Fibre Optic Sensors
Monitor Optics Systems uses Fibre Optic based sensors for their structural health monitoring solutions, typically using Fibre Bragg Gratings (FBGs) to measure changes in infrastructure. Optical Fibres are flexible optically transparent fibres that transmit light through successive internal reflections along the fibre. External actions on an optical fibre can induce variations of properties of the light travelling into an optical fibre and these can be accurately measured.
Fibre Bragg Gratings
An FBG is a periodic variation in the refractive index written into the fibre core, which reflects particular wavelengths of light, and transmits all others. This is demonstrated below where a portion of the light is reflected while the rest is transmitted. The reflected wavelength is sensitive to both strain and temperature.
Fibre Bragg Gratings Under Strain
A change in strain will modify the grating period, which causes the reflected wavelength to shift linearly. This is a reversible physical change to the fibre, and thus, an FBG is self referencing, meaning no recalibration is required.
Fibre Bragg Gratings Under Temperature Change
A change in temperature will also result in a shift in wavelength, this time predominantly due to the change to the refractive index the temperature change causes. While the fibre does expand with temperature, the change in wavelength is minimal when compared to the change caused by the refractive index change.