Experimental optimization of curvature sensors
Student thesis: Master's Thesis
A fiber-optic sensor of curvature has been developed with the sensitivity high enough to make it suitable for measurement of deflection of structures under mechanical loading. Typically, deflection-curvatures are extremely small, with the curvature-radii in the kilometre range. While this sensor is intensity modulated, as summarised further in this thesis, its sensitivity exceeds by far that of numerous long-period grating sensors of curvature reported recently, and sets it apart as the only sensor of deflection curvature. The application domain is specified in this thesis in which the deflection-curvature measurement concept offers advantages over the alternative of measuring strain in terms of the sensitivity, flexibility to choose the location for the sensor placement, and immunity to the effects of microstructural interfacing between dissimilar materials that the sensor presence introduces. Naturally, details of the sensor itself are also given. Two kinds of advancements of the deflection-curvature sensor are reported in this thesis: • key features of the earlier sensor configuration have been optimised experimentally, and • an improved configuration has been proposed with the two-way light transmission across the sensor for increased modulation effects and a more suitable integration in/on structures for more design flexibility. The features optimised experimentally are the depth and length of the device's sensitive zone. The respective dimensions of 4.5 mm and 110 μm gave the highest sensitivity for a fixed amount of bending of the structure the sensor had been mounted in/on. The second kind of improvement reported in this research is that the sensor configuration has been improved by producing mirrored (distal) fiber-ends that reflect the light back to the input fibre ends. Not only does this two-pass light transmission configuration allow for the light to be modulated twice at the sensitive zone, but also the access to the structure is needed from one location only, and the fibre does not have to transverse the entire structure. Electron beam evaporation technique was applied to produce the mirrored fibre ends. High-purity (99.99 percent) aluminium was used in this process that resulted in the 485 nm coating with 93 percent reflectivity. The stronger modulation effects of the proposed sensor configuration were confirmed by the 40 percent sensitivity gain relative to the earlier sensor configuration with the one-way light transmission across the fibre (from emitter to detector). With the improvements achieved, deflection-curvatures with the curvature-radii approaching 2 km have been measured.
- Optical fiber detectors