refractive index modulating ability and more abundant design space.Based on unplanar grid,the novel fiber grating can be designed with compact structure and outstanding property.Therefore,it provides important scientific significance and application value to further research the modulation effect and sensing characteristics of unplanar grid in theory and experiment for the design,fabrication and development of fiber grating.In this dissertation,a series of long-period fiber gratings based on the unplanar grid has been designed and fabricated,and their transmission spectra and sensing characteristics,including bending,torsion,temperature and refractive index,have been tested and analyzed.The innovative research achievements in this dissertation are listed as below:1.Based on the abstract space geometrical model and mathematical group theory,the geothrical symmetry on fiber structure is analyzed.Multiple optical fiber devices with different symmetries and sensing characteristics are systematically summarized,and the relation between symmetry and sensing characteristics of fiber modulated structure is analyzed.According to the symmetry principle,when the structure includes C₂ rotating axis or mirror plane perpendicular to the bending direction,the fiber device would equally respond to the opposite bending directions in wavelength;when the structure includes mirror plane,the fiber device would equally respond to the opposite torsion directions in wavelength.2.Based on traditional model of tilted planar grid,the unplanar grid model is established by introducing the geometrical analysis and optical path condition.Using the firstly reported arc grid as the example,the refractive index modulated effect of unplanar grid is semi-quantificationally calculated,and the modulation change from its deformation under bending status is also simulated.The distinguishing ability of bending direction is expounded,and the result is mainly consistent with the reported data.3.The micrometer-scaled unplanar microstructre with V-shape,arch-shape and tilted-arc shape are designed and fabricated on the fiber,which surmounts the planar structure limit of traditional fiber grating,and provides new design thought and realizing route.According to the symmetry analysis,the V-shaped and arch-shaped grid can both distinguish a pair of bending directions the same as and opposite to the open side,and the tilted-arc shaped grid can distinguish the opposite torsion directions.4.A series of unplanar long-period fiber gratings based on the periodically distributed V-shaped,arch-shaped and tilted-arc shaped microstructures are fabricated on single-mode fiber with CO₂ laser scanning technology.The bending and torsion sensing characteristics are experimentally tested,and the results agree with the theoretical expectation.Both the V-shaped and arch-shaped long-period fiber grating can distinguish multiple bending directions,and each of the highest bending sensitivity is more than 22 nm/m^-1.The tilted-arc shaped long-period fiber grating can distinguish the opposite torsion directions,and the highest torsion sensitivity is more than 0.5 nm/(rad·m^-1).Each of the sensitivity is enhanced twice or triply than the traditional fiber grating device.5.The ZnO-coated arc long-period fiber grating is fabricated by sintering,and the change of spectra and temperature and refractive index sensing characteristics of the coated grating device are experimentally tested.After coating ZnO,the temperature sensitivity of this grating device is reduced about 44%,but the refractive index sensitivity mainly remains,which agrees with the theoretical expectation.