Multimode interference based optical sensors for structural health monitoring application / Nur Hotimah Yusof

Nur Hotimah , Yusof (2019) Multimode interference based optical sensors for structural health monitoring application / Nur Hotimah Yusof. PhD thesis, Universiti Malaya.

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Abstract

This research work focuses on developing new optical fiber sensing probes for structural health monitoring (SHM). Two types of optical sensor probes; microfiber and single mode-multimode-single mode fiber (SMS) structure were successfully fabricated using a flame brushing and fusion splicing, respectively. The non-adiabatic type of microfiber produced a periodic behavior at the output spectrum due to the oscillations as a result of back and forth coupling between core and cladding modes. On the other hand, the SMS structure generates a sufficient bandpass spectral response for a given wavelength range. Both non-adiabatic microfiber and SMS device are capable to be used to demonstrate both temperature and strain sensors. Temperature and strain sensors were successfully demonstrated using non-adiabatic microfiber and SMS structure as a probe. The proposed microfiber and SMS sensors were shown to be able to measure temperature stably up to 400°C with sensitivities of 6.0 pm/◦C and 11.1 pm/◦C, respectively. Both probes were also established for strain measurements. The strain sensitivities originate from the difference of the effective refractive index of the core and the cladding modes, which induces resonant wavelength shift against the strain. The strain sensitivity of the microfiber and SMS sensor was -1.36 nm/n and -2.06 nm/n with a linearity of more than 0.99 and 0.97, respectively. Both sensor designs are simple and thus they can be mass-produced with a relatively low cost. Finally, both microfiber and SMS sensor probes were successfully employed for measurements of compression strain in both steel and concrete beam structures. For instance, the microfiber sensor exhibits linear sensitivity of 0.0094 nm/kN for the compression strain measurement. The optical sensor was also able to detect the elastic limit of the steel beam by its significant change of the peak center wavelength. On the other hand, the SMS sensor exhibits linear sensitivity of 0.0323 nm/kN for the compression strain measurement. The steel beam achieved its elastic mode limit at a compressive load of 130 kN, which agrees well with the conventional electrical resistance (ER) strain gauge measurement. The microfiber sensor was also successfully demonstrated for measuring compression strain and to the breaking point of a concrete beam. It exhibits linear sensitivity of 0.0454 nm/kN for the compression strain measurement. The concrete beam achieved its modulus of rupture limit at compressive load of 8.0 kN, which agrees well with the conventional ER strain gauge measurement.

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