Study On Fabricating Microstructures With Solid Material Employing Femtosecond Laser

Micromachining of solid materials employing near-infrared femtosecond laser and its application in fabricating microstructures is mainly studied in this dissertation.1. The multiphoton ionization, tunnelling ionization and collisional ionization induced inside wide-band-gap transparent bulk dielectrics by femtosecond laser are revealed, and evolution of the density of plasma localized in femtosecond laser-matter interaction area is discussed. Theorectical calculation shows that the time for avalanche ionization induced breakdown in transparent bulk dielectrics is very short, typically several tens to hundreds of femtosecond. The refractive-index-change induced by nonlinear optical effects producing in transparent bulk dielectrics by femtosecond laser, such as self-focusing, plasma defocusing and filamentation, are also discussed.2. Optical waveguide is directly written inside fused silica employing Ti: Sappire femtosecond laser amplifier (775 nm central wavelength, 150 fs pulse duration, 1 kHz repetition rate), by adopting transverse and longitudinal scanning modes, respectively. Merits and defects for two scanning modes (transverse and longitudinal) are both elucidated. The reason for elliptical cross-section of waveguides written by transverse scanning is analyzed and improvements of beam shaping beforehand by cylindrical lens or slit is proposed. The influences of incident average laser power, scanning velocity and iterative scanning times on writing results are experimentally investigated, and condition for writing optical waveguide with good performance is determined. A simple experimental setup is founded to characterize the written optical waveguide.3. Fundamental microstructures such as vertical micro-hole, smooth micro-concave surface and micro-channel are fabricated with a photosensitive glass (FOTURAN) employing focused irradiation of near-infrared femtosecond laser, followed by subsequent heat treatment and ultrasonic etching in dilute hydrofluoric acid solution at room temperature. Influences of incident femtosecond laser parameters on morphologies of fabricated microstructures are experimentally investigated, and appropriate condition for fabrication is determined. Saturation of dimensions of irradiated area with incident femtosecond laser paremeters (including fluence and number of pulse shots) is found during experiment, and it is explained based on fully reacting exhaustion.4. Employing focused near-infrared femtosecond laser direct writing, periodic structural change is induced inside non-sensitized photonic crystal fiber (PCF) and single mode communication fiber. Influences of incident average power and exposure time on the induced structural change are investigated, which facilitates the direct writing of long-period-fiber-grating by near-infrared femtosecond laser.5. Micromachining system based on Yb-doped large-mode-area photonic crystal fiber (PCF) femtosecond laser amplifier is founded. The motion control for fabricaiton of micro-pattern using three-dimensional translation stage in the system is improved and location of laser focus spot on the surface of a sample is solved by co-axial monitoring with helium-neon (He-Ne) laser. Three typical types of material, including semiconductor, metallic thin film and transparent bulk dielectrics, are micromachined by the founded system. Experimental results show that lines contained in the rectangular and circular micro-patterns directly written on the surface of silicon (Si), chromium (Cr) and aluminum (Al) film, own uniform and regular morphology. In addition, contamination and heat effect are effectively reduced during micromachining, which herein, protects the substrate. When femtosecond laser with much higher average power than that of direct writing on silicon is focused and scans inside fused silica, filamentation and concomitant visible radiation are found around the laser focus spot.Based on above experimental results, it is revealed that the characteristics of high repetition rate and high average power bring advantages of high efficiency and enhanced capability to the founded system in material processing. Comparing with typical solid kilohertz-repetition-rate femtosecond laser amplifier, the micromachining system based on PCF femtosecond laser amplifier exhibits merits in compact structure, environmental stability and manipulability.