Investigation On The Fabrication Technology And Sensing Characters Of Fiber Sensors Based On Femtosecond Laser Micromachining

With the advantages of high sensitivity, compact structure, electromagnetic immunity, anti-corrosion and distributed measurement achievable, optical fiber sensing system has been applied increasingly in scientific research, production, security and other areas. Femtosecond laser has ultra short pulse width, ultra high peak power, which changed the action mechanism of laser beam with materials in manufacturing. Due to the superiority of high precision, wide range of acting materials and true 3-Dimentional fabrication, femtosecond laser micromachining has been widly used in kinds of functional device manufacturing.The thesis centered on the technology of microstructure fiber sensor fabricated by femtosecond laser. The research work was carried out on sensing mechanism, mode analyzing, numerical simulation, fabrication technologies and sensing characters. The problem of beam optimization was also investigated. The main innovations in three aspects are shown as follows:(1) The research on fiber sensing technique with inner air-cavity based on slightly tapering was carried out. A versatile, miniature, robust and low cost sensing device was acquired. Sensing principle of fiber interferometer based on inner air-cavity was studied. The two-beam interference formula was analyzed. The conclusion could be drawn that the temperature, axial strain and outside refractive index (RI) sensitivities were related to mode order. The transmission mode in cavity wall was analyzed with a simulation model. The possibility of red shift and blue shift was discussed in the axial strain and RI sensing application. The fabrication process of fiber sensor with inner air-cavity was experimentally studied. The three steps of manufacture was determined to be femtosecond laser ablation-fusion splicing-slightly tapering. The technological parameters was optimized. The sensing characters on temperature, axial strain and RI was tested. The experiment results was consistent with theoretical analyzing. Simultaneously measurement of temperature and axial strain was realized with the device. To improve the RI sensitivity, the thickness of cavity wall was reduced. The sensitivity in low RI range rose to 4202 nm/RIU, which is about 100 times higher than slightly tapered device.(2) The research on fiber sensing technique with photonic crystal fiber (PCF) based on selective infiltration was carried out. A vector bending sensor of flexible design and controllable fabrication was acquired. Sensing principle of selective infiltrated PCF coupler was investigated. The affection to sensing charactors of structure parameters of PCF and infiltrated material was analyzed with simulation model. A whole-space bending vector sensor was designed by the method of infiltrating two orthometric air holes with different RI liquid. Taking femtosecond laser as key tool, the fabrication process of infiltrated PCF coupler was investigated in experiments. The bend sensing experiments was performed. A linear response in 0～10.7m-curvature range was obtained with sensitivity of-1.20nm/m-1. The result of the experiment highly united with that of theory. A reliable basis for design and fabrication of special functional sensor was provided from the research results.(3) The research on light beam optimizing of femtosecond laser machining was carried out. A pupil filter was designed with which the transversal focal spot was reduced and the focal depth was extended. The focal intensity distribution of radial polarized beam was calculated with vector diffraction theory. The main affecting factors of focal size was analyzed and a continuous phase filter was designed. Taking transversal superresolution factor as optimization target and Strehl ratio as constraint, the filter function was optimized. The calculation results showed that a smaller focal spot (GT=0.75) was acquired with a relatively high energy utilization rate (S=0.5).