Summary and conclusion

Interferometric sensors are often used for detecting diverse measurands. The most common and simplest one is probably the strain sensor that makes it possible to watch how the structure evolves. For example it can easily be placed in a part made of composite material. The small size of the fiber makes it possible not to adulterate the characteristics of the part. Configurations with a very long sensitive area are also possible but people most often use distributed sensors. In order to watch infrastructures (like bridges and dams...) the most common method is using Bragg gratings, however interferometry can turn out to be more appropriate: using a double interferometer can be useful to watch the sensitive areas (via the OPD's control). These areas are generally air cavities positioned along the infrastructure [38].

It is also possible to measure transversal strains, but they induce birefringence in the fiber, so sensors using polarization modulation are better to detect them (cf next chapter).

All the measurands making a strain on the sensitive area are easily detectable, others quantities also are measurable like electric and magnetic fields via magneto- and electro-optic effects [39].

Interferometric techniques enable fine and sensitive measuring but they are limited by the methods you need to resort to in order to get the phase difference without ambiguity. That is why Bragg gratings are still most often used.

Optical fiber's intrinsic sensitivity to temperature is one of its main drawbacks; you have to use discriminatory techniques to distinguish between the measurands. Of course, this effect is very useful for producing a thermometer.