The Fiber Bragg Grating
Probably the most versatile fiber-optic sensor is the fiber Bragg grating (FBG). An FBG is made of a periodic perturbation of the refractive index of the core of an optical fiber along its axis. Such a fiber Bragg grating can be seen as a filter that reflects light around a particular wavelength, called the Bragg Wavelength, and that is transparent to other wavelengths. The Bragg Wavelength depends on mechanical strain exerted on the grating as well as on the grating’s temperature. In this way, mechanical forces and temperatures can be quantified by measuring the wavelength at which the FBG reflects light (or by measuring at which wavelength the FBG does not transmit light). This is an entirely optical way of performing the measurement; no electricity is involved up to now.
The absence of electricity at the point of measurement guarantees that no electromagnetic interference occurs. The low mass and low volume of an FBG-based sensor cause little thermal or mechanical interference with the unit under test.
The low mass and low volume of an FBG-based sensor also allow for easy accommodation of a number of sensors even under the stringent mass and volume requirements of a launcher. Last but not least, it is possible to cascade multiple FBGs along a single fiber. In this concept, each FBG operates around an individual Bragg wavelength. By wavelength division multiplexing, each sensor can be individually queried using a designated wavelength range.
In this way, FBG sensors can contribute to highly advanced sensory solutions on board of commercial launchers, such as the Ariane. This can be of benefit for increased situational awareness during launch and for emerging technologies such as active vibration damping. As an additional advantage, an FBG-based measurement system can be used as an overlay system that compliments established and proven techniques (such as electrical sensing) without causing problems of mutual interference.