Components and types of fiber optics

Fiber Optic Components

• Core: in silica, fused quartz or plastic. Optical waves propagate in it. Diameter: 50 or 62.5  µm for multimode fiber and 9 µm for single-mode fiber.
• Optical sheath: Generally of the same materials as the core but with additives that confine the optical waves in the core.
• The protective coating: it is generally made of plastic and ensures mechanical protection of the  fiber optics careers.

Optical fiber types

Single-mode fiber: Potentially, this is the fiber that offers the greatest capacity for information transport. It has a bandwidth on the order of 100  GHz / km . The highest flows are achieved with this fiber, but it is also the most complex to implant.

The drawing shows that only rays that have a path that follows the axis of the fiber can be transmitted, which is why it has earned the name of "single mode" (mode of propagation, or path of the single light beam). They are fibers that have the diameter of the nucleus in the same order of magnitude as the wavelength of the optical signals they transmit, that is, from about 5 to 8  mm .

If the core is made of a material whose refractive index is very different from that of the shell, then we are talking about single-mode fibers with a stepped index. The high fluxes that can be achieved are the main advantage of single-mode fibers, since their small dimensions imply delicate handling and entail connection difficulties that are still poorly mastered.

• Gradient Gradient Index Multimode Fiber - Gradient Gradient Index multimode fibers have a passband that goes up to 500  MHz / km . Its principle is based on the fact that the refractive index inside the core is not unique and decreases when it moves from the core to the shell.

The light rays are focused towards the axis of the fiber, as can be seen in the drawing. These fibers allow the dispersion between the different modes of propagation through the fiber core to be reduced. Multimode fiber with gradual gradient index size 62.5 / 125  m (core diameter / shell diameter) is standardized, but other types of fibers can be found:

• Stepwise index multimode 100/140  mm .
• Multimode with gradual gradient index 50/125  mm .

Multimode stepped index fiber: Multimode stepped index fibers are made of glass, with an attenuation of 30  dB / km , or plastic, with an attenuation of 100  dB / km . They have a passband that goes up to 40   MHz / km

In these fibers, the core is made up of a uniform material whose refractive index is clearly higher than that of the surrounding shell. The passage from the core to the roof therefore entails a brutal variation of the index, hence its name staggered index.