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How do fiber optic cables work in a broad sense?

G

genghis37

Junior Member
Hey. I don't really have any particular motive to post this question beyond sheer curiosity, but anyway, I've been wondering lately how it is the fiber optic cables work. I thought of a few assumptions on how they might work but they all appeared to have some flaw. I can't remember them either. The more I wondered about it though the more curious I've become.

So, finally, after the little background note, how do fiber optic cables transmit (not encode or decode) information across a line? I'm just wondering in a broad sense. I think some others might like to read about it as well.
 
Pulses of laser light of a single wavelength (for single mode fiber) or several wavelengths (for multimode fiber) travel through the fiber. The fiber is made of very pure glass, with a center core of glass with a certain index of refraction encased by a glass cladding with a different index of refraction. The light is reflected inside the cable by the difference in the index of refraction between the core and cladding, with very little loss.
 


<< The fiber is made of very pure glass >>


Not for TOSLink...

Anyway, to expand on what he said, the receiving device converts the pulses of light into 1s and 0s (light on, light off) - binary.

And, while optic cables resist external interference extremely well, some kinds should be used over 10m (not sure which, but definitely not the ones in the ground 😀).
 
Thanks. That gives me a good idea of how it gets transmitted through the line. I never would have been able to figure that out since I haven't take a course in physics or anything but I at least got an idea of what an index of refraction is online. I take it the light inherently wants to refract to say within the inner portion of glass. Seems pretty expensive though to generate a uniform light (through a laser?) and pure glass all throughout a long wire.
 
The basic concept is that, the fibre acts like a 'pipe' for the light - shine light in one end, and it shines out of the other.

The key process is that of total internal reflection - where light is travelling through a dense material (e.g. glass), and it hits an edge where the dense material meets a light material (e.g. air). If the angle the light makes with the boundary is very shallow, the light will be reflected. Try this at your local swimming pool - if you look across the pool from just under water, the surface will appear like a mirror.

Light enters the fibre, parallel to the sides - where the sides are no longer parallel to the light direction (because the fibre isn't exactly straigh), the light is totally internally reflected to remain within the fibre, and because the angle of reflection has to be shallow, the light continues in the same overall direction in the fibre.

You don't have to use glass, cheap fibres can be made of plastic (similar to perspex/plexiglass). For a simple demonstration you can use a stream of water, e.g. from a hose. I've done this by sticking an LED down the end of a garden hose so that it faces along the stream of water - the light is carried along the water 'fibre' and is released when the stream hits the ground.

Low-cost glass fibres are made from solid glass - no need for fancy mixtures of types of glass. Only the very expensive ultra-high speed fibres are made with a special core, and glass which gradually changes in refractive index from inside to out.
 
A lot of people have the misconception that light travels straight through the cable. They assume the 'speed of light' notion like this. But it's not. Instead, picture light inside the cable travelling in zig-zag patterns to get from one end to the other. Like the previous posts said, it's all about internal reflection.
 
In some fiber optic systems, LED's are also used as the light emitting sources. These are only capable of sending signals short distances, however (such as LAN's). But most fiber optic systems use lasers. I think optical audio cables use LED's (correct me if I am wrong).
 
Good Information here. Seems like a pretty interesting field. Thanks for the info.

One more question though: Is it possible for light to refract at an angle that is considered more than "shallow". Going along with what I think you were saying there, what if the light was travelling parrellal to the sides but then the sides changed angle quite abruptly. Is it possible (even if minutely) the transmission could get "messed up"?
 
Bends in the fiber, especially sharp bends, will attenuate the signal somewhat.

Laser-driven systems DO pretty much shoot the signal straight down the glass (very little internal reflection), multi-mode pretty much relies on the aforementioned zig-zag reflections to get to one end from the other.

Single mode fiber is much thinner glass - 8.3 micron - about 1/10 the diameter of the average human hair.

Multi-mode these days is typically 62.5/125 or 50/100 (X/Y: X = diameter of the glass in microns, Y=overall diameter of the fiber in microns, including the cladding).

FWIW

Scott
 
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