Research On Femtosecond Laser Direct Writing Small-Period Long-period Fiber Grating And Its Sensing Application

Fiber gratings sensors are widely used due to the compact size,resistance to electromagnetic interference and chemical corrosion,and easy integration.Among them,long period fiber grating sensor has high sensing sensitivity in bending,refractive index,which makes it an irreplaceable position in the fields of building health detection,food safety,environmental detection and biomedicine.Femtosecond laser inscribed fiber grating is simple in operation,flexible in design,and high in accuracy.The inscribed fiber grating is resistant to high temperature,which provides the possibility for fiber grating sensor to be put into practical application.In this article,the small-period long-period fiber grating sensor is made and studied.The main work contents of this paper are summarized as follows:(1)The mechanism of femtosecond laser induced changes in refractive index of materials is summarized,and the femtosecond laser direct writing system is introduced.We analyze the influence of the different parameters such as laser wavelength,pulse width,laser power on the production of fiber gratings.(2)A small period long period fiber grating fiber torsion sensor is proposed.The sensor can distinguish the torsion direction according to the wavelength shift direction.The maximum sensitivity of the resonant wavelength in the clockwise and counterclockwise twist directions is 0.228 nm/(rad/m)and-0.183 nm/(rad/m),respectively.The maximum distortion sensitivity of the resonant depth in the clockwise and counterclockwise directions is 2.745 dB/(rad/m)and 3.71 dB/(rad/m),respectively.At the same time,the temperature sensitivity of the sensor is only 4.85 pm/℃2 and 0.01325 dB/℃.Compared with conventional long-period fiber gratings,the temperature sensitivity of small-period long-period fiber gratings is greatly reduced.The sensor has the advantages of high sensitivity,small device length of only 3 mm and low temperature crosstalk.(3)The eccentric small-period long-period fiber grating sensor is proposed as the direction-dependent curvature sensor and refractive index sensor.The curvature sensing sensitivity of small-period long-period fiber gratings with different eccentricity and the refractive index sensing characteristics of the fiber gratings with different periods are studied in this paper.The greater the degree of eccentricity,the higher the sensitivity of the sensor to curvature.The small-period long-period fiber grating with an eccentric 2 μm shows high curvature sensing sensitivity in the bending direction of 0° and 180°,and the maximum sensitivity is-9.74 dB/m-1 and 7.37 dB/m-1,respectively.As the grating period increases,the range of the external refractive index corresponding to the maximum refractive index sensitivity shifts to the direction of high refractive index.Therefore,a suitable fiber grating period can be selected according to the refractive index range of the solution to be measured to obtain a high-sensitivity refractive index sensor within the refractive index range.