How does Fiber Optic carry data?

[ahartman]

I understand how it works inside a processor - gates and such.

I understand that fiber optic cables carry light, which carries data, but how does it work? A friend says if the light is on, that's equiv to a 1 and if it's off, it's equiv to a 0.

In addition, how do multiple signals exist in a fiber at the same time? Over a phone line, there's packets of information which enables them to be routed and interspersed with other packets going elsewhere.

I'm no engineer (I can't even play one very well in TFC), so please try to keep the explanation to where a lay person might be able to understand it.

Thanks in advance.

[Jammrock]

I work at a large US telecom company in the midst of 300 engineers so I have received a lot of insight on the subject. I will attempt to write as untechnical as I can, and explain technical terms as best I can, but no promises.

Most fiber optic systems carry information much like a radio does. Although the on/off technique is used quite a bit, it involves a lot of extremely complex timing circuits, even more so than other techniques. We are talking atomic clock accuracy! In truth, most telecom companies will use a combination of the techniques I am describing. Basically all they do is modulate (modulate basically means to alter a frequency, intensity or phase of a carrier frequency slightly to carry information) the laser using one of several techniques, in combination with or in place of the on/off thing.

A very common technique is to hook up the data lines to a coupled power transformer/toroid on the laser's power circuit. I won't go into details (it would take a lot of space), but basically it increases or decreases the intensity/power of the laser, commonly refered to as Amplitude Modulation or AM (yes, exactly like the radio AM). When the laser reaches the other side it hits a photosensitive diode (photocell). Basically they act light solar cells. The light hits them and depending on the intensity of the light, produces a voltage. When the voltage operates at normal, say 1-3 V, the receivers says its a 0, when the signal arrives a little more intense, say 5-7 V, then it calls it a 1 and sends the data along whatever comes next in the copper world.

There are several other techniques that people use, but I will not go into them much. You can also directly modulate the frequency (FM, like the radio) of the laser. Then at the other end you chop off the carrier signal (the carrier signal is the operating frequency of the laser, like 1.57 GHz) and you are left with the difference in signals. Certain fequencies are translated into different bits or bytes. FM allows you to put more data on the line at once, but is much more complex and costly to do. Then there is phase modulation (PM, but not like the time. Star Wars used PM ham radios to do those radio broadcasts on the Death Star attack scene), but I'm not going to touch that, I've typed too much as it is.

As for multiple signals on the same fiber...its time to get technical. Light operates at a specific frequency and wavelength, just like a radio signal (a common red light frequency is 1.57 GHz or 635 nm wavelength). Basically all they do is take modulated laser light at different wavelengths, say 550 nm, 635 nm, 700 nm, 750 nm, and shoot them down the same fiber. Since the wavelengths and frequencies are nowhere near each other they do not cause any signal interference. Much like there are 15-35 radio station pumping out signals at a 200 KHz difference, but they never interfere with each other unless there is a broadcasting error. At the other end they use a complex set of mirrors and prisms and who knows what the hell else they use (I'm a techie not an optical engineer) and split up the diferent wavelengths so they go to seperate photocells. Current systems use up to 40 wavelengths down the same fiber!

There is the run down. Sorry I couldn't make it with less technobabble.

Jammrock

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Athlon 650, Biostar board, 128 MB PC133 (Crucial), G400 32 MB DH, SB Live! w/ Digital I/O, 10/100 NIC, lots of case fans, etc...

[ahartman]

Thanks! That's about the level of detail I was looking for.

Andy

[Gurm]

So you weren't looking for the "they flash a little light at one end and watch for it at the other end" answer I was gonna post before the fori crashed?

- Gurm

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Listen up, you primitive screwheads! See this? This is my BOOMSTICK! Etc. etc.

[Jammrock]

They used to do that Gurm, but it caused too much eye strain =)

Jammrock

[MarkD]

Just another two cents about multiple channels in one fibre:
pumping different wavelengths in one fibre is one solution ('wavelength division multiplexing'), which is only starting to be implemented.
Another - more standard - solution however is just mixing multiple low-data-rate channels to one high-data-rate channel ('time division multiplexing'), as is also done in non-optical networks.
The combination of both techniques (WDM & TDM) allows HUGE data-rates through 1 single fibre: more than 1THz = 1,000 GHz = 1,000,000 MHz.

[Jammrock]

Working in the engineering section of a telecom company let's me see all the new technologies. Let's just say a T1 or T3 line is a drop in the bucket compared to what some of the network backbones that these companies use. In the more densely populated areas they have over 100 fibers (physical cables) all running DWDM (Division Wave Data Multiplexing). Each cable being able to handle up to 400 Gb/s. The next gen of equipment will handle in the Tb/s range. And that's just from one of 3 major telecoms!

Jammrock

[Marshmallowman]

There are 2 kinds of optic cable, mono mode and multi mode. Mono mode is optimised for one particular frequency, it cheaper to produce becasue it uses a central light conducting core with one "optically reflective" layer to keep the light bouncing down the fibre. The junction betwwen the inner core and the outer sheath appear almost perfectly refelective(kind of) to the particular frequency of laser light used.

But multimode optic cable has a more diffuse or multi layered layers to reflect various frequency's. optimised for a range of frequecies.

The both can carry very similar data rates. But the use of multiple modes can be very nice for carrying multiple streams of info.

Well thats what I was told 9 years ago!.
I have no doubt they probably have cooked up some nice new optical technologies by now.

[Cmag]

In the old office building I worked in, one of the companies installed a fibre optic backbone. I was talking to one of the installers and he was explaining to me that the 3 foot by 2 foot transmitter and receiver for their system was about a half million dollars. Well as you can imagine, I immediately put off ordering one of these for my home computer. Maybe next month

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Consequences smonsequences, as long as I’m rich


[This message has been edited by Cmag (edited 26 March 2000).]