基于螺旋形纤芯的长周期光纤光栅扭转传感器

Torsion Sensor Based on Helical-Core Long-Period Fiber Grating

提出并制备了一种基于螺旋形纤芯的长周期光纤光栅扭转传感器。首先利用激光器对单模光纤进行周期性的多面蚀刻,然后对加工完成的光纤结构进行熔融加热,在表面张力的作用下,结构表面的凹槽形状可以转化为内部纤芯的变形。当该传感器受到外界施加的扭转时,其纤芯的螺旋状态会被改变,从而使得结构纤芯和包层的有效折射率发生改变,达到提升传感器扭转灵敏度的目的。实验结果表明螺旋芯长周期光纤光栅在-18.42 rad/m至18.42 rad/m的测量范围内的扭转灵敏度可以达到0.37 nm/(rad/m),同时该传感器具有1.75 mm的紧凑长度。

Objective Torsion is one of the most important physical parameters in industrial automation control and robotics.Optical fiber sensors feature small size,light weight,and electromagnetic interference resistance,providing reliable measurement results under such extreme working scenarios as flammable and explosive,high temperature,high pressure,and strong electromagnetic properties.Among them,long-period fiber grating(LPFG)shows superiority due to the high sensing sensitivity,low transmission loss,directional judgment,and mature of preparation method.However,the sensitivity and length of LPFGs limit the applications in extremely special environments,such as industrial automatic control and intelligent robots.Thus,we aim to enhance the sensitivity and shorten the length of the LPFGs by simulation analysis and experimental optimization.Meanwhile,a new method for fabricating LPFGs is proposed and the sensing mechanism of the torsion sensors is also investigated.Finally,we hope our study can provide some ideas for the design of optical fiber torsion sensors.Methods We experimentally propose and theoretically study a torsional sensor based on helical-core LPFGs.The fabricating steps for the structure are as follows.Firstly,a high-frequency CO2 laser is employed to periodically etch the single-mode fiber to form a preheated structure,and then the fiber-preheated structure is heated.Meanwhile,the grooves of the preheated structure are transformed into the core deformation under the surface tension of the molten glass.When the sensor is subjected to external torsion,the helical state of the fiber core is changed to alter the effective refractive index of the core and cladding to improve the torsional sensitivity of the sensor.Results and Discussions The light distribution of the proposed fiber sensor is theoretically analyzed and simulated.When the incident light passes through the helical-core structure,the energy decreases rapidly within 0.8 mm.With the increasing propagation distance,most of the light propagating in the fiber core is coupled to the cladding mode,and a part of the coupled light continues to propagate in the cladding until it is completely lost.At the same time,the other part of the light in the cladding is coupled back to the fiber core by the next micro-bent core and interferes with the light in the fiber core.When the bent core is periodically arranged in the fiber axis,the light in the core is periodically coupled to form a resonance peak.Since the helical core exerts a strong coupling effect on the incident light,a relatively small number of cycles can form a fiber grating to make the sensing unit of the sensor reach 1.75 mm.In the experiment,three samples show different sensitivities during the torsion measurement.The helical-core LPFGs with a length of 1.75 mm exhibit the sensitivity of 0.37 nm/(rad/m)in the range from-18.42 rad/m to 18.42 rad/m.The structures of 1.125 mm and 2.625 mm present the highest sensitivities of 0.65 nm/(rad/m)and 0.78 nm/(rad/m)respectively.The sensor sensitivity can be improved by increasing the period number(length).However,the longer length of the spiral core results in greater insertion loss of the structure and weaker controllability of the structure.Therefore,the optical fiber structure of 750μm is selected as the main research object.The temperature response and strain response of the structure are 44 pm/℃and 4 pm/με.Conclusions We propose an HC-LPFG sensor for measuring torsion magnitude and direction.This sensor is prepared by laser etching and thermal melting method,which realizes the helical fiber core without the spiral of the cladding.The helical core improves the coupling effect of the transmitted light,reduces the grating formation period of the fiber grating,and makes the sensing unit reach 1.75 mm.Experimental results show that the torsion sensitivity of HC-LPFG is 0.37 nm/(rad/m)in the measurement range from-18.42 rad/m to 18.42 rad/m,about ten times higher than that of the LPFG written by the laser point-to-point method.This sensor features high sensitivity and compact structure and is expected to be employed in industrial automation control and robotics.