Smart fibres for structural health monitoring SHM

Optical power losses are very small, about 0.03 dB/km. This means that over 100 kilometres the optical power is halved this is an important requirement for telecommunications. Optical power loss is minimized near the wavelengths of 1300 nm and 1550 nm, where the minimum Rayleigh scattering or minimal infrared absorption is found. For sensing purposes, the losses in the fibre optics are irrelevant, but still 1550 nm is the preferred window, since the optoelectronic components are more readily available. 

This discussion is mainly limited to measurements of temperatures and strains, but fibre optic sensors are used in many other applications, as chemical sensors, electrical fields, etc. A global review can be found in Lopez-Higuera (2002). 

A sensor is a device capable of converting a physical or chemical quantity into readable information. In addition to a topological classification (local or distributed, intrinsic or extrinsic), a more basic classification can be made according to the optical parameter affected by the external factor intensity, phase, wavelength, and polarization. 

Phase modulation as optical fibre sensors, or interferometers 

Interferometry is the most accurate laboratory technique for precise measurement of distance. It should be recognized that direct measurement of the phase of an electromagnetic wave cannot be done for the optical wavelengths, as is done with oscilloscopes for low-frequency electrical signals, and only the light intensity, which is the average power of the electromagnetic field, can be measured with photodetectors. 

---