Introduction

Optical fibers are generally used for transmissions and telecommunications. Because of their low attenuation and large bandwith, they made it possible to create land and transoceanic networks to carry information for telephone, the Internet, etc. The refraction's indexes of silica (i.e. of glass) vary with the bandpass (i.e. the dispersion), the temperature, possible strain, pressure... all these influences modify the wave propagating in the fiber. These modifications are generally detrimental to telecommunications but can be used to make captors sensitive to temperature variations, strain, ...

Optical fibers are very interesting in many respects:

• small transverse dimensions, do not disrupt their environment and can be used in the medical field,

• they are insensitive to electromagnetic disturbances,

• they have low attenuation, so that the sensor can be set up far away,

• regarding light, they can be used for spatial and aeronautical applications,

• Optical fibers have good stability over time,

• and good resistance to corrosion.

However they have some downsides:

• fragility,

• related connections are complex,

• some fibers can only withstand a limited range of temperature (+85°C for plastic fibers),

• optical fiber sensors prices are (for the time being) higher than traditionnel sensors, but there performances are not higher in standard applications.

The purpose of this lesson is to give a general survey about the various techniques used to make sensors from optical fibers. We will successively detail the various techniques of modulation of information in this lesson, which means :

• intensity modulation,

• phase modulation,

• polarisation modulation.

Readers can find further information in the wide literature, particularly the reference [1], [2], [3] , and [4] .