Femtosecond Laser Micromachining Of The All-optical Fiber Sensor And Its Application

With the advantages of miniaturized in size, high sensitivity, simple to fabricate,immunity to electromagnetic interference (EMI), resistance to corrosion, andreflection-based configuration, the all-optical fiber sensors are becoming more and moredesired for various applications in many areas, such as aeronautics and astronautics,petrochemical industry, chemical detection, biomedical science and homeland security, etc.Recently, due to the characteristics of high fabricating precision, low thermal diffusioneffect and defect threshold, and the capability of three-dimensional fabrication,femtosecond laser micromachining technique has been developed rapidly and become moreand more prominent regarding to the application of all-optical fiber sensorsmicromachining. This project is of great importance in significant study and worthyapplication, and as one up-to-date research scope is currently most attracted and focused byresearch scholars.According to the mechanism of femtosecond laser nonlinear interactions withtransparent materials, a series of high performance all-optical fiber sensor probes operatedin a reflection configuration were proposed and fabricated. Meanwhile, several significantsensing measurements (e.g., high temperature, high pressure, refractive index variation, andmolecule identification) of the proposed sensors were demonstrated in this thesis as well.The main research contents and innovations are shown as follows:(1) A novel fiber inline Michelson interferometer was fabricated by micromachining astep structure at the tip of a single-mode optical fiber using a femtosecond laser. The stepstructure splits the fiber core into two reflection paths and produces an interference signal.A fringe visibility of18dB was achieved. Temperature sensing up to1000°C andrefractive index sensing application in various concentrations of ethanol solutions were alldemonstrated using the fabricated assembly-free device. The temperature sensitivity was~14.72pm/℃at specific wavelength and the low refractive index sensitivity indicated thatsuch device was very suitable for high temperature sensing measurement.(2) A novel all-in-fiber optofluidic device was fabricated by femtosecond laser (fs)irradiation and subsequent selective chemical wet etching. Horizontal and verticalmicrochannels can be flexibly created into an optical fiber to form a fluidic cavity withinlets/outlets. The fluidic cavity also functions as an optical Fabry-Perot (FP) cavity in which the filled liquid can be probed. The assembly-free micro device exhibited a fringevisibility of20dB and demonstrated for measurement of the refractive index (RI) of thefilling liquids. The RI sensitivity was~1135.7nm/RIU at specific wavelength. Meanwhile,high temperature survivability of this micro device was also demonstrated with thetemperature sensitivity of0.58pm/℃, which indicated that the proposed assembly-freeall-in-fiber optofluidic device had lower temperature cross-sensitivity and was attractive forRI sensing applications in high-temperature harsh environments.(3) We report a fiber optic Fabry-Perot interferometric pressure sensor with itsexternal diaphragm surface thinned and roughened precisely by femtosecond (fs) laserablation. The thickness of the micromachined diaphragm (~2.6μm) was closely related tothe pressure sensitivity and pressure measurement range of the proposed device. The laserroughened surface helps to eliminate outer reflections from the external diaphragm surfaceand makes the sensor immune to variations in ambient refractive index. Static pressure (upto6.895×105Pa) and high temperature (up to700℃) measurements were all investgatedusing the proposed device. The sensitivity of pressure and temperature-pressure crosssensitivity were2.8×10-4nm/Pa and15.86Pa/℃, respectively, which indicated that thesensor was useful for pressure measurement in a high temperature environment with lowtemperature dependence. The sensor has also been demonstrated for measurement ofautogenic pressures of water vapor up to200℃.Without temperature compensation, thepressure measurement results agreed well with those calculated based on the theoreticalmodel.(4) We present a new approach of simultaneously measuring temperature and pressurewith fiber inline sensor. This approach utilizes cascaded fs laser micromachined intrinsicFabry-Perot interferometer (IFPI) and extrinsic Fabry-Perot interferometer (EFPI) astemperature and pressure sensing elements, respectively. Experimental results showed thatthis sensing system can resolve temperature and pressure unambiguously in a pressurerange of0to6.895×105Pa and temperature range from room temperature to700C.(5) Different types of fibers were compared for construction of reflection-basedsurface-enhanced Raman-scattering (SERS) fiber probes. The probes were made by directfemtosecond (fs) laser micromachining of nanometer structures on the fiber endfaceandsubsequent chemical plating of a thin layer of silver. In comparison with the silica fibers,the single-crystal sapphire fiber has much lower background Raman scattering. The fs laseris found effective to fabricate high-quality sapphire fiber SERS probes for detection of weak Raman signals in a reflection configuration.