Experimental Investigation On The Fabrication Of Volume Grating And Novel Fiber Sensors By The Use Of Femtosecond Laser Micromachining

Femtosecond laser micromachining has been widely used in materials processing for its high precision, good quality and capability of three-dimensional micromachining. The heat effect can be ignored during femtosecond laser micromachining since pulse energies can be deposited into materials in the vicinity of the focal volume in an ultrashort time scale due to the short pulse duration (<1 ps). Novel micro-optic devices and features can be fabricated at the surface or in the bulk of materials with the advantages aforementioned. In this dissertation, experimental study on the micromachining of silicon wafer, fused silica and fiber materials have been perfomed by use of the femtosecond laser micromachining methods. Combined with theoretical analysis and numerical simulation, volume gratings have been fabricated in the bulk of fused silica with femtosecond laser three-dimensional fabrication technique, and novel fiber sensors have been fabricated in fibers by direct femtosecond laser ablation and the corresponding sensitivities for sensing applications have been investigated experimentally. The main contents are listed as follows:(1) The materials micromachining methods and procedures with femtosecond laser micromachining have been investigated systematically, in the case of silicon wafer processing. The morphologies, surface oxidation in the vicinity of the ablated holes and cutting kerfs in silicon wafer have been studied with the help of optical microscopy, scanning electronic microscopy (SEM) and energy dispersive X-ray analysis (EDX), and a micro-mould has been fabricated with optimal processing parameters for applications in MEMS.(2) The ablation threshold and bulk damage threshold of fused silica have been investigated experimentally. The processing parameters on the fabrication of volume gratings have been studies and optimized, and the diffraction power spectra of the gratings have been conclusively characterized by experiments and numerical simulations.(3) Refractive index sensor based on a microhole in the conventional single mode fiber (SMF-28) has been fabricated by use of femtosecond laser micromachining. The transmission characteristics and working principal of the sensor are discussed with a simple geometrical optical model and numerical simulations.(4) Long-period fiber gratings (LPFG) have been fabricated by direct femtosecond laser drilling in single mode fiber (SMF-28, from Corning) and photonic crystal fibers (LMA-10, from NKT Photonics and all-solid photonic bandgap fiber, from YOFC). The resonance wavelength shift and transmission spectrum of the microhole-structured LPFG are analyzed with coupled-local mode theory and simulation results obtained by the finite element method (FEM). And the mode profiles in the near field of the microhole structured LPFG are measured with a tunable laser and an infrared CCD camera. Furthermore, the polarization denpendent loss (PDL) of the device has been tested with an all optical parameter analyzer, and the sensitivity to surrounding refractive index of the LPFG has also been investigated by use of standard RI liquids (from Cargille Laboratory).(5) Miniaturized fiber inline Mach-Zehnder interferometer has been fabricated in single mode fiber with femtosecond laser ablation, and its corresponding transmission characteristics and sensitivity to surrounding refractive index and temperature have been discussed with the well-known two-beam interference equation. Results show that the miniaturized fiber inline Mach-Zehnder interferometer is suitable for refractive index sensing and high-temperature sensing with high sensitivity.