applications. In addition, these inexpensive and versatile sensors have been applied successfully to detect rising liquid levels, leading to the development of a wireless water level detection system suitable for monitoring flood-prone areas. POF, which is commonly used for local data networking applications, offers an attractive alternative to conventional electrical-based sensors. Firstly, being non-conducting, POFs are immune to electromagnetic interference and therefore more reliable, especially during lightning storms. Secondly, POFs do not rust nor easily fracture and this resilience makes them suitable for use in harsh environments. Thirdly, POFs are cheap and can be economically deployed in large numbers.

As damage sensors, these fibres can be attached to sections of a concrete structure prone to cracks. The earlier detection of cracks on rebar concretes and subsequent remedial actions can prevent moisture and chlorides absorption, which leads to corrosion and more severe cracking of these load-bearing structures. In this research, several sensitized POFs were bonded to the surface of concrete beams to detect and monitor the crack propagation of the host structure under load. It has been found that these fibres are sufficiently sensitive to successfully detect the initiation and propagation of a hairline transverse crack (see Figure 1).

In another application, a novel POF sensor was developed for monitoring structural vibration and sensing of very high strain. The developed signal detection system monitors the light intensity emanating from the POF sensor and shows potential for detecting vibration frequencies up to 1 kHz. Using standard frequency spectrum analysis, such as the fast-Fourier transform, it has been found that the POF sensor is capable of detecting multi-mode vibration, comparing well with other reference sensors such as piezofilms and optical fibre Bragg grating sensors. In addition, the POF sensor has also been tested and found to be capable of measuring very high strain in applications and materials where large deformations are expected. Measuring strain levels of up to 50% is possible using the intensity-based POF sensor with an error of up to 2%. In other applications where better strain sensitivity is required, a liquid medium could be introduced within the cavity of the sensor to obtain resolution of 50 microstrain. Due to the simplicity in design and cost-effectiveness, the POF sensor offers an attractive option for structural health monitoring applications.

In view of the liquid level detection capability of the POF sensor, a novel wireless flood monitoring system has been developed, arguably the first of its kind in the market. By tailoring the design, the sensor integrates seamlessly with a generic wireless network system, rendering the device small, highly portable and energy-efficient. A flood monitoring simulation has been successfully carried out in a 24m x10m x 0.9m wave basin (see Figure 2) where four of these wireless POF sensors were used to detect the rising water level in the basin, highlighting the potential of the system for real flood monitoring applications.

This work was done in collaboration with research fellow Dr KSC Kuang.