Sensing characteristics of core and cladding modes in conventional single mode fibre and photonic crystal fibre.

摘要

Optical fibre sensors are widely used for strain, temperature, surrounding refractive index (SRI) and bending monitoring as well as for gyroscopes. Fibre Bragg gratings (FBG) have been widely used in telecommunications and sensors for temperature, strain and structural health monitoring. A single FBG only has one strong core mode (Bragg) resonance in its transmission spectrum or reflection spectrum, which is sensitive to both temperature and strain perturbations or even to SRI changes if the fibre has a very thin cladding layer. Therefore it is hard to use a single FBG to sense either the temperature or strain without ambiguity. To distinguish temperature and strain effects, two FBGs or one FBG and one long period grating need to be used; it brings the complexity for grating fabrication and sensor applications.;In the thesis, novel tilted FBG sensors have been developed for strain, temperature and index sensing applications using conventional SMF fibres. The differential strain sensitivity between the core and cladding modes in SMF-28 fibre are theoretically derived and they are well matched with the experiment result. The differential strain, temperature and SRI sensitivities in the SMF fibre with different cladding layer diameters have been analyzed and tested experimentally.;To further explore the characteristics of cladding modes in multilayer step-index fibre especially the fibre with a thin metal coating, a new vectorial mode solver, which can solve the lossless, lossy and amplifying optical fibres, has been developed. The coupling coefficients between the core mode and cladding modes are calculated for the fibre with FBG and tilted FBG. The results give a good explanation why the surface plasmon polariton can be excited in a gold coated fibre with tilted FBG.;Finally strong FBGs are written in a germanium/fluorine co-doped core photonic crystal fibre (PCF). A limited number of strong cladding mode resonances are obtained with a FBG. The strain, temperature and refractive index sensitivities of core and cladding mode resonances in this PCF have been determined experimentally. In addition to the usual core and cladding modes, this PCF has a cladding mode whose optical field is confined by the air-hole region, making this mode insensitive to the surrounding refractive index changes, just like the core-guided mode. The main advantage of the PCF short period gratings in sensing applications resides in the presence of the SRI-insensitive cladding mode, whose differential resonance shift can be used to sense axial strain independently of temperature and regardless of the material in which the sensor is placed, including liquids, epoxies or structural materials such as concrete.;By writing the FBG in standard single mode fibre (SMF) with a weakly tilted angle between 3--10°, now in the transmission spectrum there is a strong core mode resonance but there are also cladding mode resonances because of the core mode coupling to the cladding modes. While the core mode resonance is sensitive to temperature and strain perturbations, the cladding mode resonances are sensitive to temperature, strain and SRI perturbations. The core mode resonance and the cladding mode resonances have the same temperature sensitivities. So by tracking the core mode resonance and cladding mode resonances, the temperature effect can be removed and temperature compensated strain and SRI sensors can be developed. Furthermore by coating the fibre with a thin (20--30 nm) gold layer, a surface plasmon polariton can be excited by certain cladding modes. This plasmon resonance is very sensitive to SRI and can be used as a biomedical sensor for bio-molecular reactions occurring at the metal surface.;As the multilayer fibre mode solver could be integrated into a commercial optical CAD software, the fibre sensors based on TFBGs in conventional fibres and FBGs in PCF are suitable for real industrial sensing applications.