Long period grating-based humidity sensor for potential structural health monitoring

This paper reports the work on the development of novel humidity sensors using long period grating (LPG) technology in an optical fibre. The sensors are created through coating a thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level, onto an LPG. The LPG written in a single mode optical fibre provides a means to change the coupling between the core and cladding modes thus to induce a wavelength shift in the attenuation bands of the transmission spectrum of the optical sensor device created. In this work different sensor configurations were created by coating various LPGs and then characterizing the resulting sensor performance. The work carried out has shown that, compared to the more familiar fibre Bragg grating (FBG)-based humidity sensors, LPG sensors show higher measurement sensitivity, with more relaxed requirements for the coating thickness and uniformity, with the potential for their development as chemical sensors through optimization of the design of the coating materials chosen. In this research, the sensors which were designed and fabricated were created for future applications in determining moisture ingress in a range of concrete materials when subjected to various environmental conditions.