Fiber Bragg gratings, although excellent sensors for many applications, are sensitive to temperature and strain only. Any other parameter to be sensed by the FBG (e.g., pressure, gas, force, inclination, acceleration, etc.) has to interfere with one of these two variables. When measuring temperature, the only concern for the engineer is to provide a good thermal conduction between the spot to be monitored and the fiber locale where the FBG is inscribed. The FBG measures its internal temperature and not the environmental temperature where it is located. It is the duty of engineers to ensure that both temperatures are equal or else the measurements will be incorrect. This circumstance is true for any other temperature sensor, and it also applies to strain.
The way the fiber with the inscribed FBG is connected to the sample to be monitored is crucial. When measuring temperature, a fiber only 125 μm in diameter presents a negligible thermal impedance, so the rise time is also negligible for most industrial applications. It is important to avoid transferring any strain from the sample to the FBG.
The measurement of strain is complex because all strain experienced by the sample must be transferred to the FBG, which is achieved by affixing points of the FBG to the sample. Of course, the same concern is applicable to conventional electrical strain gauges, but the plastic base of these sensors is much easier to fix for two reasons: the contact area and the elasticity.