Fabrication And Sensing Characteristics Of Optical Fiber Sensor Based On Refractive Index Remodulation

Optical fiber sensors are widely used in intelligent equipment manufacturing,structural health detection,biochemical and medical fields due to their advantages of small size,high sensitivity and anti-electromagnetic interference.Different preparation methods give it different sensing characteristics,so that the fiber sensor can be used in the measurement of axial strain,bending,refractive index and other physical quantities.As the monitoring target gradually develops to deep space,deep sea and other areas with complex environment,the sensitivity and integration of fiber sensors are increasingly required.In order to meet the diverse sensing needs,it is difficult to achieve a breakthrough in sensitivity by using a single preparation method.In order to improve the performance of fiber sensors in sensing with different physical characteristics,this paper uses CO₂laser for refraction remodulation of the original sensing structure to prepare three kinds of high-sensitivity and low-cost optical fiber sensors with different principles and further study their sensing characteristics.The main work content is summarized as follows:1.A four-sided modulated long-period fiber grating with high strain response is prepared by refraction remodulation of traditional D-shaped long-period fiber grating with CO₂laser.The basic parameters of the grating were determined by the beam propagation method,and the influence of the etching groove on the axial strain sensitivity of the long-period fiber grating was analyzed.The results show that the strain sensitivity of the structure is 127.5pm/μεin the range of 0～125με,and the average sensitivity is 17.7 pm/μεin the range of125～575με,which is much higher than the traditional long-period fiber grating.By combining with the stress distribution simulation,the principle is elaborated from three aspects:equivalent radius,strain sensitive area and core deviation.2.By combining optical fiber splicing and CO₂laser exposure,a highly sensitive bending sensor with vector bending discrimination ability is prepared.The assembled long-period fiber grating was fabricated by fiber splicing technology,and the refractive index remodulation was carried out by CO₂laser to make it meet the phase matching and form a resonant peak.Due to the high refractive index modulation efficiency of multi-mode fiber,the length of structure is greatly shortened and the evanescent field is increased.Meanwhile,the refraction index distribution of CO₂laser is reconstructed,which leads to the directional mode distribution under bending,and thus has the ability to distinguish vector bending.The results show that the maximum bending sensitivity is 85.81 nm/m^-1（0°）,16.18 nm/m^-1（90°）,-16.18nm/m^-1（180°）in the three orthogonal directions,which realizes the highly sensitive vector bending measurement.3.The refractive index remodulation of the single-mode-graded index multimode-single-mode fiber interferometer is carried out by CO₂laser.The self-imaging effect in the structure is destroyed and the evanescent field of the structure is greatly enhanced.Thus,the high sensitivity refractive index sensor is prepared.By using the beam propagation method to simulate the self-imaging effect of the graded index multimode fiber used in the experiment,the length of the graded index multimode fiber is selected as an integer multiple of the self-imaging length,which reduces the insertion loss.Using the same modulation power,the refractive index remodulation of the single-mode-gradient index multimode-single-mode fiber interferometers of different lengths was carried out,and the refractive index sensing tests were carried out respectively.The results show that the maximum refractive index sensitivity reaches 811.00 nm/RIU in the range of 1.333-1.419.The refractive index sensitivity of this structure is significantly higher than that of the traditional multi-mode interference structure,which confirms the potential of the graded refractive index multi-mode fiber as a sensing structure in the field of biochemical measurement.