Research On Structure-modulated LPFG Based On CO₂ Laser

In this paper,five types of structure-modulated long-period fiber gratings are proposed to improve the sensitivity of key characteristics of fiber gratings.Changes in the spatial structure of the fiber will cause the grating structure to change drastically when it is resistant to changes in the external environment,which will cause a significant drift in the resonant wavelength of the grating.In this paper,the CO₂ laser is used to etch the optical fiber,and the single-mode optical fiber is etched in different ways to form some grating structures that respond strongly to parameters such as strain,torsion,bending and so on.Firstly,in order to improve the strain sensitivity of fiber-optic sensors,two types of high-sensitivity fiber strain sensors are proposed in this paper,namely spring-shaped long-period fiber grating and S-shaped long-period fiber grating,respectively.Both gratings are formed by a CO₂ laser to etch a single-mode fiber to form a partially asymmetrical structure,spring-shaped or S-shaped structure.When such gratings are pulled by tension,the spatial structures of the gratings will change to a large extent due to the unbalanced force.Therefore,all of them have high strain sensitivities due to the relatively large resonance wavelength drifts.The spring-shaped long-period fiber grating has a strain sensitivity of 30 pm/??,and the S-shaped long-period fiber grating has a strain sensitivity of 29.3 pm/??.Compared to the traditional CO₂-LPFGs,the strain sensitivities of these two gratings are both higher about more than a dozen times.Secondly,two high sensitivity torsion sensors are proposed in this paper.The levorotation/dextrorotation fiber sheets grating is a new type of grating structure,designed by understanding and extension of the polarizing plate.The levorotation grating and the dextrorotation grating have similar resonance peaks,but have opposite and higher torsional sensitivities with-0.153nm/(rad/m)and 0.175 nm/(rad/m),respectively.Then,the pre-twisting CO₂-LPFG is an improvement to the writing process of traditional CO₂-LPFG to improve the torsional characteristic.When the grating is completed,the pre-twisting of one round will cause the spiral groove distribution after free release,which will greatly enhance the torsional characteristics and demonstrate the torsional characteristics by the experiments.And,the torsional sensitivity of the pre-twisting CO₂-LPFG is-0.151 nm/(rad/m)Finally,this paper proposes a structure-modulated high-sensitivity bending sensors.Based on the S-shaped fiber structure,it is straightened by means of fiber tapering,which causes the fiber core to be forced to exhibit a waveform state inside the cladding,so that we can measure the bending characteristic of its structure.There is a strong bending direction dependence in two orthogonal directions,perpendicular and parallel to the etching face,respectively.And the bending sensitivities are-15.98 nm/m^-1 and-4.19 nm/m^-1,respectively.The characteristics of five new structure-modulated long-period fiber gratings,proposed in this paper,show that the change of fiber structure will increase the sensing performance,and different modulation methods will lead to different superior sensing performances.The precise carving of the fiber structure by CO₂ laser can realize the objective that is difficult to achieve in traditional technology.Meanwhile,this paper also proposes that the fiber gratings with good sensing performance can be obtained by reprocessing the single-mode fiber with structural modulation.Therefore,as a kind of fiber gratings preprocessing or processing method,structure modulation is still worthy of further research.