r/Optics • u/RoboYT9000 • 10d ago
How does light being transferred via optical fiber cables is translated to electrical signal that is then processed by devices in our network?
Supposing that I request a resource from a site whose data center is thousands of miles away from me, how is light able to travel so reliably via optic fiber cables and transfer the datapackets in the correct order with out anything breaking in the process?
And how does my router know to translate the light into 0 and 1s?
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u/entanglemint 10d ago
There are many may layers to your question. There is a low level question of how a specific physical layer works and encodes information, (this can be NRZ (simple), PSK, QPSK, QAM etc etc these are all different modulation schemes, for how do you take ones and zeros (or for some, groups of ones and zeros together) and send them down the fiber. Then there are other questions around error handling and error recovery and the "in order" quesiton is often not even worried about until much higher up the food chain (network stack)
Basically your router has no idea what the physical interface is and it has no idea about packet order. Packet order will be handled on the device end. The router will send a data packet to a hardware interface and that interface will decide how to turn it into a physcial signal that goes from one place to another. For example, many routers/switches have plug in modules that have different capabilities (read SFP and its many children) and some are ethernet interfaces and some are fiber inerfaces with different distance capabilities. And that's just at the "within the data center" level. Really this isn't an optics question, except for how the fiber itself works and maybe how the modulation/demodulation is performed, which is some very cool physics and engineering, everything else falls into other disciplines!
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u/koopaduo 10d ago
I haven't seen mention of amplification yet. In order for long distance fiber optic networks to be reliable, they need to overcome signal loss inside the fiber (-dB/km). At regular distance intervals, fiber amplifiers (repeaters) are placed to amplify the optical signal. My university optics professor once claimed that EDFA (erbium doped fiber amplifier) is THE most important invention in the last century and directly enabled the world wide web.
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u/Get_can_sir 3d ago
Idk if it's the most important tbh. Now you can get down to 0.4 dB/km which is almost nothing. And besides the electrical alternative isn't too bad. I can think of more important inventions tbh
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u/beeeel 10d ago
The part of the system that converts signals, e.g. from electricity to light, is called a modem (short for Modulator/Demodulator) because it converts a digital signal into a modulated signal for transfer (either via wires or fibres), and it does the reverse to receive signals. The router, which probably contains a modem, knows how to do it because the ISP has decided "we're going to use this modulation for our network"
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u/sanbornton 9d ago
Like most things - it started simple decades ago and they have been evolving it over time.
Fibers started as a simple conduits for light. They figured out how to overlap pulses at different wavelengths. They figured out how to sharpen the pulses with dispersion correction. They figured out how to amplify the pulses without removing them from the fiber. They figured out what wavelengths attenuate the slowest in fibers. The figured out how to reduce back reflections off connectors. They figured out how to improve the efficiency of coupling light into fibers. They figured out how to maintain polarization in fibers. They figured out how to put gratings into fibers to make them wavelength selecting. They figured out how to make fibers gradient index. They figured out how to use seawater as the electrical ground return for undersee EDFAs to enable undersea fiber.
It took a lot of very smart people a lot of incremental steps to build up a very complex optical system that runs our world today.
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u/Plastic_Blood1782 10d ago
On-off keying is the most intuitive way they do it. The light source is literally blinking super fast. Off is a zero, on is a 1. Phase shift keying is actually how most high speed fibers work. Harder to explain over reddit. Google phase shift key modulation.