Research On The Preparation And Sensing Characteristics Of Long Period Fiber Grating Fabricated By High Frequency CO₂ Laser

Long period fiber gratings,a kind of passive devices with the period size of several tens to hundreds of microns,have many advantages,such as anti-electromagnetic interference,good corrosion resistance,small volume and so on.Due to the core-base mode is coupled with the cladding mode during the transmission of light in the optical fiber,long period fiber gratings are very sensitive to the change of external environment and are widely used as sensors for the measurement of temperature,refractive index,curvature and so on.With the development of preparation technique of fiber gratings,the researchers have gradually matured in the study and fabrication of long period gratings resulting in many novel long period fiber gratings with new structures and different properties.As a kind of novel long period fiber gratings,ultra-long period fiber gratings have many different performances and have been widely concerned by scholars due to the core-base mode is coupled with the high cladding mode during the transmission of light in the optical fiber.In this paper,the basic principle of single-mode long period fiber gratings was analyzed in detail.The experimental system of high frequency CO2 laser fabricating long period fiber gratings was designed and constructed.Long period fiber gratings and ultra-long period fiber gratings with different periods were fabricated and were used to measure curvature,temperature and refractive index.The main research contents were listed as following:(1)According to the couple mode theory,the mode of fiber core and fiber coating of single-mode long period fiber gratings,the coupling mode equation,the wavelength and the intensity of resonance peaks were analyzed in detail.The influences of period size and the grid number of long period fiber grating were studied by Rsoft software simulation.The mechanism of fabricating long period gratings by the high frequency CO2 laser was analyzed,which was useful to improve the preparation technique and fabricate long period fiber gratings by the high frequency CO2 laser.(2)The experimental platform of fabricating long period gratings by the high frequency CO2 laser was designed and built and the long period fiber gratings with different period sizes 400μm,640μm and 700μm were successfully fabricated to achieve the measurement of the sensing performances.The long period fiber grating with 640μm period-size,whose curvature sensitivities of exposure-face and exposure-face-down were 56.55556nm/m-1 and 57.44925nm/m-1,was qualified as a vector curvature sensor.The long period fiber grating with 700μm period-size had irregular spectral changes when the curvature was small,but they had good performances for the measurement of large curvature.Its disadvantage was that it could not recognize the directions of curvature.During the measurement of temperature and refractive index for the long period fiber grating with 640μm period-size,it had good sensitivity of temperature,which is 0.11332nm/℃.But it was not sensitive to the refractive index ranging from 1.33 to 1.377.(3)The ultra-long period fiber gratings with 1mm period-size,2mm period-size and 1mm period-size circular-arc-shaped-grid were fabricated and were used to the measurement of curvature,temperature and refractive index.It found that the ultra-long period fiber grating with 1mm period-size had good curvature directions recognition ability and curvature sensitivity,which was suitable as a vector curvature sensor.During the measurement of temperature and refractive index,its resonance peak was insensitive to the temperature but the intensity of its resonance peak was sensitive to refractive index,which was 142dB/RIU.In this paper,the experiment system fabricating long period gratings had many advantages,such as low cost,high fabricating efficiency,simplicity of operator,high fabricating quality,easy control and so on.Based on the high frequency CO2 laser,the long period fiber grating with 640μm period-size was suitable as a vector curvature sensor.But it was sensitive to the temperature resulting in the temperature-curvature cross sensitivity.The ultra-long period fiber grating with 1mm period-size was not only suitable as a vector sensor but also was insensitive to the temperature resulting no problem of temperature-curvature cross sensitivity.