Structural Health Monitoring Potential using a Long Period Grating-Based Humidity Sensor


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Title: Structural Health Monitoring Potential using a Long Period Grating-Based Humidity Sensor

Author(s): T. Venugopalan, M. Rajesh, T. Sun, and K.T.V. Grattan

Publication: Special Publication

Volume: 252


Appears on pages(s): 85-98

Keywords: humidity; long period grating; structural health monitoring

Date: 3/1/2008

In this research, a series of innovative optically-based sensors, which were designed, fabricated and characterized were created for potential evaluation for applications in determining moisture ingress in a range of concrete materials subjected to various environmental conditions. The approach taken to the creation of these novel humidity sensors is using long period grating (LPG) technology in an optical fibre. Several sensor configurations are fabricated by coating LPGs and then characterizing and cross-comparing and evaluating the resulting sensor performance. The thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level when coated onto a LPG written into an optical fibre, provides a means to change the optical propagation in the fibre and thus to induce the a wavelength shift in the attenuation bands of its transmission spectrum, which then is calibrated against the measurand, humidity. When compared to the more familiar optical fibre-based humidity sensors, using Fibre Bragg gratings (FBGs), the LPG-based devices show a much higher measurement sensitivity, with more relaxed requirements for coating thickness and uniformity.