Glass Fibre (data transfer)
|Engkish subtitles possible . . .|
Thousands of small glass fibers divide a single source of light into tiny points. In the automobile technology one expects a data transfer through these light cables at the speed of light (more than 300,000 km/s.). At the
moment this speed is still reduced in the junctions. There, the light signals must be transformed into current signals and vice versa. In this case, photonic crystals promise a direct relaying, and with it, a considerable
increase in the transmission rate. This is however, insignificant for the automobile, because glass fiber cables only have to be amplified every 400 km.
In addition, there is the advantage that fiber-optic light conductors cannot be influenced by magnetic fields. Glass fiber cables are also no less flexible than copper cables.
The first tests were made with Light Emitting Diodes which transported their light into the glass fibers. However, to eliminate the damping through the LED-
- highly concentrated laser diodes with high oscillation-constancy are used,
- glass fibers made from especially pure quartz sand are used,
- light with a higher wave-length than that of visible light is applied,
- the light-beam is directly initiated into the fiber.
|Using these components, distances far greater than those normally found in cars, are possible.
The pre-requirement for all this expenditure is a silica glass, with a light transparency which is 15,000 times higher than that of window glass. It does not depend that much on the luminosity. The central issue is, that the
receiver clearly recognises whether a bit is placed or not. Of course the luminosity defined for one placed bit may not fall short. The signal is then called indifferent. A greater danger however, is that the bits overtake each
other (dispersion). The transmitters and receivers may also not be too strongly dependent on the temperature differences occurring in the vehicle.
Now, one may not envisage modern systems in such a way, that a certain series of zeros and ones was sent out from the laser diode, and then converted by a sort of photodiode with an IC, into electric signals again. The thereby attainable bit-rates are not high enough for the technicians. This is where multiplex, the
long time standard for copper-wire networks, comes into play. Several bit-series, with different wave-lengths, (comparable with colours) are transferred at the same time. To do this, several laser diodes and several
receivers are necessary. In addition, components which concentrate the individual fibers into one, and others which separate the different wave-lengths again. The common distance is travelled by one specific impure
fiber. However, in this case, light waves (like electric signals) can also cause interference.
In the end, it also does depend largely on which cable is used. There are two different types. The cable with 1/100 mm core diameter (single-mode) allows the ray only an almost straight passage. No reflection occurs from
the glass casing surrounding the core (up to 5/100 mm in diameter) with any other refraction. This applies only to cables with up to 5/100 mm diameter (multi-mode). The cores consist, because of the very low damping,
de facto, of glass, which accordingly, would be also be breakable, if not for the special casing which distributes a possible buckling strain so skilfully, that bending radii of up to a minimum of 1mm (!) are permissible.
However, they should not be thus laid. The critical junctions must be made with a tolerance of up to 1/1000 mm and are accordingly expensive. It depends on very exact adjustment of the cable ends to each other, which
must also be cut off precisely vertically. To keep the damping as low as possible, the ends are sometimes welded together, of course only after adjustment under a sort of microscope. Additionally, of course, the welding
points must be encased again to also drastically reduce the danger of them fracturing.