Optical fiber devices fabricated by femtosecond laser micro-machining for sensing applications.

摘要

Femtosecond pulse (fs) laser has been widely applied for micro-machining due to its unique advantages, which is suitable for fabrication of micro-structure based optical fiber sensors (OFS). This thesis firstly reviews the development of fs laser micro-machining and micro-structure OFS fabricated in literatures.;The fs laser micro-machining system used in this thesis is described, which includes: 1) a commercialized fs laser; 2) an external optical path; and 3) a microscopy system. Detailed descriptions on the focusing micro objective lens and its effect on the final micro-structure dimensions are presented.;With proper micro-machining parameters, a symmetric located micro-hole can be drilled in the core center. The micro-hole introduces scattering loss that is changeable according to the external refractive index (RI). The RI response depends on the micro-hole diameter. This micro-hole can be used as RI sensor, with a linear RI response and a small temperature-RI cross sensitivity.;Fs laser line scanning is applied to fabricate micro-cavity into fiber. A Mach-Zehnder interferometer (MZI) can be formed by asymmetrically positioned the micro-cavity deviated from the core center. The micro-cavity can be applied for both high temperature (up to 1100 °C) sensing and RI sensing around water.;Structural modulated long period fiber gratings (LPFG) can be fabricated by periodically positioning micro-holes along the fiber length. The micro-holes can effectively couple light from core mode to cladding mode and enable a compact grating dimension. This kind of LPFG can be applied as RI sensor.;Integration of micro-holes into fiber with fiber Bragg grating (FBG) inscribed in advance is achieved by locating micro-holes asymmetrically on one side of fiber core. The micro-holes can partially expose the core to external RI without significantly damaging the FBG. This OFS can detect both temperature and RI simultaneously, by tracing the wavelength change and the transmission loss of the FBG resonant dip.;Selective infiltration of liquid into air holes of photonic crystal fiber is achieved by fs micro-machining with advantages of flexibility, accuracy and device robustness. Two kinds of sensor devices, a directional coupler and a MZI are developed, both of which possess a large temperature sensitivity.