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comprehensive experimental study is carried out to develop FBG based all-optical sensors to monitor tilt, vibration, strain and relative humidity (RH), which are some of the most important parameters in SHM and other engineering/industrial applications. It’s important to mention that not only a reasonably good sensitivity is required in such applications; sensor is simultaneously desired to have an ability to capture the applied perturbations strategically very effectively and without any inherent instability. The objective of the research carried out in the present thesis is to develop these sensors with manifold enhanced sensitivity, tunable response characteristics and without inherent measurement instabilities. A novel, temperature insensitive, pendulum based sensor design strategy employing four FBGs with a feature of inherent tuning capability for its sensitivity is characterized. Sensor exhibits a completely reversible linear response over a dynamic range of 10o. An excellent sensitivity (~0.0626 nm/o) that can further be nonlinearly tuned with a much improved resolution (0.008o), accuracy (±0.36o), maximum discrepancy (less than ±0.0231nm) and maximum angular uncertainty (~±0.34o) are established for this sensor. In order to capture the applied tilt perturbations strategically more effectively with enhanced stability, another novel FBG based tilt sensor, employing a non-pendulum type design strategy is realized. Being non-pendulum type, there is no possibility of any inherent friction and the limiting effects at mechanical joints. This sensor exhibits a better resolution (better than 0.004°), a better accuracy (∼±0.05°) and a lesser maximum discrepancy (∼±0.001nm) during the forward as well as the reverse tilts. Sensor response is characterized by a very high degree of reversibilityChapter 7: Conclusion of Thesis and Future Scope 142
and repeatability over the observed dynamic range. In the next step, a simple design strategy of FBG based two axial all-optical vibration sensor, employing a methodology to ensure chirp-free mechanism is proposed and experimentally demonstrated. In this design, vibration of sensor displaces the seismic mass fixed between two flexural beams, thus inducing a variable strain in the two beams. This strain variation results in a Bragg wavelength shift for corresponding FBGs attached to the beams. An excellent capability of resolving externally applied signals having frequency in 10Hz to 350Hz and the acceleration in 0.65 to 70m/sec2 range is observed. Further, if the inherent strain sensitivity of FBG can be enhanced to a very large value, it will improve the response of all the FBG based sensors. In order to achieve this, a single FBG sensor with corrugated cladding structure over the FBG carrying core is designed and experimentally investigated for strain sensitivity enhancement. The corrugated structure across the FBG section of the fiber splits a single FBG into multiple & independent FBGs, each suitable for independently sensing an applied perturbation. Further, a manifold enhancement of the strain sensitivity (over ten times relative enhancement of strain sensitivity) is observed, which is impossible to realize with a normal FBG. Finally, in the last part of research, FBG based humidity sensor is developed employing a novel mechanism of effective-index modulation with an ambition to achieve widest possible dynamic range and optimum sensitivity. To achieve these objectives, fiber diameter in the grating region is reduced to a lowest possible working limit (~ a few microns) using appropriately controlled chemical etching. A wide linear dynamic range ~3–
94 %RH with good sensitivity of ~0.082 pm/%RH is realized. The response of the developed sensor is observed to be completely reversible with an appreciably stable characteristic during repeatability tests.
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Thesis work leads to the possibility of wide range of potential research work to be carried out in the future. For example, all the sensors reported here in the thesis are characterized in the laboratory environment; however, these sensors are aimed for real-field applications such as in SHM. While deploying the sensor in monitoring locations/over the monitored infrastructures, fiber sensor should not only be protected from a possible source that may damage it but also, all the characteristic features and merits of the sensor should be preserved in such real-field monitoring applications. As an example, the reported FBG based RH sensor in the present form cannot be used to detect moisture content in concrete structures. This emphasises the need of an intelligent packaging of the sensor and then re-characterizing the sensor response in real-filed locations. Further, as mentioned for the corrugated FBG based sensor developed in the thesis work, a single FBG sensor splits into three independent FBG sensors with enhanced strain sensitivity. As a future work, one needs to employ the three sections of this sensor to design a sensor that can monitor three different parameters independently without any cross-talk.
Similarly, in the case of FBG based humidity sensor, owing to the laboratory constraints, we could not etch the diameter of the fiber over FBG region below 13.5µm, though, we observed that the sensitivity will increase if the diameter is lowered further. In addition, we also observed that if the RH variations are mapped around the surrounding RI of ~1.4, sensitivity will increase manifold. RH variations can be easily mapped at around this surrounding RI by coating the etched fiber in the FBG region with a thin film of suitable chemical composition. This leads to the possibility of designing a FBG based RH sensor employing effective-index modulation technique with a far better response characteristics.