How do adaHow do the mode setters work??
In order to clearly explain how mode adapters work, without having to resort to physics and go down to its equations, we are going to allow ourselves several licenses when describing the different phenomena that occur inside an optical fiber.
The main problem we encounter when transmitting through MM optical fiber signals at high speeds, or in other words, signals with a high bandwidth demand, is modal dispersion, because its effects can be observed even in very short fiber runs.
-Modal Dispersion:
Normally the following metaphor is used to describe the phenomenon of modal dispersion: imagine that we illuminate the core of a MM fiber, it can be observed that depending on the angle of incidence the different light rays entering the fiber travel different paths, adding different distances for each of them, adding a different time inside the fiber, which produces that the signal arrives at the opposite end decomposed and distorted due to the differences in time that the signal has elapsed inside the fiber.
The problem with this simplification is that it is not true and does not illustrate what actually happens in the MM fibers we use, because it completely ignores the wavelike nature of light. This statement would be more or less true for fibers with a much thicker core, for highly multimode fibers, for example, in POF fibers.
Although this explanation is very useful and graphical, it is far from what really happens with the propagation of light inside a MM fiber. To understand the root of the modal dispersion problem, when we illuminate the core of a fiber with a laser we excite a set of distinct propagating modes inside it. A mode is a way in which light energy can propagate inside a fiber, these modes are discrete and can be calculated taking into account the geometrical characteristics of the fiber and the wavelength of the light entering it.
When light is confined inside an MM fiber, it is only possible for it to propagate through it by means of these differentiated modes, in other words, at any time inside an MM fiber we will only find a combination of these modes inside it.
The problem of modal dispersion comes because each mode propagates inside the fiber at a different speed, distorting the signal along the fiber, generating ISI (Inter Symbolic Interference) problems, etc...
*In these diagrams you can see a cross section of the core and through which regions more or less energy propagates inside the fiber, depending on the mode.
-SM to MM adapters
It is a very simple and economical solution that substantially improves the performance of this type of optical signals transmitted over MM fibers.
As you can see in the schematic, we simply fit the SM fiber core on the edge of the MM fiber core, instead of centering the two cores axially. What do we achieve with this? By illuminating only this small region of the MM core at the input, we excite with more energy a set of modes with a similar group velocity, thus reducing the problems arising from the disparate group velocities of the different modes of the MM fiber.
