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Authors: Ruslan Lagashkin, Gebremariam Gudeta, Xingyu Gao, Ilia Kempi

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How can we organise light rays in a cable?

  1. We can put small beam of light right in the middle, so it is perpendicular to the cable interface, so it propagates always straight in the middle. This is called single-mode fiber.
  2. We can put light wavefront to the acceptance cone of the cable, so the light propagates on all cable area. This is called multi-mode fiber.

Which way is better?

It turns out, some ways are better in some applications, and some are not.

In multi-mode fiber, rays are constantly reflected from the walls of the core. When light enters the acceptance cone of the cable, rays which enter with bigger angle have much more longer total path, than light which enters directly to the center. This type of fibers is usually used to transmit optical signal to short distances (up to 2 km).

In single-mode fiber, core of cable is small enough (< 10um) that light propagates without reflections at all. But frequency of the light should be significantly higher than in multi-mode, and acceptance cone of the cable is so small that we need expensive laser light source. When producing signal for the multi-mode fiber, we can use just LED's. Single-mode fiber allows distances over 80 km ans is mainly used to transmit data at long ranges.

So, can we transmit parallel data streams in the multi-mode fiber?

Yes, theoretically it is possible: such techniques are called MIMO (multiple input - multiple output). However, this technique involves spectral modulation with different wavelengths of light, and should be much more expensive. Main reason to use multi-mode fiber is broad range of compatible light sources.

 

What is the relation between the Modes and form of the fiber optics?

The main components of a fiberoptic fiber are the core, cladding and buffer. While the first two are made from a type of glass the buffer is just for protective reasons and usually made of acrylic. In a single mode fiber optic the core is so small in size usually about 8 to 10um whereas in multimode its typically  about 50um. The following formula provides the relation between the thickness of the core, index of refraction of the cladding and the core and the wavelength.

From the above formula one can deduce that the mode is inversely proportional to wavelength and directly proportional to core radius.

 

Is there any other way to optimize signal transmission in the fiber?

Well, there is a way to balance the speed of different light rays in multi-mode transmission. It is done by making the refractive index of the core
gradually change into the index of the cladding
. In this case, light beams closer to the outer core are travelling faster than beams passing through the center of the fiber core. This results in sinusoidal shape of the path, like on the picture. Cables made by this method are more expensive, and are called graded-index optical fibers.

When talking about single-mode optical cables, those are always step-index, which means that refractive index is discretely different from core to cladding. Step-index multi-mode fibers are cheaper to manufacture, but they introduce dispersion of speed between different light incidence angles.

 


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