Coupling between two fibers

In this last part dedicated to intensity modulation we will talk about sensors coupling two fibers or more generally two waveguides. This kind of sensors includes the configurations where the light is extracted from a waveguide, interacts with the measurand and is coupled in another or in the same waveguide. This kind of sensors can have different structures with mono- or multimode fibers. Coupling two fibers can be for transmission or for reflection, as shown on Figure 4.

The main element to get a good coupling sensor is to perfectly know how to couple the two fibers you use. There are lots of articles about this topic, notably [16], [17]. These studies were to determine the leakages in the connectors' area. People often use the paraxial approximation to describe the fundamental mode because it makes it possible to simplify calculating and have very good results when using monomode fibers.

For the multimode fibers, you have to use more complicated and sophisticated methods to get good predictions. Some models as the ray tracing or the uniform illumination (that is to say assuming that the transmitting fiber uniformly lights the receiving fiber) are often used because they are simple, despite they fail when the two fibers are very unaligned.

These sensors coupling two fibers make it possible to measure:

• displacement

• pressure

• vibration

• positioning

• ...

In order to use this kind of sensors, you just have to design a mechanical assembly which can be moved by the measurand