|Title:||Optical fiber devices fabricated by femtosecond laser micro-machining for sensing applications|
|Subject:||Optical fiber detectors.|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||xvi, 128 p. : ill. ; 30 cm.|
|Abstract:||The development of optical fiber sensors (OFS) with high sensitivity and compact dimension is receiving increased attention. One promising solution is to develop micro-structure based OFS, which requires excellent performance of fabrication tools. Femtosecond pulse (fs) laser is widely applied for micro-machining due to the advantages of high precision, good quality and excellent spatial resolution, which is suitable for fabricating micro-structures over fiber. This thesis starts with a background review of the fs laser micro-machining technique, including the generation of ultrashort pulse duration and high power intensity, the advantages and typical applications of fs laser micro-machining and micro-structured OFS fabricated by fs laser in previous literatures. The fs laser micro-machining system setup is described, which includes: 1) a commercialized fs laser; 2) an external optical path; and 3) a microscopy system. The detailed descriptions on the focusing micro objective lens and its effect on the fabricated micro-structure dimensions are presented. With proper micro-machining parameters, a symmetrically located micro-hole can be drilled into the core center. The micro-hole can introduce scattering loss that is changeable according to the external refractive index (RI) and the transmission properties depend on its diameter near the core. Such a micro-hole can be used as RI sensor based on transmission detection, with a linear response and a small temperature-RI cross sensitivity, which enables temperature independent RI sensing.|
Fs laser scanning technique is applied to fabricate micro-cavity into fiber. By asymmetrically positioned the micro-cavity to be deviated from the core center, the remaining core and the micro-cavity can form a Mach-Zehnder interferometer (MZI) with a large RI difference between the two arms and enables a compact device dimension. The fringe visibility can be optimized by adjusting the deviation position. The micro-cavity can be applied for both high temperature (up to 1100 ℃) sensing with a good repeatability and RI sensing with an extremely large sensitivity around the RI value of water. Novel structural modulated long period fiber grating (LPFG) can be fabricated by periodically positioning micro-holes either symmetrically in all solid photonic bandgap fiber or asymmetrically in single mode fiber. The micro-holes can effectively couple energy from core mode to cladding mode and thus enabling a compact grating dimension. Cladding modes involved in coupling are numerically simulated and compared with the experiments. The RI sensing properties are measured for both types of LPFG. Integration of micro-holes into the fiber with inscribed fiber Bragg grating (FBG) is achieved by locating micro-holes asymmetrically on one side of the core. The micro-holes can partially expose the core to external RI without significantly damaging the FBG structure. This composite sensor can detect both temperature and RI simultaneously, by tracing the wavelength change and transmission loss of the FBG resonant dip. Selective infiltration of liquid into air hole(s) of photonic crystal fiber is developed. This technique has the advantages of flexibility, good accuracy and device robustness. Two kinds of sensor devices, a directional coupler and a MZI are developed based on this technique. Both of the devices possess a large temperature sensitivity.
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