| With the development and advancement of sensing technology,fiber grating sensors are favored by researchers due to their small size,high integration,high sensitivity,and suitability for extreme environments.And a variety of preparation technologies have also emerged.The sensors have the unique sensing characteristics as the different preparation methods and make them suitable for the measurement of strain,temperature,refractive index,torsion and other physical quantities.As the detection target gradually develops to the field of complex environment,the demand for miniaturized and high-performance sensors is increasing.It has been difficult to achieve a breakthrough in sensing performance by using single preparation technology.In order to improve the sensor performance,a novel composite modulation long period fiber grating with remodulated grating base structure is developed by combining the advantages of grating preparation technologies.Four kinds of high sensitive composite modulation long period fiber grating(LPFG)for strain,bending,torsion and liquid level measurement are designed by using frequency CO2 laser combined with fiber cone,arc discharge and polishing technology in this thesis.With the help of simulation calculation and experimental verification,the parameters of sensor sturcutres are optimized and the preparation technology is simplified.These four kinds of sensors are successfully prepared and the performance is tested.It solves the problems of temperature cross sensitivity and bending direction discrimination in practical application.The distribution of the research in this theise is as follows:1.Combining the fused tapering technique with the high frequency CO2 laser writing method,three grating sensors suitable for strain measurement are fabricated:remodulation tapered-LPFG sensor,remodulation D-LPFG sensor and local elliptical core LPFG.Because of the effective refraction index of the grating is redistributed,several resonant peaks with different properties are generated.It can realize the strain and temperature measurement simultaneously,and greatly reduce the temperature crosstalk to 0.013 με/℃.Meanwhile,based on the modulation characteristics of the local elliptical core LPFG,the influence of stress change on the core is analyzed.And the maximum strain sensitivity of the sensors is increased to-20.21 pm/με.The strain characteristic is improved by remodulation processing method.2.The arc remodulation LPFG and the embedded multi-mode remodulation D-type grating(EMD-LPFG)are fabricated by combining the high frequency CO2 laser writing technology with the arc discharge and splicing technology respectively.The grating sensor can distinguish the bending direction by inducing the uneven distribution of refractive index on the fiber profile with 10.6 μm laser.This method can enhance the effective refractive index modulation intensity of grating,improve the bending sensitivity and realize the improvement of the sensor performance by composite modulation.The maximum bending sensitivity of EMD-LPFG is 70.21nm/m-1.3.A novel fabrication method of composite modulation for torsion sensor is proposed.The spiral remodulation LPFG based on pre-twist and V-LPFG are fabricated.The influence of shear stress on torsion performance of grating is simulated and analyzed.In addition,the design method of enhanced grating torsional shear stress is proposed.By analyzing the torsional performance of the two sensors,the improved torsional performance of the sensor is realized with the composite modulation method.4.The MZI-MMI composite interference structure and the slice-shaped composite modulation LPFG(SSC-LPFG)are fabricated by using polishing technology to remodulte the grating base structure.The surface evanescent field of grating is enhanced by polishing technology to improve the response to environmental parameters.The parameters of gratings are optimized and the liquid level sensitivity is improved through simulation calculation,in which the liquid level sensitivity of SSC-LPFG is up to 7.08 nm/mm with the measurement accuracy of±10 μm.In this thesis,a variety of composite modulation long period fiber grating sensors suitable for specific physical measurement are fabricated by remodulation processing method,which combining high frequency CO2 laser with other preparation methods.The structure of the sensor is optimized and the sensor performance is improved,which provides a new idea for the design and development of highly sensitive and miniaturized grating structure. |
