How FBG sensors work …
A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. The wavelength (λ) of the reflected light depends on the spacing of a periodic variation or modulation of the refractive index of the fiber core, which gives rise to a series of dielectric mirrors for certain λ.
A fiber Bragg grating thus acts as a band rejection optical filter transmitting all the wavelengths of light that are not in resonance with it and reflecting wavelengths that satisfy the Bragg conditions of the core index modulation.
The sensing function of an FBG originates from the sensitivity to externally applied thermal perturbations of both the refractive index of the optical fiber and the grating period within the fiber. The temperature sensitivity of the FBG is mainly due to the thermo-optic effect i.e., temperature-induced change in the glass refractive index and to a lesser extent, on the thermal expansion coefficient of the fiber.
An external temperature variation thus induces a change in the reflected light wavelength, which can be monitored using a laser interrogator and a photodetector. An external calibration allows correlating the wavelength displacement with the temperature change. The correlation is precise and almost linear, so the temperature surrounding the FGB sensor can be measured with a great accuracy over a large range from room temperature to at least 800ºC.
Network of sensors by multiplexing …
Many FBG sensors can be written on a single fiber, each having a specific resonant wavelength, so they can be multiplexed into a sensor network where different temperatures can be measured simultaneously at different locations along the length of the optical fiber. Sensors can be positioned as close as to few millimeters or separated by several kilometers.
